(Updated on September 24, 2021)
Looking for an all-terrain vehicle? Perhaps you want something that will be stable in rainy, snowy, or icy road conditions. Not all AWD and 4WD systems are created equally.
If you’re looking to maximize your vehicle’s capability per dollar, you should know the difference between some of the common drivetrain options on the market that power all four corners.
Is AWD the Same as 4WD?
Although the terms are often used interchangeably, AWD and 4WD typically mean different things.
This is largely dependent on who you ask, but for the purposes of this article, 4WD refers to a on-demand, part-time four wheel drive system with a transfer case, and AWD refers to every other system where all four wheels can be driven.
What is 4WD?
The 4WD, four-wheel drive, or 4×4 designation usually indicates some level of off-road capability. These systems are commonly found on pickup trucks, Jeeps, and body-on-frame SUVs.
Body-on-frame SUVs often share a platform with a pickup truck. For instance, the Chevrolet Tahoe and Suburban use the same platform as the Silverado. Crossover SUVs typically share a unibody platform with a car and use AWD instead of 4WD.
How Does 4-Wheel Drive Work?
4WD systems have front and rear differentials, but they use a transfer case instead of a center differential. A transfer case allows the driver to select between 2WD and 4WD as conditions require.
Transfer cases have two sets of gears: one set that allows the driver to switch from low to high mode, and one that allows the driver to select 2WD or 4WD mode.
The “low” gear mode uses a reduction gear that makes your final drive very short, maximizing torque to the wheels. This is particularly useful if you are rock crawling or need to inch up steep terrain.
When 4-Hi or 4-Low is selected, the transfer case links the engine output to the front output shaft that leads to the front differential. This allows the transfer case to drive both the front and the rear wheels at the same relative speed.
If you’re going off road over rugged terrain, 4WD is your best bet. The transfer case allows for no slip between the front and rear differentials when you are locked in 4WD mode. This maximizes traction at both axles and can help get you out of sticky situations.
4WD gives you an out if you go too far down a trail in 2WD and get stuck. With this in mind, you probably don’t want to engage 4WD until you actually need it. There’s an old saying: “4WD lets you go twice as far before you get stuck”. It’s best to only use 4WD to get yourself out of a sticky situation.
You don’t want to leave most 4WD vehicles in 4WD mode if you’re not off road or driving on a slippery surface. Most 4WD systems lock the transfer case so the front and rear axles spin at the same speed.
Each wheel takes a slightly different path and spins at a different speed when you turn a corner. If you are in a high-grip environment (such as a dry paved road), you’re forcing the tires to spin at the same speed which causes excessive wear on both the tires and transfer case.
Therefore, you will want to stay in 2WD mode on the pavement unless perhaps it is icy outside and you need extra traction to get moving.
Types of 4WD Systems
4WD is much easier to explain than AWD because there is really only one type for the purposes of this article. The main differences will be in the front and rear differential technology, and any additional programming that limits wheelspin via the traction control system.
Some trucks have locking differentials on the front, rear, or both. Typically, lockers are only found on solid axles. Since many trucks use independent front suspension, you’re likely to find a locker only on the rear of your truck, if it has one at all.
The traction control system will assist in limiting slip on axles with open differentials. Traction control works by braking the slipping wheel using the ABS system’s wheel speed sensors.
What is AWD?
AWD or all-wheel drive usually indicates the vehicle is designed for on-road use. An AWD system uses computers or limited slip differentials to transfer torque from a slipping tire to a tire with traction. These systems are commonly found in passenger cars, vans, and crossover SUVs.
AWD is implemented differently on each make and model, but most models have three differentials: a front, center, and rear differential. When a tire starts to slip, the AWD system directs the torque from the engine to another corner of the vehicle that has more grip.
How Does All-Wheel Drive Work?
There are many different implementations of AWD, perhaps too many to delve into in this article. All AWD systems share a common trait: they power all four wheels.
The torque distribution will vary depending on the make and model, as will the AWD system’s effectiveness in traction limited scenarios. Some systems can move the vehicle when only one wheel has traction. Some cheaper systems require three wheels to have grip.
As a general rule, the more limited slip differentials the vehicle has, the better. Traction control systems can make up for some of the difference, but they brake a slipping wheel instead of providing additional torque to a wheel that already has traction.
AWD systems require no input from the driver, sending input to the wheels that need it most without you having to think about it. You don’t have to worry about binding when you take a corner on a high grip surface like you do with a 4WD system; the engineers design AWD systems to account for this.
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. Since boxer engines (like those in Subarus) are mounted longitudinally (that is, the pistons move in and out toward the fenders), the transmission and center differential sit just behind the engine.
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.
There are six generations of the Audi Quattro system.
In the first generation Audi AWD system, the front, rear, and center differentials were open by default. The rear and center diffs could be locked with a push of a button located in the cabin, usually near the handbrake.
The second generation changed the center differential to a type 1 Torsen differential. Torsen is a portmanteau of “torque sensing”. The torque bias of type 1 was an even 50/50 between the front and rear axles, but if a wheel slipped the differential automatically biased the torque (up to 75%) to whichever axle needed it more.
Most vehicles at this time still had a manually-lockable rear differential. Some larger vehicles instead had a Torsen differential on the rear axle for better handling.
The third generation Quattro system was only used on the Audi V8 from 1988 to 1994. In manual vehicles, the center and rear differentials were still Torsen type 1 with an open front differential.
Automatic vehicles introduced a new technology in the center differential: a planetary gear system with an electronically-controlled locking clutch. It still had a Torsen type 1 rear differential and an open front differential.
In the fourth generation, electronic differential lock (EDL) uses ABS sensors to detect a slipping wheel and apply brakes to it so that the opposite wheel can receive more torque from the open differential. This was used on the front and rear differentials, while the Torsen type 1 remained the technology for the center differential.
In 2006, the fifth generation of Quattro welcomed a different type of Torsen – type C, also known as ”self-locking”, center differential. This has a default 40/60 split and can automatically send up to 80% of the torque to whichever axle needs it.
The front and rear differentials are still open with EDL.
A sport differential became an option for the rear differential. This system used torque vectoring to increase safety and handling by using various sensors to better encourage the car to turn in the direction the vehicle is directed by the steering wheel.
The last generation of Audi AWD system, introduced in 2010, replaced the Torsen center differential with the “Crown Gear” differential. Up to 70% of torque can go to the front wheels when needed and up to 85% can go to the rear wheels.
This differential is better for all driving conditions and movements. The technology produces a more reliable and quick response while becoming more lightweight and compact.
Which is Better?
The answer to this question depends entirely on your use case.
Do you plan to head to the mountains where it can be muddy, snowy, or sandy? You might want 4WD.
Do you plan to use your vehicle around town and occasionally see ice or snow in the forecast? AWD may be right for you.