A Top Guide To Drivetrains: AWD Vs. FWD Vs. RWD Vs. 4-WD
This article will cover everything there is to know about AWD Vs. FWD Vs. RWD Vs. 4-WD drivetrains and how the various components function. I will try to keep the explanation as simple as possible but still cover all the essential concepts for you to get the best understanding of how traditional drivetrain systems work. Let’s start by looking at a basic description of a Front-wheel-drive system then progress to RWD and finally 4WD and AWD.
Let’s dive straight into it.
Table of Contents
AWD Vs. FWD Vs. RWD Vs. 4-WD (Front-Wheel Drive Explained)
A front-wheel drive vehicle is one where the front wheels are propelling the vehicle forward. It’s as simple as that. It is a two-wheel-drive concept because 100% of the engine’s power is sent to the two front wheels which is split 50/50 when driving in a straight line. The front wheels pull the vehicle forward.
What are some of the pros and cons of a Front-Wheel Drive system? Well, since the two front wheels are pulling the vehicle and they are also responsible for steering the vehicle, there is some understeer. Now this isn’t a problem anymore with power-steering being a standard feature on almost all modern vehicles today. Older vehicles that are not equipped with power steering feel the negative understeer effect a lot more when cornering at speed.
Another side-effect of FWD vehicles is torque-steer. Torque steer occurs when you accelerate aggressively in a certain direction and the front wheels lose traction causing the vehicle to drive straight thus temporarily losing most of the steering ability. Most manufacturers have solved this problem by installing electronic traction control systems to manage the amount of power being sent to the wheels, thus allowing for better control.
The main advantage of a front-wheel drive car (FWD) is that it is easier and cheaper to develop since there are fewer drivetrain components to manufacture and assemble. The front wheels have a side shaft with a CV Joint on the end to allow the vehicle to be propelled forward while steering simultaneously. This also means the vehicle requires fewer heavy mechanical components which means it is cheaper to develop and more economical.
The absence of a rear driveshaft and differential not only means better fuel economy but also means there is more space beneath and more importantly inside the vehicle. This allows for more legroom inside, especially in the middle as well as slightly more trunk space. A FWD vehicle has more control on slippery surfaces and is easier to handle when compared to a RWD (rear-wheel drive) vehicle that can lose traction at the rear wheels, thus causing the rear to spin out. In both cases, a set of winter tires can improve traction and handling on slippery surfaces such as snow or icy roads.
Advantages In Summary
- Easier and Cheaper to Manufacture
- Easier to drive
- Fewer drivetrain components
- More passenger legroom space and
- Bigger Trunk space
- More economical (MPG)
- negative effect on cornering
- More fuel efficient
- Easier Towing
Disadvantages in Summary
- Torque steer
- Slight Understeer effect
- Less sporty feel
RWD (Rear Wheel Drive) Explained
A rear-wheel drive vehicle is one where the rear wheels propel the vehicle forward. It achieves this by means of a rear driveshaft and differential. 100% of the engine torque is sent to the rear axle via the driveshaft which is split 50/50 to each wheel when driving in a straight line.
There are some advantages and disadvantages to this drivetrain type when compared to a front-drive vehicle or FWD. Many high-performance vehicles and sports cars are developed with a rear-wheel drive system. It makes more sense to develop a performance car using a RWD drivetrain since it offers a much more engaging and responsive feel.
In many cases, a rear-wheel drive vehicle is designed with the engine mounted at the rear or middle of the chassis instead of the front of the vehicle. In sports cars, this is an advantage since it allows the power to be sent directly to the rear tires without the need for a long driveshaft. This makes for a very exciting feel with quick response times and power on demand.
| Advantages in Summary | Description |
|---|---|
| Sportier more dynamic drive | Enhanced driving experience with a more dynamic feel |
| Improved driver experience | Overall improvement in the driver’s perception |
| Better layout for sports cars | Optimal design suited for sports car performance |
| Sharper steering feel | Enhanced responsiveness and precision in steering |
| Disadvantages in Summary | Description |
|---|---|
| More expensive to develop | Higher proficiency and expertise are needed to drive |
| More drivetrain components | Increased number of components in the drivetrain system |
| More advanced driving skills required | Higher proficiency and expertise needed in driving |
| Less interior space in some cases | Reduced interior roominess in certain scenarios |
When it comes to mass-produced passenger sedans, SUVs, and Pickup trucks, a rear-wheel drive car requires a slight change in driving style when compared to a FWD. You can experience some oversteer when accelerating into a corner since the steering wheel feel is a lot more sensitive. Manufacturers have made modern RWD vehicles a lot safer and easier to drive with the incorporation of traction control and steering control systems to assist the driver. This makes a modern rear-wheel drive vehicle just as safe and easy to drive as a modern front-wheel drive vehicle.
4WD (4-Wheel Drive) Explained
The four-wheel-drive system works by engine torque being sent to all wheels equally. This is achieved by a center differential that has a front and rear driveshaft. When you engage 4H from 2H these driveshafts are connected to the front and rear differentials that split the power equally and manage the transfer of power to each wheel.
When 4WD is engaged the transfer case joins the front and rear driveshafts and the vehicle’s torque and power are split 50/50 to the front and rear axles. Front and rear driveshafts send power to each axle with every wheel getting 25% of the available power. This allows all four wheels to push and pull simultaneously. The disadvantage is that it uses slightly more gas and 4WDs can sometimes get stuck in 4H due to various reasons.
4WD vehicles come mainly in 2 forms, permanent and part-time 4WD. 4WD drastically improves the vehicle’s ability to acquire and maintain traction when you’re driving on slippery surfaces like wet roads, sand, or snow. 4 is better than 2 so instead of only 2 wheels propelling the vehicle forward you now have all four wheels clawing for traction and propelling the vehicle forward, irrespective of the surface conditions. A Part-time 4WD is normally driven in 2H when on high traction surfaces or when being towed.
Example: You are in a 2 wheel drive rear-wheel driven vehicle and you are stuck with two wheels in the mud or snow. The two wheels in the mud/snow have lost traction and the vehicle cannot be propelled forward. With a 4WD vehicle, the other 2 wheels will pull the vehicle out since 50% of the power is split between the 2 wheels with good traction and the vehicle will be able to drive out.
AWD (All-Wheel Drive) Explained
This system is permanently in 4H mode and cannot be changed to 2H where only the rear wheels are propelled. Some manufacturers incorporate a center diff-lock function which allows the driver to manually lock the front and rear driveshafts in place to propel as a single unit. Most traditional AWD does not have Low Range (4Lo) functionality.
AWD is usually managed by a computerized system that controls the power split ratios. Power is fed to each wheel and certain vehicle AWD systems allow power splits to be adjusted on the fly by the driver i.e. 50/50% or 70/30% split etc.
Other AWD systems function automatically and adjust the power splits once the computer senses traction is lost on either the front or rear axles. Manufacturers make use of various torque splitting methods such as torsion splits, viscous coupling electronic clutch mechanisms, etc. Each has its advantages and disadvantages.
When to Engage 4WD
The best time to engage 4WD is when the surface traction is low or limited like snow, or rainy wet conditions where hydroplaning is a risk. Many modern 4WDs are being manufactured with 4WD-Auto or 4A to eliminate driver error and preserve your 4WD drivetrain. I always say, as soon as you leave the tarmac, engage 4WD with the diffs open.
It’s a safety procedure and the vehicle is more stable on slippery road surfaces since the rear wheels are pushing while the front wheels are pulling. Never engage 4WD when driving on dry pavement or the highway. This will result in drivetrain binding and catastrophic damage. If you start hearing any whining, or grinding disengage 4WD, stop, and inspect for anything obvious.
