Suspension oil: is it as important as your engine oil?

Most of us rightly treat engine oil in our bikes, particularly in 4 strokes, with a fear level equal to having a pet alligator loose in the back seat of the car. It's not if that situation goes bad, it's when.

A very unique metaphor for MX bike oil change frequency.

To mitigate the risk of a spontaneous engine grenade scenario followed by significant wallet surgery, we keep engine oil and filter changes as frequent as we can possibly afford. It's not unusual for engine oil on a 4st to be changed every outing, which for MX can be as little as 1hrs run time.

But how often does everyone change the oil in their forks or shock? Should it be treated any different to engine oil? Are the consequences any different?

The first thing to say is yes, obviously, suspension oil should be treated slightly differently to engine oil. The simple lack of combustion or rotating parts within your suspension internals means that oil degradation has less catastrophic consequences. Lack of suspension maintenance doesn't often end in a scenario where everything melts together and needs replacing at a cost similar to a weeks' family holiday in Spain.

However, the performance drop off of the suspension system when oil 'wear' is high is significant. It should be argued that the performance impact of worn oil is much higher than using engine oil past its use-by-date.

To explain that, we start with a very simple explanation of how suspension dampening (compression or rebound) works. As the forks (or shock) move up and down, the speed at which they compress or extend is dictated by the oil movement inside them. The oil is pushed through holes that control the speed, or the amount of oil that can flow. The bigger the hole, the softer it feels and the smaller the hole, the harder it feels. So as the oil is core to that function, you don't need to be a suspension expert to see how important it is.

The dampening process changes kinetic (movement) energy into heat. Don't ask us why, we don't know. It's far above the simple minds at A-Spec. It's to do with Isaac Newton and apples; every action has an equal and opposite reaction. All you need to understand is that heat is generated within your suspension.

Now; oils change viscosity with temperature. The hotter it gets, the thinner the oil, the colder it gets, the thicker the oil. If you've ever drained the oil on your bike on a cold winters night compared to 20mins after a moto, you'll get that!

So suspension oil must be stable at different temperatures even though it itself is creating heat? What a conundrum.

To fix this problem, the clever oil boffins add something called viscosity index modifiers. Viscosity index modifiers are added to a thin base oil to thicken and generate the viscosity needed for the oil, but the viscosity index modifiers keep the viscosity stable over a much wider operating temperature range. Cool! (pun intended).

Imagine viscosity index modifiers as neatly rolled balls of dried noodles before you cook them. At low temperatures, they float around in a ball in cold water quite happily. As the water boils, the noodles start to cook, unravel and spread out in the water they are boiling in. This is exactly the same principle with the viscosity index modifiers in oil. The higher the temperature of the base oil, the more the viscosity index modifiers unravel or extend. When that happens, they increase the resistance to flow and therefore maintain the all important viscosity.

A-Spec noodle based suspension science. You're welcome.

Now this is the really important part. As the oil 'wears', those viscosity index modifiers become less efficient and start to break-up. If they're not capable of unravelling properly or they're simply broken into smaller lengths, the viscosity characteristic of the oil will be impacted and you'll get potentially dangerous, but definitely under-performing, suspension. Not cool (again, pun intended).

So this is why you need to change the oil in your suspension more often than you may think. Ignoring daft noodle comparisons, there is a serious technical reason for it. Oil will degrade and become less able to deal with temperature, leading to a performance shortfall. This is something we are always trying to avoid with riding or racing dirtbikes.

For this reason, along with contamination within the oil from mechanical wear of bushes etc, means we recommend that you change the oil in your suspension every 40hrs depending on the type and intensity of your riding. Don't kid yourself into not properly servicing your suspension if you think you're a serious competitor.

Suspension servicing doesn't need to be expensive. A simple service is available from A-Spec Suspension for as little as £130 until the end of January. You might be surprised with how much difference it makes! Get in touch now via the website, Facebook, Instagram or WhatsApp on +447490632182.