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Warped rotors
not usually. warped rotors will cause a vibration during braking.
vibrations while slowing or braking? Get the front brake rotors checked.
This is a self-exited vibration problem.
Balance your tires!
The brake rotors are warped. Someone tightened the lugnuts out of sequence. That compressed and torqued the rotors out of spec. Replace the from rotors and pads and the rear ones too, if necessary.
Wheel bearings are the most common for causing a vibration.
What percent of braking are front wheels responsible
Your rotors may be warped. They key need to be refinished or replaced.
The braking problem is almost certainly a disk brake rotor. The vibration could be a faulty tire, a bent or damaged "half shaft" or an out of balance tire. Front end vibration is not caused by an alignment problem, but can be accentuated by worn suspension or steering components.
Check your front bumper, it might be loose, or the radiator fan might me hitting the case, or all your nuts are loose and its about to fall appart :P Could use more info like when does it vibrate, at a stop, driving along slow, driving along fast or braking.
My first guess would be the front end alignment and possibly the brakes. During braking, you engine and transmission are not under a load so they should not cause any vibration.