We have seen the stories in news of trucks losing their wheels while in motion or TV news at six of the units that flipped or had an accident due to a wheel coming off. Over the years, we’ve seen firsthand the ramifications of improperly torqued lug nut and lug bolts. We’ve seen wheels come off, brakes damaged, broken and/or stripped lug nuts, bolts and studs. And unfortunately, we have seen this time-and-again from professional technicians as well as the do-it-yourselfers – all for one simple reason – they failed to properly torque the lug nuts or bolts on their vehicles.
But what is the cause, why is it happening, what is the root cause. Torqueing Wheel Lug Nuts / Lug Bolts Lug studs and nuts as well as lug bolts are designed with a specific grade of hardware with a certain amount of “stretch.” Stretch? You might ask – Yes. Proper torque, which stretches the bolt, causes the threads of the stud/bolt to tightly mate and secure to the matching nut or threaded hole (within the axle or wheel hub) without working themselves loose. It’s akin to mechanically securing the nut to the bolt – except that you can remove and re-tighten it repeatedly, as needed., yet, most bolt grades used for this purpose will retain its original size and properties (un-stretched) when properly torqued to specifications.
Crashes caused by wheels coming off of vehicles are commonly referred to as wheel runoff crashes. Two primary failures cause wheel runoff crashes. One is a failure of the wheel mounting system, such as the wheel studs, lug nuts, etc. The other cause is a failure of the hub and wheel bearing assembly. Most commonly, these failures are related to improper or deferred maintenance, but some are also linked to manufacturer’s defects.
Wheel System Failures
Wheel system failures are primarily caused by the improper installation of a wheel that causes it to be loose or become loose. Commonly, a loose wheel causes the wheels studs to break and the wheel and tire to separate from the vehicle. Many root causes lead to loose wheels, but most of them are associated with over-torquing or under-torquing the lug nuts.
A bolted joint, such as a wheel mounting system, works by tightly clamping two surfaces together. The friction of the two mated surfaces and the force created from clamping them together with bolts (Clamp Load) allows the surfaces to resist movement. The amount of friction and Clamp Load determines the level of resistance the joint has to movement.
Clamp Load is created by tightening the bolts against the mated surfaces and is normally measured in foot pounds of torque with a torque wrench. If the bolt torque specified for a joint is applied, then the resultant Clamp Load should also be within specification. However, variations in the system such as rust or lubrication on the threads can affect the Clamp Load vs. torque relationship. Items in place between the mated surfaces can reduce the joint’s friction and also alter the relationship between bolt torque and Clamp Load. This is called a Soft Joint.
Two concepts are important to understanding how a bolt works. They are Elastic Deformation and Yield Point. Elastic Deformation is metal’s or, in this case, a bolt’s ability to stretch and spring back to its original shape. Yield Point is where the bolt has been stretched past its elastic limit and can no longer spring back to its original shape. This stretching of a bolt and its pulling back creates Clamp Load. If, however, a bolt is over-torqued, and stretches past its Yield Point it can no longer maintain the Clamp Load.
Over-torquing is likely the most common wheel system failure due to the widespread use of impact wrenches to install wheels. Using an impact wrench to install wheels commonly causes the wheels to have 3 to 5 times the specified lug nut torque. The use of lubricants and anti-sizing compounds on the threads of the wheels studs or lug nuts can cause an even higher degree of over-torquing.
The specific torque required to install a wheel varies from vehicle to vehicle. Generally, the proper torque for the lug nuts on passenger vehicles will be around 100 foot-pounds and the proper torque for big trucks will be around 400 foot-pounds. Impact wrenches commonly used to install wheels on passenger vehicles are capable of producing 300 to 500 foot-pounds of torque. Impact wrenches used to install wheels on big trucks can produce 1200 to 2000 foot-pounds or torque.
Under-torquing is just simply not tightening the wheel lug nuts enough, causing the wheel to be loose. Under-torquing can be caused by corroded and damaged wheel system components. It can also be caused by using a cheap or worn-out impact wrench or by having a low air-pressure supply to an impact wrench.
Another common cause of wheel system failures is too much wheel paint thickness. As specified by the Recommended Practice PR222B from The Maintenance Council (TMC) of The American Trucking Association “Total thickness of the dried paint coating on each side of the wheel mounting face must not exceed 3 mils(.003 inch).” If the wheel’s paint is too thick, then a soft joint is created and the system can fail.
Paint thickness defects are not only caused by original production painting but also more commonly caused by the recondition or “remanufacturing” of wheels. Wheel reconditioning generally involves “sandblasting” used truck wheels and repainting them to make them look new. The reconditioning of wheels is typically being done by tire dealers and tire retreaders who do little to control paint thickness.
The properties of the bolt has a slight spring affect when loosened.
Improperly torqued lug nuts or bolts can also result in:
- Warped brake rotors — brakes grabbing, pulsating or overheated.
- Damage to the lug nut seating surface of alloy wheels.
- Wheel hub damage — threaded wholes stripped out.
It is also important to tighten lug nuts or bolts incrementally to the final torque specification, and doing so in the proper sequence.
It is prudent to recheck the torque specifications after a test drive of the vehicle, especially with alloy or painted wheels. It is very possible to falsely torque lug nuts or bolts due to excess paint, debris, corrosion or a tight and binding centering hole over the wheel hub that allows the bolt or nut to come loose after the vibrations and rigors of driving.
When installing new wheels you should re-torque the wheel lugs after driving the first 50 to 100 miles in case the clamping loads have changed following the initial installation. This is necessary due to the possibility of metal compression/elongation or thermal stresses affecting the wheels as they are breaking in, as well as to verify the accuracy of the original installation. When rechecking torque value, wait for the wheels to cool to ambient temperature (never torque a hot wheel). Loosen and retighten to value, in sequence.
Wheel lug torque specifications are for clean threads that are free of dirt, grit, etc. If applying an anti-seize lubricant, it is important to note it can be applied only on the threads of nuts or bolts. The lubricant must not be used on either seat of the hardware of the wheel. With the seat being the main point of friction where torque is measured, extreme caution must be used if an anti-seize lubricant is applied to the threads as excess can either drip or be pushed onto the lug seat resulting in inaccurate torque values.
A thread chaser or tap should be used to remove any burrs or obstructions of the threads allowing the lug hardware to be turned by hand until it meets the wheel’s lug seat. Once lugs are snugged down, finish tightening them with an accurate torque wrench. Use the appropriate crisscross sequence (shown above) for the number of wheel lugs on your vehicle until all have reached their proper torque value. Be careful because if you over-torque a wheel, you can strip a lug nut or hub, stretch or break a stud or bolt, and cause the wheel, brake rotor and/or brake drum to distort.