I have worked some more on the spinning wheel tuning project - and it was frustrating. Sometimes, you sit down and try things and make a prototype and do the maths and then you go for the real thing and everything works beautifully. This, however, was not one of these times. About nothing in the current setup worked as planned.
The two whorls I 3D-printed are too slippery to give good traction to the drive band. (3D printing is still very cool.) So... I tried to make the drive band less slippery. I had had wonderful de-slipping success some years ago with using Seamgrip, a sort of glue originally intended to fix holes in outdoor stuff like tents. Unfortunately, the new tube of SG that I ordered made the thread I had planned to use stiff and slick, not supple with a rubbery outside.
So, after a while of fiddling and some more fiddling and lots of cursing, I have now switched back to the prototype setup from before, which was always intended to be temporary. For the current spinning tests that I have to do, this is what I will have to use - provided it will work well enough.
After that, I will have to sit down and plan again. I have not given up yet - but it looks as if there has to be some more fiddling and making of parts before my original plan can be implemented.