How to Choose Motorcycle Fork Oil
Fork oil lubricates a motorcycle’s fork tubes to slow down wear and corrosion. It also helps with the heat and friction generated from the constant movement of the tubes.
The dampening system in the fork has a rod and a number of valves that allow the fork oil to travel through, impacting both the compression and the rebound.
Which fork oil you choose affects your motorcycle’s handling and performance. When the spring compresses, it pushes against the dampening rods, and the rods force the fork oil through valves or holes.
The rate of the oil flowing through those holes controls the rate of compression and rebound, and you can control the rate by the viscosity or thickness of the fork oil. If the oil is thin, the compression and rebound rates will be quicker. You might feel the fork bottom out and the motorcycle’s front end bouncing. However, if the oil is too thick, it can’t pass through those little holes in the fork as efficiently.
That slows both the compression and rebound, which can make for a really harsh, bumpy ride, since the oil is holding up the springs from allowing travel inside the fork.
Fork Oil Viscosity
The thickness of fork oil is measured in viscosity, or how resistant the fluid is to flow.
The higher the viscosity, the slower the fork oil flows. So the higher the viscosity, the more dampening you get, since it takes more force to get the fork oil through the holes in the valves in the forks.
Lower viscosity fork oil is the opposite. It’s a thinner fluid that travels more freely through the valves, which in turn produces less dampening force.
Fork Oil Weight
Viscosity and fork oil weight don’t always line up. You can end up with lots of different viscosities that are all the same weight, and even end up with higher viscosities at lower weights.
For example, a 5W or 7W fork oil might end up being thicker than a 10W or 15W fork oil, which can make choosing one even harder.
What you need to look at is the viscosity index, which provides a measure of how consistent the viscosity of the oil is across a temperature range. The higher the number, the more consistent the oil is. It also means less fall-off in dampening capabilities as the oil heats up, since it’ll be more consistent. A lower number fork oil will thin out and offer less consistent dampening properties.
How to Choose a Fork Oil
Think about how much weight you’re going to put on the suspension, including passengers and cargo. The more weight you’re going to carry, the thicker the fork oil needs to be.
Also, consider the surfaces you ride on regularly. Are you riding on public roads? Do you ride off on trails on a dual sport? Maybe you’re racing on really smooth asphalt, so you won’t need anything too thick.
Finally, consider your personal preference when it comes to the handling on the motorcycle. A big cruiser needs something thicker to handle the additional weight, while a dirt bike might do better with something thinner and more reactive.
Fork Oil Manufacturer Recommendations
Your motorcycle’s manufacturer offers recommendations as to what kind of oil the fork should take, and it’s a good idea not to stray far from that.
You might want to move a little bit one way or the other, but make small moves. So if the manufacturer says to use a 5-weight oil, maybe go to a 5-weight oil with a higher or lower viscosity index. Maybe get an oil that’s a little heavier, but don’t make a big jump from say, a 5 to a 15 weight. Too high of a difference drastically changes what happens in the fork and how the motorcycle rides.
Moving around that viscosity index allows you to fine-tune the ride, as opposed to jumping to a much heavier fork oil and hoping for the best.