Many people have coilovers installed on their car to improve their handling and performance but are not setting them up correctly and can have many handing issues due to setting them up incorrectly. Incorrect setup is due to a massive amount of mis-information within the industry which is not helped by the fact that different coilover manufacturers also recommend the incorrect techniques to setting the coilover up correctly.
This article details the correct way to set up a coilover that has two points of ride height adjustment and for coilovers with a single point of ride height adjustment. Coilovers with two points of adjustment have damper body height adjustment and spring base height adjustment like in the image below.
Dampers with a single point of adjustment have spring base height adjustment only. Both methods allow the damper to be set up correctly. Having a damper with two points of height adjustment helps to fine tune the bump to droop ratio of the damper stroke on your car so is a preferable option for fine tuning the handing of your track car.
Before covering how to set the ride height of the damper, the effects of pre-load must first be covered. Pre-load is another area with lots of mis-information surrounding it and usually does not exist like many people think it does. Most cars use a linear rate coil spring, therefore, if the spring is compressed, the spring rate does not change. Therefore, if you compress a linear coil spring, with a rate of 10kg/mm, by placing 100kg of mass on it the rate of the spring will still be 10kg/mm and it will compress by 10mm. If you add another 100kg to a total of 200kg directly to the spring then it will compress by another 10mm and so on.
The difference between the first and the third spring in the above diagram is that spring 3 now has stored energy and has the desire to extend. However, when you place the mass of your car on spring one it will also carry a very similar amount of stored energy making the “preload” effectively redundant.
If you begin compressing the spring so much that the coils begin to bind at full droop then this will have an effect on the spring rate due to having inactive coils which will alter the spring rate. In this extreme case, you should look to either installing shorter springs of the same spring rate or removing the helper springs if they are present.
The only scenario where pre-load does exist is in a variable rate coil spring. This is a spring with sections of different rates that changes in spring rate as the spring compresses. This style of spring is a much rarer application and the amount of compression at ride height will have an impact on the spring rate at each corner. However, for most applications, and for the vast majority of motorsport, linear rate springs are used.
Setting Damper Travel
When setting up the damper on the car you need to make sure that once the car is sat at ride height, there is an adequate amount of bump and droop travel in the damper in order to optimise performance. As a rule of thumb, for most cars, the damper should be set so that once at ride height, 2/3rds of the available travel is for bump and 1/3rd of available travel is for droop.
In order to achieve this, the first step is to remove the spring from the damper and to measure the length of the damper shaft that protrudes from the damper body when fully extended. Then fully compress the damper and measure the length of the damper shaft that protrudes again. Next, use the below equation to calculate the total amount of damper travel available:
With damper travel calculated you can now calculate the amount of droop travel for your damper:
With your droop travel calculated you can now calculate bump travel for your damper using:
Next, still with the spring removed from the coilover, install the damper onto the car properly. If you race your car in a championship then it is highly likely that you will have front and rear ride height limitations in the rules and regs. If this is the case then set your chassis to the minimum ride heights allowed front and rear using jacks, axle stands or wood etc and install the wheels.
Next, adjust the damper using the damper body height adjustment to set the bump to droop ratio calculated above. In the animation below we have highlighted the damper rod to show the droop travel (red) and bump travel (green) that we desire to have from our damper. Using the damper body adjustment we need to set the top of the damper body on the line between the red and green zones as this will mean that we have the correct amount of bump and droop travel available from the damper.
Notice in the above animation how the top mount is staying in the same position, showing that the ride height is not changing at this point. The only aspect being altered is the position of the damper piston inside the damper body showing how effective the damper body adjustment is at altering the bump and droop ratio.
Next with the wheel still installed, jack the wheel up all the way until the bump stop in engaged. At this point hopefully your wheel will not be in contact with your body work or your wheel arch. If it does not contact then you can move on to installing your spring and fine tuning the ride height in the next section.
If your wheel does interfere then you have to make some decisions. One solution is to raise the ride height of your car by the amount of damper travel left before the bump stop is engaged. This is often a sensible option if you are in a situation where you are just running your car far too low. If you have set your car to meet the ride height requirements of a championship then a solution is to cut away the inner arch or fit wider body work to prevent interference and allow the wheel to recess into the body work.
