The new Mercedes-Benz CLA-Class: Aerodynamics and All-Round Safety

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V. Aerodynamics: Sleek as they come

With record-breaking aerodynamics, the new CLA-Class combines emotion with efficiency. In respect of its aeroacoustics, i.e. noise level comfort, the four-door coupé is also at the top of its game.

With a Cd value of 0.23, the CLA sets a new benchmark – both within the Mercedes-Benz model range and among all production vehicles. The drag area, Cd x A – which is crucial to aerodynamic resistance – also leads the field, at 0.51 m². The CLA 180 BlueEFFICIENCY Edition, which is due to hit the streets in June, even manages to better this value with its Cd value of 0.22, and when it comes to the drag area, the four-door coupé crosses a magical line with its 0.49 m2. This makes the CLA-Class more streamlined than any other car.

The good air flow characteristics, which are a major contributory factor to the vehicle's low fuel consumption in everyday conditions, result from numerous aerodynamic optimisation measures. These include a low A-pillar shoulder with adapted A-pillar geometry, aerodynamically optimised exterior mirrors, wheel trim designed for low aerodynamic drag or a specially-developed aero light-alloy wheel and serrated wheel spoilers on the front and rear wheel arches.

The patented wheel spoiler was developed with the help of airflow analysis. This seemingly innocuous component reduces undesirable air turbulence in the wheel arches in no less than three ways, by lowering linear turbulence, deflecting the airflow from the wheels and stabilising shear waves with its serrated edge. The wheel spoiler remains effective whatever the suspension level and wheel dimensions.

Extensive underbody panelling, additional panelling in the middle area of the rear axle and an aerodynamically optimised rear silencer followed by a diffuser improve the flow of air under the underbody.

In addition the new CLA offers the lowest level of wind noise in its segment. Numerous measures contribute to make this possible:

- Frameless doors with a multi-stage sealing concept

- The high insulating effect of the side windows

- Form-optimised exterior mirrors

- Exterior noise is also minimised by the reduced height of the A-pillar shoulder

Contribution to active safety: anti-soiling measures

Anti-soiling measures are another aerodynamic discipline – and another area in which aerodynamics specialists at Mercedes-Benz have worked painstakingly on the CLA. Soiling can arise from rain, vehicles travelling ahead and spray thrown up by the vehicle's own wheels. Keeping windows and exterior mirror lenses as clean as possible, and therefore having the best possible visibility under all conditions, makes a contribution to active safety.

In the wind tunnel the aerodynamic specialists optimise components with the help of a fluorescent liquid which makes the soiling clearly visible. The aim is to direct water away so that the side windows and exterior mirror lenses remain clean. This is influenced by the geometry of the A-pillar with its integral components and the geometry of the exterior mirrors and trim strips in the frameless doors.

For example, the side windows: by means of minor geometrical changes of the mirror housings and optimisation in detail, with seals and a special water-capture runnel, soiling has been significantly reduced.

VI. The driving assistance systems: More sophisticated collision warning system as standard

Numerous driving assistance systems offer support in the CLA and reduce the driver's workload. Standard-fit features for the coupé include ATTENTION ASSIST drowsiness detection and radar-based COLLISION PREVENTION ASSIST with adaptive Brake Assist, which now helps to protect the CLA from collisions from a speed of only 7 km/h (previously: over 30 km/h).

The radar-based COLLISION PREVENTION ASSIST system provides the possibly distracted driver with visual and acoustic warnings of identified obstacles and prepares Brake Assist for precision braking. Braking is initiated as soon as the driver steps emphatically on the brake pedal. When an impending danger of collision is identified, COLLISION PREVENTION ASSIST calculates the precise braking force ideally needed to avoid an accident and makes the best possible use of any distance remaining. As a new feature, COLLISION PREVENTION ASSIST affords protection from typical rear-end collisions in dangerous traffic situations in the CLA from a speed of only 7 km/h (previously: over 30 km/h).

