Bicycle Mechanics - Replaced chain, now chain popping off tension pulley at largest cog/chainring

I have a road bike, Shimano 105, 3 chainrings, 10 cogs. Today I replaced my chain (with a 105 10 speed Shimano chain, old chain had about 6K miles) and did some other minor maintenance, including removing/cleaning the rear derailleur pulleys.

Everything's great, except on the largest chainring and cog, where the chain pops off the tension (lower) pulley on the rear derailleur when I pedal. Looks like the chain is too low, and too far out (away from the bike) for the pulley to accept.

Anyone have any ideas? Might my new chain be too long or too short? Thanks so much.

Nick

It's possible the chain's too long, yeah. In most cases, the chain needs to be shortened by the installer to an appropriate length. With road triples, I usually pick the most chain that the rear derailleur can handle in the small-to-small combo and still not hang limp, then confirm that this is sufficient to go around the big-to-big combo successfully (if not, I add enough to make the big-to-big combo). Once that's determined, I pin the chain.

If the old chain had 6k miles on it, your cassette and/or chainrings may not play nicely with the noOb chain. Skipping under forceful pedalling in some or all rear gears, or full-circumference chain slippage on a chainring, is a sign you're coloring outside the lines with parts that have way different wear levels.

Tangentially: the latest "5700" generation of 105 chains are directional. They're also not recommended by Shimano for triple cranks; Shimano suggests the 5600-series or 6600-series chains (non-directional, in other words) for triples. If you do have a 5700 chain, make sure the non-slotted outer plates with the stamped logos are facing outwards.

Generally one doesn't use the full crossed position, even though the chain is carefully measured to make it possible. You're right in that the chain is coming from too far outside the bike (or the RD pulley is too far in). The extreme angle causes it to climb up over the side of the pulley.

Sometimes this improves if the hanger is straightened, since when the hanger is bent in (as it does if the bike is dropped) the lower pulley is inboard of the low gear sprocket.

Also it's possible there's a bit of twist in the hanger or cage worsening the problem. A worn pulley also worsens the problem, but before you spend a ton of dough be aware that often even with all three issues addressed the problem won't go away.

Best advice, do a quick check of the hanger (eyeball from the back of the bike), and if it looks good, and shifts well otherwise, simply give up riding this combination which you shouldn't ride anyway.

Oh, and one more thing: the lower pulley on a 5600-series rear derailleur is directional too. Since you removed and reinstalled it, make sure you installed it right-side-out.

Oh, and one more thing: the lower pulley on a 5600-series rear derailleur is directional too. Since you removed and reinstalled it, make sure you installed it right-side-out.

Also make sure you didn't switch the upper and lower pulleys.

I saw that happen on a new RD, with a new cassette, and a new chain on one of my bikes. Like FBinNY said, it is crosschaining. If you look directly down from the top you can see the chain is running diagonally, and as the RD cage swings fwd due to all the excess chain being taken up by the large cog/ring, it diverges further from the chainline.

Remedy: Make the chain as long as possible in order to keep the RD cage from swinging so far fwd.

I have mine chain long enough that on the small/small combo, the RD retracts all the way to the stops, and the chain goes slack. It doesn't hurt anything to run it this way.

Thanks so much for the responses all. I did lengthen the chain a bit, but still had the crosschaining issue. As you noted, I never use this gearing, and everything else works. I imagine this was always an issue, I probably just never noticed.

Thanks again.

Nick