Keeping the chassis planted on the road are V-rated run-flat all-season tires, sized 245/45R18, wrapped around 18x8 cast-alloy wheels. W-rated run-flat performance tires-sized 245/40R19, front; 275/35R19, rear-mounted on 19x8.5-in. and 19x9-in., respectively, cast-alloy wheels are optional with the Sport package. Run-flats haven't been my favorite tires of late (they make for a rather harsh ride), but the addition of the performance version makes them more than acceptable.
The SMG version I drove had the optional Dynamic Driving and Active Steering Systems, which were first introduced on the new 5 Series. For U.S.-spec cars, Dynamic Driving is called Active Roll Stabilization (ARS) and comes standard. Dynamic Driving (or ARS) is a chassis and suspension control system that uses two active anti-roll bars integrated in the front and rear axles, which convert hydraulic pressure into torsional momentum and/or exert a stabilization force through the body of the car. The result is a 3,792-lb coupe (3,781 lb for the manual) that is remarkably tossable from corner to corner, yet maintains a ride quality that is smooth and comfortable enough for jiggle-sensitive passengers. A necessary combination when you're building a luxury sports car.
BMW's Active Steering System (funny, BMW hasn't given this one an acronym) is a speed-sensitive system taken to the next level of electronic control. Active Steering affects the overall steering angle by combining the original steering angle (steering wheel turn) with that of the angle generated by the system's electric step motor. The system also works in tandem with DSC for improved yaw and pitch control. In essence, it makes slow, wide turns easier and quick, narrow turns tighter. Editor-at-large Kevin Clemens was much dismayed by the effects of the system on the new 5er, but I think he would be less critical of it on the 645Ci. It is much less apparent, making its presence known only when sudden under- or oversteer situations occur.
The 645Ci is able to weigh as little as it does through the ample use of aluminum and other lightweight materials. Weight-saving innovations include a Weight-Reduced Aluminum Front (WRAF), aluminum hood, doors and front and rear axles, Sheet Molding Compound (SMC) thermoplasitc front fenders and trunk lid, and Superlite floorpan. For comparison, the BMW coupe is a mere 6.2 in. shorter than Mercedes' CL500 (the 645Ci's height is 1.3 in. lower, the width, 0.1 in. less), yet it weighs 278 lb less-that's almost 45 lb per inch of car.
Though the coupe is lightweight for its class, there's no scrimping on safety features or luxury amenities. On the active safety front, the 6er is replete with dual-airbag SRS with two-stage Smart Airbags, front-seat side-impact airbags, front seat Active Knee Protection (USA only), AHPS side head curtain airbags, automatic front seatbelt tenioners and force limiters and automatic locking retractors on all seatbelts, and a safety steering column with a deformable-spoke steering wheel. Other safety features include the previously mentioned run-flat tires with a tire defect/low-pressure alert system, BMW Assist with automatic collision notification (also standard in the U.S.), SOS button and enhanced roadside assistance, bi-xenon adaptive headlights (also standard in the U.S.), daytime running lights (you have to program them on), halogen free-form foglights and adaptive brake lights with LED technology.