Downforce for Cars: Help and Equations

[Jach]

Hello everyone. I am new here n need some help.
Can anyone tell me what's a downforce for cars? and is there any equations for downforce?
I don understand. Help pls. Thank you.

 

[Danger]

Welcome to PF, Jach. Downforce is a term that refers to any force that counteracts aerodynamic lift in order to maintain contact with the road. It can be accomplished with spoilers, body shape, suspension height, or several other methods. There are definitely formulae involved, but each one is specific to the method in question. There are some specialists in every aspect of aerodynamics and fluid dynamics on duty, so I'll leave the details up to them.

 

[brewnog]

Aerodynamic downforce is simply lift, but in the opposite direction. As Danger said, it's a way to push the car down onto the road in order to maintain grip.

A very useful equation (which you should have seen before) is:

L = 0.5 * Cl * A * r * V^2

Where L is lift (or in your case, downforce), Cl the lift coefficient, A the area of the body in question, r the air density, and V the velocity of the fluid flow.

Sorry for the lack of fancy text.

 

[Sojourner01]

The specific downforce for any given aerodynamic adaptation (spoilers etc) is bound to be horrendously difficult to calculate accurately, right? That said, would the trick for that equation be calculating the precise value of the lift coefficient, or is it simply an approximation that doesn't account for the shape of the lifting body?

 

[rcgldr]

As posted, downforce is a downwards force on a car using aerodynamics. The purpose is to increase the downforce in the tires, which increases how hard a car can take a turn. A FIA F1 race cars generates about 1g of downforce at around 115mph, and about 2g's of downforce at 163mph. At 163 mph, this translates into about 4g's of cornering force, and because there's about 1g of aerodynamic drag, 5g's of braking force.

There are serveral ways cars generate downforce.

FIA F1 cars use the upper body and wings. The rules state that the cars must run skidboards, so no underbody tunneling is allow. Champ cars (USA) and Indy Racing League cars are allowed to use underbody tunneling.

NASCAR cars tilt the body downwards (rear is about 1.5 inches above front), and use an air dam to block air flow from entering from in front of the car. (This should drive Bernoulli fans nuts, the slower moving air (relative to the car) underneath the car has less pressure than the faster moving air above. It's because the air dam is doing work on the air behind the dam, accelerating it forwards.) The air does flow in from the sides, so a rear spoiler is used to add downforce to the rear of the cars.

Other methods have been used as well. Older F1 type cars had side skirts to prevent air from flowing underneath the car from the sides (the wing up front directed a lot of the air upwards). The high amount of downforce was rough on the drivers and prone to accidents if a skirt was damaged, so they banned them.

Chaparral used large fans at the back of a solid body Lemans type car to draw air out from under the car, but this was later banned.

Might as well post a video of a F1 car in action. David Coulthard in a 2002 F1 McLaren at Spa.

spaf1.wmv

 

[brewnog]

The specific downforce for any given aerodynamic adaptation (spoilers etc) is bound to be horrendously difficult to calculate accurately, right? That said, would the trick for that equation be calculating the precise value of the lift coefficient, or is it simply an approximation that doesn't account for the shape of the lifting body?

The lift coefficient would be measured in a windtunnel, and would account for the shape of the body. This number would then be used to calculate the actual value of lift/downforce for given flow conditions.

For actual calculation of lift coefficients, CFD techniques are now advanced enough to give accurate results.

 

[Jach]

Thank you everyone :)
Now i get it.