Anti-squat + other characteristics of a 4 link rear end

[Sam_Q]

I am doing some research into the geometry of multilink rear ends and discovering what why my car behaved different when I changed one of the pivot points.


I had previously seen the traction brackets available from vision R as shown below:





and I decided to do the same with my diff housing by welding a second set of diff brackets on which effectively gave a second set of pivots that were approx 50mm lower down:








I initually thought I was just correcting for the change in the angle of the lower arm by the car being lowered. However after reading the book 'Chassis tuning' I have found what I did is increase the anti-squat characteristics. What this is is where the twisting force of the diff causes the diff to be pushed against the road giving more traction. I found this to be quite a decent change and recomend it to people. Working out the anti-squat is shown below:



On an IRS diff the torsional foces are not transmitted to the hubs and therfore only a very small amount of anti-squat is available. So in straight line traction in this one area IRS rear ends are significantly inferior (thats a first!).

Too much anti-squat? I read that too much anti-squat can cause the back end to start hopping under hard braking, I want to see if its possible to dial that much into a Sprinter.

Potential ideas/plans: Looking at the diff I noticed that the top pivots could potentially be moved into a lower position, but if they were moved the bars coming off them would hit the housing. Now I am thinking of making some custom arms so that they have something to overcome this problem, weather it be curved arms, the bar welded to the very top of the bush tube or a combination of both. Not only would this increase my anti-squat even further but it would also correct the top arm angle to being more straight. If I manage to do that and I don't overdo the anti-squat then I want to modify my diff brackets on the bottom for a second time. I did some basic measurements and I found I can lower the pivots another 10mm again before I run into ground clearence issues, not much but call me an extremist if you will. The brackets I made up were only meant to be temporary anyway and I need to move my shocker mounts regardless, so it shouldn't effect me too much.

As far as I understand a standard sprinter and every optermised 3/4 link has the top bar(s) at a 0 degree angle. This is to reduce the change of yaw of the housing throughout the axis of vertical movement because if the bar starts on a angle then the final yaw angle on full compression is greater. I could totally eradicate this problem by running equal length arms by installing a kit such as this (bad pics):




this makes the top bar the same dimentions as the bottom which would make it impossible to have the diff yaw and I probably wouldnt find it all that hard to setup I am not prepared to lose my back seats for it.

So the questions I ask people:


- Who has experimented i these areas in general?

- Anyone ever tried increasing the bottom pivot much more than myself?

- Anyone ever tried changing the top pivot points?

- Why is the yaw change of a diff a bad thing? how does it interfere with traction/handling?



so if someone didnt get a headache off reading my post I would very much like to hear what they think. Corrections or sujestions are welcome providing they are done politely


-->Sam.Q

[TurboRA28]

This is very interesting stuff! Seeing I've just changed mine to a 2 seater I have been toying with the idea of 4 equal length arms.

I also have custom brackets for my lower trailing arms but haven't experimented moving them down lower yet.

My only feedback to offer is one of my mates who races an ae86 in improved production and does quite well at it too. He played with the lower control arm angle and said it actually upset the car. Sorry can't offer more than that.

[Sam_Q]

thats funny, I found I had heaps more traction off the line..

the theory is that it also applies in cornering under power where it has more traction without lossing anything up front.

[strat]

We have found in two different racing classes that change in this area to be very helpfull. we did a simmilar thing as you have done with the lower controll arms on a AU falcon in saloon cars, moving them down and further back 20mm helped get the car off tight turns but made it a bit oversteery on fast turns. We also did a similar thing on a S6 RX7 by changing the roll center of the diff same can be done on live axle by making the top arms adjustable the RX was a drift car and we found that rolling the diff forward would increase rear grip and rolling it back would decreace grip. i hope this makes sence i just came back from a christmass party and im half cut.

[mc68]

Sam, If you are going to go to the trouble to make equal length arms, make them adjustable in where the upper arms mount (up and down)

again, hard to get proper pics of this stuff (but you'll probably find a few escorts with it aswell)



could be a very effective way to change how the rear end handles (dramatically)

[Sam_Q]

strat: thanks for the info, thats really interesting , I have some adjustable lower arms I made up a while ago that i can put in. I never installed them due to my diff by chance facing foward. I might try leaning it down and seeing how that works out.

mc68: I know exactly what you mean, however I am not going to lose my rear seats for any level of handling gain so its irelevant. However for race cars yes its a great idea.

[oldcorollas]

don't lose the rear seats.. just "adjust" them

the ones in that last pic look abit more robust than the usual added in sections

[Sam_Q]

hahahah, to what!? being 20cm higher up? hahaha

yeah I did notice they did seem to be a little triangulated unlike the usual risers. I guess either wanting the back seat still or sticking to class rules would stop most people

[Jedi]

If you make your upper arm mounting points adjustable (i.e drill 3 or 4 holes in the mounting bracket on a vertical plane) you can adjust the degree of anti-squat you get and dial in the amount that suits you.
Something like this:



Also, if you want some good link suspension reading, check out this link: http://www.pirate4x4.com/forum/showt...ght=suspension

Its from a 4wd site but the information is excellent and still applicable to racing applications.

[Sam_Q]

thanks for that Jedi, but moving my pivot point futher down will decrease my anti squat not increase it.

now where can I get one of those rose joints? I want just the bar ends for the top bar

I am going to plow though that 4x4 link tonight and summarise all I find on here if possible.

[boyracer]

Buy using a standard lenght lower control arm and lowering the mount 50 mm wouldn't that change the pinion angle to ?? Or is that what your after ??

[TurboRA28]

It would.. Need adjustable arms.

[Sam_Q]

no by lowering the pivot point it actually made the pinion angle horizontal again, confused?

Thats because this car isn't a standard height, when my car was lowered it must of tilted the nose downwards, and when I moved the pivot down it made it level again. I never expected this and I had some lower adjustable control arms ready I made myself to correct the angle. I didn't end up using it so I just left the standard ones in there.

[Sam_Q]

here you go, heres what I made up ages ago and never used, with the exception of the pan-hard.

[Jedi]

thanks for that Jedi, but moving my pivot point futher down will decrease my anti squat not increase it.

now where can I get one of those rose joints? I want just the bar ends for the top bar
.

A trick alot of the 4x4 comp guys do is to dial in a heap of anti squat, then run a limiting strap on the rear diff to control the "hops" under brakes. It prevents the suspension from unloading and increasing the bounce. Cheap, easy and effective.

As for rose joints (or heim joints, as they are known in the states), you can get them from http://www.abt4x4.com/

If you are changing suspension geometry on your car though, they are an absolute bastard to get engineered Im told.