# How much does a vinyl roof hurt a cars aerodynamics?



## KiwiEV (Jul 26, 2007)

the slashmaster said:


> I was wondering how much you guys think a vinyl roof on a regular car or a ragtop on a convertable hurt the drag coefficient?


I doubt the type of fabric would make much overall difference, but convertibles (especially with the hood down) are normally pretty rubbish with aerodynamics. I know the MX5 (called the Miata in the USA) has quite poor aerodynamics despite the fact it looks like a greased jellybean. 
The aerodynamics should improve with the hood up and reduce with the hood down as the windscreen acts like a vacuum. That's why you always seem to get a part in the back of your hair while in a convertible. It's plain annoying when you're driving around trying to impress girls and it looks like you just go out of bed.


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## Idrag (Feb 6, 2008)

I remember when there was a lot of speculation that the grain of a vinyl top would actually improve aerodynamics by altering the boundary layer of air...or something. I believe Richard Petty used a vinyl top in Nascar about 1970 or so, although many speculate that was to disguise altered steel roof panels. 

Convertible tops really puff out at high speed, that can't be good.

Convertibles are usually heavier, as are vinyls, if only by the weight of the vinyl.

Sorry, don't know the answers for sure, but an interesting topic.


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## ronin4sale (Jan 29, 2008)

There are 2 types of drag which the roof could effect. Induced and Parasite. Induced being the far larger amount of drag which is simply the area of the car when looking at it straight on. Be it a metal or fabric roof that area remains the same and is the largest factor in determining the drag assuming they have the same shape. 

The second is parasite drag (Which is really where the difference between fabric or metal comes into play). Parasite drag is the drag that is created by a fluid passing over the surfaces of the vehicle. Without getting into too much detail the parasite drag is significantly lower than induced drag and only overtakes it at very high speeds. (speeds most likely faster than your EV will go). 

So, if you are really that concerned with aerodynamics of the car you would be better off removing the side mirrors or something to reduce the induced drag. 

An example of how Cd is determined is like adding a bunch of numbers together from different parts and different types of drag on a vehicle. It would be like adding 100 + 50 + 1+ 1 = 152 Cd. And the Cd for the skin friction drag would be the 1, of which 20% is the roof. (might be a slight exageration but you get the idea). So, its certainly something to consider if you will be spending a large amount of time at high speeds, but won't make much difference for most of your driving (in my opinion) Engineers at the car company spend a great deal of time looking at these things, I would guess there wouldn't be much improvement you could do beyond removing safety equipment or special extras on the car.


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## the slashmaster (Feb 24, 2008)

ronin4sale said:


> There are 2 types of drag which the roof could effect. Induced and Parasite. Induced being the far larger amount of drag which is simply the area of the car when looking at it straight on. Be it a metal or fabric roof that area remains the same and is the largest factor in determining the drag assuming they have the same shape.
> 
> The second is parasite drag (Which is really where the difference between fabric or metal comes into play). Parasite drag is the drag that is created by a fluid passing over the surfaces of the vehicle. Without getting into too much detail the parasite drag is significantly lower than induced drag and only overtakes it at very high speeds. (speeds most likely faster than your EV will go).
> 
> ...


Thanks Ronin, so sideview mirrors and antenna's would be more harmful to aerodynamics than a vinyl roof. Now what about when you lower your car, does that reduce induced drag? What about mudflaps? Are they only parasitic drag or would they be considerd induced drag?


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## ronin4sale (Jan 29, 2008)

Thats my understanding at least. I believe lowing the car has a benefitial effect as well as it reduces the turbulence below the car(i think thats the reason, but its for sure benefitial). Mudflaps would effect both, most likely more so induced. Basically if you take a picture of a car from straight ahead on the level with the frame. IE, you can see under it to the other side, everything you see from that view is induced drag because the air will hit it directly.

You seem interested in this enough that you might want to consider taking a basic course on the subject, or maybe buy some books to do your own research. I did some aerodynamics classes in college and this is what I remember from those.


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## the slashmaster (Feb 24, 2008)

ronin4sale said:


> Thats my understanding at least. I believe lowing the car has a benefitial effect as well as it reduces the turbulence below the car(i think thats the reason, but its for sure benefitial). Mudflaps would effect both, most likely more so induced. Basically if you take a picture of a car from straight ahead on the level with the frame. IE, you can see under it to the other side, everything you see from that view is induced drag because the air will hit it directly.
> 
> You seem interested in this enough that you might want to consider taking a basic course on the subject, or maybe buy some books to do your own research. I did some aerodynamics classes in college and this is what I remember from those.


Thanks Ronin, I'm only interested enough to get what I can get for free off the internet so I can squeeze a little more range out of my car. But if you know a drag coefficient calculator of some sort or some other website that gives general stuff on the internet I'd love to see it!


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## ronin4sale (Jan 29, 2008)

I'd snoop around NASA and university websites looking for technical papers to find any more real detail. You can also try the google scholar search.


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## dataman19 (Oct 7, 2009)

Aerodynamics? Whew you are all biting off a pretty big chew..
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Aerodynamics and under car air flow can be corrected by effective air dams (these are called lower front end spoilers). By effective, I don't mean you just block it - you re direct it. If you cannot re direct it - you minimize it's effect at the rear end (the business end of a cars aerodymanic character.
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The significant aerodynamic restrictions in a car is where the air flow flowing across, around and under the car meet at the rear..
Dah - that's why the call it drag!!!
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Look at the aptera:::
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http://www.aptera.com/
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Notice anything funny about the boxy rear end that is characteristic in all GM.Ford, Toyota, Mercesdes cars and trucks??? Dah, they have a slip stream rear design!
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This gives this body style a better than 60% aerodynamic advantage over conventional boxy rear ends. This is one of the main reasons this vehicle gets phenominal 100 MPG!
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So a body mod orredesign would give phenominal improvement, taking the mirros off will give you a $0.0000129 per gallon savings..
,,
Go figure,
dataman19


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