4WD only works good on level ground

On level, equally conditioned surface traction on all 4 tires is equal. All 4 tires can generate generate equal amounts of torque and forward movement

On pavement and other level surfaces all 4WD systems generate equally hightorque at all 4 wheels as long as the entire surface is identical. Only when the surface condition is equal under each tire, traction on all 4 wheels is equal. Only when traction is equal at each wheel equallyhigh torque can be generated. Only then all 4 tires rotate equally and enough torque is generated to move the car. Full time 4WD, part time 4WD, 4Hi, 4Lo - doesn't matter they all do the same thing.


Up to this point its easy.
When the scenario changes, 4WD becomes a more complex issue.

When the road (still level ground) has a mixed surface, like asphalt mixed with patches of mud or ice here and there, the scene changes.
If , lets say the front right, encounters a patch of ice - the front right will have less traction and the front right tire will eventually start slipping and spinning. Here is why:
Front left has good traction (asphalt coefficient of friction = 1.0) front right has bad traction (ice coefficient of friction = 0.1). The differential is a device that reacts to differences in resistance (traction) and makes it possible to have tires rotate at different speeds. Left side has high resistance - right side has very little resistance. The differential allows the right tire to rotate faster and spin. .A slipping wheel can only generate a small amount of torque - the one with more traction will get the same amount of torque. Both amounts together might not be enough to keep the car moving.

The differential was designed to do its job in a turn. To allow an inside tire (which puts up more resistance) rotate less and allow the outside tire (which puts up less resistance) to rotate faster. It is a "stupid" box. When it encounters different resistance (traction) due to surface conditions it "thinks" it is a turn. In this case it thinks the right tire with less resistance is on the outside of a turn needing more revolutions. That'll send the right wheel spinning faster than the left. Problem is, that an open differential always distributes torque equally and the slipping wheel determines the maximum of torque to be generated.
But not all is lost. Since your rear axle generates torque as well, the rear tires are able to keep the vehicle moving. But only if you drive a part time 4WD vehicle or a full time 4WD with center diff in locked position!

If the center differential of a full time 4WD were open (unlocked) in the above described situation the vehicle would not be able to maintain uninterrupted forward motion.

Here is why:
The front right wheel is slipping and spinning because it has less traction than the front left wheel. The front axle differential allowed the right wheel to rotate faster. The center differential (the one that sits inside the transfer case) acts the same way axle differentials do: It senses and reacts to resistance. It senses less resistance at the front differential. It reacts by allowing more and more rpm to go to the front axle and proportionally less to the rear. The resistance of the front axle determines how much torque can be generated (not much). The rear axle will get an equal amount of torque. If both amounts combined are less than needed to move the car - it is stuck.

Using the center diff-lock keeps the center diff from allowing independent rpm at the two driveshafts. It forces the driveshafts to rotate at equal speeeds (like a part time system would) to help maintain constant forward movement. With the center diff locked the total amount of torque generated (2 times the amount that can be generated at the point with the least resistance) is no longer determined by the axle with the least resistance - it is now determined by the axle with the most resistance. A car with center diff lock has a higher likelyhood to keep moving in mixed traction situations than one without.

To make traction management more transparent to the user and to avoid that the driver needs to make essential decisions to stop wheel spin, most manufacturers have incorporated mechanical (Torsen), mechanical/hydraulic (Gerodisc) or electronic (ETS, Haldex) traction management systems. However, they are not as capable as a manually locked center diff. That is why the best 4x4 on the market today, offer a combination of automatic as well as driver activated traction management (Mercedes G500, Mitsubishi etc.)

*Warning: motor sports and activities are potentially dangerous. The information listed in this web site does not hold its author or representatives liable for any damage or injuries sustained.

Difference between differential lock and limited slip differential?

The differential is essentially one link in the chain of components that transmit power from a car or truck's engine to the wheels. The differential transmits different amounts of power to the wheels at a right angle from the drive shaft to one of the car's axles.

