thought it would be straight forward this.

fitted new rings 22t and 36t,bash guard,new chain and cassette,to my mates bike.
front mech shifted it over to stop on middle.and its set up right to not skip off granny

now on the big ring to big sprocket,when i freewheel it back,the chain jumps down off big ring and either off the rings onto frame or onto granny,

only does it in this gear,

any ideas?

cheers.

bump for day shift 🙂

Never had this problem with my 22/36/bash setup. You shouldn’t be going big/big anyway, and cycling the cranks backwards in big/big is never somethng you would do out riding so not a realistic problem.

However I would guess the reason you’ve got the problem is that the chain’s too slack because you didn’t remove any links when you changed from 3 rings to 2. Accepted practice for correct chain length:
1) split chain and take it out of the derailleur
2) route it tight round both big rings, not through the derailleur and mark the length
3) add one full link (1″) – that’s the correct length

I get this slightly with my 1×9 setup as the “middle” chainring has to serve all 9 cassette sprockets. I will look at my chain length (good tip CM)

I don’t know if you have a spacer or two on the driveside BB cup, but if you do, you could try moving one onto the non-drive side. The chainset will then be a bit closer to the frame which may improve the chainline for a big/big combo. But you might not have enough room around the chainstay and suffer chainsuck. And it might mess up the other extreme: small/small.

To be fair to CM, he is right to point out that with double/triple chainsets, one can and should avoid these awkward chainline problems entirely as they strain and wear the drivechain.