'Perfect' aero shape but unimpressive Cd ( Mercedes C-111)
 

I was thinking over how that the Mercedes C111 seemed to be the 'perfect' shape aerodynamically, yet the numbers are rather unimpressive when compared to the new Mercedes passenger cars, and even some new cars from Mazda.

Or perhaps, I am looking at it wrong. The Cds on the new cars are so much more impressive when you compare them to this ideal car.

Just look at the car from all angles and it should be perfect aerodynamically.
A low front end sloping upwards to a windshield curved which tapers back in plan form to a tear drop shape. No wipers or external protrusions. A tiny almost nonexistent grille. Full covered wheels - both front and back, and a curve which matches the aero template perfectly.

By looks alone, this car is a streamlined bullet with all of the right aero tricks. Perfect in fact.

Yet we see that the short tailed version of the car has a Cd of .237 .

By comparison, the new Mercedes has a Cd of .22 despite looking dumpy in comparison.

http://i1271.photobucket.com/albums/.../drawings1.jpg

http://www.autoconception.com/wp-con...ineering-2.jpg

Does this make anyone else feel as though air is so unpredictable as to make modifying our cars nearly fruitless ?

Not that it would stop me all that much, but it is frustrating never the less.

Imagine taking a car and modifying it by lowering it, removing the mirrors, adding front and rear wheel covers, a blocked grille, a curved windshield, a full belly pan, as well as a tapered template matching shape, and all the other aero tricks, only to find out that your car has more drag than a car like the CLA with its gaping grille and side scoops, uncovered wheels and profile that is completely out of line with the template curve. :confused:

http://www.emercedesbenz.com/Images/...11_3Frontp.jpg
http://www.emercedesbenz.com/Images/...C111_3_Top.jpg
http://screen-wallpapers.com/wallpap...Background.jpg
http://s1.cdn.autoevolution.com/imag...gallery_25.jpg

I still don't understand how the CLA could possibly have a .22 Cd...

As well as the Lexus 430 having the same Cd as a G1 Insight :

http://upload.wikimedia.org/wikipedi...Black_Onyx.jpg

http://upload.wikimedia.org/wikipedi...a_Insight_.jpg

It all comes down to " optimization ", but as amateurs without wind tunnels, what is the best that we can do besides guess what the air is doing ?

You could do all the right aero tricks to a car and still have an unimpressive Cd

I personally think that the best tool that we have is tuft testing, since it shows the problem areas in our testing.

That, and of course CFD and lots and lots of A-B-A testing .

Cla
 

For more information.
Mercedes' 2014 CLA is the new low-Cd king

Here you can see the flat wheel ' covers ' used in the low drag version.
Note that the the area around the 'spokes' of the wheel are painted matte black to disguise the fact that there are no real spokes to the flat wheel.

http://www.autoweek.com/storyimage/C...ind-cheats.jpg

are discrepancies in Cd testing to blame?

Two things (probably among others) that are keeping the C-111 from having a lower Cd are the arched front fenders, and the slab sides. The side taper only occurs at the very back.

I agree, the Insight has tapered sides, muddying the waters in this comparison.

The two cars with the best Cd's of any "production" cars are the EV1 and the XL1.

The C-111 is obviously not "perfect" and I think it is the virtual lack of side taper that is the difference. I think that Phil has said that side taper can reduce the Cd by as much as 40%, which is huge.

Looking at the side taper of the CLR and LS 430, it doesn't seem there is much there.
For example, here is the LS 430 :
http://www.the-blueprints.com/bluepr...2003-05115.jpg

Mercedes designed that car with one goal in mind - low drag.
It's hard to believe that they would skip something so obvious if it made that big a difference.

( I do find the angle of plan taper on the C-111 to be quite excessive at the rear. Again, surely the engineers at Mercedes knew what they were doing and wouldn't have fuddged this up. )

It also could be taper (up) on the underside.

