Flat Floors - (Custom Parts)
- Thread starter Gturbo5
- 71 comments
- 26,818 views
- 1,360
- Vancouver, BC
- Awong124
I tested the flat floor on my fully modified '91 NSX with custom wheels (along with tire bug). Having the flat floor on doesn't show increased downforce in the settings menu, but the difference on track is night and day. The car is much more stable during cornering.
I have an NSX-R, and will agree, it's cake to drive with the flat floor. It tames the beast without neutering it, while adding 20pp and lowering top speed, as well as decreasing acceleration (however slightly).
That said, it still gets around most courses faster without it. Untamed beast is still the fastest, even if it's more of a handful (and has more potential for spins/slides). Even 20pp down, it's still faster.
The downforce wouldn't be reflected in the menu's, it's stand-alone downforce independent of front and rear spoilers, which is what the numbers shown in the menu specifically refers to. Same reason "body downforce" is not reflected anywhere. Some cars in this game make hundreds of kilos of downforce without any aero.
Ferrari 458 for example, 350kilos+ of body downforce near top speed, and about 150kilos around 200kph. A typical (race style) spoiler only gives ~100kilos at 200kph, stock wings are in the 40-60 ballpark (think STI, Supra, NSX, FTO style).
(Of course, I'm talking real life and not the game, just giving a point of reference. I have no idea what scale/system they use in-game.)
That said, it still gets around most courses faster without it. Untamed beast is still the fastest, even if it's more of a handful (and has more potential for spins/slides). Even 20pp down, it's still faster.
The downforce wouldn't be reflected in the menu's, it's stand-alone downforce independent of front and rear spoilers, which is what the numbers shown in the menu specifically refers to. Same reason "body downforce" is not reflected anywhere. Some cars in this game make hundreds of kilos of downforce without any aero.
Ferrari 458 for example, 350kilos+ of body downforce near top speed, and about 150kilos around 200kph. A typical (race style) spoiler only gives ~100kilos at 200kph, stock wings are in the 40-60 ballpark (think STI, Supra, NSX, FTO style).
(Of course, I'm talking real life and not the game, just giving a point of reference. I have no idea what scale/system they use in-game.)
The flat floor is paired with the diffuser. It's possible for a diffuser to choke or stall, creating drag. Witness the 458 Speciale or the LaFerrari's moveable diffuser height to prevent this from happening, to a degree.
It's also worth comparing the affect on top speed between the flat floor-diffuser versus a wing(s) with equal downforce.
It's also worth comparing the affect on top speed between the flat floor-diffuser versus a wing(s) with equal downforce.
- 198
- Frogner
- Coenraad Noorloos
I drove the new SRT Viper against my nephew and i could not find out why he was so much faster then me on Apricothill, since normaly i am the slightly faster one of us two by just a little bit. I simply could not corner with him and he came just way faster out of the corners. Well, as you guesed it, he had a flatbottom.
- 7,671
- Exorcet
- OE Exorcet
This applies more to wings. The flat bottom on a car will lower drag and increase downforce because it removes stagnation points from under the car, such as air hitting the transmission, frame members, etc and because it allows the air to maintain total pressure (http://en.wikipedia.org/wiki/Total_pressure) this allows the fast air under the car to increase in pressure in the diffuser, which leads to lower drag.
Air is typically incompressible at car speeds. It does move faster underneath, but isn't really compressed. Nor does the diffuser expand the air. Air becomes compressible around Mach .3 (~250 mph) although local air speed does not correspond to vehicle speed. At 200 mph for example, air under the front splitter may be exceeding 250 mph, but even in that case, density change is not what is driving the underbody lift and drag.
When the airflow changes direction, as it must to conform to the shape of the car, force is exerted on the air and car, but energy isn't being dumped into the air. Instead the air's potential energy is being converted into kinetic energy. This is why the pressure drops as speed goes up.
While a lower height is generally better, it is not soley ground proximity that generates underbody downforce. In fact, curved underbodies can be better than flat ones since they utilize more than Bernoulli to generate downforce.
Dimples on a golf ball do the opposite and keep the flow attached. The dimples transition laminar flow into turbulent which is less likely to separate. However, using separated flow to lower drag also works. The reason why is because in each case, a different kind of drag is being counted. Laminar flow creates low skin friction drag, but if it separate, high pressure drag is created. Turbulent flow is the opposite, which high friction but low pressure drag if it remains attached. On a flat bottom, separate flow will lower drag, but it will prevent diffuser pressure recovery.
That implies bad design. It shouldn't really happen under normal driving.
I haven't applied a flat floor and/or diffuser on the game to any car, so I'm not sure if they are separate options. If they are, "bad design" doesn't even scratch the surface. A flat floor by itself would likely create lift, even if it did reduce drag.
And the design isn't necessarily bad just because the diffuser stalls near the vehicle's top speed. Consider the track(s) required to test the vehicle's top speed vs the track(s) in which the additional downforce would be beneficial.
