Do Flat Floors Slow You Down?

[Griffith500]

Before this goes further, just to be clear, are you saying that a flat floor is the best of both worlds? More top speed and more downforce?

No, a flat floor on its own can only really reduce drag over a floor that is not flat. Tautological, perhaps, but that precision is important.

What you can do, and is what many teams do in many series, is to run that flat floor at a slight angle to the ground to basically make a really long, inefficient venturi. That's why F1 cars look like they're going around with their back ends in the air (when you look at the floor of the cars, which are effectively designed to be run at that angle, i.e. they are not "horizontal" with respect to the rest of the aero design).

If you start adding proper areas for the flow to expand or speed up, and thus, by Bernoulli, the pressure under the car to drop, you can get lots of nice downforce, too, with some additional drag. That's why F1 cars had skirts back in the '70s, to seal that low pressure under the car more effectively. They had massive underbody venturis, which are very efficient downforce generators, in terms of the drag penalty.

The trouble is that it was all very sensitive to rideheight, and thus, inherently dangerous. Which is why such "ground effect" aerodynamics are "banned" generally (initially by complicated regulations, such as not allowing venturis between the axles - cue the CLR flip, believed to be due to the underbody venturi in front of the front axle "failing" and compounding the situation). F1 makes it "easier" by stipulating a flat floor only, except the rear diffuser...

So it seems that the GT6 "flat floor" is a bit of a misnomer, if its increased downforce and increased drag behaviour is intended. Otherwise, it is not working "correctly". Of course, fitting a "flat floor" to certain cars does visually add a diffuser-like element to the rear of the car.

 

[Crispy]

@eran0004 Most the downforce is actually created by the low pressure zone caused by the car being low to the ground. The diffuser essentially slows the air back down, causing it to return to normal speed and pressure. Otherwise the air would be forced to rapidly increase in pressure, causing lift at the rear and drag due to turbulence caused by the uneven air pressures.

Essentially the closer the car is to the ground, the lower the pressure. This is because a larger sum of air is trying to be forced into a small area. The air speeds up but in order to compensate for this the air has to decrease in pressure. The decrease in pressure underneath the car (with a high pressure zone above the car) creates a vacuum, pushing and pulling the car down.

You are correct though. More downforce also means more drag. I believe PD decided to model it based on "FlatFloor=Yes then Downforce=100 Drag=50" regardless of how they visually modeled it.

As you can tell I really, really like aerodynamics!

 

[CSLACR]

No, a flat floor on its own can only really reduce drag over a floor that is not flat. Tautological, perhaps, but that precision is important.

What you can do, and is what many teams do in many series, is to run that flat floor at a slight angle to the ground to basically make a really long, inefficient venturi. That's why F1 cars look like they're going around with their back ends in the air (when you look at the floor of the cars, which are effectively designed to be run at that angle, i.e. they are not "horizontal" with respect to the rest of the aero design).

If you start adding proper areas for the flow to expand or speed up, and thus, by Bernoulli, the pressure under the car to drop, you can get lots of nice downforce, too, with some additional drag. That's why F1 cars had skirts back in the '70s, to seal that low pressure under the car more effectively. They had massive underbody venturis, which are very efficient downforce generators, in terms of the drag penalty.

The trouble is that it was all very sensitive to rideheight, and thus, inherently dangerous. Which is why such "ground effect" aerodynamics are "banned" generally (initially by complicated regulations, such as not allowing venturis between the axles - cue the CLR flip, believed to be due to the underbody venturi in front of the front axle "failing" and compounding the situation). F1 makes it "easier" by stipulating a flat floor only, except the rear diffuser...

So it seems that the GT6 "flat floor" is a bit of a misnomer, if its increased downforce and increased drag behaviour is intended. Otherwise, it is not working "correctly". Of course, fitting a "flat floor" to certain cars does visually add a diffuser-like element to the rear of the car.

In one sentence flat floors reduce drag, in another sentence they increase it.

Get back to me when you're certain.

 

[Imari]

In one sentence flat floors reduce drag, in another sentence they increase it.

Get back to me when you're certain.

