Why does your car pull to one side when you take your hands off the steering wheel?

Why does your car pull to one side when you take your hands off the steering wheel?
If your car pulls to one side when you release the wheel it could be one of a few different things. Most commonly this means it needs an alignment. There are three different alignment settings...toe, caster, and camber.

Toe is the most easily adjustable and is the most often needed adjustment. It is the measurement from the front of one tire to the front of the opposite side tire, or rear of one to the rear of the other. When you turn your wheel, the motion of the tires is the motion that toe adjustment sets, but it is in relation of one tire to the other and how straight they travel down the road in a parallel relationship to each other. This setting will cause tire wear, but it is a common misconception that it will cause a car to pull. It will not cause a pull. It will however cause your steering wheel to be off center, or crooked.

Camber is the adjustment of the top of your wheel leaning in or out in relation to the bottom of your wheel causing the tire to ride more on the outer edge of the tread, on the inner edge of the tread, or squarely on the center of the tread as the tire was intended. This setting will cause tire wear, will cause a car to pull one way or the other, and in extreme cases of being out of manufacturers specs can cause unstable and erratic handling. Some cars have adjustments from the factory for camber, though an increasing amount of cars require purchase of aftermarket cam bolts or bushings in order for the camber to be adjusted. This is particularly true of a lot of imports like Toyota and Nissan though it varies my make and model.
Caster is the least needed adjustment and is in some cases not adjustable on some makes and models. It is the measurement of the angle of a line drawn through your upper and lower ball joint in relation to the ground. Imagine a line traveling from the top of your tire to the bottom that is straight up and down. If you leaned the top of that line toward the front or the rear of your car, that would be the movement of the caster. The rake angle on a bike is the same thing. Caster adjustment affects the straight line stability of a car and how quickly and easily a car will turn into a corner. Caster does not cause cause tire wear and will not usually cause a pull, except in extreme cases of caster offset between one tire and the other.

Sometimes a manufacturing flaw in a tire will make it cause a pull even when the tire is brand new and with a perfect alignment. This is commonly called a radial pull. To diagnose a radial pull, swap the front tires and drive the car again. If it is a radial pull, the car will now pull on the opposite direction as it did before.

Some other causes are tires already irregularly worn from a bad alignment, worn shocks, or lack of proper rotation. Also, mismatched size tires on each side, uneven tire pressures on each side, and early signs of tire malfunctions such as a broken or separating belt inside of the tire carcass. Broken or separating belts are most commonly accompanied by a shake or vibration and may or may not cause a pull.