- 103
- East Sussex
- perdy450
It sucks you can't put them on 15th Anniversary cars. After seeing how ridiculously quick the new Viper was (the AI) I had actually forgotten I had the Anniversary one myself. I'm sure in the fast corners it would be much better with one, shame I'm saving for a 250 GTO or I'd buy a normal one and try it.
- 1,124
- TexasTyme214
Don't you need to add rake in the suspension to get the downforce you're looking for? I've noticed downforce in the Diablo GT2 by doing so.
- 7,671
- Exorcet
- OE Exorcet
Without the diffuser, you would still get downforce, just not as much. Drag would go up a lot (but it would be better than an open floor without a diffuser).
This simple shape represents a car with a flat underbody and diffuser. If you flip it upside down, you actually get a rough idea of what a flat underbody without a diffuser would look like. The velocity still goes up, so pressure goes down. You would still generate downforce.
Below is a stalled diffuser (and an example of why airdams and diffusers don't mix)
It's still a problem because the diffuser is going to behave the same in both cases for the most part. Below speed where compressibility matters, the flow around the car is approximately speed independent. So if it stalls at max speed, there is a good chance it's going to stall at low speed. Also in a car with a permanent diffuser, you're going to want to design in for a general case, which means you need to consider the weakest link. You'd design it so at max speed it wouldn't stall, unless you were 100% sure the car would never get there.
The first pic appears to show a difference in velocity above and below the splitter, but not the entire body. Isn't this just showing a downforce producing splitter, not a downforce producing flat floor? I'm not familiar with reading these, so excuse my ignorance.
Edit: Also, this diagram not only has a splitter, but a diffuser as well (as you stated). If they can be applied separately in-game, this pic doesn't address the potential issue with their flat floor model.
Edit2: Just reread your post and came to the model flipped upside part. I'm lost.
I'm assuming the low speed stall is acceptable because it can be mitigated by mechanical grip. I'm also assuming the speed limiting diffuser stall is only experienced at say, SS Route X or similar top speed test course. In other words, a max speed you are 100% sure the car cannot achieve on a proper circuit.
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- 7,671
- Exorcet
- OE Exorcet
It might be better to try and think of the image in terms of pressure coefficient. I just happen to have these and did not make them for this thread, which is why they're showing velocity. In terms of CP, the air under the car is at a negative CP and so is contributing to downforce. The air on top of the car is also at negative CP and is contributing to lift. That is not the underbody's fault though. The underbody of a car without a flat floor would have stagnation points and separation bubbles along its length. The stagnation points would cause lift, the separation bubbles would help with downforce, but not as much as attached flow would.
But getting on to clarifying why I posted the image:
I want you to look at it upside down because the shape is so simple that either side could represent a car's underbody. The shape is actually in a "wind tunnel" so there is some "ground effect" on the top as well as the bottom. As you noted, air is accelerated on the top as well as the bottom. So the pressure is being lowered on the top and the bottom. The top half has no diffuser, but still generates low pressure.
The thing is a GT diffuser would likely have to take SSRX into account since. Diffusers/flat panels aren't just for downforce, but for drag as well, and it's easy to have downforce and low drag using panels. There really shouldn't be a drag increase at any point with the panels installed.
Bringing this back a little more to the original questions content, why does the car go slower with the flat floor and diffuser, the thing to remember is that even though a flat floor and a diffuser will reduce drag, they also generate downforce which creates its own set of new variables, and eventually you will get to a point of diminishing returns in either direction.
Ideally, yes, a flat floor will reduce drag by a reduction in laminar flow disruption, but when you have a diffuser that is generating a low pressure area at the throat, and sucking air into it generating a low pressure under the car, that will induce a drag effect. It is less than a wing in the air, but it is still there. There is no such thing as "free downforce".
Likewise, a diffuser will help to reintroduce more stable air into the drag wake of the car, in effect reducing the drag wake, and reducing drag. However, the flip side of that is, depending on the cars shape, its horsepower, and how the air off the top of the car is separating off the back deck, the introduction of that air fromt he diffuser can create some very powerful vortices, which can sometimes be more parasitic than a drag wake itself, as again, there comes a point of diminishing returns.
And none of this even takes into effect the increased rolling resistance that any amount of downforce will also add.
Racing teams spend millions on CFD and wind tunnel testing to try and mitigate these factors for the speeds and tracks their cars will run on, and it is always a matter of compromise. Find enough downforce to go around the corners at competitive speed, reduce enough drag to maintain competitive speeds on the straightaway, and hope you are on the positive side of the curve in both cases, and not the negative side with a dog for a car that cant be fixed overnight.
Bottom line is, if you are making downforce, you are making drag, and drag will reduce speed, unless and until you can apply more horsepower. And if you have more horsepower that translates into more speed, that in turn means more drag, and more air to push out of the way. It is literally an exponential problem, with no end.
Classic example is a Bugatti Veyron. It only takes about 350 BHP to get the car to 200 MPH, it takes the other 700BHP to get it that extra 50 MPH because of the wind resistance and drag. And that is a car that was massaged in a wind tunnel from the get go as an entire package capable of 240+ MPH speeds, not a road car with a flat floor and a diffuser slapped on it.
