Aerodynamics of road cars

[J.A.W.]

At one time it was also said that people wouldn't accept mobile phones without keys. Sad that the world works this way. What one could do and what one could sell are two very different things, and it doesn't always make sense.

Did you mean to post your comment on the cell phone thread, Hollus?
It might make sense then.. quality cars are surely still - a bit more involving..

[Greg Locock]

This thread seems to be assuming that a significant proportion of new car buyers make a buying decision partly based on Cd. That's a very ...engineering... assumption and no model of buyer behavior that I have seen supports it. They do care about fuel consumption, but there are more important factors.

The opinion of people who do not buy new cars is frankly irrelevant in the context of production cars. That covers many enthusiasts (and me-I've never bought a new car in my life).

[J.A.W.]

The opinion of people who do not buy new cars is frankly irrelevant in the context of production cars.

Not necessarily Greg, since such purchase decisions - may well be subject to the views/veto of significant others..

"Ohhh.. that one looks swoopy.. but in that colour, noooo... I wouldn't be seen dead, in that.."

[henry]

The Audi A2 perhaps gives an example of what is possible in a conventional aesthetic. 0.25 in its best, 3 litre, form.

It also shows how hard you have to work to achieve this since in its larger engined, bigger tyred, versions it was already back up to 0.29.

[Greg Locock]

Some companies use ...problematic... wind tunnels that give consistently low results. I've seen the results of a proprietary round robin where the same representative vehicles were tested in many tunnels around the world, and mods fitted and removed.

The upshot is that you can't directly compare Cd from different tunnels, and different car companies in the same tunnel testing the same vehicle will get different answers.

[J.A.W.]

Ok, but they - surely - still do basic real-world 'coast-down' tests - for co-relational aero- value comparison..
(& measured power/drag top-end speed runs)..

[Tim.Wright]

Coast downs have got their own sources of errors as does any type of measurement or simulation.

Factory declared Cx values are like 0-100 times and lateral acceleration values. There are too many variables in the measurement method to render any of them reliable for comparison against anything other than the same manufacturer's previous car.

Its like the old saying: the best way to find another 20hp is to put the engine on another dyno.

[J.A.W.]

C'mon T.W, those variables are easily controlled here in the C21st..
..hell, engineers successfully put Astronauts on the Moon ( & returned 'em safe, too) nearly 1/2 a century ago..