Originally Posted by 86Scotty
why is it usually the driver's side that comes loose?
From this very interesting article:
"The mechanism for the left side wheel nuts spinning off as the vehicle travels can be understood from the geometry of the wheel and studs when the wheel is slightly loose. Since the stud holes are larger than the studs, the wheel is not perfectly concentric with the axle when the nuts are loose. When the road pushes up on the tire, the wheel tends to be pushed up relative to the axle centerline. This means the wheel centerline is slightly above the axle centerline. This centerline offset gives rise to a relative velocity vector between each wheel nut and the part of the wheel the nuts touch. This vector is in the loosening direction on the left side when the vehicle is driving forward and is the cause of wheel nuts spinning off the left side.
The right side nuts have that same relative velocity vector, but in the opposite (i.e., tightening) direction. It turns out that this vector is not strong enough to make a loose nut tight again. So, on the right side, a loose nut tends to stay loose rather than spin off. But this invites another mechanism – fatigue. When a nut is tight, the clamping force creates large frictional forces at the wheel/hub interface that transfer the vertical forces that support the weight of the car. However, when a nut is loose, there is no clamping force, and the studs now carry the vertical forces. This bends the studs up and down every time the tire rotates. Just as a paper clip breaks when you bend it back and forth a few times, a wheel stud can break when it is bent up and down a few million times. This is called reversed-bending fatigue, and is the reason that right-side studs eventually break off when the nuts are loose."