Think about it..
Ideally, yes, a flat floor will reduce drag by a reduction in laminar flow disruption, but when you have a diffuser that is generating a low pressure area at the throat, and sucking air into it generating a low pressure under the car, that will induce a drag effect. It is less than a wing in the air, but it is still there. There is no such thing as "free downforce".
Likewise, a diffuser will help to reintroduce more stable air into the drag wake of the car, in effect reducing the drag wake, and reducing drag. However, the flip side of that is, depending on the cars shape, its horsepower, and how the air off the top of the car is separating off the back deck, the introduction of that air fromt he diffuser can create some very powerful vortices, which can sometimes be more parasitic than a drag wake itself, as again, there comes a point of diminishing returns.
And none of this even takes into effect the increased rolling resistance that any amount of downforce will also add.
Racing teams spend millions on CFD and wind tunnel testing to try and mitigate these factors for the speeds and tracks their cars will run on, and it is always a matter of compromise. Find enough downforce to go around the corners at competitive speed, reduce enough drag to maintain competitive speeds on the straightaway, and hope you are on the positive side of the curve in both cases, and not the negative side with a dog for a car that cant be fixed overnight.
Bottom line is, if you are making downforce, you are making drag, and drag will reduce speed, unless and until you can apply more horsepower. And if you have more horsepower that translates into more speed, that in turn means more drag, and more air to push out of the way. It is literally an exponential problem, with no end.
Classic example is a Bugatti Veyron. It only takes about 350 BHP to get the car to 200 MPH, it takes the other 700BHP to get it that extra 50 MPH because of the wind resistance and drag. And that is a car that was massaged in a wind tunnel from the get go as an entire package capable of 240+ MPH speeds, not a road car with a flat floor and a diffuser slapped on it.
Think about it..
- 7,671
- Exorcet
- OE Exorcet
Again this pertains more to wings. Anything generating lift will have induced drag to some degree most likely, but the underbody is in ground effect which substantially weakens the induced drag, and the entire point of the diffuser is pressure recovery.
The throat is the only area of the diffuser where pressure goes down, outside of a few special cases. After the throat, pressure goes up. downforce goes down, and drag goes down.
You can see above that the pressure very quickly rises after the throat. Because of the diffuser geometry, this pressure rise goes into pushing the car. You get additional downforce from the throat, and reduced drag from the rear diffuser surface.
One of the special cases, I was mentioning earlier, vortex driven diffusers might induce drag, but usually you kill the vortex in the diffuser. This is very likely true of the kind of diffusers you see in GT since the cars they're applied to are relatively fast and don't need additional low pressure from underbody vortices.
It's not a lot.
Overall yes, but it's not difficult to design an underbody that is better in terms of drag and lift compared to an open underbody or one without a diffuser.
The entire point of a diffuser is to generate downforce, and provide flow control. The pressure recovery is the byproduct of the physics of fluid dynamics behind the application, that doesn't mean it is the "entire point".
That is not to say that some diffusers are used primarily for that reason, however. As the diffuser section on a Toyota Prius is a prime example of using a flat floor and diffuser to control the drag wake, and manage under body flow through pressure recovery. However, in a racing application, the primary goal is always downforce first, flow control second, and any other benefits that come are an added bonus.
The throat is the only area of the diffuser where pressure goes down, outside of a few special cases. After the throat, pressure goes up. downforce goes down, and drag goes down.
You can see above that the pressure very quickly rises after the throat. Because of the diffuser geometry, this pressure rise goes into pushing the car. You get additional downforce from the throat, and reduced drag from the rear diffuser surface.
Again, you make a completely erroneous statement, and even prove yourself wrong with the pic you post. If you look at the pressure reading, yes, the throat is were the highest center of low pressure is located. However, if we look forward of the throat, we can also see that the pressure rating there is light blue. Why? Because the diffuser throat, once it begins to produce the low pressure area, will generate a scavenging effect underneath the floor of the car, and a complete low pressure air pocket is generated on the flat floor. This is why in Formula one, the teams focus so heavily on creating vortexes from the leading edge of the flat floor at the radiator inlet, to attempt to seal the sides of the car to help extend the low pressure area forward further.
The pressure rise does not "push" the car. I don't know where you get that idea from. There is no such effect. Anything that is "pushing" the car is called "drag" because you are having to tow it behind you. Again, if you look at the color of the pressure readings, you will see that the air exiting the diffuser, is still coming out at a far lower pressure than the "clean" air pressure presented at the front of the car. This all follows basic principals of fluid dynamics, as velocity decreases, so does the pressure increase. But that higher pressure air out the back, has lost so much acceleration that there is no "push" effect. What velocity it does have, will simply help it to reintegrate with the flow over the top of the car, to help reduce drag wake.
