Advice: the importance of suspension servicing

Motorcycle suspension, considered a ‘dark art’ by most motorcyclists, can be simply defined as the management of weight transfer on a motorcycle chassis. And we control this motion by adjusting the suspension damping, i.e. rebound and compression.

Compression is the damping force required to allow adequate controlled bump/squat for a motorcycle, while rebound is the damping force required to effectively control the release of kinetic energy stored within the springs when under compression.

Actions and reactions
The primary components which make up a suspension system are all precision-machined parts with varying tolerances dependent upon whether they are mass-produced or aftermarket hand-built items.

Regardless of the manufacturing tolerances involved, all of these components form a system which – as already described above – allows the control of weight transfer. The intricacies of the system therefore allow for the damping effect to be altered to varying levels. And the determining factor here is the component itself, which dictates the level of in-built adjustability. For example, sports bikes in generally have full preload, compression and rebound adjustment.

Basic physics states that for every action there is an equal reaction – in this case motion. Motion or movement at its very essence creates energy which then produces heat as a by-product. Heat will cause thermal expansion of materials and cause fluid viscosity to alter and this is why a cold bike may feel different to a bike that has been operating for 20 minutes.

Looking inside
One of the most neglected and fundamentally overlooked items on a motorcycle, regardless of its use, is suspension servicing.

Suspension by its very nature is a moving/oil dampened mechanical part (with the exception of air forks), and like any moving part on a vehicle requires maintenance to be at its peak performance.

Here are some suspension parts that may be affected over time:

Bushes
These are Teflon lined, and provide a bearing surface for the fork stanchion and outer leg to move across each other. Shocks also utilise a bush around the shock’s core piston, whether it’s a solid piston found in setups like the Ohlins TTX or Bitubo, or a ported piston in Showa, KYB and Penske shocks.

Oil
Oil will go through heat cycles and it is important to improve performance and consistency with damping by using an oil with a high viscosity index. Viscosity index is the difference of an oil between cold and hot – manufacturers rate this at specific temperatures, and there is also an international rating system – and the higher the index the less the change in the oil’s damping performance. This is key to damping performance, particularly with shock absorbers as operating temperatures can reach in excess of 50 degrees Celsius.

Oil may also become contaminated with minuscule shavings of metal produced by a fork spring as it bows under compression inside the fork. These can work their way through a fork’s valving and in some extreme cases cause a deflection within the shim stack, potentially blocking a shim in an unwanted position which will alter the damping performance and system response.

Seals
They can degrade, get damaged or lose their elasticity, which may cause weeping of oil from the fork or the shock’s seal head. The rubber used in seals will vary greatly, and will be of notable difference between brands. Another thing to consider is rubber sliding up and down the fork stanchion/shock shaft, which causes stiction – and this has some effect on the efficiency of the movement. In this case, stiction is something we wish to minimise and quality seals will help diminish this effect.

Service time
How often should we be servicing the component parts?

Factory teams will do it all the time, looking for vital setup changes or even a full rebuild. Club racers will not have the luxury of such a service – but this is not to say they are not putting their machine under equal stress/duress. As a general rule, a race bike should be seeing a full service on its suspension approximately every 20 hours. A track rider would get away with the minimum of an annual service.

For a road bike, there is much debate. Ideally annually, or at least bi-annually, especially with the amount of mileage and time road riders can spend on their machines. You might think that a road bike will not tax its suspension as much as a racing bike, and as such won’t wear the parts out as quick. However, road conditions can be very poor, especially in regional areas, and there is an argument that this promotes quicker wear.

It’s generally rare that non-race bikes have their suspension serviced unless there is a visible leak which prompts the owner into action. Regular servicing of your motorcycle’s suspension really is important and keeps your bike at its best.

The difference in performance between serviced and non-serviced suspension really is night and day. A service is also not just changing fork oil and seals; it is the complete strip and check of every component part from the individual shims to the pistons, top out springs and beyond, something which will not be a routine thing for your regular motorcycle workshop.