20 spoke 700c rear wheels are near the lower limit of what's workable, but they're made and succeed in the market, otherwise there wouldn't be 20h 700c rims.
While a coaster brake has the potential to create high torque loads in the wheel, it isn't great compared to the torque produced in hard sprinting or hill climbing. Braking torque is limited by the tire's skid traction, which because of weight transfer is usually less then the maximum possible driving torque.
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Last edited by FBinNY; 05-25-12 at 07:05 PM.