How to service your forks

Published: 09 April 2016

You wouldn’t expect your engine to keep going if you didn’t replace the oil and give it a service – so don’t make that mistake with your forks. Give them a service and be rewarded with a front end that feels like new.

ervicing your forks is a very rewarding job because you can feel the difference. You’ll get more feedback, more surefooted cornering and better braking from a well-behaved front end. You’re also doing something that even a bike with the best service record may not have had done – a fork service is in very few service schedules.

How do they work?
Bike forks have two main components – springs that spring enthusiastically and dampers that dampen said spring. Dampers control the spring by forcing oil through holes on damper rod forks and through a stack of shims on more expensive cartridge ones. Over time the oil goes off and although a fork is sealed the oil will pick up metal particles from the insides and dirt gets past the seals. On cartridge forks these particles can sit between the damping shims, giving the oil an easier path and so weakening the damping. Seals start to leak and those the dampers will lose their control.
But they can be rebuilt and it’s easier than you think, as we’re about to demonstrate on our old Fireblade. We can also replace the fork seals if they’re leaking, give the damper a thorough clean and replace the fork oil.
 
Parts you may need
Fork seals, £8.50 a pair and dust seals,  £11.75 a pair from www.wemoto.com
Top and bottom bushes (if play is felt), £21 each, from www.davidsilverspares.co.uk
Bottom nut washer, £2.50, from our local Honda dealer
Fork oil, £14 per litre
Brake cleaner (buy a five litre container and pressurised bottle from a motor factor) £8
Paper rag (lots of) £4
Tea and biscuits, priceless

Any special tools needed?
A vice on a bench or a Workmate
A manual is really useful
Torque wrench
Measuring jug
Latex gloves – things are gonna get dirty
Ice cream tub
A length of exhaust pipe – straight – the same diameter as your fork seal

1 That oil residue is a sign that all’s not well. Oil is getting past the knackered fork seals and attracting dirt. Once they start to weep things will quickly get worse and oil will flow down to your brakes . You really don’t want to let that happen.

2 All forks are different but most bikes will have a bolt in the bottom yoke, a smaller one in the top yoke and an even smaller one in the handlebar. Don’t take them out, just loosen them. Slide out the fork and give it a good clean.

3  Undo the bottom bolt that holds the damper rod or cartridge in place. The spring is pushing down and should prevent the innards from turning. Make sure to turn the preload adjuster, if there is one, to max. Watch for the oil coming out.

4 Undo the top cap. Clamp the chrome stanchion with soft vice grips and cardboard and be careful not to clamp too tightly – this could oval the stanchion. Once undone, drain the oil into a container. Dispose of it at the local recycling centre.

5 You should now be able to pull all the innards out in one piece. Clean them with brake cleaner, check the spring for cracks and take notes and pictures of the way it all goes together.

6 You need to undo the top cap from the damper rod to be able to take the spring off. Hold the nut with one spanner, the preload adjuster with another, and twist the cap – not the nut.

7 Measure the thread once the top is off. This will help with set-up when you put everything back together again as it affects how much the preload adjusters tension the spring.

8 Before removing the stanchion take the dust seal off carefully with a small flat screwdriver between the fork leg and seal. Remove the circlip. Once this is out pull on the stanchion like a slide hammer. It should come out with the bushes and seal on it.

9 Lay everything on the bench so rebuilding is straightforward. Check the new and old oil – that’s got to make a difference. Clean everything with brake cleaner and inspect the chrome stanchion for straightness and any pitting.

10 Damper rod cleaning is simple but a cartridge takes more time. With the brake cleaner in a tub, push the rod in and out to suck in the cleaner and force the old oil out. Pump for several minutes with vigour to help clean those shim stacks.

11 The top and bottom bushes keep the stanchion from moving sideways. Check the face for wear. It should look like these – a grey finish with no brass colour showing through. Don’t take the lower bush off unless you’re replacing it.

12 Refit the stanchion with fresh oil on the bush and then pop an oiled top bush back on. This should just wedge into its seat – it’ll get forced in when the seal is pushed down.

13 Now oil the seal and fit it over the top of the stanchion and push down. Coat the stanchion too as the seal can easily get torn when doing this. It should be a tight fit.

14 Now put the old seal on top of the new one. Get your exhaust tubing (or pipe) and carefully but firmly drive down the seal and bush into place. Keep going until you see the channel for the circlip and then fit that.

15 After cleaning the thread and replacing the washer on the bottom bolt, re-fit the cartridge or damper rod. Don’t forget the cup at the bottom and use the right torque setting. Fit it with the spring in and screw the top on.

16 There are two ways to measure the correct amount of oil. One is to measure the amount – for our Fireblade 509cc. The other is to measure the air gap – stand the fork upright without the spring in and the stanchion fully down. Measure the gap from the top of the fork to the oil level. Ours should be 115mm.

17 Add a little oil at a time, pumping both the damper rod and stanchion 10-15 times each time. This is to get rid of any air in the cartridge or damper and will help get that smooth action. When all the air is out it should feel smooth but stiff. Slack off any preload adjuster.

18 Pop the wheel back in. Bolt the calipers back on and pump out the brake lever. Double check all the bolts. After the first run there may be a little residue oil on the stanchion. Clean this off and check for the next couple of rides. Your forks should feel smooth and slightly stiffer. Enjoy!

Words and Pictures  Matt Hull