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

[Cd]

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.





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.

[Cd]

[HydroJim]

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

[Cd]

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

[Cd]

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 .

[nemo]

The 1.6-L CLA180 BlueEfficiency Edition incorporates narrower tires (width of 195 mm/7.7 in) on 15-in wheels with aerodynamic covers, and lower ride height

Design refinements proven in the tunnel included adjustable radiator shutters and rear lamp lens fins. The raised trunk lid creates a partial Kamm-type tail. A diffuser is fitted below the rear bumper, and the car’s underbody is enclosed, including the middle section of the rear axle. The muffler also received aerodynamic attention.

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

[Cd]

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.

[HydroJim]

are discrepancies in Cd testing to blame?

[NeilBlanchard]

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.

[Cd]

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 :


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. )