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Anatomy of a fork

13:40 16th October 2008 by Bikemagic
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While only RockShox refer to their suspension products as Black Box Techology, we tend to treat all boingy bits as “black boxes” – things that we just trust to get on with their job and don’t worry too much about what’s going on inside. It’s an approach that works up to a point, but occasionally you’ll need to take at least a little look at the internals. Before we start delving too deeply, though, we’ll take a quick look at the anatomy of a suspension fork including simple things you can do to extend maintenance intervals and stuff to keep an eye on.

fork_overview (4K)

1. Here’s a fork (this one’s a Marzocchi MX Pro). Arguments will doubtless rage about whether it’s “a fork” or “a pair of forks” but we’re going with fork singular if only because it’s quicker to type. Most conventional forks look fundamentally the same as this, with two legs and a steerer tube. The only exceptions are things like Cannondale’s Lefty and USE’s Sub, which feature just the one leg each.

fork_crown_steerer (6K)

2. At the top of the fork is the steerer tube. This is the bit that turns inside the headset bearings and has the stem clamped to the top of it. At the other end is the crown, usually a forged aluminium part that holds the steerer and both stanchion tubes. Once upon a time the various tubes were clamped in to the crown with pinch bolts, but most forks these days use some sort of press fit. As a result, there’s little that can go wrong here and no bolts to check, but any movement or damage is generally beyond the realm of home mechanics.

fork_stanchions (3K)

3. The stanchion tubes (otherwise known as upper legs) are generally either anodised aluminium or chromed steel. The top end of the stanchions is one of the highest-loaded bits of a fork, so it’s the stanchions that get beefed up for heavier-duty work – you won’t encounter many forks with 1in stanchion tubes any more, but they’ll range between 28.6mm (1-1/8in) and 32mm or even bigger on some downhill-specific forks. Keep an eye out for any nicks or dings in the surface of the stanchions. They need to be perfectly smooth to avoid seal damamge.

fork_seals (4K)

4. The seals live in the top of the lower legs (or sliders). The ones that you can see are usually only half the story. They’re the wiper seals and are designed to clear any crud off the surface of the stanchions before the actual seals get there. Under the wipers there’s often an oil-soaked foam ring to maintain a thin coating of oil on the stanchions, with the actual fork seal that keeps damping oil (and air on air-sprung forks) inside. It’s a good idea to spray a little bit of light oil on the stanchions after you’ve cleaned the fork. It’ll displace any water and keep the seals lubricated. Don’t be surprised to see a thin oily ring on the stanchion at the full travel point, but if lots of oil is coming out then the fork’s going to need attention.

fork_topcap1 (7K)

5. In the top of each stanchion there’s generally some combination of top caps or adjuster knobs. Many modern forks have the springs (coil or air) in one leg and the damping mechanism in the other. Usually the damping leg is the right hand one to keep the oil away from heat from a disc brake. Knobs on the damping leg will adjust the speed at which the fork moves, while those on the spring leg will alter preload (and thus ride height). Air-sprung forks will have an air valve. You may also have some sort of travel adjust lever. All of these things should move easily, have some discernible effect and not dribble any oil (although quite often you’ll get a bit of oil spurting from air valves, which is nothing to worry about).

fork_bolts (4K)

6. Down at the bottom of the fork there are some important bolts that stop the sliders falling off the stanchions. On some forks you’ll also find damping adjusters and air valves down here too. There are usually O-rings inside the fork to keep oil inside – there shouldn’t be any significant amount of oil in evidence at the bottom of the fork legs.

fork_dropouts (5K)

7. Also at the bottom of the fork are the dropouts and disc brake mounts. Carefully check these for distortion or cracking. It’s a fairly rare occurrence but broken dropouts aren’t unheard of and can prove nasty. Quite a few forks aren’t guaranteed if you use a fork-mounting roof rack to carry your bike, which suggests that frequent dropout checks are even more important if you do use one. And make sure the front wheel’s in properly before you set off.

So that’s what all the bits are called. Check back in next week for some simple fork maintenance…

More maintenance

There’s hundreds of top maintenance tips in the BM archive.