Do Flat Floors Slow You Down?

[Bo]

Maybe I'm missing something so help me out here. You put a perfectly flat floor on a car for example, how does it have less aero drag but more turbulence than a car with a jumpled mess underneath? How does a perfectly flat surface (the underside of the car) running over another relatively flat surface (the road) create more turbulence than a jumbled mess running over the same flat road?

It's basically completely dependent on whether the said object has been specifically designed to have an aerodynamic stability aid in the complex dimply/vaney mess.

In the case of normal cars with a flat floor bolted on, a completely flat floor will always work better than some horrible mess of a floor.

Because at 'normal car speeds' (several tens of m/s), the air under the car is already turbulent. A flatter surface makes the air even more turbulent purely because the air velocity will be higher. The net effect is more energy-dissipating airflow that increase drag.

That's why diffusers are almost always coupled with flat floors, since they determine where the turbulent air leaves the car, which is the most important factor in determining a car's cd and aerodynamic performance.

 

[Johnnypenso]

Because at 'normal car speeds' (several tens of m/s), the air under the car is already turbulent. A flatter surface makes the air even more turbulent purely because the air velocity will be higher. The net effect is more energy-dissipating airflow that increase drag.

That seems counter-intuitive to me. The air is effectively stationary (in the absence of wind) and the car is passing over stationary air when it's racing correct? Intuitively it would seem that what's happening is a car that is not aerodynamic underneath, has a harder time allowing the moving car to allow the stationairy air a path through the underbelly of the car, creating more turbulence and more air resistance to the car moving through the stationary air.

On the other hand, a car at speed with a smooth undercarriage should pass over that same stationary air with less turbulence and less resistance.

 

[Firedragon]

It's basically completely dependent on whether the said object has been specifically designed to have an aerodynamic stability aid in the complex dimply/vaney mess.

In the case of normal cars with a flat floor bolted on, a completely flat floor will always work better than some horrible mess of a floor.

A flat floor will induce more downforce at the cost of more drag, yes.

That's why diffusers are almost always coupled with flat floors, since they determine where the turbulent air leaves the car, which is the most important factor in determining a car's cd and aerodynamic performance.

At the back

That seems counter-intuitive to me. The air is effectively stationary (in the absence of wind) and the car is passing over stationary air when it's racing correct? Intuitively it would seem that what's happening is a car that is not aerodynamic underneath, has a harder time allowing the moving car to allow the stationairy air a path through the underbelly of the car, creating more turbulence and more air resistance to the car moving through the stationary air.

On the other hand, a car at speed with a smooth undercarriage should pass over that same stationary air with less turbulence and less resistance.

That's the thing I'm trying to emphasize: it is counter-intuitive, because the interaction is not only between the molecules of air and the surface beneath the car that those hit, but it extends to the surrounding air molecules as well and that's where characterizing different airflow regimes is necessary to say something about drag.

The term 'turbulence' and 'laminar' describe the way the air moves around the car. It doesn't matter if the air is standing still and a car moves over it or if a car is standing still and the air moves over it; the relative velocity of the airflow relative to the object (car floor) is what is relevant.

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.

 

[Bo]

A flat floor will induce more downforce at the cost of more drag, yes.

A flat floor by itself can only ever produce downforce if the ride height of the car was to be set at such a specific height as to induce ground effects of some sort.

At the back

Yeah... that's where diffusers are... if they were at the front, they would force a mess of airflow onto a flat pan floor, creating one unholy mess of instability.[/quote]

 

[MSTER232]

And all the air in space?...

There is no air in space, it's a vacuum.

 

[Crispy]

I wonder if by flat floor PD really means a shaped underside. In theory, a flat floor covers all the mechanical bits under the car, thereby greatly reducing drag and lightly increasing downforce.

A shaped underside includes venturi tunnels, splitters, and diffusers to really increase downforce with a slight increase in drag. It creates a low pressure, high velocity zone underneath the car where venturi tunnels then increase the speed (Thus decreasing pressure) even more. Then the air is forced to the diffuser which lowers drag by gradually increasing speed back to normal. The effect is a greater downforce-drag ratio than a traditional wing.

