Kinda depends on the application IMO. If you want a lighter rotating assembly for high revving, AL is light. AL rods also absorb shock and are more plastic; they stretch about 0.010" more than a steel rod. They're machined much thicker than steel rods, so clearance might be an issue. They do have a slightly lower expected life than steel. For a street car with lots of power I'd go for forged steel. Race cars get torn down a lot more and inspected, unlike a street car which should go at least 100k without a tear down. If you're looking at GRP rods, take a look at this article: http://www.dragzine.com/tech-stories/engine/debunking-aluminum-rod-myths-with-grp/
The alloys are light, ~ 9 lbs. The steel is far better, and less of a weight penalty than you'd think. A street driven car has to survive weather, pot-holes, delays in maintenance. Next one is going to be steel.
I had a accident on the way to Goodguys several years ago and bent a Super Bell tube axle and was forced to buy a new axle while at the show,the only thing anyone had was a aluminum Beam axle,they were the "New Thing" at that time,super light maybe 17 lbs and it was on the roadster when I sold it but after about few months a bunch of us were standing around in front of my roadster one of the guys just kicked at the tire and it moved slightly. Seeing this I asked him to kick it harder and it shook more,after that I would be ridding down the road at lean out and look at the tire and watch it shake,from then on I was uncomfortable with the axle although I never experienced any other problems with it but I will never use one again. If you were to build a all show no go Aluminum is a pretty piece but for a real world drive I would advise against it. FWIW, I will only use forged Beam axles from now on . HRP
Your front axle is a poor choice of places to reduce the unsprung weight. Henry Ford never had an engineers' meeting to discuss compromise on the Vanadium steel axles he used in his cars. Fact is, he demonstrated the strength (and resilience) of his axles at the Chicago World's Fair, at the Ford exhibit...Some Ford axles were twisted in a fixture, 10 turns. Then twisted the opposite way. No cracks, There is one of those axles in the showroom of Bill Perry's Ford store in Campbell. Why anyone would settle for a cast iron axle is beyond logic. Aluminum? Beyond X3. I was also at Goodguys in Pleasanton the year of the aluminum axle. There was a narrow one, about 40"...Ed Moss was talking to someone about it, I thought it was a 'display' model for the new Magnum I-beams. Ed said "No, this one's for a digger...but I understand there'll be some 47" coming out soon..." Ed wasn't the MFR., don't know who was. Same show, there was a pair of Faux Flathead heads for a Chevy...I was convinced that someone had a Voracious sense of humor.
Nothing beats an Anson dropped original but if that's not available I've been selling So-Cal forged steel axles. I'm not a big fan of aluminum axles.
Trick question here mate! Depends on whether the I beam is bolted to a crank journal or has king-pins?
"Ford axles were twisted in a fixture, 10 turns. Then twisted the opposite way. No cracks" Holy moses. That's impressive for 1920's steel tech. I thought we were talking connecting rods for some reason. I haven't seen AL I-beam axles. If they're the same dimensions as the forged steel ones, they're not as strong. If you're going to make an aluminum part it may be lighter than steel, but it needs to be beefier to make up for the difference in fatigue strength.
From my personal experience, go with forged steel. I bought one of the first aluminum i-beams for my Deuce many years ago. The car is finally finished and we noticed it had pronounced negative camber. I called the manufacturer, and they admitted they had a problem with the early units, and the weight of the Hemi didn't help. They offered to offset grind the kingpin holes to get the camber back to where it should be, but on second thought I knew I wanted the security of forged steel. I don't want an axle that flexes going down the road. As with most manufacturers, it's hard to improve on original Henry steel. I've wicked wall hanging though.
As no doubt many will chime in on you can still get an original Ford axle and have it dropped for around the same total cost as buying an aftermarket axle. I snagged a SoCal Axle off Ebay a few years for a right price that had been someone's display model and while a bit shop worn from being handled I saved a big chunk of money on it. Great looking and beefy axle. Some may say it isn't quite as pretty as a Ford axle but I have less in it than having the Model A axle out in the shed dropped would cost. The car will have fenders so you will have to get down on your knees to see it anyhow.
I understand some over the road semi tractors have aluminum axles. I know they don't compromise on safety.
You ever notice how big and heavy the trucks are compared to hot rods? The truck axles for sure are hi-quality forged and heat treated and tested.
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