Before you start playing with spanners and ripping your hubs apart, check that they fit neatly into your frame – ill-fitting hubs won’t clamp securely into your frame or forks and will wear quickly.
Hub spacing and fitting
Almost all mountain bike hubs come with the same spacing: the distance between the inside of the dropouts (technically called the ’over locknut dimension’ – OLND). This is 100mm (4 inches) at the front and 135mm (51/4 inches) at the back. A ’dropout’ is the part of the frame or fork with a slot for your axle. On forks, there’s a dropout at the end of each fork leg; on frames, the right-hand dropout also has a hanger onto which your rear derailleur bolts. Road bikes use 100mm at the front and 130mm at the back.
It’s important that the hubs fit snugly into the space between the dropouts. If the hub is more than a couple of millimetres wider or narrower than the frame or fork, the dropouts clamp it at an angle, rather than straight on. This puts uneven pressure on the hub bearings, wearing them out more quickly and, in extreme cases, bending or snapping the axle or the dropouts.
Each face of the dropout needs to be in good condition, so that the axle and skewer can clamp securely onto the surfaces. The locknuts on the outer end of the axle and the inner clamping face of your skewer are both knurled (notched) to increase grip. If you’ve ridden the bike with loose wheels, these notches will tear up the dropout surface, leaving the wheels less secure, even if you later tighten them correctly.
Care needs to be taken with your dropouts when you transport your bike too. They’re very strong when the wheel is clamped in place, but are vulnerable when the wheel is removed. If you remove the wheels to put the bike on a plane, fit an axle support between the dropouts. You can usually pick one up at your bike shop – new bikes come packaged with them to protect the forks in transit.
Removing the front wheel and clamping the dropouts to transport your bike on a car roof rack can damage your suspension forks. Any movement between forks and clamp will twist the dropout, weakening it. Some fork manufacturers refuse to warranty forks with cracked dropouts if they’ve been carried in this way.
For me, getting dirty is a feature of biking. It might not happen every time we go out, but there’s no point pretending (as, ahem, in the photos in this book do) that we always stay clean. Muddy tracks and roads aren’t what designers working on bearings in clean studios consider as ’running conditions,’ so your bearings definitely work best if you can keep the outside world out of them. Ideally, you create a completely watertight seal between your bearings and the world. Unfortunately, one of the features of bearing componentry is that one part needs to move smoothly against another. This means that at some point there’s going to be a gap between the part of the wheel you want to move – the hub, rim and spokes – and the part you want to stay still attached to the bike – the axle.
But since we ride in grubby places, there’s the issue of trying to stop detritus entering the gap between the hub and axle. Anything you put in the gap has to be carefully chosen to keep out dirt (so it has to touch both moving and stationary parts), but it cannot rub at all (so it’s best if it doesn’t touch either part). This is contradictory but true – a great deal of physics has gone into the problem. The practical upshot is that dirt gets into your hubs, and occasionally you have to get it out.
Different kinds of hubs
One major difference between cheap and expensive hubs is how well they’re sealed. More expensive design and manufacture mean that the gap between moving parts can be made as small as possible, without the parts rubbing. Stronger, stiffer hubs also seal better – cheaper versions will flex under pressure, creating gaps around the seals. However, anything that completely keeps out the dirt won’t spin around, so no seal is perfect, however much you spend on your parts. When you service your hubs, check the condition of the seals as you go along. Tears and cuts in the seals will allow rain and dirt into your bearings.
Many hubs have a large external seal protecting the exposed portion of hub and axle. It is often in the shape of a black cone and is usually found on the left-hand side of the rear hub and on both sides of the front hub. These help a lot, but if they get dry they squeak as they turn. To cure this problem, peel back the edge of the seal, clean it if it is dirty and drop a drip of oil onto the interface between seal and hub. As for the rest, learn how to strip and rebuild your hubs, so they turn round smoothly without ever letting in a mud-grinding paste.
“Dirt gets into your hubs, and occasionally you have to get it out”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Hubs: bearings”