Re: [CR]Curved crank sets...Why

(Example: Framebuilding)

Date: Sun, 2 Jul 2006 11:19:09 -0700
From: "Kurt Sperry" <haxixe@gmail.com>
To: "The Maaslands" <TheMaaslands@comcast.net>
Subject: Re: [CR]Curved crank sets...Why
In-Reply-To: <0a6101c69df0$65e29da0$0200a8c0@HPLAPTOP>
References: <0a6101c69df0$65e29da0$0200a8c0@HPLAPTOP>
cc: CR <Classicrendezvous@bikelist.org>

The point I was trying apparently unsuccessfully to make was that from a biomechanical standpoint, the only relevant geometric consideration is the distance of the pedal hole from the crank centerline. The shape or form of the structure connecting the pedal to the crank spindle will have no biomechanical effect whatsoever discounting significant flexure.

Kurt Sperry Bellingham WA

On 7/2/06, The Maaslands <TheMaaslands@comcast.net> wrote:
>
> 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
>
>
>
> _______________________________________________
>

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