From: cma851@huxley.anu.edu.au (Brenton LeMesurier)
Newsgroups: rec.bicycles.misc
Subject: Re: Power needs on recumbents vs. diamond frames (was: Recumbents)
Date: 22 Jan 93 04:57:49 GMT
Organization: Australian National University
NNTP-Posting-Host: 150.203.2.12

I think I owe a few clarifications and partial retractions of my
previous response to Torsten Lif.

    Firstly, the definition should of course have been

power (= time rate of work) = force times "speed component in the
direction of the force",

    Next, etxorst@eos.ericsson.se (Torsten Lif) writes, in part:

>There is a world of difference between the concept of "work" as
>defined by school physics and the dynamics of actually *riding* a bike.
>Your example is flawed because it relied on the support of rigid
>structures. The chair in one case and your locked knees in the other. It
>takes very little muscular force to keep your knees straight and this is
>why you can stand up with little effort. But try this for a change:
>Remain in the "sitting" position you had on the chair but remove the
>chair so that you now have to support your weight with your muscles only
>(knees at 90 degrees). Let's see how many seconds of statich work you can do.

And he is right that producing a force through the braced joints of
straight legs is quite different than producing it directly by muscle
action.  I failed to explain the intent of my example, which was only to
dispel a misunderstanding that has appeared here before: that
producing a force NECESSARILY consumes metabolic energy at a rate
proportional to that force regardless of the mechanical work being done
by that force, or thinking along those lines.

    And that is enough to support my main point, modified to the more
reasonable case of pedalling in the saddle:

    Suppose first that you pedal on a recumbent with a certain pattern of
pedalling force through each crank revolution.  If you then repeat this
pattern of pedalling force while seated on a diamond frame, you will
still deliver the same power to the drive train.  At the same time, the
vertical component of that force will give some partial support of you
body weight, reducing the "saddle pressure".  This partial support does
not require any extra force, metabolic work, strain on joints, etc.

    On the other hand, when pedalling out of the saddle, the forces of
typical seated pedalling are less than body weight, so to hold
themsleves up riders typically change to a lower cadence, higher gear,
higher force riding style, which for me at least is far more painful.
Thus I will partially agree with Torsten's quote: sustained pedalling
out of the saddle is a pain in the thighs.  My solution is to have
low enough gears to stay seated, which are the same gears (or very
slightly higher due to bike weight differences) as would be needed to
climb on a recumbent using the same pedalling force.

    If someone suggests that the recumbent allows higher forces by
pushing back on the seat, I will reply that firstly, I like to avoid such
forces for the reasons mentioned above and secondly, the upright rider
can achieve the same thing by some combination of taking their weight of
the saddle and pulling back on the handle bars.  Perhaps better yet
would be the "leash between the waist and the handlebar stem" idea that
was banned from racing but might appeal to some recreational riders.

    Do not take this rejection of one argument for recumbents as a
general attack on them: I would like the chance to try a day ride on
one, to experience for myself the trade-offs of genuine advantages such
as aerodynamics against things like greater weight.
-- 
Brenton LeMesurier
Department of Mathematics, Australian National University
GPO Box 4 Canberra 2601 Australia
Email: B.LeMesurier@anu.edu.au  Phone: (61) 6-249-3829  FAX: -5549
Newsgroups: rec.bicycles.misc