When a shock is run on a Dyno, it is cycled at different speeds and the resistance is accu
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 10 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 10 inches per second of shaft movement whereas 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, each shock would be numbered as say 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.
Controlling wheel movement would be much easier if all we had to work with was the shocks. But in reality, our racecars 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 One 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.
A shock that moves in direct proportion to the spring moves at the exact same speed as the
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 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 promoting rebound and therefore we increase 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, 18 degrees or more, we need split valving, but we would 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.