on 12/25/03 11:36 AM, Stephen Barner at steve@sburl.com wrote:
> Todd, While it is true that forks involved in catastrophic failure typically
> bend backwards, this is a result of either direct frontal impact, or impact on
> a part of the wheel (say, 5 o'clock, in an extreme case) that is not normally
> in contact with pavement.
My contention that the fork deflects rearward has nothing to do with fork failures I have seen. As you describe, these are most often caused from impact with some object above ground level. Rather, you can see this rearward deflection as you ride on normal pavement. Look down at the fork tips and watch their movement relative to your handlebar. The movement of the dropouts will be rearward.
As I understand Jobst Brandt's argument, the fork is loaded axially when going over a bump. So, the primary force is upward towards the crown. The fork isn't deflecting forward or rearward. Of course, there is some fore-aft movement, which we have all seen. So, there must be some rearward or forward component to the force of the bump.
Jobst argues that any deflection of the fork does nothing to affect ride quality. He points out that even a large fore-aft deflection (whether rearward or forward) would only have a very small vertical component. A couple of years ago on rec.bicycles.tech, he responded to the example of a fork that deflects 1/4" at the dropouts:
"That doesn't translate to 1/4 inch of cushioning but gives mostly fore and aft motion of the wheel. The vertical component thereof being (1-COS(15))*0.25" = 0.0085" vertical motion."
Todd Kuzma
Heron Bicycles
Tullio's Big Dog Cyclery
LaSalle, IL
http://www.heronbicycles.com/
http://www.tullios.com/