Downforce/Upforce (street racing)

[NDR008]

I am am a little confused on what is the main target in the case of a street car.
Lighter cars with the same engine = great power:weight ratio, thus better acceleration.
Thus i would imagine if a car could be made lighter it would be better. So why create downforce with spoilers, etc? wouldn't that push the car down, making it act heavier?
Why not create a minor lift force to make it seem lighter?

Probably there is an obvious and silly reason as to why, but i just can't seem to see it.

Ps. i realise lift would cause less grip. but lighter cars still grip enough to turn round corners, and trash the competition with their acceleration....

[tempest]

The simple reason is that the wings do not make the car heavier or lighter.

Multiplying the co-efficient of friction between the tyre and the road by the normal force through the tyre gives you the maximum force that you can use for acceleration. Increasing the normal force can be done in 2 ways, increasing the weight (mass) of the car, or by aerodynamic devices. But increasing the weight of the car means you need to use more force to turn it. (Force = Mass * acceleration) So an aero device gives you greater normal force but keeps the same car mass.

Thing is, they only work at high speed. So at low speeds the car behaves as if they aren't there. Therefore they only help you go round corners faster, but dont affect your straight line acceleration (only your speed, but thats a different story)

[Guest]

Weight is a force.
So if in a stationary situation F=W=ma
but due to a lift force you get
F(downwards) = ma - lift
= W - lift
by 'seem' lighter or heavier weight, i mean the actual resultant force acting in the direction of the car's weight.

[marcush.]

of course,aero forces influence the cars accelerating abilities to a great extend.
F1 cars are said to be decelerating as much as a street car with fully locked brakes simply when you go out of the throttle..
So putting on downforce thru wing ,especially gurney reduces your acceleration on high speed quite substantially but of course they also help to come around the corner faster so your speed at the beginning of the straight is hiogher to start with and so you might cover the distance of the straight in a shorter time ,without reaching the same top speed as without the wing

[tempest]

I think what you are asking is why not make the car "fly" to make it lighter?
Maybe this will clarify my previous post. The force going through the tyres is related to the mass of the car but only matters as to how much grip they can provide. In terms of acceleration the mass you have to accelerate is the mass of the car, it doesnt matter that it has less footprint on the road (But i would find this really scary if it was my car). By your logic an aeroplane "weighs" nothing and has instant acceleration. Wings do not make the weight of the car change, marcush has a point that they actually make you slower in a straight line, and no matter which way they are inverted they give you drag.

The last word in this should be given to Mark Webber, check him out in the warm up at Le Mans 1999.

http://www.lemans24horas.hpg.ig.com.br/ 5.htm

About halfway down the page titled O VÔO DO MERCEDES - LE MANS 1999

[NDR008]

Hmmm, still i am a little confused. on the moon, would the same car accelerate quicker or not? Hmmmm... unfair question, there is no air.

Ok, hmmm... i am not tryin to refer to how aerodynamics effect the car in terms of how they cause or reduce drag.
But, let me have one lest go at this:

car: 1000kg (mass)
Vertical:
F=ma
a=g
g~10
F=1000*10
=10000N

OH ---! LOL i had a feeling there was super silly reason as to why....
Irrelevant to wheather the vertical force sum increases or decreses, the actual mass remains the same, and that is what will be multiplied by the horizontal acceleration forces. So it is irelevant wheather or not the Vertical force increases or decreases, that will only effect the grip the tires are providing right?

Hope i figured it out with the right reasoning

[Guest]

Yes, that is exactly why. Hope you all liked the pics

[tempest]

er...its tempest, Im at uni, gettin too used to auto sign in. I want my post to count!

[Guest]

Mass adds weight and therefore a negative force, or downforce in this case, on the tires. This sounds good until you find out that more mass contributes to more inertia. W/ more inertia it takes more energy to change direction, when cornering, accelerating, decelerating, and what else. I think I got that from Monstro

Wings and other aerodynamic devices add downforce w/o a huge mass penalty and therefore add traction to the wheels.

The simplest way to look at it is to look at the eqn:

F = (mv^2)/2

m = mass
v = velocity

and F = total available cornering force. If we say we have 1000N of cornering force (supplied by aero + mass), it can easily be seen that w/ higher mass we must reduce the velocity through the corner to get the same cornering force w/ a car with less mass.

Use downforce or lift or even negative lift, but please don't say "upforce." It makes my eyes bleed.

- West

[NDR008]

So ultimately, one should have as light as possible of a car with lots of downforce to make up for the lack of turning force.
However, there must be a draw back in gaining turning force just from aero:downforce, and not true mass/weight.

[dumrick]

Not in turning dynamics, there isn't. Less mass equals less inertia, so if you have force in your contact patch that results from the sum of weight and downforce, the car will turn better than if it came only from weight (that would mean more inertia).
The limitations of aero downforce are that it comes often with the downturn of more aero drag and that at low speeds its relative contribution is a lot lesser.

And, of course, the lighter the car, the best he is in all aspects (acceleration, cornering, braking,...) - Colin Chapman sure knew it!!!

[Guest]

I meant to say the eqn was (mv^2)/r w/ r being the effective radius of the turn.

- West

[NDR008]

Multiplying the co-efficient of friction between the tyre and the road

Though your reasoning is a good simplified version. If you actually look into the material science of tyres, you will realize that most of the grip has nothing to do with co-efficient of friction. In actual fact it is the high tech way the polymers are blended into racing tyres, that result in an uneven force distribution pull the car forward or into a corner. So much so, that tyre are constantly slipping (hence the term slip ratio, which I forget the equation definition of the SAE, i remember it was bit fudged to make the negative signs look nice) and even when turning the front wheels do not point the direction the car is going.

My view of the original question. Larger weight is larger inertia forces. Which means you need bigger efforts to turn round a bend, to brake, to accelerate.

When already moving, thanks to downforce. The inertia of the lower car means that the centrifugal force is still based on the mass of the car and the radius of turn. As tempest was leading to, the extra benefit of the downforce is relating to the tyre grip.

Oh, and one final note - on most road cars, spoilers and 'aero' devices are stuck on for the wrong reasons.

Also some cars like the Audi TT have a stock spoiler at the back for different reasons. Besides drag and downforce you have pitching moments. The Audi TT is by nature unstable, when you go over a bump the car prefers lifting its nose up, and this in turn further increases the pitching up effect. The spoiler at the back is to bring back the stability by causing a restoring moment in such a scenario.

[slimjim8201]

Oh, and one final note - on most road cars, spoilers and 'aero' devices are stuck on for the wrong reasons.

Also some cars like the Audi TT have a stock spoiler at the back for different reasons. Besides drag and downforce you have pitching moments. The Audi TT is by nature unstable, when you go over a bump the car prefers lifting its nose up, and this in turn further increases the pitching up effect. The spoiler at the back is to bring back the stability by causing a restoring moment in such a scenario.

It has less to do with moments and more to do with overall lift. Most cars produce a small amount of lift at speed. Certain cars like the VW New Beetle and the Audi TT have a rather airfoil like side profile. They produce measureable amounts of lift at speed and cornering capabilities suffer as velocity increases. The purpose of the spoiler is to disrupt the flow so that the car isn't an efficient "wing" anymore. I'm not sure pitch and dive have much to do with it.

These devices do not produce downforce, they merely decrease lift.

[Jersey Tom]

I like seeing big goofy wings on the rear of FWD cars with static understeer.