Been thinking about 4 link rear suspension. Did some searching and some guys (actually a lot on this board) seem to like more squat than anti-squat? Is this correct? Why? Many people on this board mount their lower rear links on top of the axle. With both links on top of the axle doesnt this give you alot of squat in the rear? I would think the lower links should be mounted below the centerline of the axle. This would push through the frame and counteract with the pulling forces on the frame from the upper links. I also would think that some anti-squat is a good thing. Especially climbing and keeping the front end down. It would be like re-distributing the weight back to the front instead of all the weight being in the rear. Does any of this make sense? More than anything help me understand why people mount both upper and lower links on top of the axle? :confused:

I am still VERY new to the link thing. (emphasis on the VERY! and the NEW!) Anyway it doesnt matter if the bottom link is above the pivot point of the axle under torque as long as the top one is high enough to keep it on the anti squat side of the scale. The problem is most people want the clearance. So in theory you want both links as far away from the pivot point of the axle tube because it stresses on the joinst a hell of a lot less because there is more leverage farther away from the centerline (thing a cheater bar on a wrench) (if I sound like I am simplifying too much sorry :rolleyes: ) Anyway people put them on top for clearance while sacrificing other characteristics. My personal opinion I like the idea of a torque arm setup. The arm design defines the amount of anti squat while you can keep the other links (probably 2) above the axle because all they do is position the axle all torque is applied to the arm (anti-squat applied threough this arm). Its like a traditional wrap arm and two or 3 more links for positioning. like camaros did.


I am even thinking torque arm and two triangulated links to position laterally. and torque arm takes care of anti squat and torque application on the 3rd member. I seem to have the same question as every other swinging dick around here who has thought about a link't spension... HOW FAWKIN MUCH SQUAT/ANTI-SQUAT DO I WANT??? and why???



BTW I am late its tired and make no sense sorry post questions and I will try to 'splain best I can. :zzz:

IMHO anti squat is a bad thing to have in the rear for hill climbing.

Lets say you have all the links parallel so that the torque reaction dosent produce any anti squat. In this instance any anti squat will be produced by the angle of the links going up from the axle to the chassis. The steeper the angle the more anti squat.

So you must look at what this anti squat from the angled links will do. As the axle drives forward it do so through the links and cause the links are on an angle it will produce a lifting force on the chassis. Because the weight on the back axle is carried by the springs and now that some of the weight is carried by the lift produced in the links the springs will not carry as much weight and therefore extend (this is why its called anti squat cause it stops the rear from squatting)

So you can see that the rear will not squat as much with the anti squat in the rear and the above effect in itself is not a bad thing.


Why it is bad is because of this:

Now that some of the rear weight is carried by the links it has, in effect, moved the centre of lift of the rear suspension forward. (cause now part of the weight is carried by the links and part of the weight is carried by the springs) You could imagine that if the links were angled so steeply that the axle would start to walk underneith the frame and the entire weight of the rear could be supported only through the links. So what this means is that a rear setup with anti squat when throttled (like in hill climbs) the action of the lifting force of the rear suspension is moved forward which is exactly the same as physicaly moving the rear axle forward in terms of the weight carried by the front and rear axles. With the rear axle moved forward (or a rear axle with lots af anti squat) the rear axle takes on a lot more weight and the front axle takes on less. There is no way in hell that someone would move the rear axle forward to make a better hill climbing machine. Well this is exactally the same result that anti squat will produce and this is why it is bad for extreme climbing hills. (although on some hills that arnt that steep and have a loose surface getting more weight transfer onto to rear helps as the rear digs in harder and produces more traction (a bit like running skinny tyres).

I posted some stuff on this a while back. Ill see if I can find it and repost it cause I think I made bit of sence.

Sam

OK I found the thread it was in

http://www.pirate4x4.com/forum/showthread.php?s=&postid=64777#post64777

Ill pull out what I wrote cause the thread discussed a lot of issues and I came in at the end.

Ill cut and paste in the next few replys (three of them I think)

Sam

Ill just cut and paste them all into this post. I have learnt a lot since I made these posts and I still feel that what I have said is correct.

Sam

----------------------------------------------------------------------


Here are my thoughts for what they are worth.

To set up the rear suspension to drive up hill or to hook up properly when you throttle it you dont want any anti squat. If you have anti squat the rear will try to walk underneath the rig when you hit the gas. Two things give you anti squat. Firstly having the links at too much of an angle going down from the chassis to the diff as what happens when you lift a standard truck up to much. What happens here is when the rear pushes forward it will push up at an angle on the rig and lift it and the rear will start to walk underneath and make the angle worse and so on. Long links fix this and horizontal links also fix it. This is why you should mount all the links on top of the axle to keep the links as horizontal as possible while still giving you the lift you need. Generally the higher you lift the frame off the axle the harder it is to get rid of the anti squat. The second cause is when looking from the side of the frame if the upper and lower links are not parallel and converge to a point (or converge at all). What this does (if they converge to a point) is lift the rear of the rig from the torque reaction of the diff. When you gas it the rear diff trys to rotate backwards and if the links go to a point it will lift the rig at that point and unload the rear and start to walk etc.. If the links are parallel then the rotation of the rear is controlled by the diff pushing and pulling on the frame through the parallel links and does not produce any lift (or anti squat effect)

So I believe the best setup is to have the links parallel and as horizontal as possible (when looking from the side). This means if the rig is lifted they should all go on top of the diff (just like Lances cruiser, from what I here it climbs umbelievable) and be parallel.


---------------------------------------------------------------------


OK guys - heres just a few numbers to thow at the effects of converging arms (when looking from the side) giving unwanted anti squat.

Lets say you throttle it a bit to punch up a hill with a modest 100ftlb in a modest 50 to 1 overall reduction this get a torque in the rear axle of 100x50 = 5000ftlb which has to be constrained by the rear links.

Now say your links converge and are 4ft long, to resist the torque the arms will lift at the chassis at the convergent point. This lift will be 5000 divide by 4 = 1250lb which will be lifting up 4 ft in front of the rear springs on the chassis and unloading the rear springs by the same 1250lb. This means that instead of the rear axle supporting all its weight above the axle 1250lb of it is supported 4ft in front of it which takes weight off the front axle and loads more on the back.

say there was 2500lb on the rear suspension and 1500lb on the front before you throttled it with the rear lifting up 1250lb at the 4ft mark this will (now say the wheelbase is 8ft long) unload the front suspension by 625lb and throw this extra weight on the rear so now you have 875lb on the front and 3125lb on the rear. Now this is BAD you want the weight on the front axle to drive up hill not the rear and the anti squat is doing the exact opposite.

This calc was done at modest torque and gearing and will get much much worse with more right boot.

The other thing this does is the 1250lb not going through the rear springs will unload the rear springs (say each spring is 300lb/in times 2 is 600lb/in) now 1250divide by 600 = 2 inches (this is ignoring the weight transfer effect from above cause my brane hurts) thus lifting the rear by 2 inches which increases your effective rear lift thus the rear angle of the arms giving you more anti squat by the arms pushing up on more of an angle which will lift the rear more and transfering more weight to the rear tyres etc etc . Which is again BAD.

The above calc is very rough but im sure that the numbers are about right.

Believe me if you want to get it awn and throttle it up hills you have got to get the power to the ground and anti squat is the killer. Get some videos of blokes with mega lift and buckets of anti squat throttle it and watch that back axle walk and the rig go no where.

Sam


----------------------------------------------------------------------------


More numbers!!

The effect of the angle of the links (when looking from the side) isnt as great as the effect of converging arms on anti squat.

Say the same 100ftlb at 50:1 gives 5000ftlb at the axle. If we have 36in tyres this gives a tyre radius of 1.5ft. So that the forward push on the links from the tyres will be 5000 / 1.5 = 3333lb.

If the same 4ft links are at an angle coming down from the chassis to the axle with a 1ft drop then the lift on the chassis will be about in the ratio of 4 to 1 so that with every 4lb pushing you will get 1lb lifting at the chassis mount point.

So with 3333lb pushing you will get 833lb lifting at the chassis. And if you mounted all the links on top of the axle to get the arms as horizontal as possible I doubt that you will have 1ft drop from the chassis mount point to the axle mount point. So that the 4 : 1 ratio should be much less. So that the 833lb should also be much less in reality.

This compares with the 1250lb lift caused by the convergent arms which if you do have converging arms to get the pinion always facing the transfer you will always get and there is no other way around it.

Sam


------------------------------------------------------------------------------


I threw a few numbers around just to demonstrate what anti squat actually does. When you gas it and have anti squat in the rear the effect is the same as if you moved the rear axle forward on the rig and had a big honkin rear overhang. Anti squat takes weight off the front and loads it on the rear because part of the weight the rear springs support is actually supported where the links attach to the chassis ie forward of the rear axle.

