Crank turning, but wheels aren't

[Bodom]

I just got a cheap mountain bike (off craigslist) for riding from A to B, but ate it real bad when I got home.

Not sure what happened, but now the crank will turn separately from the chain holder (sorry don't really know bike part names yet) (The pedals will spin, but nothing else will move.) if there is weight on the back tire. If there is no weight then the crank will spin the tire (Like having it upside down on the seat and spinning it.)

Any help would be appreciated!

[DaveM]

If it looks like the chain has not fallen off either of the gear clusters and is running smoothly, I think it means that the read gear cluster is broken (specifically there is a ratchet inside that is broken.) It would be pretty weird for this to break from a crash, so maybe something else is going on, but I suppose it's possible.

When it's upside down, can you shift the gears ok and all else seems fine?

[nfmisso]

Ref: http://bikeloft.com/merchant/257/images/site/bike01.jpg
so the cranks are turning but the chain rings are not?

Or when you turn the cranks, the chain moves, but the rear cogs (freewheel) does not transmit the power to the rear wheel - more likely.

I suspect that you need to replace the freewheel - always replace the chain at the same time as you replace the freewheel or cassette.

[Bodom]

I guess it's kind of hard to explain. I was riding really fast and I I think the crank gave out. Pretty much the pedals will turn the wheel, chain, chain holder all like normal when there's no weight but I can hold the back wheel and the only thing that will turn is the pedals ans crank the chain holder doesn't move. Doesn't make sense. Would replacing the crank + pedals possibly fix this?

[DaveM]

I think we need to get some terminology down to help you.
Pedals - the things your feet go on
cranks - the two arms the pedals attach to
chainrings (front chainrings) - the toothed wheel attached to the cranks/pedals
rear cogs (freewheel/cassette) - the gear cluster at the back wheel

When you pedal, which parts move and which don't?

[Bodom]

This might help.


http://www.youtube.com/watch?v=pbPdUbqnC-s


Thanks guys

[Joe_W]

The chain rings have come loose. I do not know if they can be reattached again, it looks like a one piece crank set
http://sheldonbrown.com/gloss_n-o.html#opc
http://sheldonbrown.com/opc.html
I guess either the pin is worn / broken or the crank has become loose. I'd (personally) replace the crank set and bottom bracket with something more modern, but first check if something is loose that should be fixed.

[RobAR]

Without seeing the driveside arm closer - I'm with Joe that the pin that is welded to the arm and fits into a hole on the rings has broken off. Quite a common problem.

Rob