cool windscreens, how do you make em Options

[usa4cheer]

I am working on my grassroots motorsport challenge car and want to take the windshield out of my car and want to do the mini windscreen. How do you make it???

[Mugs914]

QUOTE (skeates @ Feb 16 2006, 11:15 PM)

Andy - you are correct in saying that the frontal area of the car plays a big point in determining its CD, however....      the curvature of the body in the front plays a very large roll as well. Take for example a Jeep Cherokee vs. a Ferrari Testerosa. The Jeep has a very broad and flat frontal area wich creates a VERY large "stagnation point," meaning that one might as well be trying to run down a windy ravine holding a piece of plywood directly infront of them and perpendicular to the wind. The Testerosa on the other hand takes that plywood and tilts it back so that the airstream from the wind is not directly stopped on account of the plywood, but rather only slowed down. One can test this out on their own by buying a piece of plywood and standing with it in the wind at different orientations (BTW: even if the tilted piece of plywood is longer to make up for the decreased frontal surface area it should still create less drag than the directly perpendicualr piece). This is why the pop-up headlights on the 914 could potentially create more drag than the fender-flairs Peace, Steven

Actually, the frontal area of the vehicle doesn't factor into the CD, Though it has everything to do with the overall drag of the vehicle. Steven's second paragraph is a good description of what the CD means.

The CD is a measure of the aerodynamic eficiency (or lack thereof) of a given shape. The CD of a car (or any other shape, for that matter) doesn't change with the size of the object. The CD of the 1973 914 in your driveway is about the same as the CD of the 1/43 914 on your desk. A lower number indicates higher aerodynamic eficiency. Frontal area is factored into the equation seperately. The basic eqation for aerodynamic drag, ignoring compressability, Reynolds effects, etc. is:

Air density x CD x Forntal area x velocity, squared = overall drag

So to reduce drag you can reduce air density (move to the mountains), lower the CD of the shape ("streamlining"), reduce frontal area (sawzall), or slow down (blasphemy...).

Since no-one on this list ever wants to slow down we can eliminate that one...

So if you assume a given speed and air density (it can be tough to change), you are left with two factors that can affect the overall drag of the vehicle. It's shape and it's size.

Though the CD change with the shape of a 914 (.363 all closed up vs. .484 with everything hangin' out. A difference of .121) is pretty significant as far a CD figure goes, if you look at the equation you'll see that even a change that great will not have a huge effect on the overall drag. Lowering the CD usually means a lot of work for very small improvements. As an example, a clear "fastback" rear window over a 914's rear decklid would probably net a reduction of about .015 or so to the CD (that's just a guess. if anyone has seen actual numbers for such a thing let me know!).

That leaves frontal area. For any given change this will have the greatest effect on the overall drag. Besides, its much easier to reduce frontal area than CD! So while removing the windscreen and rear window would increase the CD by as much as a point, the loss of around 5 - 7 square feet of frontal area would mean that the overall drag had been reduced. In the same way the GT flares will add very slightly to the CD of the car, but their major drag penalty comes from an extra two or so square feet of frontal area.

Sometimes it works the other way as well. When Prosche was running the original 908 coupes they installed a raised rooftop on one car to give extra helmet clearance for a tall driver. It turned out that the (very) slight increase in frontal area resulted in a lower CD and a very, very slight decrease in drag.

(IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) Sorry to geek out on you guys, didn't mean to go on for pages. Aero is one of my favorite subjects. It's easy for me to ramble on and on about such things.... (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)