The all-wheel drive is supposed to increase the grip on the road by increasing the number of driving gears. Either it prevents too much slipping, or it distributes the available traction force to all wheels in order to improve road performance. A simple all-wheel drive requires up to three locking differentials or Torsen–differential gears. One of them might be a viscous clutch.
With all-wheel drives the entire torque is present at the output side of the transmission, and is subsequently divided amongst the axles.
Front-wheel drive/front engine -> all-wheel driveRear-wheel drive/rear engine -> all-wheel drive
Permanent all-wheel drive:With permanently connected rear axle and a longitudinal drive, the torque is transferred through the (locking) differential and the hollow transmission’s input shaft to the front. The previously not driven axle is connected via a Cardan shaft with the (locking) differential. With permanently connected front axle one has to go around the complete drive, starting at the exit of the transmission. This method is used with a longitudinal front wheel drive and automatic transmission, too. If the car is equipped with front or rear wheel drive, the conversion is easier than with front engine/rear-wheel drive.
Automatically activating all-wheel drive:Most producers connect the exit of the transmission with the previously not driven axle via a viscous clutch. If the engine lies diagonally a bevel drive is necessary.
Front engine/rear-wheel drive -> all-wheel drive
The Cardan shaft drives on the rear, the front are driven without connection to the transmission. With automatically activating all-wheel drive there is no need for a differential, because it activates only if there is enough grip of the driven wheels. If the drive is connected with all the wheels permanently, one calls this a permanent all-wheel drive. In this case, the permanent all-wheel drive has to be combined with a differential, a viscous clutch, a Torsen-differential or a Haldex-clutch.
Like with other all-wheel drives, the automatically activating all-wheel drive might sometimes cause problems with the anti-lock braking system. The problem is that the drive of the rear and back is not fully neutralized, disrupting the freewheel of the wheels when the brakes should be eased. One possibility to solve this problem is for example the usage of the Haldex clutch. Again, the effort for electronic control has increased.
Without locking the offset, the all-wheel drive is ineffectual with already one spinning wheel.
Did you know that there have been bikes with all-wheel drive? In addition to the normal drive to the rear axle the front axle could be driven by a hand crank.
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