Maximum Pressure on Car Lug Nuts

Yesterday, I stopped to help the proverbial little old lady change a flat tire on her Buick–or tried to. Problem is, the lug nuts had been tightened by Superman and, though I’m a big guy and lift weights like a fiend, I simply couldn’t budge one of the bastards. Ultimately, I put everything I had into it and sheared the entire lug from the wheel, making my whole day, not to mention her’s.

What is the maximum amount of torque that a mechanic’s pneumatic gun can put out–and can that be expressed in easily comprehended terms (rather than foot pounds)?

From this article:

So, that answers the first part of your question.

To answer the second part of your question, imagine you have a lug wrench of a certain length, L, the load you’d want to apply, F, at the very end to acheive the specified torque, T, would be F=T/L. So to acheive a torque of say, 100 ft-lb with a 2 ft long lug wrech, you’d want to be able to apply F=100/2=50 lb at the end of the wrench without it turning the lug anymore. Theoretically, you’d have to apply 50 lb to loosen it although you have some hysteresis due to friction that would probably raise this value slightly.

Rather than having been put on too tight, a careless mechanic may have crossthreaded it all the way on with an impact.

It’s possible that it was cross-threaded, but frankly I think it’s more likely that it was just overtightened.

I’ve run into this problem more than once. After having to call AAA to come change a flat on a friend’s minivan solely for the reason that none of us could get the damn lug nuts off, I decided I’d had it. I now carry a three- or four-foot length of pipe in my truck to use as a lever arm. I fit it over the end of the lug wrench, and can get three or four times the torque for the same amount of force. I’ve never had that not work, but it’s still scary how hard I have to push sometimes…

A professional mechanic who puts lug nuts on by hand and not using an impact, for just this reason, is the definition of a mechanic who thinks about what he’s doing, as opposed to a useless braindead lazy timeserving idiot, which (with the greatest of respect to good mechanics) is what too many professional mechanics are. IMHO.

Also, if no grease or oil is put on with the lug nuts, the longer it has been on, the tougher it will be to get off. Rust never sleeps, you know.

I call the ones who think about what they are doing technicians.

All modern cars have a torque specification for the lug nuts. For our Safari minivan it is 100 ft-lbs, for example. When purchasing new tires, insist that they use a torque wrench, and insist being allowed to watch the technician put the tires on your car (for proof). In fact, when I buy tires, I stand over the guy’s shoulder just to make sure a) he’s using a torque wrench, and b) he’s using it properly. And you should definitely invest in a torque wrench if you ever feel the need to remove/re-install wheels yourself (doing brake jobs, rotating tires, etc.)

I had this exact thing happen once after getting new tires. I ended up putting so much force on the lug nut when trying to remove it (to rotate my tires) that I sheared the whole lug off. Of course I was impressed with my ability to break solid metal with (almost) my bare hands, but I was kind of pissed at the shop for doing it.

I never went back there again.

As the son of a mechanic, and a former mechanic myself, I’d call them mechanics. No offense please, but “technician” sounds too much like something that a chain of shops puts in their advertising so their grease monkeys sound competent.

Demanding that they use hand tools is a good idea. The sad fact is that no installer in a tire shop will hand torque unless you insist since management wouldn’t stand for them wasting the extra five minutes per car. Tire shops are in a competitive market; it ain’t a boutique business.