The Theory And Application Of Rear-Steer GeometryThe asphalt racing surface provides a lot of traction, even on those flat "slick" tracks. Because there's 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 rearend to the right on asphalt. It's been a common practice for teams to align the rearend 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 shouldn't be necessary if the car is set up properly.
For general information on what produces rear steer, read the first few paragraphs of the Dirt section in Part One (on page XX), then come back to this section. Asphalt stock cars have three predominant rear suspension systems and all of them produce some amount of rear steer. They are:
The Asphalt Three-Link The three-link rear suspension system has two trailing arms mounted near the rear tires and one third link mounted atop the rear differential that controls rearend wrap-up. The trailing arms can be mounted parallel to the centerline of the car or angled with the front mounts closer to centerline.
RS 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, and on most flat to medium banked tracks, the left rear moves very little. This has been confirmed by studying data from onboard data recording systems during testing that shows 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; whereas the RR shock may show from 2 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 RS 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 1/3 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 mount of as small as 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 three-link system when using a pullbar upper third link. If we mount the left-side trailing arm lower than the right-side trailing arm, then as the rearend rotates under acceleration due to the pullbar extending, then the LR wheel will move rearward more 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.
The three-link rear suspension...
The three-link rear suspension system can produce rear steer in both directions. As the chassis moves down on the right side, the RR wheel will be moved back as the front mount approaches the height of the rear mounting point. As the front mounting point continues to move down, the RR wheel will be pulled forward. Normally, to produce a small amount of rear steer to the left on asphalt, we mount the front pivot point 1/3 of the total travel distance higher than the rear pivot.
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 rearend causes rear steer. If the Panhard bar is mounted on the right side of the chassis and level to the ground, when the chassis moves during cornering, the rearend will be pulled to the right and will steer to the left. This is caused by the rearend swinging around the instant center that is created from projecting lines through the arms to the front until the lines meet.
The Truck-Arm System The truck-arm system has been adapted from the design for a '64 Chevy truck and is used on many Late Model Stock cars as well as the three premier divisions of NASCAR, the Camping World Trucks, and the Nationwide cars, as well as on Sprint Cup cars. 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.
The front mounting block on...
The front mounting block on the three-link suspension system is slotted vertically so that 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. Most teams are getting away from using right trailing arm rubber biscuits. The movement can be as much as 1/4 inch forward upon acceleration, and that's way too much rear steer. Bob Bolles
As the pullbar extends under...
As the pullbar extends under acceleration, the rearend 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, then the radius on the left side will be longer from the lower mount to the axle, causing the LR wheel to move farther rearward than the RR wheel.
The added movement of the...
The added movement of the LR causes a slight amount of rear steer to the left, the exact amount depends on the difference in height of the mounting points and the amount of third-link travel. When we tried this some years ago, we calculated that the movement was around 0.050 inch and produced enough rear steer to tighten the car off the corner on a flat and slick track. We can see where a link with a rubber biscuit moving 0.250 inch would cause excessive rear steer.