Why do 250 mph (400 km/h) supercars have 0.40+ Cds?

[chillsworld]

Originally Posted by NHB

Those numbers were taken from Mercedes website (german). Most likely they are for basic model without any extra trim or spoilers. AMG-model has a bit different shape for outer decorstion for sure, but that doesn't explain a difference that huge.

I was trying to illustrate that each of those cars has a "base model" that is different. Some of them, seem to be far different from one another in terms of aerodynamics and the generation of down force. I mean one of them has radiator blocks to increase aero, but I could be entirely wrong . And simple exterior differences can make a large imapct on CD... Sometimes an increase and sometimes a decrease. Generating down force is not the same as reducing drag for FE. I don't claim to know what causes the difference in CD on these Mercedes, and you may be entirely correct, but here are some examples of how "little" differences make a big impact one way or the other. This is on the front of the car.

Data from modeling...
1. Stock 1990-1997 Mazda Miata
2. Stock 1990-1997 Mazda Miata at a 4in Ride Height
3. Small Front Air Dam at 4in Ride Height
4. Small Air Dam with Splitter at 4 in Ride Height
5. Large Air Dam at 4in Ride Height
6. Large Air Dam with Splitter at 4in Ride Height
Note: The air dam and/or splitter is 2 inches off the ground in study 3-6


The models illustrate how "simple" changes can drastically impact down force and drag at 100 mph... Much less 200+ mph. Creation of down force can cause a lower OR a higher CD value depending on how it is done. Taken from Splitter or Air Dam – Which Design is Best? | Hancha Blog.

Similarly, rear diffusers that are designed for down force, often have the opposite impact of those designed for better FE via lower CD. Most designs increase drag exponentially as the angle of the diffuser increases (gets steeper), and the angle is often quite steep in order to create more down force.



Read the following description, the diffuser only created 55lbs of downforce @175mph with an increase of .002 in CD... How much would the CD increase if it was to generate 200 or 300lbs of down force on the same car? A modern F1 race care produces over 1000lbs of down force via the rear diffuser alone, just as an example of how race cars and super cars are designed differently from normal cars.

"The Werks1 diffuser was designed using the most state of the art engineering and computer modeling techniques available today. Using these processes, a functional prototype was created and wind tunnel tested in order to verify the effectiveness of the part. During testing, it was proven that a stock 997.2 (with wing raised) produced over 40lbs of lift at the rear axel, at 175mph. With the rear wing raised and the new Werks1 diffuser installed, that number was completely eliminated, and the test vehicle not only showed no signs of lift, but it now produced 14 lbs of usable downforce, again at the rear axle. When measured at both the front and rear axles, the stock 997.2 showed nearly 110 lbs of total lift. Once the Werks1 diffuser was added, total lift was reduced to just below 75lbs. Best of all, the addition of the Werks1 rear diffuser increased the vehicle's coefficient of drag (or Cd) by only .002, from .313Cd to .315Cd. What this means is that the downforce created by the Werks1 rear diffuser is essentially "free", with no negative side-effects to be found."
From Werks1 - Porsche 997.2 Carbon Fiber Rear Diffuser

I didn't say that it's only cooling. Wide tyres have also a major effect. And of cource there are many other thing but those two are always major reasons for high Cd values.

Sorry if I implied that you thought it was only cooling, I was just trying to illustrate that little aerodynamic differences can create big differences to CD and the generation of down force at high speeds. Whether that is the case with the CLA, is unknown to me... But I feel like it would make sense

Originally Posted by freebeard

Whatever the reason, 400mph (644kph) cars *don't*.

But do they turn at 400mph???

~C

[NHB]

Originally Posted by chillsworld

I was trying to illustrate that each of those cars has a "base model" that is different. Some of them, seem to be far different from one another in terms of aerodynamics and the generation of down force. I mean one of them has radiator blocks to increase aero, but I could be entirely wrong . And simple exterior differences can make a large imapct on CD... Sometimes an increase and sometimes a decrease. Generating down force is not the same as reducing drag for FE. I don't claim to know what causes the difference in CD on these Mercedes, and you may be entirely correct, but here are some examples of how "little" differences make a big impact one way or the other. This is on the front of the car.

