[darkstar]

OK, I have searched and read until my brain hurt, and I still don't see a consensus on this. I'm putting a 4 link together on the rear of my truggy, and the best design I have come up with leaves me with the uppers and lowers parallel. As I understand it, the rest of the numbers look pretty good. I don't really know how meaningful they are, as the vehicle has changed quite a bit since I weighed it last, so there are some guesses and assumptions here.

So, who has a rear 4-link with the uppers and lowers parallel, similar to what you see in the attached images, and how does (did it work). Comments, please, before I burn this in! For once on this thing I would like to build something just once and have it work just the way I want to.

Here's a link to the build thread: http://www.pirate4x4.com/forum/showthread.php?t=696926

I'm also attaching pics of the rear of the rig that's getting 4-linked.

 

[riksnacjnow]

You need to get the lower link off the centerline of the axle tube. The forces are infinite when mounted like that. IE move your lowers down a little bit at the axle. That will also get rid of the parallel worries

 

[darkstar]

I played around with that... If I simply move the axle end of the lowers down below the axle centerline (which I can do easily) it sends the antisquat through the ceiling. To compensate I can also built a tall truss and get the uppers flat, but I'm not convinced its worth the extra work.

What are the bad effects of having the uppers and lowers parallel to each other? I can't believe no one has built and driven a 4 link with a similar design... someone post up...

 

[vetteboy79]

You need to get the lower link off the centerline of the axle tube. The forces are infinite when mounted like that.

:laughing:

No they're not.

To the OP: no way in hell the COG of that rig is a foot below the top of the tires. Where did that number come from?

 

[darkstar]

yeah, that number is probably wrong...I don't think I changed that number from what was in the spreadsheet I downloaded. the COG is probably at about floor level, at least that's a reasonable assumption (?). I'll rerun the numbers when I get to work and see what comes up.

 

[Bigger Valves]

What are the bad effects of having the uppers and lowers parallel to each other? I can't believe no one has built and driven a 4 link with a similar design... someone post up...

The very large changes in instant center location and thus other characteristics that can occur from small changes in suspension movement or construction. Just as you saw when you moved the lower's mount just an inch. Parallel is fine, just be very precise when you build and double check full droop to full bump for any ill-effects.

Forces don't become infinite anywhere, that's ridiculous. Learn when to draw the line between book and reality.

 

[Bigger Valves]

To the OP: no way in hell the COG of that rig is a foot below the top of the tires. Where did that number come from?

Of course I second this..

 

[russellmn]

What a lot of people do for the COG when they don't know where it's at, is to measure to the top bellhousing bolts from the floor and use that.

 

[X-Rated]

on the spread sheet does it want COG of just he sprung weight or the COG of the entire rig with axles and wheels.

 

[95geo]

I would try to get more vertical seperation between the axle end uppers and lowers and mount your shocks/springs as far out as you can get them.

some basics that will get your suspension pretty neutral/predictable are:
instant center at least 4' or so in front of the COG, roll axis within 3 degrees +/- of 0 and 50-70% anti squat.

disclaimer: the second part is my $.02

 

[Toolmaker]

If you can make the front mounts adjustable in height , I gave myself about 2.5 inches , and I'm glad I did .

I ran all the numbers asked questions , got no answers , finally just did it .

When I did I remembered what the four link setup on one of my buddies drag car looked like and it and it had a series of holes for adjustment , I went with three and that way I had some options .

My two cents.

 

[darkstar]

OK, here are a couple more plots. the first one is the same as above, but with a more realistic COG height (about floor level, couple of inches above the frame). The second is with me trying to get the links to not be parallel and still have a decent antisquat number. This version will be a bit harder to build, because I will need to make a tall bridge above the diff (the first version puts the link mounts right on top of the diff).

Is there really a great advantage to the second version, making it worth all the effort?

What actually happens when the uppers and lowers are parallel? No one seems to have an answer to this question...

 

[95geo]

Is there really a great advantage to the second version, making it worth all the effort?