It dosent really mater what the numbers and ration and lengths you end up with but you should understand what anti squat is, what causes it, and how to minimise it because it is the difference between building a four link that can flex and one that can flex and hook up and get the power to the ground when you gas it.

To minimise the anti squat the upper and lower links should be parallel. The biggest cause of antisquat is using links that converge to one point on the chassis (when looking from the side)

The other cause is the links coming down from the chassis to the axle on an angle although this dosent cause as bad an effect as converging links. If the links came down at an angle of 45 deg this would be really bad but if they dropped by say 6 to 10 inches over a 45 inch long arm then this would be good.

If the arms were totally horizontal and parallel then you would have no anti squat and the rig would squat on the rear springs when you gassed it (cause there is no lift from the links to help). This may not be such a good thing either cause as the rig squats on the rear springs you will again get a weight shift to the rear so some anti squat could be a good thing.

Generally to build the links and fit everything into your rig you will have the arms coming down at a bit of an angle from the chassis to the axle and you probable will have them converging a little bit as well so I feel that just fitting everything in gets enough anti squat and you should just try to minimise it.

Sam

---------------------------------------------------------------


OK thats it. If you think im full of shit then flame away. You probably should have a look at this thread and also the god of suspension one as well there is a lot of good stuff im them.

Sam

[QUOTE]Originally posted by scouter77
[B]I am still VERY new to the link thing. (emphasis on the VERY! and the NEW!) Anyway it doesnt matter if the bottom link is above the pivot point of the axle under torque as long as the top one is high enough to keep it on the anti squat side of the scale.

I would disagree here. I would think the top links promote squat because they will be pulling on the chassis when the gas is applied. If the lower links are above the axle centerline they would be pulling on the chassis as well which means even more squat. Which I would say is a bad thing. However mount the lower links below the centerline of the axle and they are pushing throught the chassis whcih promotes anti-squat. With the lower links pushing (anti-squat) and the upper links pulling (squat) they will more or less cancel each other out evenly. Does any of this make sense?

The problem is most people want the clearance. So in theory you want both links as far away from the pivot point of the axle tube because it stresses on the joinst a hell of a lot less because there is more leverage farther away from the centerline

I agree but it would much stronger and resist much more of the twisting force of the axle if there were links below the centerline of the axle and links above the centerline. The further apart the stronger as well.


.. HOW FAWKIN MUCH SQUAT/ANTI-SQUAT DO I WANT??? and why???

That seems to be the magic question. The other question is how can each be measured, before you get everything all welded up?

[QUOTE]Originally posted by Strange Rover
[B]IMHO anti squat is a bad thing to have in the rear for hill climbing.


Now that some of the rear weight is carried by the links it has, in effect, moved the centre of lift of the rear suspension forward. (cause now part of the weight is carried by the links and part of the weight is carried by the springs) You could imagine that if the links were angled so steeply that the axle would start to walk underneith the frame and the entire weight of the rear could be supported only through the links. So what this means is that a rear setup with anti squat when throttled (like in hill climbs) the action of the lifting force of the rear suspension is moved forward which is exactly the same as physicaly moving the rear axle forward in terms of the weight carried by the front and rear axles. With the rear axle moved forward (or a rear axle with lots af anti squat) the rear axle takes on a lot more weight and the front axle takes on less.

How can that be? It seems like if the rig has anti-squat more weight would be shifted to the front. Not the rear. I would say squat transferes weight to the rear. What happens when the rig squats, the front end will lift. So that must mean that when a rig squats all the weight is transferred to the back. Not a good thing when climbing.

I am thinking about the rotational forces of the rear axle. with both links on top of the axle there is no pushing force and both links will be pulling. If you put 2 links on top and 2 links on the bottom then the lowers will be pushing and the tops will be pulling therefore counteracting each other. I still think when mounting the lower links below the centerline of the axle, and with the links being long enough (smaler angles) you can transfer some weight to the front which is a good thing when climbing. Does this make any sense or should I just go back to sleep.

:)

That was actually pretty helpful... thanks Strange Rover.

Hopefully I can get started on a design for my own vehicle soon.

These threads always make my brain hurt.
Sounds like you guys got the concept down. It also sounds like you guys are up against the same questions that everyone who builds a 4 link come up against........................................... ............ If you haven't figured it out yet, the answer is THERE IS NO PERFECT SET UP!
All you can do is, look at other peoples designs, figure out what there problems are and try to improve on there design. You can learn alot by going to these rock crawling championships and watching how the different suspensions work. What you will find is, some work good for climbing but suck for sidehilling. and others sidehill good, but can't make it down an obsticle without going over.
It's a big give and take game. I have yet to see a setup that works excellent at everything.
SO GET STARTED! Lets see what you got! The more people that build 4 link setups for rock crawling, the more EVERYONE will learn about what will work and what will not.
Here's mine!

Originally posted by Strange Rover
Ill just cut and paste them all into this post. I have learnt a lot since I made these posts and I still feel that what I have said is correct.

Sam

----------------------------------------------------------------------


Here are my thoughts for what they are worth.

To set up the rear suspension to drive up hill or to hook up properly when you throttle it you dont want any anti squat.

Are you saying you want squat for hill climbing?

If you have anti squat the rear will try to walk underneath the rig when you hit the gas.

Maybe in extreme cases. Ever watch a drag race? Cars that are built right dont squat in the rear. Why? Because they build enough anti-squat into the car so the weight is transferred forward, meaning quicker launches. I dont see their rear axles walking underneath their chassis.

Two things give you anti squat. Firstly having the links at too much of an angle going down from the chassis to the diff as what happens when you lift a standard truck up to much.

This I would agree with. What I am saying is having the links long and flat, but the lower should be mounted below the centerline of the axle. This is what will redistribute weight forward but the angle isnt so much that the front will raise. And the links are long enoguh so the rear wont raise.

the rotation of the rear is controlled by the diff pushing and pulling on the frame through the parallel links and does not produce any lift (or anti squat effect)

Exactly, but dont you think in order to push and pull the links have to be above and below the centerline of the axle. Put both links on top and there really isnt any link that is pushing. The bottom half of the axle is pushing and there are no links there if you put both on top.

Anti squat takes weight off the front and loads it on the rear because part of the weight the rear springs support is actually supported where the links attach to the chassis ie forward of the rear axle.

I dont understand this. How can anit-squat put weight on the rear. what happens in anti-squat? The rear raises, usually when something raises it means it gets lighter, no?



This may not be such a good thing either cause as the rig squats on the rear springs you will again get a weight shift to the rear so some anti squat could be a good thing.

How can anit-squat (above) and squat BOTH transfer weight to the rear?


Sam

Not flamin ya Sam, just trying to learn.;)

This is all very interesting.... I have recently installed a 3 link/panhard setup front and rear on my truck. I researched basically POR and took the advice of others in building it. I understand the principles now.

I applied what I read about rear suspensions to the front, albeit with some differences. I'm wondering what everyone else's opinon is about front suspension? Dive or anti-dive? On most trucks that i've seen it's impossible to eliminate anti-squat or anti-dive. Of course we can minimize it though.

Squash

from what I have learned with drag cars, and thought about aplying these concepts to offroad I feel you need SOME Anti squat. If you have Squat effectivly your truck is trying to lift the back tires off the ground, meaning initially there will be slightly less weight on the rear tires until the weight transfer occurs. If you have SOME anti squat your truck pushes down on the tires lifting the rear, giving more initial bite. I think these initial effects are very important because started from a dead stop requires more traction then if you have some momentum. Now if you have ALOT of anti squat you will over power the traction of the tires and they will slip, or you run into the problems of transfering to much weight of the rear end.

And by mouting both links on top of the axle, is it still possible to have a little anti-squat, which in my opinion is a good thing. How is it possible?

Just keep them (LINKS) as parallel as possible. Thats it.

All very good guestions. I havent got the time now but later on today (its 7:30am hear now in OZ) I try to explain how I believe it works and I will draw some pictures and take photos of em and post them.

clc900 I can understand where you are coming from with your statements.

Sam

http://community.webshots.com/storage/1/v1/4/26/96/30642696abWUZlmXsO_ph

dont know if that image comes up for you.
the 11.4 location is incorrect it should be at the force vector intersection.
i cant see how the control arms being above axle centre line changes the anti squat calcs at all.
the forces are still trying to pivot the axle, the lower link will still get pushed and the upper pulled as the pinion trys to rise.

