Poly+Rubber rear suspension bushings (passive rear steer)

[darwoodious]

Ben has a nice thread going on documenting work on his exxe here.

As usual, his thread sparked a really interesting conversation on polyurethane bushings, some issues raised by Steve Hoelscher regarding binding etc.

Thought I'd extend it here instead of in Ben's thread and here are my thoughts...

First, I too picked up some poly bushings and looking at the geometry, I agree with Steve that there can be some binding for sure and the stress on the mounts is concerning. I started thinking: what if one were to put on polyurethane bushings on the rear a-arm pickup points and rubber on the front pickups? Binding is now not an issue as the arm axis of swing will prefer to follow the rear point's axis. BTW, the delta between the front and rear axis looks to be around 15º.

What would this do to overall suspension geometry under load is the question:

1. Under acceleration, the hub is thrust forward and the front bushing is "pushed" while the rear is pulled (slightly). This would make a slight increase to toe-in at the rear. With this front-rubber/rear-poly combo, we'd still have this but it would be less.
2. Under braking, the hub is thrust backward and the opposite occurs resulting in a more toe-out in the rear.
3. In hard cornering where the rear arm is outside, the hub is thrust towards the chassis. The poly rear bushing would deflect less than the rubber front bushing, resulting in to-in. Conversely, on the other side (inside of the corner) the hub is pulled away from the chassis and this arm results in toe-out. I imagine very slight.

This would effectively be passive 4 wheel steering?

I put together some pics of the thought. This is the arrangement (I marked up a '75 rear repair diagram)

Here's how my feeble mind imagines the loads on these:

Did some more research on these "passive rear wheel steering" setups.
* wikipedia has an article: https://en.wikipedia.org/wiki/Steering#Passive_rear_wheel_steering
* Porsche did a "Weissach axle" in the 928: https://en.wikipedia.org/wiki/Weissach_axle
* Saab did something called "ReAxs"
* I remember hearing about VW doing a similar system in Scirocco's and Corrados but didn't find an article on the webs


The Weissach system is interesting (if complicated). Here's an image of it:

Now this stiff rear / softer front bushing setup isn't anything like that, but it should make the car tighter, provide a bit of help with oversteer. Even under throttle deceleration in a corner while it won't provide more toe-in on the outer-rear wheel, it should provide less toe out on that same wheel.

Just throwing fun/silly ideas out there.

Someone talk me out of it (especially guys with race experience). I'm doing poly in the front as there's no geometry binding issues there with the trailing bar. The all-polyurethane rear bushing setup issues do make some sense to me tho.

 

[Steve Hoelscher]

A very interesting prospect. I will want to spend some time thinking about how this would work in actual application. I tend to think you have a good initial analysis of the dynamics. The one thing to keep in mind is that; for the typical street car application, the front bushing will twist as the outside suspension compresses with body roll as the car builds lateral g from cornering. The two are both somewhat progressive in that the more lateral g the more roll and the more roll the more twist. Because twisting reduces the clearance between the control arm's bushing sleeve and the bushing's housing it compresses the bushing and makes it more resistant to compression from lateral load. Thus the effect of passive steering might be limited.

There have been a number of cars that incorporated passive rear steering. The 2nd generation Mazda RX7 had such a system. I had the dubious honor of trying to setup one of these cars so equipped. This system was resistant to any functional performance setup. The solution was to use one of the aftermarket eliminator kits. Once done the car worked reasonably well. A more recent example is the Porsche 991. I have worked extensively with the 991 GT3 Cup cars and they incorporate what we term "bump steer" instead of rear steer. In this case the car is designed to have a significant amount of toe change from suspension travel. It effectively works like passive steering but is a function of suspension travel and not bushing deflection from lateral load. It pretty much irritates the crap out of setup engineers.

 

[ArtBasement]

Not that I understand all the above, but I think I can follow along. My big concern is the tearing of the mounts. I too recently put urethane bushings all around my car, before I knew about this concern.

Am I correct in thinking that the mount that fails is the front one, specifically at the rear of the front mount? And would it be stressed more by braking and accelerating or by up and down movements of the suspension? I like the imaginative thinking of keeping the front one rubber.

 

[Steve Hoelscher]

I have seen both front and rear crack but the front is probably the most prone to the problem. The specific location isn't consistent.

The binding is due to suspension motion. The stress comes from the rotation of the arm causing the arm's bushing bore to become misaligned with the body mount's bore.

 

[Mark]

Back in the Nineties, I upgraded to poly bushings from The Monte Hospital. They recommended only changing two of the four end bushings. I did as you described; only on a Scorpion.

 

[Dr.Jeff]

Very interesting indeed Darin.

I'm certainly not a suspension expert so the "rear steering" aspect is beyond my scope. But if nothing else, having one 'semi-solid' (urethane) bushing and one 'soft' (rubber) bushing should reduce the total amount of unwanted deflection (uncontrolled suspension movement) by approximately half, while still allowing for some off-axis movement of the two pivot points. Sort of a half and half compromise, which is certainly better than none.
Steve, as you spend some time thinking about it, please consider the benefits of this combined urethane/rubber idea even without rear-steer possibilities. Do you think it will offer better suspension control yet still allow sufficient 'play' to prevent failures?

On the other end of the issue. Is it warranted to add a little well placed reinforcement around the mounting points? Especially for those who already have full urethane conversions. Possibly disperse the loads over larger areas and avoid cracks. I know something has to give, but at some point that will be the urethane rather than the chassis.