Bit of a background. In early 2008 or I got a Nuvinci CVT hub and was convinced I’d be able to hack this to work bidirectionally in a unicycle application with a jackjaft. Unlike the conventional internal hubs, the friction drive against the spherical rollers should fundamentally be able to alternate between forwards and reverse torques with no slop at all.
Then you’d have the dream setup, a 300+% gear ratio with no sudden shifting.
Here’s what the whole thing looks like taken apart on the bench.
The hub casing is on the left, in the center of which is the pedal drive sprocket that connects to the input ring, which then spins the roller balls, in turn rotating the output ring which is directly connected to the hub. The shifting mechanism tilts the axis of the roller balls.
What I didn’t realize is that the compressive force that holds the rings against the balls is generated from the input torque of pedaling, I has assumed it was just pre-loaded. So as you pedal, that causes the flat cylinders here to roll up the ramp and squeeze the parts together:
This is clever since it means that you don’t have excessive friction loss with light pedal effort, there is only ever just as much friction as needed to prevent slippage. But it’s designed to work only in the forwards direction. To work both ways, the ramp plate would have to be machined so that it slopes equally on either side of the cylinder, but there was no way I could machine that on the precision ground hardened steel rings.
What I tried to do is instead is a spacer shim that would enable a certain amount of preload when the parts were all put back together. That kinda worked but not great.
It turns out that Nuvinci actually put some development effort into a bidirectional hub model for electric vehicle and scooters, where regen braking and reverse are desireable, though I got the impression it’s not currently on their program.
I pleaded with some of their engineers at the interbike trade show to get back on this as a whole unicycle community is craving it! (Apparently they get a tech inquiry about twice a month from someone wanting to use their CVT in a uni application.)