aerodynamics of the new bmw z4

Aerodynamics of the new BMW Z4 Dipl.-Ing. Holger Winkelmann BMW AG, Aerodynamics and heat engineering Abstract: Roadster vehicles have long been an integral part of BMW's product range. This is reflected in both the vehicle concept and the design of the latest BMW Roadster, the Z4. At the same time, in the case of the BMW Z4, great emphasis was placed on aerodynamic development. In comparison to its predecessors, the air resistance was considerably reduced in the BMW Z4, and a balanced suspension behaviour was achieved at a low level. This means a significant improvement in the vehicle's driving dynamics. In order to achieve this, even the aerodynamics of the vehicle underbody were optimised. Depending on the configuration, the acceleration characteristics of the BMW Z4 permit the roadster feeling to be more clearly experienced or a greater emphasis to be placed on comfort. 1. The BMW Roadster story When R. Schleicher and F. Fiedler developed the BMW 328 in 1936 together with their small team, they had no way of knowing that this roadster would inspire generations of sporty BMW automobiles. Individuality was apparently already an important element at BMW during the 1930s, and, alongside the Mille Miglia versions, two other aerodynamic bodies were developed by Freiherr von Koenig Fachsenfeld for private customers. In addition to the standard 328 roadster, Figure 1 depicts two other body designs.

The aerodynamic shape of the Mille Miglia vehicles was developed by the aerodynamics engineer Dr. P. Beibarth in co-operative effort with Prof. Kamm (FKFS). Both the 328 roadster and 328 Mille Miglia Coupe were tested in the BMW wind tunnel in 1987, achieving the values in Figure 1. Aerodynamic drag was drastically reduced by this new geometry, but the overall level is still high when compared with present-day vehicles. BMW created a further roadster legend in the 507. This car exhibits everything that is typical of a roadster in its proportions. A short overhang at the front, a long engine bonnet, a steep windscreen, a greenhouse set well to the rear with soft-top cover and a short, precipitous tail are among the characteristics which distinguish a roadster. The values specified in Figure 2 were recorded in 1987 for this vehicle in the BMW wind tunnel. The extremely low lift value of Cz2 = 0.065 recorded on the rear axle with the top closed is particularly noticeable. This value is mainly achieved through the extremely high front axle lift. However, the spoiler on the tail contributes to a certain degree. It is not known whether this element was designed in this fashion for aerodynamic reasons. The 507s classic roadster proportions were not included in the BMW Z1. This design consists of a modern wedge shape with a high tail, a silhouette not dissimilar to that of the present-day Daimler-Chrysler SLK. The vehicle achieved a value of Cx = 0.38, a respectable reading for the standards of the time, but the rear axle lift of Cz2 = 0.16 is less impressive than that which one would expect of the wedge form (Figure 3). BMW reverted once more to classical roadster proportions with the Z3 and, of course, the Z8. The Z3 represents a fully independent design, whereas the BMW Z8 from 1999, with its retro-design, can be considered a homage to the 507. The design of both vehicles evokes great emotion, but these vehicles certainly do not represent the ultimate when it comes to their characteristic aerodynamic values. It is evident that roadster proportions lead to disadvantages in the aerodynamic design of the vehicle. Moreover, particular priority was given to design when developing the Z8. Figure 4 and 5 depict the aerodynamic values recorded. The BMW Z4s design is intended to retain the typical roadster proportions of its predecessors. As with the 507, the Z4 should also have a long engine hood and a greenhouse set well to the rear. Figure. 6 shows a comparison between the proportions of the BMW Z4 and its competitors. It indicates that the Z4 has a driver position set very far back to the rear. The centre of the front axle is taken as a point of reference here. A conscious decision was made to retain the soft-top cover, as this is a typical characteristic of the roadster.

