Joined: Jun 2014
Posts: 4,628
Likes: 943
From: Ontario, Canada
Bikes: iele Latina, Miele Suprema, Miele Uno LS, Miele Miele Beta, MMTB, Bianchi Model Unknown, Fiori Venezia, Fiori Napoli, VeloSport Adamas AX
OP. You can use a 7-speed derailleur to shift a 9-speed cassette.
If you have a 7-speed cassette and want to use a 9-speed cassette on the SAME hub you can use iirc 8 of the 9 cogs. and leave off the smallest cog or the biggest cog depending on which gear you need more a high gear or a low gear.
BUT
To use 9-speed cogs you'll need both, 9-speed shifters and a 9-speed chain.
Cheers