COLLISION PREVENTION ASSIST combines with DISTRONIC PLUS to become COLLISION PREVENTION ASSIST PLUS, incorporating an additional function: when a danger of collision persists and the driver fails to respond, the system is able to carry out autonomous braking at speeds of up to 200 km/h, thereby reducing the severity of collisions with slow or stationary vehicles. The system also brakes in response to stationary vehicles at a speed of up to 30 km/h and is able to prevent rear-end collisions at speeds of up to 20 km/h.

Electronic helpers: all the other assistance systems at a glance

Active Parking Assist, which is available as an option for the CLA-Class, has been developed on the basis of the PARKTRONIC system familiar from other model series. Twelve ultrasonic sensors mounted in the front and rear bumpers measure potential longitudinal and transversal parking spaces at the left and right sides of the road. By default the system searches on the right; the turn indicator must be actuated for it to look on the left. Once a suitable space has been identified, the driver stops the vehicle, selects reverse gear and activates the automatic parking assistance system via the controls on the multifunction steering wheel. The system calculates a suitable manoeuvring pattern with a maximum of seven moves to complete the parking operation. The driver is assisted by the automatic actuation of the electromechanical steering during the parking process but still has to operate the accelerator, gearshift and brake. The speed is limited to 10 km/h during the parking process. Parking in spaces situated along slightly curved roads is also supported.

Automatic steering to exit the parking space is a new subfunction. If the vehicle has been parked using Active Parking Assist, the vehicle can be steered out of the longitudinal parking space in which the vehicle had previously been parked. For this the driver has to engage reverse and actuate the turn indicator before activating - via control panel on the multifunction steering wheel - the automatic exit from parking space assistance. Here, too, the driver has to operate the accelerator, brake and gearshift. As soon as the vehicle has assumed a sufficient angle to exit the parking space in one straight move, the wheels are set to the straight ahead position, the process ends and the driver assumes steering control again. The process is completed at the latest when the vehicle is at an angle of 45 degrees relative to the starting position.

A series of further assistance systems can be added to this:

- Adaptive Highbeam Assist: when vehicles are detected ahead of or oncoming to the vehicle, this system automatically dips the beams and adjusts the range of the headlamps appropriate to the distance. The driver benefits from a longer dipped beam range and does not generally need to switch manually between main and dipped beam.

- Blind Spot Assist: detects vehicles in the critical sector by means of two radar sensors located at the sides at the rear of the vehicle. The system warns the driver of the presence of a vehicle by displaying a red triangle in the exterior mirror. If the turn indicator is actuated the red triangle begins to flash and a warning tone is emitted.

- Lane Keeping Assist: a camera behind the windscreen detects carriageway markings and can recognise if the car is about to leave a detected lane unintentionally.

- ATTENTION ASSIST (standard): provides a warning when typical signs of drowsiness are detected. The system's highly sensitive sensors observe the driver's behaviour and can recognise – especially on the basis of how the steering wheel is being moved – if the driver is becoming drowsy or inattentive.

- Speed Limit Assist: a camera fitted behind the windscreen detects speed limit signs at the roadside and compares this data to information contained in the GPS system or the digital map. The relevant speed limit is then displayed in the instrument cluster.

- Brake hold function: when stopping, for example at traffic lights, the driver merely has to press the brake pedal slightly more firmly. They can then take their foot off the brake pedal and the brake will remain engaged until they move off again. The brake is released automatically when the driver steps on the accelerator.

- Hill-Start Assist: this function is able to prevent the vehicle from rolling back unintentionally when moving off on an uphill slope.

- Reversing camera: integrated in the handle strip on the tailgate, the reversing camera features a wide-angle lens. When reverse gear is engaged, the image from the camera is transmitted to the screen of the Audio 20 system or the COMAND system. Static and dynamic guide lines assist the driver during manoeuvring.

- DISTRONIC PLUS: the radar-based adaptive cruise control supports the driver at speeds between zero and 200 km/h by maintaining the distance to the vehicles in front. In doing so it is able to apply the brakes to bring the vehicle to a complete standstill and also accelerate it again. As a result, the system is also particularly convenient in stop-and-go traffic. If the system detects that the distance is being reduced too quickly, it warns the driver with both visual and audible signals.

