We have four wheel drive to maximise our chances of maintaining traction, but do you know why maintaining traction is important? Do you understands what traction is all about? And do you know how four wheel drive works, and what it's trying to do for us, and why we have it in the first place?
The article 'Get a Grip', is all about the concept of traction, and this article compliments it. Driving a vehicle, we are not that concerned with the mechanics that are happening, only what the vehicle is doing for us, and as long as its moving, and moving in the direction we want, at the kind of speed we are comfortable with, that's pretty much all that matters.
It's when things start getting difficult, and it's not going quite where we want it, or at the speed we want it, that it all starts to get concerning. From the drivers seat; the important thing is traction. Traction is all about the vehicles grip on the surface it's driving on, and that is what lets us push it in whatever direction we'd like to go.
The engine, is what's giving us the 'push', the steering, is what's directing that push, and the traction, or grip between the surface and wheel, is what's letting us use that push to get or keep moving.
Fundamental idea from 'Get a Grip'; traction is a limit, the point at which a wheel will stop gripping the surface. It sets the limit to how much motive force we can transmit between the wheel and the surface.
The transmission, is the thing that takes the push from the engine, and delivers it to the wheel or wheels, depending on how many of them we have, and how many we choose to send 'drive' to.
It 'transmits' power.
But; most of the time, the 'transmission system' isn't just a shaft or belt or chain, taking power from one place and putting it somewhere else; there are usually cogs and gears and couplings and things that do 'something' to the power between where its made and where its used, and in most cases, there's levers or buttons and things we can waggle or press to change some of the things that the transmission is doing!
So there's obviously a bit more to it. But, lets start in the middle!
The usual answer to this one, is "It gives you more traction; better grip". Err..... Hmmmm.... I REALLY don't like that answer, it's not entirely wrong, but its right for the wrong reasons; so I'd better explain.
Traction, grip, and how much of it you might have is not effected in any way by how many driven wheels you have. The amount of grip that there is, is entirely down to the properties of the surface and the wheel, and the force holding them together; usually the weight of the vehicle.
Little illustration; Front wheel drive car, with a rear wheel hand brake. We've all done it, set off with the hand brake still on. What happens?
Front wheels try and pull the car forward. Rear wheels try holding it back. Front wheels are trying to transmit a motive force, rear wheels try and transmit a braking force.
There are three possible outcomes; Front wheels win, and drag the locked rear wheels along. Rear wheels win, and hold the car stationary while the front wheels spin, smoking the tyres. Neither wheels win; the car moves forwards with the rear wheels dragging and the front wheels spinning; or the engine stalls. What happens depends on grip. If the rear wheels have more grip, then they win. If the front have more grip, then they win.
Thing is, the amount of grip is fixed, and in that example we were trying to do two things with it, push and pull and the same time.
OK, lets try a 'burn out'. Same front wheel drive car, at a set of traffic lights, and we give it some wellie, and drop the clutch a bit fiercely and deliver a lot of motive force to the wheels quite suddenly, in order to deliberately exceed the limit of grip, and make the wheels spin. It's like starting with the hand brake on and the hand brake winning; only the hand brake isn't on!
Back wheels have loads of grip, only it's not being used for anything, they are just sat there, not doing very much!
All our motive force is at the front wheels, and has exceeded the limit of traction, so instead of pushing us forwards, it's just spinning the wheels and making the tyres smoke,
Now, if we had four wheel drive, our transmission could deliver some motive force to the rear wheels, and make use of some of the grip that they have, but aren't using to push us along.
Pretty simple idea. But the point is, the transmission doesn't 'Find' us any traction or give us any more grip than we had to start with, just lets us use more effectively, what grip we had to begin with, to transmit motive force.
The Four wheel Drive transmission, then is all about 'power distribution', and putting motive force to where it might be most effective, in order that we can better utilise whatever traction we have or can find.
There are a number of types of four wheel drive, either permanent or part time and these days 'smart'.
The old series Land Rovers have part time four wheel drive, and normally only the rear wheels are driven. Select low range or depress the yellow know in high range though, and you engage a 'dog' clutch, that connects drive to the front axle to give four wheel drive.
