Tech! This is what you need to know about Cw-values

Posted on 30-05-2015 at 13:10 by MauritsH – 85 Comments”

Manufacturers are spreading like with the Cw-values of new models and especially if it is to verbruikswonderen. You have really something to that getalletje, or is it empty marketingpraat? We lay it out.

Most cars have a Cw-value of somewhere between 0.25 and 0.35 in. The BMW i8, for example, does it very neatly with a value of 0.26, on the other side of the spectrum we find driving in the gazebos as the Hummer H2. This cart has a drag coefficient of cw = 0,57. Fine, but what can you there, infer?

A Cw-value or luchtweerstandscoëfficiënt actually shows how ‘easy’ a given surface by a flowing medium cuts. Running medium sounds vague, but in this case we mean there is air. The Cw-value can thus be seen as a kind of quality mark, but a car with a low value does not, by definition, have little air resistance. This is also the frontaaloppervlak of interest.

Calculations

That can be illustrated on the basis of the following formula, with which the air resistance of a certain car at a certain speed can be calculated. Don’t be scared of a few figures and calculations. The undersigned has anything but a wiskundeknobbel and understands it too, so you can count on it that it’s simple.

The formula for air resistance to calculate is:

F = 0.5 x ρ x v2 x Cw x A

Where:

F = the force on the car works, expressed in N (Newton). Simply put: the car drives from A to B, then F therefore, from B to A. Or: the car drives from left to right, F right-to-left.
ρ = (pronounced ‘rho’) is the air density in kg per m3. We keep for convenience, an average value of 1.23.
v = the speed in metres per second. Mnemonic: to of km/h to m/s to go, you share the number of kilometers per hour by up to 3.6.
Cw = the Cw-value
A = the frontaaloppervlak in m2

Let ‘t take it easy on you to act and do not click right away, because it is easier than you think. Note in each case that the speed have a huge influence on the aerodynamic drag. This must be squared to the solution of the formula.

As an example, let the awesome McLaren F1. This car has a Cw-value of 0,32 (with active spoiler folded) and a frontaaloppervlak of 1,79 m2. The top speed of this car is 386 km/h (386 / 3.6 = 107,23 m/s) , while the maximum power in ‘just’ 627 horsepower. On the basis of the following calculation we are going to show you how that is.

We fill in the values in the formula:

F = 0.5 x 1.23 x 107,232 x 0,32 x 1,79. In other words: F = Nlg 4,050 N. Fine, but we are still nothing. We can use this number however, is the ability to calculate that is needed to keep this speed. Please note: this is a windless situation, and only to the aerodynamic drag. Rolling resistance and weight, we leave out of consideration, we focus only on the Cw-value.

The power that is required is a 386 km/h is fairly simple to calculate. Multiply F with the speed in m/s and you get the power in W (Watt). Divide this by 1,000 and you get the power in kW. That we multiply with 1,359 et voilà: you have the power in horsepower. Take a look!

Nlg 4,050 x 107,23 = 434281,5 W / 1.000 = 434,28 kW x 1.359 = 590 hp. Hopsa! The McLaren F1 has 590 hp needed to the aerodynamic drag at 386 km/h the boss. How is that? The Cw-value is not so low that it is? Correct! What the McLaren, however, perfectly illustrates, is that the frontaaloppervlak is at least equally important.

Modern supercars are often two metres wide at, let’s say, around the 1.20 meter high. The F1, however, is only 1.820 mm wide and that’s pretty weird for a supercar. That plus a lot of other factors contribute to the frontaaloppervlak of the Mac is very small.

For comparison: the Bugatti ” there you have ‘m’ Veyron has a surface area of 2.07 m2, and a Cw-value of 0.36 in lowered condition and with low spoilers. Enter these numbers into the formula and you will see that the Bug is a whopping 767 horsepower needs to 386 km/h! The Cw-value is not significantly higher than that of the F1, the frontaaloppervlak is that however.

It should of course be noted that the W16 of the Veyron considerable cooling need and the cooling holes are again fatal for the resistance. Therefore, tape manufacturers also as many holes and cracks to close, if a model is tested in the NEDC cycle. The less air resistance, the lower the consumption in general will fall because the engine has less to work hard in order to overcome the air resistance.

An extreme example of this is the Ecorunner V, of students at the TU Delft. This überzuinige recordauto has a Cw-value of 0,0512!

Drag area is a better benchmark

Various experts are arguing for the upholding of the so-called drag area, i.e. Cw-value x frontaaloppervlak. That way it is much easier to assess whether or not a car little or a lot of drag experience.

A huge list with numbers of the most diverse range of cars you can find by the way here, if you are busy with a calculator to the course want.

Aerodynamic drag and downforce

For street-legal car is the Cw-value is somewhere around 0.3. Nevertheless rides as Formula 1 cars soon to a value above 1. That has on the one hand, for example, with the open wheels that are against the sky have boxing, but on the other hand, with downforce. A Tesla Model S will benefit from this as little as possible air resistance to ensure that the batteries are not to be quickly drained. This car is so built to be as easy as possible through the air to cut.

F1 cars have downforce needed, be they on the asphalt printed. The more the better, because then-ie (apart from mechanical grip) harder through the turn. Very briefly stated. They are diffusers used, the air under the car as well as possible carry, but also substantial front and rear wings. These wings have the airflow needed to can do their work, because otherwise nothing happens. In other words: race cars have drag need for downforce or downforce to be able to wake up.

Check this image which perfectly illustrates what parts drag and what parts for downforce worry.

Manufacturers search, of course, have to compromise. A fast track as Monza requires less downforce and higher top speed. The tight street circuit of Monaco is required to understand as much as possible downforce and top speed is much less important. In both situations are so different vleugelafstellingen used. So also different Cw values. At Monza is around 0,75, in the streets of Monaco around the 1.25. A huge difference! The ultimate aerodynamic challenge in the racing is so generate as many as possible downforce, for as little as possible air resistance.

Finally

We would be much deeper on the matter, but for now let’s just get the basics. Bottom line of the story is, in any case, that Cw-values do indeed tell us something about the air resistance, but that the frontaaloppervlak is at least equally important.

PS: if you like the story completely out of have read, you have what to us is a beer earned. Cheers!