At 8:53 PM -0600 12/26/08, Mark Stonich wrote:
>At 12/26/2008 10:30 AM -0800, Jan Heine wrote:
>>If you want to build a lightweight mixte that offers good
>>performance, you'll have to go with a fully triangulated frame such
>>as that used by the French constructeurs. On those, an extra set of
>>stays feeds the loads from the "top" tube into the rear dropouts.
>
>That's just what I want to avoid. With cheap steel in the seat
>tube, to avoid failure due to bending loads you have to make it
>heavy or add mixte stays.
Most of the French constructeurs used Reynolds 531 for their women's bikes. For those not familiar with the design, a photo is here
http://www.vintagebicyclepress.com/
Not immediately obvious, a single top tube goes is supported by an
extra set of stays (unlike a "mixte" that has two long stays that run
all the way from the dropouts to the head tube).
>Good steel can flex quite a bit without taking a set. Mixte stays
>make the frame stiff in the vertical plane. Which is fine for
>bridges. Bicycles, not so much.
Is it stiffer than a traditional men's frame? I don't see how the extra seat stays transmit extra shock to the seat that isn't already transmitted by the standard, upper seatstays... Of course, eliminating the extra stays on the "constructeur" women's frame would cause the entire frame to flex like a giant spring when you hit a bump, front or rear. So it would create a frame that is much less stiff than a "men's" frame.
You could create a similar men's frame by moving the seatstays way
down so they attach in the middle of the seat tube, rather than the
top. In both cases, the loads of bumps would bow the seat tube in the
middle. I wonder whether that is a good idea.
>>As you suspect, the mixte that feeds loads into the unsupported
>>middle of the seat tube requires a beefy seat tube to resist the
>>bending loads.
>
>That is not at all what I suspected. I'm not surprised that some
>are reporting 27.2 mm seat posts on the highest quality frames.
Was that a conscious design choice based on calculating stresses, or was it just an issue of using standard tubing? If a standard 8-5 seat tube works even on a women's frame that bends in the middle, doesn't this mean that when the same tube is used on a men's bike, it's totally overbuilt?
The women's bikes for the technical trials all used the "constructeur" frame design (single top tube, extra stays). Either Herse, Singer and Routens did not realize they did not need those extra stays (unlikely), or the stays were needed with the superlight tubing these builders used, when ridden hard on very bumpy roads.
One issue with many women's bikes is that they aren't ridden hard. (Avid female riders usually prefer "men's" bikes). So the fact that they haven't broken does not always mean much. It would be interesting to get some data under controlled conditions. Let us know how your wife's new bike works out in the long run.
Jan Heine
Editor
Bicycle Quarterly
140 Lakeside Ave #C
Seattle WA 98122
http://www.vintagebicyclepress.com