Transmission Differentials Explained – Part–One

What is a differential? What does it do? No, don’t switch off, because this article can be of help to you as an ordinary car driver, as it doesn’t just apply to drivers of 4x4 vehicles.

A differential is a box of tricks (it’s a collection of gear wheels actually) and is an important part of your car’s transmission system as it allows the wheel at each end of an axle to rotate at different speeds. Why is it necessary to have each wheel on the same axle to rotate at different speeds? Well, if you drive around a corner the wheel on the outside of the curve has further to travel and so needs to speed up to a rate greater than that of the centre of the vehicle.

The inner wheel, however, the one you catch on that pesky kerb corner, has to cover less distance and therefore needs to slow down. If there were no ‘give’ in the middle, as in the axle was solid, there would be a lot stress build up and something might break as the two wheels fight against each other. Certainly the situation would do nothing to enhance the rate of tyre wear!

Most Cars On The Road Are Two–Wheel Drive

Most cars that you see on the road are what are known as two–wheel drive, and that means the energy from the engine is transmitted to one pair of wheels, either at the front or at the back, and that drive is introduced to the appropriate axle through a differential. The differential then distributes the power to the two wheels and so the car is powered along the road.

Now that we can see how each wheel on an axle needs to be allowed to rotate at independent speeds during cornering, let us broaden our thoughts to the concept that each axle needs to have the ability to travel at an independent rate to its opposite number. This is because the rear axle travels a shorter arc than the front, as the car turns a corner, which again is why you catch your back wheel on the kerb. However, with a two–wheel drive vehicle it doesn’t matter that the two axles travel on different arc, and therefore at different rates of speed, because there is no mechanical connection between them, but with a four–wheel drive vehicle, the situation is very different.

Each Axle Travels a Different Arc During Cornering

To have four–wheel drive there needs to be a means by which to transmit energy from the engine to both the front and rear axles, and then to each wheel on each axle. Through what we have learned already on this page, we know that in this case each axle will need to be fitted with a differential.

However, as each axle travels a different arc during cornering, in the case of a 4x4 vehicle do we not therefore need a differential in the middle of the car somewhere? Yes, of course we do.

Without getting specific or technical, with a four–wheel drive system there are three differentials, one in each axle, and one between the axles; usually somewhere at or about the main gearbox area. So what, you might ask? Well, considering the content of the above, the question for you is this; does a four–wheel drive vehicle have four–wheel drive? That is not such a daft question, and as you read on you will see why.

Going back to our two–wheel drive family car for a moment, let us say the one we are thinking of is a front–wheel drive model. If you were to park that car with one front wheel on patch of ice, and the other on a good road surface, what would happen when you try and drive away? Yes, that’s right, the wheel on the ice will spin, whilst the other will remain motionless, and the car will go nowhere.

Energy is Always Lost to The Spinning Wheel

This is the differential at work, as it has allowed one wheel to rotate independently of the other, but in this situation you don’t want that to happen. You want the drive to go to the wheel that is on the grippy surface, so as to allow you to drive forward, but why has all the energy been lost to the wheel that is on the ice?

The answer to the above is simple. It is because the energy has followed the least line of resistance, as it requires less effort to set the wheel spinning on the slippery surface than it does to move a one–tonne of motor car forward on the road.

So, how can we say this is a two–wheel drive car when only one wheel is responding to the drive of the engine? That question is your first clue to the point of this article.

Let us now take the car away from our icy puddle and replace it with a Land Rover, which of course is a 4x4 vehicle. If we park the thing with the left front wheel and tyre on the ice patch, and with the other three on the good road surface, what will happen now as we attempt to drive away? Well actually, the wheel on the ice will spin and the other three remain stationary, and the vehicle is going nowhere. Not much of a four–wheel drive is it?

Most 4x4’s Have Something That is Called a Differential Lock

The example of the Land Rover has only been used as it probably has the most recognised name in the world of 4x4 vehicles, and not because it is lacking in capability by comparison to other marques. You could be forgiven for thinking that if a 4x4 becomes floundered when you get one wheel on a slippery surface, whilst the the other three are on a grippy one, there would be little point in trying to use it to drive on rough terrain. However, most 4x4’s have something that is called a differential lock, or diff–lock for short.

A diff–lock does exactly what the name suggests, as it locks a differential. That is to say it stops one side of the differential from moving independently to the other. Going back to our front–wheel drive family car, if we could temporarily have locked the differential in the front axle, the drive from the engine would indeed have forced the wheel standing on the dry road to power the car forward, despite its opposite number being on the patch of ice.

So, here is a conundrum for you, just to get the old grey matter warmed up. Have a close look at the picture of the red Land Rover Discovery and you will see it is parked with the front offside wheel on a set of rollers.

You can’t see all of the vehicle wheels in the picture, but the others are standing on firm ground, and the reason the rollers have been used is because in the month of June, when this picture was taken, there was a shortage of ice patches. In other words, the rollers provide the same effect.

If you were driving this Land Rover, you would be quite right in your assumption that if you were to try and move it forward, the wheel on the rollers would spin whilst the other three would remain stationary, and this will mean the vehicle would be going nowhere. So the question to you all is, how can you get the car to move forward in this situation?

