Since the whole of the aerodynamics in the CO2 emission is already understood, it is important to correct evaluate the surface features of a road car and how these affect the air flow over the vehicle body. In addition, road cars have the difficult design task to attend the aesthetic and aerodynamic requirements. This article proposes an overview about the road car body design.

Aerodynamic families

To discuss the car families, Figure 1 illustrates all of these put together in graph that correlates lift (CL∙S) and drag (CD∙S). As can be seen, Figure 1 is populated from the level which there are cars that always generates lifting and the ones capable to generate downforce. Actually, road cars are at the lowest CD∙S levels, overall road cars vehicles generate lift. Some cars are able to produce some downforce, but these are only limited or sporty versions of urban cars. The high performance road cars are on the same range of value than road cars in terms of CD∙S, but producing a considerable high downforce with respect to road cars. The map envelop is populated by open-wheels (OW), prototypes and GT racing cars. However, the bubble of the prototypes is decreasing, because the balance of performance (BoP) is defined by regulations. The aerodynamic efficiency Eff must be 4, which results 4 points of CL∙S for 1 point of CD∙S. This restricts a lot the aerodynamics behavior of prototypes. In a very general term, not considering the active aerodynamic controls, the racing car families is approximately the one seen in Figure 1.

For normal road cars the aerodynamic forces are usually split in two components, the vertical and the horizontal ones, which is downforce (negative CL∙S) and drag CD∙S, respectively. As can be seen, the most part of the drag of a normal road car comes from the body (Figure 2). The main origin for the body drag is the delta pressure ΔP between the front and rear end of the car. The front end of the car is always at an overpressure condition, while the rear end of the car is at low pressure situation, because of the stall and the generation of the wake over the circulation area where the pressure is lower. The second reason is the surface effect, but in much lower magnitude than ΔP between front and rear ends of the car. The shape of the car and ΔP are the main reasons behind road cars CD∙S. In even minor magnitude there are the effects from wheels and cooling, this one it is ΔP from the radiator cores. The underbody, for different reasons can generate CL∙S or CD∙S, but it is important for its package to have a concern about the flow field to avoid an extremely turbulent one under the car. For the vertical force the situation is always the one illustrated on Figure 2. The underbody and roof always generate downforce and lift, respectively. The reason comes from the continuity equation, thus the underbody downforce is due to the section decreasing as a consequence of the velocity increasing, consequently the pressure decrease. Hence, it is possible to understand how would be important a better quality of the engineering of the underfloor package. The point is that this region is not usually visible, thus the quality in terms of the car finishing is usually high in the top part of the body and extremely low on the underbody. The reason is that for carmakers this is a cost for nothing, but in the reality, it makes a cleaner flow underneath the car.

Road car aerodynamic requirements

Hence, in general the perimeter of the road car aerodynamics is divided in three important itens:

  • Reducing the aerodynamic drag;
  • Reducing the positive contributions to lift;
  • Sizing and integration of cooling system.

The drag reduction is mainly connected to the fuel consumption and CO2 emissions. For some cars, the contribution is to have no-lift situation and the cooling sizing and optimization. In addition, for high performance cars, there is also the understanding of the control of the car behavior. Hence, probably the main parameter for racing and high performance cars is the car behavior in the aerodynamic point of view. For a road car, the main issue in terms of performance is drag reduction. When the level of performance increase, there are at least two parameters that must be considered. One is the vertical force, because this one is always upward and increasing with velocity, which intensifies the feeling of unsafety since the steering wheel is becoming lighter. This occurs, because increasing the speed, the lift increasing is a consequence. On the other hand, the behavior in terms of aero is characterized by the map envelope, which is how the vehicle oscillates when in braking, acceleration and cornering. In high performance cars if the balance is lost inside a corner, since this is a pure aero parameter that generally comes from the underfloor of the car, this makes the situation critical.

