BMW M6 Press Release no 4: The Chassis and Suspension of the BMW M6:Agile, Fast, Safe
The chassis and suspension of the BMW M6 is based on the all-aluminum chassis of the BMW 6 Series suitably modified in its kinematics to the
in-crease in engine power. Naturally, the very stiff bodyshell as well as the large number of aluminum components provide the ideal foundation for sheer driving pleasure at its best. Added to this there is the perfect balance of weight spread out on the front and rear axles and, of course, rear-wheel drive typical of BMW and keeping the steering free of drive forces.
The M6 retains the basic geometry of the 6 Series suspension: Track mea-sures 1,567 millimeters or 61.7´´ at the front and 1,584 millimeters or 62.4´´ at the rear, wheelbase is 2,781 millimeters or 109.5´´. Featuring an increase in negative camber, wheel guidance and control meets the even greater de-mands made in terms of driving dynamics and high loads.
Axle load distribution in perfect harmony is again typical of BMW, with 54 per cent of the car’s weight on the front, 46 per cent on the rear axle. Added to this there is the unladen weight of only 1,710 kilos or 3,771 lb (ECE), combining ideal dimensions and ideal weight in a particularly dynamic blend. So with the BMW 6 Series already offering the perfect rendition of agility and dy-nam-ic performance, the BMW M6 enhances this driving pleasure to a sig-nificantly higher standard than you initially might deem possible.
Intelligent lightweight technology: Less mass – but extra strength and stiffness
With the exception of components such as the tiebars, wheel mounts or bearing journals subject to high loads, the two-arm spring strut front axle is made completely of aluminum. The U-shaped front axle subframe takes
up the steering mechanism, anti-roll bar, track control arms and pushrods.
A multi-dimensional aluminum frame gives the front axle subframe maximum crosswise strength and stability, offering the advantage of particularly pre-cise response to the steering.
The aluminum frame comes complete with two NACA intakes of the same type as used in motorsport or aviation. These openings allow cooling air to flow through the front section to the transmission and other components with-out impairing the aerodynamic and streamlining qualities of the under-floor.
Servotronic with two control maps
In developing the BMW M6 High-Performance Coupe, the engineers at
BMW M GmbH have given particular attention to the steering system crucial to agile and safe motoring. Via special control maps, Servotronic steering management controls the degree of power assistance as a function of road and engine speed. This solves the basic conflict of interest between high power assistance for comfortable parking and low assistance in the interest of superior driving dynamics at high speeds.
Two different Servotronic control lines corresponding to the EDC mode
cur-rently in use allow two different configurations, one focusing on motorsport, the other on motoring comfort. In the former case with the sports-oriented control line, the steering is very direct giving the driver precise feedback un-der high lateral acceleration. This is important because of the extremely high speeds the M6 is able to reach in a bend. The comfort setting, in turn, places the emphasis on motoring comfort appreciated above all on long dis-tances.
A common feature shared by both map control lines is that they maintain the basic characteristics and feeling of the steering.
Rear axle perfectly set-up in intelligent lightweight technology
Made almost completely of aluminum, the Integral IV rear axle in the
BMW 6 Series – a configuration absolutely outstanding in terms of both directional stability and comfort – has been adapted to the car’s far higher level of per-formance through special elastokinematics as well as reinforced arms, guide bars and joints. The former rubber-dampened wheel guidance joints, for example, are replaced by rigid, solid joints offering a far higher standard of stability in the interest of even more precise wheel guidance. This also re-duces temperatures in the final drive by up to 15 °C versus a conventional configuration, dramatically cutting back the thermal load acting on the axle components.
The final drive is connected to the SMG transmission by a two-piece pro-peller shaft featuring a Hardy disc at the front, a synchronizing joint at the rear, and a central bearing. The output drive shafts are made as torsionally stiff hollow pipes in the interest of minimum weight.
M Differential Lock for more driving pleasure and extra safety.
Like the BMW M3 and M5, the two other models in the BMW M Family, the M6 comes with its final drive comprising a variable, speed-sensing
M Dif-ferential Lock. This gives the Coupe superior driving stability and optimum traction particularly when accelerating out of a bend.
