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Chain doesn't transmit drive, except in third and sixth

My bike has some problems with the drivetrain somewhere: I think it is either the six-speed rear cassette or possibly the front chainring (there is only one of these and no front derailleur to worry about!).

Bascially, the bike doesn't transmit any of my power as a rider except in sixth gear and (sometimes) third. In all the others, the chain just goes round and doesn't seem to 'drive' the bike onwards. I just sort of spin the cranks round and the chain doesn't seem to bite and move the bike forwards!

I had a new chain fitted a few months back at my local bike shop, the other one snapped while I was riding. The rear derailleur changes smoothly and looks fine, so I don't think it is that. That said, the bike has been moved around as I moved house, so it is possible that the rear derailleur was damaged and this is causing the shifting problems.

I think the problem is that the old chain has worn the teeth down on the rear cassette cluster/freewheel.

If anyone thinks it could be something else, please tell me.

Thanks for your help,

Joseph North
Try to see where the chain is slipping. If it is the front chainring (unlikely) the chain won't move at all as you pedal. If the chain is slipping over the top of the teeth of the freewheel in back, the chain will move but the freewheel will not. If it is the ratchet inside the freewheel, the chain and freewheel will move, but it won't drive the wheel forward.

Most likely, you need to replace the rear cluster. If you have a "freewheel", this will solve either problem 2 or 3. If you have a "cassette" (unlikely), this would only solve problem 2.
Hi Joe;

Dave has it correct.

One additional comment - unless you are measuring the chain wear (length increase) and replacing the chain based on a conservative measurement - ALWAYS replace the chain and freewheel (or cassette) at the same time.

Most likely you have a freewheel.

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