When to engage lockers - for example on your Mercedes G 
valid of course also on Jeep Wrangler Rubicon or other trucks with manual diff locks

by the way, a differential lock and a locking diffeential are not the same thing

Jeep 101

Answers to your most often
asked 4WD questions

Here is a good example of when and why to engage differential locks.

by the way, a differential lock and a locking diffeential are not the same thing

It is not when tires get airborne that you need lockers.
It is a common misconception and LandRover harped on that one for years by saying "with longer wheel travel your tires stay on the ground longer and you don't lose traction".

Fact is that tires will start spinning when the amount of torque generated with your gas foot is higher than the amount of traction your tires have.

For example (I'll keep it as simple as possible even if the engineers don't like it):

You drive with all 4 wheels on level ground. A 10" high rock catches your eye. You have to drive over it. We all understand. You plan on driving over it with your left wheels.

Before you hit the rock and all tires have equal high traction, all can absorb an equal high amount of torque.

Location front axle:
Left wheel gets to the top of the rock - spring is compressed (carrying more than 1/2 of the vehicle's weight)
Visual: tire sits already inside the fender well
Result: more traction than when axle was level (weight plus friction between rubber and ground = traction)

Right wheel rolls on flat ground - spring is extended (carrying less than 1/2 of the vehicle's weight)
Visual: tire moved way down from original position
Result: less traction than when axle was level

Now it gets tricky.
A rule first: The unlocked differential always distributes torque equally left and right. It allows for unequal velocity - but torque distribution is always equal.

The right wheel in its new situation can not deal with the same amount of torque as before. Depending on how hard you are on the gas it has an infinite range of reaction: From slipping slightly and rotating barely noticeable faster than the left to spinning twice as fast as before and the left stops moving completely.

The locked diff would have guaranteed that both wheels rotate at the same speed. Well, you forgot to hit the locker.

If the right slips a little but the vehicle keeps moving you are off the hook. Just remember next time to lock before you climb.

In case you are so hard on the gas that the right spins and the vehicle becomes immobile, you have a couple of choices choices:
A. Reduce the gas so far as to eliminate the wheel spin and most likely the car will keep moving.
B. Get off the gas and on the brake, engage locker and proceed driving carefully.

By the way, the rear two tires are of no help in this situation, because in the rear the diagonally opposed tire (left rear) will also slip and spin. Always! No exceptions.

More about choice A. :
Since the open differential always distributes torque equally, the left wheel will get exactly as much torque as you can generate on the right up to the threshhold before it starts spinning. If the amount of torque left and right together (plus rear left and right together) is enough to move the car - it will move.
For example, on an uphill drive, more torque is needed to move the car than on flat ground (Duh!), and if you have to drive up the same height rock while going uphill, the amount of torque generated right before the threshhold of spinning is most likely not enough to move the car.
That's why you can get through rough stuff on level ground almost with a Honda CRV - but need lockers on both axles for steep climbs.

Locking your diff - more..

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