This article will cover everything there is to know about the 4-wheel-drive drivetrain and how the various components function. I will try to keep the explanation as simple as possible but still cover all the essential 4WD mechanisms for you to get the best understanding of how traditional 4WD systems work. Let’s start by looking at a basic description of a four-wheel-drive system.
The Four-wheel-drive system works by engine power being sent to all for wheels equally. This is achieved by a center differential that has a front and rear driveshaft’s connected to 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 axles and the vehicle’s torque and power are split 50/50 to the front and rear axles. Front and rear drive-shafts send power to each axle with every wheel getting 25% of the available power. This allows all four wheels to push and pull simultaneously.
Four-Wheel-Drive vehicles come mainly in 2 forms, permanent and part-time 4WD. The next part of this article will cover what 4WD is in more detail and how all its components function. We will also look at the advantages and disadvantages of Four-wheel-drives and the differences between a 4WD and an AWD.
Let’s dive straight into it.
What is 4WD
With part-time 4WD systems, differentials are fitted to the front and rear axle assemblies. The Axle is the component that transfers the power from the diff to the wheels. The differential is the big round bell housing you see underneath your truck and is usually the lowest part of your vehicle. When driving in 2H the power is transferred through the rear propeller shaft or driveshaft to the rear final drive and the differential gears and rear axle shaft. This is similar to the system of a front-mounted engine rear-wheel-drive car.
The gears in the rear differential allow the rear wheels to turn at different speeds when the vehicle turns left or right. While continuing to transfer equal torque to each wheel.
When 4WD is engaged then the transfer case joins the front and rear axles and transmits equal power to the front and rear driveshafts. The drive is transmitted through both the front and rear axle assemblies.
4WD drastically improves the vehicles ability to acquire and maintain traction when you’re driving on slippery, uneven surfaces.
When turning, the swiveling of the front wheels creates a wider turning circle than the rear wheels and this causes a difference in the rotational speeds of the front and rear wheels. This, in turn, causes a difference in speeds of the front and rear propeller shafts (Driveshafts)
Since there is a difference in speed between the inner and outer wheels both axle shafts also turn at different speeds. Differences in speed can also be caused by uneven tire wear between the front and rear wheels or varied inflation pressures.
How 4WD Works
4WD drastically improves the vehicle’s ability to acquire and maintain traction when you’re driving on slippery surfaces. 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.
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.
When to use 4WD
The best time to engage four-wheel drive is when the surface traction is low or limited. I always say, as soon as you leave the tarmac, engage 4WD with the diffs open. It’s a safety mechanism and the vehicle will be more stable on slippery road surfaces since the rear wheels are pushing while the front wheels are pulling.
4H or 4×4 mode allows you to 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
- Slippery 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-low is mainly used to get you through the following:
- Deep soft sand where 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 minimum.
- 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 forces you to lower your speed and gear selection down to 1st or 2nd gear.
Examples 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 vehicles 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.
Each and 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 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 a Full-time 4WD system or a non-permanent 4WD system. These vehicles are purpose-built and can traverse rough uneven terrain with ease where two-wheel drives 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 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 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 drive-train allowing you to descend loose slippery surfaces with ease and in full control.
Most modern 4WD’s 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.
Snow driving is outside the scope of this article, however, I wrote an in-depth article about the differences between AWD and 4WD and which is a better option for snow conditions. You can read it here
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.
- 4×4’s also offers 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 4WD’s to have over conventional two-wheel drives
- The heavy body on frame chassis results in higher fuel consumption
- The body on Frame construction is not as safe as monocoque SUV’s (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 4WD’s 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 by means of 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 4WD’s 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 drive-train 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 it is important for the driver 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.
AWD
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 front or rear axles. Manufacturers make use of various torque splitting methods such as torsion splits, viscous coupling or electronic clutch mechanisms, etc. Each has its advantages and disadvantages.
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 vehicle 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.
(If you would like to read more detail about AWD vs 4WD, a more comprehensive article can be found here.)
4WD Components
The following are the components that make up an SFA or Solid front Axle 4WD system:
- Front axles
- Front differential
- Front driveshaft
- Centre differential
- transfer case
- Rear driveshaft
- Rear differential
- Rear-axle
There are many part-time and permanent FWD vehicles that do not have solid front axles but rather independent front suspensions. Vehicles like the Toyota Fortuner and the Mitsubishi Pajero are both Independent front suspension Permanent Four-Wheel-Drive SUVs.
4WD Transfer Case
The transfer case is coupled, usually behind the gearbox or transmission box, 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 then transferred by means of 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.
(If you would like to read more detail about 4WD Drivetrains, a more comprehensive article can be found here.)
4WD Differentials
A 4-wheel-drive vehicle, depending on if it is a solid front axle or independent suspension, will have a front differential. The center will always have a differential and a rear differential. On 2WD vehicles, a single differential sits in the middle of the front or rear axles (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” does not solely transmit power but does so by means of the gears inside of it which allows the wheels on a single axle to move at different speeds. This is necessary when you are driving around corners where your inside wheel travels less of a distance than your outside wheel. To prevent the wheels from screeching and damaging tires and other driveline components, the wheels must be able to turn at varying speeds. So the inside wheel turns slower than the outside wheel which turns quickly to keep up.
The front and rear diffs of a 4WD vehicle allow the right and left wheels on each axle to rotate at varying speeds so the vehicle avoids wheel skipping or skidding when making a turn.