I have been running Milemarker 1/2T style lockouts (purchased through ebay) on my Ford HP/Chevy knuckle/'89 Waggy outer combination axle in my XJ for a little less than a year. I have no other experiences with lockouts. They are all metal and seem to be working ok. I did crack part of one when smacking the end on a rock (damn I wish they didn't stick out so far), but it is still operational. I don't think they seal all that well though as there is only a small o-ring around the rotating knob and a small o-ring around the actual body of the hub. The bolts are not sealed in any way and I think the rotating knob doesn't seal too well even though there is an o-ring in there. HTH a little. Jeff

Sorry about that. I don't understand how that got posted under the wrong thread? I would delete it, but I'm not sure how to do that either. Jeff :confused:

I 4-linked the ft and rear of my cj7, no science, just simple math and lots of trial and error to get everything to clear all the necessary things that couldn't be moved any further... it works great... no squat no lift, it just hooks up and corners pretty flat. I think some people are working too hard at this. There is only so much room to work with. All this center of gravity/roll axis/instant center text book info. is nice, but, if there is no room for the engine, or the rig needs 12" of chassis lift to make it work... what good is it ?
The basic theories are sound, but how much of it can be applied to a crawler ? ...It's kinda of a perfect world/real world situation imho.

Make the links as long as possible, make the bars and mounts strong, and make sure that everything has a good bumpstop to hit before it bottoms on something hard.

Originally posted by dirtrod
I 4-linked the ft and rear of my cj7, no science, just simple math and lots of trial and error to get everything to clear all the necessary things that couldn't be moved any further... it works great... no squat no lift, it just hooks up and corners pretty flat. I think some people are working too hard at this. There is only so much room to work with. All this center of gravity/roll axis/instant center text book info. is nice, but, if there is no room for the engine, or the rig needs 12" of chassis lift to make it work... what good is it ?
The basic theories are sound, but how much of it can be applied to a crawler ? ...It's kinda of a perfect world/real world situation imho.

Make the links as long as possible, make the bars and mounts strong, and make sure that everything has a good bumpstop to hit before it bottoms on something hard.

THIS IS THE WORST ADVICE I HAVE EVER HEARD

There is a lot of science to designing a 4-link and they are pretty easy to fock up. The reason you design something is to get the best out of what you have. You could easily create and entire thread about how to fock up a link suspension and it would probably be 10X longer than a thread on how to built a 4-link properly

If you want a 4-link go build one but i garuntee that there is a dirrect relation ship between how well designed your link setup is and how well it works.

monoball is similar but dont think they use poly
http://www.stockcarproducts.com/index.htm

Originally posted by TRD


THIS IS THE WORST ADVICE I HAVE EVER HEARD

There is a lot of science to designing a 4-link and they are pretty easy to fock up. The reason you design something is to get the best out of what you have. You could easily create and entire thread about how to fock up a link suspension and it would probably be 10X longer than a thread on how to built a 4-link properly

If you want a 4-link go build one but i garuntee that there is a dirrect relation ship between how well designed your link setup is and how well it works.

If that is the worst advice you have ever heard, YOU HAVE LED A SHELTERED LIFE

Ok, lets hear how you have focked them up...I've built a few rears with no problems other than the strength of the joints. I've only built 1 front set-up and I can't find any other place to put the links other than where I put them...There is only a few places these things can mount and still clear the frame or engine when stuffed. We are talking REAL World here, a crawler that must be kept as low as possible with lots of ground clearence and lots of wheel travel...

There is many ways to design a rear link suspension. Trial and error is one of them. Copy someone elses is another. If you want to copy someones I think that Desert Toy has got very good design. Its got little anti squat and little roll steer (by the looks of it). And as Goat Boy said keep them parallel which again I think is a good thing.

The beauty of understanding all this is that you can have a look under the back of a rear linked rig and see how the suspension is set up and how much anti squat it has got (just by looking at it) and then see how it goes when it throttles up the hills. And then do the same on the next rig that drives the same hill and then on the next and then the next. After a while you can look at them, see how they throttle up the hills and figger out which ones work well and which ones lift front tyres and have trouble. Then you go and stick your head under the rear and say yes I can understand why that rig performed like that on the hills and make a judgement on what is the best setup and why.

From then you can build your own setup (copy the one that works well if you want) but like dirtrod said a lot of it depends on what you can fit under your rig. When you start running links and stuff you run outa room real quick. But if you understand what works and why you will know which elements and important and which you can make comprimises on and you will still get your rear end to work well.

Also different amounts of anti squat in the rear work well in different types of hills. I have seen some rigs with lots of rear anti squat absolutly idle up some stuff that in my rig (that has little anti squat) I have to drive at full noise in second. And then some types of terrain where if you have lots af anti squat you will bunny hop and almost flip over backwards and somethiung with little anti squat will drive it easy. So by understanding the principles of this stuff you can get the best setup for the type of wheeling that you do.

I will post some diagrams soon (run outa time again) cause this is actually a simple principle to understand and once you understand it you will stick your head under every rig see and you will never stop learning (jees I sound pathetic here) but any way I gotta go.

Sam

OK I have thought about it and maybe its not all that easy to understand.

Firstly I will say this that I have never read any books on this stuff and this is just what I think but I am bloody sure that I am right (but then again I could be spewing utter BS but I dont think that I am)


The first thing to realise is that anti squat stops the rear suspension from squatting and it does this by using the reaction of the rear links to lift the rear of the rig up. The two ways of doing this is by converging links that lift the rear by the torque reaction if the diff. And by links that rise to the chassis from the diff on an angle which lift the rear by the pushing of the rear links at an upwards angle on the chassis. Now because the links lift the rear of the rig not as much weight has to be carried by the springs and the springs will not compress as much (or may in fact extend because the links are pushing up so hard)

The down side to this is the links lift the rig in at more forward point on the chassis, forward of the springs and therefore shift the centre of lift of the rear suspension forward. Now because the centre of lift of the rear suspension is more forward the rear suspension will take on more weight in exactally the same way as if the rear axle was physically moved forward and this is why anti squat puts more weight on the rear axle (even though the rear springs dont compress as much)

The reason why the rear dosent compress as much is because the links arer now doing some if the lifting and the springs dont have to support as much weight with the help from the lifting links. But in doing so the rear supports the weight at a more forward point and therefore unloads the front tyres and loads more weight on the rear.

Anyone who is interested in what Im trying to explain needs to understand what I have just said. When I read my explanation I can see that it is not the best so if anyone dosent understand it and wants to just say so and I will draw some diagrams and post them.

Once the basis of what anti squat does and how it effects the weight distribution on the front and rear tyres is understood (or should I say how I understand it) we can then talk about different types of link setups and how much lift they produce on the chassis and how it is best to locate the rear axle.

Im sorry if I come across sounding like a "know it all" because I dont mean to. I just really enjoy discussing this kind a shit and I have learnt heaps in a couple of other threads of this nature and I htink that the more of this kind of tech that is discussed the better.

Sam

Originally posted by NE-RokToy
from what I have learned with drag cars, and thought about aplying these concepts to offroad I feel you need SOME Anti squat. If you have Squat effectivly your truck is trying to lift the back tires off the ground, meaning initially there will be slightly less weight on the rear tires until the weight transfer occurs. If you have SOME anti squat your truck pushes down on the tires lifting the rear, giving more initial bite. I think these initial effects are very important because started from a dead stop requires more traction then if you have some momentum. Now if you have ALOT of anti squat you will over power the traction of the tires and they will slip, or you run into the problems of transfering to much weight of the rear end.

Yes I believe this is true. Drag cars run with a fair bit of anti squat in the rear. They use anti squat to take weight off the front and put it on the rear to get more traction for the launch off the line. In four wheel drive IMO you dont need to take weight off the front and load it on the rear because the front wheels are driving as well and they can look after themselves. So I dont think that on a high traction surface you need anti squat in the rear. On a surface where skinny tyres are usefull, anit squat may also be usefull because it is another way to increase the tyre pressure on the ground for more grip.

Sam

clc900,

I dont think it matters whether the links are both above the axle or above and belov or both below the axle. If the links are parallel thet will still push and pull on the chassis. If both the links are above the axle they will still push and pull although the loads will be greater in the links because they are further away from the point of load application (the tyre contact point). But the resultant nett load on the chassis will still be exactly the same whether the links are above or below. What happens when the links are all above the axle is that the angle of the links can be made less so that they produce less net upwards lift on the chassis and hence a less anti squat effect. If the links were mounted so high that they ran parallel to the ground then they would produce no anti squat effect although because the links are so far away from the tyre-ground contact point the loads on the links would be much grearer (the pushing and pulling forces would be numerically greater but the difference between the too would be the same (the amount the tyre pushes on the ground)

The way to think of the loads on the links is to only look at the effect of the tyre push force which a horizontal push force at the tyre-ground contact point. When you think of it this way the position of the axle is irrelevant(sp?)


Damn I may not be making any sence now (or ever) I really got to go to bed.

Sam

I'll admit that I am no expert on suspension design, but I think I can give you some good advise. I am an engineer who has actually design and constructed a number of vehicles that actually work well. This thread started with a question about anti-squat and the location of the rear links above the axle. The discussion has centered around load transfer with anti-squat, but anti-squat has little to no effect on load transfer. The front to rear load transfer under acceleration or braking is governed by the height of the center of gravity (CG), the front to rear location of the CG, the length of the wheel base and the force of acceleration. Squat and anti-squat control the pitch of the vehicle. Anti-squat does take some of the load off the springs and transfers it to the control arms, but the total weight at the rear tires doesn't change. A case in point, is that you can actually design a car with enough anti-squat in the front and rear that when it accelerates that it will lift up off the ground. The car didn't get any lighter in this case it just transfer load from the springs to the control arms.