Many models are similar in that sense and it doesn't make any sense at all to offer inferior aero parts in more expensive car. I doubt that Mercedes has engineered downforce in those models at least if we don't count AMG model. This increase of Cd value hand in hand can be seen in many other cars. It is actually the Newton's forgotten law.

[chillsworld]

Originally Posted by NHB

Many models are similar in that sense and it doesn't make any sense at all to offer inferior aero parts in more expensive car. I doubt that Mercedes has engineered downforce in those models at least if we don't count AMG model. This increase of Cd value hand in hand can be seen in many other cars. It is actually the Newton's forgotten law.

Inferior? I don't believe that any model has "inferior" aero parts. They are just aero parts which have different goals. Aero parts designed for FE aren't inferior to those designed for downforce/venting. Just as those designed for down force/venting aren't inferior to ones designed for FE.

Why would we not include the AMG? You listed it as an example, it should be included. If we don't include the AMG then we shouldn't include the BlueEFFICIENCY Edition since it has changes made specifically for aero like the AMG does. IF we do that, we are only looking at a .04 increase in CD between the CLA 180 FWD (90kw) and the CLA250 4Matic AWD (155kw) models. And from what I gather, the underbody is different due to having a drive shaft and driven rear wheels which would impact drag compared to the FWD version... Of course I can't find comparison pictures for the two underbodies to see if this is true. I also assume AWD would lead to more heat/venting/drag than the FWD version.. Another factor that isn't taken into consideration when looking at CD relative to engine output.

0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)
0,25 CLA 180 90 kW
0,29 CLA 250 4MATIC 155 kW
0,32 CLA 45 AMG 4MATIC 265 kW

And I agree, it can be seen hand in hand in other cars... Just as ground effects, diffuser, exhaust, underbody, airdam differences can be seen in many of those cars. I understand the science and theories behind heat exchange and drag... I don't deny that it exists. But I'm a hands on kind of person, lets actually look at some differences in models.


CLA 180:
Front end specific FE based aero designs, wheels with lower drag



AMG 4MATIC
Massive air intakes with functional winglets, splitter, and non optimized front apron, drag creating 18" 5 spoke wheels


Again, not saying it isn't heat exchange and "tires" that cause the difference in CD... I just find it hard to believe that those are the biggest causes when I look at the differences in design on these particular vehicles side by side. Especially after reading the projects on this site where people have done simple tweaks to decrease CD for better FE. Tweaks that are the opposite of what Mercedes did on the AMG (bigger intake/vents and larger open tires for example).


Sorry to the original poster for kind of high jacking this thread... I still stand by the generation of down force for application of power and safety with handling at speed

~C

[serialk11r]

I still think tires is the biggest difference. Porsche uses the exact same bodies across a lot of models. Between the Cayman S and non-S for example, the drag is the same, but the S is over an inch lower. Lowering 1 inch takes away as much drag as adding 1 inch of tire (mainly in the back). I think the number cited for each of these changes is about 0.01 Cd, which is quite a lot (3% of the whole car's drag).

Same story with the 911, you can see the 4S models with their fat tires having a couple hundredths of Cd added, but it's only an inch or so of tire width making that difference.

For the MB CLA, we're talking a huge change in tire width from the BlueEfficiency up to the AMG, I could see that alone explaining half of the increase in drag.

[chillsworld]

Originally Posted by serialk11r

I still think tires is the biggest difference. Porsche uses the exact same bodies across a lot of models. Between the Cayman S and non-S for example, the drag is the same, but the S is over an inch lower. Lowering 1 inch takes away as much drag as adding 1 inch of tire (mainly in the back). I think the number cited for each of these changes is about 0.01 Cd, which is quite a lot (3% of the whole car's drag).

Same story with the 911, you can see the 4S models with their fat tires having a couple hundredths of Cd added, but it's only an inch or so of tire width making that difference.

For the MB CLA, we're talking a huge change in tire width from the BlueEfficiency up to the AMG, I could see that alone explaining half of the increase in drag.

