Wheelbuilder.com has just commissioned this made-in-the-USA titanium XD Driver Body to allow XX1 / X01 cassettes to run on newer Powertap mountain bike hubs.
They’re designed around the 15mm axle hubs, not the older 12mm versions, but will work with Wheelbuilder.com’s 142×12 adapter kit also. Weight is just 79g (claimed), which is substantially lighter than the 130g Industry Nine part we weighed (although that one did include the large outer bearing, too) and the 133g steel prototype Wheelbuolder made when first developing the part.
The piece comes with instructions and a replacement outer seal ring for the end cap for $175. It works with your existing end caps and should be a quick swap by just pulling the old freehub body off, swapping the pawls and popping the new one back in.
Wheelbuilder works closely with Saris, helping develop the mountain bike version of the Powertap hub. They mentioned that the key challenge in development was decoupling the braking forces from the pedaling forces, which raises and interesting point about the development of disc brake power meter hubs as they spread into road bikes. Stay tuned, the guys at WB say they’re already working with Saris on some next gen stuff!