Flat floors reduce drag over an underbody with pipes, bolts and all the other gubbins. End of.

With a flat floor, you can choose to use any number of techniques that are relatively efficient at producing downforce. They produce some drag.

A flat floor, in and of itself produces zero downforce. You need to do other things as well to get the underside of the car generating downforce.

Now what you're thinking about is the laminar regime, in which air will follow a car undertray's shape relatively well. A flat floor means less resistance = less drag, because there will be less parts to hit head-on.

However, when the air velocity is increased, the airflow quickly goes into a turbulent regime. In that case, the flat floor of course also means that there is less for the air molecules to hit, so in that way it reduces drag. However, there is an extra effect of vortices (whirls in the airflow) that only appear in the turbulent region of airflow which overpowers the first effect tremendously and the net result is an increase of drag.

Except that with a non-flat underbody, the airflow will go turbulent at a much lower speed than it will with a flat underbody, and it will be more turbulent at any given speed.

Given that you're saying that essentially turbulence = drag for the underbody, I'm not seeing how you can say that a flat underbody is more draggy than a non-flat one.

 

[LMSCorvetteGT2]

How are you in college when you're only 14?

Interesting and very enlightening conversation guys. 👍

Yeah I was wondering the same thing, I'd have to agree somewhat with @Firedragon on the subject of aero properties

 

[Imari]

Yeah I was wondering the same thing, I'd have to agree somewhat with @Firedragon on the subject of aero properties

Because college is a synonym for high school/secondary school in some areas.

Up to you what sort of aero you think people study in high school. Basic physics, I would assume.

 

[LMSCorvetteGT2]

delete

 

[LMSCorvetteGT2]

Because college is a synonym for high school/secondary school in some areas.

Up to you what sort of aero you think people study in high school. Basic physics, I would assume.

That's understandable, thanks Imari, though if that's the case I have a hard time believing the degree is as high as someone like me or others have to study at the University. Which is the way he sort of made it sound.

No, a flat floor on its own can only really reduce drag over a floor that is not flat. Tautological, perhaps, but that precision is important.

True it would reduce drag over a non-flat floor by comparison, but would still retain drag compared to that of a floor that inhibits a diffuser at the end or in the form or tunnels aided by side skirts (e.g. ground effects). Because the issue is that a flat floor alone would cause more lift than downforce and then drag due to lift would then become the issue but not nearly as problematic as a flat floor. The GT issue seems to be that downforce is generated rather than lift thus causing the top speed to be limited which is the issue the OP seems to have.

So do cars with just a flat floor perform different aero wise to those with one and a diffuser combined? Or do they perform the same? And if this can be figured out then it would obviously demonstrate a massive issue with aerodynamic physics in GT

What you can do, and is what many teams do in many series, is to run that flat floor at a slight angle to the ground to basically make a really long, inefficient venturi. That's why F1 cars look like they're going around with their back ends in the air (when you look at the floor of the cars, which are effectively designed to be run at that angle, i.e. they are not "horizontal" with respect to the rest of the aero design).

What you mean by this exactly? I'm just having trouble visualizing it in my head. Also there are many reasons why F1 cars under bodies are shaped the way they are and the leading cause being three decades of regulations setting it this way, as you're aware. Are you saying that the entire reference plane is angled or gradual steps in attack of the angle, because if this is the case I'd say that is due to trying to prevent flow separation.

So it seems that the GT6 "flat floor" is a bit of a misnomer, if its increased downforce and increased drag behaviour is intended. Otherwise, it is not working "correctly". Of course, fitting a "flat floor" to certain cars does visually add a diffuser-like element to the rear of the car.

So it seems you agree that the attempt wasn't done correctly as far as the actual concept goes.

In one sentence flat floors reduce drag, in another sentence they increase it.

Get back to me when you're certain.

They reduce drag more so obviously than a car without one, yet as I just stated above not all drag is reduced and some is still retained or created in a different way due to lift. But not to the significance of a non-flat floor.

I read his post and though I agree at times he seems to use over the top verbose when he could keep it simple so he isn't misunderstood which is the case here. Either way he basically did say or stay mainly consistent with what he was trying to tell you and what I just told you.