TL;DR-A flat floor decreases drag by covering up underpinnings that increase the amount of drag on the car, with a slight increase in downforce due to the low pressure air under the car actually having somewhere to go. A shaped-underside greatly increases downforce by shaping the air in useful ways to provide more downforce, however this comes at a small increase of drag. It's most likely that PD uses the name "flat-floor" to describe shaped floors of GT-Cars, rather than the flat-floor of an F1 car.

 

[Firedragon]

A flat floor by itself can only ever produce downforce if the ride height of the car was to be set at such a specific height as to induce ground effects of some sort.

It works whenever there is a non-zero airflow. The downforce doesn't magically disappear when the car height is above the point where the 'ground effect' is gone, especially since there is no universally defined point in which the air accelerates so much that there is significantly more downforce than to be expected (= 'ground effect').

Yeah... that's where diffusers are... if they were at the front, they would force a mess of airflow onto a flat pan floor, creating one unholy mess of instability.

Not really...

It would only reduce airflow speed and therefore making the flat floor and/or rear diffuser less efficient.

One thing I'm trying to say here is that you should leave the gut-feeling and really read into aerodynamics, but so far to no avail. Also, back it up with some real-world examples or some calculated results, because all you're doing now is saying I'm wrong without any evidence whatsoever.

 

[Johnnypenso]

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.

I think I'm going to have to give up because this again sounds counter-intuitive to the point of defying logic.

Given that a flat floor = less for air molecules to hit = less drag....

How is then possible that a car produces more turbulent airflow if the air molecules are hitting less stuff under the car?

Then the air is forced to the diffuser which lowers drag by gradually increasing speed back to normal. The effect is a greater downforce-drag ratio than a traditional wing.

As far as I can tell, at least on the RX-8, it doesn't look like there are any diffusers with the flat floor upgrade.

 

[Bo]

One thing I'm trying to say here is that you should leave the gut-feeling and really read into aerodynamics, but so far to no avail. Also, back it up with some real-world examples or some calculated results, because all you're doing now is saying I'm wrong without any evidence whatsoever.

This isn't gut feeling. I study aerodynamics to a far greater level than my college even teaches at.
I also apply it to my hobby as well, where I develop and produce aerodynamic parts for RC cars, which I successfully compete with.

You haven't given me any statistics either actually, mainly because statistics and calculated results for a topic as specific as "what effect does moving a diffuser to the front of a car which has a flat floor have?" are pretty impossible to come across.

I'm giving you points in all of my posts as well, actually.

I wonder if by flat floor PD really means a shaped underside. In theory, a flat floor covers all the mechanical bits under the car, thereby greatly reducing drag and lightly increasing downforce.

A shaped underside includes venturi tunnels, splitters, and diffusers to really increase downforce with a slight increase in drag. It creates a low pressure, high velocity zone underneath the car where venturi tunnels then increase the speed (Thus decreasing pressure) even more. Then the air is forced to the diffuser which lowers drag by gradually increasing speed back to normal. The effect is a greater downforce-drag ratio than a traditional wing.

TL;DR-A flat floor decreases drag by covering up underpinnings that increase the amount of drag on the car, with a slight increase in downforce due to the low pressure air under the car actually having somewhere to go. A shaped-underside greatly increases downforce by shaping the air in useful ways to provide more downforce, however this comes at a small increase of drag. It's most likely that PD uses the name "flat-floor" to describe shaped floors of GT-Cars, rather than the flat-floor of an F1 car.

Since every flat floor but one that i've come across has a noticeable kink in the back, or a diffuser, I think it is just a redesigned and sculpted underside.

Whatever way they've done it, the effect it has is far removed from real life.

 

[Firedragon]

I think I'm going to have to give up because this again sounds counter-intuitive to the point of defying logic.

Given that a flat floor = less for air molecules to hit = less drag....

How is then possible that a car produces more turbulent airflow if the air molecules are hitting less stuff under the car?