I think your confusion comes from looking at where the diffuser flow "narrows" at the rear. It is not a hose nozzle. This is simply flow control. What you CANT see in this angle, is that though the diffuser flow is narrowing horizontally, it is still expanding on the ramp vertically. This is why you don't see any significant pressure change in the flow until it reaches the exit. This is to prevent separation from the diffuser ramp, and to prevent unwanted vortices. There are several formulas from NACA that calculate this shape for different effect. If you were generating a "push" from the diffuser, you would need to compress the air to increase its velocity, by increasing its pressure...if you did that, you wouldn't have any laminar flow through the diffuser, and it would stall the entire floor. The fact is, you only get as much air, and as much energy, as you put in. This again is basic physics. It is also why F1 teams spent so much time investing in "blowing" the diffuser with exhaust gasses, to try and energize it further, and promote the vacuous effect even at low speed.
Amy diffuser will generate vortices, the question is where, and how big. Vortex control through strakes, and sideboard shape is most common. The further off the car you can generate the vortex, the better.
No, it isn't, but it will have a greater effect on limiting the top speed of the car, than any "acceleration push" out the back of the diffuser will.
That depends. Are we talking about putting a splitter, flat floor, and diffuser onto a car with bad under flow management, and aerodynamics? Or are we talking about a car without a flat floor that has spent time in a wind tunnel to help mitigate lift, and control underbody airflow?
For example, a Nissan 370Z, with its front splitter, and no flat floor, will manage air better than a Nissan Sentra with a flat floor and diffuser. Neither of which will be as good as a Nissan GT-R which was designed from the start to have a flat floor and a diffuser, and utilize it to its fullest extent.
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- 7,671
- Exorcet
- OE Exorcet
It took me a little while to figure out how I wanted to reply. We agree on somethings but not on others. It seems as though we agree that a diffuser can increase downforce and lower drag. I certainly think this can happen in the same diffuser design. I'm assuming you do as well, but I could use clarification. One big disagreement is this: you state that "as velocity decreases, so does the pressure". This is wrong, unarguably.
http://en.wikipedia.org/wiki/Bernoulli's_principle
I would say it's the other way around in most cases. The downforce is a byproduct of using the diffuser for pressure recovery. Even in race cars meant to take corners over going fast in a straight line, drag is still a concern. Race cars use diffusers to reduce drag, but it just so happens that diffusers can increase downforce at the same time. I don't think this point is worth spending a lot of time on. You could certainly argue one way or the other.
I'm not sure what you're saying here with the air pocket. The pressure is low under the flat floor because the air is being forced into a constricted cross sectional area and the speed of the air is being kept up by the moving ground under the car (this is a simulation with a moving ground plane). You can see that there is an inlet ahead of the flat section, a strip of high pressure (yellow/orange) air is created where the air partially stagnates on the inlet. You can see even stronger flow acceleration and pressure drop at the nose of the car where there is a sizeable dark blue region without a diffuser of any kind. The nose pressure is heavily influence by the surround low pressure under the wings though. So if you look along the center of the car, air comes from the left, hits the nose and slows down. This means pressure goes up. Some air is forced under the nose. It speeds up because it is being forced through a smaller cross sectional area. This speeding up leads to lower pressure (and hence downforce since this is under the car). The air slows again as it gets to the underbody inlet but is quickly reaccelerated under the flat floor. At the diffuser throat it is accelerated again, but then begins to slow down and increase in pressure. That increase in pressure translates to lower drag. If there was no diffuser there and the floor continued flat, you would have a much larger wake.
The low pressure ahead of the throat is aided by the throat pressure, but it does not exist because of the throat and would still be there if the diffuser was not present.
I say that the pressure rise pushes the car because of how pressure works. It only ever exerts a normal force. The surface normal on the diffuser points away from the direction that the car is traveling (for the most part). So the pressure exerted by the air is pointing in the direction of travel. This is true no matter what the pressure is, so you could technically argue that even extremely low pressure air pushes the car forward. However the pressure rise in the diffuser is intentional, so I think it's fair to think of it as a push.
The diffuser doesn't narrow. In the first image it should be clear, it's basically a wedge shape of constant width. What is happening in the second image is that the wake off the rear wheels is being sucked under the car. It's a bit of an issue with this diffuser since that stalled air is limiting pressure recovery. The vertical expansion of the diffuser aera is not to prevent separation. In fact it will tend to encourage separation. The increasing of the height is limited because of this risk. The reason why the diffuser grows in height is to increase pressure and lower drag. It also gives the throat curvature, which is why the throat is a low pressure region.
You never want to increase velocity in the diffuser after the throat if you're trying to reduce drag. Higher velocity leads to lower pressure pushing the back of the car, which means higher drag.
I assume "vacuous" to mean viscous, but that is not helpful in the diffuser at all. Higher viscoity means more drags. Blown diffusers simply expoit the fact that higher velocity leads to lower pressure.
Yes, a vortex is unavoidable in the diffuser, but you can mitigate it. By vortex driven diffuser, I meant something that is basically housing a vortex that takes up it's entire volume. Also, you would want the vortex as close to the surface as possible so that it induces the lowest possible pressure.
