However, if the chassis tuner understands how shocks work and how they affect handling, he can use shocks to gain a performance edge over the competition. The following information, gathered through testing on both dirt and asphalt race tracks, as well as on the AFCO dyno and through the use of AFCO's electronic data gathering equipment, should help you better understand racing shocks.

A shock is a valved hydraulic device that resists motion. When its shaft, and the piston assembly attached to the shaft, are moved, fluid inside the shock is forced through a series of small orifices. Some of these orifices are always open (permitting fluid to pass through during any shock movement) while others are covered and permit fluid to pass through only when the fluid reaches a certain pressure. Since there is a volume of fluid on both sides of the piston, the shock is able to resist the movement caused by suspension travel.

The size of the orifices and the pressure levels at which the closed orifices become open determine the stiffness of the shock at various piston speeds. Generally speaking, the greater the force put onto the shock the faster its piston attempts to travel. This increases the shock's resistance to movement and slows down the movement of the suspension.

This staged valving is necessary because the shock resistance required to control the suspension when a tire goes over a severe bump (referred to as the high speed control of the shock) is much greater than the resistance needed to control body sway or suspension movement caused by small bumps (referred to as the low and medium speed control). For the best handling to occur, the resistance of the shocks at low, medium and high piston speeds must be matched to the needs of the race car. On the AFCO Shock Dyno, a shock's resistance is checked at a minimum of three different piston speeds so that a more precise determination can be made of how the shock will affect overall handling. Since it is important to evaluate a shock's resistance and low, medium & high piston speeds, you should know that whenever you stroke a shock by hand you are forcing fluid only through the valving orifices that are uncovered. Therefore, the resistance that you feel is not an indication of how the shock will perform on a race car when the shock moves much quicker.

Basically, shock control at low piston speeds affects how the race car handles through the corners. Shock control at middle and high piston speeds affects how the race car handles whenever it encounters bumps and ruts. The speed of the piston, at which a shock develops a given amount of control, should always be specified. (i.e. 250# of resistance at 17" of shocktravel per second.)

Rebound control is a shock's resistance to extend. The amount of rebound control developed by a shock will affect how quickly the tire is unloaded during dynamic weight transfer and how quickly the suspension "rebounds" or returns to its original position, after the spring has been compressed.(more later)

Compression, or bump control, is a shock's resistance to compressing and is specified at a given piston speed. Compression control will determine generally, how quickly the tire is loaded during dynamic weight transfer and how the suspension will react whenever a bump is initially contacted.

Shocks that have equal rebound and compression controls are referred to as 50/50 shocks since rebound represents 50 percent of the total shock control as does compression. Shocks with unequal rebound and compression controls are referred to as "split valve" shocks. For example, a shock that has 90 percent of its total stiffness in compression control and 10 percent of its total stiffness in rebound control is referred to as a '90/10" shock.

Please note that the ratio number put on a shock does not indicate its stiffness. However, to facilitate the shock selection process, most shock manufacturers use a part numbering system that does indicate the stiffness differences between rebound and compression controls (see the AFCO catalog or shock catalog for details on AFCO shock numbering).

Like shock stiffness, the ratio between rebound control and compression control greatly affects the handling of a race car

We said earlier that the resistances delivered by a shock at medium and high piston speeds affect handling over bumps and ruts. When a fast moving race car contacts a large bump the suspension must react smoothly and with as little change in the attitude of the chassis as possible. This allows the tire to maintain compliance with the track surface. However, if the middle and/or high speed compression control of the shock is too great, or if the rate of the spring is too stiff, the race car will rise and upset the chassis set-up whenever a bump is encountered. If the suspension is extremely stiff, the whole car can actually bounce and allow the tire to lose contact with the track surface. Remember that in "bump" the spring is actually working with the shock to resist suspension deflection. In "rebound" the spring works against the shock by trying to extend the shock and deflect the suspension. Consequently, most shocks, including shocks that are referred to as 50/50 shocks, will have more rebound control than compression control at middle and high speeds.

When middle and high speed rebound controls are too stiff the shock does not allow the spring (or suspension) to return to its original position quickly enough after a bump is encountered. Consequently the tire loses some of its compliance with the track surface. The shock can literally hold the tire off the track surface for a period of time. It will do the same if the tire runs through a rut.

If the race car is shocked too stiffly the race car will tend to skate up the race track whenever bumps and ruts are encountered. Many drivers mistakingly describe this ill-handling as a "push" instead of a "skate." Consequently, the wrong areas of the chassis receive adjustments.

