The danger of too much, too little torque

When tires are rotated or when new tires and/or wheels are mounted on a vehicle, tightening the hardware to the right torque is critical. As the hardware seats naturally, those bolts don’t always stay as secure as necessary to maintain safety.

Temperature stressors and changes in metal properties can cause bolts to loosen. Add to that the stress of simply being on the road, such as distance, weight, load, vibrations and road conditions, as well as cornering and turns, and it’s no wonder wheel hardware needs checked and tightened.

Every time wheels are replaced, or routine tire maintenance is performed, the vehicle’s lug nuts must be torqued accurately –- when the new tires leave the shop, again at 25 miles, and at 50 miles.

Failure to check and re-check wheel hardware can create a significant safety issue. When a wheel becomes unstable, loose or separates (i.e. falls off a vehicle), a collision is likely.

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To compensate for any possibility of loose nuts and bolts, technicians often are tempted to over-torque the wheel hardware. The improper use of tools, such as impact guns, can result in the same dangerous outcome or worse, as over-torqued lug nuts can cause permanent damage to them.

The converse can also be detrimental. An under-torqued lug nut can loosen over time. This will redistribute the tension on the remaining lug nuts. As the nut weakens its hold, the continued stress eventually will fatigue the entire set of hardware, which can cause wheel separation.

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Too much force can cause a technician to strip threaded fasteners or stretch bolts beyond their limits, which can result in loose nuts or cause the bolt to crack and break. One study found that in 85% of wheel separations reported, the main cause was “fastener failure,” which typically means the lug nuts or bolts loosened. Many of these situations are the result of using an impact wrench to tighten hardware.

Instead, a calibrated torque wrench is recommended for ensuring accurate torque based on recommended specifications. Those same torque specs should be checked at regular maintenance intervals.

Regulating torque helps to prevent fatal accidents caused by wheel separation. It also helps increase the quality, safety and reliability of its tires, as well as helps avoid costly repairs to wheel alignments, brakes and rotors.