The pressure recovery in the diffuser would probably outweigh the increase in rolling resistance from the downforce gained by the diffuser alone, but this could vary case by case. I doubled checked quickly using engineeringtoolbox to calculate some rolling resistance numbers and estimated some drag stuff myself. A car with a 18 ft^2 frontal area at 200 mph with a .4 CD generates 735 lbf drag. If you reduce that CD by 10% (to .36) the drag is cut by 74 pounds. That's worth 1475 pounds of downforce rolling resistance on your average tire.
http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
I'm not sure what you're getting at here. The Sentra probably has a comparable or better CD than the Z if both are stock and would get even better with a flat floor and diffuser if the Z was refused both. The GT-R isn't particularly special compared to the other two and I don't see what advantages it would have over them. It might possibly have more air funneled under the car to allow the underbody to do more work, but the other two could simply recover pressure on their upper surfaces.
http://en.wikipedia.org/wiki/Bernoulli's_principle
I would say it's the other way around in most cases. The downforce is a byproduct of using the diffuser for pressure recovery. Even in race cars meant to take corners over going fast in a straight line, drag is still a concern. Race cars use diffusers to reduce drag, but it just so happens that diffusers can increase downforce at the same time. I don't think this point is worth spending a lot of time on. You could certainly argue one way or the other.
I'm not sure what you're saying here with the air pocket. The pressure is low under the flat floor because the air is being forced into a constricted cross sectional area and the speed of the air is being kept up by the moving ground under the car (this is a simulation with a moving ground plane). You can see that there is an inlet ahead of the flat section, a strip of high pressure (yellow/orange) air is created where the air partially stagnates on the inlet. You can see even stronger flow acceleration and pressure drop at the nose of the car where there is a sizeable dark blue region without a diffuser of any kind. The nose pressure is heavily influence by the surround low pressure under the wings though. So if you look along the center of the car, air comes from the left, hits the nose and slows down. This means pressure goes up. Some air is forced under the nose. It speeds up because it is being forced through a smaller cross sectional area. This speeding up leads to lower pressure (and hence downforce since this is under the car). The air slows again as it gets to the underbody inlet but is quickly reaccelerated under the flat floor. At the diffuser throat it is accelerated again, but then begins to slow down and increase in pressure. That increase in pressure translates to lower drag. If there was no diffuser there and the floor continued flat, you would have a much larger wake.
The low pressure ahead of the throat is aided by the throat pressure, but it does not exist because of the throat and would still be there if the diffuser was not present.
I say that the pressure rise pushes the car because of how pressure works. It only ever exerts a normal force. The surface normal on the diffuser points away from the direction that the car is traveling (for the most part). So the pressure exerted by the air is pointing in the direction of travel. This is true no matter what the pressure is, so you could technically argue that even extremely low pressure air pushes the car forward. However the pressure rise in the diffuser is intentional, so I think it's fair to think of it as a push.
The diffuser doesn't narrow. In the first image it should be clear, it's basically a wedge shape of constant width. What is happening in the second image is that the wake off the rear wheels is being sucked under the car. It's a bit of an issue with this diffuser since that stalled air is limiting pressure recovery. The vertical expansion of the diffuser aera is not to prevent separation. In fact it will tend to encourage separation. The increasing of the height is limited because of this risk. The reason why the diffuser grows in height is to increase pressure and lower drag. It also gives the throat curvature, which is why the throat is a low pressure region.
You never want to increase velocity in the diffuser after the throat if you're trying to reduce drag. Higher velocity leads to lower pressure pushing the back of the car, which means higher drag.
I assume "vacuous" to mean viscous, but that is not helpful in the diffuser at all. Higher viscoity means more drags. Blown diffusers simply expoit the fact that higher velocity leads to lower pressure.
Yes, a vortex is unavoidable in the diffuser, but you can mitigate it. By vortex driven diffuser, I meant something that is basically housing a vortex that takes up it's entire volume. Also, you would want the vortex as close to the surface as possible so that it induces the lowest possible pressure.
The pressure recovery in the diffuser would probably outweigh the increase in rolling resistance from the downforce gained by the diffuser alone, but this could vary case by case. I doubled checked quickly using engineeringtoolbox to calculate some rolling resistance numbers and estimated some drag stuff myself. A car with a 18 ft^2 frontal area at 200 mph with a .4 CD generates 735 lbf drag. If you reduce that CD by 10% (to .36) the drag is cut by 74 pounds. That's worth 1475 pounds of downforce rolling resistance on your average tire.
http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
I'm not sure what you're getting at here. The Sentra probably has a comparable or better CD than the Z if both are stock and would get even better with a flat floor and diffuser if the Z was refused both. The GT-R isn't particularly special compared to the other two and I don't see what advantages it would have over them. It might possibly have more air funneled under the car to allow the underbody to do more work, but the other two could simply recover pressure on their upper surfaces.
- 6,318
- West Virginia
- HBR-Roadhog
- HBR Roadhog
Well not all cars will be faster when you add parts to them unless of course you take the time to dial them in properly. Some cars do not need a rear wing and adding one will just cause the car to understeer and make your laps times slower.