Road cars aerodynamic balance

The aerodynamic balance is independent of the driver, if it is not correct in corner or straights, the corner entering would not be good since the rear downforce is reducing a lot during braking, thus the braking feeling describes a really unsafe car. This occurs, because when in braking, the load transfer goes to the front and if, for some reason, the rear downforce is already low or lost, the rear end becomes critically light, which increases the feeling of promptness to loose control. Hence, CD∙S is 95% of the road car aero requirement in a more general overview of the problem, it is considered all the parameters that refers to driving safety, thus the handling of the car, which is the management of the vertical forces. This is the lift and the distribution of the vertical forces between the two axles, because if the aero balance changes too much between a straight and a corner, this represents a car that refuses to be stable. For instance, a car with 35% front balance Fbal expected in straight line traveling and, for some reason, this condition comes from the underfloor. If this condition varies to 15% inside a corner, this car will reach its limits very easily. It will be very weak car in terms of handling since Fbal variation is accepted in straights, but not in corners. In this case, Fbal should be the correct value of the aerodynamic distribution. Bottom line, when the level of performance increases, the management of the vertical forces over the axles and the fully knowledge of the aero map is extremely important.

Road car body analysis

In the aerodynamic point of view there are some reasons to have or not a good aero. These are the general features of a road car body, which are divided in:

  • Front end;
  • Rear end;
  • Side body;
  • Top body;
  • Underbody;
  • Cooling;
  • Wheels.

All of these have specific aspect that require a careful design to provide a harmonic stile together with a good aerodynamic performance.

Front end

The front end is the main point, if it is not well designed, the car design is completely lost. Figure 3 illustrates examples of good front end designs in terms of aerodynamics. There is an evident surface treatment of the corners. On the vehicle front end there will be always an overpressure, this is inevitable. As reduced the section in overpressure is, lower will be the drag generated. Regarding to the corners, these are essential since they improve the flow management on the car sides. The expected flow field is the correct management around the front wheel contour. The reason is that the flow field separation and stall occur in the front of the wheel. This can be managed to make the separation less stressful for the flow field around a car with the proper surface treatment in corners. Over the years those corners became more and more squared. The reason is related to the separation that occurs in front of the wheel openings and this is connected to the exit tangency of the corner.

If the angle is more obtuse, which is the design made for style, it is generated a wide separation on the car side (Figure 4), which these wakes are basically loosing the flow energy on the sides and rear. Hence, it means nothing if a car has a good design in front, sides and rear. If the front end contours are not properly designed, the car aero is compromised. This is the reason why in last years the squaring effect is increased, to decrease the tangency angle (Figure 4) between the horizontal line and the corner exit (Figure 4). As can be seen on Figure 4, decreasing the angle results in a lower wake. This helps to manage the separation on the wheel opening. Hence, the objective of the aero department is to keep everything straight on the wheel contour. On the other hand, the objective style department is to have a curved surface in front of the wheel contour and define the car aesthetics. Figure 4 illustrates a cross-section made in the Z axis, the separation is inevitable, but controllable. If the angle is tending to an obtuse one, the wake thickness will increase. If a car has a very well designed rear end, but with a curved front end, the aerodynamics of this car is compromised. Hence, it is advisable to keep the tangency angle with the horizontal line more acute in order to decrease the wake thickness.

Wheel coverage

In the past the original design of any sporty car was a curved surface in front of the wheels. This design is still used in some cars today in order to create a robust appearance. The wheels, for the styling point of view, motivates a perception of 30% of the car. Hence, if the wheel contour is big, the perception of the style of the car is good. Actually, styling department always looks for a car that it is well positioned over the four wheels and surface treatments for the style of the wheel contour area are normally very curvy to bring more emphasis to the wheel. Obviously, this is not the objective for the aero department, because in this case it is better to have a squared contour. In addition, car stylists request to normally have a big and wider lower part. These two make the car look more robust. Basically, the compromise in those years is a kind of opening on the lower side part of the bumper. This is a trade-off between style and function. The front view is characteristic for the style of these kind of openings, this is a sort of wide aggressive face.

For the aero department this is a good reason to make this surface flat. This surface treatment improves the quality of the flow field, since it goes to the wheel archers. Understanding that there is an overpressure on the front face, which is the main reason of the pressure drag, this section is less critical for electric motor powered cars. Although the area of this section is similar to the one from internal combustion engine powered cars, in EM this section can be lower, because the grill is smaller, thus also the compression area is smaller. In the case of the wheel coverage, if a wider portion of the wheel is exposed, the overall drag will increase, because the wheel is in compression. Another important point is the curved lines on side bodies. Finally, the front end aero is driven by the compression area, the corner treatment and the wheel coverage. The car should look wide on the front, more robust.