Defender's Range Rover's and Discoveries, share a permanent four wheel drive system, with both front and back axles always driven from the engine in high, whether in High or Low range gears
The latest generation of Land Rovers though, have 'smart' four wheel drive, which can send the power to whatever wheel or wheels it thinks would be most useful, at any given time. This means that the car could be one, two three or four wheel drive, at any moment, depending on what the systems 'brain' thinks is best at the time.
But, the transmission is distributing the engine's power to all of the driven wheels, and it is pretty useful to have some means of controlling or effecting that distribution, or balance of how the power is divided between those wheels, whether it's done by the driver waggling levers or twiddling buttons, or a black box making the decisions and sending electric signals to do the same thing.
What you NEED to know about is a thing called the 'Differential Gear Set', or 'diff' for short.
A differential is a clever type of planetary gear set, which takes drive from one shaft and transmits it to a pair of half shafts, and allows each shaft to rotate at different speeds. And because it's a very clever piece of kit, and because these so much to know about them, I have written an entire article all about them, called, imaginatively, 'Differentials & Viscous Couplings'
The differential, or viscous coupling, is very important to the four wheel drive transmission. They work in different ways and have different properties, but both do the same thing, they take shaft power, from a single input, and split that power to two outputs.
the clever bit about them, though, is that they 'differentiate' between the two outputs and allow them to turn at different speeds to each other.
I could get all bogged down, explaining all this here, but if you need or want to know what the diffs are doing, go look at , 'Differentials & Viscous Couplings'. The important bits of that for us are that we are dealing with power distribution and the kind of power we are trying to distribute is shaft power. If you want or need to know more about that, then you might want to look at my articles, Power; Torque; Revs & Thrust', or even 'Making (more?) Power', which deal with the topic in greater detail.
Keeping things brief, Newton tells us that Power is rate of work done, and that work done is force times distance moved. So with a lot of maths, applying that to a rotational situation, we get to the highly useful equation:-
Power = Torque x Revs
Where power is power, torque is rotational force, and revs is rotational speed. Remember that, it is everything we are dealing with.
Engine, the input to our transmission system is providing us with a fixed quantity of shaft power, which is provided on a shaft as a torque and a speed.
We can change the torque and the speed we have by playing with gears. If we select a low gear, then that takes the power we are provided and delivers it at low revs, but with a lot of torque. Select a high one, same power, but delivered as more revs and less torque.
If we 'split' the power, then, it's the same deal, we can deliver it at higher revs and lower torque or higher torque and lower revs; but we only have so much power to play with, so we are only going to be sending a proportion of the power we have to each destination.
And depending on circumstance, we might not want to send the same amount of power to each destination, and what power we might want to send at different revs or at different torques.
Which is enough to make your brain hurt, and we haven't even started yet! Ok, lets start at the beginning, and make things simple!
Right, one engine, two axles, four wheels. So, lets take the shaft power from the engine, and split it between the two axles. We do this simply, by putting a propeller shaft between the two axles, and driving that with the engine. We have no differential or viscous coupling between the axles, just a rigid shaft. Now, we have to split the power between the wheels on each axle, so again, we use a rigid shaft between each wheel.
All four wheels will now rotate at the same speed, as they are all rigidly connected with nothing between them to allow them to rotate at different speeds.
Engine provides shaft power; torque times revs. Wheels go round at whatever revs they are driven, each getting an equal proportion of the engines delivered torque. Four wheels, each getting the same amount, that will be 1/4 or 25% then. 50-50 split between the axles, 50-50 split of that between the wheels on each axle.
Because the drive is rigid though, there is no way of changing that balance. It is what we get whether we want it or not, or whether any other 'distribution' might be better.
This is very useful. When a car moves in a straight line all four wheels turn at the same speed, but when you go round a corner, they don't. The wheels on the inside of the corner need to travel slower, while the wheels on the outside need to travel faster.
So having a differential between the wheels on either side of the car is useful because it allows them to do just that.
Similarly, when you go round a corner, the rear wheels will try and follow a tighter radius than the front axle, so both rear wheels will try to go slower than both the front wheels.
So having a differential between the front and back axles would be a good idea too.
And in fact, a lot of 4x4's do just that and have a 'centre diff'. Old Series Land Rovers, some of the other 4x4's and the latest offerings from Land Rover, however don't, but don't worry about that just yet.
