A[4]WD - What is AWD/4WD and how does it work?

4WD vs AWD, as big a battleground among car people as steam engines vs gas engine am I right? Well no, but its probably just as confusing if not as popularly debated, if for nothing else than the determination of manufactures to brand their all wheel control system as being the only one to prevent you from hitting that damn deer while driving your family to visit grandma on that dark and slippery country road. Seriously, how many names of AWD/4WD can you even think of off the top of your head? Symmetrical AWD, clearly better than non-symmetrical AWD right? In a rehash of an old article I wrote I will attempt to make a technical distinction, as best I can, between many of the popular methods of getting 4 wheels to the ground but let me start by saying this: it doesn’t matter what you call it, if its part time, or full time truck or sedan, etc, etc. I don’t care and neither should you.

However if you love cars and engineering, the myriad and varied ways to do it are fascinating and knowing them might just make you a better driver.

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So to start on this all wheel issue, what is it that I need to make all 4 wheels drive my car? Bearing in mind this is for explosion powered vehicles, and not taking into account fully electric or hybrid all wheel systems. Also keep in mind that I will be using commonly used terms but they aren’t locked to a technology as much as just a historical or marketing norm.

You basically need 3 things for this to work -

  • A front differential
  • A rear differential
  • Some way to connect to the two

The front and rear differentials are important, but I will be somewhat ignoring them (and assuming open) for this discussion and instead focusing on the third issue. I assume you know what a differential does, since you are here, but a quick a dirty refresher is that a differential allows for the left and right wheels to turn at different speeds going around a corner, without that the inside wheel would be going to fast and the outside wheel too slow. This is called speed biasing and the same principle applies to the front and rear axles as well, as shown here.

As you can see, the inside wheel travels less distance in a turn than the outside and the same goes for the front and rear axles. So while the obvious solution to AWD might be to lock the front and rear axles together, you can see why that wouldn’t make sense: resistance to turn, wheel binding/hopping and driveline wear or damage.

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So, here is how to do it and what names are typically associated with what systems.

4x4/4wd/part time 4wd - Despite the disadvantages illustrated above, this method of locking the front and rear differentials together with permanent engagement/no slip is exactly whats happening in this type of system. Typically done with a transfer case after the transmission in the case of longitudinal setups. You can get away with this when the surface you are driving on allows for some level of slip (snow, dirt, etc) and not on high grip surfaces. This is typically done by physically locking the shafts together using a toothed collar, actuated electronically (solenoid), hydraulically (small heater or pump), or manually with a lever.

These are typically user activated system, but can be automatic as well. Low range can be found on these systems but not always.

Pros: Cheap and easy, strong, permanent torque transfer when engaged (50/50 usually)

Cons: Cannot be used on the road, high driveline wear when used on high traction surfaces.

How does this make me a better driver? Well now you know not to drive around all day with your 4wd locked in if you wont need it; You will increase your turning circle, increase your drive wear and will generally make handling worse doing so.

Examples: Jeep Wrangler, 4x4 trucks, Nissan Xterra, Toyota Tacoma, etc.

Open Center Differential

That’s the end of the easy ways to do it, from here on out you will need a a way to solve the problems inherent with part time 4wd by solving the differentiation issue. Engine layout, transmission type and low range availability are inconsequential here as there are examples of each type, i.e. a system could be full time 4wd with a transverse layout and no low range.

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A center differential would allow for speed difference (bias) to take place between the front and rear axle.

Just like a regular differential, this is an easy way to deal with speed bias, but not torque bias as these system will always send 50% of the torque to both sides, and they cannot bias torque. Biasing speed but not torque means that the power (speed x torque) will always go to the wheel with less traction. Good news for us is that very few cars use this type of center diff and the ones that do typically have mechanical locking options or very good traction control systems.

Pros: Cheap, easy, purely mechanical, instantaneous transfer, smooth and drivable.

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Cons: Like all open diffs, The constant mesh spyder arrangement only allows as much torque to the good side a the worst side, in effect sending all the power to the wheel with the least traction. In practice this gives you a maximum of 2 wheel drive (front and rear, 1 wheel each) no torque manipulation.

How does this make me a better driver? Knowing that you only have 2wd at best (really 1wd if your other diffs are open) means knowing why you can’t really handle at the extremes so well; don’t be fooled into thinking 4wd means all 4 wheels will drive. Without a traction management system, fully open systems are only marginally better than nothing for traction.

Examples: Toyota Sienna, FJ80 land cruiser, Discovery I

Viscous Coupling (VC) -

A Viscous Coupler is usually paired with an open differential or built into the differential, and acts kind of like a torque converter, it has an input shaft connected that is fully decoupled to the output shaft and the medium of torque transfer is a fluid. In the case of most VC’s, its a Dilatant fluid, a non-Newtonian fluid that increases its viscosity with when subject to high shear load. Basically as the speed bias increases across the input and output the sheer load increasing and locks a series of discs together to increase friction and attempt to normalize the speed bias; This is torque biasing as for the first time on our list you can start to send more than 50% of the torque to any given axle/wheel. Though most VC units wont allow for full lockup without damaging the unit, they can get close (30-40% additional torque bias to the slipping wheel/axle)

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A strange but interesting example of this system is a Gerotor coupling, like Jeep Quadra Drive and Honda’s old “Real Time 4wd”. A gerotor is pump, like most oil pumps, and they are very effective small displacement high pressure fluid pumps. In a gerotor system there is a pump on the input and output shaft linked via closed loop of fluid, when the wheels/axles are spinning the same rate the pressure in the line is equalized, but if one wheel/axle spins faster than the other a pressure differential builds up and that pressure is sent to clutches to apply pressure in varying degrees to apply limited slip friction. Totally automatic and provide lockup in varying degrees, these system have been superseded by electronic coupled clutches.