4WD Lockers
A Locking differential is an amazing piece of engineering if used correctly. This single drive-train component can mean the difference between a good off-road 4×4 vehicle and a great one. If you’ve ever driven off-road with a 4×4 that has a locking rear differential you will be able to relate to how easy it makes a vehicle overcome most obstacles. But what is a locking differential used for and when does one engage the locker? What is the difference between a rear locker and an LSD (Limited Slip Diff)? To find the answer to these questions and more read on…
Differential lockers are used to overcome a limitation of an open differential in off-road conditions by locking the center, front, or rear differential to distribute power and torque evenly to both sides of the diff. By locking a front or rear differential it allows the axle to turn both wheels simultaneously at the same speed irrespective of traction differences.
A locking differential or “lockers” as they are commonly referred to in off-road circles, essentially locks an open differential, may it be the center, front or rear diffs, and forces both axles to operate as one unit and turn at the exact same speed irrelevant of the traction or lack thereof. Unlike an open differential that allows both wheels to turn at different speeds especially when cornering, the diff locker forces the axles to rotate at the same speed and is only designed to be engaged once traction is limited or almost lost.
A locking differential is like a magic button that gives a vehicle a major advantage over an open diff especially under challenging driving conditions like rock crawling or deep loose ruts. Thick muddy conditions and steep declines are also perfect scenarios to engage your lockers.
It is very important to note that locking differentials are only designed to be engaged under very extreme conditions. DO NOT ever engage a locker on the open road especially on the tarmac, and if used incorrectly can cause major damage to expensive drive-train components.
Torque is always transferred to the wheels with the least resistance.
Not all 4×4 vehicles are equipped with a rear diff locker which means you will have to adjust your driving style to compensate for the lack thereof. A vehicle without a rear locking differential could possibly still overcome a difficult obstacle, it just means you will need to use more momentum to get your axles over the obstacle. Once any of the wheels become airborne, or you find yourself in a cross-axle situation, which often happens off-road, the torque will always be transferred to the wheel with the least resistance, resulting in a temporary immobile 4×4….aka stuck! If not enough momentum is used, you could find yourself stranded with your wheels spinning in the air.
Open diff vehicles like the older Land Rover Defenders, Suzuki Jimny, and many others without any TC (Traction Control) aids will need more momentum and struggle a bit in cross-axle off-roading conditions.
Off-road Example:
If one wheel is in mud/sand/snow and the other is on the tarmac with good traction, the torque will always be transferred to the wheel with the least resistance i.e the wheel with the least traction. TC systems have been incorporated to overcome this “weakness”
(If you would like to read more detail about Differential Lockers, a more comprehensive article can be found here.)
READ: What are diff lockers used for
4WD Suspensions
The suspension of your 4WD will make a massive difference in the stability of the vehicle off-road as well as the load-carrying abilities of your vehicle. Upgraded suspensions offer increased ride height and better ground clearance. This is great for off-road tracks and trails.
Once you start kitting and customizing a four-wheel-Drive you are essentially slowly adding more weight to your vehicle. In doing so you are decreasing the vehicle’s payload capacity since your 4WD overall tare/curb weight is increasing with each add-on.
For example, when you add a front steel bumper without upgrading your suspension first you could experience adverse effects on your vehicle’s handling, cornering, and braking abilities. The additional weight could negatively impact the driving dynamics drastically.
I wrote an in-depth article on the weight effects of off-road steel bumpers here.
Also, items such as winches, roof-racks, rear-wheel tire carriers, jerry can holders, deep cycle batteries, drawer systems all add extra weight to your vehicle so it is very important to do a suspension upgrade first to ensure your payload is increased and can safely handle the additional weight. The most popular suspension manufacturers are Fox, Old Man Emu, Iron Man, Dobinsons, Bilstein. All of them offer suspension upgrades for the most popular 4WD vehicles.
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 compliment 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.
GOOD READ: Rock Crawling: Manual vs Automatic
AT (All-Terrain Tires)
These tires are a good all-round tire that allows 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 easier eject and mud trapped between the rubber lugs easier. They are a more aggressive tire pattern and offer excellent traction over an array of surfaces. Mud tires are not that great on-road since the aggressive tire pattern is quite noisy on the tarmac and concrete surfaces. Due to their wider spaces between the rubber lugs, means they are not as “grippy” on low traction wet-road conditions.
I had a set of muds on my Toyota Hilux and I found them to be difficult to drive with on loose sand and dunes. This is because the aggressive mud terrain tread pattern digs into the sand instead of gliding over it as a less aggressive all-terrain tire does.
Best 4WD’s
Technically there is no such thing as the best 4WD since each has its advantages and disadvantages. Also to be able to settle that debate we first need to determine what the actual intended purpose of the 4WD is. There are 4WD’s built purely for off-roading and extreme rock crawling conditions. Others are built for over-landing with heavy loads and equipped for self-sufficient camping in remote places for multiple days at a time. Other 4WD’s are built for a mixture of tradesman work during the week and also double as a recreational vehicle over weekends as well as the occasional camp trip. These vehicles are the most difficult to please segment since customers want a comfortable, safe, reliable Ute which can also do heavy off-road when necessary.
Worst 4WD
The worst 4WD is the one with the lowest level of reliability and build quality. Also, these 4WD’s that are expensive to maintain and are constantly in need of costly repairs are the ones to be avoided at all costs. We won’t be brand bashing here, you know who they are…
Conclusion
In summary, the simple explanation is the following: The engine sends power to the gearbox. The transmission box which is connected to the gearbox splits the power evenly to the front and rear drive-shafts. The front and rear differential in turn split the power an additional 50% to the wheels via the axle. This means each wheel gets 25% of the total available engine power.