Alignment vs. balance
One of the most common sources of confusion for car owners is the difference between balance and alignment. Wheel balance is most important on the front wheels. It doesn't affect tire wear much unless it's way off. When a car's steering wheel shimmies at freeway speeds, almost certainly your front wheels are out of balance, your front tires are not round, or both.
Alignment almost never causes a shimmy. Alignment is, however, directly connected with tire wear and is one of the reasons a car may pull to one side or the other when going down the road. Knowing the difference between balance and alignment can save you a lot of grief, but the safest thing to do when you take your car to the shop is to explain the sympoms you have experienced instead of asking either to have your front end aligned or to have your front wheels balanced. Your shop may not be sympathetic when you tell them "I had you align my car last week and it still shimmies as much as ever."
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