Aerodynamically speaking

[Gerrycan]

In a previous life and a country far away, I used to fly gliders which has resulted in a lifelong interest in practical aerodynamics (I run a mile from the mathematics).
In the 1970's the mostly German, tadpole shaped fibre glass gliders, had made a huge performance leap from the previous generation of wood and fabric. Modern materials (carbon-fibre, Kevlar etc) have contributed to improved flying performance of current gliders but the aerodynamics seem much the same indicating that some sort of practical limit has been reached.

Similarly cars aerodynamics seem to have stagnated with a drag coefficient of around 0.3 since the introduction of the Audi 100. Of course there have been many individual backward steps since that time such as the VE Commodore (0.36) and SUV's in general (0.37 - 0.45) and some forward steps with dedicated 'futuristic' shaped economy models such as the Toyota Prius, Honda Insight (0.25).

Now do not get me wrong I think my Octavia 3 is great value and performs better in most areas than many more expensive rivals and is still my vehicle of choice. However, you would think that when they introduced the new Octavia 3, nine years on from the 2, that it would have improved aerodynamics over the previous design, but it appears not as they stayed at 0.3 for the sedan and 0.31 for the combi.

I did not really pick up on this as I considered the huge and practical interior was probably the price paid for the lack of change but two things have made me consider that Skoda have missed an opportunity
1) The Golf wagon is quoted with a drag coefficient of 0.27
2) Mercedes updated B200 models are quoting 0.25 and this is staggering because it is really a very ordinary looking large and high, 5 door hatch vehicle.
There is no exaggerated styling, cowls over the rear wheels, compromised internal head-room. Mercedes have achieved this with impressive figure with excellent (but not obvious) aerodynamic engineering.

Now I'm quite miffed, how does the Golf wagon get 4 points better rating than my better looking (imo) Octavia wagon and how on earth does Mercedes make a virtual van 6 points better?

The Mazda 6 and the Hyundai I40 wagons get around 0.27-ish but they are compromised inside, have more raked screens and pretty much dedicated two wheel drive designs, so their undersides are probably where most gains are made.
Don't be fooled by their vision impairing, rising side window lines as they contribute nothing aero, just the looks.

Nope, I beginning to think that Skoda may just have been a bit lazy in this area, what do you think?

[Transporter]

How does that better aerodynamics of the Hyundai and Mazda translate to the fuel efficiency, practicality...?

[Mysticality]

How does that better aerodynamics of the Hyundai and Mazda translate to the fuel efficiency, practicality...?

Naturally aspirated petrol engines; not really something to compare...

[Transporter]

Naturally aspirated petrol engines; not really something to compare...

The aerodynamics and the fuel efficiency goes hand in hand.
Is the Skoda noisier inside than Mazda6 because of the aerodynamics?

[Gerrycan]

How does that better aerodynamics of the Hyundai and Mazda translate to the fuel efficiency, practicality...?

Dammit, now I'll have to do some maths! I'll have to do some research and get back to you.
My former 1.9 diesel Octy 2 fuel consumption did not seem to be influenced much by air-conditioner use, headwinds / crosswinds or going a bit faster (when the opportunity presented), but my petrol Octy 3 consumption is definitely more sensitive to these factors.

[Transporter]

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.

[Ramee]

Here is a video showing how Merc B class achieved that very good .25 drag coefficient. They have basically tried to reduce turbulence through streamline design of the side panels and roof so less vortex formation behind the vehicle.



But drag coefficient only shows a part of the story, What really matters is the drag it self not the coefficient. Drag is basically calculate by, Drag = Cd x A x K where A is the cross sectional area perpendicular to the travel and K is a variable depend on vehicle speed and air density.

So if the car has lager cross sectional area (e,g Merc B200 ) with low Cd will create same amount or higher drag compared to a car with lower cross sectional area with bit higher Cd. So most probably your octavia experience lower air drag than that over hyped Merc B200.

[Gerrycan]

Transporter, that is the first I have heard of the Tatra 77a and the Cd figure is excellent so thanks for that interesting info.

Ramee has supplied the formula for estimating drag and I do not disagree with anything he has stated.
All other factors being equal, a reduction of the Cd by 10% results in a near 6% reduction in fuel consumption at a steady 110kph.

I did appreciate the effort Skoda put into producing a larger and much lighter car with very efficient drivetrains, but I stand by my initial statement that Skoda should have reduced the Cd on the new Octavia as VW did with the Golf wagon.

After all the TV Mythbusters team unintentionally managed to reduce the Cd of a Ford Fusion by more than 20% just by covering it with clay and putting large golf ball type dimples into the clay.
You can usually shoot cannon balls through the holes in their methodology with regards to car fuel consumption myths but that one was particularly elegant and well executed.

[woofy]

I guess the answer as Transporter was getting at is that there are a few factors in improving fuel efficiency which is the ultimate aim here. You have two teams at odds or of disposal here, the design team who want to make things look good and the engineers who want things to be practical. Either one can try and reduce consumption, but with Skoda they would go for bang for buck. As you said the difference in Cd hasn't changed and yes some are .04-.05 lower but in real terms does this have as big an effect as lowering overall body weight and changing some engineering. They would for sure have run these in wind tunnels etc, so it depends on what was more cost effective, something MB aren't exactly as compromised on, and they have made it a big selling feature...but it could be a complete red herring and have minimal effect at the end of the day.

[Gerrycan]

I guess the answer as Transporter was getting at is that there are a few factors in improving fuel efficiency which is the ultimate aim here. You have two teams at odds or of disposal here, the design team who want to make things look good and the engineers who want things to be practical. Either one can try and reduce consumption, but with Skoda they would go for bang for buck. As you said the difference in Cd hasn't changed and yes some are .04-.05 lower but in real terms does this have as big an effect as lowering overall body weight and changing some engineering. They would for sure have run these in wind tunnels etc, so it depends on what was more cost effective, something MB aren't exactly as compromised on, and they have made it a big selling feature...but it could be a complete red herring and have minimal effect at the end of the day.

It pains me to say it but you and Transporter are right. I am looking at it from only one perspective and I have ignored (many) other considerations.
Improved aero performance will have little effect on the official fuel consumption tests which is largely stop /start and a quick dash up to 120 kph. Engine /drivetrain efficiency and overall weight are far bigger influences on these results.
It is quite likely that if the fuel consumption test format was changed to include a bigger highway cruise component then aero would suddenly become more important for all manufacturers because it would reflect in their published figures, but they have to work to current regulations.
Another consideration is that commercially, good styling is far more important than good aero (or good anything) to a large percentage of the buying public so even MB have had to be very subtle with the B200 aero improvements so it still looks like an ordinary vehicle.
Improved aero is not a 'red herring' to anyone that exceeds 80kph for any length of time.

After all that rant I have to admit to being a hypocrite as I have fitted mud-flaps to my Octavia 3, and that cannot be good for aero