Shock Selection Guide – Shock Crutches

Shock Selection Can Enhance Or Ruin A Good Setup, So Never Try To Solve Basic Setup Problems With Odd Selections Of Shocks

The basic setup on our race cars can be further improved by the proper selection of shocks at the four corners. Once all of the basic handling parameters have been sorted out, the car is getting through the middle well, and the tire temperatures look good, we can then-and only then-experiment with different rates of shocks to try to further enhance the entry and exit performance.

Too many teams try to solve basic handling problems with different shock rates or even different brands of shocks. We have learned over the past decade that using shocks to overcome mid-turn handling problems can be frustrating and futile.

The mid-turn handling problems can affect entry and exit performance, which is exactly why we always solve the mid-turn problems first. Anyone who tries to tell us different shocks can improve mid-turn handling is not being honest. The rare exception is when we are trying to overcome rough track conditions, mainly related to dirt tracks, where a different rate and build of shock will allow the tire to maintain more constant contact with the racing surface all the way around the track.

By looking at the angle of the left-front wheel as the car negotiates the turns, we can tell it is very tight. Shock changes will not solve this problem, only proper changes to the base setup and/or geometry of the car.

We will analyze each corner of the car and see where we might have been crutching the car and where we can use shocks to our advantage. Sometimes we can use one corner to effect a change, and there are times when a combination of corners has a better effect.

The left-front (LF) corner of the car is similar to the left-rear (LR) corner in that it does not move vertically much as we turn laps on asphalt. On dirt, we see more movement due to the setups used to gain bite on the drier and slicker tracks. The most movement either way comes when we are braking into the corner and when we accelerate off the corner. These two movements, compression on braking and rebound on acceleration, can be utilized to our benefit.

What usually happens is that we might try to solve a “loose off” condition by changing the rebound rate of the LF shock or a “tight in” situation by changing the compression of the LF shock. Entry problems can usually be traced to poor alignment of the rear end or front steering problems. Making drastic changes to solve entry problems with shocks is not the first consideration. We need to look at our alignment first.

Using the look-and-see method, so to speak, we know this car is very loose. The direction the car is going compared to the pavement lines in the track surface tells us this guy needs help with the basic setup. Dirt trackin' on asphalt will not get the job done over the long haul.

For corner exit, increasing the rebound resistance at the LF to “tie down” that corner will decrease the upward movement of the LF suspension, but weight is still being lifted off that tire. If there is an aero push due to excess movement of the LF suspension, then increased LF rebound may be a positive change for the aero effect. It might also tighten the car off the corner, which could cause more of a push.

On initial acceleration off the corner, a stiff rebound shock at the LF will momentarily put more weight on the right front (RF) and LR corners, causing an increase in the crossweight percentage, which tightens the car. On the other hand, if we decrease the rebound at the LF, we will de-wedge the car as well as create a possible aero push condition.

We can see where changes to the LF shock in most cases causes opposite effects in different areas. The best scenario is to take the middle road at the LF and not go too far in the direction of either high or low rebound and compression rates.

When we have corrected all of our setup problems, we can enhance the entry characteristics of the race car by utilizing the RF shock. We often try to control excess movement of the RF corner with a high-compression shock, especially over the past few years, where softer spring setups are more common.

Excess movement of the RF corner on entry usually causes the car to push due to excess and sudden camber change at the RF wheel. There’s a problem with trying to solve a push like this with a stiffer compression shock. Once we do that, we have further tightened the car by momentarily adding wedge to it. Upon braking, a high-compression RF shock will cause a greater amount of the unsprung weight to be carried by the RF and LR tires, which increases the crossweight in the car, still making push.

Excess movement at the RF can be controlled more effectively by adding an antidive design to the RF and the LF suspensions. Antidive works well to control excess dive on entry while braking and does not affect the setup at any other point on the racetrack.

We can introduce antidive to both front corners of the car so that resistance to dive is distributed onto both front tires. This eliminates the change in crossweight percentage that might make the car tight, as when using a stiff-compression RF shock.

The right-rear (RR) corner experiences the most movement on the race car. Shocks control the speed of this movement so that we can utilize it to effect changes to the weight distribution of the car while the shocks are in motion. Since the RR moves so much, we can do more with it than at any other corner.

Shock angle affects the speed at which the shock moves. If the wheel is moving at 6 inches per second, the shock, if mounted at 20 degrees, will move only 5.47 inches per second.

Rarely do we see teams try to crutch the car using different shocks on the RR corner. Suggestions to change the shock on the RR to solve tight or loose middle handling conditions should be disregarded.

