On Sat, 04 Aug 2007 18:36:01 -0500, DougC
wrote:

[snip]

With motorcycles, the reason I was told was precisely because using a
smaller rear tire allows the tire to be mounted closer under the
rider+frame, contributing to level-ground traction. On-road motorcycles
don't bother with it because for them it's not a significant advantage.

Dear Doug,

Sorry, but they told you wrong.

The thick rear 18 x 4.00 tire on a trials motorcycle isn't smaller.
It's just as tall as the skinny 21 x 2.75 front ti

http://i16.tinypic.com/4pl4yhk.jpg

And level-ground traction is not a problem on a trials machine.

Cheers,

Carl Fogel