How Does a Splitter Work.

A splitter generates downforce on the front of a car by creating pressure differentials. Actually a splitter produces a "downward force". It is not technically termed "downforce" unless the downward force is large enough to overcome the "upward" force caused by lift. But that is really just semantics.

 

To understand how a splitter creates downforce you have to know a little bit of fluid mechanics. One should at least be aware of the difference between static pressure and dynamic pressure. It is also instructive to know how dynamic pressure is related to flow velocity. This relationship is given by the well known Bernoulli equation.

The essential point to be taken from Bernoulli's equation is that the pressure inside an air stream is related to the velocity at which the airstream is moving. As the air stream velocity goes up, the dynamic pressure within the air goes down. In the case of an automobile it is instructive to consider the vehicle as stationary, with the air flow moving towards it, as opposed to the other way around. The oncoming air approaches the vehicle at V1, which is the speed of the vehicle, and a pressure P1 which is related to V1 according to Bernoulli's equation. P1 will be somewhat less then atmospheric.

Figure 1 shows that as the incoming air reaches the front of the vehicle it must come to a stop before it turns to move either up and over, down and under, or around the vehicle. The area where the oncoming air flow comes to a stop is termed the "stagnation point". Since the velocity has gone down, the pressure has gone up (the "stagnation pressure). Thus the front of the moving vehicle is an area of relatively high pressure. Most people are well aware of this already.


Figure 1

 

One of the realities of pressure is that it acts in all directions. Thus the relatively high stagnation pressure at the front of a moving vehicle pushes back on the vehicle (creating drag). Now if we stick a splitter out on the bottom of the spoiler, then the stagnation pressure will also push down on the top of the splitter. But what is the pressure on the underside of the splitter? If it is the same as the pressure on top then there will be no force on the splitter. So let's examine the pressure underneath the splitter.

Since the splitter is close to the tarmac, it creates a restriction to air flowing underneath it (a mini-Venturi if you will). Just as water in a river speeds up when the river narrows, the air that is piled up in front of the spoiler must speed up if it wants to squeeze underneath the splitter. And as we know from the Bernoulli equation, when an airstream speeds up, its pressure goes down. Thus the region between the splitter and the pavement is an area of low pressure. To recap; high pressure exists on top of the splitter, and low pressure exists beneath it. This adds up to a downward force on the splitter. And this downward force is directly proportional to the surface area of the splitter. So, up to a point, the bigger the splitter is, the more downforce it can create.

Two photos of splitter implementations are shown below. Notice the enormous size of the splitter on the Panoz Le Mans car. And on the Ford Mondeo BTCC car we can clearly see the very narrow "venturi" section under the splitter. This speeds up the air flow and lowers the local pressure.

 

Some racecar designers have maximized the use of a front spoiler venturi.

 

If we examine the spoiler on the 1996 DTM/ITC Mercedes Benz shown above we notice that it does not have any kind of splitter. In fact, it has the opposite of a splitter. The lower portion of the spoiler curves back in towards the car, instead of jutting out into the stagnation region. The designers were trying to encourage the high pressure air in front of the spoiler to flow underneath the car. This way they could maximize the venturi effect underneath the front of the vehicle. In essence, the front spoiler region of the car forms a "mini ground-effect tunnel". In the "throat" region of the venturi, the air flow speeds up and hence the local pressure goes down. This creates downforce. The longer the throat region is, the more downforce that can be created. A simplified schematic of the air flow over and under a racecar with a spoiler venturi is shown below in Figure 2:


Figure 2

 

There are many possible permutations of the splitter/venturi spoiler idea. Some racecars use either just a splitter or just a venturi. Other designers have attempted to combine the two devices in an effort to maximize front downforce under modern racing regulations (which usually require a flat under-body from the front axle-line back). Two examples of combined splitter-venturi designs are shown below. These Super Touring cars from the English and German series. Notice in particular that the Volvo S-40 has a splitter going right up and over the venturi inlet.

 

Another example

 

The lenght depends on how shiok you wan it to be.

My car? What car?

 

Too long but flexible oso not good. So need the right lenght