tastest

Help please. A problem happened several times this past weekend after starting all of a sudden with no explanation.

Riding along with my ultegra triple. Riding in middle chainring, all is fine. Shift the rear to one gear easier / bigger cog, and the chain falls off the middle chainring to the small chainring.

Otherwise, working fine.

Of course mainly happened while starting or on a hill, so grind to a halt.

Thanks

HillRider

What type of shifters do you have?

FBinNY 

The most obvious cause is poor FD trim, but this usually doesn't happen suddenly. Check it anyway just in case before wasting time on tricky stuff.

Another possibility is a slightly bent or badly worn tooth, especially the one following the shift gate. It's fine when the chain is coming from a good angle, but if the chain comes from an inside angle, the tip hits the edge of the inner plate lifting the chain a hbit before it settles in. I find this to be fairly common on gated chainrings with some wear. Some times you can see it if you pedal slowly watching the engagement as the gate comes by. (though it could be any tooth).

Another way to check is to clean the ring with solvent, dry it completely, then color it with a dry marker. Go for a short ride in the problem combination or the nearest one where the chain stays on, then look for a tooth with a shiny tip, or shiner than the others.

dsbrantjr

Whenever any sudden, unexpected change occurs with Shimano brifters you should check to be sure that your shift wire is not beginning to fray inside the shift mechanism. If it breaks off you will go through hell trying to get the broken-off head out of the mechanism.

FBinNY 

+1, a sudden change, usually a stiffening of the action or more resistance is an early warning of cables fraying. If it's bad enough, the cable will be a hair longer and the trim off toward the inside. It could be as simple as this, or bad trim from another cause, so check trim first, check cable second.

tastest

More Info. The brifters are ultegra, 7703 era 10 speed. The cables were new at start of season, maybe 1000 miles. Everything was fine for first maybe 60 miles of a 100 mile ride, then happened two or three times. Happened the next day again maybe twice on a 50 mile ride. I don't understand how trim would affect things, as not moving the front shifter/derailleur at all, and chain is not rubbing. If anything, the trim would seem to stop the chain from moving inward (at least as how it is set up on my bike). But I will check the trim.

hobkirk

What's a "shift gate"? I posted the anal post on "Ultegra Chainrings [Triples] are complicated!" so maybe the "shift gates" are what has me baffled.

BTW, I really, really like the look of clean chains and gears. But I still use Chain-L. My chains do seem to last longer, although I haven't kept good records.

hobkirk

Sporadic problems are often difficult, but they usually become more common. My triple is an FSA (I got an Ultegra 6603 in the mail today) but the only time this happens to me is when I have the trim set so the cage is NOT centered around the chain (when the chain is engaged in one of the middle rear gears). But it only happens, even then, when I shift into one of my largest rear gears.

So my take is you have several options:

1. Let the problem become more chronic and it will probably be easy to solve.
2. Take spin classes so you're comfortable spinning 130-160 RPM when this happens.
3. Or maybe one of the brilliant contributors here will nail it. (I thought the fraying cable was an excellent suggestion.)

Good luck.

As an aside, I have been riding with a 42-tooth middle chainring and 11-34 gears. I stay in the middle chainring almost all the time (about 7 MPH at 70 RPM to 30 MPH at 100 RPM). I think the 39T will cause me to use the big ring more. But I am expecting great shifting after examining all the tricky features on the Ultegra rings.

Airburst

A shift gate is a short section of a chainring where the teeth are modified in some way, usually shortening them or some more complex change to the profile, to aid in shifting.

cycle_maven

If the cables are new, then somebody may have changed the front derailleur setting. First thing I would do is adjust the derailleur.

I had the same problem with a bike that would drop the chain from the middle to the little ring when I shifted into the largest rear cog. No amount of front derailleur setting would fix that- the chainline was wrong; the crank was too far out and the angle of the chain caused the chainring to just shift itself without any help from the derailleur. I replaced the bottom bracket with a few mm shorter unit and it got a lot better. It still occasionally drops the chain, but less often. I try to avoid the extremes of my cassette when in the center ring anyway.