Middle chain ring running rough

[slcbob1]

This one is driving me bonkers. Looking for a jolt of wisdom. I am not smoking crack. But this might drive me to it!

Hardtail MTB turned commuter, XT cranks, LX front derailleur and trigger shifters, XTR rear.

I just changed my chain (SRAM 971) and did a basic clean up, lube and adjust. Everything runs smooth on the workstand. But under load, and only when on the middle chainring, I get a slight noise and chunk. It feels like a super light version of the chunk you get when your gears hang and then shift under load. But no gears are shifting. Just "runs rough" - all the time, whenever any half decent power is applied. I'm going nuts trying to isolate it to a spot in the pedal, chain, or other rotation - no luck.

I have checked for excessive wear and bent teeth. I can't see any cracks or signs of stress on the chainring. Chain seems to be fine, and runs as smooth as silk on the big and little chainrings (and all the rear cogs). This is not, as far as I can tell, derailleur adjustment -- the front is not rubbing, the back works fine on the big and little chainrings. Chainring bolts are tight. This occurs consistently, across all the rear gears, whenever I am on the middle chainring. This didn't happen (that I noticed) prior to my last bout of maintenance.

Any ideas?

I'm thinking new bike , or hare kari.

 

[kdelong]

Let me guess, you use the middle chainring, like, most of the time, right? It may be worn, even though it does not show any readily visible signs of wear. Bet if you replace just the chainring, not the whole bike, that it will be OK again.

 

[dhk2]

Let me guess, you use the middle chainring, like, most of the time, right? It may be worn, even though it does not show any readily visible signs of wear. Bet if you replace just the chainring, not the whole bike, that it will be OK again.

+1. If there is any good way to check for chainring (or cassette) wear, I sure don't know it. They can look fine, but not work when you put the new chain on.

Even if you replace the chain at proper intervals, ie, under 1% "stretch", keep everything lubed and cleaned, cogs and chainrings don't last forever.

 

[slcbob1]

Let me guess, you use the middle chainring, like, most of the time, right?

Nope! I know that's SOP around here. And I'm not in the big ring because I'm some deluded slow-pedalling not 100% honest Jan wannabe, which seems to be the other SOP around here. I'm in the big ring 75% of the time because I'm on a relatively level bike path for the last 3 years for ~4,000 miles/year, on a mountain bike. Before I pimped my ride to commuterville, I was mostly middle ring but legit mountain bike mix with some substantial small and some rare bursts of large ring time.

It may be worn, even though it does not show any readily visible signs of wear. Bet if you replace just the chainring, not the whole bike, that it will be OK again.

Yes, I guess I'm about there.

I am a faithful member of the 1% club as dhk2 states. And about at the end of my rope. But, DAMN, it amazes me how this kicked in all of a sudden like.

 

[dhk2]

It's really not so amazing....it was caused by the new chain. Last time I replaced a chain, my friendly LBS mechanic asked how many miles I had on my cassette. When I told him 10K, he told me to be prepared to come back for a new cogset. He also advised I try out the chain close to home "just in case" the new chain slipped under load on the old cassette. I was back within an hour to buy the replacement cogs.

 

[slcbob1]

Sure as heck looks as if you guys are right, me and my big ring notwithstanding. Off the bike and disassembled, I can really see an eccentric curve in the teeth that I couldn't see earlier, even with a close look.

If only my LBS had sold me the right replacement when I stopped in yesterday! Then I'd be able to prove you right for sure. Instead, I have to goof around with doing this repair twice. Guess I really threw them off the scent by bringing the bike there and saying "I want a new middle chain ring for that."

 

[dhk2]

Sure as heck looks as if you guys are right, me and my big ring notwithstanding. Off the bike and disassembled, I can really see an eccentric curve in the teeth that I couldn't see earlier, even with a close look.

If only my LBS had sold me the right replacement when I stopped in yesterday! Then I'd be able to prove you right for sure. Instead, I have to goof around with doing this repair twice. Guess I really threw them off the scent by bringing the bike there and saying "I want a new middle chain ring for that."

Sounds like your LBS mechanic isn't the sharpest tool in the box. It's bad enough to miss a worn chainring, but for him to turn down an opportunity to sell parts is inexcuseable

 

[slcbob1]

Sounds like your LBS mechanic isn't the sharpest tool in the box. It's bad enough to miss a worn chainring, but for him to turn down an opportunity to sell parts is inexcuseable

To clarify my earlier -- he sold me a part alright, just the wrong one . Pesky model year issue w/ Shimano XT, I believe, but that's a rookie mistake (me!), not a pro one.

And I just went in and asked for the part, so I can only pin the diagnostics issue on yours truly. Thanks for the nudges here to help suspend my disbelief that I really wasn't seeing a miraculous peculiarity, just wear. Live and learn. I'm still surprised the big ring isn't causing any problems. I just guess all that clean living is paying off