Car & Driver Aerodynamics Test

[sheepdog 44]

Originally Posted by Insight for life

.30 for the 2001 insight? uh think again people.

I call phooey on the whole thing! Maybe i'll go pick up some 19inch turbine wheels and call it a day.

Back to la la land i go---->

[Vman455]

Originally Posted by Thenorm

the quoted 0.23 is for the special bluemotion edition in europe with extra features such a narrow LRR tires, grill shutters, etc.

Correct; Mercedes claims .28 for the CLA250, and .22 for the CLA180 BlueEfficiency. Just goes to show how much Cd can be influenced by the details.

[California98Civic]

Originally Posted by sheepdog 44

I call phooey on the whole thing! Maybe i'll go pick up some 19inch turbine wheels and call it a day.

Back to la la land i go---->

G1 Insight drivers pretty upset at Car & Driver!

Originally Posted by Vman455

Correct; Mercedes claims .28 for the CLA250, and .22 for the CLA180 BlueEfficiency. Just goes to show how much Cd can be influenced by the details.

That's interesting, of course. The article speculates on the same theme. But .28 is still lower than the C&D tunnel offers (.30). So, I'm still wondering if the testing method makes its own noise. Did Mercedes test without the wheels spinning? That kinda thing.

[mikeyjd]

Originally Posted by California98Civic

That's interesting, of course. The article speculates on the same theme. But .28 is still lower than the C&D tunnel offers (.30). So, I'm still wondering if the testing method makes its own noise. Did Mercedes test without the wheels spinning? That kinda thing.

Originally Posted by aerohead

One SAE Paper from General Motors in the 1960s reported that the difference in Cd between spinning and stationary wheels was of such a low statistical significance that it didn't warrant the time and expense to test; and spinning tire effects could be easily simulated with trip strips in the tire/wheel area.
Exposed-wheel race cars MUST be tested with spinning wheels.

[Vman455]

Originally Posted by California98Civic

G1 Insight drivers pretty upset at Car & Driver!



That's interesting, of course. The article speculates on the same theme. But .28 is still lower than the C&D tunnel offers (.30). So, I'm still wondering if the testing method makes its own noise. Did Mercedes test without the wheels spinning? That kinda thing.

I wonder how Cd is actually calculated in a wind tunnel? Is it derived from total drag force and calculated frontal area? If so, that leaves a lot of room for fudging--er, estimating--numbers.

[gone-ot]

Fore/aft force gauges under each wheel measure drag resistance.

Vertical force gauges under each wheel measure LIFT & DOWN forces.

[oldtamiyaphile]

Originally Posted by aerohead

One SAE Paper from General Motors in the 1960s reported that the difference in Cd between spinning and stationary wheels was of such a low statistical significance that it didn't warrant the time and expense to test; and spinning tire effects could be easily simulated with trip strips in the tire/wheel area.
Exposed-wheel race cars MUST be tested with spinning wheels.

Nissan would beg to differ. They test with wheels spining and the engine running to maximise under hood airflow management too.

What was considered adequate by GM in the 60's isn't nessesarily best practise today. It's unlikely we'd have fairly conventional looking 0.22CD sedans with that attitude.

[aerohead]

Originally Posted by California98Civic

G1 Insight drivers pretty upset at Car & Driver!



That's interesting, of course. The article speculates on the same theme. But .28 is still lower than the C&D tunnel offers (.30). So, I'm still wondering if the testing method makes its own noise. Did Mercedes test without the wheels spinning? That kinda thing.

Mercedes-Benz uses the FKFS wind tunnel that Kamm built in the late 1930s.It does not have a moving floor,or provision to roll the wheels unless it's been recently modified.

[aerohead]

Originally Posted by Vman455

I wonder how Cd is actually calculated in a wind tunnel? Is it derived from total drag force and calculated frontal area? If so, that leaves a lot of room for fudging--er, estimating--numbers.

From: Drag Force = ( 1/2 X rho X Cd X A X V-squared ),
they measure the actual axial drag force,then using actual station pressure,calculate the air density at test time,then using the projected frontal area,and actual test section air velocity, squared,the coefficient falls out of the equation.
To complicate matters,the EPA does not recognize the frontal area of the side-view mirror(s).
The drag reflects the actual frontal area,including the mirrors.
If the drag is mandated to reflect frontal area sans mirror(s),then the coefficient of aerodynamic drag will be arbitrarily high.
A marketing statistician could 'cook' the numbers.
It's regulated ambiguity.

[aerohead]

Originally Posted by oldtamiyaphile

Nissan would beg to differ. They test with wheels spining and the engine running to maximise under hood airflow management too.

What was considered adequate by GM in the 60's isn't nessesarily best practise today. It's unlikely we'd have fairly conventional looking 0.22CD sedans with that attitude.

I'm not certain what Nissan's tunnel specifications are,and how their quanta compare to other facilities.
General Motors research was conducted at Lockheed's Marietta,Georgia facility.It is considered a world-class wind tunnel,good enough for the Skunk Works and all major NASCAR teams.
GM has produced Cd 0.089 vehicles.I'm not sure what Nissan's claim to fame is.
If you have some technical information from NISSAN it would be most appreciated.Hopefully it is not from the same tunnel that their 280-ZX was developed in.