 

[CSLACR]

Flat floors reduce drag over an underbody with pipes, bolts and all the other gubbins. End of.

With a flat floor, you can choose to use any number of techniques that are relatively efficient at producing downforce. They produce some drag.

A flat floor, in and of itself produces zero downforce. You need to do other things as well to get the underside of the car generating downforce.



Except that with a non-flat underbody, the airflow will go turbulent at a much lower speed than it will with a flat underbody, and it will be more turbulent at any given speed.

Given that you're saying that essentially turbulence = drag for the underbody, I'm not seeing how you can say that a flat underbody is more draggy than a non-flat one.

Okay, so if it's really that simple, why don't manufactures slap some plastic covers up under vehicles in these never-ending fuel economy wars? (or have they started very recently?)

 

[LMSCorvetteGT2]

Okay, so if it's really that simple, why don't manufactures slap some plastic covers up under vehicles in these never-ending fuel economy wars? (or have they started very recently?)

Well aero devices aren't automatic...so it doesn't work that way to where just putting a cheaply constructed flat floor under the car some how creates more efficiency. Certain speed regions need to be met before the car actually starts generating the necessary amount of force. 65mph highway speeds tend to not be the case.

 

[Bopop4]

Okay, so if it's really that simple, why don't manufactures slap some plastic covers up under vehicles in these never-ending fuel economy wars? (or have they started very recently?)

That would also make it harder to do service work on the car.

The savings at 60mph wouldn't be enough to justify it.

 

[CSLACR]

Interesting that you both picked lower than the average US Interstate speed to use as a number. (Idk what Canada does, but you used mph instead of kph, so...)

Now say 70. (or even 80, not that anyone in the world ever reaches such a crazy speed)

Sounds like you aren't sure.

 

[Imari]

Okay, so if it's really that simple, why don't manufactures slap some plastic covers up under vehicles in these never-ending fuel economy wars? (or have they started very recently?)

Some cars do have at least partial covering, particularly at the front.

However, it's more about the speeds at which the cars are going. The efficiency gains at 10kmh are going to be trivial. My gut feel is that the efficiency gains at town speeds from such a device would be negligible compared to things like having correct tyre pressures and driving technique.

On the open road...dunno. Probably there's some gains, and possibly they're not big enough to justify the added weight and complexity. It's another part, and it is kind of a pain to take the underfloor off every time you want to work on the bottom of the car.

Look at this poor Toyota Prius, for example.

It's not a totally flat floor, but they've obviously tried where possible to make it a single surface where possible. It's a reasonable compromise that still allows access to all the parts that need it, and probably provides the lion's share of the drag benefits as well.

Compared to a race car where anything is fair game if it makes you go faster, road cars have other compromises to meet.


Edit:

Interesting that you both picked lower than the average US Interstate speed to use as a number. (Idk what Canada does, but you used mph instead of kph, so...)

Now say 70. (or even 80, not that anyone in the world ever reaches such a crazy speed)

Sounds like you aren't sure.

Open road speeds in the US are 75mph? It varies here from about 100 to 120kmh, and I'm used to assuming it's 100.

Obviously the faster you go, the better a flat floor is going to be for you. Without testing, it's really not possible to say definitively how big an effect you get at 60/65/75 mph.

Although you can extrapolate from the fact that only seriously quick sports cars seem to have a full flat floor fitted, so that for whatever reason, be it fuel efficiency, cost of manufacture or whatever, car companies don't find it expedient to fit a flat floor to econoboxes.

 

[NOSWaster]

Actually the effect on the 400 km/h car would be double, as that car would have a pretty streamlined shape to begin with, and adding downforce to a streamlined body generally costs more drag than adding downforce to a less streamlined body.

I'm not hopping in to argue or anything. I'm just here to say that it's four times the effect on a car going 400KPH versus a car going 200KPH.

Speed and air drag have a quadratic relationship.

Interesting that you both picked lower than the average US Interstate speed to use as a number. (Idk what Canada does, but you used mph instead of kph, so...)

Now say 70. (or even 80, not that anyone in the world ever reaches such a crazy speed)

Sounds like you aren't sure.

Not everyone lives in the US, and not everyone has average US interstate speeds.

 

[LMSCorvetteGT2]

Interesting that you both picked lower than the average US Interstate speed to use as a number. (Idk what Canada does, but you used mph instead of kph, so...)

Now say 70. (or even 80, not that anyone in the world ever reaches such a crazy speed)

Sounds like you aren't sure.

I picked what the speed limit is here on the freeway and highway where I live...no reason to split hairs, it's not some ploy against you. Just situational awareness of what's familiar to us

And I am sure or I should be passing and obtaining my degree in the very field we are talking about. Even at 80 it would still not be enough to make it worth the investment especially if it were just as easy as slapping flat plastic under a car. It's not by the way.

Also since highway speed wouldn't be a constant situation the car deals in and there would be city as well, then the device becomes an issue. It becomes an issue because now you aren't moving fast enough to create lift or reduce drag and thus it does little to no help in the situation.

 

[Johnnypenso]

Ferrari 430

 

[ITCC_Andrew]

Speed and air drag have a quadratic relationship

I've long believed that, since at 0 km/h, there's no airflow (and therefore no downforce, excluding vacuum cars,) that it's an exponentially increasing factor. Like, a car going at 20 km/h may have miniscule drag, while a car going 400 km/h+ is likely to break its windscreen because there's soooo much aerodynamic force. Surely you can't say that it's only 4x stronger when you double the speed from 200 km/h to 400 km/h?


Also, it's not necessarily quadratic, but rather exponential, isn't it?
--------------------------------------------------------------------------------------------------------------
The point remains, this is a GT6 forum. We need to test it in GT6, as relevant to GT6. Most of us here on the forums aren't building actual race cars, we're building pixels with sounds that mimic race cars. We want to know what effect flat floors have in GT6.

 

[NOSWaster]

I've long believed that, since at 0 km/h, there's no airflow (and therefore no downforce, excluding vacuum cars,) that it's an exponentially increasing factor. Like, a car going at 20 km/h may have miniscule drag, while a car going 400 km/h+ is likely to break its windscreen because there's soooo much aerodynamic force. Surely you can't say that it's only 4x stronger when you double the speed from 200 km/h to 400 km/h?

If you want to double the top speed of a car that somehow is aerodynamically limited to 100MPH, you will at the minimum increase the amount of power output to the wheels by four times. That's a quadratic relationship. Double one, the other quadruples.
http://en.wikipedia.org/wiki/Quadratic_function

Edit: Also, the drag equation has a dependency based on velocity squared, thus air drag being a quadratic relationship.

 

[LMSCorvetteGT2]

Ferrari 430

Was the the point of this?

 

[ITCC_Andrew]

If you want to double the top speed of a car that somehow is aerodynamically limited to 100MPH, you will at the minimum increase the amount of power output to the wheels by four times. That's a quadratic relationship. Double one, the other quadruples.
http://en.wikipedia.org/wiki/Quadratic_function

Precisely. But if you want to double that speed again, to hit 400 mph, then you have to multiply the power by at least 4x once again (16x your starting power!!), meaning that the relationship between speed and drag is exponential. Drag increases exponentially as speed increases.


TL;DR: We're both right. I see what you're saying now.

 

[NOSWaster]

TL;DR: We're both right. I see what you're saying now.

Welp, that was interesting. Yay for us both being right!

 

[Johnnypenso]

Was the the point of this?

Just another picture of an underbelly of a car to go with the Prius.

 

[LMSCorvetteGT2]

Just another picture of an underbelly of a car to go with the Prius.

Too bad it's flipped, tragic

 

[ITCC_Andrew]

Just another picture of an underbelly of a car to go with the Prius.

This is nothing new. Many, many cars have flat floors. My Impreza has a rather flat floor.... Well, at least, it has aerodynamic devices everywhere, and a skidplate. But, it's not perfect, since there's a gap below the spare tire.


If even our mundane street cars have them, and F1 cars have them, can we not agree that flat floors are aerodynamically good in real life?

