Harvey,
You raise interesting points. However, my experience is different.
>JH: One question I have is how well these crowns can deal with
>twisting of the fork blades from brazed-on brake pivots - canti or
>even more pronounced with centerpulls, where the pivot is brazed
>close to the crown, with little blade to "absorb" the flexing motion
>each time the brake is applied. Obviously, this twisting action
>would stress the lower plate more than the upper one and might lead
>to failure. Thoughts?
>HS: Jan, I think you protest too much about the matter.
>Empirically, they seem to do just fine, so the stresses must be
>manageable. Indeed, has anyone heard hints to toe-in brake pads
>differently for these crowns?
I have a bike from an American KOF maker that originally had a twin-plate crown and brazed-on centerpulls. The fork crown failed after only about 3000 miles. Nobody was hurt. The replacement one-piece crown has been fine for about 30,000 miles since. Most eerily, when I showed the brand-new bike (with twin-plate crown, before failure) to Ernest Csuka, he said "Nice bike. That fork crown is going to break. We did some like that in the 1940s, and they failed." I did not know him well, so I paid little attention to the "old codger" talking disparagingly about my pride and joy. He was right, the crown lasted only 8 months!
That was years ago.
>
>JH: I wonder whether this is the reason most French constructeurs
>did not use twin-plate crowns on their production bikes after World
>War II. In the 1930s, they were found on many bikes, including
>tandems. The fact that they switched from a lighter to a heavier
>crown on their production bikes can't have been cost, so there must
>have been a functional issue.
>HS: I don't quite follow your logic. Most (but not all) the less
>expensive post-war French production tandems (particularly Gitane
>and Follis)I've seen had twin-plate crowns.
But the mass producers did not use brazed-on pivots for the brakes! My reasoning was that if Herse, Singer et al., went from a lighter (twin crown) to a heavier (one-piece) design, there must have been a reason beyond cost - even IF the twin-plate had been more expensive. (The best constructeurs did a lot of things that could have been done cheaper and more simply, so cost wasn't really an issue.)
Even on superlight bikes, Singer used a one-piece crown (machined out
underneath to save a few grams), despite the fact that at the same
time, their technical trials bikes used a modified twin-plate crown
(see
http://www.mindspring.com/
So to summarize, I am all for twin-plate crowns, but not on bikes with brakes that have pivots brazed onto the fork blades.
--
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
c/o Il Vecchio Bicycles
140 Lakeside Ave, Ste. C
Seattle WA 98122
http://www.mindspring.com/