Aerodynamically speaking

[brad]

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


Take off the mudflaps & all the stone chips will give a golf ball effect & further improve aero.

[Gerrycan]

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


Take off the mudflaps & all the stone chips will give a golf ball effect & further improve aero.

I have not found anything on the web to confirm this but you would think that the rolling road would have to be pretty sophisticated as would the configuration to take into account, total drag, vehicle weight and goodness knows how many other things such as freewheeling capability, braking energy recapture etc etc. Otherwise the fuel figures would not reflect the manufacturers investment in the technology.

I imagine there is many a temptation to tweak the figures, or make an error...and notice I did not say Hyundai ('cept then)

Wonder if anyone noticed a fuel consumption improvement after hail damage to their car ?

[team_v]

Another consideration is the safety aspect.
Given that cars are being designed more to comply with crash ratings and passenger/pedestrian safety as a selling point it makes sense that they reduce the focus on drag coefficient and instead focus on better fuel efficiency through engine technology.

[brad]

I imagine there is many a temptation to tweak the figures, or make an error...and notice I did not say Hyundai ('cept then)

holden changed the tyre pressures in the Commodore simply to gain a slight fuel economy improvement in a mid-model update. Plus they added a light to the dash to get an extra ANCAP star.

[kamold]

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.

[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