| SHOCK BASICS |
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| I think a friend of mine said it best when he referred to shock absorber tuning as a 'black art'. |
| As usual, I have a wonderful quote from Carroll Smith on the subject: |
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| "Sometimes I think that I would have enjoyed racing more in the days of the friction shock. Since you couldn't do anything much to them or with them I would have spent a lot less time being confused.'" |
| Carroll Smith - 'Tune To Win' - pg.74 |
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| The shock absorber works hand in hand with the suspension springs. Ironically, the springs are the actual shock absorbers of the suspension. The springs allow the sprung mass of the vehicle to absorb the wide variety of road surface changes we encounter when driving our cars. When our car hits a bump, the springs are what absorb the shock of the impact as the tire travels up into the wheel well. |
| Shock absorbers serve one main purpose, to dissipate energy by converting kinetic energy (stored in the springs) to thermal energy (heat) that is easily whisked away into the air. |
| If you're a Forza Motorsport veteran, you're already familiar with the adjustable shocks on your race cars. Real life shocks are no different, and their movements are broken down into two types: |
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| 1-Bump (compression) - The shock is in the bump phase when the springs compress and the wheels travel up into the wheel wells. It is in this phase of suspension movement that the spring moves from its pre-loaded or static state and stores energy. |
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| 2-Rebound (expansion) - The shock is in the rebound phase of suspension movement when the spring either 'droops' from its static state, or expands from a compressed state (the bump phase) and returns to its original pre-loaded position. In the first case, the wheel moves away from the wheel well as the spring expands. This is most common when the vehicle is in a left hand corner as the body rolls right. The inside wheels (left side tires) are unloaded and the springs expand as the tires move away from the wheel wells. In the second instance, (using the same left hand turn as an example) the right side wheels are laden (loaded) and so are the right side springs. The right side dampers are in bump, and the springs are holding a tremendous amount of stored energy. As the car exits the corner and the vehicle takes its set for the straight, the right side shocks go into the rebound phase as all that extra stored energy in the spring is released. |
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| The shock absorbers job is to absorb and dissipate that energy as its released by the springs. So, I suppose it could be said that our shock absorbers are actually energy absorbers. |
| This is why when shocks are valved by the manufacturer or by the shock tuner, the rebound stiffness is always greater than the bump stiffness. Simply explained, when the shock is in the bump phase, energy is going into the spring and when the shock is in the rebound phase, energy is coming out. Because of the tremendous release of stored energy in the spring, the rebound strength of the shock must be greater in order to dampen it. |
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| SPRING OSCILLATION |
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| The first key to figuring out what shocks do, is to understand what springs do. Springs oscillate. When you stretch an unloaded spring and let it go, it snaps back, but it doesn't neccessarily return to it's original state. It can go past equilibrium (overshoot) and compress. Then the stored energy from the compression releases, and the spring stretches again….then compresses…then stretches…etc. |
| In layman's terms the spring 'bounces' and this action of expansion and contraction is known as oscillation. An undamped spring will continue to oscillate with gradually decreasing force on each 'bounce' until it runs out of energy and returns to a static state. Now you could imagine the amount of energy that is stored and released in the suspension springs with a 3,000 lb car sitting on top of them. Without shocks (aka: dampers) the car would bounce down the road like it was on four independently moving pogo sticks. This would haphazardly load and unload the wheels and make the car impossible to handle with any degree of predictability. |
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| So, to clarify the springs on our car can be categorized as follows: |
| 1-Static - our 3,000 lb car is sitting in our driveway. The four springs are said to be 'pre-loaded' with 750 lbs of weight on each. |
| 2-Loaded - our 3,000 lb car is driving on a level surface and turns left. The right side springs compress, and are loaded with stored energy. |
| 3-Unloaded - in the same turn, our left side springs are unloaded as they expand and release energy. |
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| It's key to remember that every time we turn our vehicle, the outside wheels' dampers go into bump while the inside wheels' dampers goes into rebound. |
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| SHOCK FUNCTION |
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| It's important to realize exactly what the shock does from the perspective of adjustability. Lateral weight transfer is dependent (in part) upon our spring stiffness. Stiffer springs transfer more weight from side to side. Longitudinal weight transfer (pitch) is also dependent upon our spring stiffness as stiffer springs transfer more weight under both braking and acceleration. |
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| Our springs stiffness determines the amount of weight that will transfer. |
| Our shock stiffness determines the speed or rate with which the weight is transferred. |
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| So when we set our spring rates the weight is going to get there, but our shocks dictate how fast or slow this transfer will happen. |
| When shocks are custom valved at a shop, the valving is dependent upon the rates of the front and rear springs. Shocks are often set up to absorb energy at 65% to 70% of critical damping. |
| 'Critical Damping' isn't exactly critical to know in order to tune your shocks. But basically, a spring that is critically damped is when the spring is stretched or compressed, then returns to its state of equilibrium as quickly as possible without oscillating. In the case of the automobile, the springs' state of equilibrium is the pre-loaded state (ie: the car is sitting motionless at a stop light). |
| Adjustable shocks give us the freedom to tweak oversteer and understeer characteristics although they're an instrument for fine tuning when compared to the other adjustable parts of the race car that are a lot easier to work with, like our ARBs. I'm not saying that shock tuning isn't effective for tweaking the car's handling, because it is. But truth be told, it's a much more slippery slope to travel. |
| A much easier task to master, is getting our adjustable dampers to work in harmony with our spring rates to maintain proper tire compliance over the multitude of varying track conditions our cars will encounter. |