Cd and blunt front ends

[texanidiot25]

Originally Posted by MechEngVT

NASCAR Monte Carlos have next to zero downforce. An LMP car should produce tremendous downforce at high speeds. Increasing downforce will increase Cd (I guess the opposite of lift/drag ratio....anti-lift/drag ratio?) so at say 170mph the stock car is probably weight-neutral and pulling through the air but the LMP car's tires are about flattened and suspension bottomed and drag-limited which isn't a problem in their racing series since the fastest those cars ever go is down the Mulsanne at LeMans but typically go much slower through tight turns without banking. Different cars for different purposes.

That's pretty much the same conclusion in the article. Despite the Porsche having more power, and a much more aerodynamic shape. They did swap the rear body on the Porsche to the one with the least amount of safe down force for the track. The Monte still went 190-200+ mph.. I'll dig it up when next week..

[Kevin Cooper]

If you take a brick and round the front edges just enough to achieve attached floe, then the drag coefficient will drop from 0.80 to about 0.30. At this level, the forebody drag is close to zero abd the drag comes mainly from the base pressure and to a lesser extent from skin friction. As the edge radius is further increased, the drag coefficient changes very little more. The explanation is that the curved edges have suction pressures that pull forward while the central, flat region has positive pressures adding drag. As the edge radius becomes smaller, the edge suctions become larger, offsetting the drag increase from the larger flat region. This process of balancing positive and and negative contributions continues until the flow separates and the edge suction diminishes. See: "The Effect of Front-Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity" SAE 850288 for a detailed discussion of this behaviour.

[mar5ka]

I think about this all the time, back in the 80's saab angled the front and got 5% better aerodynamics, but how did it happen, was it because less air around the sides(smaller wake) or did the car really slice threw the air better, there is some nascar sites that talk about better aero with an angled front but they are interested in down force too. But in the end its the end that is more important how the air comes back togather after it has hit by a blunt body. Here is a web site that is really interesting, Alot of methology, trial and error and real world plus classroom math...

[mar5ka]

Chris' 914 Porsche Aerodynamic Aids

[Cd]

Thanks to everyone here that has posted.
I have found this information very helpful.

[NeilBlanchard]

Hello,

I don't think that anyone has mentioned the Mercedes Bionic "Boxfish" car, with it's 0.19Cd:

[aerohead]

Yeah Neil,I know we talked up the Mercedes quite a bit.The forum has grown so large,its become a challenge to track down earlier discussions.At Cd 0.19,the boxfish hasn't quite hit Klemperer's target of Cd0.16 back in 1922.The upshot for the M-B is that it's "producible",and with Renault Vesta-II mecanicals,and same frontal area,we're looking at 146-mpg at 63-mph.Pretty sweet!

[NeilBlanchard]

Hi,

So, is Mercedes going to produce something close to the Boxfish? If it gets anywhere near 140mpg, that would be awesome.

[aerohead]

I doubt it.The industry doesn't appear to want to let go of design obsolescence.If they do real aero,the cat will be out of the bag,and everything GM worked so hard for will be destroyed.Good design is okay for anything except cars.

[SuperTrooper]

Originally Posted by aerohead

I doubt it.The industry doesn't appear to want to let go of design obsolescence.If they do real aero,the cat will be out of the bag,and everything GM worked so hard for will be destroyed.Good design is okay for anything except cars.

No, if they did real aero people wouldn't buy them because they might be ugly. I'm sorry but most americans would find the Boxfish design UGLY. I realize some members here would drive a Pontiac Aztek that got great mileage regardless of how it looked, but the public at large wouldn't. Car ownership goes far beyond function and utility, it makes some kind of statement.

If you recall, back in the '90s cars were getting more rounded and sleek. Some of the lowest Cds to come out of Detroit were born in that era. What happened? Car buyers complained that the cars all looked alike, and worse, that they were too plain and feminine. So, out came the boxy shapes and bulging wheel wells. Car design has to satisfy many masters, which means that every design that hits the street is a compromise.