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.
1 Shock companies all have specialized equipment so that they can custom build any shock
2 It has become necessary to use adjustable shocks like the one shown. Not only do the co
3 The adjustable shocks are easily tuned and changed as to either or both compression and
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.