5. Dropout slot height. Make sure the flat on the hub is the same width as the dropout slot height. Or, buy approporiate washers to adapt. The axle on internal hubs has torque and must be secured; thus, the flats.

I ride an old Schwinn 3-speed & like it. Was a pig with an 18-tooth sprocket. Low wasn't low enough & medium was marginal for flat & no-wind. Now with the front 46 (is it 46?) and rear 22, it's about right. Low is low, medium is kinda low for flat & level & no wind, but high is usable with a slight downgrade or tailwind.

-Rich