How a Four Wheel Drive Works Explained

Educational

Mar 20th, 2019

How a Four Wheel Drive Works Explained

Let's start by differentiating between four-wheel-drive and all-wheel-drive systems.

Difference between Four-Wheel and All-Wheel Drives

Car makers that refer to a car as a four-wheel drive (4WD) usually are talking about a part-time system. Basically, the part-time system is designed to be used only in low-traction conditions, like on snow, for example.

On the other hand, an all-wheel-drive system (AWD) is a full-time four-wheel drive. Basically, the all-wheel-drive system is made for virtually every kind of surface. They can be used on-road and off-road, and a lot of these systems can't be switched off.

The same parameters may be used to evaluate part-time four-wheel-drive and the full-time four-wheel-drive systems. The best system is one that supplies an appropriate level of torque to the wheels of the vehicle. This article will explain the basics of four-wheel drives, and we will begin by getting some understanding about traction and the parts that comprise the system.

Torque, Traction & Wheel Slip

Let us consider some concepts such as torque, traction, and wheel slip in order to better grasp the various four-wheel drive systems that you can find in vehicles.

Torque

When we talk about torque, we are referring to that twisting force that is produced by the engine. This is what is responsible for propelling the vehicle. The torque is multiplied by the transmission gears and the differential. Then, they split the torque between the wheels. The first gear can supply more torque to the wheels than the fifth gear since the gear ratio of the first gear is quite larger for torque multiplication.

A very interesting fact when it comes to torque is that when you have a low traction situation, the maximum torque level that is produced is not determined in any way by the engine but rather by the traction level. So, even if your car's engine is that of a stock car, there will still be no way to take advantage of the power of that engine if the tyres of your car do not stay on the ground.

Traction

In this article, we define traction as the highest level of force which the tyre of your car applies against the ground and vice versa. You see, traction is influenced by some factors such as –

  • Weight applied to your tyre

Your car tyre will have more traction if there is more weight on it. As you drive your car, this weight is subject to change. An example is when you make a turn. At this point, more weight is transferred to the outside of your car’s wheels. As you accelerate, the weight is then shifted to the wheels at the rear.

  • Coefficient of friction

When we talk about the coefficient of friction, we refer to the friction force amounts that exist between two surfaces in relation to the force that holds both surfaces together. So, in this case, the coefficient of friction refers to the traction amount that exists between your wheels and the road in relation to the weight that rests on each wheel. 

Friction coefficient is basically influenced by the kind of wheels that the vehicle has as well as the surface on which you drive your vehicle. As an example, stock car tyres basically have high levels of friction coefficient when being driven on dry concrete tracks. Evidently, this is one reason that makes them able to corner at extremely high levels of speed. If you used the same tyres on mud, however, they would have a friction coefficient of almost zero. On the other hand, a knobby off-road tyre will not have a very high friction coefficient on dry tracks, but when used on a muddy track, they have extremely high friction coefficient.

Wheel Slip

The tyres of your car make contact with the road in two ways, either static or dynamic.

  • Static

The tyre of your car and the ground do not slip relative to themselves. In static contact, the friction coefficient is of a higher level than that in dynamic contact. Therefore, you could say that better traction is created in static contact.

  • Dynamic

Here, the tyre of your car slips relative to the ground. In dynamic contact, the friction coefficient is lower when compared to the static contact, and this gives you less traction.

Basically, you get to experience wheel slip if the force that is applied to the tyre is more than the traction that is available to the tyre. There are two ways by which force can be applied –

1. Longitudinally

The torque that comes from the engine or the car brakes produces longitudinal force when it is applied to a car tyre. This causes the car to accelerate or decelerate.

2. Laterally

As you drive your car around a curve, a lateral force is produced. A car needs force in order to change direction and lateral force is ultimately provided by the tyres as well as the ground.

If you have a powerful rear-wheel-drive vehicle and you happen to be going around a curve when the road is we, the tyres possess sufficient traction for the application of right lateral force which your car needs to stay on the road while negotiating a bend. Now, while going around the curve and stepping on the gas pedal, more torque will be transferred from the engine to the tyres which will end up creating very huge amounts of longitudinal force. 

