Open differentials are the most common in cars and come standard in
most four wheel drive trucks and SUVs (Sport Utility Vehicles). The open
differential is a system of gears that keeps the same amount of pressure
on the faces of the gears that drive each of the axle shafts (to the
wheels). When one wheel turns faster than the other, as when turning a
corner, the pressure on it is kept up (so more power is applied to the
faster turning side). This is the best possible application of power on
smooth dry surfaces and provides the best handling. In poor traction
conditions, as in sand or mud, this is the worst possible application of
power. When one of the wheels has no traction, it spins. Since the
pressure is kept the same on both axle shafts, little or no torque is
available on the side that is not spinning (and which may have
traction).
(by Chris Wilson Jan 94) A typical open differential will keep the AVERAGE rpm of both
axles constant relative to the drive shaft rpm. For example, when one
wheel is not turning (0rpm), the other wheel will be turning at 2 times
the rpm that it would be if both wheels were at the same rpm - relative
to the drive shaft. This behavior is due to the spider gears and
therefore will also be present in a limited slip design which uses the
spider gears. In a non-wheel-slip turn, a open diff will have the
outside wheel faster than in a straight line and the inside wheel slower
than in a straight line for a constant vehicle speed.
In a locker design which does not use spider gears, at least one
wheel must turn the same (relative) rpm as the drive shaft while the
other wheel can turn faster (but not slower). ( end by Chris Wilson )
The opposite of the open differential is the spool. The spool replaces a
differential with, effectively, a solid axle shaft between both wheels.
The right and left wheel must then move at exactly the same rate. To
turn a corner, one or both of the wheels must slip on the ground. This
places an enormous load on the axle shafts and wears tires rapidly, if
it is driven on dry pavement. To completely lose traction, both wheels
must lose traction and spin. Spools are often used on off-road racing
vehicles.
A number of compromises between these two extremes are available.
These are: differentials that can be switched manually between spools
and open differentials, lockers, gear driven limited slip differentials,
and clutch driven limited slip differentials. Each of these has
advantages and disadvantages.
There are two sorts of manually actuated locking differentials. One is
operated by air pressure. A compressor is installed and a solenoid is
controlled by a switch operated by the driver. Pushing the switch
connects the compressed air source and pushes a piston that moves a ring
that locks the differential. When the switch is pushed again, the line
to the differential is vented and a spring returns the differential to
its normal, open condition. This system is marketed under the name ARB
Air Locker and is available for a number of common axles. Reliability is
the main drawback to this system. Sticking of the solenoid or leaking of
the air tubing are the main causes of failure. These problems are easily
repaired in the field. A blow-out in the seal within the compressor or
the piston inside of the differential are not so easily repaired, but
are much less common. Some people object to the cycling of the air
compressor, both when in use and when in standby. This is the most
expensive type of differential.
Currie Enterprises is planning an electrically actuated locker for March
94. This is available only for Ford 9" differentials. It has the
advantages of the ARB in that it can be turned on or off, but has no air
hoses to break.
Lockers are manufactured by two companies. One brand is called the
Lock-Rite or L.A. Locker. The other is the Detroit Locker or No-Spin.
These are similar in use, but differ in application (what they fit) and
in internal details. Both of these act very similar. When driving in a
straight line the locker is normally locked. On dry pavement, when
cornering, the outer wheel over-running the speed of the axle unlocks
the outside axle shaft. Power is applied to the inside wheel only. This
can cause a noticeable under-steer. Additionally, if enough power is
applied while in the turn, the axle will lock up and the vehicle either
slips wildly, spins or straightens out suddenly. These differentials
sometimes make a clicking noise when cornering; some find this
objectionable.
This type of differential is not suitable for front axle use in
vehicles with disconnect axles in the front. Disconnect axles are used
on the front end of some newer vehicles. They are considered by some a
nice feature and by others a terrible fault. They are used with
"shift on the fly" four wheel drive. On my Wrangler, it
seperates the right side axle shaft. Locking hubs are not used with this
arrangement. The problems with using lockers with such axles are that
the drive shaft will be driven and that the side not split by the
disconnect will undergo drag, especially when the differential is cold.
This will cause a pull toward the side of the front axle that the
differential is on.
Limited slip differentials of both gear and clutch types will
transfer only a fraction of the available torque to the side that is not
slipping. This is not as good as a locker, but may be enough to get you
through. Many vehicles are available with limited slip differentials as
a factory option. Limited slip differentials are not suitable for use in
the front axle of a vehicle with an axle disconnect.
The only gear type limited slip I am aware of is made by Dyneer, the
company that makes the Detroit Locker. It works by having the speed
difference between the wheels wedge one of the gears in a pocket,
reducing the freedom of movement of that side. This transfers torque to
the other wheel as long as the speed difference is large enough to keep
the gear in the pocket. This type of differential requires no special
service and no special lubricants. The main drawback to this approach is
mechanical weakness. The True-Trak is available only for lighter
vehicles and is recommended only for light or medium duty off-road use.
The clutch type limited slip differential is the most common traction
enhancing device. It is the cheapest, but also the least capable. It
uses a clutch or clutch pack to couple the axle shafts when the speed
becomes too different between the two sides. The clutch does not
transmit all the available torque when new and becomes less effective
with time, eventually becoming effectively an open differential. The
clutch must be replaced periodically. Special lubricants must be used in
these differentials to increase the friction on the clutch.
All of these enhancements have drawbacks. The most expensive ones have
the least serious drawbacks (excepting price). They can be a big step up
from an open differential. None of them will always guarantee traction,
since all four tires can be spun at once! Preventing a wheel from
spinning not only reduces the chances of getting stuck and increases
your chance of pulling a hill, it also reduces the chance of breaking
equipment when an airborne, spinning wheel hits the ground. Preventing a
wheel from spinning is also a big part of treading lightly. A spinning
wheel can move alot of soil leading to ruts, erosion, and dust.
I have a 2WD 2000 toyota tundra I would like to put a locker or limited slip rear end in. Can you please tell me if it is a 7.5" or 8.0" The dealer can tell me it is a 4.8 ratio but not what size in width. My vin # is 5TBRN3412YS114760
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