Originally Posted by
blamester
And so as not to derail the o.p.s original question if he had an 11 speed group he wanted to use. He could just use 10 of 11 or 9 of 11 and choose the ratios he didn't want or need and keep the wheel as is.
Maybe the simplest option depending on why he wants to do it.
While I agree with this, it kinda defeats the point as his bike came with an 11s group but he wants to use a 9s wheel. He'd have to remove one cog to fit a 11s cassette onto the freehub, but then he'd have to adjust his limit screws to keep from shifting off the cassette. Having to re-adjust the derailleur every time you switch the wheel would be a bit of a PITA.
Depending on what wheel it is, I'd say a better option would be to get an 11s hub and rebuild the wheel. That's what I did with my Open Pro's. Had an Ultegra 6600 hub, rebuilt it with a 105 5800. Spoke hole diameter was the same, so I was able to re-use the spokes I had. That being said... If it's something like the Ultegra 6700 tubeless wheel, probably going to have to just replace the wheel.