If you properly tune the springs, roll bars, dampers and such to take advantage of the areo that was added then your lap times should be faster but if not then they very well may be slower
JEMcG
(Banned)
- 639
- Glasgow
It gives you more overall downforce. Try it on Monaco and you will kill it.
It's not necessarily the diffusor, some cars get the flat floor with no change to the rear. It just means the air flow is cleaned up and sped up creating more downforce. Similar to how the spoilers work, the air going under is faster than the air going over it.
It's a great addition and if people don't know about it you could kill it at twisty tracks against people online.
It's not necessarily the diffusor, some cars get the flat floor with no change to the rear. It just means the air flow is cleaned up and sped up creating more downforce. Similar to how the spoilers work, the air going under is faster than the air going over it.
It's a great addition and if people don't know about it you could kill it at twisty tracks against people online.
Clearly it did take a while. You must have quoted right before i edited that very line. When you are attempting to do things on your phone, it doesnt work so well. So once i had access to a computer, i took the time to revise that line. The new, and correct line(s) is...
"This all follows basic principals of fluid dynamics, as velocity decreases, so does the pressure increase. But that higher pressure air out the back, has lost so much acceleration that there is no "push" effect. What velocity it does have, will simply help it to reintegrate with the flow over the top of the car, to help reduce drag wake."
As a result of typing on my phone...i lost a word.
In so far as reducing drag and generating downforce all in the same design...no, i dont believe that is possible. Again using the Prius as an example, the expansion area of the diffuser is very shallow, it is there purely to keep the flow under the car from stagnating. There is no downforce produced, but it does much to reduce the coefficient of drag. When you produce a diffuser that is specifically tuned for high levels of downforce, and exploiting a very strong ground effect, you will have a much higher Lift to Drag ratio, and you will produce a not insignificant amount of drag.
For example, here is a basic diffuser flow chart, plotting drag, and lift (in this case downforce), and the lift to drag ratios.
For the most part the drag is relatively static. But as the lift increases, so does the drag, even marginally. You can also notice the effect of diminishing returns, and how tuning the diffuser for certain speeds makes a big difference in its efficiency.
It should be noted that this chart is IN the diffuser, and does not take into account the airflow AFTER the diffuser. I.E. where it is reintroduced back into the stream of the car, which can make a HUGE difference in how the drag wake is handled.
We will indeed have to agree to disagree on this. As i said in the original post, any part of a car is about compromise. If you can produce a reasonable amount more downforce, without imparting a detrimentally measurable amount of drag, they will do it. But its no point to reduce drag, if there is no downforce to actually race with. It is all about hunting at the top of that bell curve.
This isnt really accurate. A flat floor, by itself, unless heavily raked, will not produce downforce on its own. It must have some form of expansion, to reduce velocity, to generate downforce. For example...
As you can see, a flat floor by itself wont do much at all.
When you add in a diffuser to the mix, the expansion generates a low pressure area at the throat, and that low pressure area must be fed, so the the air under the flat floor literally gets sucked into the diffuser. This vacuum effect is what produces the low pressure area under the cars flat floor, and adds a not insignificant amount of overall downforce to the car. This again is why F1 trys to use vortices to seal the sides of the cars, in order to exploit this effect further.
Judging by the shape of the nose...
It actually does have a small diffuser on the nose. The two inboard wing boards, act as strakes collecting and forcing air under the nose. The nose probably sweeps up slightly, which creates your expansion area...instant diffuser. This was being exploited in F1 by Ferrari by placing the cameras low on the center section of the neutral wing. It became an open "stepped" diffuser. A technique often used on endurance prototypes to skirt diffuser rules. The Audis are almost famous for it. Also part of the pressure increase after the nose section, is because the floor opens up, as barely visable in this shot.
All i can say about that entire group of statements, is that, while they are relatively sound technically (in terms of acceleration, and pressure zones), the application of why, what, and how is completely incorrect. And here is a velocity comparison chart to prove it.
You are correct that the flat floor will produce a larger drag wake, you can see how far out from underneath the flat floor it extends, and how the low velocity air extends up into the drag wake causing a significant amount of induced drag.
But you can also clearly see that without the diffuser, there is no acceleration of the air under the car, and without that acceleration, there would be no low pressure, and thus no downforce. The low pressure ahead of the diffuser is complete because of the diffuser, and generated by it in its entirety. The only time this changes, is if the flat floor is raked.
No, totally wrong. So totally wrong. While the air in the diffuser is an expansion, and it is a high pressure zone, we need to think of it in relative terms. In so much that the air under the car is in a negative pressure, the air coming out of the diffuser, while still technically "positive pressure" in relation to the "negative pressure" at the throat and under the flat floor, is still not at atmospheric pressure, and is no "positive pressure" over atmospheric pressure.
Again using your graphic...
