You waited too long, didn't you?
Now you've got a mess and a job ahead. Hopefully you already bought the replacement parts otherwise ride day can wait. Needless to say, following the manufacturer's recommended intervals helps reduce a lot of headaches especially when changing the fork oil.
Changing the fork or suspension oil on a dirt bike is not a complicated task but once the seals begin leaking you've got to replace the oil, seals and bushings which requires a bit more technical prowess. Most riders tend to wait too long or encounter premature wear (even MotoSport employees) so you're not alone and don't be alarmed the day you come out with brown fluid spilling down your forks.
Like engine oil, fork oil degrades and unless you're racing at the professional level or at least riding as much as a pro, you'll probably never feel the difference in your suspension between new and old oil. Old fork oil does affect your suspension but it often goes unnoticed. Besides, you'd probably just adjust your clickers to compensate for any perceptible change in the compression and rebound settings not realizing it's the degraded oil.
Regardless of whether you wait until the seals leak or follow the recommended intervals, fork oil eventually requires changing just like motor oil and here is a general overview of how it's done on spring forks:
Upper Fork Tube
Changing the oil in the upper fork tube is pretty straight forward and the most common application towards routine fork maintenance, as long as the seals remain intact.
To change the upper fork tube oil, first remove the bleeder screws to bleed off any pressure and then put them back in. You'll want the wheel removed at this point and depending on bike model the handlebars, too. Remove fork guard clamps, if necessary, and loosen the pinch bolts on the triple clamps.
Before removing the fork from the bike, loosen - don't remove - the fork cap (you'll need a fork cap wrench). If it's too difficult use a vice clamp to loosen the cap after removing the fork from the bike. Once the cap is loose, the upper fork legs slides down the lower fork leg. This is full of oil so be careful! Tip the fork upside down to drain the oil. It's a slow process so it's best to lean the fork on a wall while it drains into the pan.
Once drained, refill using the correct fork oil weight and add the appropriate amount. Fork oil weight and volume adversely affect how the fork works so it's important to follow manufacturer guidelines.
Dang, no dipstick. How do you measure the correct amount of suspension oil? Remember, always refer to the manufacturer's recommendation. Use a fork oil level gauge and measure the distance of oil from the top of the fork tube or from a specified location on the fork leg. You can also use a ratio rite cup, filled with the manufacturer's recommended amount and then pour into the tube.
Reattach the fork to the bike.
Replacing Fork Seals and Bushings
If the seals leak then you have to disassemble the forks to get at the inner cartridge. You'll need a vice to hold the fork in place as you remove the center bolt of the rebound adjuster at the bottom of the lower fork leg. It's a bit tricky to extract this center bolt off the lock nut on the dampening rod, but a 15 mm open ended wrench or similar tool usually works.
Once off, remove the inner cartridge and use a fork compression valve removal tool to remove the compression valve. Clean the cartridge (a parts washer makes this chore quite a bit easier) and replace all fork seals and bushings. Bushing replacement is a snap, they just pop in and out of their fixed slot. Replacing the seals requires a bit more finesse:
- Place a plastic bag over the lip of the lower tube
- Slide a greased seal over the bag
- Put the lower and upper tubes together
- Push the seals up to seat using a seal driver to provide equal pressure
- Once seated, put the cartridge in and reassemble the fork
- Pour oil through the upper tube to fill the inner cartridge
- Pump dampening rod to remove air, add additional oil as needed
- Once the inner cartridge is full, fill the upper fork tube with oil
- Use the fork level gauge to measure the appropriate level
- Tighten fork cap and reinstall fork on bike
On your first ride with fresh oiled forks, pump the forks a bit prior to heading out and take it easy at first just in case air persists in the forks. Bleed them by removing the bleeder screw, if necessary.
Written By: AndrewT