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Old 11-29-2019, 01:32 AM
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The humps on sports car roofs.

This is a question that has occurred to me often when driving my Fit around, and now that I'm giving it to my niece and nephew and have bought a new car, it has taken a personal aspect.

I've owned several sports/sporty cars, but in recent years I've noticed a new trend where cars that were intended from the beginning to be a sports car have what you would describe as either "humps" over each front passenger position or a "canyon" between the two. I might imagine this would be for the extra room needed by a helmet in a car that might see the track, but why not just make the whole roof a little higher unless there is an aerodynamic reason for having the middle lower? Current Mustangs and Camaros have this, and the car I bought, the BRZ* does too.

Is there an honest aerodynamic reason to have that dip in the middle of the roof on a sports car? Or is it one of those things that looks racy, so everyone adopted it? If so, who was the progenitor of this design habit, and who would you consider the first offender of proclaiming its pretend racy-ness by adopting it?

(Ok, this could be IMHO or Cafe Society material, but I have hopes for a hard science answer, even though it relates to the "MORE VENTS AND DUCTS!!!!" world of sports cars. If it looks hopeless, may the mods move it as is appropriate.)

*And if I may gush, it is a pretty car. My wife, who is a hardcore art/design head and moderate gearhead, has kind of made a habit the last day of looking at it out front and saying "It is a pretty car, isn't it?" I've coerced her into driving it once, but she seems intimidated by its prettiness in a way she wasn't by the others. Like say, the hatchback WRX that only a mother would think was sexy. She'll floor that in a hearbeat, and describes it as "The car that just fucking goes!"
Old 11-29-2019, 04:41 AM
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I would say it's mostly style/marketing reasons that tested well with a focus group.
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Old 11-29-2019, 06:17 AM
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Originally Posted by snfaulkner View Post
I would say it's mostly style/marketing reasons that tested well with a focus group.
Agreed for the vast majority of cars. If the OP was talking about the extreme high end super/hyper car then it gets slightly different. These cars are carefully shaped using wind tunnels (both computer simulated and real blowey ones) for ideal aerodynamic effects of low drag and carefully modulated down force. Style does naturally still matter to the designers of such cars but the shape does legitimately affect the performance as well.
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Old 11-29-2019, 09:53 AM
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Double-bubble roofs on sports cars aren't new. They were fairly common on sports and racing cars in the sixties. The practical reason was, as you noted, to provide headroom for helmets. I don't know when they started but they are famously associated with designer Ugo Zagato in the fifties and sixties. Lots of Zagato-styled cars long after his death continued to have them. Many other car makers used them too. In fact, with Ford v. Ferrari in theaters, I thought this might be a question about the Ford GT-40, some of which famously had a bubble in the roof to accommodate Dan Gurney's height.

Auto makers probably didn't want to make the whole roof taller for a couple of reasons. First, it would increase the car's frontal area. Reducing frontal area is half of the battle in reducing drag, which is a function of the car's frontal area and coefficient of drag. We tend to hear about the coefficient of drag more but lowering a car's frontal area also reduces drag and increases speed. This is why the Ford GT-40 was just 40" tall. Second, many of these cars (particularly from coachmakers like Zagato) carried hand-hammered bodies pounded out on wooden bucks. Making the whole car taller to accommodate a taller person would have required a whole new roof buck, new glass all around, and a whole host of new changes. You might as well just ask them to design a whole new car from scratch. On the other hand, if they need to accommodate a taller driver, pounding a couple of dents into the roof takes a few extra minutes.

Why do cars have them today? For style and to associate new cars with that romantic era of racing. The roof bubbles wouldn't do much to provide space for helmets. Modern race regulations require full roll cages of roughly 2" steel bars to be built inside the car. The helmeted driver has to be seated so that his helmet ends before the roll cage begins. The roof bubble would be several inches higher than a modern drivers helmet when racing.

Last edited by Tired and Cranky; 11-29-2019 at 09:57 AM.
Old 11-29-2019, 03:06 PM
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Could it also be to stiffen the sheet metal? Putting a crease in a flat sheet metal makes it stiffer in the direction parallel to the crease. (That's why corrugated metal sheets are used in some applications.)

Last edited by scr4; 11-29-2019 at 03:07 PM.
Old 11-29-2019, 03:10 PM
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Thanks all, especially Tired and Cranky. That specific answer was exactly what I was looking for. Keeping heavy things like glass low was another reason I thought about after posting.

I would say that at least the bumps in the BRZ do actually provide more headroom, with corresponding bumps in the headliner. If I were a much taller person, it would help with track days and autocrossing, which both require helmets but don't require a cage or rollbar.

Last edited by scabpicker; 11-29-2019 at 03:13 PM.


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