In engineering, a freewheel is defined as: a device in the transmission (or bicycle hub) that disengages the drive shaft from the driven shaft when the driven shaft rotates faster than the drive shaft.
For all of us average Joes, let's say it's a cyndrilical device placed in the hub of the wheel that uses angled teeth so that when the wheel goes faster than the drive train, a ratchet effect occurs to let the wheel spin freely. This is what lets us rest our lazy feet and coast down the hill, enjoying the sunset and feeling our hair wave in that ocean breeze.
There are generally two styles of freewheel:
NOTE: the bulge in the hub indicates it is a freehub at a first glance.
As you might have guessed, freehub bodies can be more expensive than standard freewheels. Most manufacturers have converted to the freehub design, and these days there is much diversity. Interchangeability can be an issue with the freehub, which raises prices by giving some manufacturers a more specific niche to extort higher prices. The old-school thread-on freewheel remains low price.
Freehubs can range from less than $20 to top notch ceramic bodies at over $200 on Amazon.com:
Traditional thread-on freewheels are usually $20, and no more than $50 for a sprocket cluster:
A freewheel threads onto the axle, the cog cluster is threaded over top of that, and finally the chain is attached. As the rider pedals, it constantly tightens the freewheel down on the threads. Removing the free wheel isn't as easy as spinning the cassette counter-clockwise (opposite to installation), because the ratchet mechanism will just allow the cassette to spin freely.
To remove the freewheel, a special "freewheel tool" is applied. It looks like a socket with splines on the exterior. When the socket is put over the freewheel, the splines engage the non-rotational part of the freewheel, and it can be threaded from the axle. There are a four spline styles of freewheel tools, so it's important to match the splines on the tool to the correct freewheel.
Removal can sometimes be difficult because the current freewheel has been tightened and tightened and tightened to no end by the constant pedaling of the rider. This is a main reason for the switch over to freehubs. More distance away from the rotational center gives you more torque on the socket. It will come off princess, roll up your sleeves.
To remove and install a freehub body is very simple. Remove the locknut holding on the cassette on with a socket. Grip the cassette with a chain whip or something of that nature.
With the cassette removed, the freehub body can be unscrewed with just a 10mm hex key.
To install, just thread on the new freehub body, replace the cassette and cinch it down with the locknut.
The freewheel hub norm has largely shifted to the freehub in modern cycles, mostly for purposes of simplicity. Main brands such as SRAM, Campagnolo, and Sturmey-Archer have lately adopted the Shimano design (with a few minor sizing differences), so now there is a lot more compatibility than when the freehub first hit the market. Cassette sizing varies widely from around 11 teeth to 28 teeth, but freewheel threading is standardized by the ISO (International Standardization Organization) at 1.375" x 24TPI, or metric 34.92 x 1.058mm.
NOTE: These are the same ISO dimensions as the bottom bracket.
The initial freehub dimensions were standardized by the manufacturer, Shimano. Cassettes of 7, 8, 9, 10, and 11 speeds have different widths, therefore different sized freehubs are needed to accommodate the extra cogs.
Generally these are the lengths of freehub splines (with a few differences between manufacturers):