How hard should a front axle be?

[Fear]

Not being a machinist, I'm not sure how hard an axle should be. A 3/4" axle necked-down from 7/8" at the fork cap to a 3/4" hub to maybe a 5/8" NC thread over a span of maybe 7 1/2" seems to leave a lot of great places to shear/fail. Would typical round-bar mild-steel from the local steel warehouse be able to be hardened enough? What does the factory use?

A quick Google search says pre-heat treated 4130 or 4140 over typical cold-rolled 1020. I'm just wondering about machining-down pre-treated would machine-off any heat-treat and I don't know how steel reacts to being heat-treated twice. seems non heat-treated, machine, then send out to heat-treat but I don't know how much that would warp it.

[Blackbetty]

We used CR 4140 for the steerer in my springer, the heat treated stuff is probably overkill and is really hard on tooling.

The front axle on my bike is CR 4140

[bobberadam]

Yup 4140 will be more than enough for ya! Thats what i make all my axles out of. never had a problem.

[hannibalsmith1]

For your application, any steel alloy including 1020 etc. will be more than sufficient. You have a moderate tension, and a moderate shear.
A 3/4" axle is beyond ample for this application. A solid axle affords some degree of additional reliability over a hollowed out example ( not by much, extreme fiber equation).

Long story short, any steel alloy including your "cold rolled" that you ask about is way beyond what is required to do the job for this application.

I have been making axles for high horse and GP bikes for years, and we have even made aluminum ones.......(don't try this at home!)

[spidr]

Derek made my new axles from stainless, thats how hes been doing it, I used 4340 on my last one I made because it was cheaper than stainless locally.