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That didnt come out to well. Ill try it this way.

Now the horizontal drive force from the tyre causes the rig to accelerate. So that the drive force cause an opposite horizontal enertia force at the COG.

The horizontal drive force also causes a vertical reaction force at the tyre because the tyre is connected to the frame by the links through the IC (meaning drive force comes into the tyre the tyre is forced into the ground).

Now for the rig not to squat the rotational effect of the COG force must balance the rotational effect of the reaction force of the tyre being forced into the ground.

Now you can measure the effect of the rotatinal forces about any point - it doesent matter. The easiest place to measure the rotatinal effects is about the front tyre contact point.

Fuck it - Im back to the confusing part with the equations and stuff.

Ill think about it a bit more.

Sam
 

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rokcrln said:
Check out TEAM PURPLE on this pbb he is set this way front and rear. Seems to work for him for trail only.
Interesting. Found a link:
http://www.pirate4x4.com/forum/showthread.php?s=&threadid=163394

Looks like they are running portals, which means they were able to run the links much higher and flatter than you could w/ a standard axle. This helps w/ the axle steer problem, but you can still see that it has some.

Originally posted by Strange Rover
Clear as mud??
Yep. :p
I still don't understand how the wheelbase gets thrown in there, or why moving the CG forward or backwards wouldn't affect squat...

I'm picturing a dragster- really long wheelbase, but CG very far back. Now put the engine in the front of the dragster to move the CG forward. Don't you think the squat would change?

Aside from that, I do understand that here is a horizontal force and a torque force though...
 

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Hows this -

The COG height is drawn at the front tyre contact point for no other reason that is forms a graphical representation of the forces involved.

Now the COG force is applied at the actual COG but when we draw the anti squat diagram what we are actually comparing is two different ratios -

One ratio is the height of the COG to the WB of the rig

The other ratio is the vertical and horizontal forces at the rear tyre.

Now when these two ratios are the same the rig is neutral or has 100% AS.

When we draw the anti squat diagram all we are doing is comparing two triangles so that we can easily compare the two ratios.

Sam
 

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ErikB said:



Yep. :p
I still don't understand how the wheelbase gets thrown in there, or why moving the CG forward or backwards wouldn't affect squat...

I'm picturing a dragster- really long wheelbase, but CG very far back. Now put the engine in the front of the dragster to move the CG forward. Don't you think the squat would change?

Aside from that, I do understand that here is a horizontal force and a torque force though...
OK - the COG only creates a horizontal force that tries to rotate the rig on its tyres.

Now if the COG is way forward (in the drag car) there isnt much chance of it flipping over backwards. If the COG is way back then there is more chance.

Now assume that in both cases that the drag car doesent flip. In both cases the horizontal interia force from the COG is the same and thus in both cases the exact same weight will be transfered off the front axle and loaded onto the back axle.

Its just that with the engine at the front you have to transfer a lot more weight before it flips.

Sam
 

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Team purple only have the lowers triangulated so that means that they have a low roll centre and the roll axis angle is determined by the angle of the upper links.

If they also triangulated the uppers (like a reverse double triangulated) will make the roll axis very steep. If you want to triangulate the lowers with the apex at the axle to protect the drive shaft then I wouldnt triangulate the uppers - by keeping the uppers parallel then you get the flattest roll axis you can get with that configuration.

Sam
 

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Discussion Starter #87
wow, this thread is coming out much better than I expected. kick ass.

My only question right now is how to determine your COG? I know someone mentioned just using the upper bell housing bolt, but what if your building something funky, like a rear engined buggy. is there a simple way of weighing the rig and figuring it out at home? Also how do you determine where the COG is on a buggy that only exists in CAD?

Dallas
 

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Strange Rover said:


This is the reason why the line isnt through the COG.

Now again this would be easier with a picture but I will try to explain it without.

Now when the rig acceletates we get a horizontal inertia force from the COG pushing backwards.

