Dear Tom and Ray:
I live in New Hampshire, and have a 2002 Saab 9-5 Aero. About 10 weeks ago, after putting on the snow tires, I personally put the little blue "hub covers" with the Saab logo on the center of each wheel. Just for kicks, I put on the covers so they all were facing exactly the same way. The other day, I was walking up to the car and noticed that all the covers are now misaligned. I'm thinking to myself: "How did that happen? Did someone sneak into my garage at night and purposely reposition the hub covers?" This is really baffling me, and I've come up with several possible explanations: (1) Remember the movie "My Cousin Vinny" when Marisa Tomei gives her courtroom explanation of Positraction? I am thinking that maybe there's a similar system in my Saab that's causing this phenomenon. (2) Maybe the front wheels spun unequally on ice. (3) When taking a corner to the left, maybe the left wheel spins less than the right one does. In any case, please provide an explanation for this. I know it doesn't really matter how the hub covers are positioned, but it's driving me nuts. Thanks!
Ray: Not a lot to keep you busy in the dead of the New Hampshire winter, eh, Phil?
Tom: Explanation 3 is the correct answer. Turning left or right forces the wheels to turn at different speeds (although wheels spinning unevenly on ice could make it happen, too).
Ray: Here's how it works. Imagine that you could park your car on a large, clean surface, like the beige wall-to-wall carpet in your living room.
Tom: And imagine you could dip each tire in ink so you can track where it goes. In fact, you might want to try this, Phil. Your wife'll probably get a kick out of it.
Ray: If you then turn the front wheels to the left and drive the car in a circle, the right, front wheel will make the largest circle -- near the outside wall, behind where your sofa used to be. That means it's traveling farther -- and faster -- than any other wheel. The left, rear wheel will make the smallest circle, and travel the least distance. And the other two wheels will be somewhere in between.
Tom: This is exactly what happens every time you make a turn of any kind on the road. The wheels all travel at different speeds, and travel different distances.
Ray: And because each wheel is turning a different number of revolutions, the little Saab logos end up pointing in random directions.
Tom: It didn't take 10 weeks for them to get out of alignment, either, Phil. If you reset them now, and then move the steering wheel at all while you're backing out of your driveway, they'll be out of alignment again.
Ray: Of course, if you keep driving, chances are that at some point, they'll end up back in alignment again. Right? They have to, at some point.
Tom: Sure. So here's your new winter project, Phil. Stop the car after every few turns, hop out and have a look at the hub covers. And write to us and let us know as soon as all the planets align again.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)