Once we removed the shocks,...
Once we removed the shocks, we were surprised to see this damage. The car had height spacers installed, a common procedure to create a desired ride height. But with the stock mounting holes in the frame, the shock body is too close to the spacer, and on this car the shocks rubbed severely. We even thought the shafts were bent due to the stiffness we saw with the simple task of trying to push them in.
I don't know if someone chose really stiff compression numbers, or if the shocks might have been bent, but these units were not helping the situation. On dirt, we must have shocks that will allow the car to run over the holes and bumps and move fairly easily. They must control the springs, but not be so stiff as to cause the tire to skip or bounce.
I inspected the front geometry and noticed that the car builder had used the taller Impala spindles, which reduced the lower control arm angles. He also lowered the upper control arm mounts to gain more angle. Both of these changes are highly recommended for this class. Using taller spindles actually improves both upper and lower arm angles.
The geometry I saw looked a lot like previous cars I have fooled with in the stock classes, and I knew the moment center was close to where it needed to be, at or very near the centerline of the car. Had this work not been done, we could have expected the MC to be somewhere outside the car making the front end overly stiff.
By using Impala spindles,...
By using Impala spindles, you can increase the upper ball joint height and create more upper control arm angle. This is good for creating a more centered location for the moment center. It also reduced camber change on dive and roll in the turns, and helps keep a flatter tire contact patch. This also creates a high angle of the ball joint. Make sure on your car that the ball joint shaft does not contact the control arm during upward travel.
The cambers were not what I would have expected or chosen. The LF was a little less than 1 degree, and the RF was around 5 degrees. This is typical of a car that transfers a lot of load to the RF, as that tire is forced to carry most of the front load and work hard to turn the car. The LF doesn't need camber because it isn't working very hard. We needed to make camber changes to reflect how the front end would work after the re-design.
We reset the cambers to a positive (+) 2.0 on the LF and minus (-) 3.5 at the RF. Once we re-spring the car to allow a more balanced setup, the LF tire will definitely be working harder and take some of the load off of the RF tire. So, we needed more LF camber and less RF camber. Our tire contact patch will be optimal with these changes.Inspection of the caster settings revealed that the RF caster was in the negative range (upper ball joint forward of the lower ball joint) and the left side was positive. That would cause the steering to want to turn right, maybe good for sliding the car through the turns, but we were going to make this car turn well, so I opted for a different plan.
We moved the right upper ball joint back to create about 2 degrees of positive caster that matched the left side caster. This way, the steering would be neutral and the driver could steer both directions without feeling a difference in resistance.
The car had spring adjusters...
The car had spring adjusters made by AllStar Performance installed in the front end on top of the stock springs. These were intended as spacers only, and not to jack weight around.
We were a little worried about possible binding of the ball joints with the increased upper control arm angles. So we cycled the spindles beyond what they would see on the track and found we had plenty of clearance. Always check to see if there is any binding or tightness in your suspension while you have the springs and shocks off the car.
Next, we checked out the spring rates that were installed in the car. The fronts were marked as: LF = 1100 lb./in. and the RF = 1200 lb./in. The front springs had a half round of the coil cut off, so the actual rate, although we did not measure them, was obviously higher than they were marked.
The rear springs were: LR = 225 and the RR = 150. The rear had too much spring split, even for a Metric 4-link rear suspension with a high roll center. With the front spring rates being so high, this combination in the rear caused a lot of load to transfer to the RF on entry and through the middle of the turns overloading that tire. That would definitely help cause a push.
We re-used these adjustable...
We re-used these adjustable height spacers, and after re-setting them for our new spring heights, welded them so they became actual spacers. The advantage in using these in this way is that you can adjust the height to where you need them for proper ride height and then tack-weld them to satisfy the rules.
After careful consideration, we installed the following springs: LF = 900, RF = 850, LR = 225, and RR = 175. The front reverse spring split with the softer RF spring helps corner entry and promotes front roll angle, while the reduced spring split in the rear facilitates the high metric moment center while not going too far to help control the rear roll.
Here is a further explanation of this. For a balanced setup, we need for each end of the car to desire to roll to about the same angle in the turns. We have discussed this concept for over four years now in Circle Track. A high moment center reduces the desire to roll, so if we don't soften the RR spring, the rear will be stiff and not allow compliance.
That would overwork the RR tire and cause a loose off, if not totally loose, condition. Running a RR spring that is too soft compared to the LR spring would have the opposite effect. The rear would want to roll over more so than the front and the car would be tight, or tight/loose off. There is an optimum spring split, RR softer, that will keep the car close to a balanced dynamic state and help the car on entry, through the middle and provide more bite off the corners.