As described above in section 8.4.1, a traditional differential allows the
left and right wheels of a drive axle to spin at different speeds. This is
necessary in order to allow the vehicle to turn. A traditional differential is
also called an "open" differential.
An open differential splits the torque evenly between each of the two
wheels to which it is connected. If one of those two wheels comes off the
ground, or is on a very slippery surface, very little torque is required to drive
that wheel. Because the torque is split evenly, this means that the other
wheel also receives very little torque. So even if the other wheel has plenty
of traction, no torque is transferred to it. This is a major disadvantage of an
open differential.
An improvement on the open differential is a locking differential. In a
locking differential, the driver can operate a switch to lock the left and right
wheels together. This ensures that both wheels together receive the total
torque. If one of the two wheels is on a slippery surface, the other wheel
could still receive adequate torque and provide the longitudinal traction
force. Thus a locking differential provides better traction on slippery
surfaces and can be used when required by the driver.
Yet another type of differential is the limited slip differential (LSD). In a
limited slip differential, a clutch progressively locks the left and right wheels
together but initially allows some slip between them. This allows the inner
and outer wheels to spin at different speeds during a turn but automatically
locks the two wheels together when the speed difference is big so as to
provide traction help on slippery surfaces.
From the above discussion on differentials it is clear that the ratio of
torque transmitted to the left and right wheels is determined by the type of
differential. In an open differential, the torque transmitted to both wheels is
always equal. In a locked differential, the speed of both wheels is equal and
both wheels receive the total torque together as one integrated system. In a
limited slip differential (LSD), more torque can be transferred to the slower
wheel. This increase in torque to the slower wheel is equal to the torque
required to overpower the clutch used in the LSD.