Raised High Spoilers

[ToyotaObsession]

Originally Posted by Dimman

No.

Aerodynamics only deal with CL, which is coefficient of lift, and can have a negative value.

The car as a whole has a CL, if the total is positive the car is said to have lift, if it's negative it's said to have downforce. Semantics.

Secondly your positioning of your 'vacuum' is the opposite of where it should be. It is about the locations of high and low pressure. Lower pressure (your 'vacuum')on the top of something is what creates lift. It is about pressure differentials. High pressure on one side, low pressure on the other, makes a force that pushes towards the low pressure side.

Also you understanding of drag is wrong too.


What the heck do you think reducing drag does? It makes your car easier to move through the air. Again it's about pressure differentials.

I stand by my need to keep a facepalm smiley handy.

You're right. I got my vacuums mixed up.

But that does not invalidate my point.

Reducing lift doesn't create downforce. That's like saying that reducing the amount of pulling suddenly creates pushing.

The coeffecient of lift doesn't become downforce until it reaches a negative number. Same type of force different application. Hence the different names. If it was as simple as you say it is, we wouldn't call it downforce just negative lift. In which case you would be right. But as it stands now, you can add a spoiler, add downforce and do nothing to address the issue of lift under the car. Therefore the amount of lift is the same, but you are counteracting it with a greater amount of downforce than before. Making the lift you do have less effective.

Excuse me, I was thinking of Drag as the physical car moving through the air, as in a blunt object vs. a pointed one. A brick and an Arrow for example. I was confusing the turbulent air behind the vehicle being smoothed out as something else. But you're right it is in fact drag.

[serialk11r]

Originally Posted by ToyotaObsession

ROFL yes you should. Because you make no sense. Actually he was making sense. Mostly everything.

Yes lift and downforce are the same force applied in different ways, but it's how they are applied which makes them different. Lift is created when you have a Vacuum under something so the atmosphere in an attempt to equalize it rushes in which creates lift. Downforce does the opposite. It's a vacuum on top of an object forcing it down. Vacuum=lower than atmospheric pressure, or negative barometric pressure. You have it mixed up, since a vacuum on top would be pulling the car up.

Reducing lift does not mean creating downforce, it just means reducing the difference in the atmospheric pressure so that's it's more equal. Ideally neutral. You can have 0 lift pressure but that doesn't mean you've suddenly created downforce. So in a strict sense, yes I suppose. If you add a wing to a car, you are counteracting the lift that is already there with the wing. However in effect, reducing lift is the same as creating downforce, it's the same net force on the car.

Since lift is created under the car you can counteract it with a Spoiler, but you won't reduce it. I'm not sure this is entirely true, sure the velocity of air is higher on top of the car so you'll always get a bit of lift like that unless you have some serious bodywork added, but I think you can design a spoiler (at least on some body designs) to change the flow over the top of the car in a way that reduces lift.

Now the less lift you have the easier it is for downforce to work because it has less to work against. Sure, ceteris paribus, if the method of creating downforce doesn't affect what's producing the lift. So a high mounted wing would do that more or less.

...

Reducing drag with a spoiler usually ins't very effective. Spoilers will help smooth out the air coming off the rear so that it isn't spilling over and creating a vaccum. But it's not so much reducing drag as it is making it easier for the car to travel though the air. Again, same power, just easier to utilize it. That was a bit self-contradictory. If it helps smooth the air coming off the back, in vague terms, it's probably reducing drag. But most spoilers don't reduce drag, they are there for aesthetics, or to provide desirable aerodynamic characteristics usually at the expense of more drag, but not necessarily so. Spoilers differ from wings in that wings are sorta standalone devices for increasing downforce and spoilers are meant to modify airflow over the body. The only way I can see to interpret "easier ... to travel through the air" is lower drag.

The air coming off a spoiler if everything is working correctly won't interact with the air coming off the bottom because everything is coming off smoothly. In a strict sense, it does interact with the air coming off the bottom but yea it doesn't really do much.

So to summarize, increasing downforce isn't reducing lift. It's simply bullying lift to have less of an effect lol.So not really, there are a lot of ways to produce downforce, and any reduction in lift is an increase in net downforce. I don't know if you read my previous response, but a diffuser decreases pressure below the car, producing a downward force. Now since the top of the car can still have a lower net force, you might say the diffuser is just reducing lift. But it doesn't really matter, downforce, lift, they're just forces. Anything changing the net force (due to air) does just that, it changes the net force.

responses in bold...and if you missed my previous post, I explained why a diffuser can cut drag.
oops you guys posted while I was typing that. well it still applies.

[Dimman]

Originally Posted by ToyotaObsession

You're right. I got my vacuums mixed up.

But that does not invalidate my point.

Reducing lift doesn't create downforce. That's like saying that reducing the amount of pulling suddenly creates pushing.

The coeffecient of lift doesn't become downforce until it reaches a negative number. Same type of force different application. Hence the different names. If it was as simple as you say it is, we wouldn't call it downforce just negative lift. In which case you would be right. But as it stands now, you can add a spoiler, add downforce and do nothing to address the issue of lift under the car. Therefore the amount of lift is the same, but you are counteracting it with a greater amount of downforce than before. Making the lift you do have less effective.

Excuse me, I was thinking of Drag as the physical car moving through the air, as in a blunt object vs. a pointed one. A brick and an Arrow for example. I was confusing the turbulent air behind the vehicle being smoothed out as something else. But you're right it is in fact drag.

