From: jbrandt@hpl.hp.com (Jobst Brandt )
Newsgroups: rec.bicycles.off-road,rec.bicycles.tech
Subject: Re: Power at bottom of stroke (was: STX, STX RC or Deore LX)
Date: 27 Aug 1996 00:33:17 GMT

Dennis O'Connor writes:

>> Nonetheless, there is a point in each stroke--no matter how fast or
>> smoothly it passes--where power cannot be applied, and that is at
>> the bottom of either stroke.

> I was taught that at the bottom of the stroke you wanted to be
> "scraping your feet".  This does provide power at that part of the
> stroke, and certainly helped me to smooth out my spin too.

It provides "force" at that part of the stroke at a certain cost.
Actually there is a large part of the stroke that transmits no power,
especially when riding fast on flat ground.  A concern is often
expressed that for speed and efficiency, power should be delivered
smoothly and continuously to the cranks.  This is untrue for both
motors or humans.  All work or power put into the movement of the
cranks goes into moving the bicycle and is integrated into its kinetic
energy, it cannot leak out anywhere.

The power strokes of various top bicycle riders have been measured.
In a TT, pedal input is typically in an arc from 1:30 to 3:30 in clock
sense, 12:00 being at the top.  The rest of the stroke serves to keep
the pedals turning and bring the other foot over the top.

The reason this works is that engaging other muscles than the
principal pushing ones distributes cardiovascular supply, that is the
limiting factor, to other muscles, each with its own overhead and
losses.  The most efficient output is to concentrate the effort in the
main muscle groups and to not pull up, back, or shove forward over the
top.  This is not to say that in the short term these efforts can
increase output but they are only anaerobically so.  Aerobically it has
been demonstrated, that for instance, turning cranks with the hands
decreases the effective power available over pedaling only with the
feet.

To assess this concept, a simple experiment can demonstrate this
effect if you have a speedometer that has 1/10 mph resolution.  Ride
at a brisk continuous pace requiring heavy breathing then change the
pedal action so that you push only every third time each foot comes
over the top of the stroke, counting one-two-three-one-two-three-...
with every stroke, pushing only on "one".  You will find that your
speed barely diminishes if you keep the same effort when in fact,
according to common belief, it should diminish to 1/3 the effort.

This is consistent with the one legged time trialist we had here a few
years ago who was about as good as the best in the club.  He, as the
rest of us, was out of breath when he went fast, and fast he went.  He
had one strong leg and a polio crippled other leg that was not much
more than a pedal return device.

A similar effect is found in automobile engines where the highest
cylinder pressure occurs near top dead center when the piston acts on
the crank with practically no leverage.  What is overlooked by this
perception is that the gas pressure is expanded against the rotating
crank and that the gas expansion work passes entirely to the output
shaft.  No system of levers can extract more power from that stroke
than a simple crank.  In bicycling, the same is true.  Inventors build
strange lever mechanisms that appear to apply the foot force to a more
effective mechanical advantage.  Force times distance is still the net
output and employing other muscles, different strokes or levers,
pushing and pulling, do not improve the output of heart and lungs that
are, after all, the limit of performance.

Forget "round pedaling" it's a theoretical bugaboo.

Jobst Brandt      <jbrandt@hpl.hp.com>