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Four Wheel Driving System
There are roughly as many diverse four-wheel-drive systems as there are four-wheel-drive vehicles. Every producer has several different plans for providing power to all of the wheels. The way different carmakers use it can sometimes be a bit confusing, so before explaining about how they work, here are some terminology to get familiarized:
- Four-wheel drive - Four-wheel drive is mostly referred 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 occasionally called full-time four-wheel drive. And they are designed to function on all types of surfaces, both on- and off-road, and a majority of them cannot be switched off.
A better knowledge of torque, traction and wheel slip will be helpful to understand the different four-wheel-drive systems found on cars.
The twisting force that the engine produces is known as torque. The torque from the engine moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels. In the first gear, more torque can be sent to the wheels than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque.
The remarkable fact 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. Even if your car is equipped with a NASCAR engine, if the tires won't stick to the ground there is simply no way to harness that power.
We can define traction as the maximum amount of force the tire can apply against the ground or that the ground can apply against the tire. These factors that affect traction are:
The twisting force that the engine produces is known as torque. The torque from the engine moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels. In the first gear, more torque can be sent to the wheels than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque.
The remarkable fact 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. Even if your car is equipped with a NASCAR engine, if the tires won't stick to the ground there is simply no way to harness that power.
We can define traction as the maximum amount of force the tire can apply against the ground or that the ground can apply against the tire. These factors that affect traction are:
- The weight on the tire - The tire has more traction when it has more weight. The weight can be shifted as a car drives. Example, when a car makes a turn, the outside wheels have most of the weight. 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. Here it means the amount of traction between the tires and the road to the weight resting on each tire. The coefficient of friction is mostly a function of the kind of tires on the vehicle and the type of surface the vehicle is driving on. The coefficient of friction for the tire in mud would be almost zero. By contrast, massive, knotted, off-road tires 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 - static and dynamic are the two kinds of contact that tires can make with the ground.
- static contact - The tire and the ground are not slipping comparative 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 tire is slipping relative to the road.
In simple terms, wheel slip happens when the force applied to a tire exceeds the traction available to that tire. Force is applied to the tire in two ways:
This is of great help in a variety of situations such as:
- Longitudinally - Longitudinal force is made available from the torque applied to the tire by the engine or by the brakes. It tends to either accelerate or decelerate the car.
- Laterally - Lateral force is produced when the car drives around a curve. Force is needed to make a car change direction and eventually, the tires and the ground provide lateral force.
This is of great help in a variety of situations such as:
- In snow - The car needs a lot of force to push through the snow. The amount of force obtainable is restricted 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, but a four-wheel-drive car can utilize the traction of all four tires.
- Off road - In off-road conditions such as when crossing a stream or mud puddle, wheels will be in a low traction situation. With four-wheel drive, the other set of tires still has traction, so they can pull you out.
- Climbing slippery hills - Climbing hills need lot of traction. A four-wheel-drive car can utilize the traction of all four tires to pull the car up the hill.
