Joined: Feb 2012
Posts: 19,320
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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
I don't think you will find any chart or published data as to how long a pad a rim size can handle. Some pads have a curve built into them, usually about the same curve that a 700c rim side has. Some pads can be filed/ground on their edges to relieve tire contact potential.
I know of nearly no one who ever talked about the aspect of the disk (a rim is a disk after all, just with a second job of holding the tire too) being so much smaller with a 20" wheel, compared to a 26" or larger wheel. The leverage, when one moves from a 26" to a 20", is about 75% of what the 26" wheel would produce.
BTW the leverage VS disk diameter thing reverses when a hub mounted disk is used. The 20" wheel with a disk (hub mounted) brake has about 130% more leverage on the tire then a 26" wheel would have. Andy
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AndrewRStewart