How to create lift using the underside of your car

[CobraBall]

Double your speed and your drag increases FOUR FOLD.

Sticking a wing, spoiler, etc on the rear of a car is an attempt to make up for a aero design deficiency. A perfect example of good aerodynamic design (not perfect design) is the soap box derby racer bodies (do a Google Image search). Notice no spoiler, wing, etc.

Lift, whether negative lift or positive, creates drag. And a wing at zero angle of attack still produces parasitic drag, which increases 4X as speed 2X's.

[skyl4rk]

In order to enjoy any benefits from lift, you would first have to lighten the car and build it like an airframe. Even then, to get any reduction in friction you would have to build wings on the car to allow the lift to reduce its weight on the ground by any significant amount.

[Bicycle Bob]

A car body designed for low drag will usually generate some lift. This causes more drag than leaving the weight on the tires, but less than spoiling it deliberately. Even with efficient wing shapes, aircraft have to fly where the air is very thin to get reasonable economy.

[CobraBall]

Aircraft fly high for two reasons. Less air with the same air to fuel ration burns less fuel. Higher altitude (colder) the higher the true airspeed.

[Blue Bomber]

You DON'T want lift. Trust me, I have an MR2, and the front end aero is pretty poorly designed, and known for creating lift, and being unstable on the highway. The car can get pretty twitchy when you don't have all of the stock underpanels on to reduce lift.

[Bicycle Bob]

Originally Posted by CobraBall

Aircraft fly high for two reasons. Less air with the same air to fuel ration burns less fuel. Higher altitude (colder) the higher the true airspeed.

That just means that they have fewer HP available. A throttle does the same thing. Piston engines often have superchargers that would blow the heads off at low altitude, that are used to help burn more gas up higher, for a faster, but less efficient cruise speed. The altitude does help with reducing drag and avoiding weather.
Colder air is denser, which gives both more drag and more lift.

[metromizer]

Old aircooled VW's have lift... rounded roof, relatively flat belly pan, in many respects they are like one giant airplane wing.

A drag racing buddy of mine wanted to confirm what many of us thought we already knew. He had a guy video tape his bettle-bodied race car at the top end of the quarter mile, while he went through the traps at 120mph. The observed separation between the top of the tire and the inside fender wheel arch was greater at 120 than standing still, by about two inches! Definately lift. Definately dangerous in a crosswind situation.

[Twerp]

I'm no flight engineer, but from what I've learned about Bernoulli's principles I'd guess that a basic car should be able to generate some lift with minor modification. Basically if you cause air to move faster on the upper side of your car than it does on the lower side of your car, you will create a lower pressure above than below. The result: lift. Since cars already are pretty flat on the bottom, and less so on top, the design isn't too far off from an airfoil, depending on the car.

So, if you were to put a full flat belly pan on the bottom of your car and completely eliminate air flow resistance, the air underneath you would remain relatively static. If you raise your bumper/air dam height just a little, you could actually compress air between the ground and your car (and change your angle of attack) which would increase the air pressure. The air flowing over the top of your car will have an increased distance to travel and will flow much faster than the air underneath your car, creating a low pressure area. If you streamline the top of your car to reduce turbulence, the flow will be smooth, fast and uninterrupted. If your car slopes down toward the back in a boattail fashion it should maintain this flow. If your car ends abruptly like most hatches, the turbulence behind the car will prolly create drag which may cancel out some of the lift.

A spoiler is usually put on race cars to interrupt the smooth flow of the air, reduce lift and add down force. Since your goal isn't to take sharp corners at 100+mph, you prolly won't need as much down force. However, if your car is light to begin with, expect to be pushed around by wind gusts when you are at speed. Also, sharp high speed maneuvering may be a little more difficult. If your car is decently heavy, it may just take some pressure off the wheels.

I remember reading an article in Car and Driver many years ago about the effectiveness of spoilers on common sports cars (not race cars). They found that most spoilers didn't make much of a difference at normal to excessive driving speeds with the exception of the Porsche 911, which became effective at around highway speed. If you look at the shape of that car when the spoiler is down, you can see how it might be able to generate some lift at speed.

Again, I'm no flight engineer, but the theory kinda makes sense, doesn't it?

[equation112]

Originally Posted by Twerp

I'm no flight engineer, but from what I've learned about Bernoulli's principles I'd guess that a basic car should be able to generate some lift with minor modification. Basically if you cause air to move faster on the upper side of your car than it does on the lower side of your car, you will create a lower pressure above than below. The result: lift. Since cars already are pretty flat on the bottom, and less so on top, the design isn't too far off from an airfoil, depending on the car.

So, if you were to put a full flat belly pan on the bottom of your car and completely eliminate air flow resistance, the air underneath you would remain relatively static. If you raise your bumper/air dam height just a little, you could actually compress air between the ground and your car (and change your angle of attack) which would increase the air pressure. The air flowing over the top of your car will have an increased distance to travel and will flow much faster than the air underneath your car, creating a low pressure area. If you streamline the top of your car to reduce turbulence, the flow will be smooth, fast and uninterrupted. If your car slopes down toward the back in a boattail fashion it should maintain this flow. If your car ends abruptly like most hatches, the turbulence behind the car will prolly create drag which may cancel out some of the lift.

actually, adding a bellypan will increase downforce thanks to the venturi effect - the car would become like a tube with open ends of greater area than the middle. however doing so will also reduce drag by a substantial amount because typical car underbodies are catastrophically unsmooth

A spoiler is usually put on race cars to interrupt the smooth flow of the air, reduce lift and add down force. Since your goal isn't to take sharp corners at 100+mph, you prolly won't need as much down force. However, if your car is light to begin with, expect to be pushed around by wind gusts when you are at speed. Also, sharp high speed maneuvering may be a little more difficult. If your car is decently heavy, it may just take some pressure off the wheels.

I remember reading an article in Car and Driver many years ago about the effectiveness of spoilers on common sports cars (not race cars). They found that most spoilers didn't make much of a difference at normal to excessive driving speeds with the exception of the Porsche 911, which became effective at around highway speed. If you look at the shape of that car when the spoiler is down, you can see how it might be able to generate some lift at speed.

Again, I'm no flight engineer, but the theory kinda makes sense, doesn't it?

everybody here needs to look at the autospeed articles where they tuft-tested several different cars

[MazdaMatt]

For some particular car, can anybody give an example of the speed at which rolling resistance is equal to aero drag? I know that it takes a LOT longer for my car to go from 10 to 0 than it does to go from 50 to 40.