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Old 02-27-16, 12:39 PM   #1
JBHoren 
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Minimum Chainring Tooth Difference?

In general, is there a minimum chainring tooth difference for C&V front derailleurs?

Specifically, can a Shimano Dura-Ace FD-7400 shift correctly between a 40T outer chainring and a 34T inner chainring? It would be paired with a Shimano Dura-Ace 7401, which will be shifting (friction) across a 6-speed freewheel (13-21).
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Old 02-27-16, 12:53 PM   #2
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Should work great. Probably need to shorten the chain if it's in the original length intended for a 52/40 chain ring combo.

Set it up with the new chain rings and see if the chain looks saggy on the small/small combo. If so, take out a link pair and check it again. Still saggy? Take out another link pair. Even if you rarely use the small/small combo, it should be useable. With such a small difference up front, this should be easy-peasy.
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Old 02-27-16, 12:57 PM   #3
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Should work great. Probably need to shorten the chain if it's in the original length intended for a 52/40 chain ring combo.
Thanks! It's a years-long build, which seems to be nearing completion; will sport a new 8-speed chain.
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Old 02-27-16, 01:00 PM   #4
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maximum, yes. minimum, no

a 40t big ring is smaller than most of our inner (small) rings. even with a 13t rear cog, that's not a very big gear.

velobase shows your rd has a maximum chain wrap of 28 and max rear cog size of 26:
VeloBase.com - Component: Shimano RD-7401, Dura-Ace (7sp)
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Old 02-27-16, 01:06 PM   #5
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Thanks! It's a years-long build, which seems to be nearing completion; will sport a new 8-speed chain.
New chains are nice (provided your freewheel/cassette cogs aren't all worn out). It'll probably come with 114 links and need to be shortened. And if you haven't got one already, you can get a cheap but functional chain breaker on the big auction site for $5, or spend a little more for a good one at your LBS.
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Old 02-27-16, 01:08 PM   #6
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maximum, yes. minimum, no

a 40t big ring is smaller than most of our inner (small) rings. even with a 13t rear cog, that's not a very big gear.

velobase shows your rd has a maximum chain wrap of 28 and max rear cog size of 26:
VeloBase.com - Component: Shimano RD-7401, Dura-Ace (7sp)
Thanks! I'd read the Velobase listing and knew that the numbers worked, but wanted to be sure about any issues with minimum tooth difference. Yeah, I know it's not a big-gear-inch setup, but it's what I can turn -- even on South Florida's flat terrain -- so I bow to the realities of old(er) age.
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Old 02-27-16, 01:13 PM   #7
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I show that there will be less than 1/2 inch difference in size between the two chainrings. The inner cage may strike the inner ring before the shift to the large ring can be completed. You may have to raise the derailleur higher than the usual amount to stop this.
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Old 02-27-16, 01:13 PM   #8
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The only problem will be a somewhat non-ideal shape for the lower edge of the front derailleur, which is almost always curved to match the 52-tooth default size of the outer ring. However, my Peugeot works great with 45-42 up front and a 13-26 "ultra" 6 speed in back.
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Old 02-27-16, 01:48 PM   #9
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The only problem will be a somewhat non-ideal shape for the lower edge of the front derailleur, which is almost always curved to match the 52-tooth default size of the outer ring. However, my Peugeot works great with 45-42 up front and a 13-26 "ultra" 6 speed in back.
Thank you! I appreciate the first-hand information... and the photo, too.
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Old 02-27-16, 01:50 PM   #10
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I show that there will be less than 1/2 inch difference in size between the two chainrings. The inner cage may strike the inner ring before the shift to the large ring can be completed. You may have to raise the derailleur higher than the usual amount to stop this.

Well, forewarned is forearmed.
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