- Cruise control with SPEEDTRONIC: the cruise control maintains the preset speed. The additional SPEEDTRONIC function ensures that the stored speed is not exceeded. Both functions come as standard with the 7G-DCT dual clutch transmission and are optionally available for the models with manual transmission.

VII. Passive safety: A protection programme for every case

"One star is all you need" – the Mercedes-Benz safety philosophy also applies to the new CLA-Class. Its safety equipment includes belt tensioners and belt force limiters for the rear seats, too, the PRE-SAFE® anticipatory occupant protection system and the active bonnet for pedestrian protection. The line-up also includes seven airbags as standard and the Mercedes automatic emergency call system.

The new CLA-Class has passed the brand's rigorous programme of crash tests. This includes not only some 30 different impact configurations, which are laid down as requirements for safety ratings and international type approval, but also nine proprietary crash tests, such as the roof-drop test or the pole impact test, developed by the brand itself.

The bodyshell structure provides the basis for the high standard of passive safety, both in terms of material and with regard to its structural design. Key features of the front-end structure are the systematic implementation of an available crash length, load distribution over several planes, the new bulkhead and floor concept and the subframe as a deformation element.

A total of three longitudinal member planes – consisting of the straight front longitudinal members with inserted crash boxes made from extruded aluminium box sections, a second, upper plane as prolongation of the load-bearing structure at the A-pillar node in the mirror triangle sector and a third plane at the bottom in front of the subframe – enable the controlled reduction of impact energy.

The subframe provides the torque support for the transverse engine/transmission block and serves to attach the components for the front axle and steering. It consists of several steel plates, some in ultra-high-strength steel, and a hydroformed tube. In order to optimise its deformation properties, the subframe is connected via two aluminium struts leading forward to the aluminium radiator mount located under the front end. In the event of a frontal impact, forces can be discharged at an early juncture into the subframe via this third load path, in order to ensure the best possible energy dissipation.

A plastic crash wedge which is fitted at the rear of the front wheel arches helps to ensure that the wheels do not slide under the front doors in the event of a high-impact crash, irrespective of the turning angles. This means that it will be possible to open the doors, even after a serious accident.

The bulkhead also incorporates a special feature: so-called "skate runners" in front of the two middle longitudinal members discharge forces into the floor. The continuous floor structure consists of a total of four straight longitudinal members. The tunnel roof reinforcements at front and rear combine with the tunnel to provide a further closed profile supporting the front end.

Protection in side-on crashes through controlled deformation

Rigid side structures and defined deformation management help to safeguard the survival space for occupants in the event of a side impact. Elements here include a member fitted diagonally in the rear footwell between centre tunnel and floor side wall which is intended to prevent the floor from being torn open in the event of side impact with a tree.

In the upper sector of the B-pillar high strength steels are used in order to ensure that as little intrusion as possible occurs, and that the passenger cell remains as intact as possible in the event of a side impact. In the lower sector, on the other hand, the B-pillar is designed to be softer, in order to enable energy reduction. In this manner, deformation is directed to a few critical areas beneath the seating surface and transferred to the other vehicle side via the tubular structures in the front seats, transversal members and massive transmission bridge in the main floor.

Extensive pedestrian protection measures including Active Bonnet

The protection of those road users who are most at risk has always been a top priority during the development of Mercedes passenger cars. The new CLA-Class features numerous protective measures designed to help lessen the risk of injury to pedestrians. In order to reduce the loads which arise in the event of a pedestrian impact on the bonnet of the vehicle, the deformation space between the bonnet and the components beneath it has been optimised. This is achieved in part through the appropriate positioning of components such as control units or fluid reservoirs in the engine compartment.

In the CLA-Class an active bonnet is also used. In the event of a pedestrian impact, sophisticated sensors combined with intelligent algorithms trigger actuators in the area of the bonnet hinges. These raise the bonnet by 60 millimetres. The additional space which this creates between the bonnet and the components in the engine compartment means that a head is subject to comparatively low internal acceleration forces on impact.

The deformation characteristics of the bonnet have been developed specifically to meet these requirements. Reductions in the impact loads can be achieved by using aluminium and reinforcing the inside face of the bonnet.

