Carlo_Carrera

This race car produced a ton of downforce but clearly needed some lift reduction devices on it.

rs10

You are both right. On the one hard, it's perfectly normal to say "this helps the car produce downforce" where it in fact merely reduces lift, thereby enabling the downforce produced by other aero bits to give the car as a whole downforce. On the other hand, as already noted several times in this thread, there is a distinction between reducing lift and producing downforce. And although reducing lift while decreasing drag is often easy, producing downforce while reducing drag is not.

However, does that mean it's impossible? I think not. Something that cleans up otherwise messy airflow can reduce drag and increase downforce. There is probably an alternate shape that would reduce drag more and not produce downforce, but that's the answer to a different question.

Examples? I can think of two candidates: First, I understand lowering a car often reduces drag and can increase the effectiveness of underfloor aero (or decrease it - it all depends ... ). Second, can a lip spoiler have the same effect? (I understand lip spoilers can sometimes decrease drag by reducing airlow to places under the car where the airflow gets messy.)

T10Chris

I am aware of how things work and there is a difference in the two. However, in the scenario of how it applies to automobiles:

Does reducing lift not equate to more downward pressure on the tires? Does downforce not apply downward pressure to the tires?

Explain to me how more pressure on the tires provided in whichever method or another has a different effect. When manufacturers quote downforce numbers, it is net of both reduced lift by whatever means and actual downforce by whatever means.

Carlo_Carrera

This whole thing started with a disagreement over a comment made by Berra back in post #11. He correctly noted that any method of producing downforce comes with some drag penalty. This is true.

Saying that reducing lift can be achieved without a drag penalty is also accurate, but reducing lift is not the same as creating downforce. It is not adding downward pressure to the tires.

The results are not the same.

Creating downforce produces a force that can be much greater than the actual weight of the car. Using air to force the car into the ground. Something reducing lift can never do. Reducing lift adds no pressure to the tires. It restores the car to its natural gravitational equilibrium, while downforce in a broad sense creates extra gravity. The static gravitational equilibrium of the car, the car just sitting there not moving, is the correct reference point.

Some types of downforce producing devices, like a rear wing, can reduce lift as they are creating downforce but there is a drag penalty. A lift reducing device, like vents on top of the fenders, can do its job without adding drag but it can never produce downforce.

Carlo_Carrera

I order to really get into this I need a very specific example. Aero is a nose to tail sort of thing. Any change made anywhere on the car will effect how the entire system functions.

autobonrun

I've attached a link with a discussion on the subject of lift and downforce we had over on Pelican about 9 years ago (still going). Some of the posts contain data for both Cd and lift (post #24 and 46 in particular). Several of the posts are of for the older 911's but the concepts are the same for the new ones; just the figures change. You might be surprised about the effect front and rear spoilers have on lift, but not Cd. The discussions also cover the 996. Some of the devices simply create turbulence and reduce lift, others actually add down force. You'll also notice that the lift at the rear of a 911 is greater than at the front (see post #12), so spoilers and airfoils at the rear will have a greater impact on reducing overall lift than a lip spoiler at the front. Lift at the rear is about twice that at the front. That's why many older 911's can run a rear spoiler with no front lip but never vice versa. The factory recommends always have both. Anyway, the link for the technocrats that enjoy this stuff.

http://forums.pelicanparts.com/porsc...ront-rear.html

T10Chris

Right. Anything that produces downforce creates drag, sometimes extremely small, but yes.

Correct again, I agree with all of these concepts you are saying. I'm not saying flat floors create downforce- Downforce itself produces an amount of what can translate to negative lift. Lift reduction allows the downforce to be more effective at putting the force to the ground...

Example (very simplifed, I know that F/R balance, different amounts of downforce and lift across the car as a whole, changes on one end can effect the other, etc etc): car creates 50 lbs lift, has aero that nets to 300 lbs downforce (negative lift), so net force at tires is 250 lbs negative lift... Now we install theoretical flat floors which reduce the lift to 30 lbs while downforce stays the same... So now we have 30 lbs lift and 300 lbs negative lift, so 270 lbs negative lift/downforce at the tires. It all works together. Again, this is very simplified example..

I think we are in agreement on what is happening just going about saying it in different manner? If I'm wrong then I've really screwed up some stuff in my career

Dock

^This

Carlo_Carrera

Yes we are saying the same things but our baseline points of reference were different. Now we are at the same reference point.

Do you work in aviation?

T10Chris

I've done some consulting work in aviation, but most of my experience is automotive aero. Most of my work is "theoretical" I guess you could say. I'm basically a software developer, but was in a unique niche.

I did physics design and controls feedback design for simulators.. I've worked on arcade style racing sims all the way up to sims that professional race teams use, and also did some work for pilot sims for an aviation manufacturer, and electronics control feedback that actually went into a plane.. Pilots like things to feel manual even if it is only connected to wires.. Same with high level/professional automotive stuff.

I do different stuff now unfortunately, been out of that for about 6 months now. I miss it, new position is very dull in comparison.

Carlo_Carrera

The sim work sounds interesting. What are you working on now?

T10Chris

It was really cool doing the sim stuff. I've been around cars, people, and experiences I would have never otherwise came across. I drove the Aston DB11 before it was publicly released, saw the recent GTR refresh months before it was public, several Porsche cars, plus contacts with McLaren, Ferrari, etc. It never felt like I was going to work, which made the long hours easy to do. I miss it.

Revenue generating software.. more effective ways to use cookies and data that are captured when a user visits a website to deliver targeted ads, or to bring relevant products offered by the vendor forward for greater exposure.

More pay, less work, but not even close to as interesting. Trade offs I suppose.

Carlo_Carrera

A thought just crossed my head and wanted to make sure we have fully put this aero/physics thing to bed.

Example (very simplifed, I know that F/R balance, different amounts of downforce and lift across the car as a whole, changes on one end can effect the other, etc etc): car creates 50 lbs lift, has aero that nets to 300 lbs downforce (negative lift), so net force at tires is 250 lbs negative lift... Now we install theoretical flat floors which reduce the lift to 30 lbs while downforce stays the same... So now we have 30 lbs lift and 300 lbs negative lift, so 270 lbs negative lift/downforce at the tires.

Rerunning the above scenario in italics again but with a car that produced no lift. What effect would adding the lift reducing flat floor would have?

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.

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.