Perhaps you've seen Ferrari's zoomy video depicting the 4RM all wheel-drive system in the 2012 Ferrari FF, but felt it was lacking in specifics.
You're not alone. In order to better understand the nuances of 4RM, Ferrari's first production AWD system, we sat down with Ferrari technical director Roberto Fedeli who explained the gory details of the company's twin-gearbox, nine-speed all wheel-drive system.
No doubt, an all wheel-drive Ferrari is a puzzling -- some might say sacrilegious -- concept to the Tifosi.
It's perhaps no surprise that the company's first foray into four driven wheels was carefully considered. If you can remember the terminal understeer present in the first AWD Porsche 911, the late-'80s 964 Carrera 4, then you understand the concern.
Philosophically, Ferrari's 4RM system in the FF is more of a traction aid than a handling aid. Fedeli explains, "We don't want to use the four wheel drive in order to enhance the performance in the dry because we think that the best solution for the sporty car is to have all the torque at the rear."
Mostly Rear Wheel-Drive
The amount of torque you're trying to transmit directly dictates the size of the bits needed to transmit it, and in the FF the front wheels receive a maximum of just 20% of the engine's torque. The mild torque split is borne out by the puny diameters of the front driveshafts compared to the monstrous ones out back.
That the requirement of relatively low torque-handling resulted in relatively small and light front-drive bits was just the beginning. Ferrari located the engine completely aft of the front axle line which presented an opportunity to get nutty with the AWD layout.
Were the FF to employ a traditional AWD system, power would flow down the driveshaft to the rear-mounted transaxle and on to a center differential, from which a second driveshaft would then redirect power to the front differential and then on to the front wheels.
This is a needlessly heavy -- and space-robbing -- layout when you're only trying to send a maximum of 20% of the total torque to the front wheels. "We don't want another driveshaft between the gearbox and front axle because this means weight and space. Space means, again, weight, and if you don't want to lose the space inside you have to make the car bigger and bigger," says Fedeli.
Instead, the FF's front wheel-drive equipment is completely divorced from the main gearbox. A familiar rear-mounted seven-speed transaxle handles the power to the rear wheels, while a second gearbox with two forward speeds (plus a reversing mechanism) manages the power to the front wheels.
Gearboxes Front And Rear
Yes, the FF has two gearboxes. The front gearbox is driven off the nose of the crankshaft and the entire unit is largely integrated into the engine block. According to Fedeli, the front power take-off hardware weighs "35-40 kg [77-88 pounds], while saving the same amount of kilos compared to a traditional [AWD] solution."
Power for the front wheels is thus taken directly from the crankshaft, through one of the two gearbox ratios and then on to two electronically-controlled wet clutch packs, each of which can vector torque to a halfshaft connected to the front wheels. There is no center differential.
The two ratios in the front gearbox work like this -- the first ratio is used when first or second gear is selected in the main gearbox. The second cog up front is selected when the main gearbox is in 3rd or 4th gear.
When the main gearbox is in fifth, sixth or seventh, no power flows to the FF's front wheels since the car is not traction-limited in such high gears. Fedeli elaborates, "In 5th gear, all the torque that our engine is able to give us we can transmit only on the rear axle. In all conditions, also in snow when the surface is 0.3, 0.2 mu, we only need the rear axle in order to transmit all the torque."
Due to the nature of gearing and the desire to vector torque variously to the front wheels, it is imperative that the front gearbox ratio always be taller than the corresponding ratio engaged in the rear gearbox.
First gear in the front 'box is some 6% taller than second gear in the main gearbox. Second gear in the front gearbox, too, is 6% taller than fourth. The mismatched front-rear drive ratios require that the front clutch packs always operate with some slippage in order for the front and rear wheelspeeds to match.
Most of the time the FF will behave like a conventional rear-drive car. When there's plenty of traction present, the front clutches will be open, transmitting zero torque to the front wheels. Predictive algorithms derived from throttle position, steering angle, yaw and wheelspeed sensors allow the ECU to close the clutches to vector torque to the front wheels as needed.
This "on-demand" strategy avoids constant slippage of the carbon fiber-lined clutch packs. Still, the system is said to be able to withstand hard use, as the heat liberated in the clutch packs is dissipated by an independent fluid cooling system.
In-House Brainchild
So, Ferrari just licensed this clever AWD concept from a supplier, right? No. The 4RM system was developed and patented by Ferrari, and the various gears and bits are manufactured by Italian supplier Carraro.
In terms of performance, 4RM indeed improves launch performance, although Fedeli stresses "this is not the main goal of this system." The FF hits 100 kph (62 mph) in 3.7 seconds and would be "two or three tenths" slower without AWD.
We'll be driving the FF in the coming weeks, where we'll hopefully address the pressing question of what burnouts look like in an AWD Ferrari.
--Jason Kavanagh, Engineering Editor