Frame stiffness

Tom Ace said:

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Quoted message said:

Imagine you're tightening a bolt with a ratchet and socket, and you have a long extension that's
not stiff torsionally and winds up with each stroke. You won't turn the bolt as many degrees
with each stroke as you would with a stiff extension. Where does the energy go that was stored
in winding up the extension?

Quoted message said:

Quoted message said:

Oops! There is a rebound with that example and one that doesn't advance the bolt. In a bicycle,
there is no rebound, the foot never retreating from its most advanced position of the pedal
revolution. This is the point I made earlier.

Quoted message said:

Bear with me please, I missed any earlier post where this was mentioned.

What sort of newsreader do you have? My ISP has articles way back, and TIN can fetch them with the
"show all read articles still in the buffer" command.

Quoted message said:

I'm still not convinced. There isn't the same kind of rebound as in the ratchet example, but there
is a stroke, and downward force on the pedal has different effects at different parts of the
stroke. You can't apply much drive torque near the bottom of the stroke. If the frame springs back
at that point, is the energy returned really going primarily into driving the bicycle?

Make sure you observe the force time distance effect required for work done. At the bottom of the
stroke, typically when standing, frame flex may let the foot move laterally but that does not
require extension or contraction of any muscles. The cranks do not cease rotating at a uniform rate
prescribed by bicycle forward motion.

You must draw your envelop around the effective elements and make use of the integrating effect of
the crank-chain-wheel unit and what the legs are doing. Similarly, those who think their arms are
part of propulsion work are ignoring that arm movement takes place at the top and bottom of pedal
strokes when there is no load. Arm holding force is applied during the stroke but the arm muscles do
not expand or contract during that phase.

Jobst Brandt [email hidden]

"Tom Ace" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Quoted message said:

Quoted message said:

Imagine you're tightening a bolt with a ratchet and socket, and

you

Quoted message said:

Quoted message said:

Quoted message said:

have a long extension that's not stiff torsionally and winds up

with

Quoted message said:

Quoted message said:

Quoted message said:

each stroke. You won't turn the bolt as many degrees with each stroke as you would with a
stiff extension. Where does the energy go that was stored in winding up the extension?

Oops! There is a rebound with that example and one that doesn't advance the bolt. In a bicycle,
there is no rebound, the foot never retreating from its most advanced position of the pedal
revolution. This is the point I made earlier.

Bear with me please, I missed any earlier post where this was

mentioned.

Quoted message said:

I'm still not convinced. There isn't the same kind of rebound as in the ratchet example, but there
is a stroke, and downward force on the pedal has different effects at different parts of the
stroke. You can't apply much drive torque near the bottom of the stroke. If the frame springs back
at that point, is the energy returned really going primarily into driving the bicycle?

The frame springs back gradually relative to the decreasing pedal force. It doesn't save it for a
dump at the bottom of the stroke. We can analyze your bolt tightening (loosening) example similar to
the crank. When loosening the bolt, the torsion force on the extension will gradually reduce (forget
about ratcheting) so it torsionally unwinds as the bolt loosens. Where did the energy go? If you
integrate a torque and angular rotation plot you will find that the energy stored in the extension
was not wasted.

Phil Holman

[email hidden] wrote in message news:<[email hidden]>...

Quoted message said:

Tom Ace said:

The losses can be in your legs. The frame may be close to 100% elastic (no significant
hysteretic loss) but the energy doesn't necessarily go into driving the bicycle.

Quoted message said:

Imagine you're tightening a bolt with a ratchet and socket, and you have a long extension that's
not stiff torsionally and winds up with each stroke. You won't turn the bolt as many degrees
with each stroke as you would with a stiff extension. Where does the energy go that was stored
in winding up the extension?

Oops! There is a rebound with that example and one that doesn't advance the bolt. In a bicycle,
there is no rebound, the foot never retreating from its most advanced position of the pedal
revolution. This is the point I made earlier.

Jobst Brandt

Dear Jobst and Tom,

Forgive a layman's intrusion, but I'm not sure that I'm following both examples. Is this somewhere
near what you two are saying?

Turning a stiff bolt with a springy-handled ratchet would waste energy because the repeated flexing
and rebound of the handle as it is bent and then pulled back doesn't help turn the bolt. (Somewhat
like the inefficiency of a piston?)

The bicycle pedal, on the other hand, is a constant spin against the slightly springy resistance of
the the frame flexing under the first quarter of the downstroke and then snapping back during the
second quarter of the downstroke, restoring much of the effort that was lost. (Somewhat like the
efficiency of a turbine?)

Carl Fogel