4H or 4×4 mode is designed to help drive over and through very slippery, low-traction surfaces where you still need to maintain normal driving speed. Ideal 4WD conditions are the following:
- Loose Sand
- Snow roads
- Ice-covered roads
- Gravel roads
- Wet roads
- Low traction surfaces
- Muddy tracks
When to Engage 4-Lo
4WD Low-range or 4-Lo is a very powerful function of a 4WD and allows you to accomplish feats no 2WD or even AWD ever could. 4-Lo is mainly used to get you through the following:
- Deep soft sand where the surface offers high resistance
- Driving over tall sand dunes where traction is low and maximum power is needed
- Up steep uneven hills where surfaces are slippery and traction is low
- Down difficult mountain tracks where surfaces are slippery and traction is minimal.
- It will even get you through deep mud or thick soft snow
- It allows you to drive over boulders and deep ruts with ease.
4-Lo when Descending Steep Hills
The lower gearing also improves your 4×4’s stability and control since it utilizes the vehicle’s engine braking, which helps to control your progress and speed while going downhill.
This advantage allows for more controlled handling without placing all the stress on the brakes alone. The Low ratio gearing also places less stress on the gearbox, engine, and clutch by assisting the engine and clutch to overcome slippery surfaces where high engine torque is needed to conquer it.
As mentioned earlier, 4-Lo is used only when you require optimal traction at low speed and maximum power availability. This is only to be engaged when the terrain is really difficult and driving conditions force you to lower your speed and gear selection down to 1st or 2nd gear.
Examples of when 4-Lo can be engaged.
- Extreme rock crawling when you need to negotiate each obstacle slowly and precisely will call for 4-Lo.
- Driving through thick loose beach sand with a load will require 4-Lo to prevent damage to your vehicle’s clutch or to decrease the risk of overheating the gearbox if you are driving an automatic 4WD
- When doing any deep river crossings you are better off engaging 4-lo to ensure you are getting optimum traction and using the power available to you sufficiently.
4-Lo is used when you require optimal traction at low speed with maximum power availibility.
Every off-road situation or obstacle needs to be carefully calculated and evaluated by the driver before he decides to take it on. However, a good rule of thumb is always to judge the speed vs the difficulty, and ground clearance you are required to succeed. Always remember “As slow as possible and as fast and necessary”, no more, no less.
Generally, if you cannot conquer it in 4-Hi, 2nd gear, or 3rd gear to cross an obstacle or if you feel the obstacle will place too much mechanical stress on your vehicle, risking damage, then you are better off engaging 4-lo and rather take it slow and steady.
4WD Advantages
Most vehicles that are produced for off-road applications are either equipped with an AWD, Full-time 4WD, or a non-permanent 4WD system. These vehicles are purpose-built and can traverse rough uneven terrain with ease whereas 2WD can easily lose contact with the ground surface, spinning, and leaving you in a temporarily immobile state….aka stuck!
When the rear and front axles are engaged and all 4 wheels are locked in 4H, the traction is significantly improved and the vehicle can drive out of that challenging situation with relative ease.
Let’s explore some more of the advantages.
Four-wheel drives offer superior grip and traction over a variety of surfaces.
Power is also utilized better since all 4 wheels are propelled in 4WD giving you a more sure-footed and confident feel.
When conquering challenging terrains such as mud, snow, rocks, and other difficult driving environments the 4WD comes into its own by maintaining traction under a variety of low traction surfaces.
The additional weight the body on the frame chassis brings is actually an advantage when venturing off-road.
Advanced 4WD Technology
4WD vehicles have evolved drastically over the years and car manufacturers such as Land Rover and Jeep have spearheaded the technical evolution of this segment. For example, Land Rover has the Pneumatic Air Suspension & Terrain Response System incorporated into their luxury SUV Four Wheel Drives. This is a highly sophisticated electronic system that controls the behavior of the vehicle over various surfaces. This affords the driver options to select the perfect setting for the specific terrain he is driving on.
This TRS system can make a novice off-road driver look like a pro since the intelligent software sets all the correct throttle, ride height, braking sensitivity, and other traction adjustments for you, at a simple flick of a button.