Ah, you see, air doesn't have to hit anything to become turbulent. It could even become turbulent when it flows through a perfect tube...the only thing is that the air velocity needs to be high enough. Now this whirly, chaotic airflow is the reason that there is drag, because - now my explanation gets a bit more non-scientific - there is less 'movement space' for other, new molecules to navigate around the car. The correct way would be saying that the turbulent airflow dissipates energy; it creates a barrier for the other molecules to flow underneath the car.

This isn't gut feeling. I study aerodynamics to a far greater level than my college even teaches at.
I also apply it to my hobby as well, where I develop and produce aerodynamic parts for RC cars, which I successfully compete with.

I guess the Formula Student team at my uni is then just screwing around then. And their GAMBIT CFD software must be rubbish too, as well as one of the main teachers at the Thermo-dynamic Fluids department.

I hope you're aware that the effect of aerodynamics on RC cars is way less than on real cars compared to other factors that define 'success' (speed, handling, other performance parameters)?

 

[Bo]

I guess the Formula Student team at my uni is then just screwing around then. And their GAMBIT CFD software must be rubbish too, as well as one of the main teachers at the Thermo-dynamic Fluids department.

I hope you're aware that the effect of aerodynamics on RC cars is way less than on real cars compared to other factors that define 'success' (speed, handling, other performance parameters)?

If the FS team isn't screwing around, then is it using your theory of sculpting a chassis full of dimples and vanes to gain that all important advantage?

Also, the aerodynamics put in place on RC cars can are almost identical to full size cars. Hence why bodyshell and aerodynamic extensions are so strictly regulated.

 

[Johnnypenso]

Ah, you see, air doesn't have to hit anything to become turbulent. It could even become turbulent when it flows through a perfect tube...the only thing is that the air velocity needs to be high enough. Now this whirly, chaotic airflow is the reason that there is drag, because - now my explanation gets a bit more non-scientific - there is less 'movement space' for other, new molecules to navigate around the car. The correct way would be saying that the turbulent airflow dissipates energy; it creates a barrier for the other molecules to flow underneath the car.

I understand that whirly, chaotic air creates drag. But that doesn't explain how with two cars, one with a flat floor, and one without, the one with the flat floor would have more, not less turbulence. What mechanism is in play that the air flows with less turbulence in the car without the flat floor?

 

[ITCC_Andrew]

I think @Famine could do one of his physics write-ups for us... To definitively answer this.


Personally, I think flat floors - in real life - add to the top speed and cornering grip, but in GT6, aids cornering grip while slowing down the top speed.

 

[Firedragon]

If the FS team isn't screwing around, then is it using your theory of sculpting a chassis full of dimples and vanes to gain that all important advantage?

Also, the aerodynamics put in place on RC cars can are almost identical to full size cars. Hence why bodyshell and aerodynamic extensions are so strictly regulated.

No, you're imagining that they are putting a lot of extra stuff on the car, let alone that that is my theory and certainly that is not what I'm advising. That example is purely to explain that in the world of fluid dynamics, there is nothing that you can explain purely based on reasoning. The only valid things are either simulation or actual results.

What I am saying and what has been found true in simulations, is that a flat floor contributes to more downforce, but also increases drag. And somehow you're not willing to accept it, based on you 'studying the subject far beyond your teacher's knowledge'. Hmm.

I understand that whirly, chaotic air creates drag. But that doesn't explain how with two cars, one with a flat floor, and one without, the one with the flat floor would have more, not less turbulence. What mechanism is in play that the air flows with less turbulence in the car without the flat floor?

The 'rate of turbulence' can be expressed with the Reynolds number, which increases with increasing speed and flow area. The flow area is the car's floor area and since it's about the same car, the airflow speed is the only parameter that comes in play.

Now this doesn't mean that a higher speed/Reynold number per se mean a higher drag, because the object's size and shape definitely have to be taken into consideration. Certainly because sometimes making an object's surface less 'slippery' creates less overall drag. It can be locally increased due to more molecules hitting the addition like a small winglet, but alter the airflow over the entire car in such a way that there is less energy dissipation, resulting in less total drag.

