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Discussion Starter · #1 ·
When I crank hard in the middle ring, the chain "pops" and skips up off of the teeth. The chain only pops off of the middle front chainring - never the cassette or either of the other front rings. It's a brand new chain, cassette, and crankset, so I don't know what the problem could be. As far as I can tell, everything's set up fine, and it's been shifting and performing great otherwise.

My first thought is that it's the fault of the chain, but I've never heard of a chain causing this before. It's a SRAM chain in an otherwise Shimano drivetrain, so I'm thinking about swapping it out for a Shimano chain to see if that helps. (side note: I've been using Sachs/SRAM chains since '97, and have never had a single broken chain... or any other problem with them) My second guess is that maybe it's the derailleur... my rear derailleur is 10 years old, and so maybe it's worn out and just not keeping as much tension on the chain as it ought to?

Any thoughts or suggestions? Has anyone else run into this problem before... and successfully fixed it?
 

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My guess is maybe an improperly adjusted front derailler that is compensated for on the other rings by the limit screw. Or a more likely scenario the old rear derailler is screwing up the chainline or something, but the fact that it shifts fine in the rear makes that scenario seem a bit odd.
 

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Discussion Starter · #3 ·
Well, I tried an experiment...

I set my bike up with the front tire against the wall (to keep it from rolling), and pressed down on one pedal while holding the rear brake. I set up cameras to film the front and rear gears, and marked the chain so that I could see how much it moved.

The result was that when I pressed down on the pedal, the chain "popped" and skipped off of the chainring - just like how it does when I'm riding - only with zero movement in the chain, cassette, or wheels. The video confirmed that the problem is definitely caused by the chain popping off of the front chainring. The front derailleur is ruled out as a culprit, because the chain was not moving through it in this experiment.

I slowed the video down, and saw that after pressure was applied to the pedal, but before the chain skipped, a small bit of slack appeared in the the chain between the bottom of the chainring and the lower derailleur pulley. Shouldn't the rear derailleur have enough tension to prevent this?

Also of note: I did extensive searching (both here and on Google), and have found a lot of posts from people who've had this identical problem. Nothing, anywhere, mentions a cause or solution, though. I think that's part of why it's bugging me so much.

So I think the plan is to pick up a new chain tomorrow afternoon, and if that doesn't work I'll try adjusting the chainline (not sure how that would fix it, but hey, swapping BB spacers doesn't cost anything, right?)
 

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are you sure you have the proper chain length????

are you sure the front derailleur hight is proper?

are you sure the derailleur is perfectly paralel to your chain rings?

have you tried turning the barrel adjuster on the shifter the proper way to combat which way it is switching towards?
 

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nice method

I like your experiments, very crafty... i really doubt that your problem is the result of using a Sram chain on a shimano drivetrain, its a proven combo. however it may be a faulty chain/powerlink. How about the crankset, are all chain rings properly installed, correct orientation and torque on the bolts?? I would say derailler tension could be an issue, but I would think yhou'd have more problems on the small ring, not the middle.
Good luck on this one...
 

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Shimano Chainrings might be the culprit

If you are running XT/LX rings (they are the same part#) the middle ring is made from a combination of aluminum and soft butter. Check for premature wear and broken teeth on this ring, if everything else is ok try a new chainring, even if yours look ok to the eye.
 

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RobNC said:
If you are running XT/LX rings (they are the same part#) the middle ring is made from a combination of aluminum and soft butter. Check for premature wear and broken teeth on this ring, if everything else is ok try a new chainring, even if yours look ok to the eye.
I am having similar issues with my chainrings. I just figure it is time to replace the chainring since the cassette and chain are new. The chainrings are original and have about 1200 miles on them. If shimano are made from soft butter, what are some kick ass alternatives? Is it OK to start mixing up drive train components?
 

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cbchess said:
my new shimano middle rings lasted just six months. I replaced them with Raceface.
I would look to get a new rear derailer too.
my LBS just sold me a small and middle Race Face chainrings. They said they don't even stock Shimano chainrings because, like someone mentioned, they are made from Al and soft butter alloy.
 

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Harky
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Tick...
How old is the chain and how long has it been used on the middle ring??
I had a similar problem that really bugged me cause it would happen in a hard climb! And only with the middle ring. When I compared the teeth of teh middle ring with those of the low or high rings, I could see that they we worn out. Duh...that's the ring that I run in most of the time. An I had run it for years.
When chains wear out, they stretch. When a worn chain is run on an aluminum chain ring, it wears out the teeth. You throw a new chain on and it runs OK for a little while then goes to s**t. Get a new chain AND and new chainring. They are both shot.
 

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Discussion Starter · #13 ·
harky said:
How old is the chain and how long has it been used on the middle ring?
Chain, cassette, and crankset (including rings and BB) are all less than a week old. That's what confused me... brand new parts, properly installed and tuned (I was a bike mechanic through high school and college), should not have problems like this.

Now, the good news is that I think I fixed it!

I run an 8-speed drivetrain, with the sole exception of 9-speed front chainrings (I've never had any trouble with this setup). This afternoon, I took the 8-speed chain off and installed a new SRAM 9-speed chain, then tried the brace-the-bike-against-the-wall-and-push-hard-on-the-pedals test. No slippage now! :thumbsup:

Still need to test it out on the trail though, but I think it should be good.
 

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I have a similar problem. New shimano chain, new LX cassette, new xt crankset and a SRAM powerlink. Mine shifts fine in the rear and I think it only started "popping" when pedaling hard up hill in the middle ring after installing a SRAM powerlink. At least that is the only thing different in the setup that I can tell.
 

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Discussion Starter · #15 ·
I suspect (and keep in mind that I just pulled this theory out of my a**) that the cause of my drivetrain slippage was a lack of sufficient friction between the middle chainring and the 8speed chain. The 9-speed chain fit the chainring more snugly, so the problem went away.

I also suspect (and this would go along with Oldskoolbiker's experience) that chainline has a part to play in this, too. My chainline is good (50mm), but maybe a less-good chainline would have introduced more friction to the front chainrings, and therefore helped fix the problem too? It's worth considering, especially since with the new outboard-bearing BB's, chainline adjustment just involves moving a spacer from one side to the other.
 
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