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Old 07-16-13, 07:30 PM
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Location: New Rochelle, NY
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Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

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Front derailleurs (especially indexing front derailleurs) are actually more sensitive to chain width than RDs. The issue is chain clearance within the cage, which is the difference between cage inside width, and chain width.

Put a wide chain into a narrow cage and you'll be trimming like crazy to adjust for the incoming angle change when you change the rear. Or put a narrow chain in a wide cage and you'll have lots of lost travel with every shift, and if the difference is enough the index system simply won't move the cage enough.

The issue with RDs is clearance between the cage plates, and there's some forgiveness since the chain is kept centered by the pulleys. RDs with floating upper pulleys may experience some chain/cage rubbing during a shift, but if the underwidth isn't extreme, this is a rarity.

The other RD cage issue is entry angle at the bottom where a chain coming on an angle may rub on entry. This is easily fixed by filing inside clearance at the front bottom of the cage.
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