The Asphalt Three-Link. The three-link rear suspension system has two trailing arms forward of the rear end and mounted near the rear tires, and a third link is mounted atop the rear differential, which controls rear end wrap-up. The trailing arms can be mounted parallel to the centerline of the car or angled with the front mounts closer to centerline.

Rear steer in this system is caused by chassis movement, which can produce several secondary effects. Usually, the RR corner of the chassis moves more than the left rear (LR), and on most flat-to-medium banked tracks, the LR moves very little. This has been confirmed by studying data from onboard computer systems during testing that shows shock travel amounts in the turns. The LR shock mostly seems to move between 1/2 inch in rebound and up to 1/2 inch in compression during the entire lap, whereas the RR shock shows 3 to 4 inches or more of travel, depending on the spring rates used.

That would mean that on most asphalt three-link cars, the RR trailing arm mostly controls rear steer due to body dive and roll. We usually need to position the angle of the trailing arm so that the front mount is higher than the rear mount by roughly one-third the distance that the front mount will move down during cornering. The variation of height for the RR trailing arm is very small. Changes in the height of the front of the trailing arm, which are as small as 1/4 inch, can be felt by the driver.

A way to produce rear steer only under acceleration is by staggering the height of the two trailing arms in the three-link system when using a pull bar upper third link. If we mount the left-side trailing arm lower than the right-side trailing arm, as the rear end rotates under acceleration due to the pull bar extending, the LR wheel will move rearward more so than the RR wheel, causing rear steer to the left to a small degree. This promotes forward bite without causing the car to be tight on entry or in the middle of the turns.

Another configuration that promotes rear steer is when the rear trailing arms are angled from a top view with the front mount closer to the centerline than the rear mounts. With this design, lateral movement of the rear end causes rear steer. If the Panhard bar is mounted on the right side of the chassis and level to the ground, then the rear end will be pulled to the right and the rear end will steer to the left when the chassis moves during cornering.

The Truck Arm System. Adapted from the design for a '64 Chevy truck, this system is used on many Late Model stock cars as well as vehicles in the three premier divisions of NASCAR: Craftsman Trucks, Busch cars, and Nextel Cup cars. This system only steers to the left and has a limited amount of steer. The roll of the chassis and the movement of the Panhard bar influence the amount of steer in these systems.

As far as the geometry related to rear steer is concerned, this is an ideal system. The amount of rear steer due to body roll is regulated by the height of the front mounts of the arms, which are always mounted lower than the rear axis of rotation, which is the axle. Rear steer amounts due to the Panhard bar angle are regulated by the angle, the same situation that exists with unparallel three-link lower arms. A downside to using the truck arm system is that a narrow spring base forms when the springs are mounted directly on top of the truck arms, resulting in a narrow spring base at the rear that doesn't resist roll very well.

The Metric Four-Link System. The metric four-link is a widely used system that is built into some models of stock automobiles. It uses four links, as the name implies, that are not parallel to the centerline of the car. The top links are angled from a top view with the front pivots wider than the rear pivots. The lower links are angled from a top view with the front pivots closer to centerline than the rear pivots.

With this system, the rear end stays located by virtue of the opposing angles of the upper and lower links. There is also steer to the left, and because of the width of the front mounts of the lower controlling links, rear steer can be considerable. Under most current rules, there is no adjustment for amounts of rear steer with these systems.

Leaf-Spring Systems. The leaf-spring rear suspension system locates the rear end fore and aft as well as laterally using the leafs. There can be some amount of rear steer as the chassis rolls and squats, but it is usually minimal and mostly fixed as far as adjustability. The advantage of this system is that it keeps the rear end squared up and the thrust under acceleration straight ahead, if that is what is needed.