How much weight does a cyclist push with each pedal stroke?
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  1. #1
    plx
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    How much weight does a cyclist push with each pedal stroke?

    Say a cyclist pedals at 60 rpm and 90rpm, how much weight is he pushing at 300watts? 400watts? 500watts? 2000watts?
    Is it possible to calculate that or do an approximation?

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    Torque T is Power P divided by rotational speed, in rad/s (which is 2Pi*rpm/60). In order to get the force you are looking for, you would need to divide the torque T by effective moment arm L. All of this is straightforward in principle, but torque varies strongly as a function of crank angle, and so does the moment arm. However, using the average power in the above formula to get an average torque, and then using your crank arm length for L (say, L=172mm) should give you a decent estimate.

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    plx
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    Can you exemplify for 500watts at 60 rpm? I don't need it to be super accurate just would like to have an idea of how much weight we're talking about.

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    Quote Originally Posted by plx View Post
    Can you exemplify for 500watts at 60 rpm? I don't need it to be super accurate just would like to have an idea of how much weight we're talking about.
    It's not as simple as you're questions suggests, and it's not about weight - it's about power vs. the sum total of resistive forces, of which aerodynamic drag is often the greatest component.

    This site, and the associated calculators can be enlighten you. Play with it/them, and read a bit.
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    Roughly 470 N, or about 100 lbs as a very rough guess. Very, very hard on your knees. Not many people (in their right minds) would output 500 W at such a low cadence, however.

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    Quote Originally Posted by Pirx View Post
    Roughly 470 N, or about 100 lbs as a very rough guess. Very, very hard on your knees. Not many people (in their right minds) would output 500 W at such a low cadence, however.
    Well I'm standing here and can do it for about all day. Which would be about 85lbs on each knee.
    When I'm standing on the bike (& when I do I'm probably at about 60rpm), I would say that the force would be about that, sometimes I pull on the handlebars, so I would say that 100lbs would not be that hard on the knees.

    Now if you're talking about providing 100lbs force angular through the full downstroke, that would be a different matter. I push 100lbs straight down, but probably don't get any where near 500 watts. Maybe 250w.
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    plx
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    AWhy is it not linear the downforce with power output? How much do you reckon you have to push downwards to get 500watts?

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    It is linear in torque, which is linear in force. The issue is as you go around the circle the force varies from very low levels when the crank arms are near the 12 o'clock/6 o'clock positions, and maximizes around the 3 o'clock position when viewed from the right side. It looks something like a sine wave , like this pic (don't try to use this for direct comparison).

    Leg_Power_Tot.jpg

    So the instantaneous power varies as the force varies around the circle. It is customary to talk about power (and torque) averaged around the pedal circle. If you want 500 watts of average power, your peak power and torque/force will need to be appreciably higher, or with a peak power of 500 watts your average power and torque/force will be appreciably lower.
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    plx
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    That's really cool. You can't get me another one of those graphs but with higher watts or can you?

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    That comes from a research effort. To be meaningful you would have to measure it for whatever setup you're interested in. It's just meant to be representative. You can find variations on that theme in the literature.
    Last edited by ibericb; 07-19-2015 at 03:05 PM.
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    Quote Originally Posted by ericm979 View Post
    That's a pretty neat calculator, thanks!

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    Quote Originally Posted by plx View Post
    Say a cyclist pedals at 60 rpm and 90rpm, how much weight is he pushing at 300watts? 400watts? 500watts? 2000watts?
    Is it possible to calculate that or do an approximation?
    The average effective pedal force on 170mm cranks for both legs combined, converted to the equivalent "weight" (i.e. Newtons / acceleration due to gravity) is as follows:

    Power 60rpm 90rpm
    300W 28.7kg 19.1kg
    400W 38.2kg 25.5kg
    500W 47.8kg 31.8kg
    2kW 191kg 127kg

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    Quote Originally Posted by Alex_Simmons/RST View Post
    The average effective pedal force on 170mm cranks for both legs combined, converted to the equivalent "weight" (i.e. Newtons / acceleration due to gravity) is as follows:

    Power 60rpm 90rpm
    300W 28.7kg 19.1kg
    400W 38.2kg 25.5kg
    500W 47.8kg 31.8kg
    2kW 191kg 127kg
    And here is another way to put out about a KW or two with your legs (mostly) over a couple seconds (462KG raised a couple feet in a couple of seconds).

    https://www.youtube.com/watch?v=w95Yi9HcQ7s

    dave

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