Side Project Questions: What Steel?

I'm currently working on a side project to get my own Longboard trucks built. For those not familiar with skating, here's a basic rundown on what the device needs to do:

1. When a rider leans the board, the force turns the Hanger, which is suspended between Polyurethane Bushings to allow it to return to center easily.
2. Hangers have conventionally been made out of cast aluminum. These ones don't handle speeds past 60mph very well, and have a tendency to bend during slides.
3. Conventional axles are made of undersized non precision ground steel. The exact ally I don't know.


So here's what I'm hoping people can help me work out: What kind of steel should I go with for my axles? I'm guessing some sort of heat treating is going to be needed, but if that can be avoided it would save cost. A bit of math that I have yet to learn might help out here...

With my current Hanger design, the axle mounts to the hanger with a 5/16 nc thread, 3/4" in depth. Let's assume the hanger is invincible for the sake of simplicity. The axle extends out about 2.5 inches, with the load spread across the 2 inches closest to the hanger. We've got a 200lb rider (safety gear is heavy) going 60mph down a hill, and the rider attempts to do a 90 degree turn at that speed. We're dealing with some fairly large forces...

Anyone who really knows their steel and could give me some advice would be totally bribed with cookies if I ever met you irl.