Ford Fusion aero mods: Cd from 0.34 to 0.21
 

OK, so it's a Bonneville car. But it's instructive about the basics what to do to improve aero:

http://www.blogsmithmedia.com/www.au..._0865e-450.jpg

- grille blocking
- smooth wheel covers
- narrower tires
- deleted mirrors
- deleted wipers
- lowering, in conjunction with...
- full undertray
- rear decklid extension (improved roof to trailing edge angle)

PS - judging from the photos at ABG, the exterior sheetmetal is stock. Nothing else is.

PPS - It's a hydrogen fuel cell powerplant.

Quantified:

0.34 - original Cd

-.08 - lowering (that's massive! and it's the only time I've seen it quantified)
-.025 - full grille block
-.015 - mirrors deletion (doesn't mention wipers though)
-.01 - full belly pan

I suspect they couldn't mod the door handles or add skirts in order to keep within the "stock" definition for bodywork. Probably a class thing.

EDIT: the slide isn't exactly comprehensive. It doesn't address the decklid extension or the smooth wheel covers...

Why is the original car's aerodynamics so terrible to begin with? Some thoughts...

- High off the ground . It's as high (or higher, depending on the source) as my hatchback Flea!

- The bodywork around the rear wheel arch is terrible. The wheel sticks out dramatically - looks like the edge of the tire is exposed for as much as as 2/3 of its height. Which means any attempt to boat-tail the bodywork after that point is a wasted effort since the air is already disturbed (and further churned up by the chunky alloys).

- the wheel arches are also pretty large, with a big gap between the tire & bodywork.

If you look through the ABG Fusion 999 gallery, you can see they used wheels with a different offset (and narrower tires) that let them completely "hide" them (from airflow from the front) within the wheel wells.

Obviously the overall shape of the car is fine if they got it down to .21. It's on the details where Ford messed up.

Fusion 999 aero
 

If you look at the pictures in the ABG 999 gallery referenced above, the front nose piece is pretty much completely smooth - the frontal features are apparently decaled, a la NASCAR. A production Fusion, even with grill blocks, still wouldn't be this sleek. However, the rest of the body appears to be standard production pieces, which interests me - the front fenders have a sharp crease at the top leading down to the nose, as does my Focus. Apparently there isn't much of an aero loss with this, since it would have been relatively easy to put a mild radius on the fenders/nose to eliminate this character line & smooth out the airflow even more. I wonder how much the fences on top of the roof are worth, too...

Good point. Much smoother than stock, too.

Here's a good pic that shows it - note the lovely radius...
http://www.autobloggreen.com/photos/...esting/313361/

The fences on the roof aren't there to contribute to reduced Cd - they're safety features meant to prevent lift from airflow across the roof should the car slew sideways at high speed.

This Mazda doesn't appear to have them, for example. And as a result we get a nice look at its wonderfully smooth undertray... several times!

[YOUTUBE]http://www.youtube.com/v/SWgWk_evROk[/YOUTUBE]

Metro, after looking at the gallery, I assumed wrongly that they ran at Bonneville with those stock door handles.
The door handles were flush at Bonneville.

That's a great flat full-length underbody pan on that Mazda - nice of him to flip his car & show us. :)

lowering
 

This is the first ever for me time that the effect of lowering had been assigned a numerical value.Its new grist for the mill.

Perhaps one of us could visit a Ford dealership and post some images of the underside of a stock Fusion.
It must be pretty messy to create that much drag !

MetroMPG -

Hrmmmmmm, I want to say "now I am glad I never did a belly pan". But, I think the Belly Pan benefit is being effected by the lowering. Would the belly-pan benefit have been more if they had never lowered it? Do you know what I mean? Did they test/simulate each mod independently?

CarloSW2

You have to keep in mind that this is a different application than we are aiming for. They are aiming for low drag, but also with a high emphasis on controlling lift. Therefore, some things are not going to apply to us. Number one is that super low front. We simply can't do this if we want to drive on the road. This, along with the lowering is definitely going to lower the effectiveness of the belly pan. The other thing is that rear spoiler.

Are you sure that lowering wouldn't make a smooth belly pan even more important? As the gap under the car diminishes, doesn't airspeed increase?

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: http://www.automallusa.net/2006/lexu...0/reviews.html

http://images.automotive.com/reviews...06-ls430-1.jpg

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.

I think it does. If my Camry, built in 92, has a .34 drag coefficient, I don't see why, with the inward curving c-pillars, taller rear end, dual exhaust, lower ride height, underbody covers and spoilers, etc... All supposedly optimized in a wind tunnel, the Lexus couldn't see .26.

MetroMPG -

But if you are lower to the ground, isn't the "smaller frontal opening" to the undertray of the car allowing for less air to travel to get all turbulent and messy?!?!? I have no idea. The more I learn about aerodynamics the less I know. There are general principles like removing side view mirrors, wheel skirts, and keeping the window closed, but it seems like without a wind tunnel,YMMV (Your Mileage May Vary).

