I have a 1984 Specialized Expedition. I have set it up with more modern parts. Everything is close to running well now except when I am on the middle chainring(36T) and the largest cog in the rear (30T), the chain has a tendency, or inclination to move to the small chainring. It rarely actually makes the jump, but the chain keeps getting out of the middle chainring moving toward the small chain ring. It seems to only to have this problem when on this gear combination. The cassette is new, SRAM 12-30 8-speed, and the front is an older sugino AT triple crankset.
please shed some light on what might be the cause of this.
My Bianchi Bergamo has a single 32-tooth chainring and a 12/32 7-speed cassette. It also wants to shift off the chainring when in the lowest gear. There was a burr or manufacturing defect on the chainring that was making it worse, but even after filing it off and dressing the area it still isn't totally reliable in the lowest gear. The problem seems to be due to a combination of equal sized "sprockets" on both ends and a relatively "crossed" chain. Bianchi deserves some of the blame for using a derailleur type chainwheel in an inappropriate application. I have assumed that a front deraileur used as a chain guide might cure my problem, but it doesn't seem to be helping you.