Rear Axle Movement - Roll Steer And Its Effects

Now let's go back to those rubber bushings that locate the rear suspension. This is how they work: There is a steel tube centered in the bushing. The tube is bonded to the rubber bushing. The bushing is also bonded to a steel tube on the outside. This outer tube is pressed into the trailing arm. When the bolt through the inner bushing tube is tightened, the inner steel tube is clamped to the chassis mount. At this point, with everything tight, there is no rotation in the bushing; there is only torsional flex. The bushing does not rotate. It actually becomes another spring. This spring has some slight effects on vertical movement. It is more effective in tightening up all the rear suspension movements.

GM made the car for street driving. For racing purposes, we often want the suspension freed up so we can control it with springs and shocks. Given what the rules allow-and what our pocketbooks will stand-all these rubber bushings might be exchanged for spherical bearings. For pavement racing, this might work very well, as we don't want the rear end flexing on rubber and moving around a lot.

For dirt bullrings, we sometimes need the rear roll steer and/or other movements to help us around the track. These cars can sometimes be too loose. More roll steer to keep the rear end tucked in behind the front might be helpful here. There are lots of things one can do with the rubber bushings to achieve this. One is to check the inner steel tube in the bushing. Some have teeth on the ends. If so, grind them smooth. This is to keep from wearing a hole in the chassis and to reduce rubbing friction.

I know some racers who comb the salvage yards for worn-out bushings. These can be loose or even torn bushings. Bushings can be softened by drilling holes parallel to the inner steel tube, making the bushing more compressible. One method I've seen is to heat the outer steel tube while it is still in the trailing arm. Using a torch to do this will destroy the bond of rubber to tube. After the bushing has been on fire a few moments, it is dumped in a bucket of water. The bushing is then completely loose in the tube. This frees up the rear suspension but does not change the compressibility of the bushing.

By softening the compressibility of the bushing on the right-rear trailing arm, you might find roll steer loosening the car on entry. This would reverse with the power on and tighten the car on corner exit. The soft bushing allows the right-rear wheel to pull rearward under braking. Then, with acceleration, the right-rear will push forward.

Before you loosen up all your bushings, you might want to be selective. Let's say you want more roll steer to tighten up the car on corner exit. Then you would soften the bushings on the right lower trailing arm. This would allow them to compress on the right side during acceleration, thus pulling the right-rear forward. It would then steer inside the right-front, keeping the rear tucked in tighter. Softening the left side lower trailing arm bushings would offset much of the right side compression. This would loosen the car. Softening both might be an aid to forward traction. Doing this would allow the rear to move forward in the chassis, thus adding weight to the rear wheels.

This effect of roll steer would be increased if the chassis ride height were adjusted so that the lower control arms tended to raise the chassis under power. Raising the chassis ride height so the rear of the lower trailing arm is below the front will push the right-rear tire rearward under acceleration. This will loosen the car on exit. Raising the ride height will also increase any roll steer tendencies.

Sorry, I can't tell you how little or how much, if any, of the steering effects of the trailing arms you need in your car. The best I can do is give you the tools and show you how to use them.

If the rear axle pivots so the right-rear pulls forward, this will tighten the car. If the left-rear pulls forward, then the car will be looser. Softer rubber bushings will allow more rear axle movement. Use locknuts on the trailing arm bolts. Don't tighten them all the way; leave some space for the bolt to float. This will free up the rear suspension. Don't be afraid to make adjustments to your car. Even if it is the wrong adjustment, you will learn something. A hint might be to bring several lower and/or upper links to the track. Some, of course, will have soft or loose bushings. You might treat one race night as a practice and change links each time you get on the track. A lot could be learned.

Now for a good idea. Go to the track with a handful of trailing arms. A selection of lower and upper arms might have drilled bushings on one or both ends. Change them and note the differences in handling. You may find a combination that works better with wet or dry tracks.