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As shown in the image below, when a vehicle turns the distance travelled
by the outside wheels is greater than that of the inside wheels. Therefore
the outside wheels have to turn faster to keep pace with the slower-turning
inside wheels. Further, the arc described by the front wheels is larger than
that of the back wheels.
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Outside wheels describe a
larger arc than inside wheels |
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Using the image above as a reference, it is evident that the front right wheel
needs to turn faster than the back right wheel. It follows that the front left
must also turn faster than the back left. In a 2 wheel drive vehicle when
either the front or rear wheels are not under power, differentials are employed
to counteract these conditions, delivering the correct amount of torque to both
wheels as required. The difference in front and rear rpm is academic as 2 of
the 4 wheels "free-wheel". However, with all 4 wheels under power, this poses
a problem.
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In a 4x4, the transfer case will power the front and rear driveshafts with
the same amount of rpm, and is thus not able to satisfy the front axle's
requirement for more rpm. This effectively slows the front wheels down,
resulting in the need for much larger turning circles and dangerous handling
on dry, paved roads. This is condition is commonly referred to as driveline
binding, or diff wind-up. Forcing a hard turn under these conditions results
in broken driveline parts (driveshaft, transfer case, axle shafts,
differentials, etc.).
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To overcome this problem 4x4's come equipped with a center differential, which
links the front and rear drive shafts.
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Let's assume we're driving a 4x4 off-road with all 4 wheels under power.
This implies that each wheel receives 25% of total torque. Now let's
further assume that the front left wheel enters a deep rut and loses
contact with the round. With differentials as described above, the wheel
with the least amount of resistance (in this case the front left) suddenly
receives 100% of available torque, resulting in 100% loss of traction on
the 3 wheels still in contact with the ground.
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The logical conclusion is that the ability to disengage (or lock) the
differentials when driving over difficult off-road terrain is desirable.
Locking the differentials will force all 4 wheels to turn at the same rate,
ensuring that traction is not lost on those wheels still in contact with
the ground. This is why differential locks (commonly referred to as
diff-locks or lockers) are installed on 4x4's. The two most commonly
found alternatives are limited-slip differentials and locking differantials.
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| Locking Differentials |
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Diff-locks are a great help in difficult terrain because they force all
wheels to turn at the same rpm. Thay can be fitted to the front, back,
or both axles. When fitted to front and back, it is advisable to lock the
rear differential before the front. This will avoid spinning the back of
the vehicle out when under power.
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| Limited Slip Differentials |
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Limited Slip differentials are generally cheaper than diff-locks.
Although less effective than lockers, they effectively limit (or delay)
slip, which is of tremendous help in difficult terrain. An interesting
albeit annoying effect with limited slip diffs is that sometimes the
wheels are turning (slipping) too slowly for the lock to engage. The
driver then engages the lock by going on the gas, forcing the wheels
to slip/spin faster, thereby allowing the diff to sense the loss of
traction and engaging.
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