It has been a while since we presented shock tech. In this issue, we will provide a more entry level explanation of the function of shocks. This is a very good read for even veteran racers that will help to explain the foundation of all shock setups. Along with this presentation we will provide the other side of shock tech, the spring tech.
Springs and shocks go hand in hand and need to be evaluated together because they influence each other directly. You can't think out your shock setup without knowing how the springs will affect the shocks, and vice versa. So, once you get through this course on shock basics, please go directly to the spring article and read how your choice of spring rates will directly affect your choice of shocks.
First up, we need to understand just what shocks can do, and what they can't do. Shocks are a tool that we use to support and enhance our spring setup. Shocks are influenced by the spring rates and help provide that all important balance we are trying to achieve that allows all four tires to work to their maximum capacity.
What Shocks Don't Do
1. Shocks do not support the car.
2. Shocks do not control the amount of load transfer or steady state load distribution.
3. Shocks do not affect chassis balance and handling at steady state mid-turn.
4. Shocks are not a cure-all for basic handling problems.
What Shocks Do
1. Shocks control and limit the speed of the motion of the suspension.
2. Shocks allow more or less rapid movement of a suspension corner in reference to opposing corners.
3. Shocks regulate the amount of time it takes for a corner of the car to assume a new ride height when the load it carries changes.
4. Shocks can be used to redistribute the amount of load on the four corners of the car as the car is in transition on corner entry and on corner exit.
5. And, shocks can be misused to "tie" down one or more corners of the car. This can be thought of as over-control.
How a Shock Works
Shocks resist suspension motion, either compression or rebound, by using a piston that must move through a fluid of thin oil. As the suspension moves, so must the fluid must pass through openings, valves, and slots. Varying resistance is created when the oil is forced through different sized openings and at different speeds. The resistance amount is necessarily different for compression (inward motion) and rebound (outward motion).
All racing shocks are of two basic designs, twin tube and mono-tube, and can be either gas pressurized or "low" pressure. The twin tube has literally two tubes, the inside tube is where the piston is located and where the work is done and the outside tube is a reservoir that holds extra fluid and a flexible gas-filled bag.
Shocks are Spring Dampeners
Shocks are installed in race cars, as in any car, to primarily control oscillations caused by the displacement of the springs, especially coil springs. If we place a load on an un-dampened spring, such as when it supports the car, and then push and release on that corner of the car, the spring will compress and rebound in a series of diminishing oscillations over a relatively long period of time. There is no known advantage to this condition and many disadvantages, so we use dampers to control and slow down the movement of the springs.
One of the most important things to understand about shocks and springs is the following:
In our race car, both the shocks and the springs resist compression. As to rebound, the springs promote it while shocks resist it. The control of these two motions, compression and rebound is the primary function of the shocks.
Compression and Rebound
The compression control side of the shock resists the following: 1) the bump movement of a corner of the car when we hit bumps (rises in the track), 2) movement due to the transfer of load to the front end during braking and deceleration, 3) movement due to the transfer of load to the rear upon acceleration, 4) compression movement of the right side springs when the lateral forces are applied as we deviate from a straight line and turn left.
The rebound control side of the shock resists the following: 1) rear chassis movement upon deceleration and braking, 2) front chassis movement on acceleration, and 3) left side movement caused by load transfer from left to right side as we negotiate the turns.
The second most important thing to know about shocks is contained in the following statement:
1 Basic shock design utilizes...
1 Basic shock design utilizes a shock shaft and piston that must travel through a fluid (shock oil) as the shock cycles through compression and rebound. The fluid travels through low speed holes drilled in the piston and through high speed slots molded into the piston and past valve discs attached to the piston face. What is not shown is the gas space whereby as the shock shaft moves into the shock body, the added displacement of the shaft compresses the gas to provide room for the area of the shaft.
2 As the piston moves inside...
2 As the piston moves inside the shock body, the fluid must flow through the piston, first, using the slow speed bleed holes drilled into the piston body. Then as the speed and pressure increases, the discs are forced open and the fluid is free to flow through slots cast into the piston and the flow is increased. The thickness of the disc as well as the number and diameter all help determine the exact amount of resistance to compression and rebound.
3 The mounting of the front...
3 The mounting of the front shock some distance from the ball joint on the lower control arm creates a motion ratio. This means that the shock moves at a speed slower than the wheel. Here we see the shock mounted inboard of the ball joint and the sway bar arm mounted inside of the spring. The sway bar has a different motion ratio than the shock. The angle of the shock also creates a motion ratio that makes the shock motion slower still than the wheel movement.
The amount of resistance that the shock provides with each movement, compression and rebound, increases proportionally to the speed at which the shock is forced to move. Low speeds create low resistance and high speed movement creates higher resistance.
Shocks are rated by their resistance in pounds of force for the speed of movement measured in inches per second. For a shock that is rated at 300 pounds at three inches, it would take 300 pounds of force to move the shock at a speed of 3 inches per second.
So, a shock, any shock, will record a rate of resistance at three inches per second of movement that is less than its rate of resistance when moving at 6 inches per second. This is primarily because with faster movement, more of the fluid must pass through the shock piston and the pressure increases causing more resistance.
We generally think of two ranges of the speed of shock movement when planning out our shock setup. These are low speed and high speed. The race car will usually experience each at different times during a lap around the track. And each is caused by different conditions.
Low Speed Control
Low speed shock movement is defined as shaft speeds that are between 1 to 5 inches of movement per second. Many racers use three inches per second to evaluate the shocks low speed resistance. These lower speeds are mostly associated with suspension movement caused by chassis roll and chassis dive at turn entry where the loss of forward speed is moderate. The low speed control dictates much of the handling side of the shock design and racetrack performance gains related solely to chassis balance and load distribution.
Each shock has a piston mounted on the end of the shaft and one or more open holes in the piston allow fluid inside of the shock to flow freely from one side of the piston to the other. The size of these "bleed" holes regulate how quickly the fluid will flow back and forth and that is one way the different levels of resistance are created for low speed control.
The bleed holes can be either permanently drilled holes or adjustable needle type valves. Most of the low speed adjustments on shocks that are built with the adjustment capability work by changing the size of the bleed opening to control the amount of slow speed flow. This adjustment can be designed to allow for both rebound and compression adjustment, or just one of those.