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Old 04-07-06, 11:39 AM   #1
BubbaDog
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Chain Shifting To Granny By Itself - Why?

After taking my new ride out last weekend, I noticed an unnerving quirk about the Ritchey WCS crankset wanting to shift by itself from the middle chainwheel to the granny. This happens when I'm on the third largest rear cassette sprocket or above, and the only way to prevent it is to index the FD so it basically holds the chain against the center sprocket on the crankset. If I index the FD so it's centered on the chain, the chain will drop to the granny in short order. I noticed that the WCS crankset has a number of teeth on the middle and large chainwheels 'shaved down' in height, only about half as high as a normal tooth. Is this allowing the chain to walk off the middle chainring when I shift to a sharp angled chainline from front to back? If so, I guess I need to either replace the whole crankset (to one with all full height teeth) or at least the 52/42 chainrings. Don't know if it makes any difference, but the cassette is a Shimano 105 12-27 and the chain is a Shimano Ultegra 10 speed chain. This stinks on a brand new ride, any help or insights are greatly appreciated....

B'Dog
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Last edited by BubbaDog; 04-07-06 at 07:05 PM. Reason: More Descriptive Title
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Old 04-07-06, 12:02 PM   #2
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Your problem is likely due a loose shift cable. Try turning the barrel adjustor 1/2 turn counter-clockwise. The teeth on the middle ring of a triple are shaped with ramps, bevels, and pins to assist in shifting from the granny gear to the middle. I suggest you leave it as is.
After a new bike has some miles on it typically you need to return it to the dealer for cable adjustments and wheel truing.

Al
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Old 04-07-06, 07:01 PM   #3
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Originally Posted by Al1943
Your problem is likely due a loose shift cable. Try turning the barrel adjustor 1/2 turn counter-clockwise. The teeth on the middle ring of a triple are shaped with ramps, bevels, and pins to assist in shifting from the granny gear to the middle. I suggest you leave it as is.
After a new bike has some miles on it typically you need to return it to the dealer for cable adjustments and wheel truing.

Al
I don't think that's it, I just finished building this bike last weekend and have only had it out once. The cables are all tight, but I'll check the barrel adjuster and see if a little tweaking will help....

B'Dog
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Old 04-07-06, 08:59 PM   #4
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Sounds like your front chainrings are too far out from the centreline of the bike. When the chain is on the middle sprocket, what cog at the back gives a straight chainline?
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Old 04-08-06, 02:42 AM   #5
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The shorter shaved teeth is to help pick up the chain on small to bigger chainring shifts. Nothing to do with the issues you're having.

Cable-friction is the problem you have. Shifting from smaller to bigger chainrings is fine since the force from the lever overcomes the friction and moves the derailleur into the correct position and shifts the chain. However, when going from big to smaller chainrings, you end up having to shift the lever more than necessary in order to get the derailleur to move. But over time with jarring and vibration, the derailleur's spring-tension eventually moves it into the final-position dictated by the shifters, which is in too far. This lazy time-delayed shifting in one direction is a sure sign of cable-friction or friction in the derailleur pivots themselves.

You can confirm this yourself by putting the derailleur & chain into the middle-ring. Then slowly move the lever to shift into the granny and you'll see that the derailleur doesn't move immediately. Or it moves in non-linear motions that's not directly correlated with the shifter's movements... but only from big into smaller chairings...
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Old 04-08-06, 04:32 AM   #6
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Originally Posted by AndrewP
Sounds like your front chainrings are too far out from the centreline of the bike. When the chain is on the middle sprocket, what cog at the back gives a straight chainline?
I get a straight chainline from the middle chainwheel to the fifth cog up from the smallest one....

B'Dog
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2003 Kona Humu Humu Nuka Nuka Apua'A Singlespeed - Gone, but not forgotten
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1990 Centurion Cavaletto 'Gaspipe Beater' Roadie
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Old 04-08-06, 04:34 AM   #7
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Quote:
Originally Posted by DannoXYZ
The shorter shaved teeth is to help pick up the chain on small to bigger chainring shifts. Nothing to do with the issues you're having.

Cable-friction is the problem you have. Shifting from smaller to bigger chainrings is fine since the force from the lever overcomes the friction and moves the derailleur into the correct position and shifts the chain. However, when going from big to smaller chainrings, you end up having to shift the lever more than necessary in order to get the derailleur to move. But over time with jarring and vibration, the derailleur's spring-tension eventually moves it into the final-position dictated by the shifters, which is in too far. This lazy time-delayed shifting in one direction is a sure sign of cable-friction or friction in the derailleur pivots themselves.

You can confirm this yourself by putting the derailleur & chain into the middle-ring. Then slowly move the lever to shift into the granny and you'll see that the derailleur doesn't move immediately. Or it moves in non-linear motions that's not directly correlated with the shifter's movements... but only from big into smaller chairings...
Danno,

I'll give it a look, thanks....

B'Dog
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2004 Salsa LaRaza aka "Mmmm, Blue Salsa" (853 steel is real, baby!)
2003 Kona Humu Humu Nuka Nuka Apua'A Singlespeed - Gone, but not forgotten
1998 Diamondback Outlook MTB
1990 Centurion Cavaletto 'Gaspipe Beater' Roadie
1978 Volkscycle 'Frankenbike' CroMo Tourer
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