And you quoted it out of context. It's in context to the weight in cars, not the molecular density of the metal. Seemed pretty clear to me, guess not...

Tensile strength of AHSS is around 500-800 MPa. Aluminum (typically 6061, 6111 alloys, but also 5XXXX alloys) is around 200-325 MPa. That's what evens out the BIW weight somewhat, and why the weight differential isn't as huge as you'd expect.

First off, you said that your AHSS steel is 15% heavier than aluminum. It isn't. It's about 188% heavier.

Now lets talk about component weight.

Let's use simple round bars 12" long. Using your strengths. One in 800 MPa (116000 psi) AHSS vs 325 MPa (47000 psi) aluminum.

The steel is 2.47 times stronger than the aluminum. So the cross-sectional area of the aluminum needs to be 2.47 times that of the steel.

Using a 1" square cross section area, the steel has a diameter of 1.128"
Using a 2.47" square cross section area, the aluminum has a diameter of 1.773"

This gives you the larger bulk that you are talking correctly about.

Now the steel's 1" area x 12" gives 12 cu inches.
The aluminum's 2.47" area x 12" gives 29.64 cu inches.

Divide these by 1728 to convert to cubic feet.
Steel: .00694 cu ft
Alum: .01715 cu ft

Now multiply by the densities for component weight:
Steel: .00694 x 490 lbs = 3.4006 lbs
Alum: .01715 x 170 lbs = 2.9155 lbs

And this gives you the steel component's weight penalty of 16.6%. close enough to your 15% statement.

See the difference, Aki?

We've gone at it before about your lack of clarity. You said the steel itself is 15% heavier, when it is the component. ^This is how it works.

The other issue, is that steel is almost never used in its maximum strength condition, due to its lack of toughness. Aluminum is quite often used in its maximum strength condition if it is heat-treated.

You could've just used 2.88/2.47 :bonk: