Aerodynamics might not be a region we „ordinary“ petrolheads spend too much thought on, mostly just putting a big wing, side skirts and a front lip on our car (or removing the same thing, depending on your taste) to make it look better and meaner. But still, it might be good to have a basic understanding of what you’re doing to your car, even more so if you spend hours deciding which tires, oils, pistons etc. you should use to get maximum performance, meaning ruining the gained performance by unfortunate body parts would be a shame. I will try to keep the simplified theoretical ramblings short and mainly show a few nice pictures that show what happens where. 😉
The foundation of the science of fluiddynamics, of which aerodynamics is an important application basically are a few very complicated governing equations, called the Navier Stokes Equations. Don’t worry, we’re not gonna solve any mathematical exercises in this article, I just wanna give you an impression of the kind of complexity we’re talking about.
If you solve these, you will know everything you ever wanted to know!
Now, trouble is, no one’s actually really solved these equations. People have found possible solutions, but noone’s found the definite one. This means that even the brainiest scientists can’t completely predict what a flow of fluid, in our case air, will do. Which is why Formula 1 teams have wind tunnels, some of them several, running around the hour to test their aero parts, because while computations can give an idea of what will happen, only a test will give approximate certainty. What I actually want to say is, how should we „normal“ people have an idea what parts to put on our car if the science is this complicated. For that reason, a host of simplifications have been introduced, the details of which I’m not gonna bore you with. But what this means is that we can have a basic understanding of what shape will result in what behaviour.
There are two main aerodynamic forces, resistance and lift, being distinguished only by their direction. Resistance (or drag) is created by the shape of a body (due to pressure differences) as well as by friction, which depends on the surface roughness of a body (smoothness of paint or also small stuff sticking to the surface). You probably will have heard the term „Cr-value“ (german: Cw), this is a dimensionless term for the resistance due to aerodynamic force, enabling comparison of shapes regardless of their size and of the speed and composition of medium they’re travelling through. This is why an SUV could have a better „Cr“ than a Mini, but this doesn’t mean it has less resistance, it’s just a better designed shape/surface!
Very big but very aerodynamic, the concorde has a low Cr! (But let’s not get started what’s different at supersonic speeds..)
To get an idea of friction, we need to know the following: The important flow phenomena happen in a layer close to the surface it flows along, this is called the boundary layer. The flow in this boundary layer is laminar at first (the entire flow medium runs in an uniform direction) and turns turbulent at a certain point (eddies occur due to surface friction). The rougher a surface is the earlier we will have turbulence and therefore friction.
The transition from laminar to turbulent flow visualized.
While turbulence in itself increases friction, the main worst thing in case of aerodynamic resistance is flow separation. This happens when the flow can no longer follow the shape of the body, resulting in chaos, basically. This generally happens when the flow would have to follow sudden steep declines 0f the body, for example at the rear window of a saloon car. This creates lots of resistance and happens at the rear of every car, the part of the car that is actually more important than the front. Because turbulent flow is more resistant to separation, in some cases you deliberately create a turbulent boundary layers to delay separation.
separation visualized for the example of a simple wing profile.
Critical regions on a scooby: separation at the rear plays the most important role.
These phenomena can be observed well when dragging a spoon through a cup of coffee with cream. 🙂 The same thing happens with the air and this creates resistance. While you can’t really influence the basic shape of your car, the only thing you can do here is to try and keep the surface clean with as little useless resistance creating obstacles as positive.
Moving on to the topic that’s much more interesting for performance cars: lift (or if achievable negative lift, i.e. downforce) is created by the shape of a body the flow tries to follow, which results in a change of pressure around it. These pressure differences create aerodynamic forces which most famously make planes fly. The simplest way to imagine it in case of a simple wing is, that if the air has to take the „longer way“ round it it has to be faster, which results in lower pressure and vice versa for the „shorter way“. In case of an airplane wing, this creates low pressure above the wing and high pressure underneath it, resulting in lift.
pressure difference profile of a wing profile.
The wing on the back of your car is exactly the same thing, just bottom up. 😉 And all kidding aside, the higher it is the better the flow around it will be. One of the main concerns on cars is the influence of the different aerodynamics parts on each other, or rather the struggle to avoid it as far as possible!
Not only for bragging rights!
Increasing the angle of attack of a wing will increase it’s lift and also it’s drag, up to a certain angle where we will have the aforementioned flow separation at a point close to the nose of the wing, resulting in mainly drag and almost no lift. This situation is called stall and is a very bad thing for airplanes and a not so great thing for rear wings on cars, because in this case all they’ll do is brake!
The influence of the angle of attack on the flow around a wing profile.
This can of course be turned into a benefit if the wing is made automatically adjustable and used as an air-brake.
