RE: [CR]30's Bike Geometries


In-Reply-To: <061520071312.16703.4672903A000643000000413F22007637040B029A019C0A0A9D00@comcast.net>
References: <061520071312.16703.4672903A000643000000413F22007637040B029A019C0A0A9D00@c omcast.net>
Date: Fri, 15 Jun 2007 08:28:10 -0700
To: freesound@comcast.net, Donald Gillies <gillies@cs.ubc.ca>, classicrendezvous@bikelist.org
From: Jan Heine <heine94@earthlink.net>
Subject: RE: [CR]30's Bike Geometries


At 1:12 PM +0000 6/15/07, freesound@comcast.net wrote:
>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.
>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!

Head angles in the upper 60s were common in the late 1920s and early 1930s, so your calculations are likely to be in the right ballpark.
>The trail is not really very low, but also not very high. My Masi
>GC has 60 mm, and seems both crisp and stable.

The handling will be very different from your Masi GC. When you calculate the flop factors for these bikes, assuming 60 mm trail for each, you get 22.3 mm for the Legnano (66 degree head angle) and 16.8 mm for the Masi (73 degree head angle). However, the weight distribution may be different, with less weight on the front of the Legnano than a modern bike, so that reduces the amount of wheel flop... (However, the long chainstays put more weight on the front, so it's hard to know what the weight distribution...)
>This plus the trail I think would tend to counteract the slower yaw
>capability of the longer wheelbase.

The wheelbase of 106 cm is only 1-3 cm longer than that most 1960s bikes, and only 4-5 cm longer than most modern bikes. I doubt increasing the wheelbase by less than 5% has a very noticeable effect on the handling. You can test this with an older bike with long horizontal dropouts. You can move the wheel 2 cm fore and aft. I doubt you'd be able to tell the difference if you had somebody else move the wheel and didn't look before you went for a ride.

If anybody has a bike like the Legnano in a large size, which they are willing to lend for a full BQ test, that would be interesting. In the current issue, we report on testing a 1957 Cinelli Supercorsa, which was interesting only because it feels so modern. An early 1930s bike would be very interesting indeed!

The closest to a 1920s/1930s geometry we have ridden extensively was a Jamis Commuter 3.0, with 71 degree head angle, 50 mm offset and 67 mm trail, flop factor 20.6 mm - about half-way between a modern bike and the 1920s/30s bikes. The Jamis wasn't bad, but it wasn't great, either. Mark Vande Kamp was the main tester, and he wrote: "The bike is not eager to change direction, and the extra-wide handlebars require large steering inputs... The handling is fine as long as one stays far away from the limits."

When I rode the Jamis, I found the wheel flop to be a problem only at very low speeds, such as rush hour traffic, where the bike became difficult to maneuver. This would not be a problem on a racing bike, but for a commuter, it isn't quite ideal. However, the Jamis had a very upright seating position, so the flop was less than it would be on a racing bike. -- Jan Heine Editor Bicycle Quarterly 140 Lakeside Ave #C Seattle WA 98122 http://www.bikequarterly.com