In the 100% AS case the force from the tyre acts through the IC of the rear links. Now this line passes through the 100% AS and the line is the direction of the resultant force from the tyre (this is comprised of the horizontal driving force - Fh and the ground reaction force - Fv .... (this force comes about because the driving force must be transfered by the links through the IC which is upwards at an angle and thus it creates a force at right angles to the ground)

Now the drag force from the COG is the same as the drive force Fh.

So the for the rig to not squat (or anti squat) the rotational effect from the COG force must be the same as the rotational effect from the ground reaction force - Fv.

So if we compare these rotational forces in relation to the front tyre contact point (so that we dont have to worry about the front forces) we get an equation -

COG force x COG vertical height = rear ground reaction force x wheelbase.

now COG force = Fh
and ground reaction force = Fv

so that means

Fh x COG vertical height = Fv x WB

this means that Fh/Fv = WB/COG vertical height

So that for a 100% AS the ratio of the vertical and horizontal forces at the rear tyre is the same as the ratio of the height of the COG and the wheelbase.

Now drawing the height of the COG at the front axle is just a easy way to graphically represent and compare these two ratios.

Clear as mud??

Sam
Here's a little more...

The marks on the picture are just a visual representation of some math/algebra.

You've got five variables:

COG height, (probably a guess for most of us)
Wheelbase, (easily measured)
IC height (Scale it off of a pic of the rig, or design it in)
IC position (front to back in truck, measured from center of rear axle, scale it off or design it into the rig)
(AS%) the final desired goal.

So you've got some math:

AS%= (COG Height * IC Position)/(Wheelbase*IC Height)

Now lets get down to designing a suspension.

Assume you want the basics, flat roll axis, 100% AS in a rig with 40" tires.

If you are going to run a standard triangulated four link, it is not terribly hard.

figure you'll want around 22" of belly pan clearance, so the frame ends of the lower links will be about 24" off the ground, to center.

This works out well, since the links mounted to the top of the pumpkin will be about the same height, once you get the tires aired down a little. Wallah, you've got your flat roll center.

Your lower links are going to be mounted around the bottom of the axle tube, so call it 18" off the ground to the center of them. They're going to be the standard 36" long that everyone says is a good rule of thumb. Now all the sudden your lower links are locked in. Make them mount as wide on the axle as you can, and as narrow on the frame as you can and you're done.

You've also already got one point on your upper link locked in by the top of the pumpkin and your desire for a flat roll axis and a flat belly with 22" of clearance.

the other end of your link is the thing you can work with to tune your suspension, this is why you see a lot of guys with adjustability built into their rigs here.

Chances are your chassis will constrain you on the length of the upper link, and even if it doesn't, you want to go with the rule of thumb of 75% of lower link length for the uppers. Bang, there's your link lenght, 27". Now you can place the end of your upper anywhere on an arc 27" from the top of the pumpkin, but WHERE you place it will determine your instant center and consequently, your antisquat.

You can figure with a 22" high belly, your CG height is gonna be around 40" at best, so you can draw a picture of your rig on graph paper with all these points plotted in, and pick your own instant center.

Draw the line through your fixed lower links, and you know the IC will be on this line.

Draw a line from the rear contac patch to the COG height over the front axle. For 100% AS, you want the IC to be along this line also.

You now have two lines, and you want the IC to be on both of them... obviously your desired IC is the intersection of the two lines.

Draw a line from the IC you just found to the top of your rear pumpkin. Scale off 27" along this line and there's your "sweet spot" for the rear upper frame mount. Make a couple more above and below this sweet spot to account for math error or handling quirks, and you are ready to start fabbing.

I just ran through this on paper, and came up with 27.5" high and 27" in front of the rear axle for the rear upper mount. This was with some really bad drafting though, so I would not use the numbers as gospel, I just wanted to give an idea. All I need to do is take the dimensioned diagram and draw a rig over it... Then build it!

