Originally Posted by seely
No, the hub on a 9/8spd is wider, so it won't fit in a 7spd frame. With some elbow grease on a 7spd steel frame you can safely fit a 9/8spd but on aluminum I wouldn't risk it even if you could mash the wheel in there.
I don't get what your question is about singlespeeds... they have horizontal dropouts and no derailleur hanger so you can't run gears.... most are spaced for a 8/9spd width hub I believe...
Not trying to be a jerk, but WRONG! the Overall hub width is the same - 135mm. The frame will accept a 8/9 speed hub without any problems.
Your Freehub body on a 7 is shorter than an 8/9, but overall hub width is the same.
To try to NOT to confuse you, here's a bit of history. After 7 speed was developed, the desire was to go to 8-speed. The centerline to centerline spacing in the cogs for 7-speed and 8-speed are the same, thus requiring a wider freehub for 8-speed cassettes. What they did to keep the overall width constant they narrowed the hub flange to hub flange width. So, the industry started making 8-speed freehubs and hubs. You see, if you use a small spacer on an 8-speed freehub, then you can use a 7-speed cassette, but not the other way around.
Then, when the move was to go to 9-speed, no one in the industry wanted to go through that whole ordeal again, so then went through a completely NEW ordeal. The freehub width between 8 and 9 is constant, but the centerline to centerline of the cogs was more narrow. This required a more narrow chain to work, then a front derailleur with a more narrow cage, then the shifters had to be all new so they would pull the cable the proper length for 9-speed. The saga continues....
So, regarding upgrading. If going from 7-speed to 8-speed, you may or may not need a new freehub body, depending on it's width. Then you need new shifters, but your front derailleur will work and you'll need a new rear derailleur, your chain should be the same.
If going from 8 to 9, a new hub/freehub is NOT needed, but you will need a new cassette, new chain, new shifters and a new front derailleur. Not to mention new chainrings too. Now, when I upgraded to 9 speed, I used my 8 speed front derailleur and chainrings and it worked. It may not have worked AS WELL as a complete 9 speed system, but $ is $ and I could only upgrade bit by bit.
L8R