Cornering is also superior as the power is equally transferred to all wheels allowing the load to be reduced on each wheel.
Engine compression braking is improved as it works in conjunction with the 4WD drivetrain allowing you to descend slippery surfaces with ease and in full control.
Most modern 4WDs have technology like hill descent control and hill climb assist as standard, which eliminates some of the guesswork for the driver. It is designed as a safety feature allowing the vehicle to be driven much slower and more controlled under extreme descents.
If you live in a part of the world that gets regular thick snowfall during winter, an AWD or 4WD will be perfectly suited for those conditions. They offer many advantages in snow and ice-clad roads.
In a 4WD you have more confidence to travel on low traction surfaces like snow and ice since power is transmitted evenly across all 4 wheels the amount of grip you have on snow is double that of a 2WD.
- 4WD is more balanced and sure-footed on slippery roads since power is distributed evenly.
- You can confidently pull off from a standstill without spinning your wheels in 4WD mode.
- The higher driving position allows you better visibility over traffic.
- High ground clearance offers a good vantage point to spot danger up ahead.
- The excellent ground clearance allows you to drive over instead of through thick snow with ease.
- 4WD offers excellent straight-line stability due to the superior grip offered on multiple surfaces.
- 4x4s also offer a sense of safety and confidence to explore uncharted territory more comfortably
4WD Disadvantages
Four-wheel drives are not the holy grail of the motor world unfortunately and it all comes with a trade-off. Below are some of the disadvantages I’ve noted that 4WDs have over conventional two-wheel drives
- The heavy body on the frame chassis results in higher fuel consumption
- The body-on-frame construction is not as safe as monocoque SUVs (Although the safety of Ladder Frame Utes has greatly improved in the last few years)
- A Hi center of gravity negatively affects your overall MPG and results in higher wind noise inside the cabin.
- Bigger longer vehicles are not easy to maneuver and parking in shopping mall lots, as well as city driving, can be tricky in areas where roads are narrow.
- Bigger vehicles take longer to stop.
- More components to service make for increased service costs.
- More expensive purchase price over 2WD vehicles.
- Higher maintenance costs as they are purpose-built and some require specialized components and tools to repair.
- Ladder frame 4WDs do not offer good on-road handling in terms of body roll and cornering dynamics.
4WD vs AWD
This part of the article will discuss the differences between permanent and part-time 4WD and explain the fundamental advantages and disadvantages between the 4WD and AWD drivetrains.
Permanent 4WD
With a permanent 4WD drive-train configuration, the power is ALWAYS split between the front and the rear axle using a transfer case and viscous coupling. The viscus coupling allows the front and rear driveshafts to rotate at different speeds when needed, especially when cornering to prevent drive-train “binding”.
Most traditional permanent 4WDs have low-range functionality. Many have a center differential that can be locked. This mode can be engaged manually to lock the front and rear driveshafts to distribute engine power 50/50 between the front and rear axles.
Many modern permanent 4WD vehicles make use of traction control or rear diff lockers for conquering challenging off-road conditions. The drivetrain is always in 4H in an open state and can never be removed from 4WD mode and placed in 2WD.
Non-Permanent 4WD
These vehicles have 2H/4H/4Lo functionality which means the power is normally sent to the rear axle in 2H for safe daily driving on a variety of surfaces. 2H also returns decent fuel economy since there are fewer drive train components involved in propelling the vehicle forward. Centre diff-lock (4H) can be engaged and power can be split evenly between the front and rear axles.
The drive-train mode is decided 100% by the driver, based on driving conditions, so the driver needs to be informed of the correct mode for each unique driving surface. Non-permanent 4WD has low range (4Lo) functionality for conquering challenging off-road conditions. Modern 4WD Utes usually have a rear diff locker or Traction control or both for difficult off-road driving conditions.
