On Sunday, June 2, 2019 at 11:40:05 AM UTC-5, AMuzi wrote:

I would expect a hypoid gear set in an oil bath to be
relatively efficient compared to a chain. Any ME comments on
an open crown & pinion set?

Back in my BMW motorcycling days it was widely accepted that the shaft drive was less efficient than a chain. Not enough to make a difference in routine or even "sport" riding, but there were few shaft-driven bikes in the races. (I saw John Long place 4th the "Superbike" class at Daytona behind 3 Suzukis)

I am not a mechanical engineer, but I *did* stay at a Holiday Inn Express a couple times. Nevertheless, it seems to me that the exposed nature of that "ceramic" drivetrain puts it at risk for all sorts of wear issues, not least of which the "cogs" are aluminum which wouldn't tolerate much abrasive road dust I'm thinking. Also, I would bet that a strong rider would be able to get that largest "cog" to bend by stomping on the pedal. It's a pretty thick piece, but the little bearings must have pretty good mechanical advantage, and the gear would slip and might even be permanently deformed.
Not very impressive for a theoretical 1% improvement in efficiency.