As setups continue to evolve, with dirt and asphalt racing over the past 5 to 10 years, dirt setups have become more like asphalt setups, while asphalt setups have struggled. In today’s racing world, we have available pre-built shocks with any number of combinations of disk design, bleed, and valving. Some types of designs give us the ability to adjust shock rates quickly at the racetrack. So, it would seem that we have what we need to choose the exact rate of compression and rebound that is necessary for a particular set of conditions if we know a little about the affect of those rates on the setup. This wide range of choices can either be an advantage, or the cause of getting dialed out. The more we can learn, the better we can make decisions regarding shock selection. The information we present here is intended to be a guide to help you understand the basic principles of shock technology and the art of track tuning with shocks. The exact rates for the shocks that you need for your car are necessarily dependent on how your car is constructed, set up, and driven, as well as factors such as load distribution and racetrack characteristics.

We would really like to give you the exact shock values that will make your car as fast as it can be, but that would be impossible due to many variables. That is exactly why you must work with your particular car and try not to follow what others are doing. Each car is a little different than the others and each driver has his/her own style of driving.

An important aspect of shock technology is that there is no computer program or chart that will tell you exactly which shock rates to run on your car. This is, and always has been, a trial and error process. Where we have tell-tale signs and computer software that point us in the right direction for the chassis setup, with shocks we don’t. It’s all driver feel and stop watch feedback that tells us when we are making progress or going backwards. And that’s not to say that a good shock technician can get you started on the right path quickly.

What We Learned

Part one of our shock saga dealt with the basic construction of the racing shock. We learned that the two strokes of the shock, rebound and compression, are looked at separately and perform functions related to different areas of track tuning. If we deal with rebound and compression separately, then we need to be able to tune each independently. There are also different designs of shock pistons including the linear design and the digressive design. Again, we are able to achieve varying results by utilizing all of the variables of shock design.

For most situations, we would use split valve shocks. Split valving means that we have different rates of resistance for rebound and compression because each movement has a very different job to do than the other. We can also rate the two movements differently for each corner of the car to further tune the setup.

If we want, we can buy (at a greater expense) shocks that have external adjustments for rebound or compression, or both (called double adjustable). In addition to that, we also see that four-way adjustable shocks are available to tune both low and high speed characteristics of rebound and compression.

In this way, we can experiment with different shock rates without removing them from the car. Regardless of how we arrive at the different shock rates, we do need to know beforehand what we are looking for and how to get there.

How Shocks Are Rated

Shock companies provide a system of numbers or letters to reference the rates of rebound and compression. Most of these companies try to provide a cross reference so that their numbering system can be compared to the other systems that are used by competing shock brands. The ultimate match between shock brands is not exact because of differences in design of the valving and the fact that each company may rate its shocks at a different shaft speed.

If for example a "3" shock or an "A" shock were rated by each manufacturer at 100 pounds of resistance, then comparing them would depend on what speed of movement each company rated that 100 pounds. We know that the rate of resistance is directly related to shaft speed. Company X might rate the 100 pounds at 5 inches per second of shaft speed where company Y might rate the 100 pounds at only 3 inches of shaft speed. We can see where the two would not feel the same to the driver. The X shock might well be 150 pounds of resistance at 3 inches per second of shaft movement where as the Y shock might be only 75 pounds of resistance at 5 inches per second of shaft movement.

To simplify things, we will use the numbering system to relate to the amount of rebound and compression, the smaller number representing less resistance. Because we are not telling the exact rate in pounds of resistance for each number, the comparisons and trends will be good for either a dirt or asphalt stock car. The difference in the two types of racing as far as shock selection is concerned is that generally speaking, the dirt cars require a softer overall package than the asphalt cars.

A basic starting shock setup for a medium banked racetrack might be a pair of 6 shocks on the front and a pair of 5 shocks on the rear. These should not be true 50/50 shocks where the resistance in either direction, rebound or compression, would be equal. Rather, we need more rebound control than compression control.

If the shocks were the split valve design, a shock would be numbered as a 6/5, which for our purpose means that the first number will represent the rebound resistance and the second number will represent the compression resistance. Since the springs help resist compression, that number should be smaller than the rebound number.

Shocks Work With Springs

Controlling wheel movement would be much easier if all we had to work with was the shocks. But in reality, our race cars are supported by a set of springs. If we wanted true equal resistance to wheel movement with the shocks installed along with the springs, we would want the rebound resistance to be greater than the compression resistance (reference Part 1 where we said that springs promote rebound and help provide resistance to compression). As we install stiffer springs, we would naturally increase the rebound resistance and decrease the compression resistance.

The more balanced shock layout might well look like this: Front shocks = 6/5, rear shocks = 5/4. The greater rebound helps control the force of the preload on the spring as it is released and the softer compression works along with the resistance to compression provided by the spring.

