Dynamic performance, agile handling, active safety, and superior driving comfort - by tradition, BMW has always offered its customers the highest standard of suspension and chassis technology in every respect.

Within its segment the new 6 Series certainly holds pole position, no other 2+2-seater convertible offering the same level of agility and dynamic performance, plus the excellent comfort features boasted by the 645i Convertible.

This superiority rests on a sound foundation: BMW's all-aluminum chassis and suspension with its well-known benefits such as front-to-rear axle load distribution of almost 50:50, rear-wheel drive and, as a result, smooth and responsive steering not affected in any way by drive forces. Other equally essential factors are the wide track wheelbase measuring 2780 millimeters or 109.5", and - in conjunction with a low center of gravity - relatively low overall weight of 1815 kg or 4002 lb (manual gearbox model).

Precise steering with Servotronic featured as standard

Driving pleasure, motoring comfort and driving safety depend to a large extent on the accuracy of the steering system and the feedback it gives the driver. In the new 6 Series, steering assistance is masterminded via a control map operating independently of road speed, based on the hydraulic rack-and-pinion steering principle further enhanced by Servotronic. This makes maneuvering smooth, comfortable and convenient, and reduces the risk of "tearing round" the steering too hard at high speeds.

Active Steering enhancing both driving pleasure and handling

BMW's innovative Active Steering available as an option ensures even greater driving pleasure and handling on the road. Benefiting from a more direct transmission ratio, the Convertible shows a far more agile and precise response when driven in sporting, dynamic style up to medium-range speeds of approximately 120 km/h or 75 mph - for example on fast and winding country roads, where the driver is hardly ever required to cross over his/her hands on the steering wheel. Then, with road speed increasing, Active Steering successively cuts back the steering angle and power assistance in the interest of even smoother directional and driving stability - which, for the driver, means a significantly higher standard of comfort on the road.

In town Active Steering facilitates parking, with less than two revolutions of the steering wheel serving to turn the front wheels completely from left to right lock. By comparison, the rack-and-pinion steering featured as standard requires almost three turns of the steering wheel from lock to lock. Active Steering therefore reduces the driver's steering effort significantly.

Superimposed steering angle

Active Steering is based on the principle of a superimposed steering angle, a step motor on the steering box increasing or cutting back the steering lock predetermined by the driver according to specific requirements and driving conditions. In practice this means that the step motor moves in the same direction as the steering position predetermined by the driver up to medium road speeds, thus turning the front wheels somewhat further and reducing the steering forces required.

At high speeds the step motor acts against the predetermined steering position, reducing steering lock on the wheels. This makes the steering transmission ratio more indirect, while at the same time Servotronic increases the level of steering forces required.

BMW Active Steering combines steer-by-wire steering with mechanical transmission of power and an authentic feedback, thus setting the standard in agility, comfort, and safety. And with the steering wheel and steering box still being connected to one another mechanically, the steering still maintains its full response and control functions should the electronic assistance system ever fail to operate properly.
A further important feature is that Active Steering communicates consistently with the DSC control unit, intervening via the steering angle on the front wheels to provide extra stability and modify the yaw angle wherever required. Indeed, this intervention in the steering is faster and is less noticeable to the driver in the lower DSC response range than when the DSC control system itself applies the brakes on the wheels.

Outstanding chassis plus DSC: the perfect team

The wide range of control systems fitted as standard and interacting with one another under the overall heading of DSC Dynamic Stability Control enhances the already outstanding "basic" chassis and suspension to an even higher level of dynamic performance and active safety in order to cope with the most demanding situations on the road. Permanently monitoring driving conditions, DSC and its sub-functions reduces drive forces or activates the brakes as soon as it detects the risk of the car swerving within fractions of a second.

The main components making up the DSC control system are ABS anti-lock brakes incorporating CBC Cornering Brake Control applying the brakes slightly on the inner rear wheel in a bend at medium to high speeds in order to enhance driving stability, as well as ASC Automatic Stability Control intervening in the brakes and electronically controlling engine torque, which enables the vehicle to set off smoothly on slippery gradients without the drive wheels spinning. In practice, therefore, ASC acts in basically the same way as a limited-slip differential.

A further feature is DTC Dynamic Traction Control which the driver is able to activate manually in order to increase traction on the drive wheels especially when driving in winter. DTC slightly raises the response threshold of the stabilizing DSC brake intervention effect in the interest of enhanced traction and drive forces, promoting a more sporting and dynamic style of motoring in the process. But it does not in any way detract from the fundamental safety functions maintained by DSC in all cases.

Finally, Dynamic Brake Control helps to build up optimum stopping power in a potentially dangerous situation even if the driver fails to press down the brake pedal all-out.