 

[Johnnypenso]

This is nothing new. Many, many cars have flat floors. My Impreza has a rather flat floor.... Well, at least, it has aerodynamic devices everywhere, and a skidplate. But, it's not perfect, since there's a gap below the spare tire.


If even our mundane street cars have them, and F1 cars have them, can we not agree that flat floors are aerodynamically good in real life?

The question isn't whether they are "good", the question for the last few pages has been, do they work in Gran Turismo relative to the way they are modeled, as they would in real life? In other words, take a real car, slap a flat floor on it and nothing else, no diffusers or other aerodynamic aids, and will it increase or decrease drag/downforce/topspeed?

 

[ITCC_Andrew]

And what I'm trying to say is that yes, I believe it should. But, I'm not quite an expert. Another thing, is that it's obviously poorly modeled in GT6, given that it negatively affects top speed, while most auto manufacturers (à la Prius) are making cars with flat floors to improve aerodynamics in order to improve gas mileage or (electric-powered) range.

 

[Bo]

This is nothing new. Many, many cars have flat floors. My Impreza has a rather flat floor.... Well, at least, it has aerodynamic devices everywhere, and a skidplate. But, it's not perfect, since there's a gap below the spare tire.


If even our mundane street cars have them, and F1 cars have them, can we not agree that flat floors are aerodynamically good in real life?

Flat floors reduce drag over an underbody with pipes, bolts and all the other gubbins. End of.

With a flat floor, you can choose to use any number of techniques that are relatively efficient at producing downforce. They produce some drag.

A flat floor, in and of itself produces zero downforce. You need to do other things as well to get the underside of the car generating downforce.



Except that with a non-flat underbody, the airflow will go turbulent at a much lower speed than it will with a flat underbody, and it will be more turbulent at any given speed.

Given that you're saying that essentially turbulence = drag for the underbody, I'm not seeing how you can say that a flat underbody is more draggy than a non-flat one.

Both of you have got it spot on 👍

What was said about the turbulent air under a flat floor in comparison to a messy floor was confusing. As I said up above, the only way a flat floor could produce more drag than a messy floor is if the floor was specifically designed to work with the rest of the car in some sort of a niche window.

Also, as said before and backed up by @Imari, just having something completely flat will not push the car down with any force at all, unless:

a) It's coupled with a diffuser or another aerodynamic aid to produce the downforce, and it's this that will cause the drag.

b) The floor isn't really flat and is set up in such a way to achieve a ground effects design.

The question isn't whether they are "good", the question for the last few pages has been, do they work in Gran Turismo relative to the way they are modeled, as they would in real life? In other words, take a real car, slap a flat floor on it and nothing else, no diffusers or other aerodynamic aids, and will it increase or decrease drag/downforce/topspeed?

Anyway, talking of GT6, I think that PD modelled the flat floor correctly. However, the diffuser that goes with it has been modeled very badly indeed, as they have probably applied the basic "downforce = drag penalty" at the biggest rate possible, so when you get a high rate of airflow passing under the car, it's coded as if you have a big GT wing underneath your car.

 

[Johnnypenso]

Anyway, talking of GT6, I think that PD modelled the flat floor correctly. However, the diffuser that goes with it has been modeled very badly indeed, as they have probably applied the basic "downforce = drag penalty" at the biggest rate possible, so when you get a high rate of airflow passing under the car, it's coded as if you have a big GT wing underneath your car.

I only checked on an RX-8, and the back is black and it's hard to see, but I didn't see any diffusers of any kind in the rear when I installed the flat floor.
Here's one from the Official GT6 Photo Sharing Thread and it has no diffuser either..

 

[Bo]

In the case of those, maybe they've modeled them without consideration for whether it has a diffuser or not? So as long as there's some kind of aero improvement to the chassis, a drag penalty must be applied?

Whatever way it is, it's wrong and way off from real life. Basically.

 

[Ridox2JZGTE]

A simple way to test if PD modeled the diffusers + flat floor correctly, buy R34 GTR '99 ( IRL this early version model year has no rear diffuser or front flat undertray ), the compare it to R34 GTR Vspec II ( use the premium '02 ). I doubt PD will differentiate the top speed or acceleration, even if it does, it will be very small, but we can compare the handling stability