Here's a pic of the diagram: http://www.foleydevelopments.com/cic/000_0360.JPG
 

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Ok, I just re-read that and I think it makes sense....

Now, who wants to take into account what happens when you tilt the picture up 45 degrees??? HOW do you do this? do you multiply COG height by SQRT of 2 or what???
 

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Just had a brainwave, or maybe just a brain fart. When is it better to have squat or when is it better to have anti squat? What works best for going up hill? I read that too much of one or the other will induce bouncing when climbing hills. Would it make any sense to build in an "on the fly" anti squat adjustment of some kind so that the postion of the upper link frame mount could be changed while driving? Or is there one particular amount of a/s that is perfect for everything? Is it 100%?
Adjustable mounts would require a bit of fabbing, but the quest for ultimate performance may make it worth while. Then again, I may be a blithering idiot.:confused:
 

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Good suspension thread!

I have a question. Don't you have to use the sprung CG for the anti-squat calcs to be perfect? On most trail rigs there is so much weight in Dana 60s and 40" krawlers that the CG info if you do the regular calcs is going to be a little bit off....

I have no idea how to work around this, other than try and find a way to measure your rigs weight without all the sprung weight. I am going to be using some CAD programs to help me find mine while doing my buggy design. It won't be perfect, but it will be very close. I just have to assign some values to things inside CAD and I can have the computer tell me just about where the CG is going to be. Its just a better way to guess. I don't know if it even matters. When I think about it.....it seems that the sprung CG is going to be above the standard CG value affecting the calcs to make you have different AS numbers.....

Just a thought...
 

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Possible BS - get your shovel

Sam - I think the simplest; most dumbed down way (if that is redneck enough) for imagining the need for that line from the rear contact patch to the front axle center is to look at the general forces going on:

Any force you impart to the rear axle is transmited to the body through the supension and a lesser degree throught torque roll of the driveshaft/drivetrain...

Whatever magnitude and direction these force vectors may be is not all that important to realize that the only other place the vehicle can pivot off the ground is at the front axle center.

The COG and the design of the links all have impact on the vector direction of what ever forces you have applied to the axle are oriented but in its simplest form you have only one other force holding up the vehicle besides the rear end - and that is the front axle

Think of it as a lever - a long WB vehicle with steep links is going to have a much higher AS value than a short WB vehicle with the same COG height and same link set up and the virtual pivot point is a very different height...

Just my 2ct's

Matt :D
 

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foley said:
Ok, I just re-read that and I think it makes sense....

Now, who wants to take into account what happens when you tilt the picture up 45 degrees??? HOW do you do this? do you multiply COG height by SQRT of 2 or what???
Ive been thinking about this and I dont think that it make any difference because all the forces (the cog force and the drive force and the ground reaction force) are all still parallel and at right angles to the ground.

Now IMO the only reason that the rear starts to hop as the climbs get steeper and steeper is because as the rig in inclined more of the rear weight is supported by the rear links and not by the springs. now since there is less weight on the rear springs the springs extend (unload) which puts the rear suspension at a bigger "lift" and this the rear will have more anti squat and it is this that causes it to hop.

Now the softer the rear springs the more the rear springs will unload and the worse the rig will hop on the steep climbs.

So if you set your AS level to between 50% and 80% at ride height on the steep climbs it hopefully wont go beyond 100% and become unstable.

