Rear steer (roll steer) occurs...
Rear steer (roll steer) occurs when one rear wheel moves forward when the chassis heels over to one side in a turn.
The rear axle of your race car doesn't just go up and down. It twists, it turns, it points-it does all sorts of things. And it moves in directions that may or may not have the effect you want.
Obviously, the rear end moves vertically as a function of the weight of the car and the track's surface. But springs and shocks only control the vertical movement that occurs. The way in which the rear axle moves in other directions is controlled by the geometry of the rear suspension.
The linkage that attaches the rear end to the chassis indexes its movement. For this article, we will use the GM metric four-link as a working sample. If the chassis were to be lowered evenly from side to side (as in straight line acceleration) then the rear axle would stay square with the chassis. It might move forward or rearward, but it would do so equally on each side.
I used this setup to find...
I used this setup to find suspension points and track their movement. The Craftsman Laser Trac made the job much easier. If the lower trailing arm is leveled or slightly higher in the front, there will be little roll steer. Raising the chassis so the front of the trailing arm is somewhat higher will induce roll steer and cause the car to be looser. Raising the chassis this way can increase forward bite. Choose your compromise.
At the end of the straight, we use the brakes. The rear end rises slightly. The axle moves rearward. Then we direct the car into a left turn. The chassis rolls over to the right. So the right side goes down and the left side comes up. When the right goes down, the axle will be pulled forward on the right side. This happens because the lower trailing arm follows a radius around its front chassis attachment point. As it follows this arc, it shortens the distance between the axle and the chassis. The axle can also be pushed rearward on the left side when the chassis rises up. Both of these actions angle the rear axle in the chassis. The axle will be pivoted to point toward the inside of the track. This is known as roll steer. When the car rolls to one side, the rear axle steers toward the other.
If the class of car you race allows modifications to the rear suspension, roll steer can be reduced to near zero. However, you may not want to reduce the roll steer to zero because roll steer can be a chassis-tuning tool.
When you are allowed to modify the rear suspension, many things can be changed. There are many different suspension link combinations that can make all sorts of things happen with vertical movement of the rear axle. It may help to look at it this way: Vertical movement of the chassis is a power source for moving the rear axle fore and aft and in twist.
The rubber bushings found...
The rubber bushings found in the GM four-link trailing arms do not rotate. They are held in torsion. In this photo notice the position of the white line.
On our shop example of a metric chassis, using common track rules that require stock rear suspension links, the first thing we need to do is find out what happens when the rear axle is cycled through its movement. First put the car on jackstands. Then remove the springs and shocks. At this point, the axle will droop. You will see that when you pull down on it, it will spring back up slightly. This is caused by the rubber bushings in the trailing arms.
It will be difficult to cycle the rear end through its movement with these bushings tight. To begin, loosen all the trailing arm bolts, backing the nut off about one quarter. This will free up the suspension somewhat by loosening the center of the rubber bushings.
In this photo the center has...
In this photo the center has not moved. The white line was bent around while the trailing arm was cycled down. These bushings act as a spring, much the same as a torsion bar.
Since we had a bare chassis in the shop, it was easy to block it at different ride heights. This allowed us to lower the right side and/or raise the left, simulating the condition of the car rolling over into a turn. When jacking the chassis, we had to stack weights on the chassis to keep it from lifting due to the stiffness of the rubber bushings even with the bolts loose.
I found that simulating chassis roll by using 3 inches of right-rear down and 1 inch left-rear up, the metric chassis pulled the right-rear tire forward almost 3/8 inch more than the left. When rolled into a turn, this would have the effect of tightening the car on corner exit or anytime the power was turned on. Depending on your setup, it might not tighten it enough. If the tail of your car is hanging out on corner exit, it may look good from the stands, but your car could be faster if all of it were going forward.