The transmission used connected to the 3.2 V6 JTS engine type is permanent four wheel drive with a Torsen Twin C type centre differential and front differential built into the Torsen assembly casing.The torque supplied by the engine enters the F40 gearbox, with specific ratios to ensure optimum exploitation of the power and torque to maximize the efficiency of the four wheel drive.At the gearbox output, the torque reaches the centre Torsen differential, by means of a crown wheel and pinion reduction with helical teeth and the drive torque is distributed between the front and rear axles with a ratio of 56/44.The torque distributed to the front axle enters the "open" type front differential incorporated in the Torsen casing and is shared between the two wheels on the front axle.The torque distributed to the rear axis on leaving the Torsen on the other hand enters the rear axle from the torque rear idler (PTU: Power Trasmission Unit).The PTU comprises a crown wheel and pinion reduction with helical teeth and a bevel pinion that transfers the engine torque to the propeller shaft.The propeller shaft, in turn, directs the engine torque to the rear differential (RDA: Rear Differential Axle) where the distribution to the two wheels on the rear axle takes place.
The transmission is assembled on the 3.2 V6 engine using new fixing bolts; the PTU, in turn, is secured to the transmission by the centre differential by means of four bolts and centered through two bushes on the gearbox casing; there is an o-ring between the flanges matching the PTU and the gearbox which is replaced at each dismantling.The tightening of the PTU is then completed by two further bolts that join it directly to the engine.
Gearbox
The gearbox used on this transmission system is the F40 with six speeds, designed to ensure optimum matching with the 3.2 V6 JTS power unit in terms of maximum exploitation of the engine torque and, at the same time, adjusting the ratios to the four wheel drive system.Gear engagement1st speed engagement
The centre differential is the Torsen Twin C type and also contains the transmission front differential in the casing. It comprises two half shells on the outside of which the crown wheel that meshes with the gearbox final gear is tightened; the casing also houses the eight planet gears with helical teeth that rotate with the casing and transmit the drive for the rear axle to an outer ring gear and the drive for the front axle to an inner ring gear connected directly to the front differential by means of ribbing.
A. Meshing line between planet gear and rear axle ring gearB. Meshing line between planet gear and front axle ring gearC. Drive entry into centre differential (coming from the gearbox)D. Drive output to rear axleE. Drive output to front axleThe sharing of the torque between the two axles takes place by means of a series of metal rings that control the drive for the two axles.When one of the axles "tends to rotate more than necessary" following variations in the grip conditions or critical connections connected to the vehicle dynamics (understeer, oversteer, etc.), the planet gears which, in normal circumstances rotate with the casing, without rotating around their own axis, "tend instantly to rotate around their own axis" producing axial thrusts typical of toothed wheels with helical teeth; these thrusts are discharged to the inner rings of the Torsen which instantly oppose the movement between the outer ring gear (rear axle) and inner ring gear (front axle) thereby transferring the torque from the axle that is rotating faster to the slower axle.Obviously, depending on whether it is the outer ring gear that wants to rotate faster or, alternatively, the inner ring gear that wants to rotate faster, the direction of the axial thrusts changes, involving different rings to discharge the forces involved transferring the torque from one axle to the other depending on requirementsThe possibility of locking the Torsen, which allows these transfers of torque to be controlled, is not infinite, in effect when the maximum torque transfer percentage from one axle to the other is reached the Torsen "opens" and allows slipping between the two ring gears; in these Torsen opening conditions the planet gears rotate around their own axle and the locking power, once the maximum value is reached, no longer increases. This is, for example, the condition produced for a fairly tight bend where the kinematic trajectories imposed by the steering manoeuvre lead the Torsen to become unlocked.
Transmission of drive to rear axle (PTU)
The casing is made from aluminium and the assembly weighs 22.2 Kg. The function of the idler unit, PTU, is to direct the engine torque to the rear axle. The PTU is connected to the gearbox by means of four bolts and to the engine by two bolts; as has already been stated, there is an O-ring between the PTU-gearbox flanges that ensures the seal and the insulation between the gearbox-centre differential internal chambers on the one hand and the PTU on the other.The PTU is lubricated internally by its own fluid and not by the same fluid that lubricates the gearbox and differential.There is an assembly inside the PTU that has the task of transferring the power from the axle coming out of the Torsen ot a second shaft with the helical wheel, on the one hand, meshing with the one on the axle coming from the Torsen and, on the other hand, the conical gear meshing on the pinion transferring the drive to the propeller shaft.
Final drive ratio 1.520 (38/25)Crown wheel and pinion ratio 2.067 (31/15)Final drive reduction ratio section
Idler final drive section
Propeller shaft
The propeller shaft comes in three parts and is connected to the bodyshell by means of two supports. The shaft has the function of transferring the PTU output power to the rear differential RDA.
Rear differential (RDA)
The casing is made from aluminium and the assembly weighs 17 Kg. The rear differential (RDA: Rear Differential Axle) is the open type and has the function of directing the input drive, coming from the propeller shaft, to the rear axle wheels.
Crown wheel and pinion ratio 3.130 (47/15)
Vehicle dynamics
Driving in a straight line
When driving in a straight line, if the grip conditions are even, the vehicle travels with the torque shared between the axles around the nominal rate; small variations in this nominal distribution are due to variations in the rolling circumference between the tyres on the two axles and the weight distribution between the axles. When the grip conditions between the two axles are no longer the same, as can be seen in the diagram, the torque is moved from the axle with less grip to the one with more grip to counterbalance the slipping of the axle which, from time to time, finds itself in poor grip conditions.Driving round bends
Round bends, when the radius is fairly consistent, the front wheels cover kinematic trajectories that make the actual wheels rotate faster than the rear wheels; in these conditions the Torsen removes torque from the front wheels and sends it to the rear wheels. When the maximum internal locking limit is reached, the Torsen opens allowing the vehicle to follow the trajectory imposed by the steering manoeuvre without the wheels slipping.Understeer
In the case of understeer the front wheels tend to rotate faster following the slipping typical of this manoeuvre, normally due to the grip limit being exceeded and allowing the engine torque to be "drained" to the ground; this usually happens either through excess torque discharged to the ground or through a decrease in road surface grip conditions. In this situation, as the tyre side road holding margin no longer remains (all the grip is used for the longitudinal part of the tyre) the vehicle tends to extend the trajectory and not follow the bend. In this situation the Torsen removes torque from the front axle and distributes it to the rear axle, returning the "side" grip margin to the front wheels and thereby restoring the stability of the vehicle, obviously within the physical limits of the vehicle dynamics.Oversteer
In the case of oversteer the rear wheels tend to rotate faster following the slipping typical fo this manoeuvre normally due to the grip limit that allows the engine torque to be "drained" to the ground having been exceeded. This usually happens either through excess torque drained to the ground or through a decrease in the road surface grip; in this situation as the tyre side road holding margin no longer remains (all the grip is used for the longitudinal part of the tyre) the vehicle tends to pitch at the rear making the vehicle unstable. In this situation the Torsen removes torque from the rear axle and distributes it to the front axle, returning the "side" grip margin to the rear wheels and thereby restoring the stability of the vehicle, obviously within the physical limits of the vehicle dynamics.
