High/Low Range
To enable a 4WD to travel at lower speeds while traveling on rough terrain it needs lower gear ratios. Not all 4WD’s have low range gearing and this restricts their ability to tackle rough terrain. However 4WD’s that lack low range gearing are generally not built for severe off-road conditions or sometimes have a "crawler" 1st gear to compensate for the lack of low range gearing.
The high range ratios in 4WD mode are the same as the gear ratios in 2WD. When low range 4WD is selected, the gear ratios are approximately half that of high range, although the exact ratio varies for each vehicle manufacturer.
For example this means that if an engine speed of 3000 rpm in high range fourth gear is 65 mph, then in low range at the same engine speed and the same gear, the speed would be around 30 mph.
Some points to note about low range gearing are:

You cannot select low range in 2WD mode in most cases.
You do not have to use low range as soon as you put the vehicle in 4WD,
but only if the terrain requires it.
On most vehicles you have to be stationary when changing from high to
low range, check your owners manual for your particular vehicle.

A handy hint when reversing with your vehicle while towing is to select low range 4WD to be able to move very slowly without having to slip the clutch. However you can only do this if you have a constant 4WD or your part-time 4WD is fitted with free-wheeling hubs AND they are not locked in, otherwise you will cause transmission windup.

Diff' Locks.

The differential action while essential for normal driving can leave a 4WD stuck with just two spinning wheels. The most obvious cure is a diff' lock. This is a dog-clutch of some kind that prevents the differential action when it is engaged. The most common arrangement is a dog clutch on one half-shaft, which can lock it to the diff' centre. This also locks the other half-shaft, indirectly, via the planet-gears. The dog clutch is engaged by some sort of external actuator. This system is mechanically simple. The Maxi-Drive (right) is of this type.

Many full-time 4WD vehicles use a (centre) diff' lock of this kind in the transfer-case, although we then have front and rear transfer-case output shafts in place of axle half-shafts. It is important to note that a centre-diff' lock does not lock the axle diff's.

The Roberts (now ARB) diff' lock is internal to the diff' centre. A dog clutch locks the side-gears to the centre. The clutch is engaged by an annular piston driven by compressed air and is disengaged by springs. The problem is getting the air to the piston inside the rotating centre! The air is carried in from outside through a rotating seal, similar to devices used to inflate tyres on the move on some very heavy duty offroad vehicles and some military vehicles. The result is ingenious but complex. A disadvantage of the design is that the "warning light" shows when the diff' is intended to be locked not when it is actually locked.

The simplest diff' lock of all is made by McNamara. A lengthened half-shaft can be slid in or out by a distance of a couple of inches by means of a spring and bolt on the modified wheel hub (right). The half-shafts are splined to the diff' side-gears, and also to the hub driving plates on fully-floating hubs as used on most serious 4WDs. When the lengthened half-shaft is moved inwards, it protrudes through the side-gear and engages with splines in a modified `spider' that carries the four planet-gears within the diff' centre. This locks the half-shaft to the centre and hence locks the diff'. It is necessary to stop and operate this diff' lock with a spanner, and it can only be fitted to the rear axle of a vehicle with fully-floating axles, but it is very simple which is no bad thing.

In some ways a diff' lock is the ultimate traction device. All wheels revolve at the same speed when engaged. A vehicle with locks on all axles has traction if just one wheel does. Unfortunately a diff' lock is on or off; there is no in-between. This can make the vehicle understeer (tend to go straight ahead in corners) or veer suddenly sideways if one side looses traction, on ice say. It also means that all of the engine's torque, amplified by low-range, can go through one half-shaft and wheel. The components had better be able to stand the strain. Some diff' locks come with strengthened half-shafts and other components for this reason.

