How does opening your sunroof affect drag?

18 March 2014

Question

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

Answer

Chris - The weather has been dramatically better all of a sudden, hasn't it? So, 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 he's 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 - if you imagine it - all the molecules, all the stuff in the air that the car is hitting. So, that's increasing drag. What's interesting, if you go on from this though, if you factor in air conditioning, it's a constant worry isn't 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, "Right, at that speed, I need to lower the windows and over that speed, I need to turn the air conditioning on." 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 it's going to be the same thing - 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, that's 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 it's knocked I think probably 5% since I added the roof rack. It's not huge.

Richard - But that's quite significant, given how light the car is and a tank of petrol costs so.

Chris - Diesel.

Richard - It's a lot of diesel, yeah. Costs even more, yeah 5% of that.

Chris - Yeah, but it's more miles to gallon on a diesel car because it's more efficient fuel Richard - Yes, but 5% of that.

Chris - Yeah, it is. It's quite a lot, so I'll have to take my roof rack off.

Richard - You will, yeah.

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