Freehubs Explained: What are HG,XDR and Micro Spline?

Micro Spline, Shimano HG, Sram XDR, XD, Campagnolo – there are nearly more drivetrain and freehub possibilities than can be counted on one hand. As drivetrain options continue to evolve and expand, these different configurations only become more confusing.

In navigating the myriad of options, it’s important to identify what the right option is for your next BOOSTOR wheelset.

First, it’s best to describe what a freehub is, and its function. Essentially, the freehub is the part of the rear wheel, and specifically the hub, that holds the group of rear cogs, or cassette as it’s commonly referred. The freehub contains the drive mechanism that allows coasting or backpedaling on the drivetrain. The cassette is attached to the freehub by means of a threaded lockring. Special tools are required to remove or attach the cassette to the freehub.

Depending on which brand your drivetrain componentry is, the freehub body style will drastically change. While there is some overlap and cross-compatibility amongst drivetrains, it is best practice to completely match drivetrain componentry. Also, should you swap drivetrains throughout the course of owning a bike, most rear hubs, including BOOSTOR, allow for a complete freehub swap without the use of tools or re-dishing of the wheel.

Below is an explanation of each of the modern freehub body options available on BOOSTOR wheels.

• HG (Hyper-Glide)
  HG, short for ‘HyperGlide’, is the most common and traditional freehub body design available. Created by Shimano, it is the standard used for cassettes that have at least an 11-tooth cog as the smallest cog on the cassette. HG Specific cassettes are available in 8,9,10, and 11speed options. Most drivetrain manufacturers have cassettes designed for the HG format, including Shimano and SRAM.
• Shimano Micro Spline
  Shimano recently launched a new freehub body design to allow for a wider range 12 speed cassette with a 10-tooth cog. The new freehub body system is called Micro Spline, and is available with 2020 Shimano SLX, XT, and XTR drivetrains. This unique freehub is only compatible with Shimano produced SLX, XT, and XTR/Micro Spline specific cassettes.
• SRAM XDR
  XDR (“R” is for Road) is the road version of the XD freehub body. It looks similar in appearance to the XD freehub body, but is machined 1.85mm deeper where the cassette threads onto the freehub. This is in an effort to match the flange spacing for road hubs, so a re-dish is not required. Similar to the XD design, XDR allows for less than a 11-tooth cog, for a wider gear range on the cassette. While XDR is road specific, a 1.85mm spacer (Supplied by SRAM) can be used on the XDR freehub body to accommodate XD cassettes. XDR driver bodies will fit 11 and 12 speed drivetrains with XDR specific cassettes.
• SRAM XD
  XD is a style of freehub body that was created by SRAM for a larger range of gears, and allows for less than a 11-tooth cog on the cassette. XD is most commonly used for mountain bike applications, but can be applied for gravel, cyclocross, and road bikes as well. XD driver bodies will fit 11 and 12 speed drivetrains with XD specific cassettes.
• Campagnolo or ‘Campy’
  The Italian’s answer to freehub design is one of the more radical in appearance. Similar to HG – it utilizes splines to hold cassettes that have at least an 11-tooth cog as the smallest gear on the cassette. Campy freehub bodies are sometimes referred to as “CA”, and will fit 8, 9, 10, 11, and 12 speed Campy/CA specific cassettes.

Overall, it’s easy to see how having this many options on your wheels can become confusing. It’s important, however, to match your cassette and drivetrain to your freehub body on the rear hub and wheel. Also, checking with the drivetrain or cassette manufacturer is the best confirmation of which freehub to pick for your next BOOSTOR wheelset.