gforeman 

This is a new Trek 1.5T. I picked it up Friday, and tried to put some miles on it today. First off the chain rubbed the front derailleur something awful, took it to the shop, they adjusted it. Seemed fine.

Three times (maybe 4?) I would be in the large front sprocket, smallest rear, really getting some speed, and the chain jumped the front sprocket outside. Stopped, got it back on, rode for a while, same thing again. I am not shifting when this happens. Chain seems to be riding on the sprocket fine, no rubbing. The reason I say maybe 4, is it jumped, but before it completely left the sprocket, I back peddled, and got it back on without stopping.

This is scaring my right crank all up. Not nice on a brand new bike. Shop is closed tomorrow, so if I ride, I'll have to stay at least one gear down in the rear. I tried that and it seemed okay.

Any ideas? Just an adjustment problem? I'll let the shop handle it Monday probably, I'm just curious if this is something that happens often. I've been riding my Mtn. bike for a few years, and I never had an issue like this.

mechBgon

One possibility is that the large front sprocket has a burr on one of its teeth, or a tooth which is bent inwards or outwards. Burrs, gouges and bent teeth may heave the chain overboard. It isn't the front derailleur's job to keep the chain on the sprocket, just to deliver it there when you shift, so if you're already riding in that gear when the chain jumps, it's not the derailleur per se.

There might be other issues, but that's where I'd start. If you want, you can check for bent teeth by sighting down the chainring from directly above, while slowly backpedalling and looking at the teeth for ones that lean in or out. You might see large burrs or gouges that way too, and feeling the teeth with your fingertips will usually help you detect smaller ones.

Tabor

You should be able to adjust the limit screw to keep this from happening. The limit screw on the front derailleur that is.

gforeman 

If the front derailleur is not rubbing, does it still need to be adjusted?

erader

try replacing the chain with one of better quality.

ed rader

Tabor

I would guess so. When you are on the biggest chain ring and the smallest cog, the derailleur should be almost touching the chain. This way if it tries to jump ship, the derailleur will stop it (hopefully). YMMV

EDIT- That said, I would probably take it back to the shop you got it from and insist they fix it.

Wordbiker

Are you a heavy rider?

I don't mean to be insulting, but heavier riders get much more flex out of a crankset and bottom bracket which could cause the shifting issue you described. It's tough to adjust the front derailleur to compensate for a lot of large chainring movement. At times it can be a choice between occasional rubbing or occasional chain-throwing depending on how much flex the BB has.

DannoXYZ 

MOST DEFINTELY! It's not a binary all/or-nothing thing with rubbing-not-rubbing. THe actual position of the front-derailleur makes a HUGE difference in shifting performance. My guess is the shop guy got lazy and just unscrewed the outer-limit screw a little to get it to stop rubbing This makes the FD sit way too far outwards when it's shifted into the big-ring. Then when you're riding fast, with a slight downhill, there's little tension on the chain and little bumps can cause it to hop to the side slightly and fall off.

If the rubbing sound originally wasn't constant, like it would rub only at one position on the rotation, that could mean a loose bottom-bracket or bent chainring. The correct solution is to adjust the bottom-bracket bearing and/or true the bent chainring, not to adjust the FD to make up for something else.

If you look at the outer-cage of the FD when it's shifted furthest to the outside with the chain in the big-chainring (smallest cog in back), it should be AS CLOSE AS POSSIBLE to the chain without rubbing. So we're taking about 0.5mm or so here. That way, shifting to the small chainring will occur quickly and easily.

Retro Grouch 

Without looking at your bike that would be my guess. When your bike is in the largest front chainring and the smallest rear cog the chain should just barely miss the outside blade of the derailleur cage.

gforeman 

What ever it was, they made it good today. I took it out and did some high speed runs. About 4 miles, shifting through the full range and all is good.