Mid-turn is mostly a steady condition, which means the four corners of the car are holding steady and not moving quickly or a significant distance. If a shock is not moving, it is not affecting the setup or weight distribution on the four tires. Shocks must move to affect the handling of the race car.

On entry, we might want to increase the rebound rate on the RR (in conjunction with the LR shock) to help keep the car’s center of gravity lower so less weight transfer will occur from the rear to the front of the car. By causing less weight transfer, we maintain more weight on the rear tires and a more even weight distribution on all four tires in combination, which gives us more traction for enhanced braking. The faster we stop, the deeper we can go into the corner.

On exit, we can enhance bite to help control wheel spin as power is applied to the rear wheels. Using a softer compression shock at the RR will momentarily increase the load on the RF and LR tires while the shock is compressing and weight transfers to the rear on acceleration. The increased crossweight percentage tightens the car at the critical moment of throttle application when we need to stop the tendency of the rear wheels to break loose. An example of a crutch would be increasing the RR compression to loosen a car that is tight off the corner. Most likely the car is tight off because of spring related setup problems or steering problems, not because of the RR shock.

The installed motion ratio as well as shock angle affects the shaft speed of the shock compared to vertical wheel speed. This directly affects the rate of the shock because shock resistance is determined by the speed that the shock shaft is moving and how quickly the fluid in the shock is forced to move through the valving and bleed holes. The shock will always move slower than the wheel in vertical displacement.

Example: Using a RR spring that is too soft will cause excess travel at the RR corner. We might be tempted to increase the compression at that corner to help control the movement, when the real problem is the soft spring. That corner will still move excessively with the soft RR spring, but it will take longer to get there by increasing the compression rate.

The left-rear (LR) shock may be the most abused shock on the car as far as crutching is concerned. By far, the most common shock crutch is increasing the rebound in the LR shock to attempt to eliminate a “tight in” condition.

The car is probably tight in because of front end geometry problems, causing the front of the car to have no grip. When we increase the rebound of the LR shock, we promote weight transfer off that corner, which de-wedges the car. We now have two ends of the car that have less grip. We have effectively balanced the car at the expense of performance.

Neutral handling is not necessarily related to performance. Overall grip on the four tires is what creates performance by allowing more speed into and through the corners. What we really need is a properly designed front end that will turn in and through the middle so we don’t have to crutch the LR shock rates.

Once we have that, we can use the LR shock to enhance turn-exit performance. By using split valve shocks, we can increase the compression of the LR shock and, at the same time, maintain a more moderate rebound rate to enhance turn-exit performance without messing up the entry performance.

Remember we softened the RR shock compression to effect the same improvement. When we increase the LR shock compression and decrease the RR shock compression, we have used two corners of the car to improve exit performance.

On dirt or asphalt, we can look at the compression and rebound rates on the left side of the car to effect the overall attitude of the car.

Lower overall compression for asphalt or dirt cars on high-banked tracks will allow the left side of the car to compress more quickly and enhance aero downforce. Once the LF corner is down and the aero effect is enhanced, more downforce will keep the left side down after the effect of the movement of the shocks is over. Very stiff overall left-side compression is never desired.

Stiff overall rebound on the left side of the car helps to maintain the aero effect when coming off the corners by reducing the tendency for the left side to lift as we accelerate. This helps to continue the low left-side attitude that creates high aero downforce.

All of the above is related to asphalt tracks or even dirt tracks with high banking and a higher degree of traction. We need to think a lot differently when dealing with shocks for dry, slick dirt tracks. The trend has been to allow the car to jack up on the left side. A very low setting for rebound on the left-side shocks allows a sudden and rapid movement of the LF and LR suspensions so the left side of the car is up through the middle.

This attitude promotes weight transfer from the left side to the right side, the theory being that more weight on two tires will bite into the dry slick better than distributing the race car weight among the four tires. The high-left-side attitude does more than promote weight transfer; it allows for more rear steer by causing the LR wheel to swing up and under the forward mounts of the trailing links, and that action steers the rear end to the right.

For dirt racing, we need to also complement the softer left-side rebound rates in our shocks with a lower compression rate for the right-side shocks on flat and dry-slick types of race tracks. This allows the right side to drop quickly on turn entry as the left side rises up.

The soft and slow motion of the shocks at both sides of the car help to reduce the “shock” to the tires during transitional periods of entry and exit. This helps the tire contract patch maintain grip with the racing surface.

As we are always saying, solve your setup problems first before experimenting with shocks. Never crutch a bad setup. Most shock technicians will tell you to put standard rate shocks on the four corners until the setup is sorted out. Make changes to one corner at a time to see and feel the results. The most important task is to make the driver more comfortable, and that will translate to faster lap times.