The green lines represent air pressure at atmosphere. The red locations are were the air has been compressed, generating high pressure. The blue shades are were the air has been accelerated, generating low pressure. Now if we look at the flow trajectory out of the diffuser, you will notice it is light green. A light green, shading towards blue. This means that it is still not at atmospheric pressure, meaning it is still a low pressure area in relation to atmosphere. Low pressure behind the car = drag. And while it is still higher pressure than the drag wake, its still low pressure, and it is still not "pushing" the car. This is junk science.
When i make that statement about the narrowing diffuser, i didnt realize that both images were if the same Formula SAE car. That realized, i can clearly see where the flow pattern is that way because of the air management (or lack of) around the wheels.
I think you missed what i said about the diffuser height in that context. But that OK, you might as well disregard it, because what i said has everything to do with a design that isnt anything like this one. But i highly recommend you look up the NACA formulas, they are fascinating, and really fun to play with in CFD.
We already know why "pushing" on the car is a non-thing, so i will skip the first bit.
And i meant "Vacuums", again going back up to why the diffuser generates a suction effect, we can see here that when a diffuser is blown with exhaust gasses, it introduces high velocity air into the leading edge of the diffuser, which in turn expands, just like air would, it generates the same vacuum effect under the car at the throat. This is what is referred to as "energizing the diffuser", by artificially blowing air into it to continue producing low pressure at the throat, even at sub optimal speeds.
Depends on the shape of the diffuser. Often times the introduction of strakes into the diffuser is specifically to reduce the volume lost to pressure shift because of large vortices. By controlling the air in the diffuser with the strakes, you can utilize more of the available volume of the diffuser, and improve its efficiency significantly.
It will always vary case by case. Again using the Prius, it will not generate hardly any downforce, and there will be a significant pressure recovery. On a RedBull F1 car, the pressure recover will be no where near as high, as they are willing to accept some drag for the benefit of improved downforce. Infact, examining the RedBull diffuser shape is a thesis paper all on its own. But again i still stand by the fact that there is no pushing effect from the diffuser, and downforce will reduce rolling resistance, not dramaticly, but where friction increases, so does drag.
The greenhouse design of the Sentra will cause all sorts of flow separation over the back of the car, and subsiquently leave a massive drag wake. I am not sure any amount of flat floor or diffuser can overcome that. It has a stock DC of .34. The Z uses a Kommback shape which already gives it an advantage, it has also spent time in a wind tunnel to shape its specialized aero to produce zero lift (not downforce, just counter acting lift) at speed No doubt those bits are adding some drag, it is without a true flat floor, and has a CD of .30. The GTR, no introduction needed, rests at .26, and if that isnt impressive enough, a Prius is .24.
Sorry this post got to long, and out of hand. Not trying to bust your chops, hopefully we can all walk away more knowledgeable, and for the better.
- 76
- United Kingdom
Hey guys, I just find out about this "issue" today.
I usually installed flat floors (FF) on all my cars when I fully tuned them and I thought the extra PP it added was justified in making the car faster. I was wrong, my cars are slower in both top speed and actually lap times.
I run Time Trials on the Nordschleife in the Open category > https://www.gtplanet.net/forum/threads/gt6-nürburgring-lap-time-board.294583/page-6#post-9192682
I'm now on my 3rd car to uninstall them and get a much better lap time.
I don't know how this works on other tracks, but on Nordschleife I won't use FF on any car.
I usually installed flat floors (FF) on all my cars when I fully tuned them and I thought the extra PP it added was justified in making the car faster. I was wrong, my cars are slower in both top speed and actually lap times.
I run Time Trials on the Nordschleife in the Open category > https://www.gtplanet.net/forum/threads/gt6-nürburgring-lap-time-board.294583/page-6#post-9192682
I'm now on my 3rd car to uninstall them and get a much better lap time.
I don't know how this works on other tracks, but on Nordschleife I won't use FF on any car.
- 7,671
- Exorcet
- OE Exorcet
Fair enough, it happens.
The Prius diffuser likely does produce downforce, just not a significant amount. By the act of directing the air upward, a downforce on the car must be produced. However if the upward motion induced is very small, so is the force.
Downforce and drag are linked in the diffuser because it uses both curvature and area expansion. Curvature will lower the pressure (leading to downforce). Expansion leads to increased pressure.
For example, here is a basic diffuser flow chart, plotting drag, and lift (in this case downforce), and the lift to drag ratios.
For the most part the drag is relatively static. But as the lift increases, so does the drag, even marginally. You can also notice the effect of diminishing returns, and how tuning the diffuser for certain speeds makes a big difference in its efficiency.
It should be noted that this chart is IN the diffuser, and does not take into account the airflow AFTER the diffuser. I.E. where it is reintroduced back into the stream of the car, which can make a HUGE difference in how the drag wake is handled.
The chart makes sense, but it was my understand that we were implicitly trying to account for the airflow after the diffuser in someway since the point of disagreement was the performance impact adding the flat floor/diffuser combo in GT6.
I am arguing that the downforce that comes with the floor + diffuser should not come with a drag penalty.
You are arguing that the drag penality in game is fair, correct?
