How to Measure
The most accurate way to measure a damper is to use a damper dyno. This is a machine that fixes one end to a load cell (force measurer) and the other end to a crank shaft. The crank shaft has a fixed displacement and rotates in a circular motion. As the crankshaft rotates, it moves the damper up and down, compressing and extending it. The crankshaft is often set to put the damper through its full range of movement.
The machine rotates at different frequencies to simulate the different velocities that would pass through the damper on a circuit. This then allows a graph to be plotted for the damper of force vs displacement. The graph below shows a typical plot for a test like this which takes the form of a single loop to represent the full rotation of the crank shaft. All of the data above the horizontal line shows the bump forces and all below show the rebound forces.
For adjustable dampers, the damper will be set to minimum bump and rebound settings before testing. A graph will then be plotted on the dyno with these settings installed. Next the damper is set to maximum bump and rebound settings and tested again. These different results are also plotted on the graph. This allows the user to see the full range of forces that can be put through the damper on track and everything in between the two lines can be adjusted to by the damper settings.
With this graph complete the velocity at each point through the damper can be calculated using the equation:
v= Velocity (m/s)
ω = Angular Velocity (rads/s)
A = amplitude (m)
Using a computer this equation can convert each data point into a velocity. Also, reading off the axis at each data point, the corresponding force at each point can be taken. This allows a final graph to be generated showing force against velocity for the damper which shows the complete damper characteristics. The two lines for full bump/rebound and minimum bump/rebound can be overlaid as well.
Most high end coilovers come with a damper dyno graph in the box so that a dyno does not need to be bought, allowing you to have the data and the knowledge without the expense of a damper dyno.
How to Adjust
If the information from the damper dyno shows that the damper is not capable of handling the velocities that are going to be put through it, then the damper needs to be rebuilt as the fundamental components need altering. Usually this would be either the gas pressure, valve diameter or shim thickness. This requires a lot of knowledge and specialist tools to fix.
Therefore, assuming that the dyno graph was correct for the application which it often is once bolted to a car. Only at the very high end of motorsport are damper internals altered track to track. In this case the damper is relatively easy to make adjustments to.
For many years, the method to adjust the bump or rebound has stayed the same. In order to adjust bump, there is usually a knob, alum key slot or screw driver slot or a collar to turn. This is usually located at the bottom end of the damper. If you have external reservoir dampers then this is sometimes put at the top of the reservoir instead. This valve is simply rotated clockwise to increase the bump force and is rotated anti-clockwise to set it to its softest setting.
For the rebound, there is often a slot to insert a screw driver, a knob, an alum key slot in the top of the damper. Rotating it one way, often marked on the adjuster, will stiffen the rebound and turning it the other way will soften the rebound. Once you are at the maximum and minimum setting for both bump and rebound, the valve will stop turning, letting you know you have reached the maximum setting.
Effects of Adjustment
Bump damping controls the un-sprung weight of the car, which is the wheel and upright assembly and the coil spring. The optimum bump setting for the car can be found anywhere in the adjustment range of the damper for any car. Therefore, the car needs to be tested in order to fine tune the damper to get its optimum setting for any particular track and set up.
It is important not to use the damper to control roll or bottoming of the car, this is instead done with spring rates, anti-roll bars and ride height. The ideal bump setting is achieved when side-hop (which is where the car bounces towards the outside of the corner) is removed when encountering bumps on a corner and when the ride is not overly stiff.
In order to set the bump settings correctly, the damper must first be set to full soft setting for both bump and rebound. Next, drive the car around the circuit with these settings installed, increasing the pace slightly each lap. Ensure that the driver is paying strict attention to how the car is riding over bumps through the corners and how much the car is hopping to the side and not how much the car is rolling or leaning on turn in. Next increase the bump by three clicks on each damper and send the car out again. Keep on adjusting 3 clicks in set intervals until the car stops side-walking in corners and when the ride of the car feels hard over bumpy surfaces. At this stage the bump can be backed off by two clicks.
If the ideal setting is reached on one end of the car sooner than another then continue the process on the end that is not set until optimum settings are achieved all around the car.
Next the rebound settings, currently set to full soft need adjusting. The rebound settings are the part of the damper that control how fast the car rolls in and out of corners. It does not limit the total amount of roll but the speed at which is rolls. If the rebound is set too stiff in relation to bump then a process called jacking down occurs. This is where, after hitting a bump and compressing the spring, the rebound is set too stiff to allow the spring to return to its full length before the next bump is encountered. This happens bump to bump until the damper is fully compressed on its bump stops which could cause drastic understeer or oversteer depending which end of the car it occurred on.
The method to set up the rebound setting on the damper is to make sure the rebound is still set to full soft. Get the driver to drive the circuit and see how much the car rolls when entering a turn. Keep increasing the rebound by a quarter turn on the damper every 2 or 3 laps. When the car begins to roll in a smooth controllable motion without leaning excessively; this is the point at which the damper is set to optimum rebound. Any further increase in the rebound could cause the car to begin understeering or oversteering.
Now the base setting is found, fine adjustments s can be made to fine tune the car according to driver feedback.
Having too much rebound set on the car will cause an initial loss of lateral acceleration which will cause the car to understeer or oversteer on corner exit/ entry. Bump and rebound can both be adjusted based upon driver feedback once that base settings have been found. The adjustments will come from reports of understeer or oversteer occurring on corner entry or corner exit.
If the driver is complaining about understeer then first you must find out where in the corner this is taking place. If it is on corner exit then the front rebound must be increased and made slightly stiffer.
If it is occurring on corner entry then the question must be asked whether the front of the car is rolling excessively or not. If the answer is yes then the front bump/compression settings should be increased. If the answer is no then the front rebound settings should be increased.
Oversteer occurring in corner entry or a mid-corner situation can also be controlled by adjusting the dampers. The question that must be asked here is whether it is occurring through a high speed corner or a low speed corner. If it is high speed then increasing the rear rebound stiffness will help reduce the oversteer.
If it is in a mid to low speed corner then a series of questions must be asked:
- Does the suspension bottom out or jam at any point during cornering?
- Does the car bottom out on the track?
- Does the rear end roll excessively?
If the answer to all the above questions is no then you can either increase the rear rebound rate, or you can reduce the rear compression/bump rate.
If the answer to the first two is no but the last question is yes then you can increase the rear compression/bump rate.