Having too much or too little spring tension will negatively affect suspension performance, but in different ways. Too much spring tension can make your suspension feel like it is topping out. This happens because now the shock extends to its maximum length too suddenly, and this may unload your wheels from the road surface. Not enough spring tension can make your suspension bottom out excessively. Knowing these effects can help make the correct adjustments.
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 tension 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, this 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/in 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 tension affects droop. But what is the proper amount of droop to 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 adjust the spring tension 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 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