Safety: Wheel Lug Nuts

I heard a gripping story that is appropriate to pass on.  Rear lug nuts on a bracket drag racecar were left loose by oversight.  On a 120 MPH pass, the driver hit the brakes.  That loaded the rear brake drum to slow down the wheel speed.  The brake drums sheared off the wheel studs.  A serious wheel failure caused an accident.  To illustrate, an airplane structures example is provided.

Shear Strength from Friction

Rivets are used extensively to join airplane sheet metal structures.  Rivets by themselves are usually not strong enough to retain a sheet metal joint in shear load.  The mechanism for load bearing is for the rivet to hold the sheets together with a clamping force.  Friction between the sheets from that clamping force provides the primary load bearing strength.  In one structure for example, 80% of the shear strength was from the friction from the clamping force between the sheets from retaining rivets.  Only 20% shear strength was from the rivets themselves.  In fact, the rivets by themselves were not strong enough to retain the parts in shear loading.

The same mechanism prevails throughout a race vehicle or boat structure.  Friction between surfaces is more often a primary load-bearing strength contributor.  Tight wheel lugs provide friction between a brake rotor or drum and the axle hub that supports it.  That friction may be a vital part of the shear force carrying strength of a wheel mounting.  Improperly tightened wheel lug nuts are more prone to failure.  Wheel stud failure can result, such as in the example.

Over-torque Problem

A few years ago, on two occasions, I caught highway vehicle tire installers who would over-torque wheel lug nuts.  For example, owner handbook specs indicated 100 foot-pounds for the vehicles that I was working with.  On both occasions, tire installation work orders indicated 120 foot-pounds installation load.  That would be too high.  Those mistakes occurred from a huge national tire reseller.  Afterwords, I found lug nuts that were stripped on those vehicles.  They would not screw on easily by hand.  Threads were distorted.

It made me think about wheel lug nut history in a used vehicle that is purchased.  A wheel that falls off from lug nut or wheel stud failure can be catastrophic.  It may be a good idea to have new wheel studs and nuts installed on any vehicle or trailer that is purchased used.  That would better handle a potential problem from prior tire installations.