The other question was about the physical location of the control arms. The answer is that it really doesn't matter. If the rear attachment points are both on top of the axle the lower control arm has to be much stronger than if it was below the axle because it is taking more of a load.

I also thought I would give you some practical advise about building a 4-link suspension. I would say the most important thing is that the longer the control arms the better. The other point is to keep them as close to parallel as posible. If you do both of these things the location of the instantaneous center will not move very much when the suspension moves. To make a vehicle corner well you need to keep the roll center close to the ground and this is a fairly complicated process to explain. But if you keep the arms long and parallel (and parallel to the ground) you will end up with a pretty good location for the roll center. All of this advice is good for the suspension design, but keep in mind that keeping the CG low is by far the most important thing in making a vehicle handle well.

Originally posted by Suprdlux
that keeping the CG low is by far the most important thing in making a vehicle handle well.

And how is this done?


I am starting to understand this a bit more now. I think seeing both links on top of the axle threw me for a loop. However I am beginning to see the benefits and that it really doesnt matter if they are both on top. It is actually better.
Thanks:beer:

Oh gurus, I'm about to weld in the final link mounting tabs on my CJ's 4 link and want to make one last check.
Upper arms are 11" above centerline of 14 bolt. Lower arms are at centerline of tubes. Arms nearly flat to ground. And the convergence point of the links aka instant center is 185" forward of the rear axle.
I figure that's a hair of anti-squat but not much.
Sound good?

I am thinking about the rotational forces of the rear axle. with both links on top of the axle there is no pushing force and both links will be pulling. If you put 2 links on top and 2 links on the bottom then the lowers will be pushing and the tops will be pulling therefore counteracting each other. I still think when mounting the lower links below the centerline of the axle, and with the links being long enough (smaler angles) you can transfer some weight to the front which is a good thing when climbing. Does this make any sense or should I just go back to sleep.
------------------------------------------------------------------------------------

if you keep the upper and lower arms at least 10 inches apart, you can control the twisting.... even if the lower arms are on top of the axle.... at least from what ive seen.

I dont understand this. How can anit-squat put weight on the rear. what happens in anti-squat? The rear raises, usually when something raises it means it gets lighter, no?


im no pro, but what i understand is... when you have lots of antisquat the weight is tranfered throungh the links to the ground... the body and frame have nothing to do (or less to do)with the weight under this high antisquat set up

Originally posted by badassjeepguy
I dont understand this. How can anit-squat put weight on the rear. what happens in anti-squat? The rear raises, usually when something raises it means it gets lighter, no?

From what I read on other posts and thru drag racing is when you have anti-squat the vehicle does raise but you have a equal and opposite reaction which is the rearend is forced down to the ground. So the jeep or what ever does lift up but you are putting the same force to the ground. Do you suspension guru's agree with that.

Originally posted by Donovan


From what I read on other posts and thru drag racing is when you have anti-squat the vehicle does raise but you have a equal and opposite reaction which is the rearend is forced down to the ground. So the jeep or what ever does lift up but you are putting the same force to the ground. Do you suspension guru's agree with that.



i didnt post that, clc900 did, i agree with you.....

Maybe I'm not seeing things clearly, but if you have a considerable amount of anti-squat in the rear, then your rear end will rise noticeably. If the rear end rises and the rear tires stay in the same location relative to the front tires, newtons laws included, weight will transfer to the front tires.

To help illustrate my point (which may be completely incorrect, but atleast you can understand what I'm saying and correct me) think of what happens when you throw a hilift under your rear bumper and start cranking. The front end will end up with more weight won't it? Just like when braking, the front end gets more traction at the tires because of the added weight.

I still run leaf springs so I don't know jack.

Originally posted by Suprdlux
I'll admit that I am no expert on suspension design, but I think I can give you some good advise. I am an engineer who has actually design and constructed a number of vehicles that actually work well. This thread started with a question about anti-squat and the location of the rear links above the axle. The discussion has centered around load transfer with anti-squat, but anti-squat has little to no effect on load transfer. The front to rear load transfer under acceleration or braking is governed by the height of the center of gravity (CG), the front to rear location of the CG, the length of the wheel base and the force of acceleration. Squat and anti-squat control the pitch of the vehicle. Anti-squat does take some of the load off the springs and transfers it to the control arms, but the total weight at the rear tires doesn't change. A case in point, is that you can actually design a car with enough anti-squat in the front and rear that when it accelerates that it will lift up off the ground. The car didn't get any lighter in this case it just transfer load from the springs to the control arms.

The other question was about the physical location of the control arms. The answer is that it really doesn't matter. If the rear attachment points are both on top of the axle the lower control arm has to be much stronger than if it was below the axle because it is taking more of a load.

I also thought I would give you some practical advise about building a 4-link suspension. I would say the most important thing is that the longer the control arms the better. The other point is to keep them as close to parallel as posible. If you do both of these things the location of the instantaneous center will not move very much when the suspension moves. To make a vehicle corner well you need to keep the roll center close to the ground and this is a fairly complicated process to explain. But if you keep the arms long and parallel (and parallel to the ground) you will end up with a pretty good location for the roll center. All of this advice is good for the suspension design, but keep in mind that keeping the CG low is by far the most important thing in making a vehicle handle well.

When you say that there is little load from front to rear whith the acceleration I agree with this.

But what I think is that with a rear that has anti squat in the rear will transfer weight from the front tyres to the rear tyres.

Lets ignore the COG and acceleration effect for the moment. If a rear setup has horizontal parallel links and the thing is throttled is has no anti squat effect and therefore the entire weight of the rear is supported by the rear springs in exactally the same way as when the vehicle was at rest. And I say that this causes no weight transfer from front to rear (except for the COG effect which I am ignoring)

Now lets say we have a rig with lots of anti squat (say short converging arms) that when throttled that the rear springs totally unload untill there is no weight in the springs and the entire weignt on the rear tyres is transmitted throught the rear links (and not transmitted via the springs) So now at this instant the weight of the chassis is supported by the front springs in the front (say the front dosent change) and by the rear links in the rear. Now because the link mount point is more forward then the rear will suport more weight as its lift point is closer to the centre of gravity of the rig. This is why something with lots of anti squat transfers more weight to the rear tyres (and yes extends the rear springs) and also onloads the front tyres because the rear supports the weight in a more forward position. So that when throttled it takes weight of the front tyres and loads it on the rear tyres. (this has nothing to do witht eh acceleration effrect on the COG)


Suprdlux can you see what I am getting at cause I have worked this stuff out myself and If Im wrong on this I would like to be corrected although I carnt see any holes in my thinking.

Sam

Originally posted by badassjeepguy




i didnt post that, clc900 did, i agree with you.....

Sorry about that I didn't read your answer all the way.

Look at the picture and print it out. On level ground you see the relationship between the instant center (IC, defined by the links) and the location of the CG. The IC is the intersection of the lines defind by the link angles (regardless of how they are welded to the axle housing). The percentage of the total vehicles weight bias on the rear axle is defined by these two locations. The key thing to remember is the only force you have to work with, to plant the tires, is the vehicle weight. If you cannot get the weight on the tires, they will not get traction to move the vehicle.

Play around with the picture.

#1. Put the links near parallel: The IC is way out in front of the front axle. You have a large lever arm radius between the IC and CG. You also do not have much change in the radius of this lever arm throughout the suspension travel range. It's predictable in it's performance because the lever arm radius does not change much. The only problem with such a large lever arm radius, placing the IC so far forward, is that the tires may not bite well when shock loaded. This is due to the weight load of the vehicle being placed (by the IC location) too far forward that it is easier to collapse the springs up into the frame than plant the tires. The tires carry little or no vertically downward load (all the load is compressing the links and springs) and for an "instant" there is no weight planting the tires. The weight the tires do eventually carry (spring rebound) will stall the tires as they gain traction, and then the tires release contact with the ground, and stall again (wheel hop with flexible links and in extreme cases frame shudder -- drag racing tire shake -- with fixed links.

#2. Put the links in a steep angle in relationship with each other: The IC moves back closer to the CG, and in an extreme case behind the CG. Look at the extreme case with very short steeply angled links. The IC point of lift is closely behind the CG and driving the axle raises the rear half of the vehicle, unloading the weight on the rear tires and levering the weight to the front axle (just what we do not want). The more we load the rear axle, the less weight bias it carries, and the result is a less predictable weight and traction at the rear tires. The weight on the rear tires changes wildly as the rear jacks as the lever arm radius between the IC and CG changes -- we get wheelhop (even with fixed links).