What actually happens when the uppers and lowers are parallel? No one seems to have an answer to this question...

this is your answer for the paralell... it will work fine if all of the other things fall into the "works well" window. but what you have in the first one looks like you can still polish it up some more.

The very large changes in instant center location and thus other characteristics that can occur from small changes in suspension movement or construction.

the lower suspension layout looks much better to me, flatter roll axis (rear steer), nicely placed instant center, good anti squat....

you only have 6" or so of axle seperation with the parallel link layouts, that is not enough, the forces get multiplied pretty quickly when they are so close together. 8-10 is a better number. if building a large truss or bridge over the diff is that it takes then thats what has to be one. lots of people make them and have no issues. be thankful you dont have to make on to a cast center section, that would be a little more difficult.

your lower layout will perform better than the others. :grinpimp:

 

[darkstar]

Thanks for the input... I guess it is worth the effort, and it is buildable (i.e. no existing metal pieces are in the way). Anyone else want to chime in?

 

[Bigger Valves]

What actually happens when the uppers and lowers are parallel? No one seems to have an answer to this question...

It's been answered.. What do you mean what happens?? Look at it, nothing special happens. Notta.. No magic, no unexplainable phenomenons, nothing. All it means is your normal force line slope is also parallel to your links instead of running to an instant center. Now if they're parallel and not the same length, the second it droops or stuffs a small amount you get an instant center way off somewhere that can make things defined by instant center location such as anti-squat go haywire. Doesn't have to go haywire, but it may. Find the spreadsheet with the droop and bump calc built in and see how that works. You need to worry about your suspension parameters from full droop to full bump more than ride height. With parallel links things can change a lot and quickly. That's what happens.

You may find that keeping the parameters consistent and minimal from bump to droop for one setup is much easier. I'd suggest going with that one.

 

[darkstar]

I see what you mean... the AS numbers for my first setup go from 23% or so at ride height to 123% at bump and 140 at full droop, while the second option goes from 60 to 24 to 42, (or from 75 to 48 to 56 if I move the uppers down one hole on the mount) which seems a lot more reasonable. This is starting to make a bit of sense now :idea:

 

[Booger Weldz]

paralell....LOW AS numbers....increasingly coverging @ the frame higher AS numbers

 

[loveshackle]

Hey Alex - Like you, I did a ton of reading...probably too much...then worked the calculator and scales and mock ups over and over until my head hurt even worse. In the end, I went with my gut and best guess, although I did leave lots of room for adjustment (you can see the rear, a 3-link, come together in my build thread...albeit at a standstill for who knows how long). It might be worth looking at...

http://www.pirate4x4.com/forum/showthread.php?t=564005

The real reason I'm replying is that I haven't seen this rig in about a year (I'm out of country)...:grinpimp: SWEET!

 

[vetteboy79]

Find the spreadsheet with the droop and bump calc built in and see how that works. You need to worry about your suspension parameters from full droop to full bump more than ride height. With parallel links things can change a lot and quickly. That's what happens.

You may find that keeping the parameters consistent and minimal from bump to droop for one setup is much easier. I'd suggest going with that one.

Here it is:

http://pages.prodigy.net/dmacock/4BarLinkV3-1.1c (with travel updated).xls

I don't know what the travel function will do with infinite/far IC locations, and I don't have excel on this machine to check it. I re-wrote all the travel calculations to use vector math instead of the solver so it's very possible you'll end up with some overflows or divide-by-zero situations...and that version was last modified May 9th, 2007 so I've totally forgotten all the details of it by now. :laughing:

To the OP: If you do the geometry right, you can get it so that the change in AS for travel/droop is beneficial in both cases to reduce or eliminate hopping and excessive jacking. I.E., a rising AS value as the suspension compresses, and a falling AS value as the suspension droops. Play with the numbers to put your IC somewhere reasonably ahead of the front tires, converge the links towards the frame, keep the upper a little shorter, and see what happens.

 

[alienjeep]

hey alex this thing is looking really nice, when are you planning on wheeling it? maybe we can hook up and hit the badlands or the cliffs.