Any way, there are alternatives to differentials, these are things called viscose couplings. Basically they do the same thing, but instead of using a clever gear set to allow the shafts to rotate at different speeds, they use a fluid, and I'll look at those in a minute.
If you were to look at a differential gear, you have three shafts coming in together in a 'T' shape. The two driven 'half-shafts', are the branches of the T and the driving or 'prop-shaft', is the stalk or leg of the T. At the junction you have the differential itself. Basically a drum, and the half-shafts come through bearings in the end of the drum. The shafts have a gear on the end to let the differential drive them, and inside the drum are the planetary gears that let the differential work. Around the outside of the drum, is the ring gear, which is turned by the prop-shaft.
So, as long as both wheels want to go round at the same speed, the prop-shaft turns the drum, and that pulls the half-shafts round with it.
When the half-shafts want to turn at different speeds, however, the prop-shaft carries on turning the drum, but the gears inside let one half-shaft turn slower and one half-shaft turn faster, relative to the drum.
In a viscose coupling, you have a similar arrangement. The 'diff' is still a drum, and it is still turned by a ring gear driven by the prop-shaft. But instead of there being gears inside the drum, there is a thick oil, and on the ends of the half-shafts, instead of gears, there are paddles or plates.
Now, without getting too bogged down in technical detail, as long as the wheels want to turn at the same speed as the drum, then the whole drum turns as a unit, and nothing happens inside. But when the wheels want to turn at different speeds, then the paddles on the end of the half-shafts will try and stir up the oil inside. Now, by clever design, what the coupling does, is use the 'drag' in the oil from the paddle that wants to turn slower, to drive the shaft that wants to go faster.
This, is no HUGE advantage over a conventional planetary gear differential, except that having got rid of the gears, and got a fluid system to do the same job, you use some clever paddle design to do things that are an advantage.
The first advantage, is that you can not only effect the balance of speed between the two half-shafts, but you can also effect the balance of 'torque'.
In a conventional differential, you are really limited to an equal torque split between the two shafts, which on an axle differential is not a problem, because you wouldn't want one wheel to pull harder than the other, any way, or the torque effect would try and steer the vehicle.
But, in a centre differential, between front & rear axles, it can have advantages. If you have a torque split that favours the rear axle, then the car will tend to behave more like a rear wheel drive car. If you have a torque split that favours the front axle, then it will tend to behave more like a front wheel drive car.
Except, that its not QUITE that simple, because, as we already know, the rear wheels tend to follow the shorter path through a corner, so even with a 50/50 torque split, the car will tend to behave more like a rear wheel drive car, with the back wheels trying to push the fronts.
If you use a viscous coupling, with a modified torque split though, you can put a bit of extra torque to the front wheels, and with a front torque bias between 50/50 and 60/40, you get that neutral balance where the fronts pull as hard as the backs push.
But, for the moment, this is a minor advantage, exploited mainly by high performance road and rally cars. More important in an all-terrain vehicle is not how neutral the car feels in corners, but how it manages transmitting motive force to the wheels on a loose surface.
So, the second property of a viscous coupling, is the capability to limit 'slip'. You can actually do this with a traditional planetary differential too, in a number of ways, so it is a useful feature, worth looking at.
Right, going back to a planetary differential. They suffer a particular problem in that they allow the motive force being put into them to take the path of least resistance.
The force put into the differential by prop-shaft is shared onto each of the half shafts depending on the resistance they have to turning. So, if one of the two wheels on an axle has little or no traction, or grip on the road, then the differential will allow all the motive force on the differential to drive that one wheel, and not the other.
The result is, that is that if you have three 'loose' diffs in a four wheel drive transmission, you only need to loose traction at one wheel, to get stuck.
This is NOT helpful, and in fact, probably less helpful than having just two wheel drive. Imagine that you are going up a steep climb, and half way up is a muddy rivulet. Its not wide, but just big enough to stop a wheel.
Now, given a two wheel drive vehicle, you would start to climb, the rear wheels, with good traction, would push the fronts through the sticky patch, and then, the rear wheel would get stuck in the mud.
Given four wheel drive, though, having got the front axle through the mud and back onto firm ground, they could then pull the rear axle through.
