At 12:47 PM -0400 6/26/09, Bianca Pratorius wrote:
>From what I'm reading I would imagine that round fork blades would
>be my choice and that of many others for road bikes. I feel the need
>for increased lateral stability on the tight corners but I feel no
>need for less flex in the vertical plane. I enjoy the feel of
>soaking up the bumps and I think that normal or even full out
>pedaling doesn't require increased vertical stiffness because not
>too much pressure is put on the fork during hard pedaling.
>
>Garth Libre in Miami Fl USA
First, all fork blades are round at the bottom, most do become oval near the top, and for a reason.
Round fork blades really don't make much sense on a road bike.
In corners, your bicycle's fork blades don't flex laterally. The bike is balanced, and the only force is in the plane of the bike - unless you hang off the saddle to one side. (After all, if there were lateral forces, your bike would not stay upright, and you'd crash.) Fork blades do flex when you sprint out of the saddle, but wheels flex even more, so fork blade flex isn't an issue there.
Fork blades do flex fore-and-aft when you brake. This is most pronounced at the top (longest lever arm), so there is a good reason to make the fork blades stiff near the fork crown, where a failure is most likely.
For flex and comfort, it appears better to try and get the lower portions of the blades flex by making them small in diameter. This was common on many older Italian, French and British bikes. Bicycle Quarterly's tests have show that this works well.
So a rational design for a fork blade would make the top relatively long and narrow, to resist flex at the fork crown/fork blade junction. The bottom should be round and small in diameter.
This describes the fork blades found on many 1950s bikes, especially those using the "old Imperial oval" cross-section. The Reynold 531 "Super Resilient" blades found on many Alex Singers are a perfect example. Today, the closest I can find are the Toei-special blades drawn by Kaisei for Toei.
Round fork blades can work fine on road bikes, as evidenced by the many people who ride them. But they are not an optimized solution.
Jan Heine
Editor
Bicycle Quarterly
140 Lakeside Ave #C
Seattle WA 98122
http://www.vintagebicyclepress.com