A four-bar dirt Late Model...
A four-bar dirt Late Model rear suspension is designed to have a large range of rear steer. The adjustability allows the racer to make adjustments for the changing conditions that occur on dirt surfaces. The attitude of the car on dry-slick tracks can be quite radical.
The use of rear steer has changed in the past few years. What was common in the mid-to-late '90s is not necessarily the fastest way to go in the 2000's. Technology races on, and we discover new and innovative ways to go faster. As setup philosophies change, so do other setup components that are affected by the setup evolution. Rear steer is one of those effects that must conform to the basic setup package.
Rear steer in a circle track race car is a condition caused by suspension movement and sometimes dynamic forces of acceleration. Under the right conditions, rear steer can be beneficial and enhance performance. Under the wrong conditions, it can ruin your performance. We do not necessarily need to know the exact amounts of rear steer our cars are subjected to, but we need to have a solid understanding of exactly what produces rear steer and what effect rear steer has on the handling of our cars.
The technology related to rear steer for asphalt and dirt is somewhat different. There are a few similarities and many differences in how we evaluate and use rear steer for each group, so we will analyze them separately.
With the rear end pointed...
With the rear end pointed to the left of centerline, steering causes the rear of the car to want to run left under the front end, causing a very tight condition, especially under acceleration.
First and foremost, rear steer is mostly caused by rear suspension movement. As the rear of the car dives and/or rises, and rolls, the control arms, which locate the rear end fore and aft, can cause the wheels to move forward or to the rear. Obviously, if both of the wheels did not move or moved in the same direction by the same amount, we would have zero rear steer. When one wheel moves more than the other or in a different direction, we have rear steer.
Rear steer can either tighten a car or make it very loose. Not only does the condition of rear steer affect the entry and middle handling balance, it also affects the handling under acceleration due to the thrust angle of the rear end being either right or left of the centerline of the car. In dirt racing, it may be advantageous to incorporate a large amount of rear steer under certain conditions.
With the rear end pointed...
With the rear end pointed to the right of centerline, the car is freed up going in and through the middle, and will possibly be loose off the corners with the rear end wanting to run around to the right of the front.
Is rear steer an important tuning tool? We asked Joe Garrison of GRT Race Cars and Mark Richards of Rocket Chassis if rear steer was important in dirt racing, and both said having rear steer capability in a dirt car was critical. "Rear steer helps the driver get the rear end around on dry-slick tracks without having to break the rear tires loose," said Garrison. Richards said, "If the driver has to counter steer the car a lot, you need more rear steer."
Keith Masters of MasterSbilt Race Car Chassis said, "The reason chassis builders build four-bar cars is rear steer. The entry is more important to design than the exit, as far as rear steer is concerned." That echoes what Garrison said in that keeping the rear tires connected to the ground going in and through the middle helps provide traction offthe corners, too. So a smooth entry provides a faster exit.
We also talked with Sandy Goddard of Warrior Race Cars, and he told us that his group of racers work with rear steer less than some other dirt teams, although he feels that using a degree of rear steer is very important at times. He said, "Rear steer definitely helps get into the corners on dry-slick tracks, but you can put too much rear steer into the rear suspension. A lot of guys take that to extremes."
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 right-rear (RR) wheel moves 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 is pulled forward. Normally, to produce a small amount of rear steer to the left on asphalt, we mount the front pivot point one third of the total travel distance higher than the rear pivot.
C.J. Rayburn, of Rayburn Race Cars, is a driver as well as a car builder and echoes much of what the other builders have to say. "We have always wanted rear steer," he says. "Rear steer is important in any design of dirt car." Rayburn builds the swing-arm type of rear suspension, and those cars have multiple holes so that the angles of the control links can be altered to produce more or less rear steer.
A rear end that is steered to the left of centerline will cause the thrust angle to be to the left of centerline and make the car tighter on entry and tighter on exit under acceleration. A rear end that is steered to the right of centerline causes the thrust angle to be pointed to the right of centerline and makes the car looser on entry and looser on exit under acceleration. Knowing these basics, we need to look at each type of racing to see how rear steer affects each type of car and how we might improve our performance using rear steer.
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. The suspension should never 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 as follows:
The front mounting block on...
The front mounting block on the three-link suspension system is usually 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 right trailing arm angle, and the height of the front mount should be moved in small increments.
The pull bar upper third link...
The pull bar upper third link allows the rear end to rotate under acceleration. The movement of the rear end rearward can be utilized to produce rear steer to the left only while under acceleration. That way, the car is correctly aligned at the rear during entry and through the middle of the turns. Rear steer to the left helps promote bite on flat and slick asphalt tracks.
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, 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.