A locking differential will transmit the same amount of power to both wheels on the axle - which is very useful in 4WD applications where a truck might be stuck and have problems getting out of deep mud or snow.

Limited slip and open differentials will allow the wheels to get varying amounts of power from the engine, affecting the amount of traction your wheels can attain. For instance if you are in a 4x4 truck and you are stuck on some rocks with one wheel in the air, an open differential senses that one wheel of the axle isn't getting much traction and will send more power to the wheel in the air, which is ineffective. But if you have a locker, it will apply the same amount of power to both and you'll be able to get free.

Longer springs are better for traction

More wheel travel actually maintains traction longer

Let's assume a vehicle weighs 4,000 lbs - that means each tire is pressed down with 1,000 lbs.

A spring will expand when it supports less than 1/4 of the vehicle weight (ie when tire rolls into a small depression - visually moving down and out of the fender well). As a result the other spring on the same axle will have to take over the extra load, and compresses more (visually that tire will move up into the fender well).

Since the traction is defined by friction (between tire and ground) plus weight, traction will be less when a tire carries less weight. Because an expanded spring does not press a tire as hard onto the ground. Traction maximum is reached when a spring is fully compressed - the point of absolutely no traction is reached when a spring is fully extended.
Therefore it is fair to say, that short springs (little wheel travel, usually on vehicles with independent suspension) will make a vehicle lose traction sooner than long springs (plenty of wheel travel, usually on vehicles with live axles).

This explains why vehicles with plenty of wheel travel (Range Rover) maintain traction longer than vehicles with little wheel travel. Unless some sort of traction management (diff locks or traction control, etc) is present, a vehicle with less wheel travel will be stuck while the one with more wheel travel is still driving.
The longer spring in the picture above has more than twice the down travel of the shorter spring. At the point where the short spring loses ground contact and traction completely (reference line) the longer spring still carries more than 500 lbs and still maintains half of its traction. So, vehicles with longer wheel travel maintain sufficient traction longer - they will get stuck later than a car with marginal wheel travel.
BUT.... the simple fact that a wheel is still on the ground and has some amount of traction does not necessarily mean you can continue driving.

The wheel with less traction determines how much torque can be generated at an axle. The wheel with the increased traction will get the same amount of torque as the wheel with less traction before it starts spinning. So, if the combined amount of torque (double of what is possible at the wheel with less traction) is sufficient to move the truck - the truck moves (in a 4WD the amount of torque of the other axle is added of course).
If traction at the wheels with less traction is not high enough to create sufficient torque for movement, one or more wheels are starting to slip and spin - they are the ones with less traction and they are usually diagonally opposed

We would then say "I have lost traction" - which is not entirely correct since you have some traction, but not enough for the amount of torque needed to move the truck.

So, when Range Rover claims that a RR will not lose traction because it's wheels due to the long wheel travel almost never lose ground contact is correct. However, the fact that a wheel is still on the ground and has some traction does not mean the Rover is still moving.

If the amount of torque determined by the wheel(s) still in contact with the ground but with reduced traction, is not sufficient to move the car - it will be stuck.

So, not a wheel in the air gets you stuck, one or two wheels with insufficient traction will do the same thing.

• On level equally dry surface, traction on all 4 tires is about equal.
• On level equally wet surface traction on all 4 tires is equal as well.

Two components are involved in creating traction:

1. A specific friction coefficient between two materials (rubber and dry asphalt, rubber and wet asphalt, rubber and dirt, rubber and mud, rubber and mud, etc. - that number varies from 1.0 to 0.1 - 1.0 is best) -
2. the weight pressing the tire to the ground (more weight is better).

When to engage lockers with manual diff locks?

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 –

On dry pavement - Never! You’ll lose all ability to steer. Bad!

On pavement with snow and ice engage your locker(s) whenever you think there is a need to keep the tires from spinning as long as you are driving straight. Disengage the front way before you would get into a turn, or severe understeer would occur.

