EFBe Frame testing, and the Great Materials Debate

wrote:

If you want to visualize the forces a standing rider puts on the
pedals, try drawing a free body diagram of the rider. Gravity pulls
down on him. The reaction forces from the pedals and handlebars push
up on him.

This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.

Peter Cole wrote:
wrote:

If you want to visualize the forces a standing rider puts on the
pedals, try drawing a free body diagram of the rider. Gravity pulls
down on him. The reaction forces from the pedals and handlebars push
up on him.

This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.

Concentrate on the rider. Is his body accelerating in the vertical
direction? To a first approximation, no. If not, then the total
upward force on him equals his weight.

At a given instant, more or less of the upward force on him may come
from his forward pedal, so the pedal force will vary. But in
principle, there's no reason a constant force can't be exerted on an
accelerating object. It happens all the time.

- Frank Krygowski

Peter Cole wrote:

This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.

Actually, I don't think that's what I was saying, but I'm starting to get
confused. What I was saying was that when standing on a firm surface, the
ground pushes up on you with full force. However, normally when standing
on a pedal you sink down, i.e., the pedal moves beneath you. I took that
to mean that as long as the pedal moves down it couldn't be pushing up on
you with full force. Only when you stall out and have to actively pull up
on the handlebar and press down on the pedal could you exceed body weight
force.

wrote:

Concentrate on the rider. Is his body accelerating in the vertical
direction? To a first approximation, no. If not, then the total
upward force on him equals his weight.

At a given instant, more or less of the upward force on him may come
from his forward pedal, so the pedal force will vary. But in
principle, there's no reason a constant force can't be exerted on an
accelerating object. It happens all the time.

The question was whether the peak force on the pedal would be reduced if
the pedal was accelerating. According to Einstein, gravity and
acceleration are indistinguishable, so the acceleration of the pedal is
equivalent to a reduction of the gravity. If the pedal was accelerating
at 1 G, the amount of force the rider could apply with body weight only
would be 0 (the scenario that Robert Chung described).

Robert Chung wrote:
Peter Cole wrote:

This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.


Actually, I don't think that's what I was saying, but I'm starting to get
confused. What I was saying was that when standing on a firm surface, the
ground pushes up on you with full force. However, normally when standing
on a pedal you sink down, i.e., the pedal moves beneath you. I took that
to mean that as long as the pedal moves down it couldn't be pushing up on
you with full force. Only when you stall out and have to actively pull up
on the handlebar and press down on the pedal could you exceed body weight
force.

You mentioned an accelerating pedal, which is a different case from a
stationary (or constant velocity) one.

If the pedal is not accelerating, you can put the force of full body
weight on it. If the pedal is accelerating, the force you can apply from
weight becomes proportionally less. If the pedal is accelerating at G,
as you suggested (9.8 m/s^2), you can produce no force with body weight.
You can produce force either against the handlebars or the inertia of
your body, though.

Since there is a variation of pedal force during the cycle (peak = ~2x
average), climbing at a constant velocity will have accelerations during
the pedal cycle. Accelerations of the bike translate to accelerations at
the pedal, that's all I was saying. To the extent that the pedal
accelerates during peak force, the peak force is reduced.

Peter Cole wrote:
Robert Chung wrote:
Peter Cole wrote:

This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.


Actually, I don't think that's what I was saying, but I'm starting to get
confused. What I was saying was that when standing on a firm surface, the
ground pushes up on you with full force. However, normally when standing
on a pedal you sink down, i.e., the pedal moves beneath you. I took that
to mean that as long as the pedal moves down it couldn't be pushing up on
you with full force. Only when you stall out and have to actively pull up
on the handlebar and press down on the pedal could you exceed body weight
force.

You mentioned an accelerating pedal, which is a different case from a
stationary (or constant velocity) one.

If the pedal is not accelerating, you can put the force of full body
weight on it. If the pedal is accelerating, the force you can apply from
weight becomes proportionally less. If the pedal is accelerating at G,
as you suggested (9.8 m/s^2), you can produce no force with body weight.
You can produce force either against the handlebars or the inertia of
your body, though.

Since there is a variation of pedal force during the cycle (peak = ~2x
average), climbing at a constant velocity will have accelerations during
the pedal cycle. Accelerations of the bike translate to accelerations at
the pedal, that's all I was saying. To the extent that the pedal
accelerates during peak force, the peak force is reduced.

I haven't read this entire thread in detail, so I may be a bit off
base. But: In principle, there is no reason a rider can't apply a
constant force to an accelerating pedal.

If your weight were applied sack-of-potatoes style to the descending
pedal, then yes, the force on the pedal would be reduced if the pedal
were accelerating downward. This can be verified by - again - drawing
a free body diagram of that sack-of-potatoes with one extended, rigid
leg. Sum of forces in the vertical direction = mass-of-potatoes times
acceleration, and since the sack would be accelerating down with the
pedal, the force between the pedal and extended-rigid-leg would be less
than the sack's weight.

But a living rider can push on the accelerating pedal by extending his
leg as the pedal descends. Even though the pedal accelerates away from
him, he can (in principle) time the leg's extension to keep his body
mass from accelerating - that is, he can keep his body at the same
height, keep its acceleration zero, and keep the force on one pedal
equal to his weight as the pedal descends. Draw the free body diagram
of the rider doing this - it's obvious.

I'm not saying this is exactly what happens in real life. Standing
riders don't have exactly zero vertical acceleration, they bob up and
down somewhat; and they have three other forces acting upward on their
body (two hands plus the back leg). But they don't bob 350 mm (two
crank lengths) so what I'm describing must happen to some degree.

- Frank Krygowski

wrote:

Peter Cole wrote:

Robert Chung wrote:

Peter Cole wrote:


This is true, but the complication Robert was introducing was that,
since the bike is accelerating/decelerating during the pedal cycle
(maintaining a constant speed up a grade -- torque not being constant),
the moment of peak pedal force is also a time when the bike (and thus
the pedal) are also accelerating. It will diminish the peak pedal force
somewhat.


Actually, I don't think that's what I was saying, but I'm starting to get
confused. What I was saying was that when standing on a firm surface, the
ground pushes up on you with full force. However, normally when standing
on a pedal you sink down, i.e., the pedal moves beneath you. I took that
to mean that as long as the pedal moves down it couldn't be pushing up on
you with full force. Only when you stall out and have to actively pull up
on the handlebar and press down on the pedal could you exceed body weight
force.

You mentioned an accelerating pedal, which is a different case from a
stationary (or constant velocity) one.

If the pedal is not accelerating, you can put the force of full body
weight on it. If the pedal is accelerating, the force you can apply from
weight becomes proportionally less. If the pedal is accelerating at G,
as you suggested (9.8 m/s^2), you can produce no force with body weight.
You can produce force either against the handlebars or the inertia of
your body, though.

Since there is a variation of pedal force during the cycle (peak = ~2x
average), climbing at a constant velocity will have accelerations during
the pedal cycle. Accelerations of the bike translate to accelerations at
the pedal, that's all I was saying. To the extent that the pedal
accelerates during peak force, the peak force is reduced.


I haven't read this entire thread in detail, so I may be a bit off
base. But: In principle, there is no reason a rider can't apply a
constant force to an accelerating pedal.

If your weight were applied sack-of-potatoes style to the descending
pedal, then yes, the force on the pedal would be reduced if the pedal
were accelerating downward. This can be verified by - again - drawing
a free body diagram of that sack-of-potatoes with one extended, rigid
leg. Sum of forces in the vertical direction = mass-of-potatoes times
acceleration, and since the sack would be accelerating down with the
pedal, the force between the pedal and extended-rigid-leg would be less
than the sack's weight.

But a living rider can push on the accelerating pedal by extending his
leg as the pedal descends. Even though the pedal accelerates away from
him, he can (in principle) time the leg's extension to keep his body
mass from accelerating - that is, he can keep his body at the same
height, keep its acceleration zero, and keep the force on one pedal
equal to his weight as the pedal descends. Draw the free body diagram
of the rider doing this - it's obvious.

Yes, you're right. I was describing a "sack of potatoes" model.


I'm not saying this is exactly what happens in real life. Standing
riders don't have exactly zero vertical acceleration, they bob up and
down somewhat; and they have three other forces acting upward on their
body (two hands plus the back leg). But they don't bob 350 mm (two
crank lengths) so what I'm describing must happen to some degree.

Assuming (just for the sake of argument) the hands and other leg are
unloaded, the only thing that would prevent the rider from applying full
body weight to an accelerating pedal would be the mass of the moving
(accelerating) leg.