All Mainstream AWD and 4WD Systems Compared and Explained

Mitsubishi Evo X GSR at Atlantic Motorsport Park - Kevin Doubleday ©

If you live in Canada or the US, you'll find that plenty of people hold sacred the terms '4x4' and '4WD' to describe a 'true 4x4', where you have a butch transfer case with a low speed, perhaps a body on frame chassis, and ideally a solid axle or two. I'm not sure how that translates to the rest of the world. My extensive research into the motoring industry in Europe (which exclusively consists of watching Top Gear and The Grand Tour...) concluded that most people across the pond simply refer to any vehicle that is capable of sending any power to all four wheels as a 4WD vehicle, further muddying the waters. Where I grew up, 4x4 was more or less synonymous with 'Jeep' so that's not much help either.

However, despite all various systems attempting to do the same sort of thing - distribute power between all four wheels instead of two - not all systems are created equal, with significant differences in some cases. As a primer for a future post about handling in AWD cars, I figured I'd take the time to briefly explain different types of systems.


Old-school 4x4

This system is very simple in operation and is probably the easiest one to wrap your head around. Power goes from the engine through the transmission/gearbox just like a two-wheel drive car. After that, power goes through a transfer case that has two output shafts (drive shafts/prop shafts); one going to the front and another going to the rear.

Traditional 4x4 drivetrain schematic (front axle drive disconnected) - Rams Eye The Track Guy ©

Power going to the rear is "in-line" and is the main shaft, meaning it is always connected to the transmission/gearbox. The second (front) shaft is connected to the main axle in the transfer case by a power transfer mechanism (most often a chain, but could be a gear drive). There is a gear or "sprocket" that the chain or gear drive in the Transfer Case is meshed with and it can be engaged with or disengaged from the main driven shaft, much like different gears on a manual transmission. If it is not engaged, you are sending power to the back only and the gear is free rolling. If it is engaged, power is split 50:50 between the front and the back. It is possible to change power distribution by changing the gear ratio on the chain drive or final axle ratios, but I'm not aware of any vehicle that has an old school 4x4 and an uneven front-rear power split (6x6 vehicles are different).

Traditional 4x4 drivetrain schematic (front axle drive connected) - Rams Eye The Track Guy ©

Once you engage/mesh the front axle drive gear, power is split between the front and the rear axles, and whatever the power split is, it is always fixed. This system is often the most preferred by hard core off roaders because it takes the "guessing work" out of the drivetrain. Unlike modern/active AWD systems, you put the system in 4 wheel drive mode (4 high or 4 low) through a switch, button, or lever and it stays there. You put your foot down, and power is split in half between the front and the back without a computer guessing or deciding where to put the power. It's dependable, consistent, and robust.

The main downside to this system is that it allows no speed differentiation between the front and rear axles because they are connected together by a solid mechanical linkage (i.e. chain or gears). That's fine if you are going in a straight line but once you go to turn, each axle has to spin at a different speed (if you don't know why, you can read my introductory post about differentials here).

If you are driving off road or on snow/ice/mud, that's also fine because each wheel/tire can easily slip (very slightly) to go at the required speed for the turn. If you are driving on dry or damp roads, however, slight tire spin is not that easy and the result is binding and high stresses on drivetrain components, and it provides resistance to turning (understeer). Moreover, in high performance driving situations, you want to constantly shift power front to back (and side to side) depending on where you have grip and this system has a fixed front:rear power split.

In short, the system is robust and great off road but has a fixed power split and no way to allow front and rear axles to rotate at different speeds, making it far from ideal for high performance applications on tarmac/asphalt.


Early Part-time AWD

Early part-time systems (and their current evolution, more on that below) were by far the most common. Although the early version is almost completely replaced by the modern type (below), some still use it. Haldex is a very well known manufacturer of this type of system (old and modern), so much so that some people simply refer to this design as a "Haldex-type". It's the least expensive and most flexible type of system to fit to a car.

