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

[Cd]

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 )

[aerohead]

Originally Posted by Cd

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 )

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.
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*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.

[freebeard]

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.

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.

[oldtamiyaphile]

Don't try to understand it, just accept it

The Iveco Daily is quoted at 0.315:



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.

[Vman455]

Details, details, details.

This car has Cd .32:



While this car has Cd .22:



Chew on that for a while!

[spacemanspif]

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".

[Vman455]

Originally Posted by Cd

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

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.

[NHB]

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.

[aerohead]

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.

[aerohead]

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.