JusWanaRide

Hi all. I've recently rebuilt my Vitus 979 with an 8sp campy corsa drive train and am having a problem with the chain rubbing on the RD cage when in the large chain ring/large cog combo. I've read that the chain rubbing can be an issue when cross chaining but this is usually the chain rubbing on the large chain ring when in the small chain ring/small cog combo which I would never do but I do frequently use the combo I'm having issues with. While the rubbing is annoying, the main issue is that it causes the chain to come off when back pedalling in that combo. The problem didn't occur with the original drive train set up which was a 9sp shimano 105 rear hub and cassette with shimano 600 RD and athena cranks and chain rings. Shifting was with down tube shifters. The new drive train consists of the original Athena crank and chain rings (52/42), chorus 8sp RD and chorus or record hubs and 8sp cassette (got these 2nd hand and seller couldn't tell me what they were). I've put on a new KMC 8.93 chain (1/2" x 3/32") which is supposed to be compatible with 18, 21 & 24 sp. I have also installed a set of 1st gen ergo snifters. The indexing is correct and gear changes are fine. I have also checked the hanger and dont think it is bent. Hoping someone out there might have some thoughts as to what might be the cause. Cheers.

cny-bikeman

The fact that you have both rubbing on the side of the derailleur cage and unshipping of the chain does not mean one causes the other. It's not really expected that backpedaling when cross chained will work well, as there is nothing to guide the chain onto the top of the cog. The derailleur, being at the bottom of the cog, has nothing whatever to do with the chain unshipping at that time. I would suggest you check the front chainline, as you may have too great an angle from front to rear. That is especially true if you put the replacement crank on the original spindle.

Also, although I occasionally use the large-large on my 24 speed, it's mainly when I don't want to shift to the smaller ring because I know I'm going to be shifting back up soon. Frequently using that combo should not be necessary, as all drive trains have overlap in gear ratio between rings, so even with an 8 speed you should have a combo on the smaller ring that is close to the large-large ratio. Finally, backpedaling is appropriate when at a stop, in order to have the cranks in the right position when starting. I don't know of another reason to backpedal.

Homebrew01

Don't cross chain, and don't back pedal .... problem solved, since both are easily avoidable.
You likely have an equivalent gear in the small chainring.

jyl

Doesn't sound like a real problem. But you might be able to improve the chainline with a narrower BB.