Got a questions about metals?

[dabuilding]

steel for sure iron is a softer metal i do believe

[Roadster]

Yeah, if you can find out what kind of steel it is, that would help.

Ductile iron is just what the name says...ductile, flexible, not as brittle as standard cast iron. With hardness comes brittleness. Less hard = more flexible and more able to withstand shock. I'm not sure what the happy medium is for something like a drop spindle, as it carries a lot of weight and sees a lot of shock. Chances are you're going to want to go with the steel one either way as long as it's relatively mild, low carbon content.