So what is the answer to good IC placement, something that neither lifts the rear axle or squats at the "instant" power is applied? Both extremes suck for traction. Can we copy the pro-stock racer designs that simply leave the line with no drama? What is best for an off-road vehicle (short or far from the CG)?

This is where you have to go back to the picture, or better yet a print of the picture, and take it off-road. Rotate the picture at a 50 degree angle (something the road race guys never need to worry about). The radius arm relationship between the IC and CG can change significantly as the vehicle climb angle is increased. Remember the weight, the only thing you have to plant the tires, is always a vertical load and it's point of contact with the links is the IC location. If terrain angle places IC close to the CG, the horizontal lever arm radius shortens between the IC and CG, then you can produce the same wild variation in rear axle weight bias that the really short and steep angle links resulted in with the second extreme case presented above. The result is less predictable weight loading of the rear tires, and wheel hop even if everything on level ground is AOK. We have the same problem as having short steeply angle arms, wild changes in rear wheel weight bias induced by a combination of terrain angle and power.

So, if you follow the thread it seems the lesser of all evils off-road is the near parallel links, because the lever arm radius between the IC and CG remains predictable and stable. Eventually, steep terrain closes this horizontal component of the lever arm radius regardless of the IC placement length (so we still cannot build links that provide traction to help us climb the face of half-dome).

Once you bite into the logic of a long lever arm radius between the IC and CG (near parallel links as the lesser of all evils) then you need to look at the action under braking, and the balance of effects between having the IC above or below the CG height (the roll center height of the IC, and again, remember to rotate that picture).

Having said all this, if you do not have room to build what you want, then you have to settle for what fits. The goal that never changes is a stable lever arm radius relationship between the IC and CG, throughout the travel range of the suspension and the terrain angle. Unfortunately, terrain and long travel suspension changes vary the IC to CG horizontal component wildly (and while you can design for great climbing, downhill stability and sidehill rollcenter-stiffness may suffer).

If it was easy, Honda's Engineers would be building and selling capable off-road crawlers to the waiting public (a market niche that even Jeep seems to be abandoning). Considering that you are only building one vehicle, not thousands, you have lots of time to cut and reweld the link mounts if you don't like the performance.

Happy Trails!

Just to answer a few of the questions that I have seen in regard to my post. First on lowering the CG. I realize that this is a hard thing to do when lifting you vehicle, but it provides lots of benifits with regard to off camber driving and steep inclines, not including the factors of front to rear weight transfer and side to side transfer. CG height is something you should keep in mind when modifying your vehicle.

Now to tackle the questions regarding squat and anti-squat under acceleration and braking. There have been may posts trying to clarify the issue and I hope this doesn't muddy the waters to much. The first thing to remember is that all the forces effecting a vehicle (disregarding aerodynamics) have to be transfered through the tires. So if you look at the side of a vehicle when it is not moving there are two forces acting on it, the force at the front tires and the force at the rear tires. (I am assuming that the weight is equally distributed between the left and right side) The weight of the vehicle can be assumed to act at the CG. So using Newton's laws the sum of the forces at the tires has to equal the weight of the vehicle. (since the vehicle is not moving up or down) The vehicle is also not spinning about its CG so the moments (i.e. torques) have to equal zero. Therefore the force at the rear tires times the distance from the rear contact patch to CG has to equal the force at the front tires times the distance from the front contact patch to the CG. It may help to draw a diagram. With these 2 equations you could find the forces at the front and rear tires.

Now lets look at the vehicle in acceleration. There are three forces happening now, the force at the front tires, the force at the rear tires, and the force accelerating the vehicle, which can be assumed to act at the rear tires. (This can be assumed by cause the force accelerating the vehicle acting at both the front and rear tires lies along the same line so the can be added together and be assumed to act at any point along the line.) As with the stationary vehicle the sum of the forces at the front and rear tires holding the vehicle up have to equal the weight of the vehicle. If you sum the torques up arround the CG There is the one for the upward force on the front tire, on for the upward force on the rear tire, and one for the acceleration force at the rear tires which is equal to the force times the height of the CG. This acceleration torque is in the same rotational direction as the force on the front tires. This combined rotation is counter acted by the force at the rear tire. (I hope all this is still making since.) Now there is more torque acting to spin the vehicle in the counterclock wise direction (lets assume that the front of the vehicle is to the right) then when the vehicle wasn't moving so there has to be a larger force at the rear tires (pointing up) then when the vehicle was stationary. This is where the load transfer comes from. The suspension geometery has nothing to do with load transfer from the front to rear, the only things that matter are the height of the CG, the wheelbase and the magnitude of the acceleration. What squat and anti-squat do is regulate the amount of weight that is carried by the spring and control arm, but the sum of these two weights is always the same, no matter how much anti-squat is run. This just isn't my calculations, I actually read up on this in my vehicle dynamics book earlier today.(Race Car Vehicle Dynamics, Miliken and Miliken) I'll just give you their quote, p 617 "The 'anti's' Do not change teh steady-state load transfer at the tire patch. The total longitudinal [front to rear] load transfer under steady acceleration or braking is a function of the wheelbase, CG height, and braking force [ie acceleration force]."

To address the question about putting a jack under the rear bumper and lifting up. There would only be weight transfer to the front if you moved the CG of the vehicle forward when you jacked up. If the CG doesn't move all the weight that was on the rear tires if transfered to the jack. Just think of the last time you had to change a tire on muddy ground.

I hope all this is clear. If it is still unclear post back and I'll try and draw up a diagram.

Hold on here...I'm not a fawking scientist or math major, just a carpenter/mechanic/plumber/designer, whatever... anyway, I've driven machines with squat and anti-squat, and the ones with a little squat hook-up better that those with anti-squat..
I think that we are forgetting the effect of the engines torque in this discussion... Everybody is talking about weight on the front or rear of the machine, but, nobody is mentioning the effect that torque has on the chassis. I'm sure I could be wrong here, (not really) , but I think the reason the rear end lifts is because some of the touque is wasted pushing against the frame and pulling on the frame and the result moves the chassis up rather than rotating the axle housing...
This force (torque) has to go somewhere, and I think is going into chassis lift and not into the ground as weight shift... I'm sure there are those who see this more clearly and have the proper terms to explain it. I'm not all that concerned with that, I am concerned with traction and controling the torque into foreward movement and I can say from (dare I say ) experience that a machine that lifts in the ass when you gas it does not put as much weight on the tires as one that squats...

Ok heres a picture of what Im am trying to say.

In this pict all i am considering it the link reaction to the torque load on the diff. In the top diagram I have shown the torque load is controlled by parallel links and therefore produces no anti squat as the torque load is controlled by equat pushing snd pulling in the parallel links. In the top the weight of the chassis is supported by the springs only.

In the bottom diagram the rear is linked with converging links which can only restrain the torque by the lifting up on the chassis mount point (just like a lever lifting up) You can see that if the torque load on the diff was so large that no weight would be supported by the rear spring at all and the chassis would only be supported by the front spring and the rear link. Now as the rear link is so much closer to the centre of the rig the links will support a much greater weight than the front springs. And thus as the links are supporting much more weight (the tyres can support the weight like this cause the torque on the diff stops the diff from faling) so the end deal is in this instance the front has less weight and ther rear has much more.

Sam

Wow I think this thread is going down hill. It seems that there is lots of misinformation around. I have included a picture to clear up my explaination, but the truth is that suspension geometry does NOT effect load transfer. I understand where you are coming form, but your missing the forest for the trees. No matter where the loads react with the frame or what torques are involved the load transfer is coming from the acceleration force at the tire. If it doesn't make sense to you please believe me I spent 5 years of my life designing and building formula car chassis and I spend weeks working through the equations to prove it to myself. And I'm not one of those engineers that can't make stuff work. I actually welded the entire chassis and suspension links myself, I spent hours making parts in the machine shop. I really understand what it means to have real world application. Just to say it one more time squat and anti-squat don't effect load transfer. It is the actual truth.

Sam to clarify your diagram, the load in the control are does increase with more anti-squat, but the load it supports is coming from the weight transfer, it cannot cause weight transfer. All the forces inside the vehicles have to sum up to the forces acting through the tires and these forces are governed by the free body diagram of the entire vehicle, not just the rear suspension.

Okay I can't figure out how to add a picture to this message so you don't get to see my diagram. Sorry:confused:

Okay I got the picture stuck on one of my web sites so here it is. I included the important equations and if you work through them they should make sensehttp://grok.ecn.uiowa.edu/~jacob/pics/Squat.jpg

Originally posted by Suprdlux
A case in point, is that you can actually design a car with enough anti-squat in the front and rear that when it accelerates that it will lift up off the ground.

Duuude!!! If this is true I can just build my suspension with tons of anti-squat and my Jeep will lift up off the ground and literally fly up the obstacles in my way!

Anyway, with a four link rear suspension the lower links will always be in compression when you are driving forward and the upper links will always be in tension when driving forward no matter where they are located in relation to the centerline of the axle. Wow, that was one helluva run-on sentence.

