If shaft speed determines the shock's resistance, then the slower the shock moves, the less rate of resistance it will have. Therefore, if the same shock is mounted at 2 inches from the ball joint versus mounting it at 6 inches from the ball joint, there will be much more shaft speed and more pounds of resistance. It makes sense a much stiffer shock would be needed when mounting it at 6 inches than would be needed if mounting it at 2 inches. Many racers miss this important point.
Linear and Digressive Designs
There are two basic piston designs used in most circle track racing: linear and digressive. The linear piston has a high flow rate at low shaft speeds and hence little resistance. The resistance increases as the shaft speed increases. The rate of the shock continues to increase as long as the speed increases.
A digressive designed shock...
A digressive designed shock has a considerable amount of low-speedcontrol and builds resistance as the shaft speed increases to apredetermined point where the rate digresses and stays the same as thespeed increases. This type of design has good low-speed control and doesnot build ultra-high resistance with the high speeds associated withbumps and holes as in the case of some rough dirt tracks.
The digressive piston design has a low flow rate at low shaft speeds that provides a lot of resistance and control. The resistance rate increases with increased shaft speeds to a designed level and then tapers off. As the shaft speed continues to increase, the resistance stays uniform above a certain shaft speed. This "pop-off" characteristic works well for reducing the possibility of building excessively high amounts of resistance usually associated with sharp increases in shaft speeds due to running over bumps and holes in some racetracks.
A New Trend Takes Shape
Another development that has become popular involves using more rebound resistance than compression control. If we read the older automotive design books related to shock design for production automobiles, we see where the design criteria calls for equal resistance in both directions, compression, and rebound in combination with the action of the springs. Because springs naturally resist compression and aid in rebound (unloading of weight), we need less compression and more rebound control in our shocks to be truly equal in each direction of movement.
We cannot use a true 50/50 rated shock (where the resistance is the same on both rebound and compression) when installed in combination with a spring. Correct thinking would have us install a shock that has more rebound control than compression resistance. More and more teams are utilizing split valve shocks that are higher in rebound resistance, resulting in faster turn speeds due to a more balanced movement of the front and rear suspension systems.
A linear design shock has...
A linear design shock has a piston that is designed to start out withlow resistance and continue to build as the speed of the shaft movementincreases in a linear fashion. This type of shock has very littlelow-speed control, but very much high-speed control.
Maintenance
Shock maintenance is a must in racing. The shocks do a lot of work throughout each race and certainly through an entire season. You need to be sure to check your shocks for bent components, sticking mono-balls in the ends, leaking fluids, and loss of pressure in the pressurized types of shocks. You should find a shock maintenance facility and have the shocks run through a dyno at least once a year to make sure they hold their intended rate.
If your shocks can be rebuilt, take them apart often to check seals and valve disks. Most major manufacturers can rebuild your shock for a nominal fee. Replace the oil periodically, and always use nitrogen gas to pressurize your shocks and recheck the pressures often. Make sure the gas bag is intact and holds the gas without leaking. Check for contact between the shock body and the coilover spring or any other part of the chassis. This will have a very negative effect on the setup of the car.