I have had a problem with my drive for a while and it only occasionally presents itself. This only happens when the drive gets covered in mud. What I have found is when running biggest gear on the back the chain will try and drop to the smallest ring on the front despite rubbing on the front mech. If it drops to the smallest ring or if I shift to smallest ring the chain will wrap round the front sprocket. This happens occasionally and seemingly regardless of the gear I am in on the cassette.

I have tried to do everything to fix this. Since this problem has started i have changed cranks, rings, chain, cassette, shifters, and played with chain tension. The only original parts are the front and rear mech.

I am generally a good mechanic and apart from sticking in the middle ring if it is muddy what can i do?

In particularly muddy conditions, try to keep the chain line straighter? Eg, rather than middle ring/biggest cog, try granny ring/middle-ish cog.

Guessing you have particularly heavy, clay-type mud?

It seems to happen all over the country, Reading, Manchester, Wales, Yorkshire and Lee quarry yesterday.

I won’t happen if i use middle to second from bigest but if i straigten the chain line and use smalest to second from bigest it will happen. It never happens to the middle ring.

Have you the BB spaced correctly?

Hmmm, perhaps your cranks are spaced out too far on the drive side?
Can you remove a shim from the BB to allow it to move inwards a bit?
What cranks + BB are you running?

It happened with XTR cranks and some of the superstar cranks. Always correctly spaced (2.5 spacer on the crive side). Using different spacers too.

what chainlube?

the stuff that comes with them is like glue. they need to be degreased and regreased when new. worked on a bike recently that hadn’t been degreased and it was like tar!!

yep, the bike is washed chain cleaned and re-lubed regualy

What size BB shell has the bike got?

i will re-measure that tonight but i am 99% sure it is a 68mm in which case it will have 2 drive side spacers. I know i have got the right number of spacers in the right place.

I have measured everything and the shell and spacers are right. 68mm 2 drive side 1 non drive.

Any other ideas?