Wheel bearings ?

A bearing is a mechanism to allow two surfaces to rotate over each other smoothly. They are also designed to take some amount of force, be it paralell or perpendicular to the rotating surface, or both. They work on the premise that things roll over each other with less wasted energy than they slide. They generally take the form of balls or rods between two round housings.

If your wheel bearings go, worst case is that your wheel becomes very difficult to turn. Worst case is that it locks, binds, and you leave the bike the unpleasant way. I'd reccomend you get them changed ASAP. They shouldn't be too much to do, if they're already doing the wheels.

chaos14, with the amount of lateral force applied to the bearings, they're pretty likely to bind before you necessarily notice something "wrong" with them. If they're already in bad shape, the chance of one of the bearings ceasing to roll, causing the three surfaces (inner race, roller, outer race) to slide over one another instead of rolling is pretty high. Factor in friction caused when the wheel is rotating at > 100rpms, and it's not so "dooms day" to concieve of the bearing freezing.

I appologize; I wasn't clear. Yes, at high speeds it's more likely for the bearing to continue sliding if the bearing stops rolling. This increases friction, and once it gets to a speed where the momentum of the wheel doesn't overwhelm the bearing and cause it to slide, the bearing locks the wheel up.

My point stands; it's something that's quite important and shouldn't be delayed in being changed.