Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Automotive Car downforce: accelerating air under the car vs blocking

  1. Jul 2, 2017 #1
    As most of you know real racecars have flat bottoms, rear diffusers and in the front well... a variety. So to keep this simple. I am building a car with a flat bottom and rear diffuser, car raked at 4 deg, diffuser at 14 (all details will be adjusted based on learned info) and nothing on top or on the side of the car.

    So, here is my question: if I were to build a second diffuser and install it on the front wouldn't I be accelerating the air under the car and creating a really low pressure zone (high pressure goes to low pressure) and creating massive downforce? I know f1 cars in the 70's had similar designs which were later banned due to such massive downforce. Today's cars run diffusers and air dams and I see these as really high drag items with a great amount of downforce. Will my concave idea work and what are some downfalls?
    Last edited by a moderator: Jul 3, 2017
  2. jcsd
  3. Jul 3, 2017 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    By real race cars i assume you are talking about open wheel formula type cars like Indy cars, F2000 etc.. which is odd since all these cars do not have air dams and 4 degrees. Fender benders do in the stock car ranks. please clarify exactly what class racing you want to compete. Also please tell me ifin you are going to this much trouble to do the aero thing right, why screw things up with adding rake? We used to hike the tail end of the car up to cause down force as the air would enter in front at high speed and slow down as the area expanded. Ideal bottom is flat and level to ground and exits upward at the rear with proper channeling ( strakes, diffuser pump, etc..to direct flow where it will benefit. The front splitter on the stock cars of today does exactly that..splits the air into under car portion at high speed and air running into the high drag air dam.

    Back in the day, we figured out the car lifts the higher it goes ( we lost grip). Air flowing over the top of the car separate and goes into turbulent flow. This makes low pressure area at the rear of the car and we lose grip. This is pure straight line action. Add to this picture going into a turn at high speed..yipes!!

    We found that by closing off the air coming in under the front nose, and directing air flow to the sides, reduced rear lift. The whole idea of down force goes back to controlling air flow over and under the race car.

    Bernoulli Principle – increase in speed of air flow occurs simultaneously with decrease in the pressure exerted by the air flow.

    On the typical race car rear wing spoiler if you measure the length of the front to the rear of the wing you will find the top distance is much shorter than the bottom length. This obviously means that if you have a car speed of 100 mph, the air flowing over the top of the wing will take a shorter time since the length is shorter. The air traveling under the wing will have a longer distance to travel.

    Because of whats called a continuity equation, the rate of flow over the top and bottom must be equal. So the air flowing over the top of the wing is about 100 mph but t he air flow under the wing is much faster than 100 mph to make the same flow rate. To do this we have

    Top of the wing - low speed, high pressure = pushing downward

    Bottom of the wing – high speed , low pressure = pushing upward

    We have down force…thus GRIP.

    Last edited: Jul 3, 2017
  4. Jul 3, 2017 #3
    So, you said it, Bernoulli Principle – increase in speed of air flow occurs simultaneously with decrease in the pressure exerted by the air flow.
    This is what i don't get, If you compress the air speed increases? This is what I would like to do, let me explain

    I have a FWD 1989 honda civic (I am 42 with three kids) and i se this car as my learning tool for fun, make it fast, make it stick. I track day auto x and road coarse and don't care if I ever win a trophy. (I always leave early). I also DD this car because it's fun to drive. This car is fast for a civic so my next step would be to add aero (no wing coments please, NO WING!) so i'm starting under the car but and here is my biggest but, I don't want to put it away in the wintrer and also i'm trying to achieve MAXIMUM down force from 40 to 100 with minimum drag and if the drag is too much then ill add some active aero to reduce drag.

    To me learning this is fun and at the same time I don't want to drive by a police car screaming RACECAR.
    So, your saying no rake (but I am not slammed on the ground?) ok?? but I am trying to neutralize my center of downforce also and with a FWD all the weight is in front which means I dont want to be too agressive up front, or do I?
    Last edited by a moderator: Jul 3, 2017
  5. Jul 3, 2017 #4

    Attached Files:

    • 2.jpg
      File size:
      93.3 KB
  6. Jul 3, 2017 #5

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    you are not compressing anything. If you are refering to limiting air under the car, this is not compressing. The rate of flow on top of the wing must equal the rate of flow under the wing. This is why your car will have rear end LIFT at high speed. The length of the under side , front to rear is a lot less distance than the length of the hood, the front windshield , the rear wind shield and the trunk. So the air must travel a lot faster on top of the car to equal flow rate under the car. Without additional modification this will not happen and we have a lift condition crated under the car at high speeds. this you do not want..

    ifin you want to learn how to make your sedan run ,, read Race Car Suspension class in this very forum.. i cover FWD cars in it
  7. Jul 3, 2017 #6
    IMG_4591.PNG IMG_4592.PNG This is the direction I'm talking about. F1 knew by ramping the front of the tunnel you can increase the speed of air thus reducing air pressure = downforce. Is there any logic in this on a street car?
  8. Jul 3, 2017 #7

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    absolutely not.. for get it..you have at least 3 inch +/- suspension travel required for pot holes, speed bumps road conditions etc...no way can you seal up the front and sides to get any kind of aero on this door slammer..go for items i note in race car suspension class
  9. Jul 3, 2017 #8
    The maximum amount of down force you'll generate with a Civic... especially trying to use ground effect is approximately zero.
    If you want to go fast in it do the following:

    Make the interior look like this
    Add a good set of these:
    And if you've got the cash:

    = trackday weapon
  10. Jul 3, 2017 #9

    jack action

    User Avatar
    Science Advisor
    Gold Member

    The idea behind the 70's Formula car is to create a very large inverted wing. Being close to the ground, it create a tunnel effect that increases the effect of the wing. An important characteristic of the wing is that it has a center of pressure (CP) where all the aerodynamic forces act (the aerodynamic equivalent of a center of mass). Where you put that CP have a big effect on the front/rear load distribution. For such a big wing, you probably want a 50/50 distribution, so it is mostly centered relative to the car (the CP should be close to where it is the closest to the ground).

    If you can't create that perfect wing shape (like with a sedan with an engine and other components in the way), then you must use another approach.

    The other approach is to create a «vacuum» under the car. You do that by blocking the front and side entry with an air spoiler and side skirts. If you imagine a car moving one car length really fast, if the front and sides are perfectly sealed then the «block» of air under the car will stay still, going out the rear end, while no other air can enter beneath the car. No air = vacuum, i.e. the lowest pressure you can achieve.

    Now, perfectly sealing the front and sides is impossible, but the less air that goes underneath the car, the lower the pressure you can achieve under it. Here are some examples of front spoilers that exist solely to block the flow from entering under the car:

  11. Jul 5, 2017 #10
    If you want to play around with aerodynamic effects and simulate them before you really build something I'd recommend using a simulation tool! Get a CAD model from some platform like grabcad and upload it in SimScale to perform aerodynamic simulations. Finally get a free account at Onshape and change your CAD model and perform a new analysis. You can just copy a public project at simscale and upload your CAD model. This makes the setup of the simulations really easy.
    Examples here:
    Aerodynamic analysis of F1 car
    .. of a LMP car
    and aerodynamics of a sports car

    This comes pretty close to a civic already but maybe you can find a better model ;)

    Have fun and let us know what you come up with!

  12. Jul 6, 2017 #11
    AH.....lift at the expense of drag.

    We removed the splitter off of our Ford GT 5 years ago and are knocking on the door of 300mph in the standing mile.
  13. Jul 7, 2017 #12
    Yes, already done. Next.....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted