Axle. Valkiri did bend a 20mm silversteel axle in the first decent fight against a wel armored opponent, and that was a short shaft, 90mm, of wich 65mm unsupported.

Due shape and weight constriction, I replaced that with a 20mm surface hardened shaft, and that seems to hold well.

But for a drum of that construction, my gut says 25mm at minimum. And that in case hardened steel.