Todd,
Since we just got a foot of snow, it will be awhile before I can watch the fork tips flex, but I promise I will, first chance I get. I skipped the math and drew up the fork deflection of a 2" rake fork on a 73 degree angle. 1/4" of flex translates into .08" vertical deflection, not .008". Still a small amount (a little over 1/16"), but off by a factor of 10. Jobst isn't saying that the fork is loaded axially when going over a bump of random size. He is saying that the fork is loaded axially when going over a statistically frequent obstacle, like a board. If one watches the road ahead, this type of obstacle should be pretty rare. The normal stones and cracks in the road should be much smaller than that and should not load the fork at an angle sufficient to force the vector axially to the headset. The relative comfort of a bike is partially determined by the amount of vibration transmitted to the hands over time, not so much the occasional bump. Sure, stress is concentrated in the upper area of the fork, but this area is formed from tubing with a much larger cross section than the tips of the forks, especially in the direction of motion (on a road fork), and diameter of tubing has a great deal to do with it's abilty to resist flex. Since you have stated that you can see movement in the axle from 4' away, it appears that there is, indeed, significant movement at the dropouts that is not being transmitted to the handlebars, whatever its direction. I wouldn't be quick to dismiss comments from experienced riders that fork detailing has no appreciable effect on ride characteristics.
Steve Barner, think about skiing in Bolton, Vermont
> 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/