drnick

i have been pondering the aerodynamics involved in generating further downforce utilising the cars underbody and a few things have me intrigued: is the object of smoothing undercar airflow to allow whatever air that does enter this space to leave as quickly as possible thus minimising the total amount of air present, generating lower pressure? to that end a lower front air dam would help to exclude air in the first place and diffusers at the rear would prevent turbulent flow on exit?

as for the front air dam, how far backward can this be situated and still be effective? to just forward of the front wheels?

i.e. this area

drnick

my artistry is poorly practiced but i hope the general idea comes through. it would be straightforward to attach a folded alloy sheet across the rear edge of the front pu, just forward of the tyres, to project downward into the airstream and restrict flow.

PorKen

That's the old school method of aerodynamics.

The new way is to smooth the underbody and allow air to flow under the car so the car becomes an upside down wing.

If you can make a vortex generators at the rear underbody, to delay separation (turbulence) to be farther behind the car, so much the better.

Jim bailey - 928 International

The Carrera Gt has a beautiful full length carbon undertray with diffusers at the rear ........what you get for $440,000 . For those without a windtunnel you probably just need to exclude as much air as possible under the nose . It matters little where the air dam is located since a "bubble" of air will build up in front of it any way . If you really want to improve top speed remove the outside mirrors and close the windows !! .

slate blue

I posted some ideas on this before with some pics of the Ferrari 550 underbody. The early cars that's the ones with spoilers, have a sort of half spoiler half splitter. That's where the air dam that builds up can be put to some use, some free use too. The pressure that builds can be directed to a splitter and gain downforce on the front wheels.

What I plan on doing is adding the s4 underbody and then joining it up to the front spoiler after I study what the latest ideas are from porsche. ( I do have access to the latest factory manuals) The rear of the car also lends itself to the use of a diffuser. Which won't be of much use unless the underbody has a flat tray the whole length. It's certainly is doable. I don't know whether I would use carbon or aluminium? I think aluminium would be bettter and cheaper near the hot areas like the exhaust. The diffuser would certainly look very cool in Carbon.

I made an extended splitter for the spoiler but concluded that clearance around town would be a bit awkward at times so I have stayed with the original.

Realist D.

There is no way to make a passenger car an "upside down" wing. In fact, the very nature of the shape of a passenger car causes all of them to effectively be an "upside right" wing - and all passenger cars create "lift" at speed. If the length of the surface on the top is longer than the length of the surface on the bottom then you have an upside right wing. Think about it ... there's no way to have an upside down wing and have a place to put people inside the car (unless you literally turn the car upside down ;-). All the blather you hear about wings and spoilers creating downforce is just that - blather. The best they can hope for is to reduce the overall "lift" the car creates as it travels along at speed.

The suggestion that smoothing the bottom of the car would help reduce lift because it will help air travel past the underside at greater speed is true - but it won't match the speed the air has to travel at over the top of the car so you'll still have overall lift. The purpose of an airdam is to reduce the amount of air going under which also helps with the overall problem. A splitter simply helps the airdam be more effective by reducing the amount of air that can "spill" under the airdam.

There is no way any of us drive at a high enough speed to really make use of any aerodynamic aids. Which is what makes me laugh so hard when I see a Honda Civic with a giant wing on the back!

Jim bailey - 928 International

Perhaps the question is ..." Does a 928 need downforce ? " last I checked they weigh 3,400 lbs or so already and down force only helps for cornering at high speed . What you need is REDUCED drag 99% of the time . The vast majority of the aftermarket (ricer) body kits , "ground effects" being bolted on cars simply ......slow down the car . Wings , flares , wide tires , hood scoops usually reduce top speed doing more harm than good . Basic fact is no body thinks of a rain drop as looking fast !!!!!