Air curtains

In the engineering point of view, the ideal is to have everything flat. On the sides, for aerodynamic reasons, it can not be squared. Hence, the treatment of these areas is more important. A good turbulence reduction has been found by the side of air curtains (Figure 6). These are a slot, an opening, that connects the inlet part with the side part of the wheel in order to flow on the car sides. When it is well designed, there is a gain around 0.5 CD∙S. The problem is when this does not occur. A badly designed air curtain can result in no improvement, but also no drag increase. However a very badly designed air curtain can increase drag. Therefore, this is a more common feature in road cars to improve aero, but it is also easy to recognize if these are functional or not. The design that generates drag usually guides the air flow directly towards the wheel, which increase wheel drag. This device is a good compromise between style and function if the blowing is correct.

Air breathers

The air breather (Figure 7) is an opening commonly used in high performance cars. The wheel housing is basically an area that encapsulate a rotating cylinder. Hence, inside of it there is a turbulent flow, because rotating cylinders generate turbulence. The problem is this creates a blockage effect for the flow that is moving through the flow, or at least the front part of it. Hence, the front downforce is reduced and the lift is increased at that part of the car. This breather is supporting a more correct functionality of the front end. Sometimes there is a positive effect in drag, but what it is really expected is a better quality of the flow and the lift reduction at the front end part of the car. The influence of the air breather is in extracting some of the turbulent flow inside the wheel housing, this helps to reduce the disturbance of flow quality at the front underneath. As a result the front lift effect is reduced. In case of a car that generates downforce at the front, this effect is improved.

Side body

The car side design is illustrated Figure 8, there are good and bad examples. The request is not to have the kinds of line as the ones seen in Figure 8, because it is created zones of compression-expansion-compression, thus there is a cost in drag.

The good design is a complete flat surface (Figure 9), but it is something that the styling department does not approve.

Normally “muscles” are very pronounced on car designs (Figure 10). In sporty cars this volume is very underlined, because this is a problem in the aerodynamic point of view.

For the top body (Figure 11) it is important for the cross-section to have this kind of tapering, because it helps in closing better the volume in order to reduce the wake volume generated by the car. Hence, this tapering helps to reduce wake, thus decreasing drag.

Rear end

The rear end is the first consideration respective to where will be the separation of the flow. The separation in the rear part of the car can not be avoided, but it should be reduced in the best possible way by surface treatments on the rear end. The most critical car is the monovolume (Figure 12) and the two-volume cars.

There is a 2D separation, because it occurs in two planes, which are the ones illustrated on Figure 13. On Z0 plane, the tools of the aerodynamic design is, first of all, the line of the roof. This is referred by the surface treatment of the exit tangency angle, which means having these downward line for the Z0 section in order to reduce the volume of the separation. The same is performed at Y0 cross section (Figure 13). Cases like these are the reason why the rear end of the cars became more and more squared in their designs. This helps to manage the separation on the rear end.

In addition it is usually used surface local treatments, which are accessories (Figure 14) to better drive the air flow in different exit tangency angles into the separation area. There is also the square edge, instead of plastic accessory, even the effect is only local, it helps to manage the flow separation. Hence, the objective of the sharp corner is to keep the flow attached as much as possible up to the line where the car body and the flow will separate. If those sections were rounded, as in Audi TT, Volkswagen New Beetle or Alfa-Romeo Brera, there will be clearly recirculation area that improve the volume of the wake. Hence, to keep the flow attached as much as possible, the flow should be driven until the point the car body finishes, then the stall and the separation will occur. If the design was correct, the tangency exit angle and the wake volume will be decreased. This is the reason why the monovolume and two-volume cars have a higher CD∙S when compared to a three-volume car.

2½ volume car bodies

The 2½ volume design (Figure 15) is an improvement relative to monovolume and two-volume designs, not only because the wheelbase-height ratio l/h ratio is higher, but also due to the after volume, which is smaller, and is guiding the separation in the best possible way. Normally, if there is no separation on the rear window, it is necessary to manage the compression over the area illustrated at Figure 16.

