Let's start by differentiating between 4WD vs AWD drive systems.
Difference between Four-Wheel and All-Wheel Drives
Are 4WD and AWD the same?
When auto makers talk about a 4WD, they usually refer to a part-time four-wheel-drive system 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 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 comprising the system.
Torque, Traction & Wheel Slip in 4WD Cars
Let us consider some concepts such as torque, traction, and wheel slip to better grasp the various four-wheel drive systems that you can find in vehicles.
The engine of a car produces torque, which is a twisting force that the transmission gears and differential multiply. The resultant torque is then split between the drive wheels to move the vehicle.
The first gear can supply more torque to the wheels than the fifth gear since the gear ratio of the first gear is significantly larger for torque multiplication.
Here’s an interesting fact about torque: When you have a low-traction situation, the maximum torque level produced is not determined by the engine but by the traction level.
So, even if you have a powerful engine, all that power will come to naught if the tyres of your car don’t stay on the ground.
In this article, we define traction as the highest level of force that your car’s tyres apply against the ground and vice versa. You see, traction is influenced by these factors –
- Weight applied to your tyres – the tyres will have more traction if there’s more weight on them. As you drive your car, this weight changes. An example is when you’re making a turn. As you turn, more weight is transferred to the outer part of your car’s wheels. As you accelerate, the weight shifts to the wheels at the rear.
- Coefficient of friction – the kind of wheels and the surface of the road affect the coefficient of friction, which tells us how well these surfaces hold together. For instance, stock car tyres have high levels of friction coefficient when driven on dry concrete tracks, which is why they are capable of cornering at extremely high speeds. However, they would have almost zero friction on mud. Conversely, a knobby off-road tyre will not be as efficient on dry tracks as they are powerful on a muddy terrain.
The tyres of your car touch the road in two ways, either static or dynamic.
- Static – the friction coefficient is higher in static contact than in dynamic contact because the tyres and the road don’t slip relative to each other. So, we say that there’s better traction in static contact.
- Dynamic – here, the tyres of your car slip relative to the ground. Dynamic contact gives you less traction because the coefficient of friction is lower than in static contact.
You’ll experience wheel slip if the force applied to the tyre is more than the traction that is available to the tyre. You apply force in two ways –
- Longitudinally – the torque that comes from the engine or the car brakes produces longitudinal force when applied to the tyres, causing the car to accelerate or decelerate.
- Laterally – as you drive your car around a curve, it needs lateral force to change direction.
The tyres of a powerful rear-wheel-drive vehicle 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, 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 more than the available level of traction.
Most 4WD drivers hardly ever exceed the traction available while driving on dry ground, and four-wheel-drive systems are very effective for low-traction situations, e.g., driving in snow or on a slippery hill.
How a 4WD System Performs in Various Road Conditions
A four-wheel-drive system is indispensable in many situations. Consider some of them –
Snow: Your car needs plenty of force to get through snow, but low traction limits the available level of force. That’s why you’ll find two-wheel drive vehicles getting stuck in snow because of the limited amount of traction between the snow and the two drive tyres. In comparison, four-wheel drive vehicles have more traction under all four wheels.
Off-Road: Two-wheel-drive vehicles will typically have low traction in off-road situations, a muddy patch, for example. When it comes to 4WD cars, the extra wheel set will provide more traction, making it easy to pull you out of the puddle.
Slippery hills: Pulling you to the top of a slippery hill is a piece of cake for four-wheel-drive vehicles with the traction available to their entire four wheels.
However, there are situations when four-wheel-drive cars do not offer an advantage over two-wheel-drive vehicles.
For example, not all 4WD cars are capable of stopping when traversing a slippery hill. That’s the work of the brakes and 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 that help the car to better utilise the available level of traction.
Differentials transfer torque from the transmission to the drive wheels and help the left and right wheels turn at varying speeds as you drive around a curve.
The outer tyres will need to cover a longer distance and will have to rotate faster than the inside tyres. Likewise, the front tyres have a different path from those in the rear.
In short, all the wheels of your vehicle spin at different speeds when negotiating curves, and it’s the job of the differentials to make this possible.
Differentials come in several types, and the type of differential that your 4WD has influences how it utilises available traction in different situations.
The transfer case divides the power between a four-wheel drive's front and rear axles.
Let's take the example of turning around a curve again. We’ve talked about how the differentials are responsible for the speed difference between the outer and inner wheels.
Inside the transfer case is another device that facilitates the speed difference between the front and rear wheels. This kind of device may be a centre differential, a viscous coupling, or could also be some other kind of gear set.
Some transfer cases in part-time systems come with an extra low-range gear set that equips the vehicle with added torque and slow output speed. This allows driving up extremely steep hills.
You will find a locking hub at the front wheels of a 4WD truck. When the front-wheel drive is inactive, the locking hubs disconnect the wheels at the front from the differential, driveshaft, and half-shafts.
In other words, the vehicle is in a two-wheel drive mode. Inactivating the 4WD reduces wear and tear on the deactivated parts and enhances fuel economy.
Now locking hubs are automatically engaged when you shift to 4WD, but that wasn’t always the case. Before, when a driver wants to engage the four-wheel drive, he would have to get out of the truck and manually get the hubs locked on the front tyres.
Most 4D cars today are equipped with electronics, such as ABS for brake-traction control.
Some four-wheel-drive vehicles come with sophisticated and electronically controlled clutches to regulate torque transfer between the wheels.
How a Four-Wheel-Drive System Works
You will mostly find a four-wheel-drive or part-time system on pickups and SUVs.
A 4WD car is primarily a rear-wheel drive that allows the driver to activate the four-wheel drive when needed. Its advantage over an AWD is obvious – it depreciates slower and is better in terms of fuel economy.
So, here’s how it works.
It is at the transfer case, which is hooked directly to the transmission, where one drive shaft turns the front axle while another shaft turns the rear.
As soon as you engage the four-wheel drive, the transfer case locks both driveshafts to allow proportionate torque to each axle. The front hubs are also locked.
The front and rear axles both possess an open differential, which offers enhanced traction but with some weaknesses.
An open differential divides the torque from the engine in two equal halves, so when any of those wheels lose contact with the ground or happen to be on an extremely slippery surface, the engine torque is sent to it. In this case, even if the traction is very high on the other wheel, it will not get any torque.
We’ve mentioned earlier that an ideal four-wheel drive system should be able to transfer appropriate amounts of torque to the individual wheels of a car, sending maximum torque that will not lead to a tyre slip.
Based on that criterion, a part-time four-wheel drive system with open differential will score poorly because the maximum amount of torque it can send to the wheels is limited by the wheel with the least traction.
The best way is to use a limited-slip rear differential, which works like an open differential except that it transfers torque to the tyre with the most traction. This process is automated and does not require any action from the driver.
Another option is a locking differential, which keeps the two wheels at the rear locked together to ensure that each wheel can access the entire engine torque regardless of differences in traction. In this case, performance will be greatly improved in off-road situations because the wheel with the most traction has a higher chance of moving your vehicle out of a literally sticky condition.