Except that you wouldn't get that far. As your front wheels reached the sticky section, being driven, and so coupled to the transmission by a loose diff, as soon as one of them started spinning, all the engine's motive force would be wasted turning that one wheel in the mud, and no force would go to the back wheels to push you through.
But there is a solution. If you got rid of the centre diff and had a solid shaft instead, then each axle would get an equal amount of the engines force, and when the front wheels reached the morass, the backs would push it through, despite any spinning wheel, then when the rears got bogged down, the fronts would repay the favour and pull the backs out.
Which is great, except that we want the centre diff so that when we go round corners, the rear wheels can go slower than the fronts. This gives us a dilemma. If we don't let the rear wheels go slower than the fronts, we can get into some quite serious trouble. The major one being the phenomenon of 'Wind Up'. But I'll explain that in more detail later.
So, how do you get around this problem? You want diffs to go round corners, but they aren't always that useful. Is there some way that we can get the differential effect when we need it, but not have it when we dont?
Of course there is, and that is what I'm getting to. This is the device called a 'diff-lock'. Now, if you have a manual gearbox, you will be familiar with the clutch. In it's simplest terms it is a transmission switch. Engage it and you get drive between two shafts; the crankshaft in the engine, and the main-shaft in the gearbox. Disengage it, and the two can turn independently.
So, in exactly the same way, you can put a clutch between the two 'half-shafts' inside the differential, but work them the opposite way to the clutch on the engine, so that normally the clutch is disengaged, allowing the differential to work, but when you are in a sticky situation and likely to have a single wheel spinning, you engage the clutch, locking the two half-shafts together.
Now, that is the most common kind of diff-lock, and is used mainly on the centre diff of 4x4's with permanent four wheel drive like the Range rover and Defender, and a lot of modern 4x4's by other makers.
But, if we go back to the viscous coupling, we can use fluid properties to refine the idea a bit. This is the idea of the 'proportional' Limited Slip Differential, or LSD.
Again, you can get a proportional LSD that uses planetary gears, and is based on that technology, rather than a viscous coupling, but without going into the detail of how they work, and explaining the differences, its easier to merely explain how the viscous coupling gives an LSD effect, and then work back from there.
Right. We have got the idea of the planetary gear differential, and have realised how putting a clutch between the too half shafts of either, so as to eliminate the differential action might be beneficial, can we do the same thing with a viscous coupling? Simple answer, 'yes'.
BUT, why go to the bother of trying to construct a mechanical clutch into the system? If you have an automatic transmission, that doesn't have a clutch between the engine and gearbox, and you may be familiar with a thing called the fluid flywheel, or 'torque converter' it has instead.
Now, a torque converter is essentially a drum, full of oil, bolted onto the engine's flywheel, where the clutch would normally be. Inside the drum is a paddle attached to the gearbox's mainshaft. As the drum rotates, so it churns the oil inside, dragging the paddle round. As long as the speed of the drum is fairly low, though, it doesn't transmit much force though, and the paddle can stay stationary. As speed increased though, so the force increases, and it starts to drag the paddle faster, but not at the same speed, until the drum is turning so fast that the paddle and drum are effectively 'locked'
This is useful. First, it does exactly what you do with a manual clutch, but automatically. That is, you don't slam the clutch in and out, you ease it in and out, feeling the plates grip, and controlling the amount of 'slip' between the two. Secondly, its remarkably similar to what you have already inside a viscous coupling.
It doesn't take a huge leap of the imagination to realise that rather than putting a mechanical clutch between the half shafts in a diff, you could put in a torque converter, and that is exactly what a viscous coupling does.
By clever arrangement of the paddles and plates inside the drum, it can be arranged so that when one half shaft wants to turn slower, the drag will try and drive the other one faster, but, only as long as the wheels don't want to travel at vastly different speeds. If they do, than the oil can be made to effectively lock the two shafts together completely.
And that gives you a viscous proportional limited slip differential, and the basis for a 'smart' four wheel drive transmission.
Now, going back to the planetary gear differential, the proportionality of the locking effect provided by a viscous clutch can be useful. Apart from the fact that it has an automatic action, it is also progressive.
