Science Questions

How does opening your sunroof affect drag?

Tue, 18th Mar 2014

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ibn yusuf asked:

Does opening the sunroof of a car INCREASE or REDUCE drag?


Chris -   The weather has been dramatically better all of a sudden, hasnít it?  A carSo, this question is highly appropriate.  Yusuf wants to know Richard, does opening the sunroof on the car increase or decrease drag?  I presume he means while heís actually driving along.

Richard -   Yes, is the short answer.  It will increase your drag because what you're doing by opening the sunroof of a car, you are disrupting the airflow.  You're increasing drag and drag is essentially Ė if you imagine it Ė all the molecules, all the stuff in the air that the car is hitting.  So, thatís increasing drag.  What's interesting, if you go on from this though, if you factor in air conditioning, itís a constant worry isnít it, for people with air-conditioning.  You turn the air conditioning on then you are using more energy, but if you open the windows and you're increasing drag.  So, is there a magic point?  Is there an actual point where you can say, ďRight, at that speed, I need to lower the windows and over that speed, I need to turn the air conditioning on.Ē  Yes, there is and General Motors have done this research.  It depends on your type of car, but roughly, around 40 miles an hour.  So, if you're under 40, so in a built-up area, generally, it is more efficient to lower your windows.  Above that certainly on a motor way, it is better to shut your sunroof, shut your windows and use the air conditioning.

Chris -   There you have it, but they have the same thing about sunroof?

Richard -   No, but itís going to be the same thing Ė windows, sunroof, same idea.

Chris -   But also, sunroof has a confounding factor which is, if you go up or fully back.

Richard -   Yes, you could do that.  So, suppose if you went up, thatís going to disrupt the air flow more than if it was open and the air is going to come into the car.  You always get the annoying whistle although they haven't really factored that in either.

Chris -   I've got a roof rack so I get that annoying whistle.

Richard -   Yes, so you're really driving inefficiently.  You should take that off.

Chris -   Actually, I've looked at this because my car logs its mileage consumption and itís knocked I think probably 5% since I added the roof rack.  Itís not huge.

Richard -   But thatís quite significant, given how light the car is and a tank of petrol costs soÖ

Chris -   Diesel.

Richard -   Itís a lot of diesel, yeah.  Costs even more, yeah 5% of that.

Chris -   Yeah, but itís more miles to gallon on a diesel car because itís more efficient fuel

Richard -   Yes, but 5% of that.

Chris -   Yeah, it is.  Itís quite a lot, so Iíll have to take my roof rack off.

Richard -   You will, yeah.


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Has all of this been confirmed with empirical testing?  When I was driving my parent's Prius, I could see a significant change in MPG with our without the AC, much greater than I was seeing by rolling down the window.  Of course, I often roll down the window and stick the whole arm outside to increase air flow inside.

The sunroof creates a gap in the air flow, but if it doesn't have a scoop effect, it may not create significant drag, perhaps like the golfball effect, although the SA article seems to discuss how the dimples affect the air flow behind the ball.

I have seen reports comparing pickup mileage with 3 cases:
Standard Tailgate in upright position: best mileage
Standard Tailgate in down position: second
Mesh Tailgate: worst mileage.

Anyway, it was interesting that the standard tailgate up position did remarkably well, pulling along trapped turbulent air with the vehicle. CliffordK, Wed, 2nd Apr 2014

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