Dynamic Drive to prevent body sway

As an option the BMW 6 Series Convertible comes with the Dynamic Drive chassis and suspension control system combining sporting driving dynamics with supreme motoring comfort: With the driver and passengers remaining largely unaffected by bumps and unsmoothness on straight stretches of the road, Dynamic Drive suppresses body sway in bends and thus ensures superior agility and stability under all driving conditions. The significant improvement in driving safety, nimble behavior and steering precision ensured in this way sets standards in suspension technology.

In city traffic, for example, Dynamic Drive eliminates body sway and roll, and even reduces the roll effect to a significant extent on winding country roads at high speeds. In such dynamic driving situations including lane change or evasion maneuvers, Dynamic Drive exerts precisely the influence and forces required on the car's steering characteristics, in the process improving both steering precision and load change response which, in practice, is tantamount to an even higher standard of active safety on the road.

Dynamic Drive largely eliminates the conflict of interest between handling and vibration comfort, since it allows the springs and dampers to be set primarily to a high standard of comfort in providing exactly the right vibration behavior. The key elements of Dynamic Drive are two active anti-roll bars integrated in the front and rear axles in lieu of conventional mechanical anti-roll bars.

These active anti-roll bars are made up of a hydraulically operated swivel motor with the swivel motor shaft and swivel motor housing each linked to one half of the anti-roll bar assembly. Their job is to convert hydraulic pressure into a torsional and, via their connection to the body, a stabilization force.

When driving in a straight line, the two halves of the anti-roll bars are released in order to avoid unpleasant sway movements on asymmetric road bumps.

Aluminum - the foundation for incomparable driving dynamics

With the exception of a few components typically subject to high loads such as the tiebars, wheel bearings or bearing pivots, the spring strut thrust rod front axle of the new BMW 6 Series Convertible is made completely of aluminum. The front axle subframe accommodates the steering box, anti-roll bar, wishbones and thrust rods. Shaped like the letter U, the front axle subframe is reinforced by a thrust plate ensuring a higher standard of lateral stability and, as a result, more precise response to loads and forces.

The integral IV rear axle is the ideal configuration for primary chassis and suspension functions such as maintaining wheel camber and ensuring superior comfort. The four control arms holding the wheels on the integral rear axle are not fastened directly to the body, but rather rest elastically in an axle subframe also supporting the final drive mounted on rubber bearings.

The subframe, in turn, is connected elastically to the body of the car via four extra-large rubber elements. Fine bumps and vibrations transmitted from the road on to the tires therefore first have to pass through these elastic bearing points before being able to reach the passenger compartment of the new Convertible in the first place.

Another reason for the excellent roll comfort of the 6 Series Convertible is the front swinging arm bearing on the rear axle subframe providing an important longitudinal suspension effect in guiding the wheels. The bearings on the rear axle subframe allow additional longitudinal suspension on the entire rear subframe, making a significant contribution to the car's roll and noise control.

Lightweight brakes with aluminum swing callipers at the front

Extra-large swing-caliper brakes with inner-vented brake discs ensure superior deceleration and resistance to fading on all four wheels of the BMW 645Ci Convertible. The configuration of the discs with a friction ring made of highly carbonized gray-cast iron and the disc support as well as the swing calipers made of aluminum, reduces unsprung masses and cuts back the float effect occurring also when applying the brakes hard by approximately 20 per cent in the interest of long service life. The result is an even higher standard of agility, driving comfort and safety on the road provided by the brake system, the innovative lightweight brake design principle being covered a worldwide BMW patent.

Runflat tires for puncture-free motoring

The new 6 Series Convertible comes on runflat tires, without even requiring a spare or emergency wheel. Reinforced outer walls on the tires with additional inserts and a particularly temperature-resistant rubber compound enable these tires to cover a distance of at least 150 kilometers or 90 miles under full load at a maximum speed of 80 km/h or 50 mph, even with the tires completely empty and without pressure. Carrying a lighter load or with some air left in the tires, the remaining mileage increases significantly. ABS, ASC and DSC remain fully functional in all cases, and thanks to special double-rim design (Extended Hump 2/EH2), the runflat tires cannot jump off the rims even under a sudden loss of air. The result, obviously, is an even higher standard of safety particularly at high speeds and on winding roads. And if necessary, conventional tires may easily be fitted on these rims.
Tire defect indicator providing a puncture warning.

Fitted as standard, the TDI Tire Defect Indicator consistently monitors air pressure in the tires by comparing wheel rotation speed on the basis of the ABS sensor signals. The system immediately recognizes any significant loss of pressure due to the change in tire circumference and the resulting deviation in the speed of rotation, giving the driver an optical warning signal and sounding a warning buzzer. This warning is generated automatically as of a road speed of 15 km/h and with a pressure loss of more than 30 per cent.

Source: BMW AG

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