roof bubble on most cars

[winkosmosis]

Originally Posted by Frank Lee

Yes, because you WILL roll over or go under a semi!

Well I was thinking "the odds of rollover or going under a trailer are low". Then I realized there are a lot of trucks with bumpers high enough to slice his head off.

[Frank Lee]

My suggestion would be to leave a bit more following distance, and don't do distracted driving

[aerohead]

Originally Posted by winkosmosis

Conventional wisdom is apparently wrong. Making a roof curved does reduce drag, even though frontal area increases. Obviously the best thing would be to keep roof height the same but curve the windshield but that's not usually practical. <warning! scribd link which slows down some browsers!> In This Chapter the Modifications That Were Carried Out To

Roof: Roofs are designed with a convex shape to ensure sufficient rigidity. For stylistic reasons an attempt is made to keep the convexity as small as possible. Fig 5.9 shows this for a medium sized notchback car. If the convex shape is designed so that the frontal area A of the vehicle increases, the aerodynamic drag of the vehicle decreases.



On the other hand, if the original roof height is kept constant the front and rear windows must be curved into the roof contour to eliminate obstruction of the view. This leads to expensive windows but results in lower drag.

The measurements plotted in Fig 5.10 (after Buchheim et al) show the same tendency for a car with a fastback. Here the chord length of the roof arch was used as the reference variable for the curvature.



Aerodynamic drag reduces with increased convexity for two reasons. First, the higher convexity allows for a larger radius at the transition from the windshield to the roof. This results in a less pronounced suction peak at this location. The momentum loss in the boundary layer during the following less steep adverse pressure gradient is therefore smaller and the boundary layer itself is less endangered by separation. Second, the convexity provides for gentle deflection of flow at the rear and the pressure rise at the rear end is therefore enhanced. The convexity of the roof and the rear end shape must be carefully matched.

"If the convex shape is designed so that the frontal area A of the vehicle increases,the aerodynamic drag increases." Hucho

[winkosmosis]

That statement can't possibly be true 100% of the time, because a teardrop shape can be more aerodynamic than a cylinder with the same frontal area.... Like I said, if the Cd improvement is bigger than the frontal area increase, drag goes down.

[Delmar]

This is one of the first aero mods I am building for my 92 metro... for me its a precursor to the boat tail or kammback. with the roof bubble, the angle at the existing rear of the car will be increased, providing opportunity for a shorter length boat tail.
I will also be stacking functions, in addition to the aero benefits, the roof bubble will have covered surfboard racks

[euromodder]

On the other hand, if the original roof height is kept constant the front and rear windows must be curved into the roof contour to eliminate obstruction of the view. This leads to expensive windows but results in lower drag.

Seeing that this kind of panoramic windows is currently being introduced on many European designs, I'd say the extra expenses have dropped so much that it can be made mainstream.

Unfortunately, they're currently not being introduced to help the aero qualities of the cars.

Previous GM / Opel Astra (though still an expensive option in late 2005) :

[winkosmosis]

You'll pay the cost when you need to replace your windshield. I'll take conic section windshields any day.

And on the Jeep Wrangler they switched to a slightly curved windshield, which I doubt has any real aero benefit. But that means in a post apocalyptic future you can't just replace it with a sheet of Lexan.

[capnbass91]

Originally Posted by winkosmosis

Chop off your windshield. You wouldn't have any rollover protection and will probably die a painful death though.

...yeahhh....uhhh...ok....

[aerohead]

Originally Posted by winkosmosis

That statement can't possibly be true 100% of the time, because a teardrop shape can be more aerodynamic than a cylinder with the same frontal area.... Like I said, if the Cd improvement is bigger than the frontal area increase, drag goes down.

Hucho's Cd curve for roof camber shows the drag reduction asymptotic when the elevation of the 'bump' is around 0.11 X car height,yielding a drag reduction of 5%.
If the 'bump' adds no more than 5% to frontal area,then aerodynamically it's a wash.The drag saving is canceled by the increased frontal area.CdA remains constant.
The challenge is to get the optimum roof camber with 5% Cd reduction while adding less than 5% frontal area.

[euromodder]

Originally Posted by donee

Please put a warning before any Scribd links. Something like - "WARNING Scribd Link! > " . That service is so FUBAR that it takes 10 minutes to get rid of their slow loading, useless response time browser pages that spawn over multipe browser instances and get on with something useful.

Seems like a user-side issue.
It opened in 1 new page, and near-instantly.

YMMV.