Admittedly, I don’t often work on flatbar setups, so this one has left me somewhat intrigued. A long time client recently dropped off his daughter’s bike, a Trek Marlin, set up with Shimano Altus all round. One of the issues she had been dealing with was chain drops at both ends (inside/outside).
Resolving the issue of the chain dropping on the inside (onto the BB) was easily sorted out with a slight adjustment to the limit screw. It’s the chain dropping on the outside that has gotten me befuddled.
The shifter is specifically, an M315-2L variant, while the FD is an M315-TS, both designed for 2x chainrings. On closer inspection, the shifter is an exact replica of the M315-L 3x variant, without the “3” printed onto the optical display. I’ve found that you could actually shift past the “2” on the M315-2L shifter into what should theoretically be the “3” position. In doing so, the FD actually shifts the chain off the big ring, leading to chain drops. This leads me to the suspicion that that was what my client’s daughter has been doing - overshifting, so that the derailleur moves a little too far outboard, causing the chain to fall off.
Is this normal for the M315 2x setups? I’ve resorted to some degree of finessing the “H” limit screw so that it prevents the FD from moving into the vacant “3” position, but outboard enough so that shifting from the small chainring to the big one is still pretty straightforward.
Thanks for the photos - I couldn’t get around to uploading them
Messing about with the “H” limit works pretty well, but not as much as I’d like, and kids grow out their bikes quite quick as well, and I’d hate to have my client spend money on new parts only to have his daughter grow out the bike in a few months.
Circling back to the FD, I’ve now got the chain running right in the middle of the FD cage, to minimise chain rub. It would have been sweet to have the FD actually be a dedicated 2x setup, so there’s less of a gap between the chain and the outer plate. Currently this isn’t possible, if I had kept the 1mm or so gap between plate and chain, since this results in the chain failing to jump onto the big ring smoothly.
I work at a Trek dealer and was pretty glad when they switched the Marlins to 1x gears. This front derailleurs are pretty finicky for the amount abuse those bikes take.
The high limit has to be set very tight, almost or actually rubbing the chain. I set the derailleur as low as possible above the chainring, usually 1-1.5mm.
The shifting in the work stand always seems terrible, but it usually works ok when you actually ride it. It seems to rely on the force of pedaling to get the chain to engage the large chainring. Additionally, a lot of those 2x cranks have chainrings with a lot of lateral runout, aka they wobble, which doesn’t help.
For the most part, I find it peculiar (and assumed I was mistaken, right off the bat), but glad to hear I was on the right track. That FD sure does have quite an excessive bit of swing, which led me to suspect that it was a 3x design, instead of 2x. That was the party that got me confused, spec’ing what seems like a 3x FD (and a 3x shifter that looks 2x at first glance) for a 2x chainring setup.
I eventually got the setup working, with liberal use of the high limit screw, and a fair amount of finessing, i.e. pedal a little, tweak the screw a little. Too little turns on the screw and the chain falls off the outboard side. Too much, and the chain fails to stay on the big ring.
I’ve also found that keeping the chain running right down the middle of the FD cage works with zero chain drops, save for a slightly annoying chain rubbing noise when in the Big-Big gear, because of the steeper than usual cross chaining, due to the shorter chainstays. That angle also seems to have the effect of “pulling” the chain off the big ring, adding to the noise.
