There is some confusion at work here and I will attempt to clarify it.
There was some conversation about shooting structural foam into the sills.
Some people were worried about the corrosion potential. I inquired whether
the increase in stiffness, which is expressed as pounds of force per inch of
torsional deflection from normal was enough to offset the concerns. The
increased stiffness achieved with foam was stated as 300 lbs/inch.
I then inquired whether this was a big increase since I didn’t know the
stiffness of the standard car. Tom Owen amazed me by saying that it was
around 1100 lbs, again PER INCH OF DEFLECTION, foaming thus making a 20+ %
improvement in T stiffness.
I then inquired if the 1100 lb figure was for a coupe or a roadster, and Tom
confirmed that it was for a roadster, and that the coupe is about 200
lbs/inch of deflection stiffer, or approximately 1300 lbs/inch of
deflection. Thus if you add foam to the sills of a roadster, the torsional
stiffness should improve from 1100 lbs/inch to 1400 lbs/inch. For a coupe
which weighs about 50 lbs more than a roadster as Tony has confirmed below,
the torsional stiffness goes from 1300 lbs/inch to 1600 lbs/inch which is
pretty good.
I doubt that there is any difference in torsional stiffness between a series
one and a series two. I also doubt that a series 2 is 50% heavier than a
series one.
I’ll bet a series three or a 2 + 2 is less stiff than either a series one or
a series two.