We will only deal with the Shimano freehub here. This is the single most common type and the most commonly available replacement part. You have to remove the rear axle for the job, so it’s worth servicing the hub at the same time. A freehub doesn’t last forever and can get clogged up with muck. It can also suffer badly if you catch it at the wrong angle with a jet hose.
The first sign of trouble is usually a regular slight slipping as you pedal. This feels very similar to a worn chain and cassette, so check those first, but if you measure your chain and it’s in good condition, the freehub is the next suspect. Take your back wheel off the bike and turn the cassette gently anticlockwise. You will hear the pawls of the ratchet click. Try to turn the cassette clockwise again – you should feel the pawls catch and stop you from turning the cassette. If the pawls catch, turn the cassette another click anticlockwise and test again. You should be able to turn the cassette a complete turn anticlockwise, testing after every click, and the pawls should catch every time. If they slip, so that you can turn the cassette clockwise more than a tiny bit, you have sticky or broken pawls in your freehub body. Replace your freehub.
Compatibility between freehub bodies
Back in the early days, back wheels had old-fashioned freewheels with six sprockets. After a few years, this standard was replaced by a stronger design – a freehub with wider-spaced bearings and seven sprockets. Space was found within a couple of years for an extra sprocket by making the freehub body longer.
Seven-speed cassettes would not fit on the longer eight-speed freehub, nor would eight-speed cassettes fit on seven-speed freehub bodies. However, since the distance between each sprocket was actually the same, the two types could both work with the same width chain.
The next sprocket, increasing the cassette from eight to nine speeds, happened in a different way. Nine-speed sprockets are narrower, with less space between each one. So nine-speed and nine-speed cassettes will fit on the same freehub, but the ninespeed system uses a narrower chain that it fits into the reduced gap. Eight-speed chains won’t work on nine-speed sprockets, nor will nine-speed chains work on eight-speed sprockets.
Follow the instructions for servicing a rear hub. Once you’ve removed everything – locknuts, cones, axle and bearings – follow these steps to replace the freehub. Then finish the hub servicing procedure.
“If you measure your chain and it’s in good condition, the freehub is the next suspect”
Step 1: The freehub is bolted into the wheel with a 10mm Allen key bolt (A). The bolt is recessed deep in the hub, so you’ll have to wiggle the Allen key in there carefully from the right-hand side of the hub. Engage it securely.
Step 2: The freehub bolt should be done up very tightly, so you’ll need to be firm with it to undo it. Stand with the wheel at your feet with the freehub facing away from you. Turn the wheel so the Allen key is horizontal, on your right-hand side. Hold the wheel with your left hand and push down hard on the Allen key with your right hand. You may find that you need more leverage. If you can find a tube that fits over the Allen key, use that for extra help.
Step 3: Once the freehub bolt (B) is loose, undo it completely and pull it out. Clean it; you can reuse it for the fresh freehub (C). Clean the area exposed behind the freehub, line up the new one and pop the bolt through. Tighten it really hard – as tightly as the one that came off. Use your ’extension tube’ again if you need it to get the bolt off.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Removing and refitting cassettes for rear-hub services”