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   Vehicle Recovery Points
Offroad Driving Techniques
Vehicle Recovery Techniques
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.
Why differential is used
Outside wheels describe a
larger arc than inside wheels
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.
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.).
To overcome this problem 4x4's come equipped with a center differential, which links the front and rear drive shafts.
A - Front Differential
B - Back Differential
C - Main Gearbox
D - Center Differential
E - Transfer Gearbox
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.
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.
Locking Differentials
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.
Limited Slip Differentials
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.