March 30, 2023 2 min read
A benefit of height adjustable coilovers is that they usually use standardized coilover springs. This means you can easily replace them if you’re looking for a softer or stiffer suspension.
Coilover spring specifications can be confusing, but it usually comes down to spring rate, inner diameter, and length.
Whether you should replace your springs and exactly what specs to use is for another time. Working with a good suspension tuner is crucial for dialing in a set up that works for you.
Spring rates are easy for linear coilover springs. A 400 lbs/in spring compresses 1 inch for every 400 lbs of load applied. The different units can be confusing, but it’s easy enough to convert. There’s lbs/in, kg/mm, and occasionally N/mm.
1 kg/mm = 56 lbs/in = 9.8 N/mm
One thing to consider: a linear spring is never 100% linear. Hyperco advertises that their springs have less than 4% linearity tolerance for most of the spring’s travel. Poor quality springs have much wider tolerances.
This Eibach coilover spring is rated at 650 lbs/in. That’s about 11.6 kg/mm.
Spring length refers to the length of the spring without any load applied (i.e. sitting on your desk). Long springs have more spring travel, meaning you can compress them more before they’re fully closed. A shorter spring may fully close before the shock bottoms out, which is one way to break stuff.
Most manufacturers list the amount of travel for their coilover springs, so make sure you have more spring travel than damper compression travel. Springs that are too long may cause clearance or ride height issues.
The upper and lower spring perches on your coilovers are designed to work with a specific inner diameter (ID) of spring. These are most commonly 60 mm ID and 2.5 inch ID.
Conversions are possible, but most of the time you can get what you need in the right ID.
It’s straightforward to measure either the perches on your coilovers or the ID of your existing springs if you have your suspension disassembled.
Barrel shaped springs are used to get more spring travel for a given spring length. Often used for dampers with lots of travel or when there’s just not enough space for longer springs.
Helper springs are short low rate springs used to keep the main coilover springs captive when the shock is extended. Tender springs are similar, but have more spring rate to allow for more tuning options when the shock is extending.
Coilover part numbers can tell you a lot. For example, a 60-170 KW spring is 170 mm long with a 60 N/mm rate. KW springs are 61 mm ID.
A 0600.250.0800 Eibach spring is 6 inches long with a rate of 800 lbs/in and an ID of 2.5 inches.
Swift uses different colors to denote different IDs and have the rate printed on the spring.
If in doubt, contact the manufacturer.