Also, what I am taking for granted is that the 'non-flat floor car' is like a normal car....not entirely flat but macroscopically, reasonably flat instead of having big holes or cavities in which air can be trapped, which I assume is the case for most of the cars in the real world. Only then it's the case that the increase of drag due to turbulence is greater than the decrease of drag due to an uneven floor surface. Ergo, a flat floor on a car(-sized object that has the typical ground clearance and speed of a car) should introduce more drag and downforce.

 

[CSLACR]

I think I'm going to have to give up because this again sounds counter-intuitive to the point of defying logic.

Given that a flat floor = less for air molecules to hit = less drag....

How is then possible that a car produces more turbulent airflow if the air molecules are hitting less stuff under the car?

As far as I can tell, at least on the RX-8, it doesn't look like there are any diffusers with the flat floor upgrade.

More air goes under a car with a flat floor, more gets deflected around without.

 

[Griffith500]

Yes.
(well, it can, I would never say anything definite on this subject)

For the same reason an un-dimpled golf ball doesn't go as far.

Eh, no. The dimples shift the flow regime into the "better" minimum in terms of drag, by allowing the flow to "separate" sooner, reducing "skin drag", which would otherwise dominate. It's very carefully "designed" to operate in the specific Reynolds number range of the game of golf played with golf balls, where there are potentially two drag values for the same Re value, depending on the exact flow regime. The more turbulent regime typically has a lower drag coefficient, for a given geometry.

The jumbled underside of a motor vehicle is in no way comparable to the dimples on a golf ball!

The local velocities around the discontinuities of the jumbled mess will be much higher than with the flat floor, increasing turbulence (much higher Reynolds number range than a golf ball, typically, just because of the "characteristic length" scales involved, if nothing else), increasing drag. That is, at speeds where aero drag is important for a car, the flow under the car is already fully turbulent and there is no benefit in increasing Reynolds numbers further by making the flow / surface interface massively uneven.

 

[CSLACR]

Eh, no. The dimples shift the flow regime into the "better" minimum in terms of drag, by allowing the flow to "separate" sooner, reducing "skin drag", which would otherwise dominate. It's very carefully "designed" to operate in the specific Reynolds number range of the game of golf played with golf balls, where there are potentially two drag values for the same Re value, depending on the exact flow regime. The more turbulent regime typically has a lower drag coefficient, for a given geometry.

The jumbled underside of a motor vehicle is in no way comparable to the dimples on a golf ball!

The local velocities around the discontinuities of the jumbled mess will be much higher than with the flat floor, increasing turbulence (much higher Reynolds number range than a golf ball, typically, just because of the "characteristic length" scales involved, if nothing else), increasing drag. That is, at speeds where aero drag is important for a car, the flow under the car is already fully turbulent and there is no benefit in increasing Reynolds numbers further by making the flow / surface interface massively uneven.

Just popped in to say cars aren't like golf balls eh? Good information.

 

[Griffith500]

Just popped in to say cars aren't like golf balls eh? Good information.

You made the comparison, not me.

 

[Bo]

Just popped in to say cars aren't like golf balls eh? Good information.

Well they're not. So it's a bad example to give in the discussion, especially since it's just an example of aerodynamic stability in a very specific manner.

 

[GTP_CargoRatt]

This isn't gut feeling. I study aerodynamics to a far greater level than my college even teaches at.
I also apply it to my hobby as well, where I develop and produce aerodynamic parts for RC cars, which I successfully compete with.

You haven't given me any statistics either actually, mainly because statistics and calculated results for a topic as specific as "what effect does moving a diffuser to the front of a car which has a flat floor have?" are pretty impossible to come across.

I'm giving you points in all of my posts as well, actually.


Since every flat floor but one that i've come across has a noticeable kink in the back, or a diffuser, I think it is just a redesigned and sculpted underside.

Whatever way they've done it, the effect it has is far removed from real life.

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

Interesting and very enlightening conversation guys. 👍

 

[CSLACR]

I didn't compare a golf ball to a car.
Quote me.

 

[Griffith500]

I didn't compare a golf ball to a car.
Quote me.

