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Discussion Starter · #1 · (Edited)
How many of us take Cd (coefficient of drag) into consideration when looking at purchasing new wheels and tires? Or it something that doesn't matter?

Read the following article in "Automotive Enginnering International" to see why I'm bringing this up.

Smoothing the 300

Under the large grille of the Chrysler 300 is a secondary air inlet to
provide the engine cooling airflow. The stylish openings at the sides, next to the front fog lamps, were blocked off because they were found to
increase drag. Notice the front underbody cover, one of seven shields used.
Airflow management engineers on the Chrysler 300 added flexible plastic panels held by pins. Shown are the top front sealing panels.
Airflow management is a multi-faceted area of vehicle engineering, and the new Chrysler 300 rear-drive sports sedan reflects DaimlerChrysler Corp’s most
comprehensive overall effort in that area. The way the air flows over and through and under the car affects more than just the coefficient of drag (Cd). The airflow also has to be guided into the right areas,
apportioned for cooling the engine, brakes, and other underhood and
underbody components. Airflow management engineers for the 300 (and the
Dodge Magnum wagon) had to carefully craft solutions that also supported styling decisions and safety engineering.

An ultra-low Cd is a publicly accepted sign of fuel-efficient design in Europe and other places where fuel prices are high. As Stanley Surratt, Chrysler Vehicle Development Engineer, explained, a 0.26 is the new unofficial standard for Europe.
He added that an engineering rule of thumb at DaimlerChrysler is that a 1%
reduction in Cd will produce a 0.4% improvement in fuel economy. The 300
has a respectable Cd of 0.33 to 0.35, and it took a lot of effort to achieve
that. “The 300 doesn’t have the budget of European luxury cars to pay
for such electronic features as pivoting airflow flaps and variable-ride height,”
said Surratt.

Much advanced work was done with CFD, often described by its advocates as a virtual wind tunnel, but there was still a lot learned from wind tunnel testing. The large 300 grille, for example, dictated a through-grille airflow for the front-end cooling module (condenser and radiator),
and there is a large air intake at each side of the grille, locked in by styling. However, in wind tunnel testing the side intakes were found to increase Cd by
about five counts (i.e., the third decimal number, so 0.331 would go to 0.336). Result: The side intakes were retained but blocked off completely.

The engine, exhaust manifolds, and today’s close-coupled catalytic converters
are not movable. So if anything is too close to be cooled by the available
airflow, and can’t be moved, it has to be shielded, and/or protected with thermal wraps. In addition to the life of the rubber parts, the underhood air temperature management has to prevent overheating of parts that would suffer performance issues,

In the 300, that meant shielding the engine mounts, the starter motor and its wiring, even the engine knock sensor. Front to rear, the 300 has seven body-inwhite shields, eight shields for the chassis,
two on the driveline, ten for electrical system components, three for the
powertrain, one at the exhaust system, and one for the rear fascia. And several hoses have some thermal wrapping.

Of the seven underbody covers, four are in front, and three in the rear. In front, one is full-width under the front fascia, and incorporates airflow ducts for brake cooling. Another just to the rear covers the powertrain cradle. Two others—one cover at each side, are just behind the front wheels. Each closes off that underbody area, preventing drag from turbulence that otherwise would develop from airflow around the frame rails.

One almost insurmountable problem is with the wheels. Large, wide tires with
open-spoke wheels make a powerful styling statement in the U.S. However, they allow huge air plumes to flare out through the openings, which produce drag. The plain wheels on the base-model 300s have much smaller openings, and so most of the airflow goes straight through to the rear, much better for aerodynamics. The low Cd wheels are available, but Chrysler doesn’t expect to sell many.

At the rear, there is an underbody cover in front of each rear wheel, and the cover has a vertical air dam that prevents underbody airflow from hitting the tires and causing turbulence that would result in drag. Instead the airflow is forced down, and it spreads out, away from the tire, according to Surratt.

The underbody design isn’t all plastic covering. The muffler has a
shape designed to provide a path through which airflow goes around, up
and over the differential banjo for cooling. That clearly isn’t as good for
underbody aerodynamics as flat plastic covering, but the 300 is a highperformance car and the rear axle does have to be cooled.

The final cover is at the very back, just under the spare tire well, where it
provides an aerodynamically flat surface under the tilted floor pan on which the spare tire sits. The tilt is designed to force the spare tire up in a rear crash to protect the gas tank. Without the underbody cover, the airflow into the crevice area formed by the tilted surface would add approximately six counts to the Cd.
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Interested in your thoughts after reading the article :)

Audrey
 

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Kewl 'Nilla Hemi said:
How many of us take Cd (coefficient of drag) into consideration when looking at purchasing new wheels and tires? Or it something that doesn't matter?

Read the following article in "Automotive Enginnering International" to see why I'm bringing this up.
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Interested in your thoughts after reading the article :)

Audrey
Audrey,

Before now I hadn't given it any thought. However now that I am aware of it I probably will since the gas prices have gone sky high.
 

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I pulled the lower vent covers off to open air flow to the front brakes through the neat channels designed in the frame.

Brake cooling is more important to me with my style of driving.
 

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Take it all with a grain of salt, these same designers gave us the right pull and exhaust resonance. Funny (not ha ha) they figure all this yang out on a computer somewhere and NEVER road tested to find how terrible the Conti tires are or that the car leads right or at best is overly crown sensitive. I think little to nothing of all the theory discussed in that article.
 
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