Limited Slip Differentials have been around for a long time, and haven't been restricted to all terrain vehicles. They have in fact mainly been used on racing cars and rally cars or high performance road cars, but until the advent of the viscous coupling in the mid nineteen seventies, they were relatively crude. They tended to use mechanical solutions, usually spring loaded friction plates between the half shafts, which meant that they tended to behave like a solid shaft until they were forced to slip, and then were almost unconstrained. This was one of the main reasons that LSD's were not widely used on 4x4's.
However, with the advent of the viscous coupling, a hybrid of the two technologies has appeared, and that is a viscous proportional limited slip planetary gear differential. Essentially an ordinary planetary diff, with plates on the ends of the half shafts inboard of the bevel gear, running in the oil that lubricates the diff. And these plates work just like a fluid flywheel.
So simple is this idea, that in fact, you can buy a kit to retrospectively fit it to a Land Rover diff, but I'll look at that in more detail later.
Right, quick recap:-
That, I think explains what differentials and viscous couplings are, and what they are for.
If we follow a little of 4x4 evolution, the earliest systems were 'part time' four wheel drive, like the Series Land Rovers. Viscous couplings weren't available to the designers of these vehicles, and neither were locking diffs. So they had to choose between planetary diffs or locked shafts. The compromise they chose was to use loose diffs in the axles so that the near-side and offside wheels could rotate at different speeds, but then used a solid shaft between the front and rear axles so that a spinning wheel on one axle would at least not prevent drive being transmitted through the other axle.
Now, it was a compromise, but in actuality it wasn't a bad one, as long as you could de-couple the front axle from the transmission on hard surfaces. Of course this meant that most of the time, like on the road, the vehicle was only two wheel drive, but until things became a bit sticky that was no real drawback, and saved the wind up problems.
When locking differentials became available and affordable though, it made permanent four wheel drive viable. The next generation of 4x4's like the Range Rover were fitted with a permanent four wheel drive system that still used loose diffs in the axles, but had a locking centre diff, so that both axles could be driven all the time, even on hard surfaces, but the centre diff could be locked so that on loose surfaces the car would work like the earlier ones, with drive to at least one of the axles.
And that has until relatively recently been the limit of 4x4 transmission development by the major manufacturers.
The question which might strike you, is with the advent of locking diffs, why were these not utilised in the axles? Well, to be honest, they have, just not on production vehicles.
The ability to lock the axle differentials is a useful feature, but the necessity is probably only found if the extremes of the vehicles abilities are explored. So they have generally been the preserve of the enthusiast, determined explorer or competition driver. For the more typical driver, the lack of a locking centre diff would probably just be a case of getting stuck and after the vehicle was recovered, remembering not to push it that far again. And while the manufacturers certainly could have fitted locking diffs as standard or as an option, it is likely that they have refrained from doing so, because incorrectly used, by say locking the centre diff of the font axle on tarmac, you could almost render the steering inoperative.
However, some manufacturers have offered vehicles with proportional limited slip centre viscous couplings, which work just like an automatic locking centre diff, and even in some cases proportional limited slip planetary differentials in the rear axle, that have given them if not an auto locking facility, something approaching it.
However, lets look at 'Traction Control'. This is 'smart' or 'intelligent' four wheel drive, or power distribution by computer. To explain how it works, let me tell you first though, about 'Twiddle Brakes'
Twiddle Brakes were something that some competitive 4x4 drivers started employing quite early on, in the days of the part time four wheel drive systems, and was basically taken from tractor technology, but that is an aside.
Right, step back and look at your Land Rover. You have three loose planetary diffs in the transmission, and having got one wheel stuck in the mud, it is spinning wildly, and you are going no where.
Now, there is very little you can do to find that wheel some traction, and you haven't got the benefit of a locking differential any where. What else could you do?
Well, the problem is that because that wheel hasn't got any grip, it is free to spin, and that is taking all the engines power. If you could put some force on it, to stop it spinning, then the differentials would have something to work against, and could transmit some force to the other wheels, right?
So how can you do that?
Well how about if you put the brakes on?
Hmm, but that would stop all the wheels turning, wouldn't it. But if you could put the brake on, on just that one wheel, then taht would stop it spinning, and then the diffs would be able to load the other wheels. So how can you do that?
