Help! Cycling Physics Question...



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"W K" <[email protected]> wrote in message news:[email protected]...
>
>
> Nope. There will be a difference between adding one gram at the pedal end or one gram at the heel
> end. Some of the force to keep the heel going round in (near) circles will be from the cyclist.
>
Yep shoes aren't point masses and the center of mass of the shoe isn't at the end of the crank.

I 'd guess center of shoe mass is about 3cm off the end of the crank, I guess the work done is a bit
like shaking the shoe 6cm back and forth 60 times a minute (or 90 for roadies). More like the
vibration shaker in a playstation gamepad than a planet.

This corresponds well with the fact we all know it is much harder to ride in big heavy boots.
 
"W K" <[email protected]> wrote in message news:<[email protected]>...
> "James Annan" <[email protected]> wrote in message Therefore no work is involved.
>
> Nope. There will be a difference between adding one gram at the pedal end or one gram at the heel
> end. Some of the force to keep the heel going round in (near) circles will be from the cyclist.

Why do you think this makes a difference? What extra work is being done, and where is the energy
dissipated? If there is substantial ankling motion, then I suppose it's a theoretical possibility,
but not if the foot and lower leg are held at a fixed angle relative to one another.

James
 
>> Nope. There will be a difference between adding one gram at the pedal end or one gram at the heel
>> end. Some of the force to keep the heel going round in (near) circles will be from the cyclist.
>>
> Yep shoes aren't point masses and the center of mass of the shoe isn't at the end of the crank.
>
> I 'd guess center of shoe mass is about 3cm off the end of the crank, I guess the work done is a
> bit like shaking the shoe 6cm back and forth 60 times a minute (or 90 for roadies). More like the
> vibration shaker in a playstation gamepad than a planet.

No, I don't think that's the case. If it was, there'd be a simple solution: move your feet forward
on the pedal by 3cm , and the effect would be gone.

My understanding of mechanics gets a bit fuzzy here, but I don't think the position of the centre of
mass of your shoes/feet matters, because your legs and feet hinge on your pelvis, which of course
sits pretty on the saddle.

Imagine a plastic model of a cyclist and bike sitting on a trainer. Get the pedals turning. If it
wasn't for friction, they would keep on turning forever, since there's nowhere to go for the kinetic
energy of the system.

Now imagine attaching lead weights to the feet of the cyclist, no matter where, and repeating the
experiment. The result would be the same, because again there's nowhere for the energy to go.

> This corresponds well with the fact we all know it is much harder to ride in big heavy boots.

There are still plenty of other reasons, why heavy boots are a disadvantage:

- Their weight actually does hold you back mechanically, but not significantly more than the same
weight in any other part of your bike.

- They tend to restrict the movement of your ankles, i.e. increase friction.

- They're not as well ventilated, causing your feet to get hot and uncomfortable more quickly.

- [careful, amateur psychology :) ] The brain is wired primarily for walking and running, and there
of course carrying big lumps of anything around on your feet does slow you down. Cycling is quite
similar to walking or running (you're still moving your legs up and down, and back and forth), so
the brain might wrongly apply the same instincts here.

Andy
 
"Andy Koppe" <a n d y @ d c s . e d . a c . u k> wrote in message
news:[email protected]...
> > However, Plan 36B: If we are on about a bike on a flat track, at
constant
> > speed, exactly the same for the two cases - lighter and heavier shoes, then why not take the
> > bike out of it altogether and think about me sat sitting on front of the television moving my
> > feet in a circular fashion watching Le Tour. The lighter shoes have gotter be easier.
>
> Yep, that's right. Trouble is, you can't just take the bicycle out of the equation, because
> thereby you take out the cranks and pedals, which force your feet into a circular motion. In your
> couch potato experiment of
course
> you have to provide that force yourself.
>
> Here's another, admittedly rather less scientific, thought on the topic.
If
> the weight on a rider's feet was so crucial, the pros on the Tour would surely ride barefoot. Come
> to think of it, the contenders for the yellow jersey would probably have their toes amputated, in
> order to save that little bit extra :)
>
> Andy

I have a nasty feeling you may well be right.... Ha ha, wait a minute, I notice you're not a proper
Physicist - you're just in their building. Oh well, moral victory to me and my "Metaphysics of
Savages(tm)" argument then :)
 
"Andy Koppe" <a n d y @ d c s . e d . a c . u k> wrote in message
news:[email protected]...
> >> Nope. There will be a difference between adding one gram at the pedal end or one gram at the
> >> heel end. Some of the force to keep the heel going round in (near) circles will
be
> >> from the cyclist.
> >>
> > Yep shoes aren't point masses and the center of mass of the shoe isn't
at
> > the end of the crank.
> >
> > I 'd guess center of shoe mass is about 3cm off the end of the crank, I guess the work done is a
> > bit like shaking the shoe 6cm back and forth 60 times a minute (or 90 for roadies). More like
> > the vibration shaker in a playstation gamepad than a planet.
>
> No, I don't think that's the case. If it was, there'd be a simple
solution:
> move your feet forward on the pedal by 3cm , and the effect would be gone.
>
Yep but this isn't a critical factor in design, position of foot makes more difference to efficiency
and shoes can be made light enough for it not to be significant.