If the tyre is hitting the outer wheel arch then check your camber settings too as if you haven’t got round to aligning the car yet you might have it set to positive camber or not enough negative camber at ride height and this could be causing the interference.
If none of the above situations are available then you might have to remove some of the bump travel using the lower damper height adjustment. This is an absolute last resort as it will reduce the amount of bump travel and increase the amount of droop travel upsetting the ratio calculated earlier.
Setting Ride Height
With the bump and droop range set correctly on the damper you can now remove the coilover and re-install your coil spring and bump stop on your damper. With the complete coilover re-installed to your car you can now lower the car back down to the ground to sit on the floor. With the mass of the car now sat on the spring and damper, your ride height will almost definitely be in a different position to your set ride height before due to the spring compressing. This is to be completely expected.
You can now use the lower spring platform to set the ride height of the car. Turning the lower spring platform upwards with raise the ride height of the car and turning the lower spring platform down the damper will lower the ride height of the car. The aim is to use the lower spring platform to raise or lower the base of the spring until the ride height that you set before based upon regulations, experience, calculations or data is achieved.
In the animation below it shows our damper from before with the red and green zones on the damper rod. We have now installed our spring which is compressed with vehicle mass upon it. You can see that the damper piston is now sat lower in the body than we want. As we raise the spring platform you can see how the damper piston is rising back to where we desire it to be but this time the ride height is also being raised (shown by the lifting top mount). Once the ride height is set using the spring platform, the damper will be back in the correct position with the correct bump to droop ratio.
Don’t worry if you compress the spring when doing this as it will not affect anything. The only time to have concern is if you are compressing the spring so much that the coils are binding and becoming inactive. In this case install a shorter spring of the same stiffness.
Once the ride height has been set, jack the car back up, lifting the wheel off the floor so that the damper is at full droop. At this point check that the spring is still captive between the upper and lower mounts. If it is then lock off the lower spring perch collars as the damper is now correctly set. If it is loose then you will need to install a helper spring to keep the main spring captive under full droop. If you do need to install a helper spring then you will need to lower the spring perch by the same distance as the compressed thickness of the helper spring. Once installed and adjusted, re-check your ride height as it might need a further tweak of the spring platform.
With this final adjustment complete you have now set up your coilover correctly and it is ready to use.
Many people have been taught how to set up a damper in a totally different way. This is due to a lot of mis-information surrounding the topic, some of this information even coming from the coilover manufacturer and being recommended on documents that they provide with the coilovers. However, this is an unfortunate case where, although the quality of the product is very good, it is effectively a copy of another product on the market and the background knowledge hasn’t been there on the production line when making these products and providing fitting guides etc. This doesn’t mean that the products aren’t up to standard, it simply means that the information provided with them is sometimes incorrect.
If you look at brands such as KW or AST they often sell coilovers without the damper body ride height adjustment and just have the spring platform adjustment. This is because, they design an ideal ride height for the application the damper is suited for, effectively pre-setting the bump and droop ratio for the car and pre-determined ride height. The spring platform can then be used to adjust the ride height to suit your personal requirements. Ohlins often supply their coilovers with the damper body adjustment as it is an added bonus being able to tune the bump and droop ratio of the damper for your set up.
As an example of why not to use the damper body to adjust your ride height, imagine a situation where you have a 26Kg spring installed on your coilover and you are now moving to a 8Kg spring of exactly the same unsprung length. This is an extreme example but helps to show the reasoning. When moving to the softer spring with all height settings in the same place as before, the car will now sit lower as the softer spring will compress by a greater amount than the stiffer spring. The way in which the car has lowered is by compressing the damper and pushing the damper piston further into the damper body (reducing bump travel). Therefore, if you were to now use the damper body to raise the car back up, this reduced bump travel would still remain as the spring would still compress by the same amount at ride height.
However, if you raised the ride height by using the lower spring platform, once the softer spring was installed, you would have raised the car and pushed the damper piston up inside the damper body, increasing the bump travel back to where it should be set. This way, you have achieved the ride height change and you have set the damper ratio back to where it should be.