It is not clear to me how the original was made. Some pictures seem to show one spoke flange is integral with the main body. And the other flange is a separate piece (probably welded, so not cast iron) .
Gotta think the original material was not particularly exotic, so if they are trouble free, then any steel of similar hardness would be fine as long as the geometry is faithfully duplicated in regards to fillet radiuses and thicknesses.
Are you trying to duplicate the "look and feel" of the original, or the 'as close as I can get" original or the "historically accurate" original, or merely trying to "make it safe" to ride?
The originals were made from a steel casting. Because of the age and rarity it is hard to find one let alone one that is in good condition. My goal is to make a close copy that'll be safe to ride and use modern bearings but that's for another post.
The spool consists of two pieces brazed together. The hub piece with one flange is a malleable iron casting. The other flange is a steel stamping. SAE 4130 would work, but is overkill for this application. SAE 1018 would be fine. Fl9018--why go to the trouble to make one? The various vendors of Indian repo parts have CNC hubs for sale. I have one myself.
The pre-'46 hubs are one piece. The '46 and later one are made of several pieces similar to a Harley. I don't think the vendors have the earlier version available. Plus I want to install a modern bearing rather than go with the loose rollers.
Thanks for all the input. So what I'm gathering from those who weighed in is that 4130 is overkill and 1018 or maybe a 1020 (A36 MOD) would be strong enough.
P.S. To Tmoose: Yes, I am trying to replicate one just like the front three in your photo. My apologies for not responding sooner.
And don't try to use a free machining steel like 1117. The inclusions in those grades run with the bar axis and would have a short transverse grain direction in your spoke holes which could initiated cracking.
