We often get questions about spring pre load. We also see coilovers that have improper spring pre load set. So we wrote up a quick guide on how to properly set spring pre load on your coilover kit. Hope this helps!
So you’ve purchased a set of coilover shocks for your car with spring pre load adjustment, but do you know how to properly set it? Maybe If your coilover system does not have independent ride height adjustability, you may have just set it to yield a desired ride height and are now just hoping spring pre load is within proper range. Or maybe, your coilovers do have spring pre load adjustability that is adjusted independent of ride height adjustability, but you are unsure of how spring pre load affects performance. In this article we will describe the effects of spring pre load and how to properly set it.
Having too much or too little spring pre load will negatively affect suspension performance, but in different ways. Too much spring pre load can make your suspension feel like it is topping out. This happens because too much spring pre load will make the shock extend to its maximum length too suddenly, and this may unload your wheels from the road surface. Not enough spring pre load can make your suspension bottom out excessively. Knowing these effects can help make the correct adjustments to spring pre load. It is important to understand that spring pre load does not affect spring rate of a linear spring (most coilover systems come with linear springs). For example, increasing spring pre load WILL NOT increase the firmness of your linear spring. Increasing spring pre load WILL increase the amount of compression stroke you have which increases bottoming resistance.
Let’s define a few terms to help understand spring pre load effects.
The amount of stroke the spring consumes at static ride height from the weight of the vehicle is called “droop.” And the amount of stroke left over at static ride height is called “compression stroke”. The total shock stroke is droop and compression stroke combined.
Total Shock Stroke = Droop + Compression Stroke
It is important to understand that spring pre load does not affect the spring rate of a linear spring (most coilover systems come with linear springs). For example, increasing spring pre load WILL NOT increase the firmness of your linear spring. However, increasing spring pre load WILL increase the amount of compression stroke you have which increases bottoming resistance.
Springs on most coilover systems have to be pre loaded in order to retain a desirable amount of compression stroke at static ride height. For example; if you have a coilover with a 200 lbs/inch spring rate carrying 800 lbs of weight, without any pre set spring pre load, the coilover will compress 4” just from the static 800 lbs of weight acting on it. If this coilover has a total of 5” of stroke, this only leaves you with 1” of compression stroke left over! In this scenario you must pre load the spring to insure you have more than 1” of compression stroke. There is way too much droop in this scenario.
So we now know that spring pre load affects droop. But what is the proper amount of droop you should have? This varies depending on how much total stroke your coilovers have, so we treat the desired droop as a ratio of total shock stroke. In order to have an appropriate amount of droop, we recommend setting droop to be 30-40% of the total shock stroke (see equation below). Now you know that you have to pre load the springs on your coilovers to yield 30-40% droop!
Desired Droop = Total shock stroke x .35
How to set spring pre load:
You must first measure the total shock stroke of your coilover (including the bump stop length). Then measure how much the coilover compresses when the vehicle is at static ride height. Subtract the total shock stroke from the compression stroke at static ride height from the total shock stroke to find the droop amount. Adjust spring pre load until suspension droop is between 30-40% of total shock stroke.
Droop = Total Shock Stroke – Compression Stroke