Does anyone know what the mechanisms are used to keep the lug nuts from backing out from the wheel?

This was after a discussion. I thought that it might be the centripetal motion of the wheel that causes a torgue on the nuts that drive it into the car. Someone else said that lug nuts are usually rounded or tapered and so are the the wheels, this causes increased surface area and thus increased frictional force that help the nuts from backing out.

When you say "backing out", I assume you mean loosen and rotate in the off direction.  

The mechanism that prevents this on a wheel joint is the same as it is for any threaded fastener joint: frictional forces between the contact surfaces of the fasteners and the clamped members.  The frictional forces are the result of preload.

Modern automobile wheel joints use either conical or spherical contact surfaces between the lug nut and wheel.  The use of these nonplanar contact surfaces removes play between the wheel and hub, and allows shear force transfer between the wheel and hub in the absence of preload, which is for enhanced safety.  

This joint is one of the few safety-critical joints that an end user actually loosens and retightens, and with the large variations present in threaded fastener joints, it is important to provide a larger operating envelope.

Regards,

Cory

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I dont know, though I have assumed that maybe the flex of the rim (it pulls in a bit as you tension the bolt / torque the nut) and provides some constant preload to facilitate retaining tension within the connection, similar to the action of a spring washer.
Regards,
Lyle

Steel wheels often do have a built-in 2D belleville washer curve with relief at the bolt/nut, and contact at a 2 diameters, larger and smaller than the bolt circle.  It looks to me like that would do 2 good things.  First provide some stretch during torquing, and second to prevent a rocking action at the bolt/stud when the relatively thin steel wheel flexes during operation.

truck wheels have a maintenance re-torquing schedule. Gotta think if the studs were 6 diameters long they would have enough spring action to maintain torque even on the solid wheels

I think the mechanism behind torque bolts / nuts / etc. is fairly well established.  This is used in many parts of the car - head bolts and connecting rod bolts immediately coming to mind.

If I rememeber my college classes right, stretching the bolt generates a HUGE!!! clamping force and most of the force that is resistant to "backing off" is in the threads - friction - which is why you will see much discussion about the use of oil and antiseize compounds on torgued bolt threads.

Read a post on this site in another section, which described the use of lock nuts in general as a farce, which questions the suggestion I provided earlier.
Apologies and regards,
Lyle