Middle chainring/small cog problem

[Batbike]

Adjusting a triple chainring with a 9-spd cassette -- shimano. It runs smoothly through all gears and up & down chainring EXCEPT on the middle chainring and small cog -- the chain gets caught on inside of the chainring, rubbing and literally picking-up chain on inner connectors causing chain to pop-offf chain ring and back down again.

I have done the following and cannot get the problem to stop:
1. adjusted front derailleur soit runs smoothly between small, middle, and larger ring
2. adjusted cable tension for front derailleur
3. tightened BB as much as I can turn

Anyone know what is going on and what I need to do?! THANKS!

[KnottedYet]

Is the chainring bent?

[Batbike]

it could be bent -- just one spot on chainring where rubbing/pop-up happens. hard for me to tell if bent there or not, but seems logical that may be the case.

do you know of way to tell if chainring is bent? how can a chainring bend -- a fall, torque, bump to area ... ???

[DebW]

It's very possible, even likely, that middle chainring-small cog is just a combination that shouldn't be used. Putting the chain on the outside rear cog results in a severe chain angle that sometimes simply can't avoid having the chain hit the large chainring. It will depend on chainstay length and lateral spacing of the cogs and chainrings and the size of the chainrings whether this gear is usable or not.

Shimano chainrings have ramps and pins built into the inner surface of the large chainring. That may be what is catching your chain. It's built that way to make shifting easier. To check if the chainring might be bent, drop the chain to the inside and eyeball the teeth from above as you turn the crankset. You can hold something like a large screwdriver so it just touches the teeth while you turn to see if they are all aligned.

[Batbike]

yes it is the inner pins that is catching the chain ... thanks! I will check if bent.