Mercedes showing its aerodynamic gadget.
For the whole car, the situation is a bit more complicated:
Pressure profile of a basic car shape.
When looked at very simplified, the shape of a car is that of a wing of an airplane. This means the basic shape of a car will create lift.. bollocks! To be able to compare lift forces across different sizes of bodies as well as air speeds, the „Cl“-value (german: Ca) is utilized, analog to the „Cr“-value for resistance. This doesn’t really give an idea of absolute lift but enables me to give you some compareable values: while a normal street car has a Ca-Value of +0.3 (meaning it creates some lift), a touring car has up to -1.0 and a Le Mans prototype is able to reach a Cl-value of -4.0! These very high downforce values will also result in an inevitable increase of drag but the increase in possible cornering speeds by far outweights this. The corner speeds of Le Mans or Formula 1 cars can never be reached by conventional vehicles. Light wight is also an advantage in cornering, but sophisticated aerodynamics starts to be dominant from a certain point. Actually, for race cars the limiting factor almost always are regulations. And just to avoid confusion, the Cl-value has no connection to the car’s weight, -1.0 doesn’t mean the downforce equals the weight of the car! (Cl = Lift Force / (Front Area * (speed)^2 * density / 2), in case you are wondering.. 😉 and by the way, there’s also a formula for calculating maximal corner speed.. )
A whole lot of downforce is comming at you!
One particular problem with high downforce cars is that extracting a lot of forces out of a flow results in high turbulences behind the car, which in turn changes the flow around the following car dramatically. This can have scarily spectacular results, as seen in the following picture, in this case helped by the fact that lift also depends on the angle of attack of a vehicle (nose up means more lift). This is actually the main reason why racecar suspension is very stiff, to achieve stable aerodynamic conditions. For good mechanical grip, stiffness is not really essential!
A Mercedes CLK GTR flips at Le Mans due to an unfortunate combination of a bumpy road and turbulence off another car.
But how do these racecars get these downforce value when their basic shape creates lift? (except for a formula 1 car, which isn’t really a car anymore..). What they do is design every part of the car in a way that they create downforce. This is pretty easy at the front, where the flow is uniform, known and nicely available. The easiest thing at the front is to add a lip spoiler, resulting in reduced resistance and reduced front lift. One thing that has to be considered here is providing enough flow for all kinds of cooling the car needs.
Downforce measures also makes a car look incredibly good.. one of the nice laws of nature 😛
Influence of front lip length (downwards) on resistance (Cw) and lift at the rear (AH) and front (AV).
What the front spoiler actually does is influence the flow underneath the car. Above the car, pretty much all you can do is keep the flow as nice and steady as possible to keep resistance low and incident flow on the rear wing as uniform as possible. But most important for downforce is the underfloor flow. Now front lip and therefore a big gap at the bottom of the front basically results in a lot of air under the car, resulting in high pressure and lift. Good underfloor flow can only be achieved when there’s a floor as flat as possible. This is done increasingly on modern cars but wasn’t really pursued until recently. Having achieved a nice low pressure underneath the car you don’t want to let it escape easily, so you could use sideskirts to „seal“ the underfloor area against the surroundings (yes, they have an aerodynamic reason.. 🙂 ).
A DTM Mercedes CLK, the underfloor almost perfectly sealed, resulting in very low pressure.
The final part to complete a good underbody flow is a diffusor. This will only really make sense if the flow up to it is in good shape, so the floor should be as flat as possible. A diffusor will the expand the flow, creating low pressure and therefore suction to the ground (in a way, imagine pulling something apart, it will „pull“ against you). Here, more angle will create more downforce up to a certain point, where the flow will separate (as discussed when we talked about wings). This means a diffusor should be as long as possible and as steep as feasible. Diffusor-like things seen on fast modern hatchbacks running round everywhere won’t really work (except maybe as a confidence booster for the driver :P), something like this however definitely will:
The Panspeed RX-7 FD3S, wouldn’t be as fast as it is without aerodynamics.
Oh and just to have said it, rear skirts as seen on certain kinds of „tuning“ cars create quite a lot of lift by creating high pressure under the rear of the car.
Avoid this! all of it! 😉
get this instead!
These are the main things that I can think of right now. There is a huge number of more things to consider, most important among it the influence of every part on each other, which is why formula 1 cars look the way they do. One example might be that skillful design of shape and placement of the rear wing will influence the flow separation at the rear of the car towards lower pressure which in turn will reduce the pressure of the flow underneath the car, which increases downforce. So it doesn’t even only go in the direction of the flow. All you can finally do is try things. And if you don’t have the possibility, I hope I’ve been able to give you some information about the topic, maybe you’ve found it interesting, maybe helpful.. if yes, I’m glad to have helped! 🙂Aerodynamics might not be a region we „ordinary“ petrolheads spend too much thought on, mostly just putting a big wing, side skirts and a front lip on our car (or removing the same thing, depending on your taste) to make it look better and meaner. But still, it might be good to have a basic understanding of what you’re doing to your car, even more so if you spend hours deciding which tires, oils, pistons etc. you should use to get maximum performance, meaning ruining the gained performance by unfortunate body parts would be a shame. I will try to keep the simplified theoretical ramblings short and mainly show a few nice pictures that show what happens where. 😉
The foundation of the science of fluiddynamics, of which aerodynamics is an important application basically are a few very complicated governing equations, called the Navier Stokes Equations. Don’t worry, we’re not gonna solve any mathematical exercises in this article, I just wanna give you an impression of the kind of complexity we’re talking about.
If you solve these, you will know everything you ever wanted to know!
Now, trouble is, no one’s actually really solved these equations. People have found possible solutions, but noone’s found the definite one. This means that even the brainiest scientists can’t completely predict what a flow of fluid, in our case air, will do. Which is why Formula 1 teams have wind tunnels, some of them several, running around the hour to test their aero parts, because while computations can give an idea of what will happen, only a test will give approximate certainty. What I actually want to say is, how should we „normal“ people have an idea what parts to put on our car if the science is this complicated. For that reason, a host of simplifications have been introduced, the details of which I’m not gonna bore you with. But what this means is that we can have a basic understanding of what shape will result in what behaviour.
There are two main aerodynamic forces, resistance and lift, being distinguished only by their direction. Resistance (or drag) is created by the shape of a body (due to pressure differences) as well as by friction, which depends on the surface roughness of a body (smoothness of paint or also small stuff sticking to the surface). You probably will have heard the term „Cr-value“ (german: Cw), this is a dimensionless term for the resistance due to aerodynamic force, enabling comparison of shapes regardless of their size and of the speed and composition of medium they’re travelling through. This is why an SUV could have a better „Cr“ than a Mini, but this doesn’t mean it has less resistance, it’s just a better designed shape/surface!
Very big but very aerodynamic, the concorde has a low Cr! (But let’s not get started what’s different at supersonic speeds..)
To get an idea of friction, we need to know the following: The important flow phenomena happen in a layer close to the surface it flows along, this is called the boundary layer. The flow in this boundary layer is laminar at first (the entire flow medium runs in an uniform direction) and turns turbulent at a certain point (eddies occur due to surface friction). The rougher a surface is the earlier we will have turbulence and therefore friction.
The transition from laminar to turbulent flow visualized.
While turbulence in itself increases friction, the main worst thing in case of aerodynamic resistance is flow separation. This happens when the flow can no longer follow the shape of the body, resulting in chaos, basically. This generally happens when the flow would have to follow sudden steep declines 0f the body, for example at the rear window of a saloon car. This creates lots of resistance and happens at the rear of every car, the part of the car that is actually more important than the front. Because turbulent flow is more resistant to separation, in some cases you deliberately create a turbulent boundary layers to delay separation.
separation visualized for the example of a simple wing profile.
Critical regions on a scooby: separation at the rear plays the most important role.
These phenomena can be observed well when dragging a spoon through a cup of coffee with cream. 🙂 The same thing happens with the air and this creates resistance. While you can’t really influence the basic shape of your car, the only thing you can do here is to try and keep the surface clean with as little useless resistance creating obstacles as positive.
Moving on to the topic that’s much more interesting for performance cars: lift (or if achievable negative lift, i.e. downforce) is created by the shape of a body the flow tries to follow, which results in a change of pressure around it. These pressure differences create aerodynamic forces which most famously make planes fly. The simplest way to imagine it in case of a simple wing is, that if the air has to take the „longer way“ round it it has to be faster, which results in lower pressure and vice versa for the „shorter way“. In case of an airplane wing, this creates low pressure above the wing and high pressure underneath it, resulting in lift.
pressure difference profile of a wing profile.
The wing on the back of your car is exactly the same thing, just bottom up. 😉 And all kidding aside, the higher it is the better the flow around it will be. One of the main concerns on cars is the influence of the different aerodynamics parts on each other, or rather the struggle to avoid it as far as possible!
Not only for bragging rights!
Increasing the angle of attack of a wing will increase it’s lift and also it’s drag, up to a certain angle where we will have the aforementioned flow separation at a point close to the nose of the wing, resulting in mainly drag and almost no lift. This situation is called stall and is a very bad thing for airplanes and a not so great thing for rear wings on cars, because in this case all they’ll do is brake!
The influence of the angle of attack on the flow around a wing profile.
This can of course be turned into a benefit if the wing is made automatically adjustable and used as an air-brake.
