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Behold The Hyundai Uni Wheel. Transportation May Never Be The Same
(cleantechnica.com)
This is a most excellent place for technology news and articles.
Isn't the ring gear the wheel body (or whatever it's called), that is, even a fixed axle would have weight there, the gearteeth even provide stiffness. CV joints also contain unsprung mass, I'd say there might be a bit of a difference but nothing drastic. With modern fancy biomimetic wheel body geometries and everything you'll probably definitely be lighter than 80s steel rims. What happened to spoked wheels, anyway.
The CV only contributes half it's mass to unsprung weight, and this system still requires a drive shaft and either a CV or u-joint. But with more mass at the hub.
Typical hubs are still lighter than this, because this setup still requires the hub structure, it's just adding gearing out there.
Half of the hub is unsprung, the transition between sprung and unsprung is at those fancy articulating planetary gears. The drive shaft is definitely sprung, it and the motor is completely static relative to the battery and everything.