These specifications governed by design represent a challenge from the aerodynamic point of view, especially when one considers that the intention was to achieve considerably improved aerodynamics with the Z4 (in comparison to those realised in the predecessor model). The lift behaviour of the vehicle in particular became the subject of intense focus during development. The Z4s design exhibits clear parallels to the BMW X Coup concept car, but classical elements were also drawn upon. The harmonious swing of the side panel is, for example, similar in shape to that found on the BMW 507. Dynamics are not only expressed through proportions, but also in surfaces. These curve into each other, with convex and concave surfaces contrasting with hard edges. They thus create a progressive impression of light and shadow pure dynamics, in other words! (Figure 7) This new design language means that solutions must be found within the design specifications which prevail here to conflicts arising in the objectives being striven for in aerodynamics and design. The comparison between the tail design of the Z3 and the new Z4 should serve as an example here (Figure 8). While the extremely rounded design of the Z3 leads to a very high rear axle lift and is, consequently, not ideal when it comes to wind drag, an almost perfect geometry is achieved with the Z4 design without the need to compromise. Typical BMW roadster characteristics are retained, with the tail design exuding strength and dynamism. 2. Aerodynamics Considerably improved aerodynamic values are realised in the new BMW roadster when compared with its predecessor (Figure 9). The Z4 achieves a Cx value of 0.35, a positive value for vehicles in the roadster class. A front surface of A = 1.91m2 means that travelling drag Cx x A is 0.669. Front axle lift is Cz1 = 0.11, the rear axle lift of Cz2 = 0.12 representing a considerable reduction in direct comparison with the predecessor car. It means that the vehicle is excellently balanced at a lower level, relative to its lift forces. The Cx value remains below Cx 0.40 with the top opened, the level of lift being once again reduced considerably in this state. The improvement in aerodynamic performance of the Z4 is indicated clearly in the Cx-Cz2 diagram (Figure 10). The Z4 not only demonstrates that it is a clear improvement over vehicles with classical roadster proportions, such as the Z3 or Z8, but also exhibits considerable superiority in comparisons with the wedge-shaped Z1 in terms of both drag and lift behaviour. The Z4 itself should also be categorised as a classic roadster with a modern design. The vehicles lift behaviour was the subject of intense focus during aerodynamic development of the BMW Z4. The effects of aerodynamics on driving stability were examined with the aid of simulation programs, long before prototypes were available. The two following standard cases were defined as evaluation criteria for high-speed stability. In case one the vehicle is driving in a stationary circular track with a curve radius of R = 400m at V = 160 km/h. The vehicle is then decelerated out of this motion at 4m/s2, without the driver attempting any steering countermeasures. Case 2

involves a similar manoeuvre, but a more extreme speed of V = 200 km/h and circular track radius of R = 800m are selected. BMW philosophy dictates that the vehicle must exhibit inherent stability during this trial (i.e. utilisation of control systems such as DSC -Dynamic Stability Control- or CBC -Cornering Break Control- is not part of the simulation). Figure 11 depicts the time-related [s] yawing behaviour [degrees/s] of the vehicle subject to different rear axle lift, the front axle lift being maintained at a constant level. It indicates that the vehicle first exhibits inherent stability in the first trial case from a value of Cz2 < 0.13 onwards, this only occurring from a value of Cz2 < 0.12 onwards in the second case. This simulated behaviour was checked in comparison with the predecessor vehicle in a hardware trial as well as with Z4 prototypes later. Suitable trial speed and circular track diameter conditions were created for the simulation. All control systems were also deactivated during this trial (Figure 12). Various values from Cz2 at a constant Cz1 could be set with an adjustable spoiler. Video sequences 1 and 2 show how vehicle behaviour changes from unstable to stable through the reduction of the Cz2 value. Sequence 3 and 4 indicate that even the relatively minor difference of Cz2 = 0.02 can lead to unstable driving behaviour in border cases. A rear axle lift of Cz2 = 0.12 was defined as the objective for the Z4 as a result of these simulations and trials, this also being realised during development. Underbody design was also subjected to particular attention in order to achieve a noticeable improvement in characteristic aerodynamic values, particularly in the Cz2 range (Figure 13). The underbody at the front of the vehicle and in the thrust zone is smoothened by engine chamber shielding, the front wheel housings being shielded with wheel spoilers from the air flow. Considerable attention was paid to achieving plate design in the central area that is as smooth as possible. Cable ducts have been covered with additional casing. The air conducting plate over the rear axle transmission is particularly conspicuous. This was necessitated by the fixing elements of the rear vehicle reinforcing which impeded flow. The casing reduces Cx. However, Cz2 in particular is also reduced, and rear axle cooling provided via an NACA intake. The tail of the car has been drastically smoothened over the components located in this area and endowed with a diffuser angle which is typical for this vehicle. Development of the hard-top was spurned on by the desire to reduce both air resistance and the rear axle lift still further. Initial investigations showed that the rounded geometry of the hard-top tended to produce a higher lift on the rear axle. Utilisation of a flow-through spoiler (which raises the pressure level on the tail) proved to be the solution to this problem. Simulation of the spoiler airfoil indicates how the air flowing through the spoiler is deflected onto the surface of the rear window (Figure 14). The hard-top thus not only achieves lower air resistance, but also a rear axle lift reduction of Cz2 0.02.

