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Does anyone consider the manufacturers drag coefficient any more when choosing a car, or has this evolved to the claimed mpg as per the manufacturer. With the introduction of more and more brick shaped SUVs etc it would appear to be still very relevent in achieving the optimum mpg. Also has any contributor noticed that even if they give their car a good polish it seems to cut through the air that little bit better or is it all in the mind!
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Drag coefficient is one thing but frontal area is also against the taller vehicle.
The Outlander I drive has a claimed Cd of 0.30, not bad at all. But the total drag is proportional to CdA, A being the frontal area. I have never seen the frontal area quoted.
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But the total drag is proportional to CdA, A being the frontal area. I have never seen the frontal area quoted.
ISTR being told the easiest way to improve the CdA was to hitch up a caravan.
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Drag coefficient is one thing but frontal area is also against the taller vehicle.
The Outlander I drive has a claimed Cd of 0.30, not bad at all. But the total drag is proportional to CdA, A being the frontal area. I have never seen the frontal area quoted.
As a car obsessed youth I used to often send away for brochures and I remember getting some Alfa Romeo ones, including for the 75 and 90. They had the cda figures quoted, though I don't remember what they were, and went on to point out how much more important they were than the cd figure.
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Does anyone consider the manufacturers drag coefficient any more when choosing a car, or has this evolved to the claimed mpg as per the manufacturer.
The only time I remember a car advert quoting the drag coefficient was for the Audi 100 from the 1980's.
Advert on youtube (anyone remember it?): www.youtube.com/watch?v=vSNoqeYHBQ0
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I remember it very well. A pal of mine who sold them came to my house in one, and I took it for a run. It wasn't a very powerful car, a 2 litre I think, but where most cars just hit a wall of air resistance in those days and topped out well below the ton, the Audi just kept going in what at the time was a really unexpected way,
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The worse car i've ever driven for this wall of air effect was the little Citroen C2VTS we owned for a while, it was amazingly economical up to about 70mph, but once you got to 80 you could see the fuel use increased massively on the instant readout, and vehicles like the large C5 can be more economical on the motorway with the same engine.
It's no longer possible to travel fast enough on the roads most of the time to worry about such things as Cd, so no we don't consider it.
Our cars being both full time 4WD there is more transmission drag than streamlining could make up for.
For a living i drive tankers now, pressure tanks rounded off smoothly at each end, these cleave through the air easily only marred by the catwalk on top.
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My old Transit Connect van had a similar thing, cruising up to about 50 or 60mph was fine, but once you go above those speeds the bluff shape, particularly the windscreen really start to affect how much throttle was needed to maintain 70 or 80mph. And of course at this point, the economy starts to suffer dramatically.
To be honest, i don't think many people pay much heed of the drag coefficient, and rightly so. Unless you happen to live in Germany you are not going to be travelling at the sort of speed where it makes a meaningful difference. I was amused by the advertising of the Mitsubishi Mirage described as being the most aerodynamic car in its class, as if any of its typical buyers would care!.
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In my estate car, on a clear run, upwards of 50mpg is easily achievable with my bikes inside, with the bikes on the roof, it drops to about 32mpg on a similar journey. Same weight, different drag.
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That does surprise me Alby, i mean i knew there would be a difference, but i didn't think it would be as much as that. I'd have to assume that is motorway speed?.
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As has been mentioned, the Cd figure is not as important as the CdA figure (as Cd does not take into account the frontal area), but i still find this Wikipedia list of cars and their Cd figures interesting,
en.wikipedia.org/wiki/Automobile_drag_coefficient
Of particular interest to me is some of the older cars whose designers clearly had a much better understanding of aerodynamics than others. The Rumpler Topfenwagen, as an extreme example of this, is a car produced nearly 100 years ago (1921-1925) which has a lower Cd figure than BMW i3. And what an amazing looking thing, imagine the reaction if you dropped the kids off at school in this!,
upload.wikimedia.org/wikipedia/commons/0/00/Rumple...g
Also the 1935 Tatra T77A which has the 3rd lowest Cd figure listed for a production car (after the VW XL1 and General Motors EV1). Though apparently there is some debate as to whether or not the test was done on an actual car or a 1:5 scale model.
www.supercars.net/blog/wp-content/uploads/2016/04/...g
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>>I'd have to assume that is motorway speed?.
Yes of course, motorway journeys are the worst affected, but even just pottering about on A roads there is a significant difference. A physical manifestation of it is very noticeable if you open the sunroof. Without the bikes "upstairs" there's very little wind noise even with the roof open until you're really bowling along, but at anything over 30mph with bikes on and the roof open, the wind noise is pretty loud.
I could of course get a towbar mounted rack which would help with both issues, but that in and of itself presents another physics problem, in the form of my own inertia ! ;-)
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Yes, i recall taking bikes on the roof to France in an Astra Sri estate and being horrified at the drop in economy. Similarly a roofbox on top of an Octavia knocked 5 or 6 mpg off. Towing a small trailer on the other hand had negligible effect in comparison.
We live in a cheap oil phase at the moment, though the price is rising. Hence there is less consideration of fuel economy when choosing vehicles. Also, vehicles including SUVs are considerably more economical than 20 years ago. I don't think the price of fuel hurts motorists as much as it has in previous years, though another oil shock would be interesting to observe, especially as diesel sales and indeed choice are on the decline and will likely continue so.
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I really noticed the difference that CdA made with a Vauxhall Omega and a Vauxhall Calibra - both automatic. Whilst fuel consumption was much of a muchness around town the Calibra was over 5 mpg better on a long run.
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For a living i drive tankers now, pressure tanks rounded off smoothly at each end, these cleave through the air easily only marred by the catwalk on top.
Much better than those car transporters GB? As a caravanner I would say that those things cause more turbulence than anything else I can think of - I give them a wide berth on motorways as far as possible.
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Much better than those car transporters GB? As a caravanner I would say that those things cause more turbulence than anything else I can think of - I give them a wide berth on motorways as far as possible.
You don't notice it in the transporter so much MT, many modern transporter configurations are unstable anyway so the effect of wind is quite a long way down the agenda of things to worry about, unless the wind starts a caravan style pendulum sway going. :-)
Yes indeed, tankers are rock solid on the road, i would expect them to be pleasant enough to be near with a caravan because when we pass trees etc we hardly cause any draught to speak of.
Edited by gordonbennet on 07/05/2018 at 14:10
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