You may not realize that your hub is loose for some time. However, your riding will probably feel “off” – cornering becomes sloppy and descents feel sketchy. On the rear wheel, shifting may be off. You may also feel or hear a “clunk” that seems to be coming from your suspension (fork or shock). You may find that tightening your quick release skewers seems to help temporarily, but the problems quickly return…
Often, this is a sign that your wheel hub is loose. When the hub is loose, the wheel bearings can move from side to side, whether they are ball or cartridge bearings. Even though this is only a small amount of movement, the distance to the rim amplifies the wobble.
Hold your bike steady, grab the wheel at its highest point and move it from side to side. If it moves at all, sometimes with a clunking noise, your hub is loose.
If your hub has press-fit cartridge bearings, it might not have any way of adjusting the hub tension. In that case, you need new bearings.
For all other loose hubs, you must tighten the bearings back down. However, it’s easy to overdo this tightening and end up with a wheel that doesn’t spin freely, causing premature bearing wear.
Hubs are normally tightened using cone nuts. “Cone” refers to “cup and cone” – the two pieces that loose bearings sit in. Even with the move to cartridge bearings, the nuts are often still called the same thing. On some wheels (Mavic, among others), the nut may not even look like a nut – it may be a flat disk with holes in it that a pin spanner fits into.
What you are looking for is a pair of nuts on the axle near where the spokes attach to the hub. They will be thin and typically are placed side by side – the outer nut “locks” the inner nut in place.
The trick is to loosen off the outer nut, tighten down the inner nut, and then re-tighten the outer nut without moving the inner nut at all. On the trail with nothing but a big fat crescent wrench, this is going to be a curse-worthy task. If your wheels have a different system (such as the Mavic one described above), you will most likely need to improvise a tool from Allen wrenches or a screwdriver. These systems often have no lock nut.
As one hub manufacturer puts it, “The correct amount of play is just slightly more than no play.” Or, in real terms, tighten the cone nuts and then spin the wheel while holding the hub axle in both hands. You should feel no grinding or binding, but you should also feel no wobble. This is normally the equivalent of tightening the cone completely and then backing it off between one sixteenth and one eighth of a turn, but it will depend from hub to hub.
Tip: If you realize that your bearings have been loose for a while it may be time for a hub rebuild. With any luck, you won’t have scored the bearing race inside the hub.
Tip: Some front hubs are press-fit. They use cartridge bearings behind push-in axle end caps and rely on the quick release skewer to hold them in place. If your hub has no cone nuts or other adjusting mechanisms and your wheel is wobbling, it’s probably time for new cartridge bearings. Luckily, it is unlikely that worn cartridge bearings will do any lasting damage to other components so feel free to ride out.