[Jonjt]

Originally Posted by Evan135

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You don't understand the characteristics of the aluminum and steel alloys that are available to the auto industry. Or, I should say, you don't understand why materials are chosen for specific purposes, and how those purposes influence the design process. And, you need to do a bit of research to understand exactly what stiffness and strength are, with respect to engineering materials. You're confusing them.

You can correctly design aluminum that is stiffer and lighter the high strength steel. If you could not, no one would use any aluminum as it typically would be more expensive.

The idea that an aluminum part will always be strong and lighter than a steel part is incorrect. Available steels are stronger, stiffer and have higher ultimate and rupture strengths, better fatigue properties, etc. Making the jump from the material properties to a well designed part is tenuous, because design criteria, resources, goals, etc, play a huge part in why certain materials are chosen. However, suffice to say that one cannot just assume that aluminum parts are always going to be stiffer and lighter.

The younds modulus for typical aluminum is 10 million psi and steels modulus is 30 million psi. The weight of aluminum is about .1 pound per cubic inch and steel is about .3 pounds per cubic inch. Hence they appear to be relatively equal from a strenth to weight evaluation.

No. You are talking about STIFFNESS per unit weight, not strength. In material property terms, there are many different "types" of strength, the definition of which depend on how you are defining material failure. Wikipedia has a good article on this.

But the reports are that BMW is using aluminum and as I said you can design an alumium structure liughter and stiffer than steel. It is all about the material and how the design uses it. The correct application.

Again, "can" does not mean "always". As you acknowledged, it depends on the design. Usually, in this regard, one characteristic is optimized, while the other is not. It's very rare that a designer can replace a steel part with an identical aluminum one that is lighter and stiffer. Usually, when that happens, it's a consequence of a poor previous design and/or improvements in design and manufacturing technology. To use a more equal comparison, if you were to design optimized parts, one aluminum and one steel, for the same application, at the same time, to the best of your engineering ability, the steel part will be STIFFER and STRONGER but the aluminum one will be LIGHTER.

Regardless of the execution on the materials BMW has been reported to be using aluminum to stiffen and reduce weight on the next M3/m4 subframes. The weight loss has not been reported so far to be from light weight carbon fiber plastic composites but the subframes.

You're making some serious assumptions about the particulars of the design of the subframes, without knowing anything about them except that BMW is using aluminum. Worse yet, you're comparing subframes from two different generations of BMW cars! There are so many ways to make a subframe for one car lighter and stiffer than a subframe from another, when the two cars are of a different design! The newer car could have a physically smaller subframe, it could be redesigned to be more rigid in torsion, bending, compression, tension, or any other complex loading condition, etc. Saying that the subframe is lighter AND stronger just because it's aluminum is myopic, at best and just downright wrong, at worst.

BMW's marketing department, maybe, or an insider has leaked this information just to pique interest. No one is going to release a full design brief on the subframes, unfortunately. And, that would be needed to fully understand HOW BMW is making the subframes lighter and stiffer. Saying it's just because they switched to aluminum, when aluminum is no where near as stiff, as strong or as fatigue resistant as steel, is just wrong.