Wot, No Diff–Lock?

Rather bizarrely, Land Rover, when the company introduced the Series III Discovery, which is the vehicle in the picture, they did not equip it with a centre diff–lock, whereas its predecessors, the 200 and 300–series, were equipped with one.

The diff–lock on most 4x4’s operates on the centre differential, as in the one between the propeller shafts (propeller shafts are the two pole–like items that transmit the drive power from the engine and gearbox to each of the two axles). You will remember we said that a 4–wheel drive needs a differential in the middle of the vehicle to allow both front and rear axles to run independently of each other?

When you lock this differential by engaging the diff–lock, the front axle has to always keep pace with the rear. That will get the vehicle off those rollers, as it will be pushed forward by the drive from the rear axle.

Yes, that is true, but let’s get you thinking some more. Let us put another set of those rollers under the back wheel. Doesn’t matter which side you so this, but to keep it simple, let us say we keep to the same side of the vehicle. How are you going to drive it off now?

Whilst you may be able to lock the centre differential you need to remember there are two more differentials in the system, one in each axle. Whilst we can stop each axle from driving at independent speeds to one another by use of the centre diff–lock, the differentials in the axles still allow each wheel on each axle to move independently of its partner.

With the car on two sets of rollers, even if it had a centre diff–lock, it still is going nowhere, because all the energy is immediately lost to the two free–spinning wheels. So, what now?

Energy Will Always Follow The Least Line of Resistance

You have to think of is what we said about our family car on the frozen puddle. The energy will always follow the least line of resistance, and the same applies to our Land Rover here. That is why the wheels are spinning on the rollers and the others remain still. So as to get the opposite numbers to come into play, we need to make the spinning wheels more difficult to drive. If we can do that the now motionless wheels will do some of the driving and we can get the vehicle moving.

The way in which you do this is to accelerate with the right foot on the gas pedal, and with the left foot apply the footbrake.

Yes, that’s right, you need to apply power and to brake at the same time.

What this does is cause resistance in the free spinning wheel side of the front differential, and with that corresponding wheel being now more difficult to drive, some of the drive energy is transmitted to the wheel on the other side. This will happen more at the front of the vehicle, because it is always the front that has the most powerful brakes. However, be assured, if you were to try this experiment, it is amazing how much power you can get into the front wheel that is off the rollers, and the vehicle will drive forward.

Just to recap a second on what we have learned

A two–wheel drive car is actually a one–wheel drive, because when on the icy puddle, only one wheel will be doing any work when you try and drive away. A four–wheel drive vehicle is only a one–wheel drive vehicle if used without a centre diff–lock engaged, a two–wheel drive vehicle with a centre diff–lock engaged, and a two and a half wheel drive when using a centre diff–lock and left–foot braking.

Brakes On a Car Are Bigger And More Powerful At The Front

Incidentally, you can use the left–foot braking technique with your front–wheel drive car on the icy puddle to get a one–and–a–half–wheel drive situation, but this won’t work with a rear–wheel drive. The reason for that is the brakes on a car are always bigger and more powerful on the front, so if you are braking and driving with a rear–wheel drive, all you are doing is making it worse, as you are making the car more difficult to move.

You can, however, try driving off with your handbrake on, BUT BE CAREFUL. Only of your handbrake (parking brake)is of conventional design, acting at each wheel unit. Tranmission brakes, and other more specialised system can be severely damaged by trying to drive with the handbrake in the on position.

If you do have a conventional handbrake that acts on both the rear wheels, this might help you gain traction enough to get you out of where you are stuck. Don–t leave the hand brake on for more than a few yards either, as this will also cause a lot of damage.

There are 4x4’s on the market that have more than one diff–lock, depending upon how much you want to pay. Some have a centre and rear axle diff–locks, so this would give you three and a half wheel drive with left foot braking being used.

There are a limited number of 4x4 vehicles that have as many as three diff–locks, one in each axle and one in the centre. That really is the only configuration where you can say the vehicle is truly a four–wheel drive car, as in that scenario and with all diff–locks activated, there is not any one wheel that can rotate independently of the others.

The Good Old Mechanical Equipment on Vehicles is Disappearing

With ever more clever advances in technology, some of which uses computerised electronics, much of the good old mechanical equipment on vehicles is disappearing. But with the increased sophistication comes increased maintenance and repair costs. For the money, and for good all–round off–road 4x4 driving fun, you still can’t beat the old Land Rover, and the vehicle still remains popular among off–road driving enthusiasts.

Before we finish, and just to counter those clever people who are itching to protest about the reference to the missing diff–lock in the Series III Discovery, it is true that model had a form of traction control that simulated the left–foot braking technique, and did so electronically. However, to get it to work, you had to floor the accelerator, screaming the engine, and perhaps at a time and place where you were in a situation that really didn’t want all that power going on.

It really was very Neanderthal to use. It is significant, therefore, that before the model was discontinued by Land Rover, the company began to fit a mechanical means of operating a centre diff–lock on them, and this was a much better arrangement.

Julian Smith
Ride Drive Limited

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Part–1 Transmission Differentials Explained
Part–2 Transmission Differentials Explained

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This page was last updated
Thursday, 27-Jan-2011

Transmission Differentials Explained – Part–One