Four-wheel drives

Depending on the vehicle’s design, engine power is transmitted by the transmission to the front wheels, the rear wheels or to all the four wheels. The wheels receiving power are called drive wheels. They propel the vehicle forward or backward. Most automobiles either are front-wheel or rear-wheel drive. In some vehicles four-wheel drive is an option the driver selects for certain road conditions, others feature full-time, all-wheel drive.

In rear-wheel drive, the driveshaft runs under the car to a differential is on the front axle and the connections to the transmission are much shorter. Four Four-wheel drives-wheel-drive vehicles have drive shafts and differentials for both axles.

There are almost as many different types of four-wheel-drive systems as there are four-wheel-drive vehicles. It seems that every 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 before we get started explaining how they work, let's clear up some terminology.

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

All-wheel-drive systems are sometimes called full-time four-wheel drive. They are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off. We need to know a little about torque, traction and wheel slip before we can understand the different 4WD systems found on cars.

The main parts of any 4WD system are the two differentials (front and rear) and the transfer case. In addition, part-time systems have locking Four-wheel drives hubs, and both types of systems may have advanced electronics that help them make even better use of the available traction.

A car has two differentials, one located between the two front wheels and one between the two rear wheels. They send the torque from the driveshaft or transmission to the drive wheels. They also allow the left and right wheels to spin at different speeds when you go around a turn. In all-wheel drive, the speed difference between the front and rear wheels is handled by the transfer case. Transfer case
is the device that splits Four-wheel drives the power between the front and rear axles on a four-wheel-drive car. The transfer case on a part-time four-wheel-drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like this should only be used in low-traction situations in which it is relatively easy for the tires to slip.


1. What does the term “drive wheels” mean? 2. What is the difference between rear Four-wheel drives-wheel and front-wheel- drive vehicles? 3. Is it true that there are different types of 4WD?

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