Well, that is 'Twiddle Brakes'. Basically a set of taps on the brake lines, one for each wheel, so that you can close three of them leaving just the one wheel you want to stop spinning connected to the brake pedal. Turn the taps, press the brake pedal, and the other three wheels start turning.
OK, so its a fiddle, and a faff, and its not that refined, but it does the job, without resorting to expensive differentials, its just four valves in the brake lines, right?
OK, so we are now up to the nineteen nineties, and 'Anti-Lock-Braking' or ABS systems are becoming almost universal fitment to ordinary saloon cars, and even four wheel drive, all terrain vehicles.
ABS is a microprocessor based system, and basically what it does is read the speed of each individual wheel and compare them. If any one wheel is turning very much slower than the rest, or isn't moving at all, then that wheel is locked or skidding. A skidding wheel isn't transmitting braking force, so what the system does is actuate a valve in the brake line, and bleed off some of the pressure from the brake caliper on that wheel until it starts turning again. Hence the wheels wont lock, no matter how hard you brake, and you will always have maximum braking and steering traction available.
But, err, valves on the brake lines, that's awfully like 'Twiddle Brakes', isn't it? Could you not kind of combine the two to make up a sort of automatic twiddle brake system?
Absolutely, and that is exactly what Traction control is. The computer brain, is constantly monitoring wheel speed. So if you get the computer to look not just for a wheel that stopped or turning noticeably slower than the rest, but one that is turning noticeably faster than the rest, then it will detect wheel spin.
Now there are already valves to relieve brake pressure, and they are already coupled to the computer, so if you can get the computer to close the right ones, when it detects wheel spin, and apply a bit of braking pressure, then it will work just like a twiddle brake and slow or stop a spinning wheel.
Big advantage, apart from doing it automatically, it can also react very quickly so keep applying and releasing the brake on a wheel very quickly until the system is holding it at the same speed as the other wheels.
And that basically is it. It is all about how the power is distributed to each of the wheels, and what controls there are to manage it.
Enabling speed differentials between the wheels are differential gear sets, or viscous couplings, and to manage how and when the power is distributed, through the differentials, you have locking devices or slip limiting devices.
So, its time to look at the phenomenon of 'Wind-Up' and similar effects.
So, what are the alternatives. We know that the wheels want to turn at different speeds in corners, and as long as we have differentials, that's not a problem until a wheel looses traction, but we can get that by using a limited slip or locking differential, right? Correct, so 'Wind Up' is only going to be a problem if you don't have differentials or viscous couplings.
OK, now, imagine if you didn't have any differentials at all. You have a permanent four wheel drive transmission that has a solid shaft back axle, and a solid shaft front axle, and a solid driving shaft between the two. All four wheels are going to try and turn at the same speed.
So, when you come to a corner, the inside wheels are going to try and go faster than the road underneath them, and the outside wheels are going to try and go slower than the road underneath them.
Likewise at the back, the rear wheels will try and push the fronts, and make the front wheels go faster than the road beneath them, while the fronts will push back and try and make the rear wheels go slower than the road beneath them.
This gives you a number of possibilities. You have road fighting with wheels and wheels fighting with transmission. If the transmission is strongest, then either the car wont go round the corner or some of the tyres are going to have to slip. If the tyres are stronger, then again, either the car wont go round the corner, or the shafts in the transmission are going to have to twist and take up whatever force they cant transmit to the road.
This 'twist' might sound pretty dire, but actually isn't that uncommon, and in fact some springs are actually shafts that are designed to twist like a candy stick.
Any way, this twist is 'Wind-Up'. Now, between the near-side and offside wheels, wind up wont tend to be much of a problem. I mean, for every right hand corner you go round, you will sooner or later go round a left hand corner, so the wind up from one will eventually be taken away by the wind up from the other.
OK, before any-one gets any ideas, that's not strictly true. If you were to drive a racing circuit, in a clockwise direction, you would have to go round more right hand bends than left hand ones. And in this country, we drive on the left hand side of the road, and all ways go around roundabouts in a clockwise direction, so for any given journey, even if it is to go up and down the same road, we will still have the nearside wheel travel a little bit further than the offside wheel. If you don't believe me, have you not noticed that the near-side tyres tend to reach the legal minimum tread limit before the offside ones? Not much before, granted, but it is the first to go.
