Shims - What They Are And Do

Everybody seems to talk about shims and only few really know what they are. But once you have an understanding of how complex the re-valving business actually is you will not let anybody near your suspension who is not an absolute expert in this field. When the internals of your suspension are messed up it is a mission to get them back on track and the handling of your bike will suffer enormously.

So let’s check out how suspension settings are done:

Most production suspension is designed fairly similarly from manufacturer to manufacturer, and the basic principles along which it works are almost identical. Sure some brands have advantages over others when it comes to friction reduction and efficiency of the system, but these differences get smaller and smaller as systems are refined.

The biggest differences come in the final set up. Final set up involves spring choice and internal damping settings to suit the motorcycle and its intended purpose (e.g. 2-stroke, 4-stroke, enduro, mx).

Getting the correct damping setting inside the suspension is an extremely time consuming and complex subject. Damping is controlled by a number of internal valves restraining the oil flow. And those valves are adjusted by means of stainless steel washers called Shims.

These washers flex and are made in many different thicknesses and different diameters. They control the opening and closing of these valves. How they are stacked and configured directly translates to how much resistance the oil needs to pass the valve and the speed the oil is forced through these valves.

Let’s say you take a big jump on an mx track. The movement of your wheels in relation to you and the bulk of the bike are controlled by those valves and their washers. All this energy and that movement - quite a big task! They are the core and heart of your suspension and they are that important.

Simplified Shim Stacking Illustration:

There are up to 150 of those washers in one fork and with all the different sizes and diameters there is literally millions of ways to configure the valves. This means that finding the perfect valve settings is almost as hard as winning the lotto!

Manufacturers spend months testing with each model to get this right and generally they do a pretty good job. The technicians doing this work have a good idea of how to tune these valves efficiently. But don’t expect the average dealer to know how to re-valve your suspension and actually make it better. Even if they claim that they are suspension specialists. You should be able to trust them with a service or to change springs but don’t let anybody mess with the internal settings of your suspension!!

There are only really 2 cases in which you need to re-valve:

  • Your you are using your bike not for its intended purpose e.g. mx bike for enduro
  • You are a top rider and you need your suspension to work 100% in a perfect personalised set up for your riding style

If you are a hobby rider or even a hobby rider with ambitions it is more than enough to get the right spring rate, set the correct sag and have your suspension serviced properly and you will have a perfect ride.


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