Mercedes showing its aerodynamic gadget.
For the whole car, the situation is a bit more complicated:
Pressure profile of a basic car shape.
When looked at very simplified, the shape of a car is that of a wing of an airplane. This means the basic shape of a car will create lift.. bollocks! To be able to compare lift forces across different sizes of bodies as well as air speeds, the „Cl“-value (german: Ca) is utilized, analog to the „Cr“-value for resistance. This doesn’t really give an idea of absolute lift but enables me to give you some compareable values: while a normal street car has a Ca-Value of +0.3 (meaning it creates some lift), a touring car has up to -1.0 and a Le Mans prototype is able to reach a Cl-value of -4.0! These very high downforce values will also result in an inevitable increase of drag but the increase in possible cornering speeds by far outweights this. The corner speeds of Le Mans or Formula 1 cars can never be reached by conventional vehicles. Light wight is also an advantage in cornering, but sophisticated aerodynamics starts to be dominant from a certain point. Actually, for race cars the limiting factor almost always are regulations. And just to avoid confusion, the Cl-value has no connection to the car’s weight, -1.0 doesn’t mean the downforce equals the weight of the car! (Cl = Lift Force / (Front Area * (speed)^2 * density / 2), in case you are wondering.. 😉 and by the way, there’s also a formula for calculating maximal corner speed.. )
A whole lot of downforce is comming at you!
One particular problem with high downforce cars is that extracting a lot of forces out of a flow results in high turbulences behind the car, which in turn changes the flow around the following car dramatically. This can have scarily spectacular results, as seen in the following picture, in this case helped by the fact that lift also depends on the angle of attack of a vehicle (nose up means more lift). This is actually the main reason why racecar suspension is very stiff, to achieve stable aerodynamic conditions. For good mechanical grip, stiffness is not really essential!
A Mercedes CLK GTR flips at Le Mans due to an unfortunate combination of a bumpy road and turbulence off another car.
But how do these racecars get these downforce value when their basic shape creates lift? (except for a formula 1 car, which isn’t really a car anymore..). What they do is design every part of the car in a way that they create downforce. This is pretty easy at the front, where the flow is uniform, known and nicely available. The easiest thing at the front is to add a lip spoiler, resulting in reduced resistance and reduced front lift. One thing that has to be considered here is providing enough flow for all kinds of cooling the car needs.
Downforce measures also makes a car look incredibly good.. one of the nice laws of nature 😛
Influence of front lip length (downwards) on resistance (Cw) and lift at the rear (AH) and front (AV).
What the front spoiler actually does is influence the flow underneath the car. Above the car, pretty much all you can do is keep the flow as nice and steady as possible to keep resistance low and incident flow on the rear wing as uniform as possible. But most important for downforce is the underfloor flow. Now front lip and therefore a big gap at the bottom of the front basically results in a lot of air under the car, resulting in high pressure and lift. Good underfloor flow can only be achieved when there’s a floor as flat as possible. This is done increasingly on modern cars but wasn’t really pursued until recently. Having achieved a nice low pressure underneath the car you don’t want to let it escape easily, so you could use sideskirts to „seal“ the underfloor area against the surroundings (yes, they have an aerodynamic reason.. 🙂 ).
A DTM Mercedes CLK, the underfloor almost perfectly sealed, resulting in very low pressure.
The final part to complete a good underbody flow is a diffusor. This will only really make sense if the flow up to it is in good shape, so the floor should be as flat as possible. A diffusor will the expand the flow, creating low pressure and therefore suction to the ground (in a way, imagine pulling something apart, it will „pull“ against you). Here, more angle will create more downforce up to a certain point, where the flow will separate (as discussed when we talked about wings). This means a diffusor should be as long as possible and as steep as feasible. Diffusor-like things seen on fast modern hatchbacks running round everywhere won’t really work (except maybe as a confidence booster for the driver :P), something like this however definitely will:
The Panspeed RX-7 FD3S, wouldn’t be as fast as it is without aerodynamics.
Oh and just to have said it, rear skirts as seen on certain kinds of „tuning“ cars create quite a lot of lift by creating high pressure under the rear of the car.
Avoid this! all of it! 😉
get this instead!
These are the main things that I can think of right now. There is a huge number of more things to consider, most important among it the influence of every part on each other, which is why formula 1 cars look the way they do. One example might be that skillful design of shape and placement of the rear wing will influence the flow separation at the rear of the car towards lower pressure which in turn will reduce the pressure of the flow underneath the car, which increases downforce. So it doesn’t even only go in the direction of the flow. All you can finally do is try things. And if you don’t have the possibility, I hope I’ve been able to give you some information about the topic, maybe you’ve found it interesting, maybe helpful.. if yes, I’m glad to have helped! 🙂