Spoked to Death
Join Date: Jul 2004
Location: Boulder, CO
Bikes: Salsa La Cruz w/ Alfine 8, Specialized Fuse Pro 27.5+, Surly 1x1
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You can usually disassemble a 9 speed cassette, and use 8 of the 9, with the 9 speed spacers, on a 7 speed body. Sheldon Brown calls it 8 of 9 on 7.
Then you use a 9 speed shifter, and then the limit screws lock out the last shift position.
But that is a lot of trouble, for not a lot of benefit. If you already had all the parts in your junk box, I'd say go for it, but don't bother with that kind of mess if you are going to have to purchase all the parts new. Just pay a few more dollars and get an actual 8/9 speed hub.
On Shimano, its easy, you can get 7 speed hubs, and then swap out the freehub bodies, which are the same for a lot of models. I recently put the freehub body from an 8 speed Sora onto a 7 speed 105 hub, and I'm running it with a 9 speed group. But then, I had everything already, and only needed to buy a Sora hub to cannabalize the freehub (12$). Then, less than a week later, I found a good 8 speed freehub body in a friend's junk box. Doh.