Ha ! ....This thread is holding its own very well, when things go downhill on this BBS it really gets ugly...

I think we are talking about different forces at work here...On a rig with steeply angles lower bars the chassis will try to rise upon applying power, whether the machine is accelerating or not.
My contention is that the torque of the engine is holding the machine up, and any weight is transferred closer to the front and to the motor mounts...

could be wrong

Poorly worded I must admit..

thank you suprdlux and Ed A Stevens.
never noticed either of you online before but i definately like Eds post trying to apply it to our high cg long travel wants.
very well written.
I will be re reeading that and chewing on it for a while.
Thank you

The links dont have to be 10 inches apart if they both are on top of the axle. Mine are only 6.5" inches apart!

Originally posted by nuttzack
The links dont have to be 10 inches apart if they both are on top of the axle. Mine are only 6.5" inches apart!


on top or bottom it doesnt matter, as long as they are far enough aprt to prevent the twisting from torque.... i say 8 to 10 from what ive seen.,... 6.5 could be just fine, but ill put a little bit more distance in mine

Originally posted by dirtrod
Ha ! ....This thread is holding its own very well, when things go downhill on this BBS it really gets ugly...

I think we are talking about different forces at work here...On a rig with steeply angles lower bars the chassis will try to rise upon applying power, whether the machine is accelerating or not.
My contention is that the torque of the engine is holding the machine up, and any weight is transferred closer to the front and to the motor mounts...

could be wrong

Poorly worded I must admit..

im not quite following you here.... ill say this, i know from personal experience, you got some good designs and you shit always seemed to work pretty damned good.... where i got qwhat you disagree with is... yes a book, now im not taking it all as gospel, but worth considering.... trying to do alot of figuring before i start tearing apart.... im gonna weigh out all book shit, engineer shit, and real world shit, and hopefully come up with shit for my rig that works....lol thats alot of shit :D


excerpt from the "book"

when a car accelerates forward, there is weight transfer from the front of the car to the rear. this weight transfer is dependant on the weight of the car, the height, cog, and on wheelbase length. because of the springs weight transfer can often be seen at the rear of the car as it "squats" during hard acceleration.

it is possible to arrange the rear suspension links so that the driving force of the rear axle counteracts this squatting force. this characteristic is called anti-squat. anti-sqaut can counteract the squat force to keep the rear of the car level, and it can be made strong enough to actually raise the rear of the car during acceleration.

because any force that can raise the rear of the car will need to have and equal and opposite force pushing against the pavement, you can use anti-squat to increase the tire load during acceleration


this is where i got the idea that anti squat would put more "load to the ground, therefore increasing traction..... this is all good but if you have to much the you got axles walkin under the hehicle which isnt good for what we do......

so, get a set up a rig that will not let the axle walk under it, and hopefully youll have some anti-sqaut to aid in tire loads?


i dont know shit, just trying to figure it all out...

either way ill be drinkin a beer and shootin the shit at tellico in a few months yeahhh!

Suprdlux has it 100% right. anti-squat/dive do not change the load on the front or rear axle.

Suprdlux,
what were you designing this formula car for?...I am in the middle of designing CalPolyPomona's FSAE car's suspension (which BTW: will have no anti-dive or anti-squat).

FSAE and Nascar Late Model are the two cars that I have exstensive experience with, i.e. I actually worked on a design that was simulated in DADS. I have also done some "consulting" for my friends in a wheeling club. I hope my comments helped in peoples understanding of weight transfer and squat/anti-squat.

badassjeepguy said,

"because any force that can raise the rear of the car will need to have and equal and opposite force pushing against the pavement, you can use anti-squat to increase the tire load during acceleration
this is where i got the idea that anti squat would put more "load to the ground, therefore increasing traction....."

My disjointed thoughts tell me that "equal/opposite force" is found in the drivetrain, and it is the twisting force that you can see in the motor mounts flexing. (torsion bar effect)

It's hard to tell the truth from what I've seen quoted from books, but ,"seat of the pants" says th rear is UN-loaded...

Yea, we'll get this all sorted out in parking lot at the motel, we'll get Gordy's brain working on it..and that fawkn roofer will have some theories for sure...

Suprdlux does not have it 100% right.

While yes the conservation of forces in free body diagram etc. etc is 100% correct but this is only valid for steady state calculations. And last time I blasted my rangie up a hill it was anything but steady state.

On the drag strip or on a flat track yes what you are saying is 100% correct but it dosent have a lot of relevance here IMHO.

Even what Ed A. Stevens has said makes a lot of sence but it again is based on street cars although his final conclusion about running with little anti squat was a good one. He said that with this setup you dont get as good a launch off the line but when blasting up a hill you dont need a spectacular launch (although it helps) what you need is to keep the tyres on the ground where they can drive the rig forward.

I have seen lots of different rigs with varing degrees of anti squat blast up hills. What I have noticed is that those rigs with little anti squat drive up well and those with lots of anti squat lift front tyres and hop and bounce and tend to drive on there back wheels. I have discovered this from observation and would hope that everyone else would see the same thing. Anti squat is the killer on hill climbs and so Im am trying to produce some theory as to why.

Suprdlux statement about the force balance thing on the free body diagram has made me think "shit hes right and im wrong" but then I thought that if hes right then something with lots anti squat should do as well as something with little anti squat on the climbs and all it has to do with is the position of the COG and the wheelbase.

But I know that this is not correct because I have observed it to many times. And as such I have realised where my logic was lacking. So here I go again.

What everyone else is talking about is steady state theory and I dont think that this applies to us blasting up hill. In effect my description applies to the results of the shock loads that are applied the the rig when is blasts up the hill and the torque on the tyres is applied and released as the tyres bounce over stuff and fight for traction.

OK so that if you are blasting up the hill and the rear tyres suddenly get good traction heres what happens-

I rig with lots of anti squat will lift up on the chassis and drive the rear wheels downwards thru the anti squat loads applied to the chassis. With lots of anti squat the rear will lift but with the sudden application of torque to the rear axle the chassis must be accelerated upwards before it gets to its final lifted position and therefore the force required to accelerate the chassis upwards is the cause for the increase weight transfer to the rear axle. It is not because of the steady state forces as Suprdlux has described but because the chassis must be accalerated upwards to get it to the final lifted position and the chassis has inertia and therefor the rear is pushed down as it is trying to lift the chassis

Now because the rear links are lifting on the chassis forward of the rear axle, the chassis at the front is also lifted and the lifting the front will unweight the front tyres because the front springs are extending as the chassis moves away from them.

Now if this torque was kept constant and everything settles down then the final result will be an identical weight distribution (with the slight variation fron the COG effect) to what there was before the torque was applied.

But again this is not what happens because the sudden application of torque on the rear is now lifting the chassis (via the anti squat in the rear links) and the chassis will now have an upwards velocity. While the rear axle is still appling the torque before steady state is reached the rear will continue to be pushed downswards and the front will get lighter (because the chassis is accelerating upwards) and if the torque application was hard enough then the upwards velocity achieved will lift the front tyres off the ground before steady state is achieved.

Now heres the killer-

with the chassis at the rear moving with an upwards velocity it will unweight the rear springs and at some point the rear tyres will slip (for whatever reason) so the rear tyres spin and the torque load comes out of the rear so that there is no longer the anti squat effect on the rear links to help support the rear springs so now the rear chassis is too high for the springs with no anti squat effect so the chassis will start to move back downwards with some momentum. Now the tyres are spinning and the chassis lands on the springs and compresses them and drives the tyres into the ground and makes them grip real hard and puts a large shock torque load on the rear diff and away we go again. The sudden increase in torque load on the rear causes the rear links to push up again on the chassis, acclerating it upwards etc etc etc.

This is why rigs with lots of rear anti squat will hop and bounce as they throttle up the hills. When the rear lands it loads the torque in the axle which lifts the chassis up and makes the tyres spin again when they get light and the process repeats. And all the time the front tyres get light cause the chassis keeps on being lifted away from them by the rear links. So the front tyres come off the ground and the rears keep bouncing away.

Now I am even more sure this is why anti squat is bad and why it makes rigs bounce and hop up the climbs.

Rigs that dont have anti squat when they spin and land dont have the sudden reaction to lift the chassis back up into the air and thus dont get into the bunny hopping problem.

Anti squat is the killer. Make the links as parallel and as horizontal as you can get them.

I know this explination is a bit clumsy but can anyone see what i am getting at????? I think that what I have said here is getting closer to real world.

Sam

can someone relate any of this information to a suspension that actually exists and works...

i have been looking at Desertoy's and really like it with one exception... when i build mine i am going to try to align the frame mounted 4 links instead of outers up and inners down

this should make the rear diff point at the transfer case throughout up or down travel

Gawd dayum my frickin head is just a achin'! Where did all these engineers come from? Someone want to speak english to me?

