Aerodynamically speaking

[Gerrycan]

If the engineers had the last word rather than marketing, then cars might be covered in dimples like a golf ball...
Mythbusters showed they got better mileage when they put dimples all over a car.

Yes, I referred to Mythbusters before and they surprised themselves with its success. Interesting I can find no one that has done anything to prove or disprove their experiment, ironically at this stage they have created a new myth of their own

[kamold]

Yes, I referred to Mythbusters before and they surprised themselves with its success. Interesting I can find no one that has done anything to prove or disprove their experiment, ironically at this stage they have created a new myth of their own

Lol sorry should have read the whole thread.
Agree that a single experiment doesn't necessarily prove the efficacy of the dimples but it's certainly interesting that the effect does appear to scale up to vehicle size...

[Ramee]

Somebody got a hail damaged car ? Could be a nice testbed for that experiment !

Anyway resistance from air (drag) come in two ways , by flow separation (i.e, vortex generation) and skin friction.

In the case of a golf ball, we all know it has to be round (it can't be in any other shape) so flow separation create by the ball traveling through air is high. By creating dimples on the surface of the ball, it encourage the flowing air to stick to the ball so that flow separation is minimal and less wake behind the ball. But these dimples create micro vortexes along the surface and will increase the surface friction. For a ball,having the lowest surface area of any 3D shape and also being not very streamlined shape, drag create by flow separation is few magnitudes higher than drag created by skin friction. So dimples on a golf ball can will still benefits from less wake formation regardless of little increase in surface friction.

In the case of a car we have the ability to alter the shape of the car so that it can be more streamline through the air and create very less wake meaning less drag. But when you create dimples on a already streamlined car it probably leads to more drag because now skin friction has increased but no further improvement in flow seperation. So there is a fine balance between selecting proper technique or techniques for given application when talk about aerodynamic optimizations.

[Mountainman]

Don't they do the fuel economy test on a rolling road? Therefore aerodynamics doesn't come into it at all?

Maybe that's why the 52kg lighter, narrower, lower, more aerodynamic Golf Mk7 only gets the same fuel efficiency figures as the Octavia when running the same 103 TSI engine and DSG. Same with the 71kg lighter Golf 110 TDI. Less weight, smaller frontal area and better aerodynamics should only mean lower fuel consumption if engines, gearboxes, gearing and tyres are the same.

[Gerrycan]

Re Mythbusters:
I found an interview with the Mythbusters duo and they confirmed that Ford duplicated their test in a wind tunnel but did not get the same results and they admitted they may have to revisit this. Wish I had checked that before mentioning it.

Re Fuel consumption test I found the following:

The exact nature of testing is dictated by Australian Design Rules - ADR 81/02 Fuel Consumption Labelling for Light Vehicles, to be specific. It draws its procedures from United Nations Economic Commission for Europe (UN ECE) Regulations.

The vehicle travels nowhere. It goes for a 20 minute spin on a chassis dynamometer - the same kind of rolling test bench used to measure power (kW) and torque (Nm) figures.

Fuel consumption testing is divided into two sequential phases. Phase one, the 'urban' cycle, emulates stop-start traffic, while phase two, the 'extra-urban' cycle, emulates acceleration to a higher peak speed. The commonly quoted combined-cycle figure takes in both phases.

Much effort is put into making the test as realistic as possible. The dyno is calibrated to simulate aerodynamic drag and inertia - resistance to movement caused by the vehicle's mass. A fan set up ahead of it imitates the flow of air into its front-end intakes at varying speeds.

But there's a natural trade-off between standardisation and real-worldness, and for these purposes it's standardisation that's more important. After all, this is first and foremost about providing a specific yardstick against which vehicles can be tested. That means removing all the variables - road quality, weather conditions, driver behaviours, topography etc - that influence fuel consumption and emissions in the course of individual journeys by individual vehicles.

It's this standardisation that allows us to use the test and its results as a reliable ready-reckoner in comparing the environmental performance of different models, a spokesman from the Department of Infrastructure and Transport told motoring.com.au.

But bear in mind that standardising it reduces the results to the level of the theoretical, he adds. "It allows objective comparison between vehicles, but obviously no one procedure can simulate all real-world driving conditions. Actual on-road fuel consumption and emissions will vary depending on traffic conditions, vehicle condition and load and how individuals drive."

The official test takes 1180 seconds - just under 20 minutes. From the viewpoint of a manufacturer, it's not a big thing, says Ford spokesman Peter Fadeyev. "It's a small formality for our people with the arrival of each new model. The whole thing - setting up, running the test for 20 minutes and dismantling it afterwards - takes about an hour. Our people do it on site - the big car companies do. But of course we have to get our labs accredited independently."

The urban component of the test cycle, taking up about 800 seconds (a little over 13 minutes), assumes an average speed of 19 km/h. Some 30 per cent of this is spent idling, but that's divided up into 13 intervals, interspersed with a series of stop-starts designed to mimic heavy urban traffic.

Our departmental spokesman says this clear division between urban and extra-urban cycles is important, with the majority of drivers spending much of their time at the wheel in city traffic. "So it gives a more accurate pointer to the fuel consumption they'll actually experience than the combined figure."

The extra-urban cycle takes up the last 380 seconds - a little over six minutes. It assumes a relatively high average speed of 63 km/h. It peaks at 120 km/h, but doesn't sustain such speeds for any length of time because when the test was conceived it was decided a more elastic acceleration and deceleration envelope would better approximate real-world driving conditions and therefore fuel consumption. (If you have a trip computer, it's easy to confirm the principle behind this. Next time you take a long trip, reset it and check your average speed when you've finished - chances are you'll find it's much lower than it seemed.)

