It does spin true, but no, the spin times are not that great. The aluminum disk, with all the cut-outs, make the derailleur pulley very light...not adding a lot of momentum. I could wrap a piece of bike chain around the disk, but it would probably destroy the balance. More experimentation is needed.
Beautiful piece, Ed!
Tops with perforated rotors like this one can produce wonderful visual effects, both at rest and at speed, and especially under pulsed lighting. The spoke-and-ring structure of your derailleur pulley falls into this category. In my mind, rotors consisting only of spokes also qualify. I'd expect this rotor of yours to generate spectacular patterns under a high-speed strobe and interesting video frame-rate effects to boot.
Most of the tops in the videos below have perforated rotors of one kind or another. (No need to watch either one past 2:00.)
Unfortunately, some are lucky to stay up 15 sec with a hard twirl, and their rotor holes are largely to blame.
Rotor holes appear to reduce spin time in at least 3 different ways:
(i) Holes greatly increase the braking torque due to aerodynamic drag at all angular speeds, but especially right after release, when speeds are highest. This is a big deal because drag accounts for most of the deceleration during spin-down on a low-friction surface like a smooth counter.
(ii) Compared to a solid rotor of the same material, the absolute axial moment of inertia (AMI) lost to the holes reduces the top's ability to resist the total braking torque due to both drag and tip friction. (The bigger the hole, and especially the more peripheral the hole, the greater the loss of AMI.) The end result is a greater deceleration rate during spin-down at all speeds.
(iii) The associated loss of AMI per unit top mass increases the "critical speed" at which sleeping or steady precession becomes unstable and significant nutation sets in. (The top falls shortly thereafter.)
Each effect can cut deeply into spin time on its own. In combination, the loss of spin time can be quite severe.
