The freehub body is one of the most complex technical marvels on your bike. It contains a tiny but strong mechanism to allow you to drive the wheel while pedalling, and freewheel while coasting. Your cassette attaches to the freehub body, and there are now a number of different standards for how it can attach.
A short history
Once upon a time, pretty much all bikes had the same type of freehub body - the venerable Shimano Hyperglide (HG). Some bikes with Campagnolo groupsets used the Campagnolo freehub body standard, but that was really the only outlier.
Then came 11-speed road cassettes, and there wasn't enough room to fit all those cogs onto an HG freehub. The 11-speed road HG freehub body was released, which was 1.85mm wider (with a corresponding shorter end cap).
The popularity of 1x drivetrains saw manufacturers pushing the limits of cassette sprocket size in both directions, in order to deliver a larger-range cassette. That meant 50-52 tooth large sprockets (which put an unprecedented amount of force onto the shallow splines of the HG freehub body), and the push for smaller sprockets at the high end (HG is limited to a minimum of 11 teeth).
Why is it important?
When you're replacing your cassette (either because it's worn, or because you want a different range) you need to match the new cassette to the freehub body that's on your rear hub - otherwise it won't fit on!
We often find that people are looking to increase the range of their drivetrain, to give them the ability to climb hills more easily. For example, many gravel bikes came with 11-speed SRAM systems with 10-42 or 11-42 cassettes. With the use of a Ratio Tech kit, you can upgrade this to a 12-speed system with a 10-52 or 11-50 cassette.
Hyperglide (HG): The enduring standard
The vast majority of bikes produced today still come with an HG freehub body.
The standard HG freehub body is 35mm, and will fit the vast majority of cassettes - including 11- and 12-speed MTB cassettes, which are able to offset the large cassette sprockets over the hub flange in order to make them fit on the narrower freehub body.
The HG road freehub body is 36.85mm, and is needed if you're fitting 11-speed road cassettes (although Shimano's road/gravel cassettes above 32t will fit on a regular 35mm freehub body).
When the cassette is installed, you can identify it by the large lockring at the end of the cassette. With the cassette off, you can see the shallow splines (one of which is narrower, so the cassette can only be fitted in the correct position).
A new standard from SRAM: XD
The XD standard was developed to solve the two problems of HG: to resist the wear on the shallow splines that can be caused by large cassette sprockets, and allow smaller than 11 teeth sprockets (you can fit a 10-tooth sprocket on an XD cassette).
The XDR freehub body is 1.85mm wider, for the same reasons as the HG and HG Road difference.
You can identify XD cassettes when they're installed on the bike by the fact that they don't have a lockring (the tool fitting is inside the end of the cassette). When the cassette is removed, you can see some short splines at the base of the freehub body, with some threads above those.
Shimano's answer: Microspline
In order to deliver the same benefits as XD, Shimano invented their own standard: Microspline. It uses many thinner splines, and a shorter body to allow a 10-tooth cog.
When installed on the bike, a Microspline cassette can be identified because the lockring is much smaller in diameter than an HG lockring.
Looking to upgrade your cassette?
If you need bigger gear range on your bike to help you get up hills, please get in touch - we can help!