Originally Posted by
RTDub
I have done some reading and know that 7 speed cassettes are a little narrower than 8 speed, for obvious reasons. My question is, are current 7 speed rear derailleurs capable of the range to support a span of 8 cogs? I'd like to go Tourney to keep it on the cheap, otherwise I'm into Sora.
Yes, a 7-speed rear derailleur is fully capable of shifting over an 8-speed (or 9 or 10 for that matter) cassette. You will have to adjust the limit screws to match the width of the new cassette but otherwise no problems.
I'm currently using a 7-speed STX and an older 7-speed RSX rear derailleurs on two different 8-speed bikes with no problems. BTW, the Sora would be a worthwhile upgrade over the Tourney, which is the lowest line rd Shimano makes. The cost difference can't be much.