Wheel bearings -- even under the best conditions -- bear a great deal of stress. The smoothest road imaginable still sends minute vibrations through the tire, vibrating the bearings' internal components and accelerating wear. Bumps in the road send massive shock loads through the assembly, and cornering forces try to pull them apart on a consistent basis. It's almost amazing that they manage to spin or bear the car's weight at all.
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Shock loading and lack of lubrication have to tie for first as the wheel bearing's primary assassins, but hard use can break them outright. Shock loading means subjecting the suspension to more sudden movement than it has to experience. This could happen over a long period of time, by operating the vehicle at high speeds over rough roads or no roads at all, or it could happen in one major blow by hitting a pothole, speed bump or curb with sufficient force.
Lack of Lubrication
Even so-called "sealed" bearings require service from time to time, generally in the form of greasing. Grease installed at the factory will eventually begin to break down at a molecular level, which causes a reduction in its ability to lubricate and control heat build-up in the bearing. When the grease thins out, dries up or leaks out, the bearing material will begin to grind, wearing it away and increasing clearances in the bearing. These increased clearances allow the bearing to bind, further accelerating wear and eventually leading to failure. Bear in mind when servicing that using too much grease or the wrong type of grease will cause failure just as surely as using none at all.
Sustained high speeds will do the same things to wheel bearings that sustained high rpm will do to a motor. The difference is that bearings don't have an oil-circulation or cooling system like the engine does, which limits how quickly they can get rid of heat. Sustained high speeds -- meaning anything past highway speeds -- will cause heat build-up in the bearing, thinning the grease and temporarily reducing its ability to lubricate. Thin grease accelerates wear. High speeds, over 100mph or so -- can cause the bearing to reach thermal meltdown and fail entirely.
Manufacturers engineer a car to handle only a given load, and the tires usually dictate how much of that load the chassis actually sees. Put simply, the tires are supposed to slip before anything else gives or breaks. Installing large, sticky tires will increase your car's handling, braking and acceleration limits, possibly past the chassis' ability to handle the forces involved. The car's suspension helps to protect the rest of the chassis, but wheel bearings are much closer to the action. This is particularly true when installing low profile tires, which flex less and transfer far more force to the bearings than they would otherwise see.
Huge Heavy Rims
Now that 2003 has come and gone, somebody needs to tell the masses: huge, heavy rims aren't just passe, they're horrendously stupid where performance and longevity are concerned. Those 20-plus inch "dubs" add an enormous amount of unsprung weight -- weight not controlled by the suspension -- and rotational inertia, both of which constantly hammer on the bearings and suspension. The low profile rubber bands that "playas" tend to wrap around their spinning discs only exacerbate this problem, practically guaranteeing premature bearing and suspension failure.
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