how areodynamic are convertibles?

[modmonster]

hi

i just saw an add on telly for a convirtable and the angle from wind screen to back was about 12deg. i bet with a cap it would be pretty areo?

[redyaris]

I suspect that convertables are only marginaly more aero than the average sport bike! Which is not that good.

[aerohead]

The Karmann Ghia convertible,with top up,was one tick lower than coupe,0.38vs0.39.
It's the only example I know of where a convertible scored lower than hardtop.
Of course,with the top down,they're sky-scrapers.

[brucey]

I have access to a convertible. I think I'll run a test in a few days with the top up and the top down to see how the mileage is.

[MetroMPG]

I have a convertible now as well.

Expect some aero testing and aeroshell prototyping later in the year.

saw an add on telly for a convirtable and the angle from wind screen to back was about 12deg. i bet with a cap it would be pretty areo

I bet your bet is right.

The potential to reduce drag by aeromodding a convertible using a properly shaped hard top/tonneau is probably greater than the equivalent hard top car it's based on - at least within the vehicle's existing length.

[MetroMPG]

And to address the question in the title of the thread, I sure wish I had my Hucho book (BEN! Bring it to the GGP in Watkins Glen next month please) there's an entire section about convertible aerodynamics.

Here's a neat tidbit: the drag coefficient of the convertible version (with the top down) of the world's most slippery-shaped production car is 0.14 higher than the coupe it's based on:

The E-class coupe carries what Mercedes claims is the lowest drag coefficient of any production car, ever: 0.24. The convertible slips by at a slightly worse 0.28 with the top up. Drag increases to 0.33 with the top down.

Source: 2011 Mercedes-Benz E-class / E350 / E550 Cabriolet - Official Photos and Info - Car and Driver

[gone-ot]

...short answer: NOT very aerodynamic!

...the late Fireball Roberts (of Daytona Beach fame) noted that the first races at Daytona raceway had two classes: convertibles and hardtops, but due to the "poor" aero of the open cockpits, nobody wanted to run convertibles...such that on race-day, French was "paying" drivers something like $2,000 to cut the tops off their hardtops and run them as convertables!

[lunarhighway]

while driving with my boss in his audi A4 convertible we where dicussing milage. he has a friend who has the same car with the same engine but as a hardtop, and he noted that even with the top closed he got slightly worse milage on average than this guy. of course they may have a different driving style etc but the science is there to support these observations.