Body and aerodynamics: There is strength in serenity

Mar 5, 2015
A high body strength is decisive not only for optimum protection of the occupants; it is also the basis for quiet running and low vibration levels. Both were achieved with lower weight through the use of higher-strength and high-strength sheet steels plus aluminium and through elaborate joining techniques. A further plus point is the aerodynamics: With a cd value of 0.26 the CLA Shooting Brake sets the benchmark in the compact estate segment.
Even from 60 km/h the wind resistance exceeds the sum of all other types of driving resistance. This motivates the engineers at Mercedes-Benz to achieve optimum values for aerodynamics on each new model. By way of example, the CLA Coupé with a cd value of 0.23[1] is world champion in this discipline. The CLA Shooting Brake with its estate tail end is no exception here and with cd = 0.26 it also achieves an optimum value in its segment. Its drag area (cd value x frontal area) of 0.57 m² is also exemplary. The aerodynamically efficient shape also reduces the occurrence of wind noise, and together with sophisticated seals the interior of the car is thus effectively shielded from wind noise.
Alongside an aerodynamically efficient basic shape of the body, a series of measures also improves the aerodynamics. These include:
  • A low A-pillar level with specially adjusted A-pillar geometry
  • Aerodynamically optimised exterior mirror housing
  • Seals in the front bumper(closed radiator section incl. headlamp section)
  • Radiator shutter for on-demand control of the cooling air, partial cover in the radiator grille
  • Serrated wheel spoiler with shoulders on the wheel arches of the front wheels
  • Comprehensive flow on the underfloor through generous covering of the main floor panel, additional covering in the centre section of the rear axle plus the spring links and an aerodynamically optimised rear silencer with a connecting diffuser
  • Aerodynamically graduated roof side and tail lights
A multitude of details also help to reduce the wind noise, for example:
  • Frameless doors with a multi-stage sealing concept
  • High window insulation due to 5 mm side windows
  • Sealing of the rear-end door joints
  • Shape-optimised exterior mirrors, which are secured to the door edges
On the full-glass panoramic roof with the sliding sunroof running on the exterior, many wind guidance measures such as a wind deflector, covers and seals with adapted geometry ensure that the noise level is as low as possible when the roof is open. The wind deflector suppresses the throbbing noises which would usually occur and also facilitate virtually draught-free driving.
Rigid body minimises noises and vibrations
Intelligent lightweight construction through robust, innovative materials (68% consist of higher-strength and ultra-high-strength sheet steels, the bonnet and front wing are made of aluminium), stress-appropriate wall thicknesses and to a certain extent innovative joining techniques such as structural adhesive create the basis for a stable body. All the sheet metal panels used are galvanised for protection against corrosion.
In addition to the overall vehicle rigidity the connection points between the body and the chassis/drive system are decisive for noise- and vibration-related comfort. Considerable increases in the introduction rigidity have been achieved, which in turn lead to a reduction of the noise level in the vehicle interior.
For damping the bodyshell's structure-borne noise, acoustic masses are automatically applied to the body. To reduce the noise impact to the passenger compartment, the insulation of the firewall is designed as a high-quality injection-moulded part. Compared with deep-drawn components this has the advantage that constant wall thicknesses can be realised and that there is no thinning of the material in the case of large breaks in contours. Thus the risk of acoustic weak points can be minimised. The irradiation of sound is also minimised by an absorber on the tunnel roof.
It was possible to achieve high standards of noise insulation of the rear through the targeted use of absorbers beneath the seating surface and also in cavities in the body. The wheel arch lining also dampens noises in a targeted manner.
In order to cut the tyre noise the focus was on the dynamic transfer response of the individual axle structure and steering components and also on the de-coupling capacity of the chassis mounts. Through a detailed operating vibration and component analysis the affected chassis and steering parts were configured in such a way with regard to their natural frequencies and dynamic rigidities that the occurrence of bursts of noise into the body was significantly lowered.
[1] CLA Blue Efficiency: cd = 0.22
CLA Shooting Brake, Aerodynamics