In particular, this compression will generate a possible effect on drag, because it generates a force component that acts favorable for drag reduction. However, this compression must be coherent with the design scope of the car, because if there is an excessive compression, it will be also generated an upwash and then, drag. Hence, for a real sporty car, a rear spoiler is mandatory in order to generate downforce, but the upwash generation comes with drag. The design of the Y0 section is extremely important for road cars.

3 volumes car bodies

Those effects can also be generated on three volume cars (Figure 17). Obviously this is the best situation, with the same l/h ratio, because this volume is smaller that in two volume car. Hence, it reduces the wake volume. Also in this case, the treatment of the Y0 section is extremely important in order to manage in the best possible way the compression at the rear volume. These details and features are in function of the performance level of the car. For instance, in mid-high performance cars, the compression region should be increased. If the car is an urban road one, it is better to guide the design process into the drag reduction. The design of the Y0 section is a function of the performance expected. If it is generated more compression at the rear end, obviously it is created more downforce and drag, because it is also generated upwash with the spoiler. It must be noticed that in very downwash designs, the focus of the design is on the drag side, not in the vertical one. For example, the Porsche 911 is clear example of downwash rear end volume. It request an active device to break the downwash effect. Although this is not a problem at low speeds, at high ones the downwash effect creates lift. Therefore, the design of the rear end is clearly a function of the level of performance expected.

Aerodynamic tools for rear ends

Figure 18 illustrates a good example of rear end. It can be noticed a quite evident balanced situation between style and function. However, this surface treatment is supporting all the contours of the car body, thus the wake management.

For the same reason, it is applied surface treatments to plastic and side edges of the lights as seen in Figure 19. The function of these edges is to provide a better exit tangency angle in order to improve the rear end wake management.

The rear underwing is sometimes adopted in high performance cars. The objective is to provide a good interaction between the underbody and the vehicle roof. Basically, the wake volume seen behind the car is impacted by the matching of the flow that comes from the roof and the underbody.

Underfloor

Since the underbody is a region which has a very low perception, the underflow is a lot disturbed due to package reason. This is not good for aerodynamics, but it is difficult to find ways to improve the flow quality. The solutions to improve the underflow are the assembling of local or complete panels. However, in the economic point of view, the local panel (Figure 21) is a sort of trade-off between the part cost, production cost and the flow quality.

The complete panel (Figure 22) is even more difficult to implement in terms of cost. This is usually applied for high performance and top class cars.

Figure 23 illustrates a plastic spoiler in the front bumper, this function is to limit the compression on the wheels. A wheel without this device would rotate fully compressed, or rather the compression is split between the car body and the wheel. The target is to end up with the sum of the compression on the body and the wheel, which is lower than a fully compressed wheel.

The point is that this component efficiency is very sensitive to the ride height and this highlights the complete understanding of the car aero map, even on road cars. The reason is that, if the car is driven with five occupants, instead of just the driver, there will be more weight on the car, thus a lower ride height. Hence, the situation will change and this device can work or not, or work in a negative way. For this reason, it is mandatory the knowledge about the ride height map.

Cooling

The radiator air flow management it is based on the mass flow rate through the grills. There are a couple of solutions to decrease the impact due to the inlet. One is the inlet duct in order to better recover the pressure in front of the radiator. The other solution could be an active closure of the grill.

The objective is to command the opening and closing of the grill according to the situation. For instance, in a highway the need of cooling is lower. This is a difficult solution for mass produced cars, thus the cost is primarily important. Therefore, an active device application should justified by the car cost.

Wheels

The wheels are a sort of a problem that became worst over the years. There are three main reasons, the tire width, rim design and size. In terms of rim design, its cover should be closed for aerodynamic reasons and very likely tires should be thin. However, the wheel cover is usually open for a sporty look. The rim size is something that increased a lot since the last 20 years. An uncovered and big wheel increase the outwash flow.

Active aerodynamics

The active aero is something that is improved year by year, but usually it is not a solution for mass produced cars. One of the late concepts is an active system developed by Renault that make a the car longer in order to increase the l/h ratio. The objective is to explore the correlation between l/h and CDS. Basically, a longer car reduces the drag in some amount. A common device is the active rear spoiler. The objective is to reduce the rear lift as the speed increases.

References

  • This articles was based in the lecture notes written by the author during the Industrial Aerodynamic lectures.