[CR]Trail and tire sizing..., was Re TIRES.


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Date: Mon, 07 Jun 2004 21:31:10 -0400
From: HM & SS Sachs <sachs@erols.com>
To: freitas1@pacbell.net, classicrendezvous@bikelist.org
Subject: [CR]Trail and tire sizing..., was Re TIRES.

Bob Freitas asked:
>> Anyone want to tackle an explaination of " TRAIL" and tire
>> sizing? I have always been more comfortable with a wider tire
>> in a downhill situation but as we all know WIDE tires (+25MM) are
>> getting very hard to find.
>>
> Some answers: 1) Trail is just a distance on the ground, measured between two points. The first is the extension of the steering axis (through the center of the steerer tube) to the ground. The second is a vertical between the hub center and the ground, which is always behind the intersection of ground and steering axis. Why important? If you push (or ride) the bike straight, if the wheel contact is moved sideways away from its "natural" intersection at directly below the hub center, then as the bike moves forward it really wants to pull the wheel contact back toward the "correct" position. The more trail, the more it wants to move back. More or less.

2) Want a better answer? Read Chapter 8, by Jim Papadopoulos, in the just-published third edition of Bicycling Science by Wilson (and Papadopoulos, who got shafted on coauthorship).*

3) Want the "right" answer about how bikes steer? It turns out to be more complex than one thinks, and there is more work to be done -- I guarantee that I won't and can't do it.

4) Want another perspective? recent Rivendell Reader empirical approach to varying trail by changing offset. Offset is the perpendicular distance between the steering axis, extended to the ground, and the wheel's hub center. Affects trail, but not the same animal.

harvey sachs mcLean va *I've promised a fuller review of Bicycling Science as it pertains to CR list members, but it will be a few days; happy if someone scoops me.