Re: Fast pedalling, why? Re: How many gears are enough?

On 2009-07-09, carlfogel@xxxxxxxxxxx <carlfogel@xxxxxxxxxxx> wrote:
On Thu, 09 Jul 2009 04:08:56 -0500, Ben C <spamspam@xxxxxxxxx> wrote:

On 2009-07-08, carlfogel@xxxxxxxxxxx <carlfogel@xxxxxxxxxxx> wrote:
On Wed, 08 Jul 2009 16:16:49 -0500, Ben C <spamspam@xxxxxxxxx> wrote:

On 2009-07-08, Johnny Twelve-Point presented by JFT <usenetremove@xxxxxxxxxxx> wrote:
[...]
You're using energy making your legs change direction -- the force is
pushing the mass of your legs down, then up.

As one leg falls under its own weight, it lifts the other one. It's
counterbalanced and total energy expended just moving your legs is
therefore zero, _apart from_ all the losses incurred by your legs being
full of water etc. sloshing and squishing around, although that might be
quite a significant amount.

If you were a terminator (with the living bits all shot or burned off,
as tends to happen eventually) you could spin all day at high cadence
only losing a tiny amount of energy to your bearings.

Dear Ben,

Legs bend significantly back and forth at the knee--a good deal of the
mass of each leg accelerates back and forth horizontally as well as
rising and falling vertically.

| \ | \ |
| / | / |
| | |

True, but it still doesn't cost you any energy (apart from the losses
because your legs are damped).

Consider just one leg attached to a pedal pushing down once. You will
accelerate the mass of your leg, but your foot is attached to the crank,
which is constrained to move in a circle. The pedal will fly round the
bottom and start to come back up again, accelerating your leg back in
the opposite direction, i.e. slowing it down again. But when it gets to
the top, instead of flying upwards, it will continue to go round as it's
still attached to the crank.

There is a continuous centripetal force in the crank which accelerates
the leg continuously to keep it moving around. But there is no net
transfer of energy out of the system (apart from damping, friction,
etc. which result in heat). This is why cranks work so well.

The situation is just the same horizontally as vertically, or in any
other direction.

I don't think I've explained this very well. But if you are still
thinking energy is required to keep your legs turning apart from because
of damping losses, you have to ask where that energy goes and how it
leaves the system. Fogel radiation?

You will lose more energy at higher cadences though due to damping,
because it probably depends quite a bit on leg velocity, but of course
there may be physiological benefits to keeping the peak forces a bit
lower.

So I would be surprised if lower cadences weren't more energy efficient,
but that doesn't necessarily mean they're better. It's also just a
matter of individual preference.

Dear Ben,

As far as I can see, the situation involves accelerating the heavy
thigh back and forth.

The heavy thigh does accelerate back and forth, but muscle force is not
required to cause those accelerations (apart from to overcome losses).

Force is required to accelerate a heavy beam back and forth, pivoting
from the hip joint.

The thigh does not move in a circular fashion--it waves back and
forth, what Sharp called a vibratory motion.

Yes, although many different kinds of oscillation are possible without
any net energy entering or leaving the system. A car engine for example,
with the spark plugs removed, would theoretically spin for hours once
set in motion with the pistons and valves and things all going up and
down happily with no loss of energy except to the usual friction and
drag.

Both thighs come to a complete stop in their waving motion (in
opposite directions) every half-cycle of the pedal motion.

Yes that is an interesting point. Balance is also important here-- if
you think of the engine spinning, is there any net force on its
mountings? If so then whatever it is attached to is part of the
oscillating system.

A bicycle rider's upper body may rock a bit, but generally the motion
seems to be quite well-balanced, although I don't know all the details
of how all the different leg-parts are moving at each point in the
stroke.

The greater the acceleration, the greater the force required.

And that's just the simple thigh motion, waving back and forth from a
fixed point.

The motion of the lower leg, from knee to foot, is an even more
complicated waving motion, since it isn't fixed at either end.

Grab a yardstick in the middle and start twisting your wrist to wave
the ends of the yardstick back and forth.

The faster you wave the ends of the yardstick back and forth, the more
force it takes to accelerate and decelerate them. The whole system
(two thighs or a single yardstick) is constantly accelerating as it
reverses direction.

The yardstick is different because you have to use your muscles to keep
slowing it down and speeding it up.

Pedalling would be like that if you didn't have a bottom bracket. You'd
need toe-clips or something but you could still turn the cranks round--
it would be similar to lying on your back and pedalling in the air.

All changes of direction would have to come from forces applied from
your leg muscles, so you'd have to do braking work to maintain control.

But if you do have a bottom bracket, it's not the same, because you
don't have to slow your legs down-- the cranks do that for you.

Having said all this, I can't say for certain that there is _no_
muscular deceleration involved in pedalling. There doesn't have to be,
but some riders may do counter-productive things like pushing down on
the up-pedal which is making one leg work against the other and is just
wasting energy.

Related point: I was watching boxing on TV and the commentator (a former
boxer) pointed out that you tire yourself out much more if you're
swinging at the other guy without hitting him than if your punches are
landing. That's because if you miss you have to do work to slow your
fists down again.
.

Relevant Pages

  • Re: Fast pedaling, why? Re: How many gears are enough?
    ... heavy thigh back and forth. ... basically a reciprocal motion into a rotary motion. ... Please tell me the details of any observations of energy lost to ... expended in lifting the rising leg using force from the descending leg ...
    (rec.bicycles.tech)
  • Re: Fast pedaling, why? Re: How many gears are enough?
    ... heavy thigh back and forth. ... basically a reciprocal motion into a rotary motion. ... energy went. ... expended in lifting the rising leg using force from the descending leg ...
    (rec.bicycles.tech)
  • Re: Pushing off from the heels?
    ... The leg movement on the ... swing were used) caution against making a large leg movement. ... even two straight legs spans little ... Small-scale aspects don't store sufficient energy. ...
    (rec.arts.dance)
  • Re: Pushing off from the heels?
    ... The same effect occurs in complete dancing as well, ... preclude restraining the travel of the first step. ... not that the leg needs to be straight. ... you should not be attempting to analyze the energy conversion. ...
    (rec.arts.dance)
  • Re: Pushing off from the heels?
    ... leg division, then the lowest point will naturally occur when the body ... But if knee flex dominates the lowering, ... while the standing leg is straight. ... of kinetic energy to potential energy conversion ...
    (rec.arts.dance)