Wednesday, 19 November 2008 17:47
First off, the difference between cassettes and freewheels.
The Freewheel - a group of gears that has a ratcheting mechanism built in. A freewheel threads onto the rear hub. The only bikes you see using a freewheel now are old bikes or cheap new bikes usually using 7 or 8 gears in back. Shop for freewheels here.
The Cassette - a group of cogs. Cassettes mount onto a splined freehub body, the freehub body has a ratcheting mechanism built into it. Bicycles mainly use cassettes nowadays. In the picture to the right, the black part on the hub is the 'freehub body'. The freehub body is typically replaceable or rebuildable if the ratcheting mechanism fails. Shop for cassettes here.
The differences between Shimano compatible 7,8,9, and 10 speed cassettes.
The following refers to Shimano compatible cassettes, not Campagnolo. A 7 and 8 speed cassette have the same amount of space between each cog. An 8 speed cassette is slightly wider than a 7 speed cassette because of that added gear. When 8 speed came out they had to make the freehub wider in order to fit that extra cog. The old time mountain bikers may remember when they wanted to upgrade their 7 speed drivetrain to 8 speed they would have to purchase a new rear wheel to fit the wider cassette on.
9 speed cassettes added a cog to the 8 speed cassette while maintaining the same overall width. The gap was reduced between the gears which required a narrower chain. The same thing happened with 10 speed. It still uses the same overall width of an 8 speed cassette - in fact 10 is ever so slightly narrower.
The important thing to note is that if you have an old road bike with 8 gears on back, you can go to 9 or 10 speed without having to change the rear wheel - the other way around works, too.
Simply put - 8, 9, 10 speed cassettes all fit on the same hub. A seven speed cassette will fit on an 8 speed freehub with the use of a spacer. (One notable exception is the Dura Ace FH-7801 hub with an alloy freehub which will only accept 10 speed Shimano cassettes - the newer Dura Ace hubs can run 8/9/10).
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