Destiny2002: Yup...Cd is only part of the total drag equation. Dynamic pressure (density * velocity squared / 2) and some measure of area are the others. For a car, usually the area is the frontal area, or the area of the cars sillouette from the front (or rear...) Multiply these all together and you get the drag force. Using coefficients is a way to compare things of different sizes in different environments. The same shape will ideally have the same Cd whether it's moving at 100 mph or 5 mph, but the drag force will be much different. A 1/10 scale model of the S2000 with have the same Cd as the real one, say, for wind tunnel tests, but again, the drag force will be much different.

UL: A small lip spoiler can certainly have a benefit on drag, but it's not necessarily creating downforce...it's destroying lift. I think I've covered this to some degree ealier, but basically...any time you use the airflow to generate a force, you will also create drag. Conversely, if you do something to lower that force, you'll lower drag (Like pulling the flaps up on a wing...lower lift...lower drag). You can think of some of these spoilers in that way.

And yes, I've taken a course or two on aerodynamics... And worked in a wind tunnel or two.

 

ejis,

With a typical 3-box sedan (are there any of those left?), you do see lift reduction and drag reduction simultaneously. But as the rear lip spoiler crosses some case dependent height, you begin to create downforce. Now, that does create more drag as you've noted, but you won't actually return to the original drag value (sans spoiler) until you increase the spoiler height even more. The reference cited in Forbes Aird's book (that was the one) noted that drag decreased up to a spoiler height of 1", but did not return to the original levels until a spoiler height of 2" was reached, while lift reduction and then downforce increased as spoiler height was increased.

What is interesting to note is experiments have shown that as rear downforce increases (or rear lift decreases) your coefficient of lift on the front increases. A good reason to avoid major aerodynamic mods on one end of the car only - IOW, get a front spoiler/splitter and a rear wing/lip/diffuser together. Although, makes one wonder if a front downforce producing device wouldn't actually increase rear downforce by changing the angle of attack of the car.

UL

 

On the basis of this thread, I put the optional Honda rear spoiler back on my S2000. I do not have the front lip. I doubt I will notice any performance gain, but if there is one albeit unnoticeable, I'm happy to have it.

 

UL,

Those findings coincide pretty well with what I would expect. Until you get to the amount of downforce generated as you originally had lift (ie the magnitude of the force), you'll have less overall drag. I'd love to see some pictures of this in a wind tunnel so I could really visualize what's going on...

The change in the front is interesting... In a real car with suspension, I could see this since as downward force is applied to the rear of the car, the angle of the car will nose upwards... This would increase the lift on the front end, like you said. Without that though, it would have to be a result of changing the airflow patterns... Now I'd really like to see some pictures!