AWD vs 4WD (What’s the Difference?)
Looking for an off-road vehicle? You may need one that can stay stable in rainy, snowy, or icy conditions. Not all AWD and 4WD systems are created equal.
If you want to maximize your vehicle’s performance per dollar, you’ll want to know the difference between popular powertrain options on the market that deliver power to all four corners.
This varies widely depending on who you ask, but for the purposes of this article, 4WD refers to on-demand, part-time four-wheel drive systems with a transfer case, and AWD refers to other systems that can drive all four wheels. .
What is 4WD? The designations 4WD, four-wheel drive, or 4×4 usually indicate some level of off-road capability. These systems are often found on pickup trucks, jeeps, and body-on-frame SUVs.
Body-on-frame SUVs often share their platform with pickup trucks. For example, the Chevrolet Tahoe and Suburban use the same platform as the Silverado. Crossover SUVs typically share a self-supporting platform with a car and use all-wheel drive rather than all-wheel drive.
How does four-wheel drive work? Four-wheel drive systems have front and rear differentials, but use a transfer case instead of a center differential. The transfer case allows the driver to select between two-wheel drive and four-wheel drive depending on the conditions. The transfer case has two gear sets, one that allows the driver to switch from low to high mode, and one that allows the driver to select between 2WD and 4WD modes.
The “low” gear mode uses a reduction gear, which results in a very short final drive, maximizing torque at the wheels. This is especially useful when crawling over rocks or needing to climb steep terrain.
When 4-Hi or 4-Low is selected, the transfer case connects the engine output to a front output shaft that goes to the front differential. This allows the transfer case to drive both the front and rear wheels at the same relative speed.
Benefits of All-Wheel Drive
When traveling off-road or over rough terrain, all-wheel drive is the best choice. The transfer case prevents slippage between the front and rear differentials when in four-wheel drive. This maximizes traction on both axles and helps you get out of sticky situations.
If you go too far down the road in two-wheel drive and get stuck, four-wheel drive can get you out. For this reason, it’s best not to turn on four-wheel drive until you really need it. There’s an old saying that “with 4WD you can go twice as far before you get stuck.” It’s best to only use 4WD to get out of a sticky situation.
Disadvantages of 4WD
Most 4WD vehicles should not be left in 4WD mode unless you’re going off-road or on a slippery surface. Most 4WD systems lock the transfer case so that the front and rear axles turn at the same speed.
When going around a corner, each wheel follows a slightly different trajectory and turns at a different speed. In high grip environments (such as dry pavement), you would end up spinning the tires at the same speed, causing excessive wear on both the tires and the transfer case.
Therefore, you should stay in 2WD mode on the road unless it’s icy outside and you need extra traction to get going.
Types of 4WD Systems
For the purposes of this article, 4WD is easier to explain than AWD because there is really only one type of 4WD. The main difference is in the technology of the front and rear differentials and additional programming that limits wheel spin via the traction control system.
Some trucks have locking differentials on the front, rear, or both. Typically the lock is only present on the rigid axle. Many trucks have independent front suspension, so if the truck has a lock, it may only be on the rear of the truck.
The traction control system limits the spin of the axles on an open differential. Traction control works by applying the brakes to the spinning wheels using the wheel speed sensors in the ABS system.
What is AWD? AWD or all-wheel drive usually means that the vehicle is designed for use on the road. AWD systems use computers or limited-slip differentials to transfer torque from the spinning tire to the tire that has traction. These systems are commonly found in cars, vans, and crossover SUVs.
How AWD is implemented varies from make to make and model to make, but most models have three differentials: a front, center, and rear differential. As the tires spin, the AWD system transfers torque from the engine to the other corner of the vehicle that has more grip.
How does four-wheel drive work? There are many different implementations of AWD, probably too many to go into in detail in this article. All AWD systems have one common feature: they drive all four wheels.
Torque distribution varies by make and model, as does the effectiveness of AWD systems in traction-limited scenarios. Some systems can move the vehicle even when only one wheel has traction. Some less expensive systems require three wheels to have grip.
Generally, the more limited-slip differentials a vehicle has, the better. Traction control systems can make up some of the difference, but they slow down the spinning wheels instead of transferring additional torque to wheels that already have traction.
AWD Advantages
A four-wheel drive system requires no input from the driver, and without any thought, it sends the input to the wheels that need it most. You don’t have to worry about locking up when turning on high-grip surfaces, as you would with a four-wheel drive vehicle. Engineers develop four-wheel drive systems with this in mind.
AWD Disadvantages
AWD is more expensive to maintain and uses more fuel due to parasitic drivetrain losses. In other words, it takes more power to move the vehicle, and therefore more fuel.
There are more drivetrain components in an AWD system than an equivalent 2WD model. AWD systems often have strict tire requirements, where the tread patterns and diameters of all tires must match. The difference in tread depth can be no more than 2/32 inch (1.5 mm) in many cases.
Types of AWD Systems
Most AWD systems are full-time, meaning all four wheels are always powered. In newer models, sometimes these full-time AWD systems have a center differential that controls the torque sent to each axle.
Some systems only send torque to one axle during a slipping condition. These are known as part-time AWD systems. This is typically done to maximize fuel economy.
Below are a few examples of how drastically different AWD systems can be, using Subaru and Audi as examples. Most makes use several different types of AWD systems, so it’s best to do your research before purchasing.
Subaru Symmetrical AWD
Subaru is legendary for its AWD systems, honed and perfected through competitive rally on dirt roads. Depending on which year and model you buy, there are several different types of symmetrical AWD systems. The vast majority of Subarus place the center differential and transmission in the same housing.
Viscous Coupled Center Differential
A viscous center differential is used on models with a manual transmission. This system uses 50/50 torque split between the front and rear differentials. When a wheel on one axle starts to slip, it heats up the fluid in the center differential and provides a locking effect, transferring torque to the other axle.
Active Torque Split
The Variable Torque Distribution (VTD) system is used on automatic and CVT Subarus (except the CVT WRX). This system uses a 60/40 front biased torque split with a clutch pack in the center differential to distribute the torque when wheel slip occurs.
Variable Torque Distribution
The CVT WRX uses a Variable Torque Distribution (VTD) system that distributes torque with a 45/55 split, slightly biasing torque rearward in an effort to reduce understeer.
Driver Controlled Center Differential (DCCD)
The Driver Controlled Center Differential (DCCD) is only available on the WRX STI. It uses a standard 41/59 torque split (35/65 until 2006).
There is a knob in the cabin that allows the driver to control the amount of lockup in manual mode, and which way the computer biases torque in automatic mode.
The DCCD system has historically used a combination of a mechanical and an electronic limited slip differential, but in 2018 they switched to a completely electronic center differential.