A differential lock serves to build up locking action whenever required,
for ex-ample if one of the two drive wheels threatens to spin. Such a differential lock is appreciated particularly by the sports-minded driver since it further enhances the advantages of rear-wheel drive on roads with an average or above-average frictional coefficient, where the driver prefers a sporting, dynamic style of motoring.
Superior traction in winter
On a “normal” torque-sensing differential lock the overall drive power trans-mitted to the wheels depends on the momentum and forces conveyed by the wheel running on the more slippery side with a lower frictional coeffi-cient. Wherever this frictional coefficient is particularly low, for example on snow, gravel or ice, the traction benefits offered by a conventional lock con-cept are limited by the restricted support forces. BMW’s Variable M Differ-en-tial Lock, on the other hand, offers a decisive advantage in terms of traction even in very demanding situations, for example with the wheels running on totally different surfaces with a very significant difference in their frictional co-efficient.
In combination with the carefully set-up DSC system and the well-balanced axle load distribution, this gives the M6 very good driving cha-racteristics in winter weather.
A further advantage of the Variable M Differential Lock lies in the fact that the system immediately builds up increasing locking action with an increas-ing difference in the speed of rotation on the two drive wheels. As a result, the wheel under less load – for example the inner wheel in a bend on a fast serpentine road – can no longer cause the drive forces conveyed to the wheels to “collapse”. Instead, the wheels consistently retain their drive forces and traction on the road.
Up to 100 per cent locking action
BMW’s Variable M Differential Lock incorporates a shear pump spontane-ously building up pressure as soon as the drive wheels start to run at a dif-ferent speed. This pressure is transmitted via a piston to a multiple-plate clutch conveying drive forces to the wheel with better grip as a function of the difference in speed. In an extreme case all the drive power coming from the engine may be transmitted to the wheel with a better frictional coeffi-cient.
As soon as the difference in speed between the two wheels starts to decrease, pump pressure is reduced accordingly and locking action is cut back. This self-adjusting pump system is maintenance-free and is filled with highly viscous silicon oil.
Benefiting from this advanced technology, the driver of the BMW M6 is able to set off much better and more smoothly on surfaces varying significantly in their frictional coefficient, since the wheels have far better traction.
The Vari-able M Differential Lock also improves the car’s handling and driving stability in the interest of extra safety and driving pleasure.
M Generation DSC for extra safety.
The outstanding driving dynamics and safety offered by the car’s chassis and suspension may be optimized to an even higher standard in specific situa-tions by a control system which intervenes when needed. BMW integrates these control systems within the overall concept of Dynamic Stability Control specially configured for the BMW M6.
The task of these systems is to permanently monitor driving conditions and
to intervene in the interest of extra stability wherever required. So it is fair to say that DSC is a special safety function taking the car and the driver to the very limits of physics.
Wherever necessary, DSC intervenes as required in engine management, reducing drive forces and/or activating the brakes on each wheel.
The result is a higher standard of driving safety, for example on slippery roads, in ab-rupt maneuvers, or with the car starting to lose stability in a bend.
The special M Generation of DSC developed for the BMW M5 and M6
en-ables the driver to pre-select the dynamic driving programs in the MDrive mode, and then call up the program required via the MDrive button in the steering wheel. While the first stage of DSC in this case is basically the same as in the BMW 6 Series, the M Dynamic Mode is intended for the very dynamic, sports-oriented driver.
M Dynamic Mode – an outstanding achievement in driving dynamics.
The M Dynamic Mode (MDM) sets a new standard in driving dynamics,
of-fering the sports-minded and ambitious driver a revolutionary improvement in dynamic performance: This special sub-function of DSC reserved to BMW M Cars enables the driver to capitalize on the car’s longitudinal and lateral ac-celeration by pressing the MDrive button in the steering wheel.
The driver using this option is quite literally limited only by the extreme laws of physics. DSC intervenes only when the car is driven to the absolute ex-treme, thus allowing the driver to countersteer moderately while power-sliding through a bend at a controllable angle. Clearly, this means that the M Dynamic Mode should be used only on a race track closed to public traffic.
A warning sign in the instrument panel informs the driver that the
M Dynamic function is active. And there is also a warning signal informing
the driver as soon as he deactivates the DSC function altogether.
EDC: From taut and dynamic all the way to comfortable and smooth.