> Imagine a plastic model of a cyclist and bike sitting on a trainer. Get
the
> pedals turning. If it wasn't for friction, they would keep on turning forever, since there's
> nowhere to go for the kinetic energy of the system.
>
No this isn't true, vibration would stop the system if it wasn't perfectly balanced. In order to
eliminate work you would need to balance the forces somewhere, be it the frame, or the whole earth.

> Now imagine attaching lead weights to the feet of the cyclist, no matter where, and repeating the
> experiment. The result would be the same, because again there's nowhere for the energy to go.
>
The energy goes accelerating the frame back and forth.

Hang your bike up and spin the back wheel as fast as you can. Unless your wheel is perfectly balance
you will notice the whole frame vibrate. Are you going to claim this doesn't dissipate energy?
 
>> >> Nope. There will be a difference between adding one gram at the pedal end or one gram at the
>> >> heel end. Some of the force to keep the heel going round in (near) circles will
> be
>> >> from the cyclist.
>> >>
>> > Yep shoes aren't point masses and the center of mass of the shoe isn't
> at
>> > the end of the crank.
>> >
>> > I 'd guess center of shoe mass is about 3cm off the end of the crank, I guess the work done is
>> > a bit like shaking the shoe 6cm back and forth 60 times a minute (or 90 for roadies). More like
>> > the vibration shaker in a playstation gamepad than a planet.
>>
>> Imagine a plastic model of a cyclist and bike sitting on a trainer. Get
> the
>> pedals turning. If it wasn't for friction, they would keep on turning forever, since there's
>> nowhere to go for the kinetic energy of the system.
>>
> No this isn't true, vibration would stop the system if it wasn't perfectly balanced. In order
> to eliminate work you would need to balance the forces somewhere, be it the frame, or the
> whole earth.

The system is pretty well balanced though. As long as you wear matching shoes anyway ...

Andy
 
Andy Koppe <a n d y @ d c s . e d . a c . u k> writes:

>The system is pretty well balanced though. As long as you wear matching shoes anyway ...

Few people have feet, or legs, exactly the same size; many people have one leg distinctly stronger
than the other.

--
Chris Malcolm [email protected] +44 (0)131 650 3085 DoD #205 School of Informatics, Edinburgh
University, 5 Forrest Hill, Edinburgh, EH1 2QL, UK. [http://www.dai.ed.ac.uk/homes/cam/ ]
 
>>The system is pretty well balanced though. As long as you wear matching shoes anyway ...
>
> Few people have feet, or legs, exactly the same size; many people have one leg distinctly stronger
> than the other.

Conceded, lucky I used the get-out clause "pretty well" :)

Anyway, looks like we're drifting a bit too far from the original question now ...

Andy
 
"James Annan" <[email protected]> wrote in message
news:[email protected]...
> "W K" <[email protected]> wrote in message
news:<[email protected]>...
> > "James Annan" <[email protected]> wrote in message Therefore no work is involved.
> >
> > Nope. There will be a difference between adding one gram at the pedal end or
one
> > gram at the heel end. Some of the force to keep the heel going round in (near) circles will be
> > from the cyclist.
>
> Why do you think this makes a difference?

It wouldn't make a real difference.

> What extra work is being done, and where is the energy dissipated?

Imagine theoretically if the heel is heavy. "centrifugal" (!!) force would push it outwards when it
was horizontal. As such there is extra force required to make the heel move in a circular pattern at
different parts of the cycle - ie extra downwards force around the top of the stroke and extra
upward force at the bottom.
 
W K wrote:
> "James Annan" <[email protected]> wrote in message
> news:[email protected]...

>>What extra work is being done, and where is the energy dissipated?
>
>
> Imagine theoretically if the heel is heavy. "centrifugal" (!!) force would push it outwards when
> it was horizontal. As such there is extra force required to make the heel move in a circular
> pattern at different parts of the cycle - ie extra downwards force around the top of the stroke
> and extra upward force at the bottom.

This is false, as I explained in the bit you snipped. Consider a false rigid wooden leg (from knee
down) - it won't make any difference if this follows a straight line from knee to pedal, or has a
kink like shin + foot, even though in the latter case there is a 'heel' offset from the pedal.

If you think I'm wrong, where do you think the extra energy is dissipated?

James
 
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