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Originally Posted by M3takesNYC

I don't agree re: why bmw used a CF roof. They used one on the CSL e46 and the previous M6. Plus its more like 20 pounds vs a steel roof and 45 compared to a moonroof but location of weight is everything. For example replacing the seats to lighter weight have virtually no real performance effect from a handling perspective given they are at the center of rotation of motion and really have negligble effect.

That is incorrect. You are accounting only for mass that gets rotated as the car leans, but ignoring acceleration, braking and direction changes - in other words, inertia. Removing any weight from the car affects the performance. The question is how much weight, in relation to the weight of the car. A 20lbs roof difference is simply too small to yield measurable performance increases. Replacing the power seats with lightweight racing versions can reduce the weight by as much as 60lbs per seat, and that 120lbs becomes measurable, albeit still small.

The lighter roof is still 20-45 pounds lighter and every 40 pounds counts no doubt.

The entire roof sheet metal weighs less than 30lbs so the difference is perhaps in the neighborhood of 20lbs, not 40.

More important is it really is the highest aspect of the car and using the CF effectively lowers the car and although those small increases in performance may not be something obvious alone, when you take the 10-15 little things they did such as use plastic for front a rear bumpers/body work, aluminum pieces, aluminum brake hubs and floating rotors etc, all of that adds up to some real performance value.

First of all, the other small changes are applied to both the cars with CF roof and the ones without. Second of all, there is more weight difference between an option loaded car vs. a bare model than between a CF cr and a steel roof car.

What you are saying sounds entirely reasonable and by the book. However it is countered by the fact that there is absolutely no measurable performance difference between the cars with CF roof, and the ones without. Absolutely none.

In 2001 when the CF was being tested on the old M6, they were not even thinking about the I3/8 series and the e90 m3 was simply already going to take the CF given the M6 had it so it was a decision effectively made in 2001 with the M6.

When they did the CF roof on the CSL, it made absolutely perfect sense in that type of car, because they went absolutely everywhere and took as much weight off as they possibly could, baring nothing. That car was some 400lbs lighter than the regular M3, which is very significant (12% weight savings). They literally left no stone unturned.

But 20lbs? that's just marketing, and good marketing at that because obviously you are buying it.

In 2001 BMW may have not known specifically about the i8 or i3, but they knew that in the future cars would become increasingly heavier due to the constant addition of comfort, convenience and safety features. The trend was as unmistakeable then as it is now. So they took a page out of the racing book, decided CF has definite advantages over steel, and went ahead to experiment and see how the two can be combined. And now they are almost to the point where they can make it commercially viable to make almost the whole body out of it, which was completely unthinkable in 2001.

Plus it looks great which is key!

There is no doubt about that. BMW apparently prices the CF about the same as a moonroof, but had it been an option in the sedan, I don't think I would have paid for it.

I can guarantee you that if you drive a CF roofed model vs. the steel model back to back, you will not feel absolutely any difference, and your lap times will be more influenced by a headwind than the CF weight savings.