Most modern wheel bearings last a long time. However on high-mileage bikes, or dirt bikes that see a lot of mud and water, and bikes washed with high pressure hoses, the bearings can go out sooner than you think. In the case of our example, a well-used Beta trials bike, the front wheel bearings were completely shot after two years. The bearings were so worn that the wheel could flop around about a half-inch at the rim.
Wheel bearings have a tough life. They are precision made parts subject to large amounts of stress and need lubrication. Typically, they have to rely on a little grease added at the factory, maybe many years ago. When they run dry or start to rust, they will heat up, wear out of round and develop excess clearance.
Most bad bearings aren't immediately obvious, though they will get worse quickly if you ignore the first signs. Bad bearings can show up as a faint 'clunk.. clunk' noise as you push the bike into the garage, or a creaking as you drive. Or you can feel a grittiness in the bearings when you turn them by hand when the wheel is off for a tire change. Usually bad bearings will not turn as easily as good ones and will feel 'lumpy' as you turn them, instead of turning smoothly.
Most bikes use sealed caged ball bearings. The inner and outer races are replaced along with the balls as a unit, and the entire bearing unit is pressed into the hub. Some bikes, most notably BMWs, use tapered roller bearings. These may or may not be replaced as a unit, and often need a fairly specific amount of preload built in when they are fitted into the wheel. Since they're more complex and rarer, we'll ignore the tapered roller bearings and cover the more common caged ball bearings.
Ball wheel bearings are usually a press-fit into the hub with a slight interference fit between the hub and the outer race. To remove them you may need to heat up the hub. While you can drive out the old bearings without heat if you use enough force, heating the hub to expand it makes bearing removal easier and causes less damage to the seating surface in the hub. Beware that too much heat may cause wire spokes to loosen. A propane torch or industrial heat gun (basically a huge 5000 watt blow-drier) works well to heat up steel or aluminum hubs. Oxy-acytlene torches produce too much heat for aluminum hubs, and way too much heat for magnesium hubs. Magnesium hubs should be stamped "magnesium" in a prominent place. Be careful about heating magnesium hubs as magnesium is flammable if you get it hot enough. A rag soaked in boiling water is the best method to heat mag hubs.Page 2 keep those wheels turning...
The wheel should be propped up on a stout box so that the hub can be accessed without pressing on the brake discs. Brake discs are not meant to take side loads and will bend more easily than you think. We propped our Beta's wheel on a crate. If the wheel has a drum brake, remove the brake backing plate and clean out the brake dust with a damp cloth before removing the wheel bearings. Drum brake dust may contain asbestos so don't blow it out all over the shop with compressed air.
When the hub is warm (the hottest it should get is hot enough to touch for a moment without burning yourself -- obviously use caution around anything hot) you can flip it over and drive the bottom bearing out with a drift. The inner spacer between the bearings can move a bit inside the hub and be pushed to one side exposing the edge of the inner race. Once you start hammering on the inner race the bearing is toast -- ball bearings can't take sharp side loads and will get flat spots, ruining the bearing. So don't re-use it.After one side is out, the inner spacer can be pulled out, along with any shims. Important: Note the position and order of the parts. The other side of the hub is then heated and the remaining bearing driven out in the same manner. Neither bearing should take much force to drive out.
Once you have the bearings out, you can clean up the inner spacer parts and inspect the inside of the hub. In the case of our example Beta, the reason for the trashed bearings was obvious -- there were seals on the outside of the bearings only, the insides were wide open to any water that leaked into the hub. So they rusted. The replacement bearings have seals on both sides to prevent this.
Inserting the new bearings is the reverse of removing the old. You can put the new bearings in the freezer while preparing to heat up the hub to receive them. Heat the hub up until water sizzles when a few drops are sprinkled on the hub, then quickly drop the new bearings in and seat them with a hammer. If you don't have a bearing or seal driver, use a socket or piece of pipe of the same diameter as the outer race to seat the bearing. Pounding on the inner race will damage the bearing, requiring you to remove it again soon. Make sure that the bearing starts square to the hub and doesn't get cocked sideways as you drive it in.
Bearings are normally installed with the writing (on the edges of the races) towards the outside of the wheel, and should be seated fully against a shoulder in the hub. Yours may be different: Best to check the manual if you didn't check the bearing when you removed it.
Flip the wheel over, grease and insert the inner spacers and shims, and heat the hub to install the other bearing. You're done! Put the wheel back on the bike, tighten everything up, and you're off, more smoothly than before.