Joined: Feb 2012
Posts: 19,290
Likes: 5,382
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
"What physically changes about the RD to allow a 28 tooth or 32 tooth cassette in the back? (aside from chain take-up considerations)" TrojanHorse
The distance from the axle (and thus the cogs' undersides') to the cage pivot is greater. The offset from the cage pivot to the guide pulley's axis. A smaller guide pulley (although this isn't what is done it will provide more cog/pulley gap). A greater angle to the slant of the parallelogram's movement, to better track the increasing diameters of the cogs. Placing the guide pulley further behind or more in front of the cogs. There may be more design details but these are what I think of. Some are done by most wider range ders, some not so.
One way to see this is to hold a wide range and a narrow range der of about the same era from the same brand in each hand and look at them. Andy.
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AndrewRStewart