Tests are typically conducted on a sample vehicle from a vehicle platform using each available powertrain rather than individual models. Take, for example, Volkswagen's Golf. It comes in hatch, wagon and cabrio variants (sedan, too, until mid-2011 when the Jetta was announced as an independent platform) with an array of petrol and diesel engines of varying power output married to manual and DSG transmissions. Rather than test every variation, testers put each powertrain on the dyno and use a look-up table to account for weight differences between the different body types.

Because some European models are made to run on PULP, Australian testing uses 95RON fuel for all petrol cars, to level the playing field.

There is a bit more at Fuel-consumption figures: A guide at best - motoring.com.au

[woofy]

Hmm the 95RON thing will certainly throw out some figures. Mazda probably did better with their petrol 6 when they were tuned for 95 not 91. Similarly the RS wouldn't be as happy on 95 as 98.

[Gerrycan]

Hmm the 95RON thing will certainly throw out some figures. Mazda probably did better with their petrol 6 when they were tuned for 95 not 91. Similarly the RS wouldn't be as happy on 95 as 98.

I thought that was bit unusual too, but it may just be a journalist error applying a comment beyond its intended context.
Journalists are only human...reportedly.

[pologti18t]

If you want to fiddle with aerodynamics have a look at Julians articles (some involve a Roomster)

AutoSpeed - Undertrays, Spoilers & Bonnet Vents, Part 1
AutoSpeed - Undertrays, Spoiler & Bonnet Vents, Part 2
AutoSpeed - Aero Basics
AutoSpeed - Blowing the Vortex, Part 1

[GTR27]

Would explain Hyundai's good figures that no one can seem to achieve (I find that most Hyundai drivers will put whatever is cheapest in there). Explains a lot of results really.

[Gerrycan]

If you want to fiddle with aerodynamics have a look at Julians articles (some involve a Roomster)

Julian Edgar is my hero! I have followed his motoring adventures and writings for years.
His interests pretty well mirror everyone of mine, the difference being he gets off his arse and does something (everything). a true genius in my opinion.
It is a damned shame for the public that he was ostracised by manufacturers from further car appraisals for being too honest and open in expressing his views.
Must get his book on car aerodynamics for Christmas.

[Gerrycan]

Would explain Hyundai's good figures that no one can seem to achieve (I find that most Hyundai drivers will put whatever is cheapest in there). Explains a lot of results really.

Too funny, I was a Hyundai owner for many years so that explains everything
I think there are a couple of reasons for the Hyundai's relatively poor real consumption. 1) For a number of years they just lied (Note American class action and settlements) 2) Nowadays, in order to appear to have good performance, the examples I have driven have sensitive throttles so with great care you can achieve reasonable economy but 'normal' driving can be quite punishing on consumption.

[GTR27]

Prob explains why I get as good or better from a lot of vehicles as I only use 98. Except in our jazz. I think 7 is the worst I have seen, for flogging it relentlessly.

Turbo cars vary a lot more between flogging and gentle. Just have to get the missus to realise that haha. I can see 10+ with her driving, she is a lead foot :p

[Gerrycan]

Same with my wife.
I have no issues with accelerating with reasonable aggression, it is socially responsible, the engine is operating more efficiently and the Octy 3 2nd gear is fairly long anyway. From around 3000 to 3500 rpm you can comfortably short shift to 4th or 5th for maintaining urban speeds economically.
She likes to drive around in 3rd which is fine and hardly noticeable with the smooth petrol engine but it kills consumption. I have learned to stay quiet. Open road driving, no problems.

[Gerrycan]

Maybe it will be of interest to you and to others that there were some cars designed by Czechs, which had even lower coefficient of drag than the Golf anreven B200 you've mentioned. So, they could probably do a better job in the aerodynamics on the new Octy, if they had a good reason for it.

Just Google Tatra 77a. It had a drag coefficient of 0.212 back in 1935.

Speaking with an even more senior citizen than myself recently about our respective cars, as you do (his is a BMW).
The topic turned to Skodas in general and he said that he remembered their first foray into the Australian market, his mate had a rear engine job. Initially I thought he was talking about a 110 but turns out it was the much earlier Tatra 77 and I was better informed on this than I would have been without this thread.
He remembers it as really big but a bit too heavy for the power of the engine.

[wai]

The vehicle travels nowhere. It goes for a 20 minute spin on a chassis dynamometer - the same kind of rolling test bench used to measure power (kW) and torque (Nm) figures.

And if you have ever watched a test being done, you will see there are points where the "chart" has to be stopped because the vehicle being tested has no way of ever catching up. And I am not talking of tiny engined cars. Back in the late 1970's, I worked for the DMT and one project involved having two vehicles (202 Kingswood and 202 Torana) tested by the EPA. For both vehicles, the chart had to be stopped to allow the vehicle to catch up.

Most manufacturers achieve these high CD values with many "tricks". The wipers are removed; the external mirrors are removed; the road wheels (and ventilated brakes) are stationary; the radiator intake is blanked off; brake ducts are blanked off; any external antennae are removed; etc. Now they do not remove all, but they do all do something to get the best value.

In the real world, you have turbulent air, and the direction is not always the best. You have a whole lot of other factors that come into it.

All a CD value does is give you SOME idea as to its efficiency, however unless you know how it was achieved, it does not tell you much. It's a lot like fuel economy figures, and ANCAP ratings. They do not always tell the whole story.