Ideally, we would rate our shocks on a fixture that would simulate the installed spring and include the installation ratio. Some professional teams now have those fixtures. The rating is measured at the ball joint and that takes into account the spring rate, installation ratio, spring angle, etc. When measured this way, it is an accepted theory that the rebound and compression numbers must be the same--i.e., it should take the same force to move the wheel vertically in both directions.

High-Banked Setups

For a setup that uses higher rated springs in the front and rear for a higher-banked racetrack, the compression and rebound numbers would change to reflect the spring change. We might use a pair of 6.5/4.5 shocks on the front and a pair of 5.5/3.5 shocks on the rear. That is because of what we said before about the higher rating in the springs promotes rebound and therefore we increased rebound resistance. The higher spring rates also resist compression and so we can reduce compression resistance in the shocks.

When the banking of the racetrack is very high, say 18 degrees or more, we still need split valving, but we might go up on both the rebound and compression rather than up on rebound and down on compression. The reason is that the turn speeds are much higher on the very high banked tracks and the shocks will move much faster in both the transitional areas of entry and exit, and also as the car goes over bumps and dips in the racetrack. Without the higher level of control, the suspension might well bottom out with obvious negative results.

A more realistic layout for a very high banked track might be a pair of 6.5/5.5 shocks on the front and a pair of 5.5/4.5 shocks on the rear. We have increased the resistance for both the rebound and compression as well as incorporating split valving to a degree.

Low-Banked Racetracks

For low-banked racetracks, we would necessarily spring the car much softer to increase the shock travels and slow the change to a new chassis attitude in order to help maintain traction. One of the basic ingredients needed to maintain grip at low banked tracks that have less grip is to soften the whole setup including spring rates as well as shock rates. As we soften the springs, we would also soften the front shocks to say 5/4 valving and the rear shocks to a pair of 4/3 shocks. This is the general way shocks can be matched to the stiffness of the spring setup.

One exception is when we go beyond the normal softening of the front springs into what has become known as the soft spring setups that have become popular in short track asphalt racing. These setups often use super soft springs in the front and, to help control the car on entry, employ bump devices on the shock shaft to overcome the tendency for the car to bottom out as we brake and turn left on entry into the corners.

Here is where we might go with a shock that has very little compression resistance to allow the front end to dive and a much greater rebound resistance, not to tie-down the corners of the car, but to control the high spring rate of the bump rubber.

Many teams want to promote left front travel with pro-dive, soft compression numbers, and a high rebound rate at the left front corner. They are not just trying to tie down that corner to reduce the height of the left front valance, they are also using the high rebound shock to control that corner.

This lower attitude reduces the amount of air that travels under the car and helps promote downforce. Most of the gains from this arrangement are associated with reduced drag, more aero downforce and a lower center of gravity which reduces load transfer.

In Part 1 of this story, we talked about how the shock regulates the speed at which each corner of the car moves as load is transferred on to and off of the four corners. If we split the rates between the four corners, we can effectively change the load distribution on the four tires while the suspension is in motion.

So, to utilize the resistance of the shocks to redistribute load, we must have load transfer taking place and that, in-turn, causes the needed suspension movement. If the suspension moves, then so do the shocks.

Rate Split Changes Loading

Compression rate differences between a pair of shocks helps to redistribute load between the four corners of the car where load is being transferred onto a particular end of the car. At the front of the car, compression rate differences can change the load distribution on entry to the corner while load is being transferred to the front due to deceleration. At the rear, compression rate differences can help to redistribute load that has transferred on to that end from load transfer that is caused by acceleration off the corners.

Rebound rate differences help to redistribute load between the four corners of the car when the suspension is in motion as load is being transferred off of a particular end of the car. In the front of the car, rebound rate differences can change the load distribution on exit while the suspension is in motion when load is transferred off of the front and onto the rear. At the rear, rebound rate differences between the LR and RR shocks can help to redistribute load on the four corners of the car again while the suspension is moving and adjusting to the load being transferred off of the rear on to the front due to deceleration of the race car.

We need to think out the motions of the car related to how it is driven and how the track is configured, as well as the desires we have for improvement. Tuning for corner entry can affect corner exit. Do not tune with shocks for problems that can be fixed with basic setup parameters.

A car that is tight on entry should not be fixed with the installation of a high rebound LR shock that will reduce the loading on the LR tire on entry. A car that is tight on entry most likely has geometry or setup problems. Or, the rear may be out of square, the front steering geometry is not designed correctly, or the setup is just plain tight.

Remember that the more you have to trick up your shocks and shock selection, corner to corner, the harder it is going to be to be consistent. We are getting many reports of teams going to more research and matching their shock rates to the springs and bump devices they are using and winning races due to consistency.

Conclusion

Again, think out how shocks work with the springs, how bump rubbers or other devices that limit spring and suspension travel increase the spring rate the suspension feels and what direction we need to go to control our total spring rates.

I used to think that shock companies were over-shocking the front shocks with very high rebound rates, but I came to understand that they are only controlling the very high spring rates exhibited by the bump devices. In Part 3 of our shock series, we will learn how to fine tune our shocks for different setups and track configurations.

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