The difference between cassette hubs and freewheel hubs
This article will help determine whether your bike has a cassette or freewheel system.
The rear cogs are attached to the hub in one of two ways. Newer bikes tend to use type hub called a “cassette hub.” The cassette hub uses a “freehub” sytem, which is a type of clutch mounted to the body of the hub. This cylindrical mechanism ratchets counter-clockwise for coasting, and locks clockwise for driving the bike when pedaled. The freehub body has a series of splines on the outer shell. “Cassette” sprockets slide over these splines. A lockring threads into the freehub and holds the sprockets, or cogs, in place. When the cogs are removed, the ratcheting freehub remains on the hub body. Most modern bicycles use the freehub system. See a typical cassette hub below.
Older bikes may have a large external thread machined into the hub. The cogs and ratcheting body assembly, called a “freewheel,” threads onto the hub. The ratcheting mechanism comes off with the cogs when the freewheel unthreads for removal.
To determine if a sprocket is a freewheel or cassette system, remove the rear wheel from the bike. Find the tool fitting on the sprocket set. Spin the sprockets backwards. If the fittings spin with the cogs, it is a cassette system with a freehub. If the tool fittings do not spin with the cogs, it is a threaded freewheel system.