Spool math and demo

Been trying to understand spool math to drive my motors. Found this resource to be very helpful and managed to put together a simple demo.

Hopefully this will be helpful to others. Please let me know if this is similar to methods others have tried.


I love this hand-on approach to learning about spool speeds and distances. So cool. Thanks for posting!

very helpful, I think spool math is one of the key problems to resolve.

theoretically, just one motor should be sufficient to do the take up, given sufficient tension from the other reel, granting a sensor near or at the film gate.

i recently shot 16mm on a krasnogorsk, and what suprised me the most was that it was entirely mechanical: the motor was wound up by hand, and then the tension release through a weight mechanism.. I wouldn’t be surprised if whoever filmed the krasnogorsk disassembly is part of this forum.

Good calculations, but perhaps a bit too complex.

Here is a simpler one, which provides almost exact results (based on the example at the bottom).

If the inner and outer diameters are D0 and D1, then calculate the cross-sectional area of the roll as follows (where r1=D1/2 and r0=D0/2):
Pi * (r1^2 - r0^2)

Now simply divide this by the thickness h.

Using the numerical example on this page,
approximate formula roll length: 235.4623694 m
exact formula roll length: 235.6194545 m
this formula: 235.619449 m (which matches the exact formula to 7 digits).