Aerodynamics of a convertible

[bowtie2bimmer]

So a buddy of mine and I were discussing whether a convertible is more aero with the top up or down...

My theory is that with the top down it works like the bed of a pickup. It creates a negative pressure zone and the air will flow over it. While with the top up the surface is uneven due to the top frame and there are gaps between the top and body/ windows etc.

Figured I'd get some opinions. Discuss...

[jeffnhiscars]

Actually, the bed of a pickup truck creates an ENORMOUS aerodynamic drag unless you put the tailgate down. That's why they sell those mesh tailgates.

Compare that to the back seat of a Cabrio with the top down (kind of the same effect). Now add the wind deflector that eliminates back currents (and drag) and I vote top up is more efficient by far.

[bowtie2bimmer]

Its been proven that with the tailgate up trucks are more efficient than with it down. It was on Mythbusters even...

[B-well]

Actually, the bed of a pickup truck creates an ENORMOUS aerodynamic drag unless you put the tailgate down. That's why they sell those mesh tailgates.

Compare that to the back seat of a Cabrio with the top down (kind of the same effect). Now add the wind deflector that eliminates back currents (and drag) and I vote top up is more efficient by far.

like bowtie said. gate up had less drag

they made gator nets for rednecks when they break off their old tailgate

[bowtie2bimmer]

To your point I'd say top down with the screen would do better as the air pocket inside the interior is more still with it....

[CorvallisBMW]

Top up is definitely more aerodynamic from my experience. A pull from 80-100 with the top up vs top down results in a very noticeable difference.

[andrewm]

I think the top up is going to have a better drag coeffecient.

[bowtie2bimmer]

I think the top up is going to have a better drag coeffecient.

Your mom has a better drag coefficient
:jk:

[jeffnhiscars]

Its been proven that with the tailgate up trucks are more efficient than with it down. It was on Mythbusters even...

This may apply to a newer designed trucks with a shorter bed where they actually thought about the "pillow of air". I think they disproved it as a generalization but I don't think it qualifies as the new prevailing wisdom....but then who am I to question Mythbusters

[andrewm]

lol...that's kind of a compliment

[jeffnhiscars]

MB has its place in modern science and culture.

I did search it and so many comments reference Mythbusters, which means no one knows anything more than they see on tv. When you read how they did their test, it is totally unscientific and applies just to that truck on that day in that environment with those 2 drivers.

The principles have merit but I doubt a 79 F150 w an 8ft bed will get better mileage with the gate up.
\
Mythbusters busted :-)

[brianao34]

^You're basing your hypothesis on unicorns and pixie dust, and that's supposed to trump MB's data/results? Ha

[frankenbeemer]

“Putting the tailgate in the down position tends to increase turbulence and drag of the open box” said Jack Williams, one of Ford Aero Systems Engineering team when asked this question a couple years ago during an F-150 test drive. .

“We’ve seen drag increases of between .5- to 1-percent when a pickup tailgate is left in the down position,” said Williams who spends considerable time testing vehicles in the most sophisticated wind tunnels in the world. “Those flow-through, web-like fabric tailgates tend to increase drag even more; we’ve seen increases as much as 4- to 5-percent.”

[Vivek]

MB has its place in modern science and culture.

I did search it and so many comments reference Mythbusters, which means no one knows anything more than they see on tv. When you read how they did their test, it is totally unscientific and applies just to that truck on that day in that environment with those 2 drivers.

The principles have merit but I doubt a 79 F150 w an 8ft bed will get better mileage with the gate up.
\
Mythbusters busted :-)

Lol so basically they tested a truck, you tested nothing, so therefore you're right?

[jeffnhiscars]

Lol so basically they tested a truck, you tested nothing, so therefore you're right?

read my comments. I never said "I was right and they were wrong"..only that they have punctured a hole in an age old belief but havent proven the opposite is true by considering all the variables. They are correct "on that day, with that truck etc etc"

What they do is rooted in science but is also somewhat tongue in cheek (ie infotainment)and def not controlled by any scientific standard.

None of it is conclusive regardless of what I've tested and agaim...just cause it's on tv doesnt make it so.

I'm waiting for someone to tell me Mr Rogers drove an e30 :-)