How All Wheel Drive Works: A Comprehensive Guide

Understanding the Mechanics of All Wheel Drive

All-Wheel Drive (AWD) is a popular feature in modern vehicles, particularly in SUVs, crossovers, and performance cars. Unlike traditional two-wheel drive systems, which power either the front or rear wheels, AWD systems distribute power to all four wheels. This distribution enhances traction, stability, and overall driving performance, especially in challenging road conditions like snow, rain, or off-road terrains. Understanding how all wheel drive works involves exploring the different types of AWD systems, their components, and the benefits they offer.

Two Types of All-Wheel Drive Systems

AWD systems can be broadly categorized into two types: Full-time AWD and Part-time AWD. Each type operates differently and offers distinct advantages depending on the driving conditions and vehicle design.

Full-Time AWD

Full-time AWD systems continuously send power to all four wheels. This system is always active, providing consistent traction and stability. A center differential, similar to those found in front or rear axles, allows the wheels to rotate at different speeds, which is crucial for smooth cornering and handling.

Part-Time AWD

Part-time AWD systems operate primarily in two-wheel drive mode (usually front-wheel drive) and only engage all four wheels when additional traction is needed. This activation can be automatic, based on sensors detecting wheel slip, or manual, controlled by the driver via a switch or button. Part-time AWD systems often include a transfer case that distributes power to the front and rear axles when AWD is engaged.

The Key Components of an All Wheel Drive System

Several critical components work together to ensure that AWD systems function effectively. These include the differential, transfer case, and various sensors and control units.

• Differentials— Differentials are crucial in allowing the wheels to rotate at different speeds, which is essential for smooth turning. In AWD systems, differentials are found at both the front and rear axles, and sometimes a center differential is used to manage the distribution of power between the front and rear wheels. Some advanced AWD systems feature limited-slip or locking differentials, which enhance traction by limiting the speed difference between wheels.
• Transfer Case—The transfer case is a vital component in AWD systems, especially in part-time AWD setups. It distributes power from the transmission to the front and rear axles. In some systems, the transfer case can vary the power split between the axles based on driving conditions, optimizing traction and handling.
• Sensors and Control Units— Modern AWD systems are often equipped with sensors and electronic control units (ECUs) that monitor various parameters, such as wheel speed, throttle position, and steering angle. These sensors help the system detect wheel slip or loss of traction and adjust power distribution accordingly. For example, if the system detects that the front wheels are slipping on ice, it can send more power to the rear wheels to maintain stability.

How All Wheel Drive Works

All wheel drive vehicles are similar to, but not the same as full-time 4WD systems shown near the bottom of this article. Instead of using a costly and heavy center differential, all wheel drive vehicles incorporate a viscous clutch that’s connected to the transfer case. The viscous coupling saves cost and weight and has a much simpler mechanism.

An All Wheel Drive system layout showing a viscous clutch differential

How a viscous coupling works

Viscous coupling

A viscous coupling is a sealed unit containing a series of interleaved clutch plates. The inner plates are splined to the input shaft and spin the entire time the transfer case provides rotation. The outer clutch plates are splined to the housing, which contains the output shaft.

The discs have tabs or perforations to effect cooling and allow liquid to flow between the discs.

The entire device if filled with a temperature sensitive silicone fluid. When the inter and outer discs rotate together, the fluid stays cool and remains in a liquid state.

However, when the discs rotate at different speeds, as what happens when front and rear wheel rotations are different, the fluid heats up and become nearly solid, gluing the plates together. So viscous coupling provides drive power to rear wheels while allowing different front to rear rates of rotation.

Different types of all wheel drive

Unlike 4WD vehicles where the torque is split equally between all four wheels, modern all wheel drive mechanisms can be designed to proportion power front to back based on driving conditions. For example, an all wheel drive system may provide full torque to the front wheels until it detect front wheel slip. At that point, it can divert rotation force to the rear wheels. Or, the all wheel drive unit can maintain a 60/40 split until it detects wheel slip and then vary the torque front to back.

What causes all wheel drive to go bad?

• Viscous coupling overheating caused by variation in tire diameters. For more information on the importance of tire size in all wheel drive vehicles, see this post

How all wheel drive differs from front wheel drive and rear wheel drive

Front Wheel Drive (FWD)— A front wheel drive vehicle transfers power from the engine to the transaxle, which usually incorporates an integral differential. The two front axle shafts, or “half-shafts,” connect to the transaxle. Because the wheels on a front wheel drive vehicle must turn left and right in addition to moving up and down, carmakers use constant velocity (CV) joints instead of standard U-joints. CV joints allow a great turn radius while still providing constant torque to the wheels. As with the rear wheel drive vehicle, the integral differential allows the front wheels to drive the vehicle, yet free-wheel at different rates in curves and turns.

Rear Wheel Drive (RWD)—In a rear-wheel-drive vehicle, power transfers from the engine to the transmission. The power then flows through the front U-joint at the tail of the transmission to the drive shaft. The drive shaft rotates and transfers power to the rear U-joint, which is connected to the rear differential. The rear differential allows each rear wheel to drive the vehicle forward, yet free-wheel at a different speed than the other rear wheel when going around curves and in turns.

A comparison of rear wheel drive and front wheel drive vehicles

How
Now lets take a look at the differences between part time four wheel drive (4WD) and full time 4WD. A part time 4wd transfers power from the transmission to a transfer case. The transfer case divides the power equally between the front and rear wheels. Both the front and rear wheels are driven by a differential that allows for different rotation rates in curves and turns. The problem with this setup is that the front wheels and rear wheels travel at different rotation rates in curves and turns. Since there’s no center differential in this design, the rotation difference holds the potential to bind/break the drive shafts or destroy the transfer case if the vehicle is driven in 4WD mode on dry pavement. However, on wet or snow-packed pavement, the wheels can slip, and it’s that slipping feature that eliminates the need for a center differential.

Part-time 4WD versus Full-time 4WD

In a full-time 4WD vehicle, engine power travels the same path through the transmission and transfer case. It then flows through a center differential that can accommodate  different front to rear rotation rates. So those vehicles can be driven on wet or dry pavement in 4WD mode. Newer full-time 4WD vehicles offer automatic 4WD engagement when road conditions call for it. Full time 4WD vehicles generally divide torque equally between all four wheels. However, some later model designs allow for varying torque based on road conditions.

The Benefits of All-Wheel Drive

AWD systems offer several advantages, making them popular in a wide range of vehicles. These benefits include enhanced traction, improved handling, and increased safety.

• Enhanced Traction— One of the primary benefits of AWD is enhanced traction, particularly in adverse weather conditions like rain, snow, or mud. By distributing power to all four wheels, AWD systems help prevent wheel slip and improve grip, making it easier to accelerate, brake, and steer safely.
• Improved Handling and Stability— AWD systems contribute to better handling and stability, especially in high-performance vehicles. By dynamically distributing power between the front and rear axles, AWD systems can improve cornering and reduce understeer or oversteer, providing a more balanced driving experience.
• Increased Safety— The improved traction and stability offered by AWD systems can enhance safety by reducing the likelihood of losing control in slippery conditions. This makes AWD vehicles particularly appealing to drivers in regions with harsh winters or frequent rainfall.

©, 2018 Rick Muscoplat