Intelligent restraint systems

The CLA-Class protects its passengers with up to nine airbags. The standard complement comprises driver and front-passenger airbag, a kneebag for the driver, large thorax-pelvisbags incorporated in the seat to protect the chest, stomach and pelvis area and windowbags. The windowbags extend over both rows of seats to the A-pillar triangle. They serve to protect the occupants' heads from hard contact in the event of side impact and can also help to keep limbs inside the vehicle in an accident. The driver and front passenger airbags operate in two stages, according to the expected severity of the impact. The gas generator first of all fills the driver's airbag with 80 and the front passenger's airbag with 60 per cent gas. If a more severe impact is forecast, the second stage of the gas generator will additionally be activated after a delay and the airbags will be filled at a higher pressure level. Sidebags for the rear are optionally available.

The high safety expertise of the CLA-Class's developers is reflected in a host of details. The deformable steering column yields by up to 100 millimetres when the driver exerts pressure on the airbag as a result of forward displacement in an accident, for example. The comfort features also reveal a fine touch: the driver's airbag is connected to a vibration absorber in the steering wheel to reduce vibrations, for example.

Comprehensive precautionary measures in the rear

In developing the new CLA-Class, the Mercedes-Benz safety experts have also attached great importance to the safety of the rear-seat occupants. The new model therefore meets the relevant requirements of the Japanese and Chinese NCAP institutes which also take account of factors such as the ease of use of the rear seat belts. Experts believe that Euro NCAP will extend its rating programme to include rear-seat safety in the medium term. The CLA-Class is already prepared for this and is a role model in this respect – both within and beyond its own vehicle category – with features such as standard-fit belt tensioners and belt force limiters for the outer rear seats. A torsion bar in the roller mechanism twists when the load on it exceeds a defined level. In this way the belt force is limited and with it the load on the seat occupant.

The CLA-Class's features also include the standard-fit ISOFIX child seat securing system on the outer seats. Child seats can additionally be fixed to the vehicle by means of special anchorage points with top tethers.

PRE-SAFE® anticipatory occupant protection

The PRE-SAFE® anticipatory occupant protection system is available in the CLA-Class for the first time as an option. This represents a further step in the ongoing democratisation of this innovation, which was premiered in the S-Class in 2002.

PRE-SAFE® from Mercedes-Benz uses the time between detection of a potential accident situation and a possible collision to initiate preventive protection measures, thus reducing the loads exerted on the occupants in the event of a crash by up to 40 percent.

Core features of PRE-SAFE® are reversible belt tensioning, the closing of side windows and sliding sunroof when critical lateral dynamics are detected and adjustment of the fully electric front passenger seat with memory function to an ideal position for maximum effectiveness of the restraint systems.

PRE-SAFE® is activated when one of the following parameters is met: emergency braking, panic braking, pronounced over- or understeering, critical steering movements or heavy support by adaptive Brake Assist.

Automatic Mercedes-Benz emergency call

In combination with COMAND Online, the new CLA-Class is equipped with the Mercedes-Benz emergency call system. As long as COMAND Online is connected to a mobile phone, the Mercedes-Benz emergency call system can rapidly automatically alert the emergency services in the event of a serious accident.

After the airbags or the pyrotechnical belt tensioners are triggered, the vehicle's exact GPS position and vehicle identification number (VIN) are sent by SMS to a special emergency centre, with positional data also being transmitted using the DTMF method (dual-tone multi-frequency) at the same time. Even if the occupants are unconscious or unsure of exactly where they are because they are on a holiday trip, for example, the emergency services alerted by the emergency centre will be notified quickly with accurate information. The driver can also make the emergency call manually as "MB emergency call" is always the first entry in the system phone directory.

On receiving the call, the emergency centre establishes voice contact with the vehicle occupants in a matter of seconds. A particularly practical aspect in an emergency is that communication takes place not in the language of the country where the accident has occurred but in the language which the driver has preset in COMAND Online.

Other than any mobile phone communication charges, which would only arise in the event of an actual emergency call, no charges are incurred by the driver for the emergency call facility and no contractual relationship – other than the regular mobile phone contract – is required.

Credits: Daimler AG

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