What is an XDR Driver for the Rear Cassette?
Troy's Tech Talk
For the longest time bicycles used freewheels, which are sprockets mounted to an internal ratchet system with threads on the inside. This in turn was threaded to a hub. Then came the transition to the cassette and freehub. This is the style we’ve been using since and have seen some slight changes as more gears were added. These came in either a SRAM/Shimano- or Campagnolo-compatible design.
As 1x drivetrains have grown in popularity, SRAM created the XD driver. It is a type of freehub that was created to allow the rear cogs to become smaller since the previous design was limited due to its circumference. The smallest cog that could be used was an 11-tooth gear. The XDR is the road-specific spacing version of the XD driver body. Essentially, the cassettes no longer use separate cog pieces that slide onto the hub body, but instead are now back to being one piece that threads onto the shorter hub body (XDR driver).
Where it differs from the original freewheel is that the ratchet system is not part of the cog assembly. This shorter hub body and one-piece cassette design allow manufacturers to use 10-tooth cogs and even 9-tooth cogs, with the limitation now being the axle circumference and chain.
To put this into perspective and give you an idea of how much that one tooth on the cog changes the ratio, look at this comparison: a 53×11 has a gear ratio of 4.82, and at a 90 cadence would produce around 34 mph. To get that same ratio with a 10-tooth cog you would have to match it with a 48-tooth chainring. These numbers are important when you want to expand the gear range. That 48t could be matched with a 32t (like the Praxis Zayante M30) or even smaller inner chainring without sacrificing big jumps on the cassette. Think of the best of a standard with the ease and efficiency of compact and tight gear gaps.