You said a car with an undressed underside could go faster than an equivalent with a flat floor, because of the same reason that a dimpled golf ball can travel further:

In GT6 IIRC, when you install a flat floor, it's simply that, a flat floor or underbelly to the car, no diffusers, no side skirts or nothing else to channel or alter airflow in any way. All it does is create a flatter profile under the car with the intention of "improving airflow". Are you saying that a car with a jumbled mess underneath will be faster (top speed) than one with a basic flat floor?

Yes.
(well, it can, I would never say anything definite on this subject)

For the same reason an un-dimpled golf ball doesn't go as far.

 

[Bo]

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

In the UK, certain secondary schools are registered as colleges. Generally those that are high profile or with historical importance.

I didn't compare a golf ball to a car.
Quote me.

You may not have said it exactly like that, but you gave an example of a golf ball in a discussion about car aerodynamics.

Yes.
(well, it can, I would never say anything definite on this subject)

For the same reason an un-dimpled golf ball doesn't go as far.

 

[rosckolove]

What is the in game description of flat floors?

 

[CSLACR]

You said a car with an undressed underside could go faster than an equivalent with a flat floor, because of the same reason that a dimpled golf ball can travel further:

Yes.

A sphere is super smooth and round, yet it's not the most aerodynamic shape.
Smooth and even don't always mean great airflow, as has been said at least 10 times in this thread already.

What is the in game description of flat floors?

If what was written in the game were decisive, this would probably be going differently.

I hope you're not suggesting that the game could ever be "the decider" though, particularly when speaking about physics.

 

[rosckolove]

I'm just interested in what it says. We are talking about the game, what does the game say they do? Do they do it? My opinion is this.... Besides all of the borderline irrelevant stuff in this thread, flat floors on most of the cars in GT6 should increase top speed.. They clearly don't do that. And they sure as hell don't provide downforce at levels that the game suggesting.

 

[Griffith500]

Yes.

A sphere is super smooth and round, yet it's not the most aerodynamic shape.
Smooth and even don't always mean great airflow, as has been said at least 10 times in this thread already.

And yet making something immediately less "aerodynamic" makes it have lower drag, for a particular set of circumstances, which has also been said several times already. That's what adding dimples to a golf ball does. It's about effecting a regime change, one that isn't pertinent to cars at all.

In the case of a car underbody, smooth and even is the way to go to reduce drag, every time.

In the case of the parts added in the game, there would have to be some serious venturis under the car in order to justify the drag increase (but I don't think that's borne out in the downforce increase). There is a reason "ground effect" aerodynamics are so sought after, even when they are technically "banned" by regulations.

 

[CSLACR]

And yet making something immediately less "aerodynamic" makes it have lower drag, for a particular set of circumstances, which has also been said several times already. That's what adding dimples to a golf ball does. It's about effecting a regime change, one that isn't pertinent to cars at all.

In the case of a car underbody, smooth and even is the way to go to reduce drag, every time.

In the case of the parts added in the game, there would have to be some serious venturis under the car in order to justify the drag increase (but I don't think that's borne out in the downforce increase). There is a reason "ground effect" aerodynamics are so sought after, even when they are technically "banned" by regulations.

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?

 

[NixxxoN]

Not sure what you mean but installing a flat floor increases the car PP a lot... therefore with the same PP, you have a lot less power compared to the same car without flat floor.

 

[eran0004]

I have never ever installed a flat floor, but this discussion is so interesting that I think I might have to and test it myself.

Edit: Still haven't bought a flat floor, but according to the internet it seems like the reason for fitting a flat floor on a car is that it enables the use of diffusers to create additional downforce (diffusers needs a smooth airflow in order to work, and if the air is too turbulent they won't be very effective). The amount of drag the diffusers add depends on the angle and of the overall aerodynamic shape of the car. Generally they are more effective when it comes to downforce vs drag than an external wing, because a wing adds a bigger frontal area, while the diffuser uses a frontal area that is already there.

So based on this I'd guess that the extra drag doesn't come from the flat floor, but rather that the flat floor also means that a diffuser is fitted, which creates the downforce and the additional drag.

How much the extra drag would slow you down depends, of course, on how fast you're going. It should have a much bigger impact on a car that's doing 400 km/h than on a car that's doing 200 km/h. 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.