On asphalt, we can only tolerate a very small amount of rear steer and most of the time we are better off with close to zero rear steer. On dirt, some teams incorporate lots of rear steer to the right into the suspension. The degree of steer is directly proportional to the amount and direction of vertical movement associated with the right and left rear suspension systems.
We can simulate the degree and direction of rear steer in our race cars by duplicating the movement of the rear suspension. If we take visual reference to the position of the wheels in the wheel wells during cornering, we can get sufficiently close to replicating the suspension attitude with the car in the shop. We could then support the chassis at levels similar to the way the car looks on the racetrack and then measure how far each rear wheel moved and in what direction. It may surprise many how far the rearend steers under some conditions.
If more or less steer is desired, changes can be made to trailing arm angles, and so on as you are simulating the rear steer and the results can be measured. This is an excellent way to learn how arm-angle changes relate to rear steer magnitude.
Spring Location and Angles
It is an engineering fact that the less angle from vertical that we have in our shock/spring design, the more efficient and consistent our setups will be. Many cars are designed with excess coilover shock angle. This results in a low net wheel rate and also a variable rate spring with diminishing rate over compression travel.
On many four-bar dirt cars, the rear coilovers are mounted at a high degree of angle. This does two things: 1) High angles with the tops in closer to the centerline than the bottoms reduce the spring base that the chassis sits on and increases the roll angle of the rear suspension. 2) It reduces the wheel rate causing increased roll angle due to the soft spring rate. That is exactly why some cars will lift the LF wheel off the track more easily than other cars. Moving the top of the springs out toward the wheels will offer more stability in the car.
With high spring angles in the rear, as the car rolls, the angle increases and the rate decreases. With high spring angles at the front or rear, with every inch of chassis movement vertically, the spring will move less providing a lower rate of resistance to chassis roll. This is a variable that we just don't need in our race cars. Variables mean that handling will change based on different entry lines and the different banking of the higher and lower grooves.
If we can get to know our cars before the season starts, we can then make critical decisions on the front and rear design that can produce more consistent handling and better performance. All it takes is a little time and some effort. This winter, think about your overall chassis design. Work on the areas we have discussed here.
Remember, it's your car to do what you want with. We are all scientists and inventors at heart, so live up to your natural tendencies and get to work on that car. Cut and weld if necessary. A can of paint will hide the work done and keep curious eyes from discovering why your car works better than others of the same make. That's the fun part of doing it yourself and experiencing the positive results.
We should always align our...
We should always align our car so that the rearend is square to the centerline of the chassis and where the right-side tire-contact patches are in line. Many handling problems can be traced to poor alignment. This problem will never be overcome until you properly align your car.
Rear steer can be adjusted...
Rear steer can be adjusted on an asphalt car by changing the angles of the trailing links. These angles are very critical and even small changes can have a profound effect on the handling of your asphalt car. Dirt Late Model and Modified cars' rear suspensions, as shown, are mostly either of a four-bar design or a swing arm. Both are adjustable for angles of the links so that we can easily control and dial in desired rear steer characteristics. The science of dirt car setup is as complicated and challenging as any race car that exists.
Spring angles have an effect...
Spring angles have an effect on the roll stiffness of each suspension and therefore the roll tendencies. We must take this into account when trying to balance the two suspension systems. Excess front or rear spring angle will promote suspension roll over. If related to the rear, we will see three wheeling of the car.