The Chainlink

Anyone interested in puzzles? Do you have any bike puzzles to share?

Here is one I saw in a book*. You can actually try this if you have a fixed gear at your disposal.

Below is a regular fixed gear bike. As you can see(!?), a lose yellow rope is resting on the floor where one end is tied to the left pedal spindle.

If someone was to kneel in front of the bike and pull the rope, will the crank move clockwise, counter clockwise, or not at all ?

You can assume a magic force is keeping the bike upright and front tire pointing straight. Oh, and assume the wheels do have spokes although it looks otherwise.

* Mathematical Carnival by M. Gardner

original bike photo from: http://fixedgearsclub.blogspot.com/2011/04/why-have-fixed-gear-bike...

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Haha.  Uhhh, the crank would move counter clockwise a little bit, but not much.  

**Waits to see how wrong I am**

+1. Could it be that simple?

Brendan said:

Haha.  Uhhh, the crank would move counter clockwise a little bit, but not much.  

**Waits to see how wrong I am**

It all depends on if the rear wheel is able to spin freely...

Correct. If it is a fixed gear, nothing will move. As soon as the rope is taut and the crank starts to move forward, the wheel and bike will start to move backward.

notoriousDUG said:

It all depends on if the rear wheel is able to spin freely...

You're missing an important assumption. That the back wheel cannot slide. That's most likely to happen, pull and slide.

Yeah, and let us not forget about the amount of chain slack the average fixie has either;)


Adam "Cezar" Jenkins said:

You're missing an important assumption. That the back wheel cannot slide. That's most likely to happen, pull and slide.

Wow, so many replies :)

You folks are thorough. Let`s see if I can address some of these issues.

Bike is a regular fixed gear, as mentioned earlier. You should also assume that the bike is heavy enough that the tires will not slide. In other words, if the bike is moving, the tires are rotating, so is the crank.

I think the chain can have some slack. However it will make things less complicated if you assume it does not have any slack (let`s say it has one of those nice tensioners in rear dropouts :) ).

Knock knock, any other answers?

I'm with Duppie:

Force on rope, crank turns beyond 90 deg, bike moves backwards.

I'd say it depends on how hard you pull on the rope :)

Allright, long time no new replies.

Do you folks want the hear the answer ?

So far we have some incomplete answers that seem to be on the right track (I think).

Here is a hint if you`d like,

*** spoiler alert ****

*** spoiler alert ****

in the book where i saw this puzzle,

the title read ¨bicycle paradox¨.

*** spoiler alert ****

*** spoiler alert ****

Just noticed, you can try this at home with a coasting bike too!

Same puzzle, phrased differently, or just a variation.

With your coasting bike, pull the same crank from the left (behind the bike) instead and see what happens.

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