Tighter lug nuts don’t always mean more security

Dear Car Talk:

The wife of a couple we’ve known forever was driving her 10-year-old Subaru Outback when three of her lug nuts sheared off, and the other two came out. She pulled over and, somehow, the wheel stayed on the car.

The car had just had the tires rotated at a Subaru dealership. The simple answer is that the lug nuts were not tightened enough. The dealership stepped up and paid for towing, a new wheel and a car rental.

OK, so what’s the question? Two weeks later, it happened again on another wheel. Neither the dealer nor Subaru have any answers.

If it were my wife’s car, either the car or I would be gone! Thoughts?

— Tom

Yeah. The dealership didn’t fail to tighten the lug nuts, Tom. They tightened them way too much.

Each wheel hub on the car has five “wheel bolts” that stick out. You hang the wheel on those five bolts, and then you use lug nuts to hold the wheel in place. But if you

grossly overtighten the lug nuts, you end up pulling — and then actually stretching — those wheel bolts.

And when you stretch metal and deform it, you weaken it, and — as your friend found out — it can break.

Then, once one bolt breaks, the pressure increases on the remaining bolts, making them more vulnerable to breaking, right? Now you’re trying to hold the wheel on with only four bolts, and each one has 25 percent more work to do. Then another breaks, and you’re going around a corner at 50 mph with only three bolts holding the wheel on. At some point, the rest of them just shear off, and the result is calamity.

How does this mistake happen? Well, an inexperienced technician might be given “simple” jobs at first that he “can’t screw up.” Like rotating tires.

But if he sets his torque wrench to 500 pounds instead of 80 pounds — thinking “if loose is bad, tighter is better” — he’d stretch the heck out of those bolts.

So, what to do? At the very least, the dealership ought to replace every single one of your friend’s wheel bolts. They’re all suspect, in my view.

Second, they probably ought to have their technicians use something called torque sticks, which go between the wrench and the nuts, and limit the amount of torque that can be applied.

And third, they ought to go through their service records and call in every customer whose car that technician worked on. It’s likely there are other people driving around with overtightened, failed or failing wheel bolts that all need to be replaced. And they may not be as lucky as your friend was.