Lockers can be engaged while the vehicle is moving - at any speed. But like in difficult off-road stuff you should be moving rather slowly on snow and ice. Especially when the surface is uneven, loss of traction is likely. In snow an inch of downtravel of one tire could result in a spinning tire. So, as a good driver you should engage the locker(s) proactively before wheelspin occurs.
Remember, lockers were designed to prevent wheelspin. All the new electronic stuff is reactive. Wheelspin has to happen first and then the system tries to rescue what’s lost. Manual lockers are 100 times better.

Proper use of lockers off-road provides enough material for a small book - in fact I am working on one. As a good driver you should engage the locker(s) proactively before wheelspin occurs. Especially when the surface is uneven (and isn't that part of the off-road definition?) loss of traction is likely. Depending on your spring set up (long and soft or short and hard) down travel of only one tire could result in two spinning tires - the diagonally opposed tire of the one that encounters downtravel will also move out of the fenderwell and could lose traction as well.

Even the best driver sometimes goofs. Forgets to lock the diff(s). No worries! As long as the spinning tire(s) are not much faster than the slower tire(s) or your rpm are around 1000 with a manual and below 1500 with an automatic you can still safely lock the diff(s).

The diff locks should not be engaged once you leave pavement just to have them on, in case someting could happen during the next hour or so. Lockers need to be used briefly and strategically when needed. A moment here and there should be enough. I sometimes use them only for a second or so - just enough to maintain or regain traction. I switch them off so quickly because I want to maintain maximum steering capability at all times.

Special attention needs to be put on whether to use the front or the rear locker - or both.

The Mercedes axle diff lock is a dog clutch setup with 5 big strong teeth. As long as the difference in speed between both axles sides is not too much, the teeth of the locker will find their way safely into the groves.
At 1000 rpm in first gear low range each tire rotates at 25rpm - so, even if one tire starts rotating faster, it will only be10 or 20 rpm more. Not much. The diff lock mechanism can take that.
However, if you panic when losing traction and step on the gas more (it is most people’s gut reaction) and then engage the diff lock(s) you might do some internal damage.

4*4 Roading

Difference between Full time Four Wheel Drive (4WD) All Wheel Drive (AWD) and automatic All Wheel Drive (auto AWD)?

Full time 4WD, also called permanent 4WD, (not to be confused with: part-time 4WD) is a system that powers all four wheels at all times and it can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time. 2WD is not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque when the ground is level with a uniform surface. Driver has a choice of a "4-high" - 4H (that's your every day setting) and "4-low" - 4L. Full time 4WD vehicles work very well on-road and are very capable off-road.

When "4-low" is selected the wheels create substantially more torque (on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).

Important: "4-low" does not create more traction - it creates more torque at slower speeds and that can be detrimental when the ground is slippery. Slipping tires are more likely in "low" than in "high"! On snowy, icy roads "low" would be a bad choice - some really deep snow, however, puts up so much resistance that "low" is needed to push forward. You see, using 4WD is not an easy task.

The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed. All wheel drive (AWD) is almost the same thing as full time 4WD - it is a system that powers all four wheels of a vehicle at all times as well. It can be used full time on all surfaces including pavement like full time 4WD. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range", AWD vehicles are much less capable in off-road settings than full time 4WD vehicles, but work perfectly well on-road.

Automatic AWD system is the newest kid on the block. PR agency generated names like "Real Time 4WD", "intelligent AWD" or "active AWD" are hiding the fact that automatic AWD is essentially a sophisticated 2WD system. Automatic AWD is NOT powering all 4 wheels all the time. Only on rare occasions all 4 wheels will be powered very briefly. Since AWD is only engaged on rare occasions, sometimes automatic AWD is falsely called "part time 4WD". Engineers refer to this system as an "on demand system". Since it is based on an existing 2WD steup, it is also referred to as a "hang on" solution.

Here is how they work: Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes some of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first and then the other axle will be added and try to keep the car moving and stable. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD. So, for a brief moment you had AWD.

Automatic AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. In fact, automatic AWD is almost useless beyond pavement. However, automatic AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.