Shock absorbers are really easy to change. They’re a lot easier to change than springs or sway bars. If you have adjustable shocks you also know how much fun it is fun to turn the adjuster knob – even if you have no clue about what you’re doing. Advertising has convinced us that new shocks will cure everything that’s wrong with our car and solve all of our handling problems.
The primary purpose of the shock absorber is to control spring and suspension movement. Shocks do not support your car. Coil springs, leaf springs, or in some cases torsion bars, hold your car off the ground. That’s why when you change shocks the ride height of your car remains the same.
Springs carry the vehicle weight and they establish how much the suspension will move for a given input. Shock absorbers are designed to control the speed of the suspension’s motion. Shock absorbers regulate how long the suspension takes to react to a given input. They restrain the oscillations of the spring. This means that the stiffer your springs are the more work the shock absorber has to do in an effort to restrain the spring.
It’s critical that you get the spring package right before you start playing with shocks. Once you get the springs right you can start to tune the package with shock absorbers. Koni feels that you should utilize shocks and sway bars as tuning devices only after you get the spring package correct. You don’t start with shock absorbers. Koni suggests that you regard shock absorbers as a precision tool for that final handling correction.
Rebound and Compression
The valves in the shock, or more correctly the damper, are calculated using rebound (extension) and bump (compression) numbers. The rebound characteristics control the sprung weight of the car, which is basically everything above the suspension (body, driver, engine, etc.). Bump, or compression, controls the car's unsprung weight (wheels, tires and brakes).
When your car passes over a bump in the track the bump forces the wheel up into the wheel housing. In the process, the spring is compressed. The shock absorber is now in its compression stage. Once the car has passed over the bump the shock absorber has to slow down the movement of the spring as it releases its tension. The shock absorber is now in its rebound stage.
Speed and the Shock Absorber
There are separate low speed and high speed bump and rebound controls. High speed and low speed has absolutely nothing to do with the speed of the car. It’s all about the speed of the shaft. When you encounter a lot of bumps on the track the shaft is moving quickly. If you’re on a really smooth track then the shaft will move slowly. It’s just that simple.
If you get a chance to look at a plot of shock velocity versus time you’ll see that the shock changes direction as often as 20 to 40 times per second. The general convention is that shaft velocities between 0 mm/s and 50 mm/s are considered low speeds. A shaft velocity between 50 mm/s and 150 mm/s is considered a mid range velocity. They affect the car under braking, during fast changes of direction. Speeds above 150mm/s are considered high speed shaft velocities.
Adjustable Shocks
This is one of the more complex areas. Adjustability enables you to modify the engineer's decisions to fit your driving style and track condition. Even more importantly, you can tailor your shocks to work with other performance modifications.
A general rule of thumb is that increased rebound will loosen that end of the car. An over steering rear wheel drive car can generally use more front rebound (to loosen the front) and less rear rebound (to reduce rotation) in the pursuit of balance. This is all a combination of art and science. Just remember to only work on one end of the car at a time.
Just because you can adjust your shocks doesn’t means you’re going to get it right though. You can just as easily make things worse. If you have two-way adjustable shocks (compression and rebound) you can get by with a good testing program. If you have four-way adjustable shock absorbers schedule both testing days and therapy sessions. It will get that confusing. In fact the people at Performance Shocks feel that unless you have an engineer on your team you shouldn’t even purchase 4-way adjustable shocks.
Remote Reservoirs
You see these all over the paddock. Pretty soon you start to think you need them Considering they may cost more than a new set of tires a little reflection is in order. Remote reservoirs are really a packaging solution. The reservoir offers a place to store the fluid and the nitrogen. They also help some with fluid temperature control.
Remote reservoirs though can also introduce a delay into the system. How much of a delay depends on how far you place the reservoir from the shock absorber. In some situations shocks with a remote reservoir can be inferior to a normal shock.
How Do I Buy the Correct Shocks?
The people at Performance Shocks feel the very first thing is to be totally honest about how much money you want to spend. Next, are you concerned about being period correct? Finally, how much adjustability do you actually need?
You also need to start collecting data about your car. The shock companies can’t help you unless they know what your car is currently doing. They want more than just impressions. The two most critical things are spring rates and weight. Springs are pretty easy to determine. Now how much does your car weigh and what’s the front to rear weight bias? It’s even better if you can provide the shock company with corner weights.
Another important item is the tracks you run. Sebring is going to require very different shock settings than Mid Ohio. Now what sorts of tires and wheels are you using? The tires are important because they’re really springs. The brand of wheels is important since some wheels flex more than others.
The more information you can provide the manufacturer the better they can determine your needs. You really can’t purchase shocks from a catalogue. You need the help of experts. Then you need to provide these experts with information. While there is a limit to the amount of information that vintage racers collect I am seeing a lot more data loggers in the paddock. Remember even the best shock experts in the world can’t help you unless they have some data to work with. Your job is to collect the necessary information the experts need to help you. Then let the experts pick the correct shock package for your car.
