The 4RM is an entirely new system that transmits torque to all four wheels of the car. A Ferrari patent, it is the main innovation sported by the new FF. Unlike a conventional four-wheel drive system fitted to a front-engined car, it allows the retention of the traditional mid-front engine architecture, with rear transaxle connected to the engine by a single driveshaft. Added to this is the new Power Transfer Unit or PTU for the front wheels, which is connected directly to the engine and located over the front axle.
This layout permits:
- a 50% saving in weight compared to a traditional four-wheel drive system. This greatly benefits the FF’s weight-to-power ratio and thus its performance;
- a low centre of gravity to be maintained and the retention of Ferrari’s sports car weight distribution with more than 50% of weight over the rear axle despite it being a front-engined car.
The PTU is the main mechanical component of the 4RM and manages the difference between the engine and wheel speeds. It also controls the amount of torque sent to the front wheels in general and distributes it between left and right as required. The PTU gets its power and torque directly from the crankshaft through a system of gearbox ratios. Two independent carbon-fibre oil-bath wet multi-plate clutch packs then vector the torque to a half-shaft connected to each front wheel. Thus, there is no mechanical connection between rear and front axles as they are linked to two completely independent traction systems. This means the FF can be rear-wheel drive-only so that none of the exhilarating driving pleasure associated with this kind of car is lost.
Lastly, the multi-disc clutches also deal with the different speeds of rotation between the front and rear axles, and between the individual front wheels, fulfilling the function of the centre and front differentials in a traditional 4WD system. Furthermore, the front clutches are completely independent and have torque vectoring functions to allow different amounts of torque to be sent to the left and right wheels. The amount of torque transmitted is controlled by closing the disc packs in the individual clutches. The PTU is also very compact (barely 170 mm long) and extremely light with the result that the 4RM weighs 50% less than a conventional 4WD system.
The hydraulic gear and clutch actuation system is integrated into the PTU, as with the F1 Dual Clutch gearbox, and this delivers absolutely blistering gear-shifting times and thus improved system responsiveness.