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Copyright © 1998-2008 H. Pietschmann.


Loss of traction & how ETS prevents spinning tires


If one of the four wheels loses traction, lets say the right rear, the rear differential senses this as less resistance and makes that wheel spin. A spinning wheel does not move the vehicle. The other wheel on the same axle (on firm ground with plenty of traction) gets the same amount of torque as the spinning wheel. Not much. Both wheels together do not produce enough torque to move the car. A differential always distributes torque equally. The amount of torque generated at the wheel with the least traction determines how much the other wheel will get - the same amount of very little. Little torque because traction is low at the spinning wheel.

If the vehicle is on absolutely level ground the two front wheels still have about equal traction (the advantage of 4WD) they receive equally high torque and move the vehicle.

Should the right rear wheel have lost traction due to a height difference, the left front wheel will also lose traction and spin even though the ground under the front axle is level. This diagonal wheel spin is very common.

This is valid for part time 4WD or full time 4WD with center differential locked.

If you have full time 4WD and your center differential is NOT locked, the center diff senses less resistance (traction) as well as the rear differential senses less traction (resistance) at one wheel - and makes that wheel spin. The center differential also always distributes torque equally. The amount of torque the axle with the least traction generates, determines how much the other axle will get - the same amount of very little. Little torque because traction is low. Most likely this amount of torque is not enough to move the vehicle. YOU ARE STUCK! see also ***

If the right rear AND the left front lose traction (this is the most common situation off-road where traction is usually lost on diagonally opposed wheels) your axle differentials distribute torque equally to left and right wheels. The amount of torque the wheels with the least traction generate determines how much the other wheels will get - the same amount of very little. Little torque because traction is low. The amount of torque that can be generated is dependent on the amount of traction (resistance).
So, the right rear and left front wheels are spinning.


This is valid for both part time 4WD and full time 4WD.

Limited slip differentials, the name says it all, they limit wheel slip.

They do not prevent wheel slip. They only limit wheel slip.

Traction is lost later than without limited slip. You will be stuck a little later. Traction will be lost when more torque is needed to move the car. Limited slip creates additional resistance and allows for a higher torque value to be produced at the wheel with less resistance.

Even though LS is a help in light duty situations, I think if you are serious about your options - go with real help (like 100% differential locks).

Fancy names like Trac-Loc are not differential locks - they are disguises for limited slip differentials. Also, be aware of false advertising by sales people.

Differential Lockers or diff locks are the only cure for the known shortcoming of differentials sometimes not distributing enough torque to the wheels as described above. Differential lockers and locking differentials are not the same. more...

(Only Dodge Powerwagon, Jeep Wrangler Rubicon and Mercedes-Benz G500 come standard with two diff locks).

Diff locks disable the differential's ability to sense resistance. By disabling (locking) the differential it is forced to make both wheels rotate at equal speeds - the amount of torque generated is now deteremined by each wheels's traction. The wheel with good traction will generate usually a lot of torque - most often enough to move the car.

DIFF LOCKS WILL KEEP YOU GOING! They will keep you from getting stuck.

BUT: Locking (disabling) the differential makes it impossible for wheels to roll at different speeds. Meaning, with lockers engaged on high traction surfaces you cannot make safe turns anymore, and on low traction surfaces turns will be very difficult.

This serious drawback of diff locks requires an educated driver who knows exactly when to lock the diff(s), usually only for a very brief moment, and when to unlock the diff to maintain steerability. Accidentally engaged diff locks on a paved winding mountain road can kill you! (Figure yourself why almost all manufacturers do not offer diff locks).

Aftermarket diff locks (ARB) are available for many makes and models of SUVs. They improve performance of any 4WD dramatically, and work as well as factory installed lockers (Mercedes/Dodge/Hummer/Jeep/Toyota/Mitsubishi) - but as I said, they belong only in hands of trained responsible drivers.

Diff locks will make the difference between STUCK and still DRIVING.

** Besides some Toyota (optional front & rear),Hummer (optional rear), Mitsubishi (optional rear) and Mercedes (Standard) most commercial and military 4WD vehicles have front and rear axle diff locks. The latest addition with 2 axle differential locks is the Jeep Wrangler "Rubicon".

Now the future of 4WD traction management, called 4ETS:

In Mercedes-Benz M-Class vehicles all differentials are "open", meaning no limited slip or diff lock is installed - saves money, saves weight. Plus: No negative effects on the steering. The ABS sensors of the M-Class are able to detect locking wheels as well as spinning wheels.

If the right rear wheels loses traction (remember this means less resistance for the diff), the rear differential makes the right rear spin. AFTER losing traction and spinning about 3mph faster than the opposing wheel the brake of the right rear wheel is activated - slowing it down to the speed of the other wheel. This braking action creates the SAME resistance as the traction at the other wheel. By creating equally high resistance with the brakes on the wheel with low traction the differential is then distributing equally high torque to both wheels.

In a way the rear differential is fooled to think both wheels have the same traction and now it will distribute an equal amount of high torque to each wheel, rather than sending little torque to both wheels.

The wheel with high traction will receive 50% of rear axle torque (25% of torque send from engine) and will transform it into moving action. The wheel with no traction, slowed down by the brake, receives the other 50% of rear axle torque which is transformed into heat.

Since the center differential (not visible like the axle diffs, it sits between the two driveshafts inside the transfer case) is also open (no locks, no limited slip, no viscous coupling), the front axle will receive very little torque from the moment the right rear wheel loses traction, because the center diff "thinks": "Less resistance in rear - So I'll reduce the torque to the front as well"

This means, the ML320 acts like a full time 4WD with an unlocked center diff for a split second until 4ETS steps in and keeps the vehicle moving.

If two wheels lose traction 4ETS slows down those two spinning wheels. Creating artificial traction (resistance). After the 4ETS action both diffs are fooled to think traction is equal on all four wheels, so they will send 25% of the available torque to each wheel. Transformed into moving action on the wheels with traction (resistance by ground friction / traction) - transformed into heat on the wheels with brake initiated resistance.

If three wheels lose traction, 4ETS will order three brakes to create resistance (brake friction) to fool the differentials.

The wheel with traction will receive its equal share of torque: 25%.
However, this might not be enough torque to move the car.

The other three wheels will receive their fair share of torque as well: 25% each. But the torque at these three wheels is transformed into heat instead of forward motion.