I know you guys are trying to be helpful and really you are but there are a couple of things that I still dont agree with. And I dont have any theory's or calcualtions as to why I think this but the conclusions I have is from standing back and watching shiat work and seeing what it does in the real world.

So, aniti-squat and squat are two opposites, correct? So how can two different reactions BOTH transfer weight to the rear. I just dont get it. I look at a car and when it accelerates hard and the ass end dips down (squat) that tells me that weight has been transferred to the rear. Now when the car hits the brakes and the ass end comes up that tells me that weight has been transferred to the front. If am building a drag car I dont want squat so it will hook up. All this does is transfer weight back, which is the WRONG direction. I want a little antisquat so weight is transferred forward which just happens to be the directiosn I am going. Now if I am climbing a hill in a 4x4, I still want a little anti squat to keep some weight that has transferred to the back because of the incline to transfer to the front and keep the front end down. You are telling me that anti squat will make the rear raise AND unload the front end??? I dont think so! When the rear end squats, that is when the damn front wheels come off the ground? This is just real world shit to me. This is what I see happening and I dont have to work out an equation to prove it to myself.

Look I am open minded and if I am wrong I am willing to listen. Shoot I have been wrong before and will be the first to admit it.

Just tell me how BOTH anit-squat AND squat put weight on the rear end and unload the front. So how in the world do I load the front tires and put weight back up there? Is it possible?

I think I need a few tylenol.
Ah hell maybe a couple :beer: :beer:

Originally posted by dirtrod
badassjeepguy said,

"because any force that can raise the rear of the car will need to have and equal and opposite force pushing against the pavement, you can use anti-squat to increase the tire load during acceleration
this is where i got the idea that anti squat would put more "load to the ground, therefore increasing traction....."

My disjointed thoughts tell me that "equal/opposite force" is found in the drivetrain, and it is the twisting force that you can see in the motor mounts flexing. (torsion bar effect)

It's hard to tell the truth from what I've seen quoted from books, but ,"seat of the pants" says th rear is UN-loaded...

Yea, we'll get this all sorted out in parking lot at the motel, we'll get Gordy's brain working on it..and that fawkn roofer will have some theories for sure...


lo, i just talked to the roofer, i need the # for the "cove" if ya got it handy, id appreciate it.... he just did some type of rear arm to prevent hop, said its workin great... he got some beef under that rig now...

The free body diagrams (FBD's) should isolate the forces: axle torque on the housing as it resists the traction patch at the tire contact (the external acceleration), and mass of the vehicle resisting forward and upward motion (the only other external acceleration, gravity). These are related to the lines drawn on the first scanned picture example (the textbook Anti-Squat drawing with the axle twist force acting on the vehicle through the IC link).

The second round of simple cart FBDs are good, an ideal case of a tractor drive with no suspension, as it neglects a suspension's ability to add or reduce the vertical components of axle housing torque on the vehicle mass. It's a place to start.

This is a two dimensional problem: vertical forces (vehicle mass * gravity at the CG), and horizontal forces (mass of the vehicle * linear acceleration at the tire contact patch). You cannot change the vehicle mass or gravity (vehicle weight). You can only alter the force acting on the vehicle mass due to the linear acceleration components resisted in the vertical and horizontal directions (defined by the relationship between the acceleration location and the CG, but that is way later in the game).

All the acceleration of gravity is planted through the fraction of vehicle weight on the rear tires (resulting in traction friction), and all the linear acceleration (generated by tire contact patch friction) moves the vehicle mass forward. This specialized case is represented by the second, tractor-cart, FBD.

Tell me what happens as the cart climbs an incline (take it off-road)? What happens to the relationship between h and l2 as you climb? What happens as the rear tires grip and deform over a rock (great traction) or pack with mud (poor traction)? Is the resulting FR (tire traction) consistent? What happens to FR as axle torque exceeds the value of FW*l2 (hard to do on level ground but easier as you increase incline angle)? With a variable FR (tire traction) can we assume the Fx (the linear acceleration at the limit of traction) is stable? Is an unstable traction limit good? Is the problem traction, or the control of traction?

The problem with repeatable traction is getting the tire contact patch friction (the location where axle rotation torque is converted to linear acceleration) to be consistent. Tire friction at the instant of launch (at any time) is a function of vertical forces acting on the axle: vehicle mass and gravity (and fortunately/unfortunately the vertical components of linear acceleration that are generated between the suspension IC and vehicle CG, squat and jacking, but we will start without a suspension).

Again, maybe it's better if we back up some, and use the simple tractor FBD, with history.

You cannot create additional mass out of thin air with any linkage system, and you cannot change the acceleration of gravity. The fraction of total vehicle weight directly planted on the rear axle (front or rear weight balance) however, is something we often see change, determined by the location of the CG and the vehicle wheelbase, as we move up an incline. Take the tractor FBD picture and tilt it, run the equations again, see that the weight bias changes, the rear tire contact patch carries more weight as the incline increases. The higher the CG, the more effect an incline has on changing the weight bias (anyone wheeling off-camber has felt this, and farm tractor drivers will relate tails of terror hill climbing).

If we have ideal traction (a cog wheel trolley) you can climb a near vertical incline, if the traction is poor (iron locomotive drive wheels on steel rails) then it's very difficult to climb even the most gentle grade. If you study history you realize Engineers (the kind that drove trains) created many solutions to solve traction problems. A steam locomotive has incredible torque, but poor traction. Climbing a gentle grade is near impossible, and throttling the power to the wheels is difficult (spinning the drive wheels, even on level ground is easy, just like a drag racer).

Locomotive Engineers resolved the problem with throttle control in the real world (a world with hills) by moving the drive wheels directly under the CG, the center of the tractor wheelbase, to minimize the potential change in traction produced by an incline. The traction still sucked, but they could better control the available traction they had between iron and steel. The extreme of this design direction was a locomotive built specifically to climb the Tehachaphi Pass between Bakersfield and Mojave, California. Six drive wheels per side, centered under the CG. The result was great throttle control, good traction for iron on steel, but poor rail bed flexibility as the long trolley frame did not conform to a less than ideal level road bed. The point of the history lesson is control of traction was more important than any other goal. You should find the same goal of control, over ultimate traction, is true in drag racing and crawling.

Now we need to look at a four-wheeler. We do not want to add extra axles inside the wheelbase, the locomotive solution, but we still have the control problem of changing traction as we drive up an incline. Go back to the tractor FBD, and look at what happens as we climb or add significant traction and torque.

The greatest vertical force you can consistently place on the rear tires is the total weight of the vehicle (we will ignore the possible addition of instantaneous suspension spring force storage for simplicity). If the tires can grip and axle torque can hike the CG any, raising the front axle ¼ inch (or twenty inches) in the air, this the best we can do, completely unload the resisting (FF) force on the front axle.

Can one of you engineering student's model this FBD, with the front wheel in the air?
Is this a two-force couple (since we eliminated any suspension)?
Is Fx*h=Fw*l2?
Is the torque for a steady state wheel stand = Fx* tire radius (ideal traction)?

Can one of you Engineering students tell me where the instant center is with the (non-suspension) tractor FBD?

BTW, can one of you Cal Poly Guy's tell Dr. Shelton hello for me? The Cal Poly Pomona guy's should go over the hill to watch the NHRA Winternationals (it's a good field trip).

Happy Trails!

So I've read through the posts again and some of the theory is getting out of control considering the original post was about setting up a real world 4 link suspension. So first I will give clc900 some advise on design that has seeming universal support. Make your link arms as long as possible, this gives your suspension travel more verticle movement (ie inches) for the angle that your link moves through. This will keep your suspension from binding. Second keep your upper and lower arms parallel to each other. This will keep the IC of your suspension at an infinite distance from you axle and your suspension will behave more consistently throughout its travel. Third mount the links as low on you vehicle as possible. This gives you the least amount of anti-squat and it tends to be where the frame is so you have something to weld your suspension bracket to. As for the distance between the upper and lower links, it doesn't matter. The closer they are together the stronger they have to be. If you what to mount both of them above the axle that is fine to it does not effect the motion of the suspension. That is my best advice for building your 4-link. It will provide you with good result. Don't worry about squat/anti-squat, roll centers, etc. because when the fire hits the steel most of the decisions will be made by what fits where and not by theory.

I would like to futher explain the transitional effects of accelerating uphill with off road tires on gravel right now, but I have to study for my pharmacology final. I promise that next week I'll set up a web site with a crash course in 4-link theory.

Originally posted by Suprdlux
Anti-squat does take some of the load off the springs and transfers it to the control arms, but the total weight at the rear tires doesn't change. A case in point, is that you can actually design a car with enough anti-squat in the front and rear that when it accelerates that it will lift up off the ground.


I am by no means an expert on suspension design, in fact I know absolutely nothing, and am trying to learn from this thread... but istn't this statement contradicting itself? If the links don't change the weight on the rear axle how can you then design a suspension that will lift the front off the ground? If the front is in the air, then all the weight must be carried by the rear axle correct??? please explain...