I couldn't find a general chart for tire width vs drag... But here is an example of increased drag from width.

CD --- tire & rim size (for a 1991 BMW 318i)

0.293 --- 155 R 15; 5 1/2 Jx15 St. with wheel covers
0.294 --- 165 R 15; 61 2 Jx15 St. with wheel covers
0.297 --- 175/70 R 15; 6 Jx15 St. with wheel covers
0.305 --- 185/65 R 15; 61 Jx15 St. with wheel covers
0.311 --- 205/60 R 15; 61 2 Jx15 St. with wheel covers
0.314 --- 205/60 R 15; 7 Jx15 LM
0.319 --- 225/55 R 15; 7 Jx15 LM
(http://ecomodder.com/forum/showthrea...g-cd-7475.html

Going from 6.1" (155) wide to 7.2" (185) increased by .012 CD, to 8.07" (205) is an additional .006 increase in CD... So if we use this as a benchmark just for conversation, a 1" increase will get an average of +/- .009 increase in CD. That's right in line with the .01 you cited above.

From Mercedes website, and the MB forums:
180 Blue Efficiency, stock 195/16 205/16 depending on country (7.6/8.07")
AMG model, stock 235/18 (9.25")

That's only 1.65" difference in tire size? So by your examples and the above chart, tires should/could account for less than .02 or .03 increase in CD I understand that percentage wise, that accounts for a lot of the cars drag, but it's still less than a third of the increase in CD from .22 to .32.



~C

[NHB]

Originally Posted by chillsworld

Inferior? I don't believe that any model has "inferior" aero parts. They are just aero parts which have different goals. Aero parts designed for FE aren't inferior to those designed for downforce/venting. Just as those designed for down force/venting aren't inferior to ones designed for FE.

Vents are there for cooling. More power needs more cooling. I really don't believe that downforce is that important in these machines. I don't have measurement data for CLA, but it's sister model A-Klasse was tested in SportAuto 3/2014. Even with optional AMG Aerodynamics package (AMG rear aerofoil, additional flics in front apron, larger front splitter) it didn't generate any downforce. It generated 430 N lift at the front end and 150 N lift at 200 km/h. Looks are there for sure but downforce is missing totally.

Why would we not include the AMG? You listed it as an example, it should be included. If we don't include the AMG then we shouldn't include the BlueEFFICIENCY Edition since it has changes made specifically for aero like the AMG does.

AMG model is the only model which has any change to be decigned downforce in mind.

And I agree, it can be seen hand in hand in other cars... Just as ground effects, diffuser, exhaust, underbody, airdam differences can be seen in many of those cars. I understand the science and theories behind heat exchange and drag... I don't deny that it exists. But I'm a hands on kind of person, lets actually look at some differences in models.

Ok. Let's take an example, which doesn't have any aero packages or anything to do with downforce. Here are Cd values for Skoda Octavia:
1,6 l TDI/66 kW 0,286
1,6 l TDI/77 kW 0,288
2,0 l TDI/110 kW 0,297

1,2 l TSI/63 kW 0,294
1,2 l TSI/77 kW 0,290
1,4 l TSI/103 kW 0,301
1,8 l TSI/132 kW 0,304

If I understand correctly, all of these values are listed for standard base model with 195/65 R15 tires. If the tire size wuold increase, we would see a bigger change in Cd.

[serialk11r]

Originally Posted by chillsworld

That's only 1.65" difference in tire size? So by your examples and the above chart, tires should/could account for less than .02 or .03 increase in CD I understand that percentage wise, that accounts for a lot of the cars drag, but it's still less than a third of the increase in CD from .22 to .32.

~C

Okay now that you actually pull numbers up I'm starting to doubt myself.

I suppose the lack of wheel covers adds a little bit more, and the AMG probably has some kind of brake cooling duct which adds a little bit more. Still, does a radiator block really do that much? It appears to be the only difference. I am pretty sure all CLAs have underbody paneling.

If anything, I'm very skeptical about the 0.22 number. The Lexus LS460 has a longer tail and all sorts of aero features and fails to get close to 0.22.

[chillsworld]

By the way guys, I'm really enjoying this conversation! I've actually learned a good deal of things looking up the stuff I have posted! I hope you two (NHB, serialk11r) know that!

Originally Posted by NHB

Vents are there for cooling. More power needs more cooling. I really don't believe that downforce is that important in these machines. I don't have measurement data for CLA, but it's sister model A-Klasse was tested in SportAuto 3/2014. Even with optional AMG Aerodynamics package (AMG rear aerofoil, additional flics in front apron, larger front splitter) it didn't generate any downforce. It generated 430 N lift at the front end and 150 N lift at 200 km/h. Looks are there for sure but downforce is missing totally.

I can't find an english version of that article, or a site that will translate correctly

But I found this about the A45 AMG, so I find the generation of lift you claim a little bit suspect:
"The car only comes with three options - black wheels ($490), stiffer suspension ($1990) or the AMG aero package which adds front winglets, front wing and a fully sick rear wing which sets it further apart from the A250 Sport but also adds 40kg extra downforce at 250km/h"
(2013 Mercedes Benz AMG A45 | Top Gear)

AMG model is the only model which has any change to be decigned downforce in mind.

I Understand that, but it's not the only one with modifications for aerodynamics.

Ok. Let's take an example, which doesn't have any aero packages or anything to do with downforce. Here are Cd values for Skoda Octavia:
1,6 l TDI/66 kW 0,286
1,6 l TDI/77 kW 0,288
2,0 l TDI/110 kW 0,297

1,2 l TSI/63 kW 0,294
1,2 l TSI/77 kW 0,290
1,4 l TSI/103 kW 0,301
1,8 l TSI/132 kW 0,304

If I understand correctly, all of these values are listed for standard base model with 195/65 R15 tires. If the tire size wuold increase, we would see a bigger change in Cd.

Awesome, this is great! I could totally believe that +44kw would lead to .011+ in CD. I could also believe that +69kw would lead to .01+ in CD. Like I said, I understand and believe in the idea...

But take those ratios and apply them to the +175kw of the CLA:

Average the +44 and the +69, use +56.5kw gets us .01+CD.

+175kw = .034+CD

I can totally believe this figure! This would be around 1/3 of the increase in CD for the CLA. But this number would not be the "majority" of the increase in CD.

Lets say the tires account for .025 and the KW increase accounts for .034, that's .059. What causes the other .049 increase in CD Even if the two combined account for .06 of the .1 increase on the CLA, where does the other .04 come from? It must be aerodynamics of some type right?


~C

[serialk11r]

I'm skeptical about cooling causing so much drag. Twice the power does not mean twice the cooling drag necessarily. A bigger radiator would give you a lot more cooling capacity for the same air flow. You would think that with a bigger engine and a bigger turbo that they would add a bigger radiator as well.

I still say, the 0.22 is BS, they probably measured it without a "treadmill" which reduces drag from the wheels and underbody. Other car companies have wind tunnels too, you'd think at least *some* car some other company comes up with would have great aerodynamics, especially considering how there are coupes with good plan taper out there vs. the CLA sedan. If you believe the 0.22, then MB understands some aero black magic that no one else does, but then as you can see the CLA45AMG isn't doing so well at 0.32, so there's a contradiction somewhere.

Also a little more on the original topic, the McLaren P1 has a 0.34? Cd, and that's with fat tires + cooling ducts everywhere, and the wing has that bend in it that can't be good for drag when in the down position. It's got much better downforce and stability at speed than a lot of the other supercars, so maybe this isn't so much a design goal problem as it is a problem with poor design.

[RedDevil]

I've seen a wind tunnel vid on here where a tech trails smoke into the lower side grills of the CLA 180 blufficiency, and it exits through the front wheel wells. And all the other stuff they do, the protruding A pillars with precisely curved sides, all way beyond the usual stuff on other makes.

Yes I do believe the Cd is accurate.
And I think even the AMG does surprisingly well, only .04 worse than the Cd on my car.