FWD-based AWD System Schematic with Viscous Coupling - Rams Eye The Track Guy ©

In FWD-based vehicles (i.e. transverse engine placement), power goes from the engine to the transaxle (gearbox/transmission and differential, front diff in this case). Power is then split front and back. the front transaxle (transmission/gearbox) typically has a Power Take Off (PTO) that sends an output shaft to the back. The secondary shaft (rear driveshaft on FWD-based cars) is connected to the secondary driven axle using something called a viscous fluid coupling.

In essence, the coupling (enlarged in the schematic) consists of a "thick" fluid (light grey in schematic) in it and multiple plates attached to the input shaft (red in schematic) and output shaft (green in schematic). The plates are designed to shear the fluid - or "turn/spin" it. If the car is going straight (i.e. front and rear axles are rotating at the same speed), though, the input and output plates are spinning at the same speed along with the viscous fluid inside the coupling. When everything is going at the same speed (no speed differences), nothing is trying to turn anything else (relative speed is zero) and no power is transferred to the secondary axle. If the main axle starts to spin, though, things change.

Taking a look at a typical FWD drivetrain (schematic above), if the main driven axle (front) starts to spin, the input plates connected to the front axle (red) start spinning relative to the output plates connected to the rear axle (green). The viscous fluid, in turn, starts to thicken and spin along with the input plates. The "spinning" fluid attempts then attempt to spin the output plates (green) which are connected to the rear axle. That results in power starting to shift from the front axle to the rear axle.

RWD-based AWD System Schematic with Viscous Coupling - Rams Eye The Track Guy ©

On RWD-based applications, the split typically happens using a transfer case on the output shaft of the transmission similar to a 4x4 system. The principles are the same. Rear axle starts to spin, this results in spinning the input shaft in the coupling through the transfer case, which in turn spins and shears the fluid relative to the front axle (which isn't slipping, meaning it's rotating slower), and power starts to get transferred from the rear axle to the front.

The benefit is that engine power is not all going to the main driven axle (reduces spinning at that axle) and some is sent to the secondary axle (better use of available traction). There are a few issues with the system, however.

For one, the system is reactive. You first have to spin/lose traction at the main driven axle to transfer power to the secondary axle. Secondly, unless you use different gear ratios in the front and rear axles, it can't send more power to the secondary axle than what is needed to slow the fronts down/match speed, meaning you will never get proper power distribution and all you can do is reduce spinning at your main axle AFTER it has started losing grip. The third (and perhaps critical) issue with this system is that it could transfer basically zero power to the secondary axle if the main driven axle has no traction at all.

For instance, in a FWD-based car, if the front wheels are on ice and any amount of power simply spins them, they will be spinning MUCH faster than the (stationary) rear axle. That huge speed differential overheats the fluid and renders it incapable of transferring any power. In other words, the AWD system becomes no better than a 2WD vehicle. And most earlier systems had "open" front and rear differentials which meant you only needed 1 wheel/tire on your main driven axle to have no traction to render the entire car immobile, much like a two-wheel-drive car.

On the bright side, though, due to the lack of a solid mechanical linkage to transfer power to the back (i.e. using a fluid coupling), the system allowed speed differentials between front and back to smoothly go around turns. That meant you can use it on asphalt/tarmac even if it is dry. For most high performance driving applications (i.e. track days/lapping/time trials), you could keep putting power down and increasing acceleration even after you've exceeded the limits of the main driven axle because spinning/losing grip sent more power to the secondary axle, a huge benefit to pace.

With that said, it still had a bad reputation for being a "marketing gimmick" or "no better than a 2WD car." The two main reasons for that were:

1. As mentioned above, it was just as easily defeated in extreme driving conditions as a 2WD vehicle. It was a huge convenience for lazy drivers on the road because in "mildly bad" road conditions like shallow snow and slush, you could simply put your foot down and the car would most likely move without having to feather the throttle and carefully take off. And the system did help most of the time on the road, making it easier to take off in poor driving conditions. The trouble is, you can also get by "most of the time" with just 2 wheel drive if you're careful. In extreme driving conditions (which is what most people buy AWD/4WD for) it is just as easily defeated as a 2WD vehicle.

