From a machinist on another forum. He lists alloys: I would recommend getting a piece of shafting steel of a known alloy. Truck axle steels vary- I use truck axle steel for stuff that my life does not depend upon. A motorcycle front axle is something where a failure will likely be sudden, catastrophic & final. Start with a piece of good alloy steel of known properties. 4140 or 4340 are good choices. There are also more specifically made steels like some of the "stressproof" steels. A motorcycle axle sees some interesting stresses- it as, as was noted, in shear where it enters each fork leg. It also has to hold the two fork legs in alignment with each other as well as transmitting the radial and thrust loads from the wheel bearings to the fork legs. Add loadings imposed by braking, road shocks.... and the axle is being asked to do a LOT. A steel with a high tensile strength oftentimes has a relatively high yield point strength. This translates out to the steel not having much of a "cushion" between yeild point (below which the steel is elastic) and failure. High strength is not all it may appear to be, but too low a strength steel will deform plastically under load. I would NOT recommend cold rolled steel, 1020, or anything like it for a motorcycle axle. An alloy steel with fairly high strength AND good impact strength is what is needed.
The other concern is the tolerances to which the axle is machined as well as "geometry" and surface finish. The axle has to hold the two fork legs in alignment as well as the wheel bearings. This means it must be turned and finished to a good close sliding fit int he fork legs as well as the bearing races (assuming Roller Bearings in a Hog front end). The diameters have to be turned to a good true cylinder- no taper, and must be dead-nuts-accurate for concentricity. Where diameters change along the axle, a smooth radius MUST be machined to transition from one diameter to the next. No "forming a radius" in steps by profiling/backing out the toolbit. This is a job for a well ground/stoned form tool. Overall surface finish HAS to be free from tool marks. If you can;t turn accurately with good surface finish and try to finish an axle by lathe-filing and lots of emery cloth, you are playing with fire. Similarly, when you go to thread the end of the axle for the nut, the thread has to be cut in the lathe, free from tool marks, tears, and must end at a nice transition to the axle without a stress riser. While you are machining parts for the front end, you may as well machine good axle spacer bushings from the same steel and make sure they are faced square.
I have seen a Hog front axle made in a Monarch lathe so that a fellow could make a "run" over a July 4th weekend. The amount of aftermarket garbage being sold to Hog owners is staggering and what we saw that time proved it. This fellow had, I believe, a Sportster with some aftermarket front end on it. He had some no-name aftermarket wheel and that's where the trouble began. Showed up at the machine shop on a Thursday before a July 4th weekend, stuff lashed onto the Sporty, wanting to know if we could take a look at his front wheel as the bike was wobbling. Well... the wheel turned out to have some light duty sealed ball bearings, as might be used in an electric motor. No heavy radial or thrust capability. Bearings were no-name bearings and were plainly shot. We miked up the wheel and axle and called the bearing distributor. They were able to match up bearings which would fit and had considerable thrust and radial load capacity. The bearings were put onto a Greyhound bus and sent up to where we were at. The front axle in this no-name front end/wheel was made from some garbage low carbon steel and had some visible deformation- the fork legs were "working" and this axle couldn't hold them. A new axle was machined from a piece of 4140 hot rolled bar stock. It was machined on an old War-Production Board Monarch engine lathe. The Monarch turned nice and true. The toolbits were ground to give nice radius's to transition from one diameter to the next. Surface finish with the toolbits was better than a 63 finish, looked like it was ground. Axle spacer bushing were also made from the 4140. The guy rode off that Friday night, and as far as we knew, rode without incident. I think the machinist who made the axle took perhaps an hour, including time to grind his toolbit and stone it to the radius needed. Aside from his paying for the bearings and shipping, the machinists figured the fellow was only young once, wished him well and took a promise of a case of beer for the job.
In short, a front end of a motorcycle is an indeterminate structure. The axle is asked to a LOT and has a LOT happening in it- as we engineers like to say. Your life, the life of whomever is packing on the back of the Hog and the lives of other people riding nearby (how often do you see Hogs one at a time)depend on that axle. When I work on motorycles, I treat the work as I would machine work on anything else- I use Starrett mikes, dial indicators and whatever it takes to do the job so nothing is left to guess. If I need to make a bearing driver or service tool, I take the time rather than "cobbing". You can't afford to cut a corner or leave something on motorycle work to chance.
Joe Michaels
