[MetroMPG]

Originally Posted by AndrewJ

One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.

Well, one thing Phil Knox has said in terms of the lowest extension of an air dam is that it shouldn't go lower than the lowest hanging suspension/underbody bit, or else you're just adding frontal area at the expense of Cd.

That said, your tests so far, and Coyote X's, seem to suggest that more can in fact be better.

But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study

I'd be wary of relying on 1958 data. Cars back then would have had MUCH worse undersides (basically being body-on-frame truck like construction) than today.

Here's a couple more modern references:

• Ford's hydrogen fuel cell Bonneville racer was lowered and had a full undertray added. They claimed a 0.01 reduction in Cd with the pan

• Audi improved the Cd of the A4 with a patchwork of panels applied to the underbody. The sum difference was Cd -0.019