Q4 – Autobike
This is the permanent all-wheel drive used by Alfa Romeo. The system works on the principle of a Torsen center differential, followed by bevel center differentials. It is placed in a common housing with the front differential and reacts to the difference in torque. Thus, the drive of both axles continuously distributes engine power between the front and rear wheels. Under standard conditions, 57% of the torque is transmitted through the TwinDiff limited-slip differential to the rear wheels, and the remaining 43% to the front wheels. This gear ratio is suitable for dry and neutral conditions where the vehicle has a similar character to a rear wheel drive vehicle. In extreme conditions, the Torsen differential can distribute torque from 22:78 to 72:28 between the two axles. In this way, the drive of both axles of the Q4 not only improves grip on slippery surfaces, but also keeps the track in sharp motion. The system helped eliminate understeer at the limit, so in the event of a skid, the car will not go straight, as is the case with front-wheel drive, but beautifully sideways with all four wheels. However, one should not exaggerate the speed of movement, since an experienced driver is already required to catch a slippery, and in the case of the Alfa 159, even an almost two-ton ATV. And that's a lot of weight, which devalues a bit the ability of a heavy-engined ATV. In the last comparison, a lighter handguard with a smaller and also lighter 1,75 TB, but also 1,9 JTD, respectively. 2,0 JTD is not much worse. The advantage of the Q4 system is mechanical strength, the relative disadvantage is the limited maximum tightness resulting from the very design principle. Q4 can be found, for example, in the Alfa 159, 159 Sportwagon, Brera and Spider models.
