Subaru WRX STI Excessive wear on outer edges of tire Inspection at your home or office.

Common reasons for this to happen:

Excessive wear on the outer edge of a tire is almost always because your wheels are not aligned. But even though misalignment causes abnormal wear, it may not be the root of the problem. Oftentimes the misalignment is caused by a more serious problem.

• Standard misalignment: Over time, and with generic wear and tear, your wheels will slowly shift, so that they are no longer at an optimal angle with the road. This can also happen when your tires are changed or your wheels are rotated, if a mechanic did not do a perfect job. Altering the angle of your wheels means altering the center of gravity, and altering what part of the tire makes the most contact with the road. If a wheel is misaligned in one direction, it will experience excessive wear on the outer edge, while a misalignment in the other direction will wear out the inner edge.

• Sagging springs: Springs help support and distribute the weight of the vehicle. By doing so, springs help hold the vehicle up at the right height. As springs wear and tear, they begin to sag, and are no longer able to support the full weight of the car. If you’ve ever noticed a car that appears lower to the ground than it should be, it’s likely that the springs were sagging. This lowered height and lack of weight support puts excess force on the wheels, which will often result in the wheels becoming misaligned.

• Worn out ball joints or control arms: The ball joints and control arms are vital pieces of your suspension system. The control arms connect the suspension to the frame of the car, while the ball joints (which act very similarly to human joints), connect the control arms to the steering knuckles. Because the ball joints and control arms are so close to the wheels, their function has an impact on the wheels. If the ball joints or control arms fail (the control arm bushings are particularly susceptible), then your wheels will absorb too much force from the road, and may be jarred such that they become misaligned.