I used to say brass, but everything I've seen since using fk has flipped that on its ear. To be able to use an aluminum machine that seems to vibrate no more or less than my steel one is pretty amazing. I don't know if its the coils or geometry or whatever. Between iron and steel though, if that's what you're set on, I think iron being heavier and softer than steel would soak up the vibration better. All I know for sure is that the more I think I know about machines, the more I have to figure out. A year ago I thought ANY aluminum machine sucked, bottom line, now I love them. Well, fallen kings anyways.