Racing Shocks - Shock Value

Second of two parts

Previously, we established the role of the shocks as they relate to the control of the suspension. While this is not a magical activity on your daily driver, the job is more detailed and involved on your race car. Shocks are adjusted to develop more of what your race car really needs. Need more grip? Adjust the shocks. If the car is pushing off the corner, into the corner, or in the middle of the corner, adjusting the shocks is one of the things you add to your tuning arsenal to fix the problem.

It is common knowledge that adjusting your shocks is a way to improve handling. The trick is understanding and knowing when to adjust, what to adjust, and how much to adjust. Do you adjust the rebound? Do you adjust the compression? Is it a good idea to change the valving in the shocks? In order to answer these questions and more, we spent time with Mark Bush, one of the resident shock experts at AFCO Products.

SCR: When does a racer know that the shocks need to be adjusted?
BUSH: Good question. When the race car is bouncing because the suspension is too stiff and not moving and the only real movement is coming from the flex in the sidewall of the tire, it is time to adjust the shocks. Often, it is just the sidewall of the tire flexing and the wheels not moving at all. When the tire is the only flex in the suspension, you use loose traction. At the same time, the contact patch of the tire is going through variations and the end result is loss of traction. You do not want the tire to bounce; you want the suspension to move. It is very clear that if the tire is bouncing, traction is reduced. On the other hand, if the suspension is too soft, it is also a condition where the car will lose traction.

Other things a racer can look for are the chassis staying flat in the corners, an absence of roll, and a lack of side bite. This is time to soften the suspension. This is demonstrated by four-wheel drifts and a loose condition getting into the corners. We need to soften the suspension, especially the shocks, the left-side rebound, and the right compression just to reduce that resistance to roll. This will typically help with the side bite issues. We need to be aware that with too much roll in the chassis, bad things can happen: excessive roll steer, geometry changes in the suspension. This may require stiffer springs or shocks, especially the left side, and rebound would need to be increased and the right side compression would need to be increased. What is too much roll? That is a question that can only be answered by experience.

SCR: If we are experiencing a lack of forward bite, what should we be doing?
BUSH: You can reduce rebound in the front, but you need to be careful, especially if you are dealing with a car that does not hike up a lot or you are not getting a lot of left-rear rebound travel in the corners. But you have to exercise some care. If we are dealing with cars that hike up in the rear in the corners, like Dirt Late Model cars that depend on a lot of hike on the left rear, reducing rebound in the right front to help forward bite is not very effective. In the left front it will help.

SCR: When you watch a Modified car go through the corner, the left-rear tire looks like it is moving forward in the chassis. The wheelbase looks like it is getting shorter on the left side. They have a bunch of rear-steer going on. Then the left front hikes up in the air and the car looks like it is driving weight onto the right rear.
BUSH: Yes, that is very typical. But what is happening is that there is a great deal of weight being driven onto both rear tires. The setups being used to get the left rear to hike set the trailing arms in an uphill, to-the-front angle so that the thrust angle of the axle tries to drive the left rear under the race car. When you run the links in an uphill, to-the-front angle, you are going to have loose roll steer. The left rear is going to come ahead when it goes into rebound. When you get the combination right, you get the car hiking, you get the timing right, and you can go very fast. And the reason for this is, with the car hiked up, the center of gravity is up and you get a great deal of weight transfer to the rear tires that can really help you off the corners; it really helps the forward bite. It also gets a lot of left-rear bite because the left-side trailing arms are running at such an uphill angle that the trailing arms are actually holding up the race car, not the springs. We have video that shows the springs on coilover cars are actually unloaded and there are gaps between the spring and the spring seat.

SCR: Let's say we our racing on a quarter-mile dirt track and the car is pushing. What kind of adjustment should we make to the shock?
BUSH: What you would adjust would depend on where in the corner the car is pushing. If we look at corner entry, we are off the gas or on the brake. If you are pushing, the first thing to do is stiffen the right-rear compression on the shock. It will help keep weight off the right rear. It will help keep wedge in the car, and it will keep left-rear weight in the car, which will help the car pivot when you are slowing down. Another thing you can do is stiffen the left-rear rebound. Basically, this gives you the same effect. It holds left-rear weight in the car, makes the left rear a little dominant in stopping the car, and lets the car pivot. You have to be careful with left-rear rebound when you are making that change to affect corner-entry handling. If we are depending on loose roll steer to help turn the car and we are putting rebound in the left rear to stop the left-rear suspension from moving, then we are going to lose the benefit of loose roll steer. Also, if you are on a fast momentum type of track, stiffen the rebound on both front shocks. If you are racing on a paper clip type of track with long straights where you are braking in a straight line, you can soften the right-front compression to help pivot the car.

If the push is off the corner and we have a throttle push, stiffen the right-rear rebound. Also, stiffen the front rebound, especially on momentum-style tracks. If we are dealing with a Street Stock type of car, leaf spring or standard GM four-link suspension, and you are loose off the corner, I would reduce the rebound on the left front. Right front and right rear, just soften up the rebound.