[bimmerFUD]

Originally Posted by needforspeed

I did read the link.
Air friction = velocity to the power of '2' (i.e. velocity squared)

Then in this case 2 is the 'exponent'.

Correct. And if instead v is the exponent, then then the drag would increase exponentially. The distinction makes an orders-of-magnitude difference.

Most people would be satisfied that something which increase to the power of 'x' is increasing exponentially. For example in this article it is stated that drag increases exponentially with speed:

http://www.theengineer.co.uk/liChann...erent+line.htm

They're wrong. I thought only Americans believed everything they read on the internet

Now I understand that 'x to the power of 2' is a polynomial function, but '2 to the power of x' is an exponential function.

Correct!

So I guess you would accept that "drag increases polynomially with velocity?"

In this case, the word is "quadratically."

The point is that as speed increases air friction increases in a non-linear way. The faster you go the GREATER the increase in friction for every additional mph of speed.

For someone who cannot distinguish between quadratic and exponential behavior, it's much better to simply say "if you double the speed, then there's four times the air resistance."

But saying "exponential" behavior in this case is simply wrong and just because some layman uses it, doesn't make it correct.

The distinction is not just semantics. Roughly, to a good engineer (not one who writes internet articles), exponential behavior is nearly impossible to overcome, whereas with quadratic behavior, you have some hope.

This is why gearing alone cannot make 155mph as fuel efficient as 80mph.

That's not necessarily true. Just accounting for air resistance (no other mechanical losses), taller gearing can definitely overcome increased drag. Now if it the air resistance truly did grown exponentially, I'd be more inclined to agree with you

But with taller gearing, coupled with a decrease in the drag *coefficient* (the factor in front of v * v), it is feasible to make 155mph as fuel efficient as 80mph. Even without the gearing change, you just need a decrease in drag coefficient (and/or cross-sectional area) of 1/4. Not easy, but I can give examples if you're not already bored out of your mind with my diatribe