Differentials - explained.
Differentials -- What's The Difference?
When a vehicle is negotiating a corner, the outside wheel has to travel a grater
distance than the inside wheel. Therefore, the outside wheel must turn faster than the
inside wheel. The differential is the device within the axle assembly which, in addition
to transmitting the power to each axle shaft/wheel, allows one wheel to turn at a
different speed than the other.
What types of Differentials are there?
Conventional or Open
Conventional differential uses two side gears inside the differential case. Each
gear is splined to accept an axle shaft. These side gears are in turn driven by a set of
spider gears. The spider gears, also inside the differential case, ride on a shaft which
is pinned into the differential case and through which all the power is transmitted. The
case is driven by the ring gear which is bolted fast to the case. The conventional
differential is fitted as standard equipment on most vehicles.
On paved roads this system is very successful, giving predictable handling, even tire
wear and requiring very little maintenance. However, in off road situations where traction
surfaces vary greatly, this type of differential has a major limitation. When one wheel
has greater traction than the other, all the power will be directed to the wheel with the
least traction. For example, if one wheel is in the air and the other wheel is still on a
hard surface, then all the power will be transferred to the wheel in the air. No power
will go to the one on the ground and the vehicle will not move.
Limited slip differentials (LSD's) come in a variety of designs. Most use friction
plates, cones and/or gears to reduce slippage between each of the tires. these units have
a dual power path from the differential case to the axle shafts. some power is transmitted
through the spider gears to the side gears in the conventional manner. The remainder is
transmitted by friction between the differential case and the clutch plates and the side
gears. A certain amount of "clutch preload" is built into the unit in a static
condition. Then, as load is applied to the differential, the separation forces between the
spider gears and the side gears increases this clutch loading. This increase in friction
provides for a good positive power flow from the case directly to the side gears. When
traction is available to both wheels, the power going to the differential causes the
plates to bind tightly together, giving even power to both wheels. However, in a situation
where there is little or no traction available to either one wheel or the other, the
amount of power that can be transmitted to the other wheel which has traction is dependent
on the friction or "preload" in the clutch plates. High levels of "clutch
preload" will result in good torque transfer but some chattering of the clutches
during cornering may occur. Lower levels of preload results in minimal chatter but reduced
levels of torque transfer to the wheel with traction. Because LSD's restrict true
differential action, tire wear is accelerated. Changes in vehicle handling may also occur,
particularly in short wheelbase vehicles. Wear rates on limited slip differentials are
generally higher than on other types due to the reliance on friction to reduce wheel
slippage. Also, special lubricants may be required to minimize rough and noisy operation.
Despite their limitations, LSD's are popular as original equipment options as well as an
aftermarket replace because:
- Some traction improvement off road is provided
- Vehicle handling idiosyncrasies are not excessive
- Installation is simple
- Cost is reasonable
Automatic locking differentials transmit power to each wheel through a pair of dog
clutches. Differential action, such as when cornering is provided by automatically
disengaging the appropriate clutch when one wheel rotates faster than the other. This
results in differential action which occurs in ratcheting stages rather than being smooth
and progressive. Power received by the differential is automatically directed to the wheel
with greater traction. Therefore, if one wheel is lifted off the ground, the other wheel
will receive the total power applied to the differential to maintain vehicle mobility.
Traction is far superior to conventional and limited slip differentials. While automatic
locking differential provide excellent performance off road, vehicle handling,
particularly on highway, is sacrificed. Unlocking during cornering can be sudden,
resulting in a rapid change of direction, particularly in short wheel based vehicles.
During sharp cornering an audible racheting sound usually occurs as differential action
takes place and a loud banging noise may be heard when the unit locks up again. Tire
wear usually increases. On 4WD vehicles, installation is normally considered for the rear
axles only. Front axle installations can cause extreme difficulties in steering.
Manually lockable differentials use a conventional differential in conjunction with
a mechanical locking device which can be operated at the drivers's discretion. when
locked, both axles will then turn at the same speed irrespective of the road surface. When
it is unlocked, the differential functions as a conventional differential giving
predictable handling, long service life and no increase in tire wear. It can be installed
in both the front and rear axles without compromising on-road performance. Although
manually lockable differentials are available in tractors and some military style
vehicles, the installation in mass produced recreation type vehicles have been restricted
by high cost and complexity of installation.
The ARB Air Locker has allowed this situation to be
reversed. This unique air operated unit incorporates its activating mechanism totally
inside the differential center. This avoids the need to have complicated piston
arrangements hanging off the axle housing. Locking action is controlled by the drive with
a push button switch inside the cab. Installation involves a bolt in replacement of the
original equipment differential center, a 12 volt air compressor with reservoir and an
in-vehicle mounted switch. Original equipment axles, bearings, etc. are retained.
Locking/Unlocking of the unit is extremely rapid approximately 0.1 seconds) and may be
achieved at any vehicle speed. No special maintenance is required and servicing of other
vehicle components is unaffected. Standard lubricants are used. The ARB Air Locker is the
ultimate differential for almost any vehicle providing 100% drive to each wheel, without
handling problems, or additional wear problems.