Wheels suddenly detach from moving cars and trucks more often than many people think. These ‘wheel separations’ can lead to serious injuries from the vehicle losing control, from the separated wheel colliding with another vehicle or pedestrian, or from another vehicle maneuvering to avoid the projectile.
When a wheel separation occurs on a highway, the separated wheel is hardly slowed down. The approach speed between the now bouncing projectile and oncoming vehicles can easily exceed 100 mph. Figure 1 shows damage to an SUV that was hit head on by a pair of escaped heavy truck wheels.
Figure 1. Front-end damage from head-on impact with separated dual heavy truck wheels.
The most common reason for a wheel to separate is failure of the fasteners, where wheel nuts fall off and/or wheel studs break and release one or two wheels from the vehicle. These failures generally occur 175 to 3000 miles and one to fifteen weeks after a wheel was taken off and put back on during some service, such as a tire installation.
There is a different pattern of evidence in left vs. right side wheel separations. Left side wheel separations usually occur after the wheel nuts spin off (Figure 2) whereas right side wheel separations usually occur after the wheel studs break off (Figure 3).
Figure 2. This stud is from a left side wheel separation that occurred when the nuts spun off 21 days after installation.
Figure 3. Broken studs from a right-side wheel separation, 2900 miles after wheel installation.
The wheel nuts and studs basically squeeze the wheel onto the vehicle with enormous force, called the clamping force. The clamping force is made when the nuts are tightened onto the studs. If the clamping force is lost, then the nuts loosen leading to left side nut spin-off and right side stud breakage.
The proper clamping force is made by turning the nuts to a specified tightness with a wrench that measures torque. Some ways to achieve the wrong clamping force are mistakenly applying the wrong torque value, failure to use a torque wrench, or using a torque wrench that is out of calibration.
Wheel nuts that were properly torqued can lose their clamping force. Some reasons are wear of paint coatings on brake drums, break up of corrosion deposits or dirt that was in the ‘sandwich’ when the wheel was put on, and wear of aluminum wheels.
Fortunately there is a well-known remedy for lost clamping force. It is simply to re-torque the wheel nuts after a short amount of driving. There is no need to remove the wheels or even jack the vehicle off the ground – just using a torque wrench to apply the manufacturer’s specified torque to each wheel nut after a short amount of driving will normally cure any loss of clamping force. Although wheel separations are rare, they should be entirely preventable with re-torqueing.