The perfect differential would provide little or no resistance to the differential action when the difference in rotational speed between the output shafts is small, ie. when cornering. It would provide increasing resistance as the difference in speed increases, ie. under wheel spin. Several limited-slip differentials (LSD) approximate this ideal to varying degrees.
The simplest LSDs provide some friction between the side-gears and the diff' centre. This can take the form of spring-loaded plates alternately keyed to the side-gears and to the centre and pressed against each other. The plates are naturally liable to heating and to wear in heavy use and may then become ineffective. The friction also resists the differential action at even low speed differences - and may affect cornering.
Mercedes and Porsche have developed more intelligent (and expensive) systems where the limited-slip plates are pressed together under hydraulic pressure when electronic sensors detect wheel spin.
The viscous-coupling also employs plates but these are not in physical contact with each other. Instead they are in a sealed drum full of viscous fluid (silicone based). They can contra-rotate freely at low speeds, but the resistance increases very rapidly with the speed difference. The precise characteristics of the device can be controlled by the choice of viscous fluid and by drilling holes in the plates. Range-Rovers from the late 1980s-on use a viscous-coupling in the centre-diff'. (I would be interested to know if a Range-Rover parked on a very steep, loose hill is liable to creep if the rear wheels slip.) Viscous-couplings were developed by Ferguson (of the P99 F1 car and the Jensen FF 4WD sports coupe). They are also used in some front-wheel drive cars.
Almost all open diff's use bevel gears within the centre, but this is not the only possible arrangement. Some early cars (maybe Austin 7 ?) instead used pairs of plain gears to link the two side-gears. You can see that pairs of gears are needed to allow the side-gears to rotate in opposite directions relative to the centre. All of these gears are mounted within the centre. Some modern LSDs use a variation on this theme. Pairs of gears are used but these are mounted in the centre in such a way as to provide a great deal of friction if they rotate at high speed under load. Unfortunately there is little friction under no load, ie. if one wheel is actually airborne.
Yet another arrangement uses the fact that a fine-pitch worm gear cannot be driven backwards, and that a coarse-pitch worm gear can only be driven backwards with difficulty. These LSDs use pairs of coarse worm gears, mounted in the centre, to drive the side-gears and hence the wheels. Each worm gear contra-rotates with its twin through end gears.

Lockers.

The Detroit locker is quite different from other differentials. The planet-gears and side-gears are replaced by what appear to be three plates. The middle plate is driven by the crown wheel. A number of cams pass through the middle plate and drive the side plates. These cams will allow the outer, faster-rotating wheel and its plate to overtake the middle plate. This is not a simple ratchet or free-wheel because, and this bit always seems like magic, the device also works correctly in reverse.
Lockers transfer torque to the inner wheel in a corner and the action is not always smooth so they affect steering characteristics and are often advised against for SWB vehicles and for "normal applications". However...... Mark Ritter fitted a lock-rite locker to his 1994 Land-Rover Discovery (full-time 4WD):

On the street there is no change except for a mild clicking noise as the outside wheel unlocks so that it can travel faster in a turn. [And off road] no tire spin or drama, and much more control.
[...]
I have had no "twitchiness" on the road. I think this is due in part to the Disco's full-time 4wd. The twitchiness that you have heard about is generated by the power going to the inside rear wheel when the outside rear unlocks. In a two wheel drive situation each rear wheel would provide 50% power to move the car until you come to a corner. As soon as the outside wheel unlocks, 100% of the power is now going to the inside wheel which would tend to cause understeer. With full-time 4wd the inside rear's share would go from 25% to 33% so the effects are minimal. About the only thing you notice is that it takes a little more pressure on the steering wheel to negotiate a turn.

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05 KC SE Smoke
01 Suzuki 600 GSF
88 Fiero 2M4

Pretty cool.

I hope this finds it way to the knowledge base too.

I will. I want to clean it up a bit, and find a little more info to add to it.

Someone asked about it at CT today, so I dug this up...

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05 KC SE Smoke
01 Suzuki 600 GSF
88 Fiero 2M4

I put this in the knowledge base 'cause I thought about it today. Let me know if you have additions of corrections to it.

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05 KC SE Smoke
01 Suzuki 600 GSF
88 Fiero 2M4

Looks good there. It's late so I didn't critique it.

So late it's early!

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05 KC SE Smoke
01 Suzuki 600 GSF
88 Fiero 2M4

i just found this on a google search for "what is an e-lock axle" am I awesome or what?

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05 KC SE Smoke
01 Suzuki 600 GSF
88 Fiero 2M4

haha....good job steveO!

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'04 2wd with PRG stuff......



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