How a four wheel drive works explained

Educational

Mar 20th, 2019

How a four wheel drive works explained

In the world today, you will find quite as many four-wheel drive systems just as there are four-wheel drive cars. It is almost as though car makers have various schemes which they use to provide power to the wheels of their cars. These car manufacturers usually make use of terms that can get you confused. So, before delving into the intricacies of a four-wheel drive, let's get to understand what some terminologies mean –

  • Four-wheel drive

Car makers that refer to a car as a four-wheel drive 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.


  • All-wheel drive

Sometimes, the all-wheel-drive system is referred to as 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 the part-time four-wheel drive system and the full time four wheel drive system. The best system is one that supplies an appropriate level of torque to the wheels of the vehicle. The basics of a 4 wheel drive will be explained in this article, and we will begin by getting some understanding of traction and, then, considering the parts which comprise the four-wheel drive system.

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



TORQUE, TRACTION & WHEEL SLIP

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, as well as, its 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 the engine your car is like that of a stock car, there will still be no way to take advantage of the power of that engine if the tires of your car do not stay on the ground.

Traction

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

  • Weight applied to your tire

Your car tire 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 tires 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 alt extremely high levels of speed. However, if you used the same tired in mud, they would have a friction coefficient of almost zero. But for an off-road tire that is huge and knobby, the friction coefficient on dry tracks would not be as high, but when used in mud, they have extremely high friction coefficient.

Wheel Slip

The tires of your car make contact with the road in two ways which are the static and dynamic ways.

  • Static

The tire 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 of the dynamic contact. Therefore, you could say that better traction is created in static contact.


  • Dynamic

Here, the tire 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 tire of your car is more than the traction that is available to the tire. 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 tire. This causes the car to accelerate or decelerate.


  1. 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 tires, as well as, the ground.

If you have a rear wheel drive vehicle that is quite powerful and you happen to be going around a curve when the road is wet. The tires of your car possess sufficient traction for the application of the right lateral force which your car needs to stay on the road while going around the curve. Now, while going around the curve, if you go ahead to floor the gas pedal, more torque will be transferred from the engine to the tires which will end up creating very huge amounts of longitudinal force. A combination of the longitudinal force which your engine just produced to 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.

A lot of persons 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 tires to the road.

This 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 having difficulties moving through the snow but get stuck. This is because there is only very little amounts of traction for each tire when it is snowing. Four-wheel drive vehicles have the ability to use the traction of the entire 4 wheels.

Off-Road: when you are in an off-road condition, it is normal that, at least, a set of the wheels will have low traction. 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 wheelset. This makes it easy to pull you out of the puddle.

Slippery hills: need very high levels of traction 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 times when four-wheel drive cars do not really provide any benefits over two-wheel drive vehicles. For example, cars that use a four-wheel drive system may not necessarily help you to stop when driving on a surface that is slippery. It is the work of the brakes, as well as, anti-lock braking system.

It is now time to consider the parts that you can find in a four-wheel drive system.



PARTS OF A FOUR WHEEL DRIVE SYSTEM

In any four-wheel drive system, the major parts are its differentials, as well as, the transfer case. Then again, you will find locking hubs and possibly advanced electronics which help the car to better utilize 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 order is placed between the two wheels behind. The job of the differentials is to transfer torque to the drive wheels of the car from the transmission. Then again, differentials help the right wheels, as well as, the left wheels to turn at varying levels of speed as you drive around the curve.

As you drive around the curve, the outside tires of your car do not follow the same path as the inside tires. Also, the front tires of a vehicle do not follow the same path as the rear tires. 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 wheels, as well as the inside wheels.

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


Transfer Case

The transfer case is a device which performs the function of dividing the power between a four-wheel drive's front axles, as well as, 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 wheels, as well as the inside wheels. However, you will find a device in the transfer case which enables the speed difference that exists between the front wheels, as well as, the rear wheels. This kind of device may be a center differential or even a viscous coupling. Then again, it could also be some other kind of gear set. It is these devices which 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 exactly similar levels of speed. Your tires 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. Tires 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 have your tires and drive train wearing out.

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, as well as, the half-shafts becoming inactive if the vehicle is in a two-wheel drive state. This helps to save 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 tires. 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 utilize 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 tires 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 THE 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.

Basically, the car is 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 axle, as well as, the rear axle 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 which are used. A differential that is open divides the torque which comes 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 tire slip. If you use that criterion, then, the part-time four-wheel drive system would have a very poor rating. This is down 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 the limited slip rear differential in the place of an open differential. This will ensure that the two wheels at the rear have the ability to apply, at least, some level of torque regardless of the situation. Limited slip differential functions just like an open differential. However, the only difference which exists between them is that rather than transferring the entire torque into the tire that possesses the lowest amount of traction, it transfers some of that torque to the tire that possesses 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 that is 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.