Losing a wheel can be disastrous for any type of vehicle but when a heavy vehicle is involved, the potential consequences are much more severe.
In January 2010, an 80kg wheel detached from a truck travelling on Auckland’s southern motorway. It accelerated like a missile, bounced over the motorway’s median barrier, smashed through a bus window and killed a 24-year-old passenger on the bus.
The company that had recently serviced the truck was prosecuted by the Department of Labour. It was found guilty and sentenced to pay a total of more than $70,000 for compensation, costs and a fine.
Jim McDonald, the NZ Transport Agency’s (NZTA) operations support manager says a number of forces act to loosen wheel nuts.
“If one of the wheel nuts loosens then the forces present on the wheel are distributed over the remaining nuts, which can cause those to loosen as well. The process accelerates as more nuts become loose and the overall force holding the wheel in place gradually reduces,” says McDonald.
“When the clamping force reduces sufficiently, the wheel moves on the hub. This leads to side loading and further loosens the remaining nuts which, if not spotted in time, can results in elongated stud holes, fatigue failure of studs, fretting cracks and in many cases, eventual wheel loss.”
Causes of wheel loosening
There are a number of reasons why wheel nuts can loosen, including:
- the failure to follow manufacturer’s instructions for fitting wheels. This is particularly applicable to after-market products such as aluminium wheels
- the failure to tighten wheel nuts to the specified torque, in the correct sequence, or fully tightening the wheel nuts one at a time rather than in stages
- the failure to retighten wheel nuts after a short period of in-service running (between 50 to 100km is recommended)
- over-tightening, causing stretched/broken studs or causing studs to be pulled through the hub
- damaged threads on wheel studs and nuts resulting in insufficient clamping force
- severe corrosion and/or wasting of wheel studs
- incorrect matching of wheel nuts and wheels / wheel hubs
- incorrect matching of wheel studs and wheel nuts when aftermarket wheels have been fitted reducing the stud length available for correct wheel nut engagement
- using inappropriate (impact tools) or non-calibrated equipment when tightening wheel nuts.
Reducing the risk for operators
McDonald recommends operators consider the following tips to reduce the risk of wheel loss:
- ensure drivers inspect tyres and wheels during pre-trip inspections for signs of damage
- after a wheel has been fitted, ensure wheel nuts are rechecked for correct torque after 50 to 100 km of in-service running. Do not slacken and re-tighten wheelnuts. Ensure maintenance procedures include a flag to note this has taken place
- ensure wheel nuts are regularly checked for security and are tightened, with a calibrated torque wrench, to the recommended torque
- establish causes of wear and damage on loose nuts before re-tightening
- ensure manufacturer’s written instructions are followed by correctly trained persons and that comprehensive maintenance records are kept
- consider fitting loose wheel nut indicators as these assist in the visual inspection of wheel nuts and provide an immediate indication when wheel nuts have moved.
“Vehicle owners, drivers and maintenance staff all have a role to play in reducing the risk of wheel loss. Everyone doing their part will result in less down time and cost for operators and safer journeys for all road users.”
For more information on wheel loss, check out the NZTA’s information sheet: www.nzta.govt.nz/wheelloss
