CRX vs MX-3 aero, When aero doesn't seem to work

[Cd]

This is one of my all time favorite car shapes for a mass produced affordable car : The Mazda Mx-3.
That shape...it's like looking at a beautiful woman.
What boggles my mind though, is that this car has the same drag figure ( Cd ) as the first generation CRX.
Just look at the 'boxy' proportions of of the CRX design, and compare them to the sleek MX-3. Both cars have an identical Cd, with the CRX actually being more aerodynamic, due to a smaller cross section ( frontal area )




The MX-3 has an overall more rounded front end, more curved glass, beautiful 'tumblehome' to the rear, yet something keeps this from improving on things.
I've seen this explained as one car having better aerodynamic 'optimization'.
Can you all explain this better ?
How can some cars have all the aero tricks, yet still have a poor Cd ?
Just look at this car here, the 1948 Panhard Dynavia :
From the top, this car is a perfect teardrop, yet has the same same Cd as the boxy Prius :
How is it that on some car designs, you can add a huge airdam and boattail, full wheel covers, and all the other tricks, yet end up with the same Cd as a vehicle with only a grille plug and a simple spoiler ?
When I do an overlay of some cars that have exceptional aerodynamics onto one with poor aero, sometimes the one with poor aero seems to be nearly identical.

[California98Civic]

Consider the different tunnels where testing might have been done... also what does the underside of these cars look like? And maybe those mirros and the rear window wiper cause some trouble. But basically, IDK.

[Cd]

Here is another excellent example of what I am talking about.
Of these two cars, which is more streamlined ?
The VW Fox ( I used to call them the VW Box ) :
Or the Jaguar XJ-220 :



The Jaguar has larger tires, yes, but does it make that big a difference when the car is so low ?
Consider how many cars benefit from a full width spoiler at the front of the car. Surely that adds just as much drag as the exposed tires of the XJ-220.
There is a built in spoiler on the rear of the XJ-220, but it is very subtle, and not like the huge drag inducing ones that are frequently on high performance cars. The car is wide, but that effects CdxA, and here we are just talking Cd.

The XJ-220 even has flush wheels.

When the XJ-220 first came out, I was still under the impression that a laid back windshield was good for aerodynamics. I was floored to find out that the Fox had better drag.
And with the smaller frontal area of the VW, it was actually quite a bit more streamlined than the XJ-220.

The numbers ?
Jaguar XJ-220 point 32 VW Fox point 31.
So given the same amount of power, the Fox would actually have a higher top speed. Amazing.

I just noticed a nice little aero feature of the XJ-220 - the wheel arches have a nice smoothing out at the rear of the wheel well, much like the Honda Insight did. Also, consider that the XJ-220 has a flat underbelly since it has the engine at the rear of the car.
The rear glass is flush as well, unlike many Ferraris and Lambos.

[seifrob]

Isn't the jaguar body designed to create downforce? That usually means lot of drag. (F1 cars have cd of 0.71 iirc)

[Cd]

Yes. In fact, I had read that the original design was tested at point 32. Later versions were point 36 !
"A number of small design changes for the body were tested in the wind tunnel; the final version had a drag coefficient of 0.36 with downforce of 3,000 lb (1,400 kg) at 200 mph (320 km/h).[28][41] The XJ220 was one of the first production cars to intentionally use underbody airflow and the venturi effect to generate downforce.[41][42]"

So that means that this car :
has the same drag as this truck ( both at point 36 ) :

[deejaaa]

wow, that's a lot to chew. goes to show: install smooth wheel covers, full belly-pan, front dam, rear diffusor, delete mirrors, etc.... and if not done right, increase drag. do one or two right, 2-3 wrong and never see any benefit. and never be able to figure out why.
there should be some way to test, (not down a hill for rolling distance) at simulated freeway speed, to weed out bad mods. homemade smoke tunnel? can't imagine cost being worthwhile.

[Cd]

I guess the XJ-220 vs VW Fox was actually a poor example, because I didn't factor in downforce.
What I meant to compare was the shape of the vehicle.
So let me compare that VW Fox with the MX-3 . The VW Fox was certainly not designed for downforce !

You would think the MX-3 would be sleeker, right ?
VW Fox point 31 Mazda MX-3 point 32 .

[aerohead]

Originally Posted by Cd

This is one of my all time favorite car shapes for a mass produced affordable car : The Mazda Mx-3.
That shape...it's like looking at a beautiful woman.
What boggles my mind though, is that this car has the same drag figure ( Cd ) as the first generation CRX.
Just look at the 'boxy' proportions of of the CRX design, and compare them to the sleek MX-3. Both cars have an identical Cd, with the CRX actually being more aerodynamic, due to a smaller cross section ( frontal area )




The MX-3 has an overall more rounded front end, more curved glass, beautiful 'tumblehome' to the rear, yet something keeps this from improving on things.
I've seen this explained as one car having better aerodynamic 'optimization'.
Can you all explain this better ?
How can some cars have all the aero tricks, yet still have a poor Cd ?
Just look at this car here, the 1948 Panhard Dynavia :
From the top, this car is a perfect teardrop, yet has the same same Cd as the boxy Prius :
How is it that on some car designs, you can add a huge airdam and boattail, full wheel covers, and all the other tricks, yet end up with the same Cd as a vehicle with only a grille plug and a simple spoiler ?
When I do an overlay of some cars that have exceptional aerodynamics onto one with poor aero, sometimes the one with poor aero seems to be nearly identical.

*I believe that the divergent portion of the roofline on the CRX is longer than the Mazda's.This would be a critical factor.We'd want to line up the roof apex of each car for a comparison.
*With respect to the Dynavia,I think I have Cd 0.17 for that car.

[aerohead]

Originally Posted by Cd

I guess the XJ-220 vs VW Fox was actually a poor example, because I didn't factor in downforce.
What I meant to compare was the shape of the vehicle.
So let me compare that VW Fox with the MX-3 . The VW Fox was certainly not designed for downforce !

You would think the MX-3 would be sleeker, right ?
VW Fox point 31 Mazda MX-3 point 32 .

Again,the VW (I believe) has the longer aft-body.It's aft-body aspect ratio is greater.This is the critical factor for low drag.This was at the central core of Kamm's/Fachsenfeld's research at the FKFS.

[aerohead]

Originally Posted by Cd

Yes. In fact, I had read that the original design was tested at point 32. Later versions were point 36 !
"A number of small design changes for the body were tested in the wind tunnel; the final version had a drag coefficient of 0.36 with downforce of 3,000 lb (1,400 kg) at 200 mph (320 km/h).[28][41] The XJ220 was one of the first production cars to intentionally use underbody airflow and the venturi effect to generate downforce.[41][42]"

So that means that this car :
has the same drag as this truck ( both at point 36 ) :

We pay dearly for high speed stability.
Common sense is an inverse-square relationship to disposable income.Double your income and you get four times as stupid!