Chain wheel seizure.

plowmar

When changing from outer to inner chainwheel when starting a climb it seizes up so that I cannot cycle, and have to dismount and free it by manually changing back to large ring. Resulting in having to walk the climb - they are not usually long - and restarting at the top. Test changes are then O K and subsequent climbs changes are O K .

Can anyone think of any reasons, the chain isn't physically caught so that effort is needed to release it.

cooldad

mech not indexed properly.

keef66

Change to the inner chainring before you get to the climb? Have either of the chainrings been taken off and put back on the wrong way round?

What chainset, front mech, chain?

plowmar

Thanks cooldad and keef66. Perhaps didn't make myself quite as clear as I should have. But I do switch at bottom of climb on lower gradients and after 'reassembling' (no adjustments) works perfectly well through remainder of ride and yesterday had no problems. This is the typical intermittent fault.
It is just that after changing I cannot continue peddaling as there is a stop to the circular motion.

svetty

Trying to shift at full load and very slow cadence?

jermas

It sounds like the large chainring is worn- causing chain suck which jams the crank. Look at the chainring teeth, if they're shark fin shaped it might be worn and need replacing. As a stop gap you can file (with a flat file) the hook part off the teeth which will allow the chain to disengage.
http://blog.artscyclery.com/ask-a-mechanic/ask-a-mechanic-eliminating-chain-suck/

plowmar

Thanks jermas I will check.

fenix

Yes sounds more like chain suck than anything else to me.

TheGardenGnome

Even if your parts were not worn... A dirty chain = chain sucking heaven.

plowmar

must admit had a load of gunge on chain and after wiping it clean had no problem the following day. cross fingers that that is what it was, certainly te cheapest option.