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The flywheel hub was made from a chunk of an aluminum grounding rod of some sort found by a dumpster diving friend. The material is very odd, much lighter than you would expect for the size, and the surface has a kind of powdery grainy look. It almost looks like it has been anodized or sandblasted or something. I'm nor sure it's actually aluminum, but I don't know what else it would be. The rods are a little over 1" in diameter and about 22" long. There is a 1/8" dia steel core down the center, which I presume was to provide a hard tip for pounding into the ground. There was also a copper connector on the top of each, which my friend cut off to keep for his copper scrap pile. You can sort of see the grainy surface finish in the photo. Compare that to the support posts to the right, which are 6061 aluminum.