Do wheel skirts really work?

[Bill in Houston]

I wonder if that is because the road has more water on it when the front wheel runs over it than when the rear does? But I have seen what you are talking about. It does seem like there is a lot more air whipping around in that front wheel well than the rear one.

[Dynamically Aero]

In this article about the Nissan P35 mention is made of the R89C. There are a number of interesting photos of the P35, including those accompanying both Part 1 and Part 2 of the story. WARNING! Going to other pages at Mulsanne's Corner may result in large periods of missing time. You've been warned.

As for the outlets in front of the rear wheels of the R89C, the upper one is to expel air from the intercooler, and the lower one is for the turbo's exhaust and wastegate. These can be seen in this small and this larger photo of a model of the R89C. This is a thread covering the building of an amazing R89C model.

For an example of another Group C car with a similar setup, see this photo of a model of a Porsche 956.

To see how a street version of the 956 routed things differently, this model shows the exhaust exiting out the back, most likely due to a law prohibiting side outlets.

Speaking of exhaust, in the P35 article there is this bit of information regarding underbody airflow: "Exhaust activation of the underwing had been used on Suzuka?s previous Nissan GTP cars, and it was also adopted for the P35." Here is the link to photos of the exhaust holes referred to in the above quote. Interesting, isn't it?

For photos of more models of cars which competed at Le Mans, go here.

The Jaguar XJR-9 is another race car which used rear wheel skirts/fairings. To see what else competed against it and the R89C in 1989, click here.

[MetroMPG]

Thanks for the info, D.A.

Shall we assume from your name & post that aerodynamics are an interest?

[trebuchet03]

What an awesome first post

[rh77]

Originally Posted by trebuchet03

What an awesome first post

I say...

An official introduction is in order.

Cheerio...

[CO ZX2]

Originally Posted by zpiloto

It's been a cold rain all day today. I noticed looking at the cars as I was driving around that there is significantly more spray from the front then the rear wheels. Which would make me think that some kinda of coverage on the front would decrease drag more than on the aft wheels. Has anyone run across any data that reflects that?

Here is an aerodynamic drag chart I saved from Autospeed magazine. Toecutter says it may be Volvo but it should be a decent guide for most modern cars. Front wheel drag percentage appears to be about double the percentage of rear wheel drag. CO ZX2

[lunarhighway]

front wheels add to the frontal area while rear wheels don't... i'm sure this is over simplified but still... also front wheels will take a lot of dirt and water of the road while the rear wheels will follow in this "clean" trail.

is would be very interesting to learn how much drag is associated with the fronts, sides, wells and spinning motion of the wheel.

most cars i see seem to have something in front of the wheels rather than beside them which makes me wonder if this area isn't more important? apparently car makers rather invest in plastic in front of the wheels (wich has very little impact on the cars appearance) than in plastic besides them (wich alters their looks a lot)

[repete86]

Also, these little pieces of plastic in front of the wheels only cost a few cents while a side skirt that needs to be designed to attach and detach yet still remain secure will cost much more.

[The Toecutter]

The effect is present, but small. It also depends upon the car.

From the book Aerodynamics of Road Vehicles, edited by Wolf-Heinrich Hucho, page 166:

From a series study on 14 passenger cars Cogotti deduced that flush panelling of the outer wheel naves could result in a drag reduction of ΔCd = -0.009 +/- 0.003. Inclusion of the wheel and the wheel opening in the vehicle contour allows a certain degree of design freedom. Of course, both the tyre bead and secondary recesses h1 and h2 must be selected so that the body is affected as little as possible.

Covering the rear wheels reduces the drag only on streamlined vehicles, and is effective only when the flow is attached upstream. On vehicles designed for extremely low aerodynamic drag (so-called concept cars) a 'spat' which moves with the steering may also be fitted over the front wheels, or, in more advanced cases, the body shell could flex with the steering motion of the front wheels.


Basically, it would be a good guess to expect about a .01 reduction from adding skirts to a streamlined car that doesn't have them, and little to no effect from a non streamlined car. May explain theclencher's lack of statistically significant results, while metrompg and basjoos had very noticable results.

A lot of aeromods seem to be cumulative or rely on other mods being adopted beforehand before they become functional.

I also recall reading a few SAE papers where the change in results of wheel skirts varied from 0.00 to 0.03. So, make of this what you will.

This car was made in 1999 and got 70 MPG. Why the heck is it not on the road. Looking at it you would think that it was just another sedan. Ford blew it on this one. Same space as the Taurus but 2400 pounds.

Ford developed it for the PNGV project, where our government gave them hundreds of millions of our dollars in welfare handouts to develop an 80 mpg midsize car with a cost penalty under a few thousand dollars.

The catch is, the automakers never had to sell it. The Prodigy, using a low drag composite body and diesel-electric drive got about 70 mpg, 0-60 mph ~ 11 seconds, cost penalty on par with that of a Prius.

But why sell a 70 mpg midsize car when selling an SUV will yield much higher profit margins? The consumer can't buy and demand what isn't for sale.

Even a gasoline car with a 2.5L V6 with 150+ horsepower, no fancy hybrid drive, no composite materials, no engine displacement or horsepower reduction, would see about a 30% combined fuel economy increase from that drag coefficient reduction alone(assuming average for midsize sedans about .32), with no meaningful additional cost. (See this article I wrote for details on drag and fuel economy: http://www.evworld.com/blogs/index.c...d=87&archive=0) The other things are just icing on the cake. Taken cumulatively, they would add up in a big way.

[The Toecutter]

If you make it aero, they may have an effect. You just have to find a way to keep attached flow, perhaps.

The Sidekick would probably have the same problem, but when I get the chance, I'm definately going to try. I just need a way to accurately measure FE, and need to wait for my parents to dump that POS into my hands. I have lots of ideas on how to cut the drag, but I'm thinking more radical **** akin to what basjoos has done.