Could it be the C111 appears narrow, which if true means it has less frontal area? Maybe frontal area to length ratio has an effect.

Salon: A Mercedes-Benz C111 - Road & Track

Mercedes-Benz C111 - Wikipedia, the free encyclopedia

Mercedes-Benz C111

The car was more about engines and suspensions than aero.

You get differences from one tunnel to another - and due to different sized models in the same tunnel.
Then there's the question on how the results were measured.
Rolling wheels , or not ?
And what wheels - thin, wide ?
Rolling road or not ?
Ride height ?

A wind tunnel is a bit like the EU tyre label, with OEM testing products on their own grounds rather than in the same or similar installation.
Within the laws and regulations, they adapt the testing grounds to their products.

Some tyres are quieter when tested by the OEM, then when the likes of ADAC.de compare many different ones in the same environment, they may turn out louder that tyres with a worse sound level ...

Cd 0.22
 

So the new M-B has caught up to the 1983 Ford 'Sierra' concept car.Or a little better than R.G.S.White's MIRA recipe car of 1968.
The C-111 III was a 'track' car and there were concessions to down-force to protect Hans Liebert and his driver team.
The nose was changed the very next year to the 'flachbau' blunt nose.
Hucho has criticized the C-111 III for its misuse of fineness ratio.
The plan-view reveals a lot about lost potential.
Kamm and Koenig -Fachsenfeld were only able to get Cd 0.21 out of the 'Template' when they ignored edge radii and body side camber integrating into the boat tail 'early' on the body,like the streamline half-bodies have.
When they did all the proper contours they got Cd 0.12 just like Walter Lay in 1933.
The C-111 looks good only in side elevation.It is not a proper 'Kombinationform'.The whole thing is a compromise.I drooled all over the thing in Stuttgart.I love it.But we can do better.Which is born out with the more 'modern' cars.
Blunt the nose,round those edges,narrow the rear axle and pull the body sides in to match the side contour of the greenhouse and you'll have a real 'hotty.'Or cold-rod,if you prefer.:)

discrepancies
 

The M-B tunnel is the FKFS wind tunnel from next door at the Stuttgart Technical University where Kamm and Koenig-Fachsenfeld did their famous K-form research.
Calibration models have been tested in the USA,Asia,and Europe and there's an acceptable deviation in tunnel results which was published in the 1980s.

low drag
 

Perhaps we need to assign parameters for 'low drag.'
Hucho says we can be looking at Cd 0.07 to 0.08.So we need some reference datum when we use 'low drag'.
Obviously the new M-B is not 'low drag.' It's just lower than previous models.
They've done a production-ready Cd 0.18 in the 90s.

Mercedes-Benz 1967 1968 1969 1970 1971 Auto Brochures
http://mclellansautomotive.com/photos/B29519.jpg

effect
 

Yes,very much so.It's the plan-view corollary to the side elevation effective fineness ratio.
Kamm and Koenig-Fachsenfeld devoted quite a bit of wind tunnel model studies to quantify the effects of the L/WXH ratio for both Jaray-form,and K-form car bodies.
How the body cross-section varies along the longitudinal axis,to and from the position of maximum cross-section is critical also.
Fast pelagic fish are a good model for streamlining.Their frontal area occurs in the first 1/3rd of their body length and the last 2/3rds cleans up the disturbance.

and still have an unimpressive
 

The point I would want to emphasize,is that if we mimic what has proven itself in tunnel after tunnel study,that we won't stray too far afield when we make it to the gas station.
There is such a vast body of empirical data to work with.If we stay with the good science we should be able to achieve whatever we target.
Personally,I've never had a disappointment as long as I followed the science and physics.I've had failures of course,but they were all borne out of ignorance.Which is no sin.:p

Maybe so, but this is impressive if true:

if true
 

That's what they reported.
If you like that,check out the fuel economy of the 1981 VW ARVW,granddaddy of today's XL1.

Yes. Trouble is, they had several cars and several versions of the car with the same name.