The primary function of a shock absorber is to dampen the oscillation of the spring after the wheel travels over bumps and dips.
If you've read the springs article, you know that the spring's job is to allow the tire to maintain constant contact with the road surface. It is the spring that actually absorbs the shocks of surface roughness.
So, contrary to their popular name in the U.S., the purpose of the "shock absorber" is not to absorb the shock of bumps on the road. That is the job of the springs. The purpose of the shock is actually to dampen the oscillation of the springs. Reflecting this function, those outside the U.S. refer to shocks as dampeners.
We mentioned that a spring retains energy to allow it to return to its original shape after being compressed or stretched. Unfortunately, a spring will not just return to its original shape and stay there. You've probably witnessed yourself that if you compress or bend a spring it will oscillate back and forth in smaller and smaller increments until finally coming to rest. If you have ever seen an old car bounce endlessly after going over a bump, you have seen what springs will do in a car with ineffective shocks. This is not good for safe control of the vehicle, and it's certainly not any good for effective handling while racing.
The shock's primary purpose is to control this oscillation. In a passenger car, the designer has the choice of just how fast the shock dampens the spring. If the dampening is immediate, the car will have better weight transfer rate control, but a harsher ride. If the dampening is a little slower allowing perhaps 2 to 3 oscillations, the ride will feel much smoother.
In racing, we want the dampening to be almost immediate. A vehicle's bouncing on the springs creates erratic shifts in the tire contact patches and mechanical downforce on the tires. Both of these conditions reduce the effective grip the tires have. Any bounce in the body of the vehicle must be eliminated quickly to allow full grip to return as fast as possible to the tires.
However, like springs, it is possible to have too stiff a shock. First, if the shocks are stiffer than the springs, the springs will be overpowered, and will not actually fulfill their bump absorbing function.
Secondly, a shock has a major affect on how quickly weight transfer occurs in the dynamic changes of accelerating, braking, and cornering. The stiffer the shock, the faster the weight transfer occurs. This will help the vehicle have very responsive steering, but the transfer can be too fast for the driver.
During cornering in particular, the driver must be able to induce smooth weight transfer and feel the tires reach their maximum grip. If the weight transfer occurs too fast, the driver will not feel the tires approach that peak grip, and will likely overshoot the traction capacity of the tires causing excessive sliding or spins.
When modifying your car, starting with the most awesome race hardware you can buy is not likely the appropriate starting point. Full-race shocks are going to be too stiff for the street, and will likely cause the car to bounce off of bumps. Additionally, you'll probably not have the sensitivity to feel the grip level of the tires when cornering at maximum speed.
To help with the dual purpose street/track car, and to provide some adjustability for tuning handling performance, there are several after-market shocks that are adjustable. A manual (or even electronic) dial allows selection of several settings which are progressively stiffer. These shocks can be turned to their softest setting (they're still going to be stiffer than stock) for a smooth ride on the street, and their firmest for the track to minimize body roll and increase the steering responsiveness. The adjustability also allows finer tuning of handling performance for a given track. As discussed in the weight transfer and the handling tuning sections, adjustable shocks can be used to help adjust out small amounts of oversteer and understeer.
If you can't afford adjustable shocks, don't assume that stiffer is better in a fixed rate shock. Shocks should be selected knowing the springs they will be used with. Too stiff a shock will overpower the spring reducing its effectiveness. If the shock is not adjustable, then matching the shock to the rating of the springs is even more critical. You sould consult a shop experienced in this matching.
Talk with a technician familiar with your car, and find out what shocks offer the best performance for you car's degree of modification. The "killer" shock for someone else's car may not be the best one for yours.
The shock's purpose is to control the oscillation of the spring's response to bumps and dips. The stiffer the shock, the faster the dampening. The shock also controls the rate of weight transfer. The stiffer the shock is for compression (or "bump" as it is often called), the faster weight transfer occurs which determines how fast the tires will take their "set" in a corner, and the resulting steering responsiveness of the car.
It is possible to have too stiff a shock which will overpower the springs, and results in weight transfer being too fast for the driver's experience and sensitivity to feel when maximum tire grip is achieved in a corner (generally resulting in a lot of spins).
Shocks need to be selected knowing the ratings of the springs.
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