CLICK & CLACK : In Wheel Trouble

Dear Tom and Ray: On my way to work, the right front wheel fell off my 2000 Jeep Grand Cherokee Laredo. I want to know why. All five of the studs were sheared off, and I could not find any of the studs or lug nuts near the scene when I stopped.

When I left my home, I heard a knocking sound in front of the firewall on the right side. I turned off the radio so I could listen, and I noticed that it got louder when I accelerated. I finally decided, after about half a mile, to stop and check it out. I walked all the way around the car, looked underneath it and checked under the hood. I couldn't see anything, so I got back on the road. About another half a mile down the road, I heard and felt some real banging in the front, so I immediately pulled over. As I was pulling to a stop, there was a big bang, and I thought, "Did my front wheel just fall off?" Sure enough, next to my car when I got out was my right front wheel, along with the rotor and everything else. I had the car inspected a couple of months ago, and that same day I took it in for an alignment and to have them fix a stabilizer shock-absorber-looking thing that had come loose under the front-end area. Since then, I put nearly 3,000 miles on it, but I am telling you this because that is the last time I can remember there could have been an opportunity to remove that wheel. So why would my wheel suddenly fall off my Jeep? -- John

TOM: Here's the likely scenario: When you had your stabilizer replaced 3,000 miles ago, someone overtightened your lug nuts. What that does is stretch the studs. Now, every metal has an "elastic region," a point to which it can be stretched and still return to its original shape and size. But if you stretch it beyond its elastic region, you get to its "plastic region," where it won't return and is permanently weakened.

RAY: My guess is that, after being overtightened, at some point one of your studs failed and broke off, taking the lug nut with it. You didn't notice it because you had four more. But because one was missing, those four had to do the work of five. That put more stress on the remaining studs.

TOM: Eventually, another one broke. And then another. When you set out that morning, you had two studs left. That's why you were hearing the wheel banging against the disc rotor. One more stud broke off, and the noise got louder. And then, that last one sheared off and the wheel came with it.

If you're sure the guys who did the stabilizer repair removed only that one wheel, you can replace the five studs and be fine.

Dear Tom and Ray:

My wife heard a report on a local radio station warning people not to use jumper cables on modern cars. The guy said they could cause "unstable voltage" and damage on-board computer operations. I own a 2005 Subaru and a 2007 Ford. When I contacted both vehicles' dealerships, each told me it was fine to use jumper cables. So? What's the truth? -- Rob

TOM: Your dealership's service representatives are exactly right. We jump-start cars all the time at the shop and have never had any car suffer from "unstable voltage" or computer damage when it was done correctly.

RAY: Now, there can be lots of problems if you hook up the jumper cables backward.

TOM: Right. That can cause unstable voltage, unstable angina, fried computers, angry customers and lots of explainin'.

RAY: If you're not sure how to jump-start a car, we have a diagram and instructions on our Web site at http://www.cartalk.com. Print them out and stick them in your glove compartment.

Got a question about cars? Write to Click & Clack in care of The Post, or e-mail them by visiting the Car Talk Web site at http://www.cartalk.com.

Copyright 2007 by Tom and Ray Magliozzi and Doug Berman