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Discussion Starter #1
As a newbie and potential buyer of a new Golf TDI or A3 TDI I have combed thru this Forum in an effort to educate myself as to various issues visa vis these vehicles. My sense at this time, is that in spite of the A3's apparent superiority in fit finish and componetry, ( 2 zone heating / AC, filters on cockpit air intake, leather seating, electric adjustable drivers seat ect. it seems a bit long in the tooth in its basic body shell design. Driver visibility is somewhat impaired by the high door sills, and consequent driver side mirror location ( although Audi seems to have recognized same with the lowered redesign in the 2011 model ) The manual transmission available in the Golf as well as the superior cd numbers .305 vs .32, and the basic 450 pound weight differential, given the same drive train, suggests to me, some MPG advantage as well.
Regarding these drag coefficient numbers I have a question or two:
Is there any information from VW GMBH available re basic aerodynamic design, and its potential for optimization? (It would be fun, and most educative, to see or hear about that phase of the vehicles design). And more specifically, are any of the available 'Body Gear Accessories', Front Valance (air dam?),Side Skirts, or the Rear Hatch Spoiler, really any kind of performance improvements on this cars basic design? Are there any numbers available? These optional extras, presumably would cut down aerodynamic lift, thereby improving traction (and weight) on the wheels. This should improve traction in the corners but does that weight at 65 mph compromise MPG ? Might these accessories provide any improvement? Is frontal area increased?
They might look cool but do they really work one way or another?
Are there any unseen advantages in the A3? Brakes?
Bill R.
 

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Without knowing your exact answer, a lower front air dam and lower side skirts will help pretty much any car's aerodynamics and improve mpg. I don't know if the additional weight of these piece offset the difference though. Rear wheel covers also help but they would look added on.

I believe the A3 has the same brakes as the Jetta Sportwagen which are 1 size larger.

Don't forget that Audi dealers are usually a little nicer. You'll take a bigger hit on the Audi though.
 

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Something to think about is what the Golf Bluemotion has. The Bluemotion isn't sold in North America but here are the differences:

Start/stop
Smaller 1.6L TDI engine w/fuel economy (euro cycle) of 3.8L/100 km and power of 105 PS and 250 Nm
Gear shift indicator
alternator w/regen capacity to increase engine drag when braking
low rolling resistance tires
lighter wheels w/aero minded design
lowered suspension (I hate how low VW engines are but a lower car is better for handling and aero)
taller gearing

Sideskirts, rear spoiler, and lower front fascia with a lower air dam.

0-100 km is 11 seconds which isn't that bad but you won't win any drag races.

And most importantly, blue seats.
 

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Extracting actual aerodynamic data from a manufacturer is going to be nearly impossible unless you have a friend inside that can shake off a few numbers without getting fired. Most of that data is proprietary and they don't really want the specifics to be floating out there for their competition to scoop up.

From what Chitty is saying with the differences in equipment between the standard TDI and the Bluemotion means that if we add the Bluemotion [or inspired] aero bits, lower the car via suspension, then add lower rolling resistance tires [narrower too?] with superlight wheels, that would all help.
 

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Discussion Starter #5
It's MPG not traction ?

Gentlemen, while I appreciate your responses, I still question, and wonder about, the addition of the lower air dam,side skirts - ( and rear hatch spoiler ?) as helps in upping MPG. While I can understand that they might increase performance, add stability, and maximize adhesion at speed, ( increasing down force by lowering aerodynamic lift,) would not the overall weight on the wheels be increased somewhat ? Then would not the MPG be compromised? If it's true that a lighter vehicle is more easily propelled down the road, and thus requires less energy, would not the heavier vehicle ( that with increased downforce ) require more?
If this is true the above VW 'Genuine Volkswagen Accessories' become cosmetic go-fast adornments and would not in fact contribute to the increase in MPG. Do I have this right ?
Thanks for your interest.
Bill
 

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I don't know but you have the right idea. Is the additional weight worth the better aerodynamics? Other than someone at VW or unless you have access to a wind tunnel, there's no way to know. Repeated trials in real world testing could show. Want to try it out and let us know? :D

I can tell you that the increase in drag is the square of the increase in speed. In other words, driving slower will give better mpg. If you speed up from 50 mph to 70 mph that's an increase of 40% in speed which creates a 96% increase in drag. You effectively doubled the aerodynamic drag. That's why it's so important to slow down when hypermiling. The most I've ever averaged was a little over 70 mpg and it was all highway and quite painful/hazardous driving that slow vs. other traffic. If there are any engineers out there I'm sure they would have some more to add.
 

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Discussion Starter #7
Aero Info Continued

My question answered ?
With respect to the addition of ground effects air dams and rear roof wings/ foils and the addition of drag and consequent loss of MPG, see a you tube video at

http://audimobiles.com/2010/01/07/audi-aerodynamics-of-planes-and-cars

or just google audi-aerodynamics of planes and cars.
It claims that a certain race car, complete with all the, adjustable, race car trim, air dams, wings etc. tested out in their wind tunnel, shows figures that demonstrate that the car running at 135 MPH, can produce enough down force,that it could run on the ceiling of the wind tunnel ! Down force it would seem, is a major addition of weight, ideal for tractive forces in corners etc, but not contributory to great fuel economy.
I would expect that VW has optimized the cars aero characteristics as currently designed and manufactured, in an effort to qualify it for maximum MPG. The addition of foils, air dams, rear deck lid spoilers, lowering of the car, etc. which might provide additional ' down force', and traction on the track would not, it would seem, contribute ( indeed would detract from ), toward MPG, and straight out down the road performance.
Bill
 

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Spoilers and air dams don't equal drag or downforce, you can tune them for either. spoiler tilted up = downforce, spoiler to smooth out air = less drag like on the Prius
 

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Discussion Starter #9
NJ2dc,
You are spot on right. But you'd need a wind tunnel to verify same.

But my point re the question of adding after market aero tuning bits, is that while they might give some added down force, they will probably compromise the OEM, wind tunnel optimized, body shape, resulting in increased drag and consequent diminishment of MPG and acceleration.
One could add a big wing on the back of a Prius. Do you think the cd would go down? There is a wager I'd like to place.
Bill
 

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You could increase aerodynamics by adding a camback rear to your wagen. Over at ecomodder, they've shown a significant increase in mpg by adding these, of course they are seriously ugly.

VW did not do everything they can to lower the cd as much as possible or they wOuld have implemented a camback like the prius
 

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Discussion Starter #11
Optimization

Glennco,
I think I'll have to disagree re VW not having done all they could.
Audi and VW have invested many millions of $'s / Euros, on the construction of ( their own ) wind tunnels and the attendant science involved in the optimization of the aerodynamic shapes of their vehicles. In addition, governments and cultures around the world, on becoming more aware of the impact of the use and diminishment of fossil fuels, are placing greater and greater demands on manufacturers to get their products to go further on the fuels they are burning.
VW, I'm sure, could have decided to design and produce, a Prius like camback, but their problem definition and product program called for a vehicle of greater volume and superior drivability. Given that mandate they put their models through their wind tunnels to get them as smooth as they were able, to maximize their performance, and improve efficiencies to the extent they could.
This has been an evolutionary process going all the way back to the Giugiaro 70's Rabbit, and like Porsche's 911, has resulted in a car that in my mind is pretty near optimal. I expect that future models / and design will give up some of their volume and probably engine size, they will become even lighter ( more aluminum/plastics/carbon fiber ), and they will get lower with lower cd's, and maybe even start to morph into a Prius like shape. But we will have to wait a while for that.
I'm a believer I guess.
Bill:bowdown
 
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