2. It did not change the handling characteristics of FWD car, the predominant application of this type of system. This is because a FWD-based car with this type of AWD system drove just like a FWD car without this system up until the front axle started to lose grip, then you started to transfer power to the back as needed to minimize spinning at the front. In other words, driving a FWD-based car with this type of system on a track feels exactly the same as driving one without, with the only difference being much higher traction limits. It meant going faster but not "better".


Modern Part-time AWD

Newer versions of this system (part-time) are very similar in principle of operation but use a multiplate clutch pack instead of a viscous coupling, which significantly improves the shortcomings. The clutches in the clutch pack are electronically controlled and are progressively engaged and disengaged as needed based on traction in the main driven axle (i.e. front on FWD or rear in RWD vehicles) as determined by various sensors and programming. If slip is detected at the main axle through sensors, the clutch packs are modulated or engaged to send power to the secondary axle.

FWD-based AWD System Schematic with an electronically controlled clutch-pack - Rams Eye The Track Guy ©

In the schematic, the clutch pack looks basically identical to the viscous coupling. The crucial difference is that the input plates (red) and output plates (dark grey) slide on the axles that they are connected to. They are electronically controlled and amount of lock is determined by the AWD controller. The amount transferred to the secondary axle depends on the clamping force in the clutches. In the viscous coupling, the plates do not slide, they are fixed. They transfer power through the fluid, instead of through physical locking/clamping of clutches between input and output shafts.

In a FWD-based system, zero lock sends zero power to the rear and you can progressively engage to send up to 50% at max lock (assuming equal front and rear final gearing, which is the most common).  In a RWD-based system, the clutch-pack is often in the transfer case or in the gearbox. The output shaft of the clutch pack is connected to a gear in the transfer case like traditional 4x4 above, but instead of that gear being engaged (meshed) or disengaged for on/off 4x4, it receives output from the clutch-pack. The amount of power sent to the front axle through the transfer case depends on lock % in the clutch-pack.

RWD-based AWD System Schematic with an electronically controlled clutch-pack - Rams Eye The Track Guy ©

The main benefit is that the computer can be predictive instead of reactive like the earlier viscous coupling. In other words, many modern vehicles with this type of system will pre-engage the clutches if you are stopped in freezing temperatures, if it detects rain, etc. in anticipation of needing traction to take off. That will reduce the likelihood of slip in the first place instead of waiting for slip to happen. Modern control algorithms can also predict the need for locking in high performance applications (i.e. approaching turns, downshifting, etc.). Some systems are also designed with a fixed/minimum amount of lock at all times to simulate a permanent AWD system, meaning you are never sending all power to only one axle.

Another huge benefit is that at full lock, the system operates essentially like a traditional 4x4 systems (minus low-gear/crawl ratio). It provides a solid mechanical linkage sending power to the secondary axle (rear in FWD-based and front in RWD-based) so it doesn't matter if you have no traction at all in the main driven axle, the secondary axle will still get power. This is why a lot of modern pedestrian AWD cars can claim "up to 100% of torque/power can be transferred to either axle" as needed even though you can never disconnect the main driven axle (front, most often). At full lock, if the main driven axle has zero traction, the secondary axle is still connected to power through the clutch pack and will receive all the drive torque.

One downside with this system, however, is that the only way to vary power or go around turns while it is engaged (i.e. not only sending power to the main driven axle) is by slipping clutches. That means that it can overheat in high performance/competitive driving due to the large amounts of power transferred for extended periods of time along with slippage. The same is true for varying power, because except for 100% lock (power is split evenly front and back) and 0% lock (all power going to the main axle), the system varies power by varying the clutch pack clamping force. Less clamping force means more slippage and less power transferred. That's the same principle as slowly engaging the clutch (i.e. slightly slipping the clutch) in a manual car after you put a car in 1st gear to take off smoothly instead of dropping the clutch and having power jerk the car.

