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I wouldn't think that aluminum connecting rods are optimum, where there are gazillions of cycles and relatively predictable stresses.

4340, which isn't particularly exotic/expensive, has much greater than 3X the fatigue strength of any aluminum alloy (the highest I could find was the 23 ksi for 7075-T651), so it still comes out lighter at 3X the density.

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Noah

It's a big world and optimum varies for different people and purposes. Rods are made from different steel and aluminum alloys, sintered metal and titanium. Obviously the common choice is steel. Pistons suffer high mechanical and thermal loads and the common choice is aluminum. Go figure!

People often under appreciate the tiny differences that can land up making a world of difference in an IC engine, and the tiny differences that matter depend on all sorts of things.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
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Every structure designed has fatigue figured in regardless of material, it is not exclusive to aluminum and nonsense to stand it out to need more consideration in design from others. It is exactly the same design process.



Triumph and BSA motorcycles were the 2 leading motorcycle brands in the world up till the 1970's and the bulk of them used aluminum rods (and pushrods), so regardless of what figures you offer up, some of the world's leading engineers disagreed with you.

Not sure if you guys are into dirt bikes but when Honda released their first aluminum framed motocross bikes in the late 90's, the panic and hysteria was hilarious. Amazing since they came from one of the largest and leading automotive engineering companies in the world. Even today when a KTM or Yamaha frame cracks "aluminum fatigue" is blamed totally ignoring the amount of steel frames that break.

The list of extreme stress aluminum in daily lives is extensive, MX bikes, planes, ocean going fishing boats, etc. yet the myths persist.

Agreed. I think some people have a bias against aluminum for the same reason that they have a bias against wood, or any other material. If you take three rods, one of aluminum, one of steel, and one of wood, obviously the steel will be stronger than the other two, but that is why you don't just take a design based on steel and change it to aluminum. Everything has to be designed to a specific material, using that material's properties (yield strength, stiffness, hardness, fatigue properties, etc). There are also so many different types of steel and aluminum alloys that you can't apply the properties of one to the others - the cheap aluminum forks and spoons you buy at the dollar store are not the same aluminum that's used in aircraft components.

Btw there are plenty of wooden aircraft flying these days, including in aerobatic displays.

There is a practical issue here with aluminum and fatigue. It's just something to be aware of. For parts that are presented with many load cycles, you must design to a lower stress then you would expect from working with steel. Here is a chart plotting stress and number of cycles. You can see that for the steel when the normal stress does not exceed about half it's yield, it is not required to derate for the number of load cycles. I would expect that the British engineers responsible for the those motorcycles payed attention to this.

It doesn't mean a dirt bike frame would see millions of load cycles. A connecting rod in a street vehicle would see millions and perhaps 100's of millions of cycles and it would be important to allow for that.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Not sure if you guys are into dirt bikes but when Honda released their first aluminum framed motocross bikes in the late 90's, the panic and hysteria was hilarious. Amazing since they came from one of the largest and leading automotive engineering companies in the world. Even today when a KTM or Yamaha frame cracks "aluminum fatigue" is blamed totally ignoring the amount of steel frames that break.

Actually the honda frame set the manufacturers on a path they never needed to go down. Not only were the first honda framed bikes terrible but they were not any significant amount lighter.Also KTM doesnt use aluminum in their dirt bike frames, only the swing arms.
I am only getting these facts straight. I agree that the "fatigue life" is a paranoia that runs rampant around here. I built a thermostat housing/waterneck for my project and you would of sworn I was killling puppies.
On the other hand several yamaha frames had serious cracking around the steering head. It probably has more to do with procedure than material and that can go wrong in steel or aluminum.

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The biggest thing in motocross.

Just because it hasn't broken yet doesn't mean that mechanical engineers are making things up. It's like walking out onto thin ice and saying, "See, so-called experts said that this was unsafe, and I'm just fine, so what do they know." Worse, saying that aluminum fatigue isn't a concern may lead a novice builder here to go ahead and build an aluminum chassis. Unless your sweeping statement can be backed up with numbers, it's very unwise advice.

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I couldnt imagine a novice builder on here having the welding skills to pull off an aluminuim framed car..but if the novice did manage to get it done then the guys welds would probaby break..but then again most of the welding on novice built cars looks like its going to break anyway...soooo the only real test is a profesionally built car welded by an expert ..heat treated and tested by a room full of guys in dust coats..somehow it isnt going to happen..unless your talking about ferarri or someone...but this doesnt appear to be a site with any of the above so i guess we will just build things that may or may not break.