A combination of the longitudinal force (which your engine just produced) and the lateral force (which was produced during the turn) will result in a wheel slip if their sum is above the level of traction that is available.

Most drivers hardly ever exceed the traction that is available while driving on dry ground. Four-wheel-drive systems are basically very effective for situations where there is low traction like when driving in snow or when driving on a slippery hill. The advantage of driving in a four-wheel drive vehicle is now easy to grasp. That is to say that driving a four-wheel-drive car rather than a two-wheel-drive car gives you the ability to double longitudinal force levels which are applied by your tyres to the road.

How 4WD Systems Perform in Various Road Conditions

A four-wheel-drive system is very helpful in many situations. Consider some of them –

Snow: In order for your car to get through snow, lots of force is needed. However, the level of traction that is available limits the available level of force. This is the reason that you find a lot of two-wheel drive vehicles getting stuck in snow. This is because there is only very little amounts of traction for each tyre when it is snowing. Four-wheel drive vehicles have the ability to use the traction of all four wheels.

Off-Road: Two-wheel-drive vehicles will typically have low traction in off-road situations. An example is if you drive in a mud puddle. When it comes to four-wheel-drive cars, you will still find traction in the other wheel set. This makes it easy to pull you out of the puddle.

Slippery hills: Very high levels of traction is needed to complete this task. Four-wheel-drive vehicles have the ability to make use of the traction of the entire four wheels to pull you to the top of the hill. 

However, there are situations when four-wheel-drive cars do not offer an advantage over two-wheel-drive vehicles. For example, some 4WD cars may not necessarily be capable of stopping when driving on a slippery surface. It is the work of the brakes as well as the anti-lock braking system.

Parts of a Four-Wheel-Drive System

It is now time to consider the parts comprising a four-wheel drive system.

In any four-wheel drive system, the major parts are its differentials and the transfer case. Also, you will find locking hubs and possibly advanced electronics which help the car to better utilise the level of traction that is available. 

Differentials

In every vehicle, you will find two differentials. One differential is placed in between the two wheels at the front, and the other is placed between the two wheels behind. The job of the differentials is to transfer torque from the transmission to the drive wheels. Also, differentials help the left and right wheels to turn at varying levels of speed as you drive around the curve.

As you drive around the curve, the outside tyres of your car do not follow the same path as the inside tyres. Likewise, the front tyres do not follow the same path as those in the rear. This goes to say that all the wheels of your vehicle spin at varying levels of speed. It is the differentials that are responsible for enabling the speed difference that exists between the outside and inside wheels.

Differentials come in many kinds. The ability of your car to make use of the available traction is significantly influenced by the kind of differential used.

Transfer Case

The transfer case is a device which performs the function of dividing the power between a four-wheel drive's front and rear axles.

So, let's take the example of turning around a curve again. The differentials are responsible for enabling the speed difference that exists between the outside and inside wheels. However, you will find a device in the transfer case which enables the speed difference that exists between the front and rear wheels. This kind of device may be a centre differential or even a viscous coupling. It could also be some other kind of gear set. These devices help the system to perform effectively regardless of the surface. 

For a part-time four-wheel drive, the transfer case does the work of keeping the driveshaft of the front and rear axles locked, causing the wheels to spin at equal levels of speed. Your tyres are, therefore, required to slip as your car makes a turn. A system such as this is recommended for use only in situations where there is a low traction level because it is easier for slips to occur. Tyres hardly slip when driving on dry concrete. At this point, it is recommended that you disengage the four-wheel drive so as not to experience jerky turns or wear the tyres and drivetrain.

There are transfer cases in part-time systems that usually come with an extra gear set which enables the car has a low range. The result is that the car is equipped with added torque and output speed that is very slow. This gives drivers the ability to go up a hill that is very steep slowly and smoothly.

