Chi_Z

Anyone know the cause? My FD is 2mm above large chain ring and perfectly parallel. The limit screw are adjusted correctly. as in 1mm spacing between either extreme ends.

The problem only happens when i downshift from large to small. The chain will get stuck for a second but eventually drops down to small ring as I backpedal a bit

FBinNY 

Most likely the chainrings are too far apart for the chain. Or the chain is too narrow for the crankset.

This has become more common as folks mix 9s and 10s equipment, or similar mixes in chain width.

This is easy enough to confirm. Shift partway and see if you can trap the chain between the tips of the inner ring and the inner face of the outer ring. If so, the solution is to use a slightly wider chain. It's also possible to file the tab area of the ring (inside face of outer ring is easiest) so the rings move together a bit.

You can sometimes solve this with no changes in the bike, simply by anticipating your needs and shifting the FD sooner, while chain tension is still low and your cadence high.

Chi_Z

Using SRAM red 2012 chainrings with 10sp 1070 chain. It only happens under force like you said. Never had this problem with my old bike, then I switched the components over to a new bike. Could it be a FD adjustment problem?

Seems like the chain could not shift down in time and gets stuck between the gap of large chainring and FD outer cage

Also every time it happened I was under the trim mode

Any insight and help is greatly appreciated

FBinNY 

It shouldn't be possible even with a poorly adjusted FD. However, it might help to adjust the inside trim inboard a bit so the FD pulls the chain in more. The flip side is that this can also increase the odds of chain drop to the inside. Also consider that as chains wear they also get more flexible and that can contribute to the problem. A newer chain is stiffer (sideways) and so it carries in a bit more as it's angling off the outer ring.

As so often happens here, we can only offer possibilities and things to look for. But we don't have the bike, so you need to review and observe and see if you can form a theory.

Since the crank used to shift OK, I'd focus on the FD. Try adjusting the trim, or possibly twisting it heel in or out (trial and error) a degree or two and see if that's enough to nudge the chain in a bit more without dumping it.

meanwhile, if it only happens when shifting under load, stop doing that.

headloss 

Is the chain new, on a worn chainring?

Chi_Z

The chain has about 1500 miles, the chainring i bought used so it could be worn out. Does worn out chainring cause this chain stuck problem? If so i'll order a new chainring.

headloss 

ugh... *edit* I failed reading comprehension today... I thought your issue was "chainsuck" not "chain STUCK." LOL

Yes, a worn chain ring could cause the chain to stick. It's definitely something to consider but that would be an if-nothing-else-works approach.
Most likely you can get it sorted out if you keep messing around with the FD. Something to consider, it sometimes shifts better if the derailleur ISN'T perfectly parallel... don't be afraid to experiment by off-setting it slightly.

cny-bikeman

It's early in the morning, but it seems to me the problem is being read incorrectly. It's clear to me that the OP is saying the chain is trapped vertically between the bottom of the outer plate of the derailleur cage and the top of the large chainwheel teeth. He stated that twice, and backpedaling when the chain is stuck between the chainwheels usually does not quickly drop the chain to the next chainwheel.

If my reading is correct it's a very unusual problem, but one I have seen before. First we need to confirm that ALL components - chain, derailleur and crankset are the same as on the previous bike - if not what is different, as the variable is the first item on which to focus.

If all is the same then:

• The derailleur spacing needs to be confirmed - should be measured when the outer plate of the cage is directly above the outer chainring. It should not be possible for the chain, which is 7-8 mm high, to fit completely between chainwheel and derailleur if the derailleur is properly positioned.
• Secondly, how is the outer plate of the cage in respect to the chainwheel - parallel or otherwise?
• Finally, how many miles are on the derailleur, and are there any ridges or is there any damage on the lower inside surface of the outer plate where the chain gets trapped?