Back end wobble

This is very common on any vehicle. It's caused by a broken belt in one of the tires. A few tire brands are noted for this, and some others commonly develop broken belts right at about the time the tread is worn out and it's time to replace the tires.

The people at any tire and alignment shop can identify the defective tire and show it to you. The easiest to see is when there's what looks like a hump or tumor on one part of the tread as you rotate that tire. Often the tread will squirm sideways too as it is rotated.

The harder defect to find is when the belt breaks very gradually over many weeks or months. The hump develops real slowly, then the tread wears down in that spot, so it looks perfectly fine. You have to look at the deepest part of the grooves in the tread, then you'll see that part rises and falls a little as the tire is rotated. While the part of the tread that contacts the road surface looks okay, the break in the belt doesn't support the weight of the car as well, so that axle will drop a little each time that spot on the tire hits the road. Most people feel that as an out-of-balance problem, but the clue is it occurs at low speeds too where balance problems aren't felt.

When the broken belt is on a front tire, you'll see the steering wheel oscillate left and right a little, once for each wheel revolution, and you'll see it at real low speeds, such as when driving through a parking lot.

Then a wheel is bent from hitting something. A wobble when the car is moving has to be caused by something that is rotating. All that is rotating is a tire and a wheel.

If you switched tires at the end of winter, that was many months ago. Broken tire belts can occur in one day, so to say they were fine months ago is not valid for this type of problem. That's like saying a tire can't be flat because it was fine four months ago.

Also be aware when tires are switched or rotated, we don't have a reason to suspect one has a broken belt, so we never check for that. Some broken belts are real obvious and hard to overlook, but they rarely break while in storage, and never when off the wheel in storage. No one has time to waste checking for the more subtle form of broken belt when rotating tires unless someone mentions the symptom, then we would look closer, but to see that, the tire and wheel has to be on the car so we can spin it by hand. That's the only way to see those sneaky belts that are just starting to break.

Normally we only look at tire wear patterns when we do anything with the tires, to see if we need to recommend an alignment or steering and suspension systems inspection. Conscientious mechanics take a quick peek at your tire wear patterns during other routine services such as oil changes. Doing so can catch a problem before it gets bad enough to cause the tires to wear out too quickly.