Why does my steering wheel shake?

Have you ever been cruising down the road with your car running smoothly, only to find that your steering wheel suddenly starts trembling like a hairless dog on a cold day the moment you hit the brakes? Your immediate thought might be that you’ve driven onto a quake-prone fault line. That’s one possibility, but it’s more likely that there’s something wrong with your car. Now the question becomes, “What causes a steering wheel to shake when braking?” There are several possible answers — some more likely than others.
Vibrations in the steering column can happen for a number of reasons, such as a busted motor mount, bent rims, bad CV joints or loose lug nuts [source: Distad]. But none of these automotive problems would cause a steering wheel to shake only when the brakes are applied. That’s almost always due to a problem with the rotors.

“What’s a rotor?” Good question. Rotors are discs that your brake pads clamp down on in order to stop your car. In other words, they’re pretty important. Why they may cause the steering wheel to shake when braking requires a quick review of how rotors work.
Rotors are usually made of metal and function at a specific width, depending on the make and model of your vehicle. When you apply the brakes, the rotors are exposed to intense heat that must be dissipated evenly across its surface for a smooth stop. However, rotors tend to wear down over time [source: FamilyCar]. As this happens, the metal may become thinned out or warped in certain spots. When the brake pads grip the rotor at high speeds, these imperfections cause a rumbling in the brake pedal that’s then transmitted to the steering wheel.