Aerodynamic efficiencies - convertibles

[Pints]

Presumably a convertible has the least drag when the top is up, therefore providing the best fuel economy like this.

But with the top down, is it more efficient to have your side windows up or down?

And what about the rear wind-deflector; more aerodynamic with or without?

These little things make a difference when driving at motorway speeds, so was curious as to what the facts are.
I know little to sweet sod-all about drag coefficients, etc. (apparently a Viper is more slippery than a Transit) so go ahead and educate me.

[Traveller]

I figure the answer to those questions is yes and yes, wind deflector prevents the swirling bubble of air forming over the passenger compartment, much like a pickup's load bed, thus less drag and the windows up would be less turbulent air travelling over the vehicle so less drag.

I maybe be totally wrong, terms and conditions apply, only available on Wednesday.

[jmorgan]

Never worried about about it,not raining, top down.

[Pints]

Not worried, just curious.

It feels more convertible-esque if the side windows are down, but at higher speeds the turbulence in the cabin seems much worse.

I suppose (based on my extensive knowledge on the subject) that that equates to increased drag.

[GroundEffect]

It's very hard to judge as each case is individual...but I'd presume you gain little from the windows closed as the phenomenon is that you are feeling the turbulence rather than it just occuring further down-stream.

But yes, cars with the roof down are demonstrably less efficient than the roof up (actually, a convertible which generally has a steep rear window is the worst shape you can have - even with the roof up).

[kambites]

Generally speaking, you can tell by how smooth the airflow over the car feels and sounds.

Obviously roof up, windows closed is best; in my experience that's followed by roof up windows down; then roof down, windows up; roof down windows down. I've never owned a car with a deflector to know how good they are.

[durbster]

I always thought it'd make a huge difference but I'm currently getting around 37mpg from my MX-5 with the roof down pretty much exclusively. That's as good an economy as I've ever had excluding long runs so it can't make a massive difference.

[kambites]

durbster said:

I always thought it'd make a huge difference but I'm currently getting around 37mpg from my MX-5 with the roof down pretty much exclusively. That's as good an economy as I've ever had excluding long runs so it can't make a massive difference.

It's not huge, no. Certainly <10% on the Elise, even at motorway speeds. In practice I actually average better economy on the motorway with the roof off, because I drive slower.

[DrTre]

The Saab just seems to go from Keith Floyd with the roof up to Keith Richards with it down, in terms of drinking.

[alock]

Mythbusters did a pick-up truck with the tailgate up and down. Tailgate up was more efficient than tailgate down. I would imagine a wind deflector in a convertible acts in the same way.

[Snowboy]

I would guess the roof off would be less efficient.

But, I would assume a noticeable increase in MPG would offer a similar decrease in performance.
For example, a 10% increase in wind resistance/drag would be the equivalent of driving up a 10% incline.

When I have the roof down I don't notice that the car is at all sluggish or struggling – so, I will presume that the difference in MPG is marginal.


I eagerly await the rocket scientists to prove me wrong.

[Fatman2]

jmorgan said:

Never worried about about it,not raining, top down.

Incidentally I got caught out in the rain with the top down and actually stayed bone dry until I slowed down to about 40mph. Thought it was an urban myth but clearly not (and it was lashing it down too).

Have to say the mpg thing doesn't even register for me. Not because I don't care about mpg but for the amount of times the top is down I think it's negligible.

[jmorgan]

Fatman2 said:

jmorgan said:

Never worried about about it,not raining, top down.

Incidentally I got caught out in the rain with the top down and actually stayed bone dry until I slowed down to about 40mph. Thought it was an urban myth but clearly not (and it was lashing it down too).

Have to say the mpg thing doesn't even register for me. Not because I don't care about mpg but for the amount of times the top is down I think it's negligible.

The rake on the wedge is good for that, and the loud peddle. Thing is, I know it is not good, on paper I think the fixed head version of mine could eke a few more on the top end but then I already have the aerodynamics of a brick and mpg around 15 on a good day so I go by how far apart are the stations not mpg with the lid up or down. If that makes sense.

Lid down as much as possible by the way, the TVR roof police are vicious..... point fingers at you and go "ner ner ner"