Asphalt Rear Steer
The front mounting block on...
The front mounting block on the 3-link suspension system is slotted vertically so you can adjust the angle of the trailing arm to fine tune the amount of rear steer. The car is very sensitive to changes in the trailing arm angle and the height of the front mount should be moved in small increments.
The asphalt racing surface provides a lot of traction, even on those flat "slick" tracks. Because there is very little slip of the tires on asphalt, the range of useable rear steer is very small. We never need our suspension to steer the rear end to the right of centerline on asphalt. It has been a practice for teams to align the rear end-and/or have it steer slightly to the right-to fix a tight mid-corner condition, but this goes into the category of crutches, and should not be necessary if the car is set up properly otherwise.
Asphalt stock cars have four predominant rear suspension systems and all of them produce some amount of rear steer. They are:
1. The Asphalt 3-Link - The 3-link rear suspension system has two trailing arms mounted near the rear tires, and one third link mounted atop the rear differential that 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.
As the pull bar extends under...
As the pull bar extends under acceleration, the rear end rotates back, causing the rear wheels to move to the rear. The bottom pivot point is the back end of the trailing arms. If we mount the right-side arm higher than the left-side arm...
Rear steer in this system is caused by chassis movement, which can produce several secondary effects. Usually, the right-rear corner of the chassis moves more than the left-rear. On most flat- to medium-banked tracks, the left-rear moves very little. This has been confirmed by studying data from onboard computer systems during testing that show shock travel amounts in the turns. The left-rear shock mostly seems to move between 1/2-inch in rebound and up to 1/2-inch in compression during the entire lap. The right-rear shock shows from 3 to 4 inches or more of travel, depending on the spring rates used.
On most asphalt 3-link cars, the right-rear trailing arm mostly controls rear steer due to body 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 of the distance that the front mount will move down during cornering. The variation of height for the right-rear trailing arm is very small. Changes in the height of the front of the trailing arm as small as a 1/4-inch can be felt by the driver.
A trick way to produce rear steer only under acceleration is by staggering the height of the two trailing arms in the 3-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, then as the rear end rotates under acceleration due to the pull bar extending, the left-rear wheel will move rearward more so than the right-rear 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.
...the radius on the left...
...the radius on the left side will be longer from the lower mount to the axle, causing the left-rear wheel to move farther rearward than the right-rear wheel. This causes a slight amount of rear steer to the left, the exact amount depending on the difference in height of the mounting points and the amount of third-link travel.
Another component 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, the rear end will be pulled to the right, and will steer to the left when the chassis moves during cornering. This is caused by the rear end swinging around the instant center, created from projecting lines through the arms to the front until the lines meet.
2. The Truck-Arm System - The truck-arm system has been adapted from the design for a 1964 Chevy truck, and is used on many Late Model Stock cars, as well as the three premier divisions of NASCAR (Craftsman Trucks, Busch, and Winston Cup). These systems only steer to the left and have a limited amount of steer. The roll of the chassis and the movement of the Panhard bar are the two components that 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 point of rotation (the axle). Rear steer amounts-due to the Panhard bar angle-are regulated by the angle. A downside to using the truck arms is not related to steering characteristics, but due to a narrow spring base when the springs are mounted directly on top of the truck arms, creating a narrow spring base in the rear.