I lost a friend (well, he wouldn't have been a very good one) over that very brainteaser when I was 17. Conditional probability is tricky, and yes, you do have to switch. I got that one wrong, but since then I went to college, I became a little quieter and more thoughtful, and I learned never to mouth off over something theoretical until I've followed the other person's logic exactly.

Well, I lost that friend both over that problem (often called the Monty Hall problem) and also a physics problem regarding the motion of binary star systems. I was right about that one. I appealed to a professor to back me up, which he did. I wonder what I meant to do by prolonging the argument: make him look like an ignorant loser? I knew, by the force of my logic and the weakness of his, that I was right. That should have been enough.

That summer was the one when I learned not to get into fights with people over physics (or mathematics, or mechanics, or religion, or anything else abstract) for that reason. The person who is wrong will never be convinced, and both people come off like jerks. Right now I live near the University of Chicago, and just hearing the undergrads on the bus makes me cringe, because they haven't learned this.

I will search around, cyccommute. None of the sizes listed are even close (I need 272-274 mm spokes) but someone might carry them. Thanks for your reply.

And sorry, Aeneas, for making a mess of your thread. It was a really good, germane question that lots of people ought to read. Well, maybe with this flame war we kept it near the top for awhile.