2007 BMW M6
--> Article summary:
Changes from the regular-production 650i Coupe affect the suspension system and its connections to the body structure; a special M version of Electronic Damping Control; the M Variable Differential Lock; Servotronic steering with two levels of power assist; amply dimensioned, cross-drilled brakes; and wheels and tires that give the M6 an awesome footprint.Start with suspension hardware, already an advanced all-aluminum system on the 6 Series. (The vehicle’s frontal structure too is aluminum, helping contain overall weight and contributing to near-perfect front/rear weight distribution.)At the front, modifications include the subframe and thrust plate (both still aluminum), fine-tuning of geometry, more rigid bushings and a 0.4-in. wider track. At the rear, the changes are more extensive:
- The subframe is modified.
- Suspension links are essentially the same as in the Z8.
- Bushings are stiffer.
- Axle halfshafts are hollow for reduced weight with greater strength.
is tailored to the greater torque that will flow through the whole system.
Here the track is actually narrower (by 0.4 in.) than that of a 6 Series with standard wheels, because the 285/35 rear wheels’ center plane must be farther inboard for their massive tires to clear the bodywork. But the actual footprint is wider.
And at all four wheels, specially calibrated springs and shock absorbers – the latter made variable by Electronic Damping Control, next – complete a sporting chassis that only BMW M could create.
Already offered in the 7 Series but not in U.S. 6 Series or other M models, EDC steplessly controls the shock absorbers to any level between softest and firmest, precisely adapting to road conditions and the driver’s demands at any given moment:
- Ride firmness is always optimum for current road conditions, vehicle speed and vehicle loading (passengers and luggage).
- On smooth roads, the shock absorbers are kept at the softest appropriate setting (considering the character of the vehicle) for riding comfort.
- When the vehicle is rounding a sharp corner or curve, the shock absorbers are automatically and instantly adjusted to a firmer, just-right level.
- On any irregular road surface, the shocks automatically and instantly adjust to the optimum level of firmness to control ride motions, preserve riding comfort and maintain adhesion to the road.
- Basic system advantages are calibrated to M Car character.
On the M6, there are three modes rather than the two of the luxury-oriented 7 Series EDC. Selected via the EDC console button or MDrive menu, they are –
- Normal, likely to be chosen most of the time; a judicious blend of M-style handling precision and riding comfort.
- Comfort, shifting the balance toward riding comfort. Appropriate for gentle, speed-controlled driving on relatively smooth and straight roads.
- Sport, putting top priority on the M6’s tremendous road capabilities at some sacrifice of riding comfort.
Within each mode, EDC continuously adjusts shock-absorber firmness to actual conditions.
Of the M6’s overall road prowess, Switzerland’s authoritative Automobil Revue reported in its April 20, ‘05 issue: “The elaborately engineered suspension makes for tenacious roadholding and allows enormously high cornering speeds. The M6 exhibits neutral handling, combined with direct and precise power steering.”
Mechanical limited-slip differentials belong to BMW M’s tradition of ultimate road handling. Together with the German division of GKN Viscodrive, BMW M engineers developed a more capable mechanical limited-slip differential; called the M Variable Differential Lock, the concept appeared first on the M3, then the M5; now it comes to the M6 as well.
Under dry to not-quite-dry road conditions, the traditional 25% limited-slip has some ability to improve traction; but under split-traction conditions (one wheel on slippery, the other on firmer ground) it cannot transmit more torque than the slippery side permits. On all current BMW models, electronic traction control (a function of Dynamic Stability Control) addresses this issue, though not optimally for sporty driving in the M Car sense. (Thus there’s a switch to reduce or altogether eliminate DSC action when the driver so desires.)
The M Variable Differential Lock specifically addresses low- and split-traction situations in a way that reinforces sporty handling, imparting a slippery-road ability other high-performance, rear-wheel-drive sports cars generally do not have. The main distinction between a traditional limited-slip “diff” and the M Variable Differential Lock is that where the former senses torque, the latter senses wheel speed.
Anytime a speed (rpm) difference develops between the two rear (driven) wheels, a shear pump, driven by this difference, develops pressure in the unit’s silicon viscous fluid. This pressure is applied to a multi-disc clutch that transfers driving torque to the wheel with the better road grip (this priority is called “select high”). The greater the speed difference between the two wheels, the more aggressively the clutch engages. As this wheel-speed difference diminishes, the clutch begins to ease off.
This mechanism accomplishes finely calibrated action by entirely natural means. There is no external pump, no external source of lubrication or operating fluid. The very motion to be controlled – differences in speed between one wheel and the other – generates its locking action.
|7-speed Sequential Manual Gear...||Servotronic steering with two ...|