Monday, December 22, 2014

Under the Hood: A lug nut mystery

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Question: Three weeks ago I brought my 2001 Camry to a shop to get the brakes checked because the squeaking was annoying. The front brakes had been done about a year earlier with Toyota OEM pads used and the rotors were resurfaced and within spec. The rear brakes were fine.

The brake check showed plenty of pad still left (6mm), and the rotors were smooth as glass. I asked to look at them when they were removed. The tech told me that the metal Toyota uses in the pads sometimes will make them squeak, although prior to the brake job last year they never squeaked. The pads showed no signs of abnormal wear, no scoring, ridges or pitting. They looked fine. The rear brakes and drums were still fine too. Although the shop wanted to sell me a front brake job with new rotors and new aftermarket pads to see if that will fix the squeak. I passed on the new brake job.

So now my brakes have been checked and all is well until three weeks later when my wife gets a flat tire. She pulls over and a Good Samaritan helps her replace the tire in order to get to our tire store. During the flat removal one of the lug nut studs breaks off. Two miles away at the tire store after they removed the spare another stud breaks off on the same wheel.

Needless to say I was not happy. My thought was that the shop that did the brake check over-torqued the lug nuts and fatigued the studs to failure. I'm just grateful that my family was not driving at freeway speeds when the lug nut studs sheared off. So what are the causes of lug nuts shearing off wheels and what could make two on the same wheel at the same time shear off when changing a tire? I want to keep my family (and other drivers on the road) safe. Can lug nuts studs shear off when driving?

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  • Answer: My first thought was whether the Good Samaritan may have caused this by inadvertently tightening, then loosening the nuts. My suggestion would be to obtain a beam type torque wrench and measure the torque needed to remove the lug nuts on the other three wheels. This older style, easy to read torque wrench allows one to continuously view the torque applied rather than the more popular "click type" torque wrench which indicates a preselected value has been reached.

    If you should find the other lug nuts to be excessively tight (specification is 80 lb/ft), take a photo of the applied torque just prior to one or more nuts coming loose. I'm thinking +/- 10 is reasonable, more than 20 is certainly not. Should the readings be excessively high, I would hope the original shop would agree to renew all wheel studs, and nuts- as need be.

    Excessively tightened wheel studs, besides the issue you've encountered, can warp brake rotors and damage wheels. Unless severely over-tightened I believe it would be unlikely you could lose a wheel while driving.

    Brake squeak is caused by pad vibration. Softer composition pads and refaced rotors can wait until the next brake service. Your "smooth as glass" rotors may actually be too smooth, lacking the desired surface finish. New rotors may be needed if there isn't enough thickness remaining to safely machine them. You may be better off with slightly thinner (but within spec) Toyota rotors than new cheapo replacements. Insist on good ones!

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    ABOUT THE WRITER

    Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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    Brad Bergholdt McClatchy-Tribune News Service