With that said, the system is very flexible because it can vary power sent to the secondary axle simply by varying clutch engagement but it can also fully lock in extreme driving conditions. And having multiple clutches combined with a wet-clutch design (the system is bathed in fluid/lubricant that also helps with cooling) means that the system can manage heat and slippage far better than a traditional dry clutch system like you'd expect in a manual car. When sized properly to handle heat, it can be a very powerful AWD system and the clutches can be permanently engaged (partially) so that some power is always sent to the secondary axle and it functions like a true AWD system.


True AWD

This system is rare but it is the best type of system. It is almost (almost) exclusively used on RWD-based drivetrains. It uses a "centre" differential to distribute power between the front and the rear axles. Think of a traditional differential on an axle with two output shafts going to the wheels, only difference is that the differential's output shafts are going front and back and splitting power between front and rear axles, instead of side to side on an axle.

Power goes from the engine to the gearbox/transmission to the centre differential. Rear axle (black) and front axle (light grey) are connected to the two "side" gears in the differential. All operating principles are the same. With that said, the centre differential can often be a different design than traditional bevel gear differentials - either a Torsen gear-type limited slip differential or a planetary type like in an automatic transmission.

RWD-based True AWD System with a centre differential - Rams Eye The Track Guy ©

This system is always splitting power just like an axle differential splits power between two wheels, so it provides the least likelihood of slip. It is comparable to only clutch-type AWD systems above that have minimum % lock/clamping force permanently applied to always send some power to the secondary axle.. But using a differential in the middle also means smoothly going around turns with no slip required (from a clutch pack or wheels/tires like an old school 4x4). Yet, you still have power being distributed front and back. Another huge benefit is that you can design it for a rear-biased torque split by changing the differential gear ratios using a planetary differential, for instance. That becomes very attractive for high performance applications.

On track, it is smoothest and most consistent, always splitting power according to the designed power split between front and rear. It generates the least amount of heat with the least amount of power loss. The only real downside to this system are the same downsides that I discussed in the post about why open differentials don't work on track. It will always send no more power than the low traction side can handle (you can read more about that in the linked post). As you go around turns and weight is transferred, the axle with the lower traction determines how much power the other axle gets (i.e. in corner exit, front axle has reduced traction due to weight transfer to the back under acceleration, plus it is using some grip for lateral road holding/steering until you're straight. The front axle limited traction will limit how much power you can send to the rear axle).

By the same token, the same limited slip differential technologies/systems can be applied or used in place of an open differential to limit those. You can read about those in my post about traditional limited slip diffs and more clever limited slip systems (upcoming!). This takes advantage of the benefits of a centre differential and uses limited slip technologies to to transfer power to the high traction axle as needed once there is slip.

Some high end SUV's with true off road credentials will also use this system so you have true all-wheel-drive in mild conditions (rain, slush, shallow snow, etc.) combined with a true locking mechanism that works like a true 4x4 system in extreme driving conditions (ice, mud, deep snow, etc.). Without true locking, if either the front or the rear axle has no traction, it will spin endlessly and you won't get moving like an open diff. The best high performance systems use the same strategy; a combination of a centre differential and a limited slip technology/mechanism like clutch packs (electronic or mechanical), Torsen differentials, etc. (more on that in a future post!).

Where do you find each?

The vast majority of AWD/4WD vehicles today use a form of the Modern Part Time AWD system with an electronically controlled clutch pack, including VW's, Ford's, Chevrolet's, and even small Audi's (A3/S3/RS3). Old time 4x4 systems are very limited these days. They were used in older SUVs but now they are limited to vehicles like trucks, Jeep Wranglers, LR Defender, the venerable Suzuki Jimmy, etc.

All AWD systems available on production cars use one of the above main designs, but with variations to suite special requirements and demands, like in the Nissan GTR or Mitsubishi Evo for instance.
I'll be making another post in a few days to discuss and compare legendary cars that have well known AWD systems like true Audi Quattro's, Mitsubishi Evo's, Subaru WRX STI's, Nissan GT-R's, and others, so stay tuned!



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