Hammerhead

In response to the post that claims that all passenger cars create lift and that they cannot be made into upside down wings, please refer to the following link.

http://www.mulsannescorner.com/data.htm

It's true that you cannot make a normal passenger car look like an upside down wing. However, you can play with the aerodynamics (especially the underbody aerodynamics) of the car to make it create downforce. The theory is to create suction, not make it act as an upside down wing. The Carrera GT and 360 Modena do not look like upside down wings and they certainly do not employ massive add-on wings to generate their downforce. Most of the downforce is created by underbody aerodynamics. In addition, the prototype race cars listed do not look like upside down wings, however they create literally tons of downforce.

To better understand underbody aerodynamics, please refer to the following links.

http://www.mulsannescorner.com/diffuser.htm

http://www.mulsannescorner.com/vortexlift.html

One thing to remember, the lower the car is, the better the underbody aerodynamics work. Also, if there is any disburance to the airflow under the car (severe crosswind) you lose the downforce. I believe this happened to the Auto Union and Mercedes guys when they were playing with aerodynamics on their top speed cars (streamliners).

2V4V

I think Hammerhead has probably put it best - it's more about suction.

Yes, you can make almost any car stick to the ground (realtively speaking) like glue.

Pretty simple formula (yes, a bit oversimplified, but just a bit...)

1. Lower vehicle

2. Front air dam that nearly touches pavement

3. Sides skirts that ride just a fraction of an inch off the pavement/ or touch it, even better

4. Seal up the rear, sort of

5. Put fans at rear of car to help draw air from underneath

This will stick your car to the ground so well that you will be banned from racing. F1 cars would still run (computer optimized) versions of this set-up if they were allowed to.

By removing air from under the car, you are allowing the ~14.7 PSI of atmospheric pressure to help 'push' your car down.

Remember the hardboiled egg and the milk bottle experiment from, like, 1st grade? Air pressure is a powerful thing.

Greg

slate blue

Except for Realist you are all correct,
Note that if you do this, your aerodynamic package will not work properly, this is because it you actually need air for the package to work. The important issue is velocity and pressure change. The other issue with a large air dam is it adds to frontal area, and will make run exceptionally hard suspesion that probably will hurt your lap times if not your back. This is what happened when the ground effect cars were introduced in F1.
Well here you need your diffuser, you use the air that has travelled under your car at high speed and then expand it creating a pressure drop.

I love the raindrop Jim, we haven't had enough of them over here.

jorj7

Just curious, what shape does a rain drop take at 165 mph?

Lizard928

jorj, I dont think anyone can answer that as waters max velocity is way less

Jim bailey - 928 International

George unfortunately a large rain drop at about 20.2 miles per hour develops a concave shape on the leading "edge" and the force of the air breaks it into smaller slower drops . So perhaps not the best example of aerodynamics especially at 165 mph . The 1/2 tear drop shape with Kammback tail might be better but is still devoid of wings ,scoop, spoilers !

bcdavis

Realist D.

I have to differ with Hammerhead, gbyron and Greg. Sorry guys, I know a bit about downforce cars and have actually driven a modern-era Formula Atlantic car in anger - the most down-force dependant racing car on the planet today. But we're talking about passenger cars here. There aren't any passenger cars that create more downforce than they create lift. For example, the Ferrari Enzo, which is supposed to be the most aero-sophisticated car on the road, actually weighs less at 160mph than it weighs at rest. That's because even though the sophisticated aero design creates a ton of downforce the overall shape creates more lift. Sorry, that's just a fact of science.

The way to make "net" downforce on a car (so that the car would actually weigh more at speed than at rest) would be to either create a super-efficient suction to the ground (easier to think about than to do - and as noted above, the seal on the suction "container" is easily broken), or create the proverbial "upsidedown" wing. You would create that wing by having the car flat across the top (from profile view) and curved on the bottom (so that the bottom of the car rises to meet the top at the front and rises to meet the top at the rear). Modern F1 cars come close to this, which is why they all have high noses now and flat upper body shape. The wings and the venturi at the rear complete the package on an F1 car and create "net" downforce. This profile would look ridiculous on a passenger car and wouldn't really provide anywhere to put "passengers".

I still defy you to name a passenger car that creates net downforce. There isn't one.