Careful motorcycle owners service motorcycle engine oil using factory recommendations as a starting point—at the least. Some are more fastidious than others and others are positively panic about it— logging the number of hours rather than miles since the last change, or changing early in preparation for a long trip. There's probably even an app for that.
To further complicate matters, we have the choice of standard dinosaur oil, full synthetic oil, or a blend. We have different viscosities to choose from based on manufacturer's recommendations, type of use and environments ridden in. It’s a well-known topic, and it’s possible to indulge your caution right to the brink of distraction.
But how about oil changes for your suspension? How often do you change your motorcycle fork oil? Do you think you should? Has it ever occurred to you that it must be changed?
Maybe you’ve thought about it, but looked in the owner’s manual to see no specifc recommendation to do so. Why should you change your fork oil if there’s no listed service line item? Trust me when I say you are not alone with this quandary and knowledge void as a street rider. Dirt riders, on the other hand, have fork oil services down in terms of frequency but that information is commonplace among those riders. So how did street riders somehow get left out? Fork-oil change frequency as a line item has long been missing in manufacturers documentation, and no one really knows why.
It makes perfect sense to remove old oil because, like engine oil, its performance diminishes over time. Heat, shearing and contamination are the key culprits. Forks move at a very fast rate, sometimes up to 2 meters per second. Think about that. We’re dealing with a lot of movement over the course of a bike’s life.
Refreshing even a non-adjustable shock usually entails replenishing the nitrogen gas charg
Factor in that as the fork springs compress and relax they flex against the inner fork tube surface, adding fne metal scrapings into the oil as well. Over a given period of time, say 20,000 miles, you can imagine the amount of dirt that works its way through the fork seal into the oil, the quantity of metal debris added and the number of heat cycles the oil has been subjected to.
The damping characteristics of old fork oil are very interesting in cooler weather. At first it will resemble cold Maple syrup, not easily stirred. Fork action will be minimal, abrupt and quite harsh. After a few miles the fork oil will begin to warm and change its flow characteristics, providing a little more suppleness to fork travel. That will improve considerably the longer you ride, but only to a point. If you ride for longer periods of time and/or for more miles on your average bumpy roads the fork oil will get thinner and thinner, and then you have little damping to go with your springing.
Modern suspension uses very light oils. A change in viscosity by just a couple of points makes a big difference in suspension compliance and action. All that applies to the fork also pertains to the shock (or shocks). The piston wears on the shock body, picking up metal that then travels through the oil and damping circuit. If the metal chips are large enough, they can cut through seals; even at best, they act like sandpaper to damage the seals over time.
So what’s a reasonable interval? For sportbikes, simple replacement of fuids and seals at 10,000 miles will bring new life to the bike. Touring bikes and cruisers can probably double that interval before the service will offer noticeable improvements. But whatever the interval, it’s certainly not “never.”
Ready to tackle a fork overhaul? Check out the fork rebuild how-to in MC Garage at www.motorcyclistonline.com.