Recently I purchased a Chevy Astro van with miles on...

Dear Car Talk

Dear Car Talk | Feb 01, 1994

Dear Tom and Ray:

Recently I purchased a 1990 Chevy Astro van with 66,000 miles on it. Under normal circumstances it runs fine, however, on two occassions, things have become violent. The first occasion was late last winter when I was travelling in a light snow which had just been plowed. I was changing lanes to exit when the whole unit started shaking and I could have sworn I had a flat or two. At the bottom of the exit ramp, I checked and there were no flats. The next occasion was two weeks later when I was entering the freeway. There was no snow, but the road was wet. Again, the whole machine shook. The two incidents had two things in common. I was accelerating--in fact, I had the pedal to the floor--and I was angling to the right. Personally, I don't think the turning had a thing to do with it. The machine doesn't behave like this under gradual acceleration. What do the experts have to say about this?

TOM: How would we know what the experts would say?

RAY: But I'll tell you what WE have to say about it. The Astro has a very light rear end. And on slippery roads under full-throttle acceleration, you probably lost traction. That could cause a vibration in a number of ways.

TOM: You don't say whether this truck has "limited slip differential." If it does, you could just be pushing it past its limit. That could cause this kind of behavior.

RAY: If you don't have limited slip, then you might be getting something called "wheel hop."

TOM: That's not a high school dance, Jimbo. It's when your wheels (the rear wheels in this case, because the Astro is rear wheel drive) are actually bouncing very rapidly off the ground. That in itself could cause a fairly violent vibration.

RAY: You could get wheel hop because you have a bad tire, an improperly inflated tire, or bad shocks. So it would be a good idea to have all of those things checked out.

TOM: But more importantly, why have you got the pedal to the metal when there's snow OR rain on the ground. Isn't that kind of dangerous? Even if your shocks and tires ARE OK! I suggest you lighten up on the foot, James. And my guess is your truck will stop responding so violently.

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