UKC

My rear dérailleur gear has jammed and refuses to move either up or down. Although the shift lever moves a bit, nothing much happens and the chain is still on the same cog. Any ideas appreciated before I take the thing apart?

There’s a jam somewhere then, first i’d release the cable from the mech and check it pulls & releases freely. i’ve had one partially broken within the outer cable and it was pretty well jammed

Whatever you do, don't force the shifter - breaking the internals is an expensive mistake!

Can you move the mech by hand? It might be a bit of gravel has flicked up and jammed it.

Check the cable routing around the bottom bracket - had a similar shifting problem on my bike recently and it turned out to be crud on the cable guide, solved by a good clean and lube. While you're at it, lube everything before starting to release the cable.

Has the cable outer jumped out of one of the sockets along its route?

If the bikes isn't regularly maintained, which means changing the cable outer at the rear derailleur every six months if you ride a lot in the wet and on salted roads, it's most likely the cable has rusted in the outer. This is especially likely if you haven't ridden it for a long time. Derailleurs are pretty bomb-proof and the pivots seem to last for many years; it's cables that give problems.

Replace all cables from the handlebars back; you'll be amazed at the improvement in braking and gear shifting.

Cable's probably frayed in the shifter - they don't last forever and fray and break where the cable joins the termination.  Undo the cable at the dérailleur end - if the latter's then free to move, you can be pretty certain that the cable's frayed. 

Thanks for all the advice. It's either the cable or actual lever that shifts the gears as it does nothing when I try to advance the gear. It's going into the repair shop tomorrow.

Thanks again

Sean

I should have thought with all that experience you had winning Paris-Nice, all those Classics and Tour de France green jersey etc etc you'd know a bit more about bikes. But I suppose you had team mechanics to do all that stuff.

It was all down tube shifters in his day

I remember them well.

I have literally only just upgraded from down tube shifters. 

I couldn't resist an ebay bargain. Cost me £50 all in. Need to cut down the cable outers at the front though. I got them too long, so they rub on the bottom headset.

I may have to upgrade my original Look pedals next year, I can't getget rid of the creak...