[Kevin Sevcik]

I haven't run any numbers on this yet, but I think it's entirely possible you'll want a harder steel, rather than a tougher steel. Here's the difference between the two:

Tough = more energy required to ultimately fail the material. For identical tensile specimens, a tougher steel will require more total energy to ultimately break the specimen. Typically, this is because it stretches and deforms a LOT, so you exert a reasonable force over a long distance. Tough steel is good for things like car crumple zones, buildings, and other things where you'd rather the member yield and deform while absorbing energy than suddenly break.

Hard = more force required to ultimately fail the material. This typically implies less toughness, because while the material requires more force to break it, it doesn't stretch near as much. Think of the difference between breaking off a pice of cold, hard taffy versus warm stretchy taffy. High hardness is good in situations where any yielding/bending will be just as catastrophic as a clean fracture. Typically, this is in shafts and suchlike. Additional benefits to high hardness are a higher fatigue limit. This means that the steel will better resist repeated low level stresses on it. Such as from bumpy pavement at 60mph, or a slightly misaligned rotating shaft.

So.... Long story short, I think you need to decide if you'd rather have an axle bend somewhat while you're going downhill at 60mph, or break off completely. Also, if you have or can find pictures of the remains of a board that HAS failed, it can be remarkably useful in determining what it was that caused the axles to fail.