RE: [CR]30's Bike Geometries

(Example: Events:Cirque du Cyclisme:2007)

From: <freesound@comcast.net>
To: Donald Gillies <gillies@cs.ubc.ca>, classicrendezvous@bikelist.org
Subject: RE: [CR]30's Bike Geometries
Date: Fri, 15 Jun 2007 13:12:26 +0000


Going back to the formula for trail, a laid-back head angle (numerically small) increases trail, if the radius and offset are equal. I printed the picture and scaled some numbers. Then I'll indulge in some speculative assessment.

saddle length 28 cm (this to get a scaling factor, assume length of current Pro) Seat tube c-c 53 cm TT 53 cm wheelbase 106 cm seat tube angle 71 degrees Head tube angle 66 degree, measuring TT to head tube axis Head tube angle 68 degrees, measuring hub-hub to head tube axis Wheel radius assume 333 mm, 19 mm tire rake about 8 cm BB drop about 8 cm

Taking the 68 degree angle, gives a trail of 48 mm ("sport-tour") Taking 66 degrees, trail is 61 mm (similar to many modern race bikes) Taking the average angle of 67 degrees, trail is 54 degrees.

This trend agrees with the Richard Jow article.

The head tube angles all seem really low, but the trails are plausible, and I believe the rake because of the seat length. At least I think the percent error in my rake is much less than that in my head tube angle!

So it's probably not what the Ibobbers are calling a low-trail design FWIW.

With the long chainstays, "late" fork curve, and laid back head tube, I'd guess this rides as cushy as an old Cadillac. The trail is not really very low, but also not very high. My Masi GC has 60 mm, and seems both crisp and stable. I think the low BB will give a sense of stability with a readiness to lean when countersteered. This plus the trail I think would tend to counteract the slower yaw capability of the longer wheelbase.

The front-center is about 63 cm, combined with the high-setback (16 cm) seat position, this is very different from modern bikes, but I don't have the experience to assess the effect of those dimensions. I assume the rider needed this setback (long femur?) to place his weight properly wrt the crank, and the long front center was to restore suitable wheel loading (45/55 perhaps). I assuem the chainstays are so long to give vertical compliance to the rear triangle.

So that's what I think, but it's mostly just speculation. I do want this bike, however!!!

Ken Freeman
Ann Arbor, MI USA


-------------- Original message --------------
From: Donald Gillies

> For posterity's sake, the golden Legnano has a chromed seat-lug,

\r?\n> chromed c-clamp head-lugs and chromed head-tube - with enough space

\r?\n> behind the seat tube for a pullman case (about 3"), plus enough space

\r?\n> under the downtube for 3 waterbottles. To have some toe overlap one

\r?\n> would have to wear a size-27 shoe, at least.

\r?\n>

\r?\n> Seriously, though, the bike has what looks like to me maybe a 71 or 70

\r?\n> degree head angle (swag), which means LOTS of trail (see richard jow's

\r?\n> 1970's bicycling magazine article on trail).

\r?\n>

\r?\n> To summarize that article:

\r?\n>

\r?\n> Bikes with more trail tend to go into a straight line, (think shopping

\r?\n> cart, where the wheel trails its swivel point) but when you finally

\r?\n> wrestle the handlebars into a turn there is more "wheel flop" (think

\r?\n> of a chopper motorcycle) as turning the bars doesn't so much rotate

\r?\n> the wheel around a perpendicular line coming straight up out of the

\r?\n> ground, rather, it rotates the wheel around a line through the hub

\r?\n> that is nearly parallel to the ground.

\r?\n>

\r?\n> Thus, avocet tires with the raised center ridge can be highly

\r?\n> unpredictable (hard to steer back straight again, once turned) on a

\r?\n> 1930's bike like this, with lots of wheel flop, since steering

\r?\n> straight requires lifting the front end of the bike from the low

\r?\n> sidewall back up to the raised center ridge.

\r?\n>

\r?\n> - Don Gillies

\r?\n> San Diego, CA