Gonna take a flyer here based on personal experience- tread separation I think it’s called- steel belts in the tire coming undone can cause shimmying.So I had weird scenario that I still don’t fully understand:
Rotated the tires on my outback doing the brakes a couple years ago. After rotation there was a noticeable shimmy in the front end. Had my wife bring it in to a shop to get wheels balanced. Tech said the wheels are bent and can’t be balanced. I thought that was weird but not impossible, ended up just living with it since we didn’t drive that car often.
Fast forward to this year, and now the Outback is our only vehicle, so I had to get it fixed. When getting the car inspected I brought it to a shop that does wheel straightening. Told them I need the wheels balanced, and if any of the wheels are bent maybe they could fix them if it was reasonable (tires still had a lot of life in them). So they call and say the car is ready, they balanced the wheels and didn’t say anything about the wheels being bent.
The next day I drive 200 miles to my moms house. The shimmy was better but not gone. Put the car on the lift as I wanted to replace the sway bar links. First wheel I remove, there’s a giant bald patch. Looking at the other tires, 3 of 4 of them have bald patches. Each in the spot they had added weights on the inside of the wheel (they added a lot of weights, but not more than the general maximum weight limit).
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So at this point I have no choice to replace the tires. So bring it to a local shop, get new tires mounted and balanced. They also don’t say anything about issues balancing it. One the road the shimmy is completely gone, everything feels good. That was a few months ago, everything has been fine with the tires.
So in the end I’m not sure what the actual issue was. Perhaps it was bad tires all along, or maybe the first couple shops were too lazy to start balancing from scratch and just kept adding weights on top of weights.
You can rebalance, but they’ll keep coming apart.
Used to get this when I was buying used tires from my mechanic (younger, broker me)