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Help me troubleshoot my drivetrain!(9 posts)

Help me troubleshoot my drivetrain!sodade
Jan 29, 2002 4:50 PM
I have been tring to troubleshoot this grinding sound that happens when I am in the two lowest gears. I noticed that the chain "bumps" around 2x per rotation of the gear. I can actually push the jumping link joint down on to the cluster. Hard to say whether this happens when the bike isn't on the workstand, but the grinding noise doesn't appear to be the cranks or the rear hub. This is on a nearly new Full DA bike.

The "L" limit screw is set just right and the derailieur makes clean shifts in both directions. It goes into the 21 and 19 gears just fine. The chain is super lubed and all of the links are as loose as you would want.
When it is in those two gears and under high torque, it makes a metal grinding/rubbing noise - it sounds just like the chain is rubbing on the front der. - but that isn't it. I have torqued it in all the other useable gear combos and there is no noise.

It's almost like there is a misshaped tooth on the cogs, but I can't see anything wrong. (but it is hard to tell because all of the teeth are different shapes).

The chainline looks good, the "B" screw is fine, the hub spins fine off the bike and the cranks spin fine without the chain.

this is making me crazy!!! Help!!!
re: Help me troubleshoot my drivetrain!grzy
Jan 29, 2002 5:07 PM
Sounds like you have either a stiff/twisted link in the chain or a damaged tooth (look for burrs). I know you said that the links are all nice and loose, but how do you explain being able to push the link back down? You didn't say how much time you have on the chain or what it's been through, but once something tramatic has happened the chain can be tweaked in ways that are hard to fathom. I think it's related to the chain b/c you say it takes two revolutions of the gear (which gear?) - if it was a tooth it would be every time that tooth came around. If you've had many chain drops or some chain suck you can *easily* twist a link and still have it move freely - even on a new bike. Ultimately chains are quite cheap and need to be replaced when they start acting strange.

If this doesn't work it's time to take a close look at the alignment of the deraileurs and the chainline. A drivetrain guru at a LBS could probably determine your problem in about 10 minutes - they've seen lots of strange things and you can learn a lot from them especially if they work on MTBs (where everything gets AFU).
I'm having the same problem....HmmmmmmJames
Jan 29, 2002 8:28 PM
My problem sounds exactly the same as yours. I just installed my Dura Ace about 3 weeks ago. I'm going to try my other new chain to see and i'll report back. thanks for the tip GRZY!
I'm having the same problem....Hmmmmmmtp
Jan 30, 2002 11:29 AM
I had a similar problem. Upon close inspection, I found the "new" large chain ring that I had installed the previous week was bent and causing FDR when applying high torque. I went back to the LBS where I had bought it and the mechanic informed me that he had seen a number of new, bent, Dura Ace chain rings in the last few months on both new bikes and replacement rings. We went through his stock of new 53 tooth Dura Ace rings(5) and found that they all were bent to verying degrees. We picked the best one and installed it. It was considerably better than the one I had and it fixed the problem.

By the way, the Ultegra chain ring that we checked was not bent.

Shimano must have had a bad batch of rings.
It's fixed!James
Jan 30, 2002 8:39 PM
I watched the chain slowly and i saw it was the same link everytime. So i took off the Sram chain(pc89r) and gave it a little hammer tap. Now it works great!
Correct pin in the Shimano chain???Geof
Jan 29, 2002 8:42 PM
I did not realize there was a dif size pin for the 9 speed chains. I uses a blackish pin on my XTR drivetrain and TOTALLY f-ed up the system. The clearance is so close, a mm will thro it off. Another thought... I've had to do my indexing adjustment on both the 2nd and 8th cogs to dial it just right. It could be the problem as well....
re: Help me troubleshoot my drivetrain!JimP
Jan 30, 2002 7:46 AM
I have to agree with the tight link theory. I had a similar experience last year with a DA chain that was only a few months old. I cleaned and hot waxed the chain and it still made noise. I found the culprit by watching the chain closely while turning the cranks on the repair stand. A little touch with a chain vice was all it needed.
re: Help me troubleshoot my drivetrain!curlybike
Jan 30, 2002 7:53 AM
Sometimes the special pin does not break off clean. This requires that you grind off the bad part to keep noise and jumps out of the cassette. It does it on the larger gears because it is touching the adjacent cog, smaller cogs allow the nub to fall into the gaps between teeth.
I've had this problem before...MeMyselfandI
Jan 30, 2002 8:37 AM
And I came to the conclusion that under torque the crank flexes and the chain grinds against the front derailluer.