Spring Rate vs. Preload

Outside of the physical characteristics that determine if a spring physically fits your bike or not, there are two key parameters that impact how that spring is going to perform: spring rate, and spring preload.

  • Spring rate is fundamental to the design of the spring, and is expressed as some amount of force per distance of compression. Many KTMs, for example, come with a 43 N/mm shock spring.
  • Spring preload is a simple distance measurement. If you have a spring with a free, uninstalled length of 260 mm and it is 247 mm when installed, you have 13 mm of preload on the spring

Changing the spring preload and spring rate do two different things, and the easiest way to see the difference is to consider the below graph. Using the front forks as an example (fork springs have a motion ratio of 1 – they compress the same amount as the front wheel, making this a bit easier to plot vs. the shock), we can compare a 0.42 kg/mm fork spring and a 0.48 kg/mm fork spring, each with 5mm and 15mm of preload.

What you can see is that spring preload shifts the entire line up or down, and the spring rate changes the slope. When you are changing preload, you’re really trying to set the static balance of the bike. While preload will make a bike feel marginally stiffer or softer at a given point in its travel, it is not a significant effect. If you dramatically increase preload in search of stiffness, you will find that it takes significant force to move the suspension even at ride height, which can make a bike feel stiff over smaller bumps while still bottoming in larger hits.

Moving to a stiffer spring, on the other hand, allows you to have more force supporting the bike as the suspension compresses further, with a lower effective rate towards the top of the stroke that allows for a bit more comfort.

In the above example, you can see that at around 70mm wheel travel, a 0.48 fork spring with 5mm preload has the same spring force as a 0.42 fork spring with 15mm of preload. Higher in the stroke, the stiffer spring is actually softer than the softer spring that has been preloaded, and provides significantly more support for the bike when it is compressed deeper into its travel.

This is a relatively tame example – many people run far too much preload instead of buying the correct springs, and the result is reduced comfort and harsh top-out behavior combined with insufficient support in rollers.  

How much preload is acceptable?

This is where we start getting into different opinions of different tuners. Tuned Concepts recommends a range between 5mm and 15mm of preload. If you’re unable to get the appropriate static ride height and sag within those bounds, you should consider a stiffer or softer spring.

How to I select the proper spring rate?

Most bikes come with spring rate recommendations for a range of rider weights. This will get you close. From there, setting your race sag and measuring preload will tell you if you are within the target range. Typically, there are two or three spring rates that can work for a given rider weight, dependent on rider style, use case, and suspension valving.

How does this rate vs. preload apply to air suspension?

One of the limitations of air suspension is the inability to separate preload from spring rate. Some setups will use an upper chamber and a lower chamber pressure adjustment to attempt to influence this. Others, like the WP AER forks, use a bypass channel in the air cartridge. The suspension has to compress far enough for the piston to pass the end of the bypass channel before the air will compress and start to build spring force. This makes AER forks especially nonlinear near the top of the stroke, and is also why your air pressure tends not to have as much measurable impact on your static ride height as you might expect. With air suspension, it is generally better to focus on the spring rate aspect of your air pressure setting.

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