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Thread: A question for farkers

  1. #1
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    Default A question for farkers

    Dear farkers,
    saw this pop up the other day.

    Problem:
    You have a bicycle, and your friend is holding it up so that it will not fall over, but he is not applying any pressure to it in any direction. You rotate the crank so that one of the crank arms is facing straight down, and then you tie a length of string around that crank arm. The setup will look something like this:

    With your friend holding the bike up, you pull back on the string. Will the bike move forward, backward, or stay in the same spot?

    Hint: The bike's rear wheel will not skid.
    Attached Images Attached Images
    Be careful with that, it was new once.

  2. #2
    Senior Member wkkie's Avatar
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    Quick pull - forward, long pull - backwards.

  3. #3
    I'll tells ya! johnny's Avatar
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    As the arc of the crank moves upward you will stop the bike from moving forward by pulling back on the string. So I'd say that you'd get a little bit of forward movement but very little.
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  4. #4
    Senior Member Comic Book Guy's Avatar
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    I'll just put this here:
    Kentlyn Trail Map: http://www.openstreetmap.org/?map=19/-34.06667/150.85439#map=15/-34.0592/150.8589

  5. #5
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    Why do you have string/rope in the forest? What else is in your rode bag? Or do you keep it under the saddle in one of those stupid little bags? Why aren't you and your friend riding?

  6. #6
    Senior Member bear the bear's Avatar
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    Quote Originally Posted by johnny View Post
    As the arc of the crank moves upward you will stop the bike from moving forward by pulling back on the string. So I'd say that you'd get a little bit of forward movement but very little.
    It depends... the force applied to the string must be sufficient to overcome the inertia of the bike to generate forward momentum. If the force is explosive the system may move forward under its own power (assuming the string has been let go).
    If a steady amount of force is applied, the Jonny's answer is correct as the system is constrained by the ability to apply angular force into the drivetrain.
    Something, something, something

  7. #7
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    Quote Originally Posted by pink poodle View Post
    Why do you have string/rope in the forest? What else is in your rode bag? Or do you keep it under the saddle in one of those stupid little bags? Why aren't you and your friend riding?
    They are imaginary. But valid points nonetheless.
    Be careful with that, it was new once.

  8. #8
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    Quote Originally Posted by schred View Post
    They are imaginary. But valid points nonetheless.
    Do I need to put some line in my backpack before this afternoon's ride?

  9. #9
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    It would depend what gear it's in.
    Crank ring / crank length would have to have higher leverage than the wheel size / sprocket size to move forward.
    Last edited by Chriso_29er; 17-04-2017 at 03:19 PM.

  10. #10
    Senior Member The Duckmeister's Avatar
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    At best you will achieve a small forward movement, at worst nothing. The bike will not be able to move backward because the freehub ratchets will engage and pull the crank against your pull on the string, cancelling it out.
    Just ride the bloody thing and enjoy yourself!

    I build wheels... :wink:

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