Chain folds/drops when shifting to smaller ring

[cyan]

This has happened four times in the past six weeks, twice at the same spot on separate rides. However, I can't re-produce it on workstand at home.

Symptom: press to shift to smaller ring, pedal jams, get off bike to inspect: 2-3 links fold on the chain and get stuck on either bigger or smaller ring. When it happens on the smaller ring, subsequent pedaling by hand results in chain drop to the BB area.

Any suggestion to diagnose the cause and fix it? This is on a DA9100 mechanical groupset. Thanks.

[muz]

I would go over the FD setup:
- check if cage is bent
- cage parallel to big ring
- 2mm spacing between the big ring and the bottom of the cage
- limit screws adjusted

[Peter P.]

Chain is too long.

Chainrings are toast.

Derailleur spring is weak. (some are adjustable)

Don't ride by the spot where the chain jams!

Just suggestions...

[cyan]

Things checked:

Cage is not bent, parallel to big ring, and ~2mm gap between big ring and its bottom
Chainrings just over 1500 miles and no visible damage, so probably long way to go.

Speaking of limit screws, how much gap should I leave between the cage and the chain when on small ring and biggest cog combo? At the moment, they will touch if without additional trim/micro-shifting. In fact, this also applies to the 2nd/3rd largest cog when on small ring, but to a less extent obviously.

One thing I notice on workstand is when I shift to small ring, the chain bounces once on shifting completes. Not sure if that's normal or a symptom of something else?

[Dr Luxurious]

check for chain stretch

[jtakeda]

Reading your initial response my gut reaction is FD is too high.


Your low limit sounds correctly set. It should not touch but be as close as possible

[lemondvictoire]

Tight chain link.. pin binding
Bent chain side links from chain jam/suck

[cyan]

Originally Posted by Dr Luxurious

check for chain stretch

Just did so, not stretched/worn enough for replacement.

[carpediemracing]

I have a feeling that if you replace the chain right now the problem will go away. It may not be "worn enough for replacement" but generally those guides are quite aggressive about keeping a chain on the bike.

I replace my chain every 1500-2000 miles, and I'm pretty fastidious about keeping them clean. I also almost never ride outside so I get very little grit in the chain; also my mileage tends to be 100% pedaling since I am on a trainer most of the time, and there isn't much coasting on a trainer.

I generally get DA chains (I have 3 left from my last bulk chain purchase?) but have gotten Record and also some KMC ones. I'm running a Campy 10s drivetrain.

[smead]

The first time it happened and the pedal "jammed", you may have very well bent a plate. Adjust FD according to shimano techdocs, make sure your chain is the correct length, replace w/ new chain, problem should go away. Another thought is that maybe in that particular spot where you drop the ring, are you by chance cross chained? Cross chained ring changes under load are a recipe for munching chains.