A question for farkers

schred

Likes Bikes and Dirt
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.
 

Attachments

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.
 

pink poodle

気が狂っている男
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?
 

bear the bear

Is a real bear
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.
 

schred

Likes Bikes and Dirt
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.
 

Chriso_29er

Likes Bikes and Dirt
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:

The Duckmeister

Has a juicy midrange
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.
 

stirk

Burner
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 ;)
 

pharmaboy

Eats Squid
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.
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:

johnny

I'll tells ya!
Staff member
I searched out the answer and it does depend on the gearing but the answer is bizarre
 

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.
 
Top