A question for farkers

Discussion in 'General MTB Discussion' started by schred, Apr 16, 2017.

  1. schred

    schred Likes Bikes and Dirt

    Dear farkers,
    saw this pop up the other day.

    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 Files:

  2. wkkie

    wkkie Likes Bikes and Dirt

    Quick pull - forward, long pull - backwards.
  3. johnny

    johnny I'll tells ya! Staff Member

    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.
  4. Comic Book Guy

    Comic Book Guy Likes Dirt

    I'll just put this here:
  5. pink poodle

    pink poodle Clinically Inane

    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. bear the bear

    bear the bear Likes Dirt

    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.
  7. schred

    schred Likes Bikes and Dirt

    They are imaginary. But valid points nonetheless.
  8. pink poodle

    pink poodle Clinically Inane

    Do I need to put some line in my backpack before this afternoon's ride?
  9. Chriso_29er

    Chriso_29er Likes Bikes

    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: Apr 17, 2017
  10. The Duckmeister

    The Duckmeister Eats Squid

    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.
  11. stirk

    stirk Wheel size expert

    Unless this is a strange riddle the bike will move forwards, it's how we pedal a bike. The backward pull part of the crank's cycle is the wipe the dog shit off your foot motion which is apparently an efficient pedalling technique perfected by roadies but don't ask me I'm a masher ;)
  12. pharmaboy

    pharmaboy Eats Squid

    Nice question, and I'd guess this is the answer. The picture looks typical 32 -18 or something like that which is close to 1 turn of the crank creates 2 turns of the wheel, therefore the power is with the crank, so if you pull the string 1 cm, the wheel will want to move forward double that so the string won't move untill it makes the wheel spin, so it stays still

    Edit - I'm wrong

    Cool problem though
    Last edited: Apr 18, 2017
  13. pink poodle

    pink poodle Clinically Inane

    What gear is the bike in? If it is in reverse this will all be very tricky.
  14. Comic Book Guy

    Comic Book Guy Likes Dirt

    The answer:
  15. hifiandmtb

    hifiandmtb King of his castle

    Excellent video

    Sent from my iPhone using Tapatalk
  16. rone

    rone Eats Squid

  17. Boom King

    Boom King Wheel size expert

    How long is the string?

    Sent from my LG-D855 using Tapatalk
  18. schred

    schred Likes Bikes and Dirt

    Hint: it's on an aircraft taking off on a treadmill
  19. johnny

    johnny I'll tells ya! Staff Member

    I searched out the answer and it does depend on the gearing but the answer is bizarre
  20. Nautonier

    Nautonier Eats Squid

    Why would it not skid? The forward momentum relies on weight to maintain traction, if there is not enough weight, it would simply skid backwards when the string is pulled. If there is traction how could it do anything but move forwards a little for the first quarter turn of the crank, irrespective of what gear ratio is being employed?

    This is all assuming that the person holding it is simply stopping it from falling over side to side, not holding it stationary.

    Also assuming that the string is strong enough to withstand significant force.

    And the person pulling it isn't a weakling.

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