3. To feel a Roadster In contrast to coup-style cabriolets, one does like to experience a slight breeze in the interior of roadster vehicles. The driver should be able to feel the wind in his face if desired. Its all part of the roadster feeling. A roadster should also be capable of offering the driver a high degree of draught-free comfort, even in the top speed range. The interior air flow circulation was investigated with the cover opened in the early development phase using CFD simulations (EXA Powerflow) to achieve this objective. Findings were taken into consideration in the design phase. The animations in Figure 15 show the pressure level in the interior on an XZ level compared to all-round flow. The illustrated flow lines also help identify the flow topology. The second animation illustrates speeds in the X direction on an XY level. Conclusions can be drawn from this regarding the areas affected by backflow in the interior. It is evident that the greatest backflow occurs in the area between the two roll bars in the passenger compartment. This effect was confirmed in an experiment conducted later. Detailed development of the anti-buffet screen involves use of a test dummy in the later project phase to determine wind speeds acting on the occupants. Wind speeds were recorded in real-time here. Figure 16 depicts a comparison between a vehicle without an anti-buffet screen and opened side windows, and a vehicle with anti-buffet screen and closed side windows. Oncoming air flow speed is 33m/s. It is important that pressure compensation is achieved between the interior of the vehicle and the air flowing around the exterior if the anti-buffet screen is to function optimally. This means that an anti-buffet screen produced with webbed material is far superior to a glass version when it comes to increased comfort. The test dummy graphic shows the excellent effectiveness of the anti-buffet screen in conjunction with closed side windows. The air flow at head level in the case of the simulated test person (95% man) is not dominated here by the anti-buffet screen, but rather the front windscreen. Use of the anti-buffet screen enables the Z4 driver to control whether he or she wishes to feel a breeze inside the vehicle, or would rather travel in greater comfort. 4. Thermal protection Figure 17 provides an overview of prevailing local temperatures on the Z4s underbody. The engine and exhaust system transfer heat to adjacent functional and safety components through conduction, radiation and convection. This means that the chassis, transmission, electronic and body components require shielding or cooling to protect them against the effects of heat.

The BMW Z4 has single or multi-layered pre-formed knopped aluminium parts for this purpose. This heat insulation currently represents an excellent compromise between costs, weight and environmental friendliness without a corresponding reduction in functional effectiveness (Figure 18). Particularly conspicuous is the covered rear axle transmission. This cladding was necessary to provide an aerodynamic casing for the central supports of the rear thrust struts. An NACA intake simultaneously directs the underbody air flow to the rear axle transmission, improving the thermal load of this component. Summary: As regards conception, the BMW Z4 represents a classic roadster with modern design features. BMW has successfully met the challenge of achieving good aerodynamic values despite the rather disadvantageous nature of this concept as regards aerodynamics. In all aerodynamic values the BMW Z4 produces good results and is considerably better than its predecessor. The acceleration characteristics produce a true roadster feeling. Values: CX Drag coefficient A [m2] Automobile front area CX x A [m2] Air resistance area CZ1 Front axle lift coefficient CZ2 Rear axle lift coefficient V [km/h] Vehicle speed R [m] Curve radius of a stationary circular track

BMW GroupH.WinkelmannAerodynamics& Heat EngineeringMIRA ConferenceOct 2002

BMW Roadster Tradition.1936 The BMW 328.

