Ford Fusion aero mods: Cd from 0.34 to 0.21

[graydonengineering]

Originally Posted by aerohead

With respect to the airspeed under the car,as a function of ride height,my thought is that because of the plan radius of the front airdam,that air is being channeled around the car as much as the rulebook will allow,and while the air volume under the car will be an arithmaic function of the gap between salt and dam,the actual velocity remains a fixed fraction of groundspeed,as the dam doesn't aggressively funnel air under,but rather around.

I would agree. By lowring this car so low and adding an extra low nearly dragging air dam, the air volume under the car is minimized reducing the benefit of the belly pan.



Since the underside of a car is a restricted space, car above, road below, any drag (disruption of flow) will only further increase the presure as the air must find a way out no matter what restrictions it encounters. That is why, epecially on a car that is low to the ground, you are better off trying to eliminate flow under the car alltogether. It this case, they did a very good job of that from the front, not so much around the sides. If they added side skirt air dams to further reduce underbody flow, you would see even less gain from the belly pan, aproching zero the closer the skirts are to the ground.

[winkosmosis]

Remember the Bernoulli effect thread?

If Bernoulli is so powerful, why doesn't that car take off into the air? It has a smooth curved top, and a flat bottom that sees greatly reduced airspeed thanks to the dam.

[aerohead]

Originally Posted by winkosmosis

Remember the Bernoulli effect thread?

If Bernoulli is so powerful, why doesn't that car take off into the air? It has a smooth curved top, and a flat bottom that sees greatly reduced airspeed thanks to the dam.

The lift will have to exceed the weight of the car.
Racing Beat's Mazda RX-7 managed to get airborne.Their car was over 3,000-lbs.I think it left the ground at around 185 mph.
Losing traction and getting sideways usually does the trick.

[aerohead]

Originally Posted by MetroMPG

I have also seen it in reference to the Lexus sedan, but not 0.08!



(The air suspension drops the car 1 inch at highway speeds.)

Source: 2006 Lexus LS430 Reviews, Road Tests, & 2006 Lexus LS430 Test Drives

There is an article in Mechanical Engineering Magazine about the GM PNGV Precept of 1999.In the article they report that ground clearance has a definite effect on Cd.
If you look at this year's Leaf and Leaf NISMO racing car,on the racing version they chose to lower the car.
I haven't finished my 'look' at the two cars,but the change in fineness ratio between the two cars is remarkable.
And if you'll remember,this is one of the most important criteria Hucho mentions for low drag.
Lowering adds effective 'length' to any vehicle,increasing its L/H ratio.
PS Check out the roofline on both cars.You'll see something familiar.

[aerohead]

Originally Posted by Cd

When they say that the LS430 was tested at .25, do you think that they cheated a little ?
What about those big pimpin' open spoke wheels , huge side mirrors , and overall boxy shape ?

Do you think it actually has a .25 out the showroom door ?

They could say it had a .10 drag coefficient and no one would know the difference.

The original LS 400 ( Cd 0.29 ) had very generous plan taper.About 38% of a sweet quasi-Mair 21.5-degree boat-tail.
The roof itself has generous plan taper of even greater percentage,and it's got a very clean underside even by today's production standards.
Cd 0.25 for the LS 430 should be a walk-on home run.

[aerohead]

Originally Posted by MetroMPG

I went looking for a source to quote, but didn't find one.

But I thought the rule for flow rate through an orifice (gap under the front of the vehicle) is that it's effectively proportional to orifice size for a given pressure.

And when you consider that dropping car closer and closer to the road may actually be increasing pressure at the air dam as the gap shrinks, the effect may be even more pronounced.

I think Hucho's got some material on this.I don't have my book with me here.The research work was probably done in the early 1980s.
KAMEI came out in 1982 I think.