Re: question (archive)

does a .44 Cd contribute to high wind noise at 100+mph speeds, as opposed to low .30's? or is noise determined by something else?
this is not rhetorical, i would like to know

I honestly don't know the answer for sure, but if I may speculate... Both of these are symptoms/measures of how the car deals with fluid flow over its exterior. Cd is the simpler of the two to understand. It is a measure of overall drag which is a function of all parts of the car's skin and underbelly design. On the other hand, most of the noise that you hear (other than engine sound and tires and conducted exhaust) is wind noise around the windscreen and roof. These elements contribute to the overall Cd so obviously the noise and Cd are loosely related but I doubt that the windscreen and roof design dominate the Cd. So, I would not consider the noise level a particularly good indicator of Cd level. I would bet that the contributors to Cd are widely distributed around and under the vehicle. For example, the headlight covers contribute a bit, the door handles contribute a bit, the overall height and width contribute, the wheel-well design contributes, the smoothness of the underbelly contributes, how air flows off the rear end contributes and so on. Because it is possible to reduce noise without any affect on Cd using insulation, for example, it is probably not useful to consider these two things similar.

I would guess that the difference in Cd with the ragtop vs hardtop will be small, but the difference in noise will be large. I expect that if the ragtop has excess noise, this is a symptom of complex turbulence above and beside the top caused by its shape and rigidity. There could be some resonances in this turbulence in the audible range that makes it worse. The hardtop has a smoother shape, and is considerably more rigid so the nature of the turbulence will be quite different. And the amount of noise transferred through the top will be quite different also.

The Modena 360, with its Cd in the 0.3 range is excellent because of attention to fluid flow thoughout the car, not just the roof and windows. I would be interested to know how much more wind noise comes into the spider than the hardtop. If the difference is small, this would indicate a less turbulent flow of air over the roof which would contribute a bit of improvement in Cd vs the Z8.

But then again, I am only guessing.

Harvey2

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