328 Mille Miglia Coup(1940, closed):Cx = 0.43 Cz1= 0.20 A = 1.492 Cz2= 0.27

328 Mille Miglia Coup(1940, closed):Cx = 0.43 Cz1= 0.20 A = 1.492 Cz2= 0.27

closed: open: Cx = 0.70 Cz1 = 0.43 Cx = 0.77 Cz1 = 0.43A = 1.54 Cz2 = 0.22 A = 1.42 Cz2 = 0.09


BMW Roadster Tradition.1955 The BMW 507.

closed: open: Cx = 0.44 Cx = 0.58A = 1.84 A = 1.79Cz1 = 0.38 Cz1 = 0.35Cz2 = 0.065 Cz2 = 0.04



BMW Roadster Tradition.1989 The BMW Z1.

closed: open:Cx = 0.38 Cx = 0.44A = 1.83 A = 1.83Cz1 = 0.06 Cz1 = 0.02Cz2 = 0.16 Cz2 = 0.16



BMW Roadster Tradition.The Z4 concept.

BMWZ4

PorscheBoxster

MercedesSLK

AudiTT


BMW Roadster Tradition.The Z4 design.


BMW Roadster Tradition.Aerodynamics and design.


Aerodynamic Values.The aerodynamics of the Z4 (2.5i / 3.0i).

Aerodynamic values closed open

Drag coefficient cx 0.35 0.40

Automobile front area A [m2] 1.91 1.85

Air resistance area cx A [m2] 0.669 0.734

Front axle lift coefficient cz1 0.11 0.10

Rear axle lift coefficient cz2 0.12 0.09


Z3/Z8

Z4

Air Resistance Values.The new Z4 in comparison with itspredecessor.

cz2

Z1

0.10

0.12

0.18

0.20

0.16

0.14

0.08

0.34 0.36 0.42 0.440.400.380.32 cx


czh = 0.12

Z4 Aerodynamics.Aerodynamics and driving stability.

-300 4 8 12

Dynamic response (yaw velocity)V = 160 km/h, R = 400 m, czv = 0.1170

[sec]

-10

10

30

50

[/s]

0

-300 4 8 12

Dynamic response (yaw velocity)V = 200 km/h, R = 800 m, czv = 0.1170

[sec]

-10

10

30

50

[/s]

0

czh = 0.165czh = 0.13

czh = 0.13

czh = 0.15


Z4 Aerodynamics.Aerodynamics and driving stability.


Z4 Aerodynamics. The underbody.

Wheelspoilerfor front wheels

Diffusor Underbody casing

Wheel spoiler in front of rear wheels

Air conducting plate rear axle transmission

Shear panel

Tripartite engineshielding (MAS)


Z4 Aerodynamics. The hardtop.

cx

0.32

0.34

0.36

0.30

0.28

cz2

0.11

0.13

0.15

0.09

0.07

with hardtop

0.350.34

0.120.12

0.100.10

0.01

0.02


To Feel a Roadster.Layout design with CFD.

Static pressure with stream lines

X velocity


To Feel a Roadster.Breeze factors and comfort.

Windows down, without anti-buffet screen, V

= 33 m/s

Windows up, with anti-buffet screen, V

= 33 m/s

100m/s


Thermal Protection.Underbody temperatures.

550C 400C 420C 250C 420C


Thermal Protection.Underbody insulation.

Air conducting platerear axle transmission

Front tunnel

Bottom plate

Engine carrier

Transverselink bearing

Gas tank

Back tunnel

aerodynamics of the new bmw z4

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