High Speed Control

As the shock experiences greater velocities of shaft movement, it goes into what is called high speed control and that involves shaft velocities of from 5 to 10-plus inches of movement per second. Types of suspension movement that create the higher shaft speeds include: 1) running over bumps, ruts or holes in the racing surface, 2) heavy braking on entry and hard on the throttle on exit, or, 3) a sudden change in banking angle such as transitioning from banking onto the apron of the racetrack.

The piston mounted to the end of the shaft also contains a valve mechanism that allows the fluid to flow through slots that are designed into the piston. The valves consist of discs that open as the internal oil pressure increases due to shaft movement in either compression or rebound. The discs are arranged in different sizes and thicknesses to control the dampening rate associated with higher shaft speeds.

Shaft Displacement

An important consideration in designing a racing shock is called shaft displacement. When the shock shaft is pushed into the shock body and into the fluid, it displaces the oil inside the shock. Suppose we pull the shock shaft out as far as it would go, fill the shock body completely with oil and then try to push the shaft back into the shock body. It couldn’t go because the shaft would be trying to compress and displace an equal volume of oil. Because oil will not compress, and it’s in a sealed environment, none of the oil can escape to allow the shaft to exist inside the shock body.

So, we need to create a space inside the shock and fill it with a substance that will compress. Gases such as air and nitrogen will compress. Every shock needs to have a certain volume of gas along with the fluid, in order to allow for the displacement of the fluid as the shaft moves into the shock body and takes up space.

The gas should not be air or any gas that could contain moisture. Due to the heat generated by shock/fluid movement, the moisture would expand and cause high pressure buildup inside the shock. Nitrogen is a dry gas that suits our purpose and is widely used in racing shocks.

Keeping the Oil and Gas Separate

The nitrogen gas we put into our shocks to compress and allow for the volume of the shock shaft must be separated from the fluids. The gas can be contained inside a plastic bag or separated by another piston. Because the bags used in twin-tube designs are not pressurized, these shocks are referred to as non- pressurized shocks.

In truth, as the shock shaft is pushed into the shock body, some amount of pressurization must take place due to the displacement of the shaft and resulting reduced volume of gas inside the shock body. The gas bag must contract which creates a small amount of pressure.

Installation Ratios Effect

If the speed at which the shock moves determines the pounds of resistance, then how and where we install the shock is a consideration. If the shock were to be mounted at the center of the front wheel, or on top of the ball joint, then as the wheel moved, the vertical speed created by that movement would equal the shock shaft speed. This is almost never the case in a stock car because of installation ratios.

In the front suspension, there is always an installation ratio—a shock angle that affects the relationship between the speed of the wheel movement and the speed of the shock shaft. The farther the shock is mounted from the ball joint and the greater the shock angle from 90 degrees off the control arm that it is mounted to, the slower the shaft will move in relation to the wheel movement, and the less control it will have over that movement.

If the speed of the shaft movement determines the amount of shock resistance, then the slower the shock moves, the less rate of resistance it will have. Therefore, if I mount the same rated shock at 2 inches from the ball joint verses mounting it at 6 inches from the ball joint I will have greater shaft speed and more resistance with the first mount with equal wheel speed movement. It makes sense that I would need a stiffer shock when mounting it at 6 inches than I would need if mounting it at 2 inches to have the same wheel speed control.

Linear and Digressive Designs

There are two basic piston designs used in most circle track racing. One is the linear piston (photo 9) that has a high flow rate at low shaft speeds and hence little resistance, and increased resistance as the shaft speed increases. The rate of the shock continues to increase as long as the speed increases.

The other design uses a digressive piston design (photo 10) and has a lower flow rate at low shaft speeds than the linear piston and provides more resistance and control. The resistance rate increases with increased shaft speeds to a pre-designed level and then tapers off.

As the shaft speed continues to increase, the resistance stays mostly uniform above a certain shaft speed. This "pop-off" characteristic works well for reducing the high amounts of resistance usually associated with sharp increases in shaft speeds due to running over bumps and holes in some racetracks. So, the digressive piston design might be better for dirt track applications and rough asphalt tracks.

Rebound Verses Compression Force

Because we install our shocks with springs, and we learned that springs resist compression and promote rebound, we need to use more rebound resistance than compression. The information presented in older automotive design books related to shock design for production automobiles called for equal resistance to suspension motion in both compression and rebound, in combination with the force of the springs.

Because springs naturally resist compression and promote rebound, we necessarily need a shock design with less compression resistance and more rebound resistance to be truly equal in control of each direction of movement. This is a very important principle and has added importance for modern day setups and spring arrangements.

We should never use a true 50/50 rated shock, where the resistance is the same on both rebound and compression, on a race car. Correct thinking would have us install a shock that has more rebound than compression resistance.

When the spring rates are very high, as in the case of a bump stop or bump rubber, the shock must control that higher spring rate. Therefore, high rebound rated shocks are not just incorporated to "tie down" the suspension, but to control suspension movement for the high spring rate of the bump device.

In Shock Part Two, coming next month, we will get a little deeper into shock technology and design. We will look at modern day shock graphs, and see how we might improve our performance by using different rate shocks in relation to various types of setups, racetrack designs and track conditions. Now, please go read the Spring article it is on page 36.