What is this bike part?
 

http://bikecoop.truman.edu/wordpress...1/mystery1.jpg

http://bikecoop.truman.edu/wordpress...1/mystery2.jpg

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The inside part spins one way, and it makes a freewheel-y type clicking sound. But I've never seen anything like this before. I thought freewheels were inside the cassettes... I admit, I'm fairly ignorant when it comes to bicycle mechanics. It can be so confusing.

It's a single speed freewheel. It's used on single speed bikes (obvious huh?) either road, MTB, BMX or cruiser types these days. It is threaded on a freewheel, not freehub, type hub.

That 14 tooth freewheel has a metric thread, and is meant to fit on the smaller side of a flip-flop BMX hub.

No, not that obvious to me, actually. Except for the fact that, if this is where the chain goes around, then obviously there isn't anywhere for the chain to go if you tried to shift. Does that mean no need for a derailleur? :rolleyes:

What does a "hub" look like? Just a hub.

I actually have read the Sheldon Brown site on freewheels and freehubs. Still somewhat confused. :twitchy:

flip-flop?

Sorry, a "flip flop" hub describes a hub with two sides to mount a freewheel or cog on. see http://www.sheldonbrown.com/singlespeed.html#flip-flop

Here's what a BMX flip flop hub looks like:
http://img684.imageshack.us/img684/2734/38987.jpg
You can see that the right side threading is a little smaller than the threading on the left, that's the side your freewheel would fit on. Freewheels 16T and larger would go on the larger diameter thread on the left side.

Right, hence the name "single speed". :D No derailleurs or shifters needed.

A hub that has threading on both sides. Some riders use this for a single speed bike with two different sized sprockets so they can "flip" the wheel for better climbing(more teeth) or more speed (fewer teeth) without owning two wheels.

yeah.... recently i have purchased a 2-speed Freewheel!!! Old stock and old model made in the land of the Rising Sun...

or a president who changes his mind not because of learning or principal, but because of political expediency :D

Thank you! That was a delightfully clear explanation! One more quick question - what is the reason for the different size freewheels? In what circumstances would you want a larger or smaller freewheel and does that then mean you have to change which rims you use?

That is such a cool idea. Kindof bike nerdy, lol, but still cool.

Would you mind taking a picture of it for me? Not the whole bike, just specifically the freewheel part? I'm a visual learner. :D

Different size freewheels (or cogs in general) give different gear ratios. You select the gear that works best for you.

The rim is the part that the tire mounts on. It really is not relevant to gear selection. However, the hub, as pictured above, must be compatible with the freewheel or cassette being used.

But it is! an X tooth sprocket on a 20" wheel will yield fewer gain inches than the same x tooth sprocket on a 28" wheel.