4x4’s and How They Work

The 4x4 specialist team at Sureterm Direct unravels the mysteries of driving a 4x4 and how and why it all works.

So how does the four wheel drive system work?
There are almost as many different types of four-wheel-drive systems as there are four-wheel-drive vehicles. It seems that every 4x4 manufacturer has several different schemes for providing power to all of the wheels. The language used by the different carmakers can sometimes be a little confusing so here’s some explanation of terminology you will often hear used:

• Four-wheel drive - Usually, when car makers say that a car has four-wheel drive, they are referring to a part-time system. These systems are meant only for use in low-traction conditions, such as off-road or on snow or ice.

• All-wheel drive - These systems are sometimes called full-time four-wheel drive. All-wheel-drive systems are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off.

Part-time and full-time four-wheel-drive systems can be evaluated using the same criteria. The best system will send exactly the right amount of torque to each wheel, which is the maximum torque that won't cause that tyre to slip.
To understand the different four-wheel-drive systems found on cars, you need to know a little about torque, traction and wheel slip.

So what are torque, traction and wheel slip?
Torque is the twisting force that the engine produces. The torque from the engine is what moves your 4x4.  The various gears in the transmission and differential multiply the torque and split it up between the wheels. More torque can be sent to the wheels in first gear than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque.

The interesting thing about torque is that in low-traction situations, the maximum amount of torque that can be created is determined by the amount of traction, not by the engine.

So what affects traction?
• The weight on the tyre - The more weight on a tyre, the more traction it has. Weight can shift as a car drives. For instance, when a car makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels.

• The coefficient of friction - This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together.  It relates to the amount of traction between the tyres and the road to the weight resting on each tyre. The coefficient of friction is mostly a function of the kind of tyres on the vehicle and the type of surface the vehicle is driving on. For example, huge, knobbly, off-road tyres wouldn't have as high a coefficient of friction on a dry track, but in the mud, their coefficient of friction is extremely high.

• Wheel slip - There are two kinds of contact that tyres can make with the road: static and dynamic.

• Static contact - The tyre and the road (or ground) are not slipping relative to each other. The coefficient of friction for static contact is higher than for dynamic contact, so static contact provides better traction.

• Dynamic contact - The tyre is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have less traction.

Quite simply, wheel slip occurs when the force applied to a tyre exceeds the traction available to that tyre. Force is applied to the tyre in two ways:

• Longitudinally - Longitudinal force comes from the torque applied to the tyre by the engine or by the brakes. It tends to either accelerate or decelerate the car.

• Laterally - Lateral force is created when the car drives around a curve. It takes force to make a car change direction - ultimately, the tyres and the ground provide lateral force.
 
The benefit of four-wheel drive is easy to understand: If you are driving four wheels instead of two, you've got the potential to double the amount of longitudinal force (the force that makes you go) that the tyres apply to the ground.

This can help in a variety of situations. For instance:

• In snow - It takes a lot of force to push a car through the snow. The amount of force available is limited by the available traction. Most two-wheel-drive cars can't move if there is more than a few inches of snow on the road, the recent weather conditions have been proof of that! A four-wheel-drive car can utilise the traction of all four tires.

• Off road - In off-road conditions, it is fairly common for at least one set of tyres to be in a low-traction situation, such as when crossing a stream or mud puddle. With four-wheel drive, the other set of tyres still has traction, so they can pull you out.

• Climbing slippery hills - This task requires a lot of traction. A four-wheel-drive car can utilise the traction of all four tyres to pull the car up the hill.