By Christopher A. Sawyer
In 2013, the Mercedes CLA will showcase a new 4Matic all-wheel drive variant with fully variable torque distribution, built around a front-drive architecture with transverse engines. And while this addition will be helpful in wet and snowy weather, it also will give the CLA 45 AMG more stable and torque steer-free handling.
The CLA’s 4Matic system includes Mercedes’ 7G-DCT seven-speed automated dual clutch transmission; all-wheel drive CLAs will not offer a manual gearbox. A compact power take-off unit (PTU) is integrated in the transmission, and supplied with lubricant from the latter's oil circuit. Weight reduction on the order of 25% is possible when compared with designs that use an add-on PTU with its own oil circuit. Further efficiency gains come from the use of tapered roller bearings.
An electro-hydraulically actuated multi-disc clutch is integrated into the rear-axle. When the clutch is open, most of the power flows to the front axle; closing the clutch brings the rear axle into play. Drive torque is shifted front-to-rear when the rotor pump in the rear axle detects speed differences between it and the front axle. A proportioning valve managed by the electronic stability control modulates the pressure. Most of this is pretty standard fare for transverse front-drive systems adapted to all-wheel drive Where the Mercedes unit is different is in its integration of the PTU, pump and multi-disc clutch pack.
Though the vehicle spends as much time as possible in front-drive for the greatest fuel efficiency, driving torque is channeled to the rear when appropriate. Under heavy braking with ABS, the rear axle is deactivated. On the other hand, if the CLA starts to understeer or oversteer under load, torque is distributed to the appropriate axle as necessary for stability. Only if these efforts fall short do the stability and traction control systems come into play.
Torque distribution also is dependent on the shift program for the seven-speed automatic transmission. In Eco mode, less torque is channeled to the rear axle. However, in Sport or Manual modes, the activation times are shortened, and more torque is distributed to the rear axle. This makes the CLA more responsive to lateral and longitudinal acceleration changes.
The CLA 45 AMG will come with a high-torque 2.0-liter turbocharged four-cylinder engine rated at 211 hp/295 lb-ft, sports suspension and 4Matic. As currently planned, both the CLA and CLA 45 AMG will be sold in North America.