Why do the automotive wheel nuts fail?

I was shocked when my Ford Ecosport got punctured and I was not able to open the wheel nuts with the nut opener supplied along with the vehicle when bought.

I went to the dealership and explained the situation to them. They checked and found that I was right!

How come a company like Ford India supplied the non standard wheel nuts which their spanners cannot open. Really strange!

Later when the issue escalated the dealership changed all my wheel nuts free of cost and said sorry.

I enquired this issue with a local tire supplier and he informed me that many Ford Ecosport owners are facing the same problem!! Whoa! What a quality check!!

Why these nuts failed?

In the above case, both under-torquing and over-torquing are common reasons when air impact wrenches, to tighten the nuts, are used. The actual torque that gets applied depends on so many factors and their variables.  The workshop mechanics generally have a  common approach to purposefully over torque wheel nuts, in part to combat this variability and in part based on the reasoning that a little more torque would be better.  But it should be noted that over-torquing actually reduces (and not increases) clamping force in most of the vehicles, by stretching the studs or threads beyond their ability to respond – especially when this is done repeatedly. Likewise, the process deforms the nut flats as over-torquing applies impact force on the nut flat surface and as the nut tightens against the wheel flat the extra impact rounds off the nut edges. Repeated tightening like this will ultimately lead to complete round off of the nut edges. But in the above case it was not the rounding of the nut edge, it was over sizing of the nuts on which the mechanic was not able to use the standard wrench despite being the standard supplied nuts. Over-torquing can also cause other problems such as cracked, seized, or cross-threaded nuts, which again cannot apply the appropriate clamping force, and increases the chances of stud failure and cracked wheels or the over sizing, as seen in the above case. Similarly, 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 or oversized wheel nuts very difficult.

Remedy?

  1. Possibly material science can provide better solutions like alloys which doesn’t get deformed or deform slightly when impact pressure is applied on the nuts made out of them!
  2. If the mechanics use hand wrenches instead of the air wrenches? It will cause much less damage to the nuts than an air wrench would cause but it will increase the vehicle service time which, I think, the service providing workshops can’t afford. Their productivity will fall.
  3. The mechanic stops the moment the nut hits against the wheel flat and then manually tightens the nut using a hand wrench clicking at the prescribed tightening torque values.

Other Observations, points to ponder!

  • Ford Ecosport is a mid budget SUV. What about big budget cars like Audi, Mercedes or a BMW? Do they also face the same kind of nut failures as the customer who is paying triple the price of a Ford Ecosport would expect more in terms of parts quality.
  • Does the companies like Mercedes, Audi or BMW use better materials to cope with nut failures or their wheel nuts are better designed?
  • Or they replace such parts at regular intervals as a part of preventive maintenance to avoid facing such issues by their customers?

 

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