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Discussion Starter · #1 ·
Hello

I have a problem with my front mech when downshifting from the 50 to the 34 tooth ring. It de-rails from the chainset about 25% of the time.

Basically the chain goes too far inwards and misses the 34 ring de-railing it completely.



Bike setup:

Chainset: Extralite Extralite E-Bones WRC 50/34
Chainline is 44mm on a BSA68 bottom bracket shell

Front mech: Shimano Dura Ace FD-7700-F

Shifter: Dura Ace ST-7700 for double chainset - 9 speed

Chain: SRAM PC-89R Hollow Pin



Does anyone know how I can fix this?



Images:





 

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Where are you based? If you're in the UK PM your phone no & I'll talk you through it. I have the same set up and haven't have any issues with it at all.
 

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This is not an unusual situation with compact chainrings. I have a bike with 50/36 and it did it occasionally ....... perhaps once every 50 shifts to the small ring. The limit screw adjustment isn't enough to solve it because it is caused by the chain having enough room during its fall to the small ring to move inward without any help from the derailleur, particularly in the larger cassette cogs.

I solved it by using a plastic chain keeper that attaches to the seat tube and physically prevents the chain from moving inside of the small ring. It is inexpensive, light and foolproof. Many others have found relief the same way.
 

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Adjustments

You want the lower limit screw set so that the chain "just" clears the inner cage when in the small chain ring and the biggest cog in the back. Some people even accept a little rubbing in that combination so as to get a grip on the chain drop problem. You also can play with the angle of the front derailleur cage - the range is from "out cage parallel to the chain rings" to "inner cage parallel with the chain rings." Set the FD cage so that it is as low as possible, just clearing the large chain ring during shifts. A chain catcher device may be appropriate for your setup. If your chain line is wrong, then you might look into ways to move the chain rings closer to the frame. Some people are willing to put some bends into the inner cage of the FD to manage this problem.

In some compact setups, chain drop is a fact of life, but 1 failure every 4 shifts is excessive.
 
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