A good quality spring rate...
A good quality spring rate tester is essential to finding your correct spring rates. Rate the spring at the correct installed height and not the first few inches. The spring will never be working at the first one or two inches. A 200-pound-per-inch rated spring will be compressed three inches at ride height with 600 pounds on it.
The tendency towards using softer springs on asphalt, and to a lesser degree on dirt, has increased recently and so this is a more common problem today. The primary culprit is the greater amount of dive on entry caused by soft springs, which creates excessive camber change. When the RF camber changes too quickly, that tire loses grip and the car begins to push. The best fix for this problem is to add anti-dive to both upper control arms. The best part of using anti-dive is that it has no negative affect on the rest of the setup. Touring dirt Late Models and touring-type asphalt Late Models have traditionally gone without anti-dive. That trend is changing and many teams are discovering the benefits of anti-dive for their short-track cars.
Anti-dive is created when the upper control arms are angled in relation to the ground with the front end of the pivot shaft higher than the rear end. Most upper control arms are mounted with two bolts that are six inches apart. On these types of arms, 11/42-inch difference in height of the two mounting bolts will produce a five-degree angle. This is sufficient anti-dive for the RF of the car. Use half that amount, or 11/44 inch of difference in height, for the LF upper control arm mount.
The lower control arms also contribute to anti-dive, although less so, and the opposite is true-meaning that the rear chassis mount should be higher than the front chassis mount.
Creating anti-dive in your race car should reduce the amount of dive on entry and the excess camber change that goes with it. Remember to keep the average heights of the mounting points the same to keep from altering the roll center location.
Loss of "Feel" for the Car-For a car to be fast, the driver must have a good feel for what the car is doing. A soft ride that feels sloppy does nothing to give a driver confidence in knowing what the car will do next.
Many racers think that increased compression in the shocks should help this problem, but too often that has an opposite effect. The car may indeed need more compression in the shocks, but the rebound should always be higher than the compression to control excess body sway. In most cases, just increasing the rebound resistance will solve the problem.
The angle of the trailing...
The angle of the trailing arms is adjustable by moving the front mount up or down. The greatest effect on the left rear corner is caused by increasing the angle to create rear steer as the car squats on exit off the corners.
For dirt cars where a lot of vertical movement is desirable to get more weight transfer on dry slick racetracks, reduced compression and rebound is helpful, and a lot of movement should be expected.
Tight in the Middle-The Car Won't Turn-This is the "A-number one" complaint for most ill-handling stock cars on dirt or asphalt. The problem usually lies with a combination of poor front-end geometry and an unbalanced setup.
Generally speaking, we want to first reduce or eliminate Ackermann effect in the steering system. We need to position our roll center so that the front suspension dynamically will work to make the LF tire work harder. As we move the roll center left, the front end becomes more efficient and turns better.
Finally, we need to balance the setup. What we have discovered is that a tight setup is usually related to how the front and rear suspensions are arranged. A front end that is stiffer than the rear end will produce an unbalanced car that is tight and won't turn the corner. A stiff RF spring and/or a soft RR spring will cause this imbalance.
Reversing the front springs and putting the stiffer spring on the LF is an old trick that a few modern day racers know about. [Although many dirt racers with 4-bar suspensions are doing this-Ed.] This is only good for lower banked racetracks, but really does help the car to turn.
A softer RR spring helps traction up and off the corners, but a 10 or 15 pound difference for asphalt cars is usually all that is needed to get the job done. More split (50-75 pounds) is required for dirt racing on dry slick tracks in order to produce the proper amount of rear steer, and to help load the LR tire. Excess rear spring split makes the car too tight.