This isnt really accurate. A flat floor, by itself, unless heavily raked, will not produce downforce on its own. It must have some form of expansion, to reduce velocity, to generate downforce. For example...
As you can see, a flat floor by itself wont do much at all.
One problem here is that the shape does not really represent a car. Though even in the upper image, there is sub ambient pressure under the wedge. What is missing here is the large stagnation zone at the front of the car.
Here you have a more car like shape and there is clear acceleration of the air under the nose. Your diffuserless wedge is not generating a large enough stagnation zone to push air under the car. Yes the diffuser helps by generating its own low pressure zone, which then propagates the pressure upstream and forces more air under the car, but the same effect could be created by changing the nose.
The nose is actually flat underneath though I don't have an image showing that.
The high detail model is different from the CFD model, which is simpler. Look at the suspension components in each.
All i can say about that entire group of statements, is that, while they are relatively sound technically (in terms of acceleration, and pressure zones), the application of why, what, and how is completely incorrect. And here is a velocity comparison chart to prove it.
You are correct that the flat floor will produce a larger drag wake, you can see how far out from underneath the flat floor it extends, and how the low velocity air extends up into the drag wake causing a significant amount of induced drag.
But you can also clearly see that without the diffuser, there is no acceleration of the air under the car, and without that acceleration, there would be no low pressure, and thus no downforce. The low pressure ahead of the diffuser is complete because of the diffuser, and generated by it in its entirety. The only time this changes, is if the flat floor is raked.
As before the wedge nose is unrealistic here, which is causing a problem. There is also no label of the freestream velocity, so there is no way of know whether or not flow is accelerating under the car (but if it is, it will not accelerate much).
If you are generating the images above, I'd ask that you retry with a more car like shape. I would, but I do not have access to CFD software that I can use to share images at this time.
It certainly won't be at atmospheric pressure due to viscous effects, but on a very aerodynamic shape it can come close. The car open wheel car image I posted is not such a shape.
If you want to define "push" as requiring atmospheric or greater pressure, then no the diffuser will not push anything. I was defining push not with regard to pressure, but with design intent. I was also thinking of the pressure distribution in comparison to the car without the diffuser. Certainly, adding the pressure rise from the diffuser will lower the drag on the car, agreed?
You're trying to put a definition on "push" that is not there. Now I will admit that I used it loosely and not with any specific completely defined meaning, but that doesn't mean you can replace my definition with one of your own and have it be more correct.
What is happening, is that the diffuser is a surface that has pressure exerted on it. That pressure is higher than the pressure that would have been in that region if the diffuser did not exist at all. So it is safe to say that the diffuser is lowering drag, at least locally.
Agreed. The blow diffuser simply increases the effective velocity seen by the diffuser over the actual freestream velocity.
I would argue that the F1 would have higher pressure recovery (as in a large pressure gradient after the throat) simply because the diffuser is more aggressive. The F1 is draggier overall because it is open wheeled and saddled with drag from all the wings.
With concerns over fuel economy, mass separation isn't really an option for most manufacturers. I don't know what the flow around a Sentra looks like, but I doubt it includes huge separation. A quick Google search says below .3 for CD. Assuming the car has an open floor, a flat floor will have obvious benefits and a diffuser could reduce the wake size and allow for a cleaner wake. They don't need to overcome what's going on at the top of the car, they will reduce drag either way.
I'm glad is was long, this is the type of discussion that you find on GTP that makes me come here. Very much agreed at coming at better for it once this is all said and done, but I don't mind if it drags out for longer.
- 308
- Nikk O'lass
Something needs to be pointed out...a pure flat floor doesn't increase down force, it actually make's the car more like a aircraft wing...increasing lift, a flat floor needs to work in conjunction with a diffuser in the rear of the car.
A cars shape naturally creates lift, the same way wing on a plane does, high pressure air underneath, low on top. This is due to the airspeed. The slow slow airspeed under the car goes a shorter distance, the speed on top is force over the car faster, over a longer distance.
A diffuser draws the high pressure from under the car, turning the air into low pressure, thus creating suction. Some flat floors channel the from the front to the rear to eliminate the pressure altogether.
A cars shape naturally creates lift, the same way wing on a plane does, high pressure air underneath, low on top. This is due to the airspeed. The slow slow airspeed under the car goes a shorter distance, the speed on top is force over the car faster, over a longer distance.
A diffuser draws the high pressure from under the car, turning the air into low pressure, thus creating suction. Some flat floors channel the from the front to the rear to eliminate the pressure altogether.
- 904
- York Virginia
- chilybee
I suspect it's best usage is to stabilize a very fast but twitchy car. The Enzo (as it was in GT5) comes to mind.
I haven't driven one in GT6. Same for McLaren F1.
P.S Interesting exchange of information. But what is the bottom line for flat floor in real life? Faster car, more stable car or both? I have no clue, but suspect stability.
I haven't driven one in GT6. Same for McLaren F1.
P.S Interesting exchange of information. But what is the bottom line for flat floor in real life? Faster car, more stable car or both? I have no clue, but suspect stability.
