Originally Posted by
genejockey
I noticed the effect you mentioned when I got a bike with 50/34 chainrings (my other bikes had 53/39). When in the small ring, In the 3 smallest cogs the chain was scraping on the big ring. So I'd avoid those gears. I basically used it as a Cross Chaining Alarm.
If it was just scraping on the smallest cog, perhaps, but on 3 smallest cogs indicates you have a chainline issue, the chainrings are too far in. I have 50/34 and chainstays toward the short end, and get no scraping on 34-11, however mine is 7 speed cassette (8/9/10 are all just a skosh wider), 130 OLD, 43.5mm chainline, and also the large ring is "dished" so the teeth are slightly outboard of the mount at the crank spider.