Whenever you have new wheels and tyres fitted to your car – for example, you’ve done the sensible thing and switched between summer and winter tyres – it’s essential to have the wheel nuts retorqued after the first 30 miles or 50 kilometres. Why should a responsible driver do this? The point is road safety. Lug nuts keep a wheel securely attached to the hub, which makes them a vital component of the vehicle. But lug nuts can’t perform as designed without having the correct amount of torque applied.
Torque is the unit of measurement for the twisting force that’s applied to a wheel nut. Mechanics will ensure that each wheel nut has been torqued to the vehicle manufacturer’s specifications when first fitting a wheel. But a second follow-up check is also necessary.
This is because, after a reasonable amount of time, the wheel nuts will undergo the typical stresses and forces of a car in daily use.
These stresses include supporting the weight of the vehicle, the rotational effects of driving, cycles of warming and cooling and nudges from bumps in the road. Such stresses can cause a slight shift in the seating of the wheel nuts, which may result in tightening or loosening.
The presence of dirt, sand, rust or grit between surfaces such as the threads of the nut or between a nut and the wheel can also create “false torques” during the initial fitting, where the force applied overcomes the friction but doesn’t translate into clamping force.
For these reasons, many car garages and tyre retailers will invite drivers to come back the next working day to have their wheels retorqued. It’s a quick procedure, usually offered free of charge, which involves inspecting each wheel nut to check if they are too tight or too loose.
If the wheel nuts are too tight, they can cause severe problems such as stripping the fastener threads or stretching the wheel studs. They could also warp the brake drums, brake discs or wheel hubs. With loose nuts, meanwhile, there’s a small risk that the wheel could come off while you’re on the road. The chances of this ever happening are very low – but if it did happen, your safety and that of other road users would be at risk.
When new or replacement wheels are first fitted, the person responsible must torque the wheel nuts according to the vehicle manufacturer’s recommendations. These details are included in the owner’s handbook and vehicle documentation.
After approximately 30 miles or 50 kilometres have been driven on the new tyres, the wheel nuts must be checked and, if necessary, retorqued to the same values as before.
Not every car will have the same torque requirements, however. Different makes of vehicle will require different amounts of torque to tighten their wheel nuts properly and avoid causing any damage.
To loosen or tighten wheel nuts correctly requires a special tool called a torque wrench.
The mechanic will initially set the torque wrench to half of the required torque and tighten each wheel nut according to this setting. Then the setting on the wrench is adjusted to tighten all the wheel nuts to the correct torque.
The wheel nuts are tightened in a specific sequence to provide the proper torque. The correct order for any wheel with five or ten wheel nuts is a star-shaped pattern. A wheel with only four wheel nuts, meanwhile, is tightened in the shape of a cross.