[CR]Re: Fork Blade Curves

(Example: Framebuilding:Norris Lockley)

From: <BobHoveyGa@aol.com>
Date: Mon, 3 Jul 2006 10:25:15 EDT
To: classicrendezvous@bikelist.org
Subject: [CR]Re: Fork Blade Curves

In a message dated 7/3/06 1:40:00 AM, John Jorgensen writes:


> Why the convention was to achieve fork rake with a curved blade instead of
> an
> offset crown?  Don't know that one. In the past I bet manufacturability
> probably
> favored the curved blade. Easier to adjust rake by bend rather than retool .
>

Great theory! That is probably the most plausible explaination I've heard.


>Aesthetics of a straight bladed fork are a different matter and generally beyond the CR list timeline, but functionally probably equal.

I don't know of anyone who played with straight-bladed forks aside from the off-topic Colnagos and off-road suspension forks, does anyone know of any earlier examples? Aesthetically, I react so strongly to straight bladed fo rks I've seen thus far that it would be hard to imagine a design solution that w ould be pleasing to my eye. At least on a bicycle... they seem to look perfectl y fine on motorcycles which leads me to believe that in this case at least, taste is determined in some part by what one is used to.

Harvey Sachs writes (about the PMP crank):
> I admit being surprised that the discussion on this particular triumph of marketing hype over common sense (let alone engineering) continues.

Let me see if I get this straight... the curved crank was supposed to flex o n the power stroke, storing a bit of energy and returning it at a weaker point

of the stroke? OK, I can grudginly admit that makes a bit of sense, at lea st in the case of the forward-curving arms of the eBay crank that started this discussion. But it makes no sense for the PMP... the arms are bent in the other direction. That means that on the power stroke, force would be exert ed more parallel to the long section of the arm and the area of greatest stress

would be on the inside of the "elbow" (a great place for stress cracks to de velop I would think). If there is anything to this energy storage-return idea, would it not make more sense for the arm to bend the other way so that the inside of the elbow is in compression (and the long portion of the arm is perpendicular to the direction in which force is applied, thereby allowing i t to flex and store energy)?

Goodness gracious, maybe that's why the idea never caught on... they bent th e danged arms in the wrong direction!

Bob Hovey
Columbus, GA