When fine tuning a race car with shocks, you need to understand the areas of handling that a shock affects.
Basically, racing shocks perform two functions:
1. When bumps and ruts are encountered, shocks keep the chassis settled and the tires in compliance with the race track. Without shocks, the chassis would pitch, roll, and bounce violently whenever the race car encountered bumps and ruts. The tires could lose contact with the track surface.
2. Shocks help control the rate of chassis roll and pitch caused by dynamic weight transfer. Whenever a race car accelerates, decelerates, or corners, the chassis will pitch or roll (due to weight transfer). Without shocks, the body roll and pitch would be violent and the chassis would not be stable.
Also the stiffness of a shock used on a race car has a profound effect on the rate at which weight transfer affects the loads on the tires. Because of this, shocks are a very important factor when it comes to handling. Basically, soft shocks allow weight transfer to affect loading more quickly than stiff shocks.
When a fast moving race car contacts a large bump, the suspension must react smoothly with as little change in the attitude of the cassis 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 setup. 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 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 and the tire loses compliance with the track surface. The shock can literally hold the tire off the track for a period of time. It will do the same whenever the tire runs through a rut.
In practically all cases, when a car is set up "too stiff" the result is a race car that skates up the race track whenever bumps and ruts are encountered. Many drivers mistakenly describe this ill-handling as a "push" instead of a "skate". 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).
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 and off the track. The wheel hop cycle begins when a bump causes the suspension to move upward so violently that the tire leaves the track. This causes the spring to compress excessively and store a large amount of energy. Because the shock is too soft to control the energy stored by the spring, the tire is slammed back onto the race track. The tire bounces off the track again and the spring stores a smaller amount of energy. The cycle continues until the shock can control the energy level of the spring. The dreaded 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 coarse, is to install stiffer shocks.
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. If one of your springs is stiffer than the other, the stiffer spring receives more weight than the soft spring.
The rate at which a tire is loaded or unloaded during dynamic weight transfer is determined mostly by the low speed control of the associated shock. In rebound, a stiff shock slows down and a soft shock speeds up the unloading process. In compression, a stiff shock slows down and a soft shock speeds up the loading process. However, excessively soft or stiff produce effects just the opposite. By changing the stiffness of the shocks used on a race car, we are adjusting the loading on the tires at different points on the race track.
The balance of traction between the left 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 car more than the left so the car tends to go to the right). By using a stiffer shock (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 to the outside tires slows. As a result, the left side tires remain loaded further into the corner which helps turn the car.
When making this adjustment, consider using the appropriate "split valve" shock 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 holds true by softer extending left side shocks, especially the left rear will cause the left side tires to unload sooner. The balance of traction between the left and right side tires moves toward the right tires quicker and the car becomes tighter in the corner entry.
During acceleration, the balance of traction between the rear tires can also be adjusted with shocks. A softer left rear shock (compression) will quicken the weight transfer to the left rear tire during acceleration. The result is a left rear that has added influence, initially, in accelerating off the corner. A 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.
Without a basic understanding of shocks it would be very difficult to correctly tune a chassis. Keep the following in mind when tuning a chassis:
1. As the piston speed of a shock increases, the shock gets stiffer.
2. Large bumps hit a high speeds cause the highest piston velocities, and the highest shock resistance to occur.
3. The low speed resistance on a shock controls the rate of body roll and pitch also, how quickly a tire is loaded and unloaded during dynamic weight transfer.
4. Generally, soft shocks will cause a tire to become loaded or unloaded more quickly than stiff shocks.
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