Pro: Cheap, easy LSD, simple, good drive-ability

Cons: Reactive (require a a speed bias before starting to work), durability can be an issue, not full lockup, no torque vectoring

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How does this make me a better driver? Go ahead and throw that WRX into a corner, drift that mofo...but don’t be surprised if your center diff explodes before too long. Also, that manual Forester is only going to take you so far into the forest.

Examples: Most Subaru manual transmission models (excluding STI), VW Snychro, Land cruiser FZJ80 series (with electronic locking), etc

Helical cut/Torsen - Its hard to simply explain the process behind a helical cut but it goes something like this.

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The input shaft and output shaft aren’t physically connected except at right angles by the element gears which take the place of the constant mesh spyder gears in an open diff. Because of the helical cut nature of these and the spur gears they mesh with, they allow only perpendicular motion to transfer to the outputs. This means you can rotate the carrier and both input and output move at the same rate, when you have different speeds of input and output the element gears rotate to transfer the speed difference, but remain in mesh. Torque is transferred as the worm gears act against the worm drive and increase friction by locking them against the housing. How hard they lock is defined as a Torque Bias Ratio, i.e. how much additional torque can you bias across the differential. For reference a locked diff or solid axle (without differential) has a torque bias of infinity:1. Here, let this guy explain it.

Pros: Activated by torque not speed, safe (if one shaft breaks it wont lock the drive and spin the car), can be locked with brake application, 2 way tuning (acceleration/deceleration)

Cons: Expensive, not mechanically locked

How does this make me a better driver? Torsen’s work best when you don’t wait for the system to work, i.e. just trash it and keep your foot into it, the response should be very linear leading to a very predictable learning curve for handling.

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Examples: Most Audi’s with Quattro (most VW 4motion as well), 1st gen HMMWV, continental GT, Toyota Full time Body of Frame 4wd system use a Torsen with electronic locking, Subaru STI (with clutch packs)

Friction plate/clutch - Again there are dozens of ways to do this, I will cover this generally.

The friction clutch method is very popular because, with the advance in electronics, its possible to achieve very high variability in operation, smoothness and drive-ability, and advanced torque vectoring. The simplest way to explain it is that it acts an an open differential with clutch packs on the input shaft, output shaft or both to vary the friction inside the carrier housing from fully open to fully locked. Its important to note here that just because a diff is friction type, doesn’t mean it has torque vectoring capability, or more accurately, overdrive ability. Think of this as a the electronic version of a VC unit. Some of these system are fully decoupling, meaning they wont send any torque (or speed) to the rear wheels until sensors determine a slip and apply a force to the clutches.

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Pros: Programmable functionality, variable profiles for terrain, can fully decouple the front or rear axle for 100% torque split to either axle.

Cons: Expensive, require computer control, clutches wear out or can break, can be harsh, require friction modifiers in the differential housing, many of these system lack the ability to transfer full torque for short or extended periods of time.

How does it make me a better driver? Computers are interesting, and they will do what the programmer wants and that may not be what you are expecting, learn the system before committing to full attack.

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Examples: VW/Audi with Haldex AWD, etc, Honda CR-V (current), Jeep Cherokee, renegade, etc.

Now on to the really tricky stuff. Automatic transmission use a planetary gear set to effect a gear ratio, they can do this be braking different parts of the assembly to change the ratio. In recent years this principle has been applied to center (and rear) differentials. Torsen Type 3 (c) differentials, for example, use a planetary gear set with Helical cut to achieve symmetrical wheel speeds with non-symmetrical torque loads and easy differentiation, allowing splits other than 50/50. Subaru, Audi and many others use this in their center differential as it allows for a rear torque bias without axle binding. The concept is very simple but profound in effect, in addition you can add clutch packs to vary the lockup on the differential and affect the Torque Bias Ratio up to fully locked for maximum variability. However, these systems still are limited in the sense that they don’t allow for over-driving of a wheel or axle, that is, to spin the output shaft faster than it should mechanically in a 1:1 setup.

To achieve torque vectoring you have to mechanically increase the output speed relative to the input speed and by putting planetary gear set’s on the output shafts on either side of the differential you achieve this. The idea is that if the sensors indicate an under steer event, the computer tells the clutches in the planetary set on the outside wheel to engage, “shifting gears” from 1:1 to something mild like 1:1.25 allowing the power to flow to that wheel at a higher rate of speed than it normally should, inducing a yaw to cancel out the under steer. Think of it as 2 tiny 2 speed transmission in the differential housing. This isn’t typically used in the center diff because torque vectoring doesn’t have an effect linearly.

Crazy other system?

There are wacky ways to do this, just ask Ferrari, yikes.

Done nerding out?

Let me leave you with some takeaways.

  • Are there different ways to drive all 4 wheels? Yes. Do I care what they are called? No, and neither should you.
  • However, knowing HOW your system works can be a boon to understanding how your vehicle will react in adverse conditions
  • I advise against using this information at a party.
  • Engineering is cool.

Did I miss something? need a point clarified? Ask Google. Nah I’m just kidding let me take a whack at it before I ask Google.