Frame stiffness

[Jobst Brandt]

Tom Ace writes:

>>> 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.

> 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.

> 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 protected]

 

[Phil Holman]

"Tom Ace" <[email protected]> wrote in message news:[email protected]...
> [email protected] wrote:
>
> > > 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.
>
> Bear with me please, I missed any earlier post where this was
mentioned.
>
> 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

 

[Carl Fogel]

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

> Tom Ace writes:
>
> > 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.
>
> > 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