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Old 01-13-22, 07:44 AM
  #37  
Road Fan
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Join Date: Apr 2005
Location: Ann Arbor, MI
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Bikes: 1980 Masi, 1984 Mondonico, 1984 Trek 610, 1980 Woodrup Giro, 2005 Mondonico Futura Leggera ELOS, 1967 PX10E, 1971 Peugeot UO-8

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Originally Posted by seypat View Post
This right here. It's easy to figure out what size sprocket the RD will take. The upper jockey wheel will start hitting the sprocket when you've reached the limit. Now you have to fiddle with the chain length until you get something that works on both ends. Also of importance is the cage length of the FD. If it's too short, the chain will drag on the cage plates bolt while on the small ring.
True dat! The front derailleur also needs to match the chainset range, as well as being designed to work with the chosen gearset and chainring shifting aids to get the smooth reliable shifter response we paid for.

With an extreme front spread like 40/20, some experimenters like Jan Heine have designed short, stout cages to push the chain around and stay free of the chainstay (big chainstays, wide tires >42 mm and low, 8 cm BB drops) and achieve very low Q-factor for his randonneur bikes. So it's also a choice to go back to such 80-year old designs (Herse, Alex Singer, and other French constructeurs in the 1940s, plus innovators like Hirose), relying on modern chainrings and chains to assist shifting the wide range.

But this is pretty costly, being nearly 100% custom and shop-made.
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