I was having all kinds of shifting problems on my 2010 Trek 4.7 which has a full Rival groupset (crank, shifters, front/rear derailleurs, brakes). I replaced the chain with a KMC X10SL. Now everything shifts great. In fact all of my shifting problems have been solved except for one - I'm throwing the chain off the outside of the big ring when I'm in the smallest rear cog.
With the old chain I was actually throwing the chain off the big ring in the last two smallest cogs and very occasionally when in the 3rd smallest cog but with the new chain it just happens on the smallest cog. No matter how I adjust (and I have adjusted the rear derailleur high limit screw so that the guide pulley is as far inside as possible and still have it shift to the smallest cog) I can't get the chain to stay on the large chainring up front.
The rear derailleur appears true and straight but a couple months ago I did have the bike fall on the expensive side when a gust of wind decided it shouldn't be leaning again the rail where I had set it. The only things that makes any sense to me are that somehow the tension pulley is too far to the outside, the tension on the slack side of the chain is too low, or the big ring is too worn to keep the chain from falling off. I did remove links on the new chain so that it had the same number of links as the old chain so I guess the only way the tension should be too low is if the spring in the rear derailleur has worn and seeing as the bike is only 9 months old that seems unlikely.
So am I missing something? Anyone have any ideas? Could it just be because there's no resistance on the rear wheel (the rear wheel is up in the air while I'm spinning the crank with my hand to test)? Is it just the nature of trying to cram 10 gears into a single cassette - there's always going to be a pretty severe angle trying to throw the chain off when in the smallest and largest cogs on the cassette.
Honestly 50 / 11 is not a combo I'm likely to use - at least not around the flat lands of North Texas. I'm guessing that I'd only need it going down steep hills and there just aren't any around here. Still, it's as much a matter of understanding the ins and outs of the drive line as it is a principle thing that makes me want to understand and hopefully fix the condition.