Old 12-26-16, 10:52 AM
  #5  
dabac
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You can't put the 9-speed cassette on the 7-speed rear wheel. You'd need another freehub body at the least. Unless you decide to run 8-of-9 on that 7-speed hub.
Going to a 42 biggest will be partially offset by the drop to a 11T smallest.
A 26" bike with a 42/11 will spin out at about 25 mph. On a 700C bike, it'll be about 28 mph. Unless you spend important time in that speed range, don't worry about it.
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