The difference in names is only a non-engineering automotive use. Also like the 'sway' of sway-bars, when the force should be correctly called roll and the bars anti-roll bars.

From a physics point of view you are only dealing with a single force: Lift.

The engineers do call it negative lift.

You can't actually 'add' downforce if the overall vehicle CL is still positive after the change, it is reducing lift.

[ToyotaObsession]

Originally Posted by Dimman

The difference in names is only a non-engineering automotive use. Also like the 'sway' of sway-bars, when the force should be correctly called roll and the bars anti-roll bars.

From a physics point of view you are only dealing with a single force: Lift.

The engineers do call it negative lift.

You can't actually 'add' downforce if the overall vehicle CL is still positive after the change, it is reducing lift.

Let's look at it from the point of the springs. I'm going to pull some numbers out of my ass.

100 pound springs for instance move 1 inch for everyone 100 pounds applied.

In a car moving it creates 100 pounds of lift. Effectively making the car 100 pounds lighter. The springs raise 1 inch.

When the car is static the springs sit level.

Now you add 100 pounds of downforce. You haven't reduce the lift it all because it is still in effect under the car. The springs stay at the same height because effectively you have canceled out the force of the lift with a separate and opposite force. Add 200 pounds of downforce, the springs will move 1 inch because you still have 100 pounds of lift in effect. You are overpowering the effect of the lift, but it's still working it's magic on the car.

Remove the lift and add 200 pounds and the springs move 2 inches down effectively making the car 200 pounds heavier (in terms of force applied on the springs). Till the car returns to a static position in which no forces are being applied to it. Other than it's own weight and gravity.

So I don't see you can say that reducing lift is akin to creating an opposite force on the car. Just because you have reduced lift, does not mean you have created an opposite force. You have created a static position.

It's like spending less. It doesn't mean you're earing more, it just means you can put more in the bank.

So you'll have to show me some literature where it says that reducing lift = creating downforce. I would be very interested in reading that.

[serialk11r]

Did 2 of my posts seriously get ignored? I said that it depends on how you make modifications to the car, but there is a net force on the car and whatever you do changes it in one way or another. So it doesn't matter. However when you usually "create downforce" you are adding bodywork that has the effect of pushing down while not affecting other aspects of aerodynamics.

[Dimman]

Originally Posted by ToyotaObsession

Let's look at it from the point of the springs. I'm going to pull some numbers out of my ass.

100 pound springs for instance move 1 inch for everyone 100 pounds applied.

In a car moving it creates 100 pounds of lift. Effectively making the car 100 pounds lighter. The springs raise 1 inch.

When the car is static the springs sit level.

Now you add 100 pounds of downforce. You haven't reduce the lift it all because it is still in effect under the car. The springs stay at the same height because effectively you have canceled out the force of the lift with a separate and opposite force. Add 200 pounds of downforce, the springs will move 1 inch because you still have 100 pounds of lift in effect. You are overpowering the effect of the lift, but it's still working it's magic on the car.

Remove the lift and add 200 pounds and the springs move 2 inches down effectively making the car 200 pounds heavier (in terms of force applied on the springs). Till the car returns to a static position in which no forces are being applied to it. Other than it's own weight and gravity.

So I don't see you can say that reducing lift is akin to creating an opposite force on the car. Just because you have reduced lift, does not mean you have created an opposite force. You have created a static position.

It's like spending less. It doesn't mean you're earing more, it just means you can put more in the bank.

So you'll have to show me some literature where it says that reducing lift = creating downforce. I would be very interested in reading that.

No.

It is a single force. It is simply called 'downforce' when lift is negative. From 'Competition Car Downforce' by Simon McBeath:

At the first stage, with just the front airdam/splitter, the car gave the following coefficients:

CD = 0.364
CLf = -0.037
CLr = 0.138

After the fences, skirts, rear spoiler, and cooling area reduction had been added, the figures were:

CD = 0.384
CLf = -0.099
CLr = 0.020

Now note that the title of his book is about 'downforce' but the engineering data that is used to represent it is in negative lift.

Same thing. Downforce is only when the lift is negative. Enough that the force of lift pushes down on the car...

[chulooz]

Adding bodywork always has multiple aerodynamic effects.

[serialk11r]

Originally Posted by chulooz

Adding bodywork always has multiple aerodynamic effects.

Sorry what I was trying to get at was, if you add say a wing, the wing has relatively little effect on the overall flow of air over the body. But I guess that's not bodywork lol...woops. The other thing would be like, if the sides of a car are shaped to channel air in some way, that may not have much effect on the flow of air over the top of the car. What ToyotaObsession and Dimman are arguing over is terms which are more or less arbitrary.

[Dimman]

Originally Posted by serialk11r

Sorry what I was trying to get at was, if you add say a wing, the wing has relatively little effect on the overall flow of air over the body. But I guess that's not bodywork lol...woops. The other thing would be like, if the sides of a car are shaped to channel air in some way, that may not have much effect on the flow of air over the top of the car. What ToyotaObsession and Dimman are arguing over is terms which are more or less arbitrary.

Not quite. He seems to think that the two terms are actually two different forces.

[serialk11r]

That's what I meant, the terms are just describing vertical component of force on a car, with a sign flip. He seems to think they are independent.

[ToyotaObsession]

Originally Posted by Dimman

Not quite. He seems to think that the two terms are actually two different forces.

I think an object in motion can have several forces working upon it at the same time in different areas.

Changing the underbody won't affect forces on top of the car and vice versa.