Adjusting Electronic Damper Control, the driver is able to set the characteristics of the chassis and suspension as required from taut and sporting all the way to comfortable and smooth. This is done by the three Comfort, Normal and Sports programs selected via the MDrive button in the steering wheel or the push button next to the SMG shift lever. For all prac-tical purposes, therefore, the driver is able to convert his BMW M6 at the touch of a button into a thoroughbred driving machine, a sports car for every-day use, and a luxury coupe for grand touring in grand style.
EDC ensures ongoing, infinite control of elec-tronic damper forces over a very wide range of different settings. In the Normal program damper forces are automatically adjusted as required to specific driving conditions. The system provides an optimum combination of motoring comfort and driving safety.
The driver then has the option to pre-select the Comfort or Sports programs, varying damper characteristics accordingly. In the Sports mode the chassis and suspension respond to bumps on the road and uneven surfaces by building up higher damper forces to reduce body motion and give the M6 a much firmer grip on the road. In the Comfort mode, by contrast, EDC re-duces damper forces in the interest of a higher level of motoring comfort. In bends, when applying the brakes and when accelerating, higher damper forces enhance driving safety in all modes, improving body sway and dive behavior of the M6 accordingly. A further benefit is the consistent mainte-nance of good vibration characteristics regardless of the load the car is carrying throughout
its complete lifecycle.
High-performance brakes like in motorsport
Reflecting its outstanding performance, the BMW M6 comes with extra-large high-performance brakes featuring weight-optimized compound brake discs carried over directly from motorsport. Optimum arrangement and configura-tion of the holes drilled into the brake discs ensure excellent stopping power both in the dry and under wet conditions.
Brake disc dimensions are 374 x 36 millimeters (14.72 x 1.42´´) at the front and 370 x 24 millimeters (14.57 x 0.94´´) at the rear. The aluminum double-piston swing calipers, optimized for low weight and enhanced stiff-ness, significantly reduce unsprung masses and contribute accordingly to the car’s agility, safety and comfort on the road. As a result, the BMW M6 offers stopping distances otherwise seen only in motorsport, coming to a halt from 100 km/h (62 mph) within just 36 meters or 118 ft, and from 200 km/h (124 mph) within less than 140 meters (459 ft).
Diagnostic system measuring brake pad wear.
Determining brake pad wear at specific measuring points, a sensor trans-mits the data monitored to the DSC control unit. Then, as a function of the driver’s style of motoring, the system determines the current state of the brake pads and forecasts the remaining mileage until the brake pads have to be replaced. This information is used for Condition Based Service in order to determine meaningful and appropriate service dates.
The wheels – simply outstanding in their looks and technical features.
The wheels on the BMW M6 are extra-large from the start due to the
large brake discs. In terms of their looks, in turn, the19-inch forged aluminum wheels developed exclusively for the BMW M6 give the car even greater presence and appeal on the road. With their five contoured double-spokes, these wheels not only look delicate, but are 1.8 kilos lighter than conven-tional cast aluminum wheels.
Like the wheels, the car’s tires do not come off the rack, but were rather developed exclusively for BMW M in an elaborate series of tests: The front tires measure 255/40 ZR 19, the rear tires are 285/35 ZR 19. In their
rub-ber compound and dimensions, the tires are designed to precisely convey high lateral and longitudinal forces with a relatively high standard of roll com-fort on both dry and wet surfaces. They also offer appropriate feedback to the driver, allowing the car to be pushed to its physical limits whenever, without sacrificing safety in the process.
Tire repair system takes the place of a spare wheel
The tire repair system comes complete with BMW’s Tire Pressure Indicator as well as the second-generation M Mobility System. Providing both a vi-sual and acoustic signal, the Tire Pressure Indicator warns the driver of a sud-den or gradual drop in pressure in one of the tires.
Featuring appropriate hump geometry, the rims prevent even an empty
tire with zero pressure from “jumping off” the wheel, thus allowing the driver to safely stop the car in the event of a puncture. The M Mobility System then serves to seal holes in the tire of up to 6 millimeters, allowing the driv-er to proceed safely to the next workshop.
This means that in practice the driver is able to repair virtually all punctures without having to change the wheel right away, so that the BMW M6 does not require a spare or emergency wheel. The result is a decrease in weight of more than 20 kilos compared with a full-size spare wheel, obviously help-ing to improve the car’s power-to-weight ratio and driving dynamics.