Originally Posted by T10Chris

In my experience, there isn't a car that produces zero lift without having flat floor already.

There is always some form of lift somewhere in the body of the car, the goal is to negate it by making more efficient, or by taking a drag penalty and adding more downforce so the effective force on the tire is greater. I had hundreds of pages of data readouts on various race cars (a few F1, LMP [the 919 is INSANE, much more going on than meets the eye]) and more production cars than I can count that I had to review when I was working on algorithms to create code to get desired simulation effect, I wish I still had access to share. OEMs collect every piece of data you can imagine... It gets very tricky to keep the example in layman's terms, or even simple mathematical terms, because you can get into boundary layer flow and vacuum induced drag/lift rather than only frontal area drag. Anyway...

In theory, if the car had zero lift created by the body, the flat floor itself would not add any downforce on it's own, but it would allow downforce producing devices to be more effective, I imagine the diffuser would benefit greatly smoother air/increased airflow speed.

Originally Posted by Dock

Are you asking as it relates to a hypothetical situation where there is no lift (?), because the 996 essentially exhibits the characteristics of a cambered airfoil, and zero lift in this case exists only when the car is moving parallel to the zero-lift axis, which it never would be as long as the tires are on the ground.

Originally Posted by Carlo_Carrera

Perfect, we are in complete agreement. And now I understand were the confuse between us started. From your experience you begin your thinking with a car having lift.

I am used to building the aero from the ground up with a baseline of zero lift. Basically setting all the measuring equipment to zero with the car parked the garage. Then assembling aero bit by bit. Countering lift as it builds with speed and adding downforce in most effective/efficient manner.

The reason we start from a zero baseline is so we never forget that when we done aero tuning the entire car that there is still lift hidden underneath it all waiting to bite us if downforce is suddenly lost. That is what happened with that Mercedes at Le Mans in that video I posted. The engineers forgot, or misjudged, the suppressed lift that was lurking.

Originally Posted by T10Chris

.....Didn't a Nissan GTR GT3 class (maybe not GT3, but full out race car) have a similar incident at Nurburgring in the last few years?

Originally Posted by Carlo_Carrera

Yes, it got airborne cresting a hill on a fast straightaway, and itmight not have been the only car. The safety ***** went nuts. Imposed an actual speed limit through the area, even during races. They lifted the speed limits recently. But there still are issues with lift even with street going GTRs.

Most racing specs now mandate anti-lift venting in critical areas, mainly front wheel wells, on all professional level race cars.

Originally Posted by T10Chris

Yep... LeMans prototypes have to cut out above the rears as well. Not sure the regs on other series.

Originally Posted by Carlo_Carrera

No lift, and no air effects in gerneral, is the baseline for all measurements.

Originally Posted by Dock

Then that would mean the car isn't moving through the air. Once it's moving, Bernoulli has a say in the matter. Like Chris said, there is always some lift occurring on some part of the car.

Originally Posted by Berra

More downforce will always increase drag.

Originally Posted by squid42

That is not universally true. Placing a downforce element so that it gets hit by air that is disturbed anyway can even lower drag.

Originally Posted by Carlo_Carrera

True, but the effect downforce produced by that device would be severely compromised and make it almost a secondary purpose. Most race engineers would consider that application to be primarily a flow modifier rather than a true downforce element, but if it was allowable within the rules of the series they would go for it.

Originally Posted by squid42

I'm not so sure about that. It also gets more complicated when you mix rear spoilers/wings and underbody rear diffusers.

Consider this:

• 911 body shape. The rear does not have a cutoff edge to drag a pile of vortex behind the car (like a car with a vertical rear does). The rear has a slope downwards which air flows down on, creating vortexes much lower to the ground than a normal car.
• Somebody went through great length to have reasonable under-body air management and then there is a serious diffuser under the rear underbody.
• If you let the air wash over the 911 rear body it will flow right into the difffuser's work area, making the diffuser less effective.
• Now consider the 911 standard spoiler (the body panel coming up), or the 991.2 sport design rear spoiler (same thing but sharper cutoff).

Deploying that spoiler will give you a huge improvement in downforce, mainly because it makes the diffuser more effective, not so much because of force on the spoiler itself.

On the other hand, the deployable body panel spoiler on a standard 911 up or down is very unlikely to make a noticeable drag difference. It is sitting in a mess of air either way.