Why do my wheel nuts / bolts keep loosening?

IS YOUR CAR READY FOR THE FESTIVE SEASON?  | 

If you'll be driving for a long distance this festive season, even just down the road to fetch your amazing DeWalt Hammer Drill they borrowed a couple of generations ago; you need to ensure that your wheels are in perfect condition. This article will focus on reasons why lug nuts come loose and the solution for that. 

Our friends at Burquip say many people use air impact wrenches to fit the wheels to trailers, cars, and trucks. Both under-torquing and over-torquing are common when air impact wrenches are used because the actual torque that is applied to the stud/bolt depends on so many variables. 

What is over-torquing?

This video by BOLTIGHT explains bolt behaviour and elastic limits. 

A common approach is to purposefully over-torque wheel nuts, in part to combat this variability and in part based on the reasoning that “more is better”.  However, over-torquing actually reduces (not increases) clamping force in many cases, by stretching the studs or threads beyond their ability to respond – especially when this is done repeatedly. 

Over-torqueing can also cause other problems such as cracked, seized, or cross-threaded nuts (which cannot apply the appropriate clamping force), and increases the frequency of stud failure and cracked wheels. In addition, it damages the chamfer around the stud hole in the wheel disc and this causes an improper mating surface.

 

Differential thermal contraction can occur when wheels are mounted at low temperatures in cold climates.  As the wheel components warm to ambient temperatures, clamping force is lost.  Wheel nuts that experience any rotation during this time will not regain their original torque / clamping force values. The opposite also applied when wheels are fitted at very high temperatures.

Improper mating surfaces include both damaged and contaminated mating surfaces. Proper clamping force cannot be achieved with non-flat mating surfaces such as damaged or bent hubs and wheels, or worn or elongated bolt holes (raised metal).  Contaminants such as excess dirt, sand, rust, metal burrs, and paint on mating surfaces can wear away with use, causing a settling effect.  When present on the threads or between a nut and the wheel surface, these contaminants can also change the clamping force/torque relationship, resulting in “false torques” where much of the torque applied is used to overcome friction and is not converted into clamping force.

A loose wheel nut can originate from any of these sources individually, or more probably, from a combination of these sources – which makes the task of eliminating all loose wheel nuts/bolts very difficult indeed.

While in service wheels on vehicles are subjected to a bewildering variety of forces, including vertical forces from the vehicle and its cargo, road vibration and shock forces, cornering forces when the vehicle turns, and rotational forces from the turning of the wheel, especially during acceleration and braking. When a wheel nut loosens these forces are redistributed among the remaining nuts and studs, but preferentially to the nuts and studs adjacent to the loose nut, causing these nuts to back off at reduced wheel force levels.  This loosening process accelerates with each successive nut that loosens, as the total clamping force drops and the stress concentration at the remaining nuts and studs increases.  At this point, these studs can fracture due to fatigue or overstress, accelerating the process further – especially with heavy wheel loads.

When the wheel forces exceed the clamping force of the remaining nuts and studs, the wheel will move relative to the hub which results in side loading and loosening of the remaining nuts, bending fatigue failure of studs, elongated bolt holes and wheel pilots on the hub, fretting fatigue cracks between the bolt holes, and wheel separation if not detected quickly. The rate of this process, and thus the potential for detection prior to wheel separation, depends on the type and magnitude of the wheel forces being experienced.

 

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Burquip recommends that the correct wheels be used with our axles. Also that the toque of wheel nuts/bolts be monitored regularly so that corrective action can be taken before expensive damage occurs. This is even more important in the case of hire trailers, off-road and other trailers that are operated by more than one person and where the operating conditions are variable and the loads are not consistent.

For more on Burqip Trailers Click here... 

BurquipLug nutVehicle safetyWheel safety

5 comments

Zoltan

Zoltan

The science in your writing is factual! The illustrations / video aid are just perfect. You can see that great research went into this.

Jason

Jason

Information overload. Satisfactory!

Dylan

Dylan

Wow. Thorough. Excellent work. Had no idea thats how lug nuts work

Fran

Fran

You guys don’t ship assembled trailers to Stadtkreis hey?

James

James

First time here. I’m liking this.

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