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

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 )

http://i877.photobucket.com/albums/a.../Photo0721.jpg
http://i877.photobucket.com/albums/a...-ad-better.jpg
http://i877.photobucket.com/albums/a...yb17/CRX-1.jpghttp://i877.photobucket.com/albums/a...azda-mx3-7.jpghttp://i877.photobucket.com/albums/a...b17/CRXFB2.jpghttp://i877.photobucket.com/albums/a...r_20080212.jpghttp://i877.photobucket.com/albums/a...33.03%20AM.jpg
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 : http://i877.photobucket.com/albums/a...-09point26.jpg
From the top, this car is a perfect teardrop, yet has the same same Cd as the boxy Prius : http://i877.photobucket.com/albums/a...s-top-view.jpg
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.

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.

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 ) : http://blog.caranddriver.com/wp-cont...swagen-Fox.jpg
Or the Jaguar XJ-220 : https://en.wikipedia.org/wiki/Jaguar...nt_JagMENA.jpg
https://en.wikipedia.org/wiki/Jaguar...re,_Gaydon.jpg
http://cartype.com/pics/3700/full/ja...j220_side1.jpg

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.

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

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]" http://www.jcna.com/library/news/200.../jcna0030f.jpg

So that means that this car : http://preview.netcarshow.com/Jaguar...92-1600-08.jpg
has the same drag as this truck ( both at point 36 ) :
http://media.caranddriver.com/images...-s-429x262.jpg

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.

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 .
http://blog.caranddriver.com/wp-cont...swagen-Fox.jpg

https://upload.wikimedia.org/wikiped.../Mazda_MX3.jpg

explain
 

*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.

Fox vs MX-3
 

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.
http://i1271.photobucket.com/albums/...titled4_18.jpg

Jag vs F-150
 

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!

Thanks Phil !
Aerodynamics really seems to be a 'black art' as I have seen you describe it in the past.
It's as if the air does whatever it wants to. ( I think i've expressed that same reaction in previous threads ! )
What baffles me is that a low drag car versus a high drag car can be only millimeters apart in their sheetmetal, yet the air will cling to one and not the other.

I was under the assumption that wake size was the main factor in a low Cd.
How can this be, if for example the MX-3 has an overall smaller wake than something like a Scion Xb, yet both have the same Cd of point 32 ? ( I have included a tiny thumbnail image because this thread is getting image heavy ) http://www.kbb.com/car-images/2015-s...ltsize=160x120

drag
 

here's something to consider:
*any car built in 1968 or later can achieve Cd 0.25 with detail optimization.
*any car that you can think of!
*thousands of body types can achieve Cd 0.25.
*and getting there can involve any and all the parts of the body and wheels.
--------------------------------------------------------------------------
*low drag,by other's definitions,begins at Cd 0.25,going lower.
*and as Hucho reminds us,there is a point where you're stuck,unless you address the aft-body,which means that the wake isn't the primary criteria until you're at about Cd 0.25.(which is essentially where Kamm quit,due to practical length considerations).
*from Cd 0.25,down to Cd 0.13 we can use elongations which comply with something like the 'Template.' Both in elevation and plan-view.
*to go below Cd 0.13 requires integration of the wheels into the body,or copious amounts of wheel fairings.
*with this,Hucho says that we can approach Cd 0.08 for the bare form.
*and for 'practical' considerations,we'd be talking about 'active' aerodynamics.

Whew, my '71 just scoots in under the wire. Good thing I'm not driving the '67 sunroof.

0.25 is my goal. Air curtains, fender skirts and a boat tail. Progress is slow.

Don't try to understand it, just accept it :)

The Iveco Daily is quoted at 0.315:

http://liveimages.motoring.com.au/mo...Pad&width=1050

It has a ladder frame chassis underneath with no apparent attempts at smoothing flow. It's much more aerodynamic than an E-Type Jags ~0.45, which basically matches the brick like aero of a Jeep Wrangler.

I think the mainstream media are to blame for pushing the idea that curvey looking designs are aerodynamic. It's an easy trap to fall into - but it seems that a certain level of boxiness is the way to go for aero. Look at how boxy the Nissan GT-R is for it's 0.27 CD.

Details, details, details.

This car has Cd .32:

http://coolmaterial.wpengine.netdna-...A-45-AMG-5.jpg

While this car has Cd .22:

http://blog.mercedes-benz-passion.co...-cla-180-B.jpg

Chew on that for a while!

In terms of the MX-3 vs CRX I think what you are missing is the rear body stuff aerohead mentioned. Yes the MX-3 looks very sleek and streamlined but you are not looking at it against the "template". The CRX most likely fills the back area a little better and maybe even has a rear attachment point with that little spoiler on the back. The MX-3 most likely has detached flow starting at the B-pilar and all the "sleakness" of the back end means nothing. Makes one wonder how much an MX-3 could benefit from a pro-stock style spoiler and reaches "template".

Cd .36 sounded suspiciously low for the Lightning--equivalent to the current Ram Ecodiesel (which uses active suspension and grill shutters to achieve it), and significantly lower than any contemporary pickup--so I did some searching. This forum poster claims to have checked with SVT directly, who quoted Cd .44; this figure is reported by another poster as having been "published in the past." Magazine reviews of the Lightning don't report a drag coefficient, unfortunately, but they do report a drag-limited top speed of 142 mph. Plugging the approximate weight (4700 lbs) and frontal area (31.5 sq ft) into the aero calculator shows a speed somewhere north of 165 mph for the Lightning's 380 hp with the lower drag coefficient, and just over 150 for the higher. I'm going to bet the actual Cd of that truck is .44, not .36.

XJ220 has also many times more power that VW. Huge power means huge cooling and a lot of drag. Cooling, tires and downforce usually make fast cars not as aerodynamically efficient as they look. Those cars have to look sleek but actual Cd numbers are not that that important.

Cd 0.36 pickup
 

Dodge claimed Cd 0.36 for their 2014 RAM 1500 Standard Cab with active aero and factory tonneau.
I think that the Chevy Avalanche is in there close.

1st-gen CRX
 

Here's the '84-'87 at top.
The HF had the narrow 165/70-R13 tires,single side mirror, factory AC condenser blank-off panel,and antenna delete to get Cd 0.32.
I don't have a true-length image for the MX-3 for a comparison.:p
http://i1271.photobucket.com/albums/...d2/Scan102.jpg

CRX / MX-3 overlay :http://i877.photobucket.com/albums/a...34.00%20PM.png
Template overlay :
http://i877.photobucket.com/albums/a...20template.jpg

Crx / mx-3
 

The CRX 1.5-liter has two side mirrors,175/70 SR tires,and I'm guessing the AC is operating,so more cooling flow.It's got Cd 0.35.That would make the Mazda the winner @ Cd 0.32.
What's that do for you?
At this coefficient there could be thousands of minute differences in architecture which would add up.
Laboratories are the only way to 'know' about these things.

This is too true. My Escort ZX2 (which has ties to the MX-3 and other Mazdas), Cd sits at 0.269 as of Spring 2015. This is down from 0.33 Cd starting out bone stock, a 18.48% reduction is drag!

With future optimization planned, I think the ZX2 can achieve Cd 0.24x before I get to work on the boat tail.