If the so-called "push" only occurs over bumps and ruts, then the problem is a "skate" and softer shocks are usually the fix (assuming the springs are not too stiff).

However, when shocks are too soft and bumps are encountered, a cycle referred to as wheel hop or tire flutter can occur.

During wheel hop, the tire actually bounces on & off the track. The wheel hop cycle begins when a bump causes the suspension to move upward violently. This upward movement of the tire and suspension causes the spring to compress excessively and store a large amount of energy. If the rebound control of the shock is too soft to control the energy stored by the spring, the tire is violently slammed onto the surface of the race track. The tire bounces off the track and the spring stores a slightly smaller (but still uncontrollable) amount of energy. The cycle continues until the shock can control the energy level of the spring. Wheel hop can be caused by any major deformity in the racing surface or by violent rear axle wrap during acceleration or deceleration.

Wheel hop can easily be felt by the driver and, if extreme, can be seen by those watching the race car. During wheel hop, the tire bounces up and down uncontrollably and causes the handling to be very unstable. The fix, of course, is to install stiffer shocks. Keep in mind that wheel hop to any degree, whether felt by the driver or not, reduces traction.

When discussing chassis tuning in depth, a basic understanding of dynamic weight transfer and its effect on tire loadings is necessary.

Dynamic weight transfer is the transferring of weight from side to side during cornering, from rear to front during deceleration and from front to rear during acceleration. The distribution of weight that transfers is affected by the rates of the springs used in the chassis. Basically, if one of a pair of springs receiving weight is stiffer than the other, the stiff spring receives proportionately more weight than the soft spring.

The rate at which a tire is loaded or unloaded during dynamic weight transfer is affected by the low piston speed control of the associated shock. In rebound, a stiff shock slows down and a soft shock speeds up the unloading process (unless rebound control is extremely stiff). In compression, a stiff shock slows down and a soft shock speeds up the loading process(unless compression control is extremely stiff). However, excessively soft or stiff shocks can produce effects opposite to those started. Consequently, by changing the stiffness of the shocks used on a race car, we are adjusting the loadings on the tires at different points on the race track. If done correctly, good handling will result.

The traction capability of a tire determines that tires influence on the race car. Traction capability is greatly affected by the load put onto the tire.

The balance of traction between the left side and right side tires determines to a great extent how the car will handle while decelerating through the corner. For example, a race car will tend to push (not turn) whenever the left side tires do not have enough influence in stopping the car (the right side tires are slowing the vehicle more than the left so the vehicle tends to go to the right). By using stiffer shocks (especially a stiffer extension control on the left rear, and to a lesser degree, a stiffer extension control on the left front), the unloading process of the inside tires (due to dynamic weight transfer) to the outside tires slows. Consequently, the left side tires remain loaded further into the corner which helps to turn the chassis.

When making this adjustment, consider using the appropriate AFCO split valve shocks so as to not increase the compression control of the left side shocks. This change should allow the chassis to roll back onto the left side tires more easily during corner exit.

Also, the opposite of the above example holds true. Softening the extension of the left side shocks, especially the left rear will cause the left side tires to unload sooner during cornering. The balance of traction between the left and right side tires moves toward the right tires more quickly and the chassis becomes tighter on corner entry.

During acceleration, the balance of traction between the rear tires can be adjusted with shocks also. A softer left rear shock (especially compression) will quicken the weight transfer effect to the left rear tire during acceleration. The result is a left rear tire that has added influence initially in accelerating the race car off the corner. A race car will tend to be tight off the corner whenever the balance of traction between the rear tires favors the left.

Forward traction can be enhanced by softening the extension control of the front shocks. This enhances the front to rear weight transfer process and helps to load the rear tires for improved forward traction. The appropriate split valve shocks can be found in the AFCO Racing Shocks Catalog. Keep in mind that a softer left front shock (rebound) may tighten corner entry handling also!

Remember, shocks are a compromise like any other suspension component. Be careful when using split valve shocks with soft rebound controls so that handling over bumps and ruts does not suffer. Generally, side bite (cornering ability) can be improved by softening the shocks (and/or springs). This adjustment can stop the race car from skating up the corners on slick, smooth tracks.

We hope you have figured out that the heading to this information is a bit misleading. There really is no mystery to shock function and tuning. However, there are complexities and qualities that need to be considered when choosing shocks for a specific application. By keeping this basic information in mind when troubleshooting handling problems, you should be able to install the correct shocks for each situation. This should also enable you to have the confidence to make shock changes with fairly good expectations for the results.

Above all, remember that chassis tuning is a compromise and shocks, though a very important part of the set-up, are still only a part.