I was doing some research on bikes... my son needs a to-school
commuter. I'm thinking of a single speed since he murders wheels,
derailleurs and derailleur hangers. So I was looking for bombproof.
And ugly. (to keep it around for a for while.)

I ran across two links... and thought they might be useful here, given
recent discussion. Perhaps they've been posted before... but if not,
they are good, short reads.

On shimmy...

http://www.sheldonbrown.com/brandt/shimmy.html

On trail and gyroscopic stability...

http://www.dclxvi.org/chunk/tech/trail/