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Chain dropping in new drivetrain

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Chain dropping in new drivetrain

Old 04-27-16, 03:31 PM
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Chain dropping in new drivetrain

I've just replaced the whole drivetrain of my mountain bike (jockey wheels, chainrings, cassette, chain). I kept the SRAM X9 9sp shifters and derailleurs, but upgraded the rear trigger shifter from 9sp to 10sp by replacing the ratchet mechanism with the Ikomar's one.

I've installed a 11-36 SLX cassette, and 24/32/44 SLX chainrings (the middle chainring is the steel/composite one).

Rear derailleur with 10sp upgrade works flawlessly, and so do the shifts between the middle and big chainrings. The problem is that there is no way to make the small chainring work. The bike previously shifted perfectly with 22/32/44 Truvativ chainrings and 11-32 XT cassette.

With the new components, shifting from the small chainring to the middle one is not very smooth, I have to press the lever quite hard for it to upshift, but it's acceptable. However, when shifting from the middle chainring to the small one, sometimes it refuses to shift, sometimes it needs 2 or 3 crank revolutions, and sometimes (maybe one in five), it drops the chain when it finally shifts. It's way less reliable when the rear is on the 36t, and it works better when it's on a smaller sprocket.

If I adjust the stop so that the chain doesn't drop, it simply refuses to shift. If I adjust it so it reliably downshifts, then it reliably drops the chain.

I'm really confused because everything is contradictory. It's somewhat difficult to upshift and very difficult to downshift, but when it finally does it, the chain goes past the chainring.

I'm starting to think it would be a good idea to put some washers on the small chainring, to place it closer to the frame, so its where the chain tends to go when I adjust the derailleur stop for a reliable shift.... but I think I'm missing something here...

Any idea?
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Old 04-27-16, 03:36 PM
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Am I right when I say that you didn't make any changes in the crank front derailleur or front shifter?
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Old 04-27-16, 03:49 PM
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Originally Posted by Retro Grouch
Am I right when I say that you didn't make any changes in the crank front derailleur or front shifter?
Right. Same cranks, same derailleur and same shifter.

I forgot to say that the 32 and 44 chainrings are 9sp, and the 24 is 10sp (I was unable to find a 24t 9sp chainring). I know that 10sp chainrings sit a bit closer, but I thought it would be a non issue because I've been running an 8sp road bike with a triple 10sp chainring for ages without issues.

I'm also thinking that maybe the issue is caused by the composite (plastic) ramps of the 32t... maybe the chain slides over them making it pick up enough lateral speed to go past the granny ring.
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Old 04-27-16, 04:04 PM
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What are the condition of your cables? Downshifts in front are by spring tension and any cable friction will cause difficulties. Having non-matched chainrings probably doesn't help any, especially with upshifts; the shift gates may not be phased correctly. Is the derailleur accurately parallel with the chain rings?
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Old 04-27-16, 04:09 PM
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Originally Posted by dsbrantjr
What are the condition of your cables? Downshifts in front are by spring tension and any cable friction will cause difficulties. Having non-matched chainrings probably doesn't help any, especially with upshifts; the shift gates may not be phased correctly. Is the derailleur accurately parallel with the chain rings?
Cables are perfectly smooth. Quite new XTR cables and housings.

You may be right about the shift gates, tough I've been running a mix of Truvativ and Shimano rings in other bikes with no issues... and anyway, the only non matched chainring is the granny, that doesn't have shift ramps or anything special.

I'm more concerned about the chain dropping and unreliable downshift than about the "not very smooth" upshift.
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Old 04-27-16, 06:03 PM
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If it was my bike, I'd take a long hard look at the position of the front derailleur on the seat tube. In particular, I'd look at exactly where the inside of the derailleur cage contacts the chain on the "iffy" shifts.

The front derailleur on my recumbent was driving me batty until I raised it up about 2 mm so that the shapes on the derailleur cage would hit it differently. Suddenly it started shifting flawlessly.
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Old 04-27-16, 11:24 PM
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Originally Posted by Retro Grouch
If it was my bike, I'd take a long hard look at the position of the front derailleur on the seat tube. In particular, I'd look at exactly where the inside of the derailleur cage contacts the chain on the "iffy" shifts.

The front derailleur on my recumbent was driving me batty until I raised it up about 2 mm so that the shapes on the derailleur cage would hit it differently. Suddenly it started shifting flawlessly.
I tried moving it, but I didn't raise it to compensate for the slightly bigger inner chainring. Yours is a good idea. I'll try it as soon as I get back from work!

I did take some measurements though and found that small and middle rings are 8mm apart, while on my triple road bike they are 9mm apart (It has 10sp cranks) . What's the correct spacing?
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Old 04-27-16, 11:42 PM
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Did you cut the chain to the correct length?
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Old 04-28-16, 12:20 AM
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What about making sure that all the rings are facing the proper way? Are the pin on the large chainring and grooves on the other two lined up with the crankarm? Since you have upgraded from 9 to 10 speed have you tried spacing the bottom bracket differently?
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Old 04-28-16, 12:58 AM
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Originally Posted by AnkleWork
Did you cut the chain to the correct length?
Of course. I'm completely sure it's not the problem.
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Old 04-28-16, 01:00 AM
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Originally Posted by domridesabike
What about making sure that all the rings are facing the proper way? Are the pin on the large chainring and grooves on the other two lined up with the crankarm? Since you have upgraded from 9 to 10 speed have you tried spacing the bottom bracket differently?
Yep. Rings are facing the correct way and the grooves are aligned. I can't space the BB differently. I'm running GXP cranks, where the spindle, spider and right crank arm are the same piece.
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Old 04-28-16, 01:22 AM
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Originally Posted by Amt0571
Yep. Rings are facing the correct way and the grooves are aligned. I can't space the BB differently. I'm running GXP cranks, where the spindle, spider and right crank arm are the same piece.
GXP bottom brackets come with spacers (washers) when you're using them for mountain bikes. No spacers for road bikes. You didn't remove that bottom bracket did you?
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Old 04-28-16, 01:30 AM
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Originally Posted by TrojanHorse
GXP bottom brackets come with spacers (washers) when you're using them for mountain bikes. No spacers for road bikes. You didn't remove that bottom bracket did you?
Yep, you're right about the bottom bracket spacers. I replaced the BB 2 years ago (with an identical one) and put the spacers exactly the same way as the ones the bike came with. It has never given me any problem until I installed the new chainrings. I even ran it for a while with mixed 22 and 44 Truvativ rings and a 32 Deore ring without issues. Chainline seems perfect to me.

I'm a bit confused with all this issues, because small chainrings don't have shifting ramps or anything special on them, so I'm not sure why it's not working as it should. I thought that changing from a 22 to a 24 would make shifting better because of the smaller difference between granny and middle ring.
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Old 04-28-16, 02:03 PM
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This afternoon I added 0,75mm thick washers between the small chainring and the crank arms and it has solved all problems.


It's now shifting really smooth, with no dropped chains.


I have concluded that the problem was caused by a combination of factors: 9sp derailleur with 10sp chain, 9sp middle chainring with 10sp small chainring and middle chainring with thick composite shifting ramps.


Thanks to everybody that tried to help
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