Locking Hubs

The wheels of your vehicle are attached to a hub. You will usually find a locking hub positioned at the front wheels of a four-wheel-drive truck. When the front-wheel drive is inactive, the locking hubs perform the task of disconnecting the wheels at the front from the differential, driveshaft, and half-shafts. This results in the differential, the driveshaft, and the half-shafts becoming inactive if the vehicle is in a two-wheel drive state. This reduces depreciation on those parts while enhancing fuel economy.

There was a time when manual locking hubs were the trend. When the driver wanted to engage the four-wheel drive, he would have to exit the truck and manually get the hubs locked on the front tyres. Today, with the advent of innovation, you will find automatic locking hubs which automatically engage as soon as the driver activates the four-wheel drive mode. With this system, the engagement can take place even while the car is in motion. Whether it is a manual or automated process, what these systems basically utilise is a sliding collar which gets the half-shafts at the front locked to the hub.

Advanced Electronics

In a lot of four-wheel-drive cars today you'll find advanced electronics which play a very vital role. The ABS system is used to apply brakes only to tyres which begin to skid. This is referred to as brake-traction control.

There are other cars that come with clutches that are very sophisticated and controlled electronically. These clutches help to regulate the transfer of torque between the wheels. As we progress in this article, we will take a look at one of these advanced systems in the four-wheel drive. 

Now, let us consider how the four-wheel-drive system works.

How a Four-Wheel-Drive System Works

Here, we will be looking at how the part-time system, which is mostly found on pickups and SUVs, work. 

The part-time four-wheel drive is a system that gives the driver the option of activating the four-wheel drive only at the point where it is actually needed. When it is not activated, it basically functions the same way a two-wheel-drive vehicle does. A huge advantage of a four-wheel-drive car is that it helps in fuel economy.

A 4WD car is primarily a rear-wheel drive. The transfer case is hooked directly to the transmission. It is from here that the front axle is turned by one drive shaft while the rear axle is turned by another drive shaft.

As soon as you engage the four-wheel drive, the front and rear driveshafts are locked by the transfer case in order to enable each axle to get a split proportion of the torque that is produced by the engine. The front hubs also get to lock at this very point.

The front and rear axles both possess an open differential. Even if this system happens to offer enhanced traction than you would get from a vehicle that operates using a two-wheel-drive system, there are two downsides. Earlier, we talked about one of the downsides which are that the transfer case which is locked makes it difficult to use this system on-road. 

Now, there is another problem which arises from the kind of differentials used. An open differential divides the torque coming from the engine in two equal halves so that the two wheels that are connected to it get one half of the torque each. In the event that any of those wheels lose contact with the ground or happen to be on a surface that is extremely slippery, the torque being sent to the other wheel automatically becomes zero. In this case, even if there is a very high level of traction on the other wheel, it will not get any torque.

At the beginning of this article, it was mentioned that a very ideal four-wheel drive system has the ability to transfer appropriate amounts of torque to the individual wheels of a car. That means that the system is capable of sending the maximum level of torque which will not lead to a tyre slip. If you use that criterion, then, the part-time four-wheel drive system would have a very poor rating. This is due to the fact that it sends only the amount of torque to the wheels which will not result in the slipping of the wheel that has the least traction. 

It is possible to make improvements for this particular system. There are ways to do it. One of such ways is to use a limited-slip rear differential instead of an open differential. This will ensure that the two wheels at the rear have the ability to apply some level of torque regardless of the situation. Limited-slip differential functions just like an open differential except that it transfers some of that torque to the tyre that possesses the least traction. This process is automated and does not require any action from the driver.

A second option that can be applied is to make use of a locking differential. What the locking differential does is to keep the two wheels at the rear locked together. The reason for this is to make sure that each of those wheels can get access to the entire torque produced by the engine and that makes its way into the axle whether or not any of the wheels are off the ground. In this case, performance will be greatly improved in off-road situations.

A locking differential is actually a step ahead in the game. What it does is to ensure that each wheel gets equal amounts of torque regardless of the differences that exist in traction. What this accomplishes is that it enables the wheel which most likely possesses the most traction to have a higher chance of getting your vehicle moving even if you get into a low traction condition.