CarloSW2

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 would guess that although the pressure may be greater just below the airdam on a lowered vehicle, and the airspeed may be higher, there would be a lower volume of air flowing under the vehicle, so the effect of the tray would be less. (just a guess)

It's hard to believe that the difference between the Lexus and the Fusion is only .05 cd. I mean compare the two pictures, maybe the cd for the fusion is lower?:confused:

If the same volume of air passes through a smaller cross-section or channel, the speed of that air will increase while passing through. It may be counter intuitive but the pressure at that same point will decrease (the Bernoulli Effect). Think of it as a venturi.

In the case of lowering an air dam and the ground, I don't think the volume will be the same as before lowering, since the gap is more of an orifice than a venturi. If the front of the car channels air under the car (unlike an air dam), however, the airspeed probably does increase and Darin has a good point Doesn't he always? ;).

2000 Honda Civic Hatchback = .38 :eek:

Bonneville Fusion
 

Sorry! I'm late to the party on this one,but I'll throw my two cents in.How fast did the car go? If its over 200-mph,compressibility issues would be looming up on the crew.As mentioned already,since this is a pure racing environment,safety may play as big a role as drag reduction and speed.The Fusion's roofline approximates an airfoil form,and the bellypan may be there to minimize static pressure under the car in an effort to forestall takeoff.

References state that air velocity under a car is always a fraction of road speed,therefore Bernoulli's theorem would come into play,dictating that pressure would have to be higher than the surrounding flow field.Not good! The smooth underbelly,acting like a traveling duct, would better allow low pressure from the wake,to be communicated under the car to help spoil the lift created by the more stagnant flow.

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.

The lowering also improves the fineness ratio,as well as the rear deck foil,both of which reduce drag,and the rear foils horizontal projection moves separation and lift completely behind and under the foil,a great boon to lift reduction and directional stability,creating a pressure weather-vane,if you will.

And I agree that the fences are also only for safety,should the car loose traction ( not an unusual event at Bonneville ),and yaw into a more wing-section attitude,sideways,as NASCAR vehicles which we may have all seen literally fly off the track.

Being a fuel-cell car,the nose can be sealed,and while not a great drag reducer for race cars ( they can be idealized for low drag ),again,it cuts off airflow "through" the car,for added stability and safety at high speed.

The 0.08 Cd reduction is really impressive and tailshadows Ford's active suspension contribution to their PROBE-IVand V concept car low drag numbers.Since the PROBE-V achieved Cd 0.137 in the 1980s,the Fusion's 0.21 doesn't carry the same wow-factor,buts its still instructive,and of great value to anyone who lives between the lines.Liked the article,thanks!

I had a '91 Subaru XT that had an adjustable air suspension. The car would drop about 1" at 50 MPH. Hmmmmm... The car was very aero for it's time: 0.29 Cd. Flush door handles.

I wonder how difficult it would be to add an air lift system. There are plenty out there for the low-rider crowd.

Anybody have any experience with air lift suspensions?

air ride can be pretty tough; you need to be good at customization and welding.

Hi All,

The picture on the first page is not the .25 Cd LS-430. There is one of the newer ones in my work parking lot most days. Its much more aero-looking, and even similar looking to the Fusion Aero. First off the grill work is allot smaller. And there are wheel dams, like on the Prius et al. Which tends to obviate the effect of the wheel spokes. At least a little bit. One of the more obvious aero features on the LS-430 is the rear trunk lid shape, which is scooped out and blends perfectly in with the surounding body work.

That exact same thought crossed my mind.
A sreet car (mass-produced) can only be SOOOO low.
Look at the work that just Audi put into the underbody panels.

I would think that there is an invers relation betweeen the amount of space and how smooth the bottom of the car needs to be. IMHO

I just reviewed my old Speedweek pics of the 999 from Bonneville. Mine show stock looking door handles covered in green/white/black vinyl.

One of the aero tricks that I really liked was how they jacked the 999 up. It was sort of like the jacking holes that rally cars use, except much larger and you had to open the rear doors for access. Once a rear door was opened, a foot-long pipe with an upside-down trailer ball was pinned in place with an aircraft pip pin. Then a floor jack would lift the upside-down trailer ball. They went to a lot of effort to hide the jacking points.:D

The whole rear of the car was one big science project. It looked like NASA. The fiber tanks were protected by a roll cage that even extended to the trunk. (Trunk cage?) It was all pretty slick, except the snorkel exhaust pipe that stuck out beyond the rear of the car. That was goofy.

Thanks for letting me know about other good stuff !

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.

http://www.blogcdn.com/www.autoblogg...-999_1958e.jpg

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.

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.

take off
 

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.

GM
 

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.

LS
 

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.

gap
 

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.