Originally Posted by
steelbikeguy
It's interesting that the deflection varies as the cube of the beam length, so a small increase in length produces a much larger change in deflection.
Of course, with a curved fork, the distance from the head tube to hub axle is the same as for a straight fork, so maybe the ratio of the tube length to the cross section ("area moment of inertia") increases, and that's what makes it more flexible?? I suppose this is similar to the idea for using a coiled spring instead of a short straight piece of metal.
As I stated earlier, two forks, one straight, and all else being equal, if the fork crown to dropout measurement is the same, if you straightened out the curved fork the blades are longer. So the
material distance of the fork blade is
longer than the straight one, similar to a coiled spring. The curved fork will flex more. I'm not sure if it makes that much of a difference, but plug it into your formula and see how much.
Originally Posted by
steelbikeguy
Going further out on the tangent, since a longer beam is more flexible, and since it is theorized that much of the fork flex is actually flexing of the steerer tube...
Steve in Peoria
That better not be true! Think of what that would do to the bearing contact in your headset, not to mention the havoc it might play with your head tube. A steerer tube is at least 1" diameter, steel fork blades are significantly smaller, especially down in the taper area. Check wikipedia for tubing flex equations, diameter has an effect, tubing wall thickness as well. Simple explanation: I can bend 1/4" tubing with my bare hands quite easily. No way I can bend and deform a 1" thinwall top tube without power assistance.
Framebuilders know that forkblades come longer than needed. If you want a stiff fork, you can trim from the tapered end, leaving more of the blade in a larger diameter. Conversely, a less stiff fork (which I prefer) you remove material from the tapered end.
Framebuilder Supply here in Portland has a nice explanation of this.
I distinctly remember looking down at my front hub while powering through that washboard back at the Tour of the Palouse. It was almost scarey watching how much the fork blades were flexing.