4WD vs AWD Comparison
A Four Wheel Drive (4WD) is optimized and designed for challenging driving conditions like rock-climbing, fording deep rivers, and conquering steep hills with loose, low traction surfaces. The fact is that many 4WD owners seldom need this extreme capability that a 4WD offers unless you are a serious off-road enthusiast.
With that being said, in many cases, an AWD sadan like a Subaru does not have the high ground clearance offered by a 4WD UTE or SUV. This means when snow conditions get really bad your AWD might not have enough clearance to plow through thick snow and you might find yourself bogged. There is very little benefit of all four wheels turning without enough traction or ground clearance to clear the obstacle.
4WD Components
The following are the components that make up an SFA or Solid front Axle 4WD system:
| Component | Description |
|---|---|
| Front axles | Axles located at the front of the vehicle |
| Front differential | Differential at the front axle, distributing power to front wheels |
| Front driveshaft | Shaft transferring power from the transmission to the front differential |
| Centre differential | Differential distributing power between front and rear axles |
| Transfer case | Mechanism distributing power from transmission to differentials |
| Rear driveshaft | Shaft transferring power from the transmission to the rear differential |
| Rear differential | Differential at the rear axle, distributing power to rear wheels |
| Rear-axle | Axle located at the rear of the vehicle |
Many part-time and permanent FWD vehicles do not have solid front axles but rather independent front suspensions.
4WD Transfer Case
The transfer case is coupled, usually behind the gearbox or transmission, and splits the power from the engine 50/50 to both the rear and front axles. It accomplishes this by means of front and rear driveshafts.
So, when the vehicle is a part-time 4WD, it spends most of its time in 2H which means 100% of the power is always sent to the rear differential and then split 50/50 to each wheel via the rear axles. All engine power is sent to the transmission first and then transferred using an output shaft to the transfer case/box. The output shaft that’s inside the transfer case is connected to the rear driveshaft. The rear-drive shaft then sends the torque to the rear differential. The rear differential then propels the wheels moving the vehicle forward.
4WD Differentials
A 4-wheel-drive vehicle, irrespective if it is a solid front axle, independent suspension, or a combination of the two, will have a front, center, and rear differential. On 2WD vehicles, a differential is located in the front or rear of the vehicle (depending on if the car is front- or rear-wheel drive). Power from the drive shaft is transferred through the differential to each wheel, causing them to turn. On a 4WD vehicle, because all four wheels are getting power, it needs two differentials — one for the front axle and one for the rear axle.
The “diff” manages power using gears inside which allows the wheels on the axle to rotate at different speeds when cornering. This is necessary since your inner wheel travels less of a distance than your outer wheel.
4WD Tires
Four-wheel drive tires are probably one of the most important components that make up a capable 4WD. The level of grip and control caused by the tires will either complement or restrict your four-wheel-drives capabilities.
There are two main types of tires used for off-road and those are AT or All-Terrain tires and MT which represents Mud Terrain tires. There is a third type of tire which is a very aggressive soft compound tire called a trail grappler. These are used mainly for Rock Crawling applications and competition trucks.
AT (All-Terrain Tires)
These tires are good all-rounders that allow you to traverse many different terrains confidently. They are commonly comprised of 2-3 ply sidewalls. The strength of the side walls in conjunction with an upgraded suspension allows you to increase your vehicle’s payload (Payload is the amount of weight your vehicle can safely handle) 3-ply tires also allow you to decrease your tire pressure more with a lower risk of sidewall damage or punctures. All-terrain tires are fantastic on gravel, sand, and rocks. They also offer decent on-road performance on wet and dry low-traction surfaces.
MT (Mud Terrain Tires)
“Muds” or “Muddys” as they are commonly referred to are off-road tires with bigger lugs and spaces between them. This allows the tire to eject and mud trapped between the rubber lugs more easily. They are a more aggressive tire pattern and offer excellent traction over an array of surfaces. Mud tires are not that great for on-road since the aggressive tire pattern is quite noisy on the tarmac and concrete surfaces. Their wider spaces between the rubber lugs, mean they are not as “grippy” on low traction wet-road conditions.