Any way, that little argument aside, this is not true of the differential between front & rear axles. The rear, un-steered axle, whether going forwards or backwards will always try and take the smallest radius and hence travel less distance than the front wheels.
Consequently, wind up tends to occur in the prop-shafts between the axles. And it is aptly named, the tension builds up just like winding up a watch spring.
Now, as you apply more and more tension to the prop-shafts, ultimately, something is going to give, and ultimately, either the tension on the prop-shafts is going to be so great that it will cause one or more of the wheels to slip, or something is going to break. Commonly, a half-shaft or a universal joint.
So that is wind-up, succinctly, it is the tension that builds up in the transmission because of not having any differential movement between wheels.
The 'associated' effects that come with it is the tendency for the tyres to try and slip or 'scrub' on corners, and for the car to try and fight going round a corner.
None of these effects is particularly desirous. If the tyres are forced to slip because of tension in the transmission, then effectively their grip on the road is reduced, and if the car fights going round corners, then your steering control and response is going to be seriously impaired.
So wind up should be avoided, but it can be tolerated. Off-road, where traction is limited, any pent up tension is going to be dissipated very quickly through slip between the wheel and loose road surface. On tarmac, provided that the scrub effects can be tolerated, most 4x4's will tolerate a LOT of wind up, before they see any permanent damage.
A series Land Rover is most like to suffer wind up, if the car is used on tarmac for any length in Low Range or High Range four wheel drive. Simple reason being that the Series Landies do not have a centre differential, they have a solid shaft. That is why they have only part time four wheel drive, so that you can disengage the drive to the front wheels, and not risk wind up, then engage four wheel drive only when necessary, presumably on a loose surface where wind up wont be an issue.
Just as a note, the first indication of significant wind up on a Series Landie is not being able to move the range selector lever into high range, or, if in high range four wheel drive, with the yellow knob down, moving it down to low range so you can move it back up to release four wheel drive.
This is because the tension on the prop-shafts is putting pressure on the dog clutch that locks the shafts together, so you need to some-how relieve the tension. You could do this by jacking one wheel up, so that it was free to turn. In dire cases, you can do this, but the tension will tend to be released with a bit of a snap, that isn't all that good for the transmission. The wheel will typically snap back something like quarter of a turn or more, and may oscillate for a while just like a weight on a spring.
If you have to jack the wheel to relieve wind up tension, then do it slowly, and make sure you aren't resting on the wheel. Letting the tyre pressure down to about 1/3, maybe 8 or 9 psi first so that the tyre has to deform as it slips. This can give a bit of damping when you release of tension.
Otherwise, take the vehicle to somewhere where you can safely drive on a loose surface, like a park with a gravel car-park, or a muddy field, and just drive slowly for a few minutes and then see if the wheels have slipped enough to release the tension through lack of grip.
I think that some of the Japanese 4x4's have a similar part time four wheel drive system to the Series Land Rovers and may be similarly prone to the phenominon.
Range Rover's, and the coil sprung Land Rovers, Defenders, and Discoveries, all have permanent four wheel drive with a centre differential. I think that some of the later Classic Range Rovers and the P38 Range Rover probably had a viscous coupling centre diff, rather than a planetary gear diff, which may or may not have had diff-lock. However those with a centre diff had a diff lock on the centre diff, of the simple clutch type.
These can run into wind-up if they are driven with the diff-lock engaged on hard surfaces. Likewise permanent four wheel drive 4x4's from other makers.
If you have a vehicle with 'Traction Control' or some other 'smart power distribution' system, then you are unlikely to have a manual diff-lock, and wind up and scrub are unlikely to be a problem as the system should compensate by whatever means it has been programmed to.
And that, really is about it. Knowing what type of system you have on your 4x4 is important, as is knowing what controls you have, and not just how, but also when, and when NOT, to use them.
I hope that you have a good idea now of what's what, why you have so many levers and not just how they work, but why.
So, back to the beginning, at the end of the day, its all about giving you the best chance of finding traction, so, if you didn't come from there you might like to go find out more about traction at:-Traction Theory