Dear Car Talk:
Decades ago, when I took driver's education, we were taught to carefully adjust each mirror before getting underway. I've noticed that on long trips, my rearview mirror eventually will move out of adjustment. This is something of a mystery to me. Does my spine compress, or does the car just shake things out of adjustment? Or is there something else at work here? The same problem occurs on all cars I drive: The side mirrors are always OK, but the rearview mirror needs readjustment.
This has bothered me for a while, and thought you might have some insight. Thank you! -- Brent
Hm. You might have a case of "Mirror Droop," Brent. This affects older cars where the ball and socket that hold the rearview mirror in place begin to fail and loosen up, and before you know it, you're looking at that coffee stain on your right pant leg.
But I'm guessing there's another factor at work here, because I experience this too, even in newer cars. So I think the problem is related to FBSC -- Fat Butt Seat Compression.
Here's what happens: You get in the car, you're alert, you're sitting up straight, you set the mirrors and you drive off. And then ... you relax. You slouch a little bit. The seat compresses a little bit. And 10 minutes into your trip, the rearview mirror is adjusted too high, and you have to fix it.
The same thing is happening to the side-view mirrors, but you don't notice the change there because they're adjusted primarily for a side-to-side view, not up and down.
With the rearview mirror, you're aiming more precisely to see out of one little sweet spot: the rear window. That's a much narrower target. So when you drop down into your seat half an inch, or an inch, you really notice it.
So I think we just need to consider this phenomenon part of the human condition, Brent. And if Shakespeare were alive today, he'd probably lament it in "A Midsummer Night's Droop."