Sam
 

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Flat50 said:
Just had a brainwave, or maybe just a brain fart. When is it better to have squat or when is it better to have anti squat? What works best for going up hill? I read that too much of one or the other will induce bouncing when climbing hills. Would it make any sense to build in an "on the fly" anti squat adjustment of some kind so that the postion of the upper link frame mount could be changed while driving? Or is there one particular amount of a/s that is perfect for everything? Is it 100%?
Adjustable mounts would require a bit of fabbing, but the quest for ultimate performance may make it worth while. Then again, I may be a blithering idiot.:confused:
If you have more than 100% then as the power comes on the links will lift the rear up more than what the COG inertia effect will try to compress the rear springs. Now in this case if you apply enough torque suddenly the links will actually throw the rear of the chassis up into the air. As the chassis rises it gets to the point where it pulls the rear tyres off the ground - this causes them to spin and also removes the torque load out of the rear axle. So as there is not torque load on the rear axle the links no longer support the chassis so that the chassis begins to fall and lowers the tyres back to the ground. The spinning tyres again bit into the ground - get the torque load again - lift the chasis again and away we go - hopping like a mofo on the steep climbs. Im sure everybody has seen this effect. What you will find that if you rig does start to hop from too much AS then you can back off the throttle and reduce the size of the instant torque load so that you only lift the chassis slowly and smoothly then you can keep increasing the torque load back up to the full level without the rear starting to hop. BUT if you do start to get any wheel spin then the hopping will start again and again you have to back off the throttle and smoothly apply it again to keep the hopping under control. Again Im sure everybody has seen this effect.


Now if you have way less than 100% AS (say like 20%) then all that will happen is when you drop the clutch on the steep climbs you will get more wheel spinthat a rig that say has 60% AS but the rig will remain stable and it wont hop so you can just keep the boot in. The rig that has the more AS will hook up better and get a better initial launch when the power is suddenly brought on.

But to me the most important thing is for the rig to not to start hopping. Once you have a rig like this it is the most fun thing to drive because you can just lay into the throttle and keep it planted and just move the wheel back and forth trying to find the line that will keep you moving forward. And not having to back off the throttle cause the rig starts to hop.

This is why I think that as a first try at building a rear linked suspension that people should aim to get as low a AS level as they can so that the end result is a rear linked suspension that is stable and doesent hop. Also if you can build in some adjustability so that you can later increase the AS level if you think that you rig can handle it.

This is also why I dont think that finding the exact position of the COG or really trying to determine the exact AS percentage is necessary. Just try to make it as small as you can and then work you way to bigger AS levels.

Sam
 

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Well I guess I gota start canceling my subscribtions to all my 4x4 mags.....

I dont think I get this much crap explained in one thread then I can in a 10 year subsribtions to all the gay 4x4 mags. oh well wish I knew about this place before I subsribed to the mags.... perfect waist of money. :rolleyes:
 

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Strange Rover said:


So if you set your AS level to between 50% and 80% at ride height on the steep climbs it hopefully wont go beyond 100% and become unstable.

Sam
I believe this to be very true. I always shot for under 80% AS.
 

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Foley - with your "magic" numbers of 36" and 75% of that for the upper link, the pinon will rotate DOWN as the suspension drops, right?

What you wrote was a GREAT primier on link building, but as Air Ride mentioned... with long-travel suspenions (unlike roundy-round cars that only move 2 or 3 inches vertically) it's probably a good idea to keep the links pretty close to the same length, so you don't bind the rear driveline at the diff, isn't it?
 

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TNToy said:
Foley - with your "magic" numbers of 36" and 75% of that for the upper link, the pinon will rotate DOWN as the suspension drops, right?

What you wrote was a GREAT primier on link building, but as Air Ride mentioned... with long-travel suspenions (unlike roundy-round cars that only move 2 or 3 inches vertically) it's probably a good idea to keep the links pretty close to the same length, so you don't bind the rear driveline at the diff, isn't it?
Yeah, the magic number of 75% was mentioned earlier on this thread, I don't particularly agree with it, I just used it as a starting point. I spent about 5 minutes on that picture (but about 20 minutes typing!). I'll have to do some modelling on AS changes through susp. travel to get a better number, but I imagine it will be closer to equal length.

The biggest constraint is going to be the fact that I don't want to mount the link to my kidney, and the driver's seat needs to go somewhere.

I'm glad you liked it, I was hoping to get some people to see the basic steps to it, and I THINK I have a handle on them now.

Mike
 
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