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MountainBikeReview.com's Forum Archives - General Discussion
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What's wrong with this chainline, drivetrain..xpost-sorry (7 posts)
|  What's wrong with this chainline, drivetrain..xpost-sorry | Gateway
Jan 16, 2002 8:19 PM | | Arrgh!
In the largest cog (34T) and middle ring (34T) -my favorite combo, the chain seems to be pushed outside by the chainring. If I backpedal when it's in this combo, after 1-5 rotations the chain drops 1-3 cogs.
Not always a big deal, but when you have to backpedal at a crucial moment and it jams -it's a big deal.
When I backpedal slow, I can see how the chain hugs the cog on the inside (showing how there's tention to put it outside).
What's going on? |
| I just had a similar problem. | Finch Platte
Jan 16, 2002 8:41 PM | | My der. was jumping all over the place. Altho, I didn't notice that the der. was being forced to the inside/outside. But- it doesn't take long to check one thing- is your rear derailleur (hate to say it) swinging freely? In my case, if you took it and pushed it back, it would stick in this position. It took a little lube and a little loosening of the bolt that fastens the der. to the dropout.
Hope you figure it out- for such 'simple' mechanisms, there sure is a lot that can go wrong. Wish the internal hubs would get better.
fp |
| re: What's wrong with this chainline, drivetrain..xpost-sorry | Hank
Jan 16, 2002 8:45 PM | | hmmm, you might try adjusting the little-known and largely ignored "b-screw" which is that little screw near the top of the der that goes right against the hanger/dropout. This is usually backed out pretty much all the way, but if you dial it in a bit (don't go too far) you will buy your der a bit of breathing room from that 34 tooth cog. I'm guessing that right now the the top jockey wheels is pushing the chain up against the 34 tooth cog a bit too much, and when you backpedal the various ramps, etc. on the cassette are catching the chain and causing it to drop down to a lower cog. Hope this makes sense. If you have a fairly stock bike and are using Shimano cranks, your chainline is probably fine (or as good as it will get with a modern mtb). Most shop wrenches ignore this b-screw adjustment, however. Good luck. |
| also... | Hank
Jan 16, 2002 8:47 PM | | of course you should check your der for damage, check the chain length and make sure you der hanger is properly aligned. |
| re: What's wrong with this chainline, drivetrain..xpost-sorry | xerxes
Jan 16, 2002 9:42 PM | | Could be as easy as needing to adjust the cable tension on your rear derailleur such that it still shifts but doesn't drop. You would need to turn the rear derailleur adjustment knob at the rear of the derailleur by a quarter turn or so clockwise. In the alternative, it could be as simple as a bent tooth on your 34 cog. Check to see if it drops on the same tooth each time. That one could be bent, probably to the left, such that it won't engage properly and pushes the chain to the right and down to the higher gears. Could be a slightly kinked chain link. Spin the crank backwards and look at the pulley wheels for a jump in the chain. If it jumps, that link is bent and should be straightened/replaced. Report back with the fix, if anything works. Good luck. |
| Update | Gateway
Jan 17, 2002 7:00 AM | | Thanks for the tips. The b-screw seems like it could be turned a little to give the rear der some space, but it's not the rear der that seems to be the issue. And the cogs are not bent either.
When the bike is clamped into the stand and the crank is turned backwards -the chain jumps down the cogs from the top (of the cog), NOT from the rear der.
It seems like there's the kind of tension on the chain that you get when you cross-chain.
When I'm in that combo (big cog 34T and middle ring 34T) and I turn the crank slowly forward, I can see the chains inner link hugging the cog then it sort of snaps slightly outward as if the chainring is too far outward or the cogset is too far inward.
Any ideas? I think I might check what my LBS thinks. |
| Chainline is the likely issue | Calvin Jones-Park Tool
Jan 17, 2002 8:12 AM | | You are describing a common problem. This may sound silly, but bikes are not designed for backpedalling. When you pedal forward, the upper pulley guides the chain the rear cogs, it is even called the Guide Pulley. The body screw (B-screw) is used to get this pulley close to the cog. When you pedal backwards, the guide pulley effectively is the front chainring, which is 17-20 inches away, hardly a useful guide.
It is likely you would be better off if you could move the front rings inward. Check the clearance from the frame to the rings. You may also be limited by the spindle lengths available. If you move too far inward, you may end up having trouble shifting easily to the smallest ring. Moving the front rings inward will straighten the chain a bit more and alleviate your backpedal derailling woes |
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