The big debate: what is the difference between an all-wheel-drive system, a four-wheel-drive system and a 4x4 system. In this episode of How Things Work, Peter unpacks the four-wheel-drive system to find out if there really is a difference. Let’s settle the debate, once and for all
Estimated reading time: 8 minutes, 33 seconds.
Welcome to our latest instalment on how things work on Let's Talk Automotive. In today's segment, we're going to be talking about four-wheel-drive systems and we're going to give you a basic introduction into the various systems that are out there. Now, one of the things that I enjoy about this topic is that because there's so much confusion out there about how all the systems work, that means there's a lot of debate. And one of the debates is what is the difference between an all-wheel-drive system, a four-wheel-drive system and a 4x4 system?
Now from a marketing point of view, the marketers will tell you that an all-wheel-drive system is typically used on a road car, and in America, a 4x4 system is called a four-wheel-drive system and in South Africa, we know our 4x4 system typically as an example on a bakkie. But I've got a different view on that because ultimately I think they’re all the same thing and we can actually see that the different systems use each other's components.
In future segments, we are actually going to be talking in more detail about each of those components, and it will make it even more clear for you.
But next, I want to show you the different layouts of all-wheel-drive and four-wheel-drive systems. So our first system that we're going to go through is one that most of you are familiar with, and that's our typical 4x4 system that we find on our 4x4 bakkies and here's the basic layout.
So here are my four wheels, here's my engine, my gearbox, and what we have over here is a transfer case. We have a prop shaft going to the front that goes through a differential and our side shafts go to each wheel. Here's our centre differential with our prop shaft going to the rear, another differential, and then the side shafts going to our two wheels. The way that we engage four-wheel drive on this is done initially when we are two high, the power is going through the engine, through the gearbox, through our rear prop shaft to our rear wheels. When we engage 4x4, what we’re doing is we actually select the transfer case, which has a chain that connects the gearbox to the transfer case and we now engage that chain. So now the chain turns this shaft, which now turns our front wheels. So very basic, very robust, very reliable system.
The disadvantage with this system, however, is that once we engage four-wheel drive, we physically have locked our power distribution to be 50/50 between front and rear. And that's a bit of a disadvantage because it means that we really can only travel pretty much in a straight line. And if we make any turns, we've got to make sure that we're on very loose gravel or mud conditions, otherwise, we're going to damage our transmission system.
So the next system, which is our all-wheel-drive system, I'll show you how those systems overcome the shortcomings of traditional 4x4. So all-wheel-drive systems were designed because engineers realised the advantages of having traction going to all four wheels. The challenge was, how to create a four-wheel-drive in a road-going car and not add all the weight that's traditionally associated with the 4x4 system.
So some very clever components were designed, and we now have this kind of layout in an all-wheel-drive system that we typically find on a road car. So first things first, you'll notice that we still retain our front-wheel-drive layout in the system. So our engine is transversely mounted, which saves space. Our gearbox is a trans axle gearbox. In other words, the axle moves through the gearbox itself.
And here, we have our system now going to the rear with our prop shaft. We have a coupling over here and this coupling is the essence of the design. So this coupling engages the back half of this prop shaft and provides drive to the rear wheels through this differential over here. Now, as I said to you, the most important part of this system over here is in fact, this coupling.
We can get this coupling at the rear, or indeed we can have this coupling in the front as well in more complex systems. And this coupling is made up of two different types of systems. We either have a viscous coupling, or we have a multi-clutch coupling. Now as I said to you in future episodes, we'll go into more detail in terms of how this coupling works, but I wanted to tackle the advantage that this has. Because now, when we engage this coupling, which was made up, for example of a whole lot of clutches, we can allow those clutches to slip slightly; or we can have those clutches engage fully. And the effect that that creates is that we can have technically, in one scenario, a hundred percent power to the front and zero to the back, or we can vary the amount of drive going between front and rear.
So I might have a scenario where I only have 30% of my drive going to the front and 70% of my drive going to the rear, depending on what my traction requirements are. So it's a very, very clever system. The other advantage of systems like this is that because they're electronically controlled, we can now integrate them into our ESP systems and so they form part of actually the overall safety and traction of the vehicle itself.
You would have also seen in vehicles that have these types of systems that you have different driving modes. So you have a normal driving mode, you might have a snow mode, you might have a sand mode and a mud mode. And that talks to how we manipulate the electronics on the engine, on the gearbox, as well as with our couplings and how they all talk to each other and develop the optimal traction for the conditions that we find ourselves in.
So now we've just gone through the two different types of systems that we use to create drive to all four wheels. And as you can see, there's a lot of complexity in those systems. But the bottom line is, is that modern off-road vehicles use a lot of the shared components that we find between the two systems. So the lines are blurred in terms of which system is actually better than the other because ultimately they're the same thing. So for me, it comes down to what are the factors and characteristics that determine how good a vehicle is off-road and there are basically seven elements; there's more, but they're the main elements that define what a good off-roader is.
So let's go through these. The first one is the chassis. There's a world of difference in ability between a ladder frame chassis and a monocoque chassis when we off-road. The preference, when we off-road is to have a ladder frame chassis, the same is true for suspension.
There's a big difference in ability between a vehicle that's got independent suspension and a vehicle that's got a solid actual suspension front and rear. Then tyres are critical. And in fact, there are tyres that are designed for specific traction conditions. So we get mud tyres, we get snow tyres, we even get sand and rock tyres.
So having the right tyre is going to make a huge difference in your ability to off-road.
A very, very important part is our ability to lock our wheels, to make sure that we've got power going to the wheels that have got traction. So if we've got one wheel up in the air, we need to be able to lock that wheel so the power goes to the wheels that have traction. And again, here, there are different ways of achieving the effect of a diff lock. So we can have electronic diff locks, which we see in the form of braking each wheel independently, utilising our brakes, or we can have a limited-slip diff, or we can have a full mechanical diff-lock that we see typically on our bakkies.
The fifth element is ride height. That's obvious. We want a nice ride height, but what's not necessarily obvious and what goes with ride height, is our entry angle. In other words, the overhang that we have on the front of the vehicle; our dwell angle, and that's the middle part of the car, so, how easy is it for our vehicle to go over an object, and then our exit angle at the rear, which talks to the overhang at the rear of the vehicle.And that's all bundled under ride height.
And then, if we're going up very, very steep inclines, we want to have low range. And low range, as the term suggests, typically will increase our gear ratios to such an extent that we can really crawl up slowly up steep slopes or down steep slopes with high engine revs.
What's the seventh factor though? And for me, this is perhaps the most fun part.
We can have all of these in place, but for me, the most important part of a good off-road vehicle is, in fact, the driver. So at the end of the day, we can have the most competent vehicle on earth, but if we haven't really been trained properly on how to drive off-road, these things are going to be pretty useless to us.
So that's it for this week's episode on how things work. We've taken you through an introduction into four-wheel-drive systems. On future episodes we're going to go into much more detail and explain each of the components that make up these systems, which these days are truly incredible and packed with technology.
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