darwoodious
Darin Nelson
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.
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.