John Wood wrote:
But the more I rode it the more impressed I was. Not only comfortable and stable, as I expected it to be, but also a very nimble handler, which I didn't expect.
That matches our observations of these geometries.
Is there any reason that a touring bike with 6.0cm of rake and ~4.6cm of trail is a bad idea now? Is there something with the new tube sets that is incompatable with old style geometry? Or is it simply a matter of style, or the quest for ever lighter and stiffer frames? I notice that even the newer Singers highlighted in recent issues of VBQ have more "modern" trail numbers. Can somebody enlighten me please.
The answer is simple: The "old-style" low-trail geometries require relatively wide tires to work well. Check the iBob archives on "pneumatic trail" for more information. Most racing bikes from the 1940s and 1950s had similar low-trail geometries (check the CR archives for Chuck Schmidt's measurements.) When tires got narrower (and thus had less pneumatic trail), builders increased the geometric trail to compensate. Interestingly, this seems to have happened not just in France, but also in Italy and Britain starting around 1960.
Touring bikes followed the fashion of racing bikes, even if their wider tires did not require the added trail. Exceptions include some Trek 520s from the 1980s, which had low-trail geometries.
For a detailed explanation and sample geometries, check out the "Front-end geometry for different speeds, loads and tire sizes" in Vol. 3, No. 3, as well as the J. P. Weigle test in Vol. 4, No. 2. That bike was tested with two forks, one resulting in 41 mm trail, the other in 51 mm trail. Between the samples in the geometry article and the ride report of the Weigle, you should find a geometry that matches the ride characteristics you would like.
If your measurements are correct, your Braxton should ride well with tires of 27 mm or more, but if you put 20 mm tires, it would lose much of its stability.
Jan Heine
Editor
Vintage Bicycle Quarterly
Seattle
http://www.vintagebicyclepress.com