How to adjust spring preload on your coilovers
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Team Driftworks
- Guides & Featured Products
- 7 Sep 2024
- Reading time: 5 minutes
What is spring preload?
Simply put, spring preload is the amount the spring is compressed in the suspension when there is no weight on the suspension.
OE suspension tends to use a lot of preload and this is down to the use of progressive springs and the amount of droop that standard cars run but when compared to some aftermarket items this is greatly reduced. Some even run no preload at all.
This is due to a change in spring design from a progressive to a linear spring which is a set rate and does not need to be pre-loaded as such.
Does adding spring preload increase the spring rate / spring stiffness? It is a common misconception that adding spring preload increases the stiffness of a spring. It does not. It increases the force required to overcome initial spring movement. Which can result in a more bouncy initial feeling in the car, that some people attribute to a firmer ride. But it doesn't increase spring rate (spring stiffness). The car will still pitch and roll the same amount.
Excessive spring preload can actually cause damage to the dampers. So in general we do not recommend it. If you require a stiffer spring, we can supply stiffer springs for many of our coilover suspension kits and help advise on the correct spring rate for your application before purchasing.
Measuring Coilover spring preload
Here is a set of HSD Coilovers for a Mazda MX5. These use a 175mm free-length front spring and you can see in the third image it isn't compressed when installed onto the coilover. This measurement of 175mm is also the same in the second image when comparing the standalone spring next to it.
This is what is referred to as zero preload where the spring pans are set at the exact height of the spring so it has no compression on and is sat at its full free length.
You can add preload to this which is where you compress the spring slightly whilst there is no weight on the car. You can see this in the 4th and final image, there has been 5mm of preload added so the height is now compressed to 170mm.
Some manufacturers still recommend a very small amount of preload to be used just to stop the springs sliding on the pans and making noise; BC Racing for example suggests 5mm of spring preload.
Adding some spring preload like this will affect both ride height and ride comfort as when you preload a spring you are putting it under tension and the more tension you place on the spring the more it will try to push back and the harder the ride in the car will get.
We do not recommend running any more than 10mm of preload on this type of coilover as a lot of the time the damper does not have a top stop (a bump stop but for full extension) and the spring will be trying to push the damper to the full extension which may cause damage if driven on set like this.
If you find that at this point the car still feels too soft you can swap to a harder spring.
Some TEIN and KW Suspension kits can only be adjusted on the preload but they tend to have either a progressive spring or a helper spring fitted which means you can preload the springs and adjust the height without adding huge amounts of force into the damper.
Using a helper spring with or without preload
A helper spring can be preloaded as much as you like. It is there to allow you to tune ride height versus damper stroke and droop correctly, and to stop the main coilover spring moving off the spring perches during droop (damper extension).
Here is a 200mm spring with a 50mm helper and a 3mm spring seat. With zero preload this measures 253mm between pans in the left image.
The right image shows the same coilover with almost 20mm of preload added and this has compressed the helper spring far more than the main spring.
As you compress this setup the helper spring will compress before the main spring so the rate will only have a very minor effect on ride comfort until you have completely compressed the helper spring and are essentially only using the main spring.
A progressive spring works similarly but is one spring with a varied rate, not 2 springs with different rates.
The amount of preload needed depends on the type of coilover kit. Normally, each manufacturer will include some instructions that will suggest the amount of preload to use.
However, it is difficult to say if preloading your kit is a good or bad thing as it all depends on your exact suspension setup and application.
Hopefully, this guide gives you some good tips on how to best setup spring preload. However, if you're still undecided or would like to know more about any of our coilover springs, then click the button below to see our complete range.
Alternatively, please speak to our friendly customer support team at [email protected] or call 0121 792 2000.
We're here to give you the most up-to-date knowledge on frequently searched topics gained from years of first-hand experience modifying our cars. We'll always aim to give you the answers straight without adding over complicated, unnecessary detail.
