Under the title "foam filling the chassis" by Mike Kojima used in Nissan 300zx,  it has been mentioned that by using structural foam with density 2 lb/ft3 will increase the chassis stiffness upto 40 percent.


For calculating the torsional stiffness of foam filled chassis frame, i used the formula, k=CJ/L
Where, k- torsional stiffness Nm
C-Shear modulus N/m^2
J-Polar moment of Inertia m^4
L-length of the frame m

I calculated the torsional stiffness for chassis frame (steel) & foam individually and added up the two values. Whether we have to just add up torsional stiffness of both frame & foam individually or some different formulae we have to follow.
Properties of foam I used:
Shear modulus C=230x10^5 N/m^2
Density=32 kg/mm^3

by calculating this way, I found there is not noticeable increase in torsional stiffness.

Hence I would like to know the mechanical properties of Structural polyurethane foam which has been used in the Nissan 300zx.

Please guide me in this regard