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Force distribution on Pedals...

Rocket69

Hi all... looking for a website or an answer to this question.

If a rider pedals at 90RPM and is generating 300 watts, what is the force distribution on the pedals? (in pounds or kilos)?

Thanks to anyone who helps or gives direction...

Rocket69

SLS

rollers

Originally posted by Rocket69
Hi all... looking for a website or an answer to this question.

If a rider pedals at 90RPM and is generating 300 watts, what is the force distribution on the pedals? (in pounds or kilos)?

Thanks to anyone who helps or gives direction...

Rocket69

I'm not sure what it is you're asking for when you say force distribution, but maybe this will help you.

To get watts multiply ft-lb-rpms by .1420 (sorry, all you people out there in metric lands, we'll just never quite get it here in the US)

Substituting your numbers yields just shy of 23.5 ft-lbs. This figure would be an average torque required. Torque is a function of force and crank length. Presumably your crank length does not change as you pedal, but the force you apply to the pedals will. Thus the torque will vary with crank position. (not exactly a startling conclusion, but in thinking of what you might mean when you say "force distribution" I figured it might be helpful.)

dhk

Believe we can do some back of the envelope here to calculate the average force applied to the pedals:

First, 300 watts = 221 ft-lbs/sec of power. If you have 175 mm cranks, each rev moves your foot through a 3.6 ft circle, which would be a foot velocity of 5.4 feet/second @ 90 rpm.

To simplify, assume each foot is applying force on it's own for exactly half a rev, and that all foot force is applied normal to the crank. Then, average force on the pedal (normal to the crank) would be 221/5.4, or 41 lbs.

In real life, the peak forces will be higher, since we deliver most of the power through considerably less than the forward half of the pedal circle. If you only push from say the 2 o'clock position to 5 o'clock, you'd need double the force from each leg....82 lbs....to average out to 300 watts @ 90 rpm.

Now I know why I can't do this for very long. Someone please jump in if I'm off base.

Dan

dhk

Originally posted by rollers
I'm not sure what it is you're asking for when you say force distribution, but maybe this will help you.

To get watts multiply ft-lb-rpms by .1420 (sorry, all you people out there in metric lands, we'll just never quite get it here in the US)

Substituting your numbers yields just shy of 23.5 ft-lbs. This figure would be an average torque required. Torque is a function of force and crank length. Presumably your crank length does not change as you pedal, but the force you apply to the pedals will. Thus the torque will vary with crank position. (not exactly a startling conclusion, but in thinking of what you might mean when you say "force distribution" I figured it might be helpful.)

We crossed msgs here, but our numbers agree. To apply 23.5 ft-lbs of torque through a 175 mm (.57 ft) crank arm takes 41 lbs at the pedal. Agree the big unknown is the distribution.

Dan

rollers

Originally posted by dhk
... If you only push from say the 2 o'clock position to 5 o'clock, you'd need double the force from each leg....82 lbs....to average out to 300 watts @ 90 rpm.
...

Dan

Good post. I especially like this part about the estimated force required to reach 300 watts with a notably inefficient pedal stroke. What a great argument for learning to pedal more efficiently.

dhk

[QUOTE]Originally posted by rollers
Good post. I especially like this part about the estimated force required to reach 300 watts with a notably inefficient pedal stroke. What a great argument for learning to pedal more efficiently. [/QUO

Thanks, always good when a couple of us agree on numbers.

It does seem if you could apply the force for 180* with each leg, lets say from the 2 o'clock to 8 o'clock, you'd have a steady application of torque at the crank. Using more of the quad muscles and hamstrings at lower force over a bigger portion of the pedal stroke would seem to be better for endurance at higher cruising speeds. Guess it would take an expert on pedalling mechanics to really sort out.

At lower cadence on hills, sometimes I work on pulling back at the 6 o'clock and then up with the hamstrings to get that extra kick through the bottom of the stroke. Seems to relieve the tired quads and keep me going up a long hill.
Dan

rollers

Originally posted by dhk
[QUOTE]Originally posted by rollers
Good post. I especially like this part about the estimated force required to reach 300 watts with a notably inefficient pedal stroke. What a great argument for learning to pedal more efficiently. [/QUO

Thanks, always good when a couple of us agree on numbers.

It does seem if you could apply the force for 180* with each leg, lets say from the 2 o'clock to 8 o'clock, you'd have a steady application of torque at the crank. Using more of the quad muscles and hamstrings at lower force over a bigger portion of the pedal stroke would seem to be better for endurance at higher cruising speeds. Guess it would take an expert on pedalling mechanics to really sort out.

At lower cadence on hills, sometimes I work on pulling back at the 6 o'clock and then up with the hamstrings to get that extra kick through the bottom of the stroke. Seems to relieve the tired quads and keep me going up a long hill.
Dan

I try to push/pull the pedals all the way around the circle. I know I'm not as good as I'd like to be at it but when I do the math on wattage and RPM like above it just convinces me all the more that recruiting as many muscles as possible is the only way to add speed. Applying more force on the down stroke is not an option for long term success.

andrewbradley

Originally posted by rollers
I especially like this part about the estimated force required to reach 300 watts with a notably inefficient pedal stroke. What a great argument for learning to pedal more efficiently.

How do you mean "inefficient pedal stroke"? Where do you suspect energy is wasted?

The argument over "circular pedalling" is about whether the application of extra power in zones not commonly observed can be fuelled aerobically.

andrewbradley

Originally posted by rollers
Applying more force on the down stroke is not an option for long term success.

But this could well be what happens in general according to one study. The better riders tended to get their extra power via a higher peak in the torque curve.

What's wrong with that?

rollers

Originally posted by andrewbradley
How do you mean "inefficient pedal stroke"? Where do you suspect energy is wasted?

In the post I was refering to when I wrote that, the author had made some rough calculations based on force being applied to the pedals only on a part of the downstroke. I labeled that as inefficient. You make a good point though, because in such a stroke energy isn't neccessarily wasted, implying inefficiency.

Maybe I should have called it a sub-optimal pedaling style. (?) :)

rollers

Originally posted by andrewbradley
But this could well be what happens in general according to one study. The better riders tended to get their extra power via a higher peak in the torque curve.

What's wrong with that?

Hmm, maybe I should have said
Applying more force on the down stroke is not an option for long term success for me.

No doubt there is truth in both what you say and in my statement. I know, for instance, that when I throw down I'm applying more force on the down stroke as well. What I'd REALLY like to be able to accomplish is a big increase in force all the way around the circle including the down stroke AND a big increase in cadence while I'm at it.

:)

Squint

Originally posted by rollers
Hmm, maybe I should have said

No doubt there is truth in both what you say and in my statement. I know, for instance, that when I throw down I'm applying more force on the down stroke as well. What I'd REALLY like to be able to accomplish is a big increase in force all the way around the circle including the down stroke AND a big increase in cadence while I'm at it.

:)

You should tie strings to your feet then you can pull up with your arms when your feet are in the down position and help eliminate the dead spot. You would recruit all the shoulder girdle muscles and therefore ride faster.

Pros aren't faster than us because they're fitter but because they're more efficient and go faster while generating the same amount of power as us.

P.S. Don't forget to get some ceramic bearings.