So, just why did the wheel fall off of John's Jeep Grand Cherokee?

Dear Car Talk

Dear Car Talk | Oct 01, 2008

Dear Tom and Ray:

Something interesting happened to me today. I was on my way to work, and the right front wheel fell off my 2000 Jeep Grand Cherokee Laredo. I want to know why it fell off. 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 this morning, I could hear a knocking sound in front of the firewall on the right side. I turned off the radio so that I could listen a little closer, 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, sitting right there in the dirt. 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 that time, I have put nearly 3,000 miles on it, but I am telling you this because that is the last time I can remember that there could have been an opportunity to remove that wheel. So here is my main question: Why would my wheel suddenly fall off my Jeep? I can think of only a few scenarios.

No. 1: The last time the wheel was off the vehicle, they did not put the lug nuts on tight enough. Good theory, but how was I able to go 3,000 miles before having an incident like this one?

No. 2: The last time the wheel was off the vehicle, they put the lug nuts on too tight and stretched the studs to the point of almost breaking. Again, how did I make it 3,000 miles?

No. 3: My wife hired a hit man who bungled the job. Not a very good theory either, because I already know that she thinks I'm pretty much worthless, so she wouldn't have much to gain by offing me.

Should I check my wife's fingernails for dirt? Please do your best Car Talk CSI work on this for me. I am really depending on you. This could be life or death if I'm wrong. -- John

TOM: Well, so far, I think your wife is the leading candidate, John.

RAY: Actually, it could have been lug nuts that were too loose OR too tight. They could loosen up and fall off one by one over 3,000 miles, but that is an unusually long time. More likely, the lug nuts were overtightened.

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 it stretches 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 this 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 off with it.

RAY: And you should get down on your knees and thank your lucky stars that you were going 4 mph when it happened instead of 40.

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

RAY: But if there's a chance that those animals overtightened all four of your wheels, then I'd recommend replacing all 20 studs. And just to be safe, check your wife's bedside table to make sure she hasn't taken out an "accident only" insurance policy that you don't know about.

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