Shocks come in an assortment...
Shocks come in an assortment of designs for different types of race carsand applications. The right shock brand and design for your type ofracing depends on both your budget and your ability to understand how toeffectively use your shocks to improve entry and exit performance.
Many racers say one of the most mysterious parts on the car is the shock, or motion damper. It is often heard that there is potential for performance gains by experimenting with different shocks.
Going fast is not simply a case of bolting on different shocks. While we do agree that performance gains are possible and performance loss often comes with running the wrong shock rates, the first order of business is to work out any basic chassis setup problems you might have before experimenting with shocks. With that in mind, let's look at what shocks do and don't do.
1. Shocks do not support the car.
2. Shocks do not control weight transfer.
3. Shocks do not affect chassis balance at mid-turn.
4. Shocks are not a cure-all for basic handling problems.
1. Shocks control (limit speed of) motion of the chassis and suspension.
2. Shocks, with varying designs of resistance, allow more or less rapid movement of a suspension corner than opposing corners.
3. Shocks regulate the amount of time it takes for a corner of the car, while in transition, to assume a new ride height.
4. Shocks can be used to redistribute the amount of weight on the four corners of the car as the car is in transition on corner entry and exit.
Shock travel indicators give...
Shock travel indicators give us an idea of how much travel incompression and rebound our shocks experience while on the racetrack.
Shocks resist motion by using a piston and valves that are mounted on the end of a shaft and that move through a fluid of thin oil. The fluid must pass through holes, valves, and slots in the piston as the shaft is stroked in and out. Resistance is created when the oil is forced through the openings on each cycle. Basically, all racing shocks are of two designs: twin tube and mono-tube and can be either gas pressurized or "low" pressure. The twin tube literally has two tubes: The inside tube is where the work is done and the outside tube is a reservoir that holds extra fluids.
Shocks are Spring Dampers
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 weight on an undamped spring, such as when they support the car, and then push and release on one corner of the car, the spring will compress and decompress 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 the movement of the springs.
In our race car, the shocks and the springs resist compression or bump at any one corner of the car. When that same corner tries to return to its normal height, the spring promotes that motion and the shock resists. The control of these two motions, compression (bump) and rebound (droop), is the primary function of the shocks.
The compression control side of a shock resists: 1. the bump movement of a corner of the car when a driver hits bumps (rises in the track); 2. the movement due to the transfer of weight to the front end during braking/deceleration; 3. the movement due to the transfer of weight to the rear upon acceleration; and 4. the tendency of the body to roll when the lateral forces are applied as we deviate from a straight line and turn left.
The rebound control side of the shock dynamics resists the following: 1. rear chassis rebound vertical movement caused by deceleration on entry; 2. front chassis rebound movement on acceleration; and 3. left-side rebound vertical movement caused by weight transfer and lateral loading as we negotiate the turns.
Basic shock design utilizes...
Basic shock design utilizes a shock shaft and piston that must travelthrough a fluid (thin oil) as the shock cycles through compression andrebound. The fluid travels through low-speed holes in the piston andthrough high-speed slots molded into the piston and past valve discsattached to the piston face.
The amount of resistance that each movement, compression and rebound, generates does increase with the speed at which the shock is forced to move. Low speeds create low resistance and high-speed movement creates higher resistance. So we have two areas related to resistance: low speed and high speed.
Low-speed shock movement is defined as shaft speeds that are between 1 to 10 inches of movement per second. These lower speeds are mostly associated with suspension movement caused by chassis roll and possibly chassis dive at turn entry where the sudden loss of 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 weight redistribution.
A mono-tube, gas pressure...
A mono-tube, gas pressure shock has a separator piston that provides aseal between the pressurized nitrogen gas and the shock fluid. As theshock shaft is pushed into the shock body and fluid, it displaces someof the fluid under the separator piston. The gas will compress as theseparator piston moves up to make room for the area of the shaft.
Each shock has a piston mounted on the end of the shaft, and one or more small holes in the piston allow fluid inside of the shock to flow from one side of the piston to the other. The size of the "bleed" holes regulate how quickly the fluid will flow back and forth, and that is how the different levels of resistance are created for low-speed control. All low-speed adjustments on shocks built with that adjustment capability work by changing the size of the bleed opening to control the amount of flow.
As we experience the greater velocities of shaft movement, we go into what is called high-speed control with shaft velocities from 10 to 25 inches of movement per second. The types of suspension movement that cause the higher shaft speeds in our shocks are: 1. bumps or holes in the racing surface (creating very high shaft speeds); 2. the driver stabbing the brakes 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 valving mechanism that allows the fluid to flow through slots designed into the piston. These valves consist of disks that open as the pressure increases due to more rapid shaft movement in either compression or rebound. These disks are used to control the damping rate associated with higher shaft speeds.
An important consideration when designing a racing shock is called shaft displacement. When the shock shaft is pushed into the shock body and the fluid, it takes up space. Suppose we pull the shock shaft out as far as it will go, fill the shock body with oil, and then reseal the shock body. If we tried to push the shaft into the shock body and the volume of oil, it would not go. The shaft would be trying to displace some of the oil and oil cannot compress, so none of the oil can escape.
We need to create a space inside the shock and fill it with a substance that will compress. Gases will compress, so 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 cannot be air or any gas that contains moisture (water) due to the heat generated by shock/fluid movement. The moisture will heat up and expand and cause high-pressure buildup inside the shock. Nitrogen is a dry gas that suits our purpose and is widely used as a gas filler in racing shocks.
As the piston moves inside...
As the piston moves inside the shock body, the fluid flows through thepiston first using the slow-speed bleed holes drilled into the pistonbody. As the speed and pressure increases, the discs are forced open andthe fluid is free to flow through slots cast into the piston and theflow is increased. The thickness of the disc, as well as the number anddiameter, all help determine the exact amount of resistance tocompression and rebound.
Keeping the Oil and Gas Separate
The nitrogen gas we put into our shocks to allow for the volume of the shock shaft must be separated from the fluids in twin-tube shocks. The gas can be contained inside a plastic bag, and the plastic provides the seal between the gas and the fluids. Because the bags 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 smaller volume inside the body. The gas bag must contract, which creates a small amount of pressure.
In mono-tube shocks, separation is accomplished by installing a second separator piston, which provides a seal that separates the fluid from the gases. A valve that is installed in the shock body at the gas chamber end of the shock allows us to pressurize the gas inside the shock. This pressure ensures that the gas will be forced to remain separate from the fluids at all times. The seal on the piston will usually allow air to seep past the piston from the fluid side to the gas chamber, but seals the heavier fluid from escaping into the gas chamber.
Installation Ratios Effect
If the speed at which the shock moves determines the number of pounds of resistance, then how and where we install the shock is also a consideration. If the shock were to be mounted at the center of the front wheel or on top of the ball joint, then the vertical wheel speed would equal the shock shaft speed. This is never the case in a stock car because of installation issues. In the front suspension, there is always an installation ratio and shock angle that affects the relationship between the vertical wheel movement and the shock shaft velocity. The farther the shock is mounted from the ball joint and the greater the shock angle from 90 degrees off the control arm it is mounted to, the slower the shaft will move in relation to the wheel movement.
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.
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.
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.