Originally posted by clc900
Gawd dayum my frickin head is just a achin'! Where did all these engineers come from? Someone want to speak english to me?

I know you guys are trying to be helpful and really you are but there are a couple of things that I still dont agree with. And I dont have any theory's or calcualtions as to why I think this but the conclusions I have is from standing back and watching shiat work and seeing what it does in the real world.

So, aniti-squat and squat are two opposites, correct? So how can two different reactions BOTH transfer weight to the rear. I just dont get it. I look at a car and when it accelerates hard and the ass end dips down (squat) that tells me that weight has been transferred to the rear. Now when the car hits the brakes and the ass end comes up that tells me that weight has been transferred to the front. If am building a drag car I dont want squat so it will hook up. All this does is transfer weight back, which is the WRONG direction. I want a little antisquat so weight is transferred forward which just happens to be the directiosn I am going. Now if I am climbing a hill in a 4x4, I still want a little anti squat to keep some weight that has transferred to the back because of the incline to transfer to the front and keep the front end down. You are telling me that anti squat will make the rear raise AND unload the front end??? I dont think so! When the rear end squats, that is when the damn front wheels come off the ground? This is just real world shit to me. This is what I see happening and I dont have to work out an equation to prove it to myself.

Look I am open minded and if I am wrong I am willing to listen. Shoot I have been wrong before and will be the first to admit it.

Just tell me how BOTH anit-squat AND squat put weight on the rear end and unload the front. So how in the world do I load the front tires and put weight back up there? Is it possible?

I think I need a few tylenol.
Ah hell maybe a couple :beer: :beer:

Yea my head is achin too. OK this is what I think.

quote :I look at a car and when it accelerates hard and the ass end dips down (squat) that tells me that weight has been transferred to the rear

Weight is transferer to the rear bacause of the accleration is trying to tip the car over backwards. If the springs are very stiff the rear wont squat but there is still weight being transfered to the rear because the extreme acceleration is still trying to tip you over backwards.

Now if you have soft springs and want to stop the the rear from squating from the acceleration we build in some antisquat into the rear where the links lift up on the chassis and stop the rear springs from compressing. So while you are accelerating the rear is lifted up higher (supported by the links). Now although the rear is lifted up higher via the anti squat the rear will not support more weight through this effect. (this means that the front and rear weight distribution is the same as if there was no anti squat)

So that whether the car as lots of anti squat or none the weight transfer to the rear wheels is due to tha accleration trying to tip the car over backwards. Now I can see that if the car squatterd so low in the rear and the front lifted really high because it has such sofrt springs then yes this squating would cause weight transer but the weight transfer from this would be small compared the the acceleration force trying to tip the car over backwards.

When you are blasting up a hill I think that the best that you can hope for is that the tyres arnt lifted off the groud. Now if you have rear anti squat and if the tyres spin and then bite hard the anti squat will lift up on the chassis via the links and so gives the chassis some upwards momentum as the links push up on the chassis. The problem here is the chassis is moving away from the ground and will therefore can start to lift the tyres off the ground which causes them to spin loosing the anti squat effect and the chassis comes back down and the process repeates making the tyres lift and the rig bounce up the hill. You dont want this.

OK Ed A. Stevens posts are really starting to make sence to me now. what I think he is saying that is the above effect wont be unstable until the COG is moved back far enough by making the incline so steep.

I guess that this is why some rigs with lots of anti squat drive the less steep loose hill better because the rear is instantaniously jammed into the ground when the torque comes on but it wont lift the cassis so quickly to throw the rig away from the hill. But drive these rigs up something steeper and the anti squat force will be to great and will start the rig bouncing because it will unstable.

Ed A. Stevens am I making sence here (BTW im not a student I did my mech eng deg 10years ago and I do not work in this feld)

But like everyone is saying you dont want anti squat. Make them parallel and horizontal. Because one day you will try to drive something really steep and the anti squat that helped you on the less steep hills will come back and bite you on the arse and make your rig bounce.

Sam

Originally posted by apeters89



I am by no means an expert on suspension design, in fact I know absolutely nothing, and am trying to learn from this thread... but istn't this statement contradicting itself? If the links don't change the weight on the rear axle how can you then design a suspension that will lift the front off the ground? If the front is in the air, then all the weight must be carried by the rear axle correct??? please explain...

I think what he is saying is that you can build in enough anti squat (steeply angle convergent links) that when the vehicle is accelerating it will sit higher off the chassis (with springs more extended) than at rest. Now while the vehicle is still accelerating the change in weight distrubution is only from the acceleration force trwing to tip the car over backwards and is not from the anti squat.

Sam

Very interesting thread,
But i like clc900, live in the real world.
I ain"t got no fancy papers :rasta:
Alot of what you see bunny hopping up the hill ,
is caused by high horsepower in a light weight vehicle.

Would it be nice if you could take the simple cart (a tractor) and make it more stable close to the limit of traction (keep the front wheels on the ground), or more responsive to throttle input on an incline (less prone to tipping backwards)? This (and more) is what adding suspension provides to the dynamics of a vehicle.

Strange Rover, you are making sense of the effect of the moving CG creating problems as an incline increases (your not going crazy, ave another beer).

Underdog is also seeing it, with the wheelhop from high horsepower in a lightweight vehicle, but does not know why (and may not care to know why, fine by me). What he sees, is a vehicle suspension that is no longer working (except for tire sidewall flex) due to the light weight and suspension geometry with the incline angle, compared to the high power and traction forces. If he watched the same vehicle try the climb with high tire pressure in treadless pizza cutters he would be watching it spin, and saw at the ground, rather than wheelhop (something to test if you think this suspension theory is all BS).

Suprdlux, I hope you realize the free body diagrams you posted are for a special case: a non-suspended vehicle (tractor, go-cart, etc.). This special case is unique, as all the weight is unsprung, with none of the vehicle weight being held up by springs. Without springs, there is no need for a four-link (or any links), as the frame and suspension is static. This special case however demands understanding, because if you ever run all the travel out of your suspension (drive on the stops, or climb perpendicular to the links, or corner at the spring bind limit) the vehicle handling dynamics revert to this example. When you run out of suspension, all the fancy design advantages of moving the instant center (roll center, anti-dive, anti squat, and link angle detail) goes out the window and the handling dynamics revert back to a vehicle that is all unsprung weight.

clc900 sees the light regarding the benefit of taking the simple unsprung cart and adding springs, weight transfer without lifting the front wheels off the ground and losing control. A suspension can take advantage of weight transfer without hiking a tire off the ground. It does not matter if the rear squats or jacks, as long at the weight is transferred to the rear axle (something that can happen if both the front and rear ends of the vehicle are lifting off the suspension, with the front lifting more than the rear). It's easier to see weight transfer as the rear end squats, but it the front jacks ten-inches and the rear jacks one-inch it's still weight transfer.

By adding suspension, we are limited to moving only the sprung weight, but the control improvement is way better than trying the wheel with a simple unsprung cart. Rather than having the traction force acting directly on the CG through a fixed frame, we add suspension to the body and direct the traction force through something that can store and release energy, springs. To make use of this (in modeling what works, and why) we also have to alter our free body diagrams to separate sprung and unsprung weight (toss that unsuspended cart FBD).

This is where theory can help, taking advantage of where and how we control the sprung weight of a vehicle through the suspension (and helps you predict what happens when you run out of suspension or bind the links).

If your trial and error has already educated you past any need for theory, then keep building what works (good results are better than any theory). If you want to learn (or teach) why it works, then get the modeling correct.

Happy Trails!

Hey all,

A few moments ago I was laying on the floor getting a few measurements of my front bumpstop locations and comparing them to the shocks lengths, double checking the clearances around the links...
I took the time to jot down a few things I noticed under there, and I also have a smartass question.

But first the observations...
With the left front axle on the bumpstop and the right rear shock at full ext. I get the following conditions...

The left lower bar is 7/8" below the framerail.
The left upper bar is 3" below the harmonic balancer and 2 1/4" below the fanbelt at one point (this get much closer when the ft. axle is on the Center bumpstop).
The tie rod is 1 1/2" below the radiator .
The right upper bar is 3/4" below the starter and is only 1 1/4" to the right of the oilpan.
The front driveshaft is right in there by the oilpan but I forget to get the measurement.

Those are just a few of the issues that I'd like to see some calculations on, and some detailed cad drawings in 3D animation would help also...

My question.... How much further should I move my links UP ? (to get them parallel to each other and the ground)


BTW guys...
When I'm talking trial and error, I'm talking about mock-ups with hiem joints threaded into plumbing pipe and brackets tack welded ect. ...you know, the actual fitment of the components in the chassis that I'm gonna use for many years...Full size model : )

Dirty work, but I don't know any other way to get it done. And be absolutely sure I can bonzi without clearance "issues".