[CR]Curved crank sets...Why

(Example: Events:Cirque du Cyclisme:2004)

From: "The Maaslands" <TheMaaslands@comcast.net>
To: "CR" <Classicrendezvous@bikelist.org>
Subject: [CR]Curved crank sets...Why
Date: Sun, 2 Jul 2006 11:58:44 -0400


Kurt wrote:

"The problem with that theory is that the pedal isn't indexed to a round chainring in any particular way. You can put a 90 degree (or any other, it makes no functional mechanical difference) bend in the crankarm, a curve or a double loop-de-loop and from a mechanical standpoint nothing really changes except you've simply moved the 'dead spot' relative to the chainring and weakened the arm for a given amount of material used."

in response to what I wrote earlier: "The idea had something to do with overcoming the 'dead spot' at the top of the pedal stroke. As we all know, there is a dead spot in the pedal stroke that occurs when the pedals are at 12 and 6 o'clock positions. The claimed purpose of these cranks was to store enough energy in the rotating crank to allow the rider to get through this dead spot with a minimal amount of energy that was 'stolen' from another part of the revolution. This was achieved by displacing the energy transfer from a direct transmission to a slightly offset point of the rotation. The PMP cranks placed the actual pedals at the maximum strength position of the rotation (namely at between 2 and 4 o'clock and 8 and 10 o'clock respectively) whereas the energy transmission was located at the 'dead spot'."

Now, playing the devil's advocate, as I do not personally espouse the theory that I outed in my post, I must point out that you are not at all correct in your reply to it. For your statement, namely that the pedal is not in any way indexed to the round chainring, to be correct, would require a constant force and leverage. If you did have constant force and leverage, your statement would indeed be correct. The theory that I outed, but do not believe, claims that in the dead spot of the crank rotation, the 'delayed' leverage, supposedly created by the curved crank, will fall precisely in this dead spot, thereby transferring some of the force from the points of full power to a point of lesser power.

I believe the curved crank theory to be a crock because I instinctively believe that the "delay" in the leverage of the crank is but an absolutely minor portion in the overall "delay" occurring during pedalling. In fact, I believe the "delay" in leverage of the chain, tire and spokes will all greatly outdo the "delay" caused by the cranks.

Steven Maasland
Moorestown, NJ