Replacing twist grip shifters

[zenith1]

Hi All- my wife just had one of the shifter bodies break on her old Cannondale CAD1 and I was thinking about replacing it with a new set of trigger shifters(SRAM or Shimano). The bike has a 7 speed cassette and has Acera derailleurs. If I replace the shifters with a new set of say, nine speed shifters, then I also must replace the cassette and rear derailleur and possibly the chain is this correct? If so what combination should I look to?

[Joe_W]

You'll have to replace the rear hub, at least the body (and then the axle too and respace the rear triangle which really should only be done on a steel frame). Probably you'll also have to replace the crank set.