Behold the wretched object of your scorn and hatred

Look, it’s 4WD if it drives all 4 wheels. There. There are some debates in car world where there are certainly some gray area topics:

What exactly is a shooting brake?

Does a truck need a ladder frame to be a “truck”?

Are the pleasures of drinking washer fluid worth the risk?

But let’s not get hung up on this 4WD/AWD issue, one that seems to be a major point of contention with a good many car people. I mean, doesn’t 4 Wheel Drive literally mean the car drives all 4 wheels? So how about this, let’s collectively decide we are better than this and let it go. I am going to refer to anything in here that drives 4 wheels, 4WD. Can you handle that?

Now, in the past, I’ve written about 4WD systems, as well as traction devices and differentials, how they differ and why that matters. I wont be going back over that same material here so take a time and read those if you want a primer on 4WD technology or traction systems before we move onto what I want to talk about today

“That’s not REAL 4WD!”

I know you’ve all said it, or at least heard it said. What does that mean? As far as I can work out it seems to mean its not the kind of 4WD associated with rugged, old 4X4 vehicles, i.e. a part time 4 wheel drive system. OK


So, before I go any further I think it’s important we break down this all wheel traction business into nice digestible chunks we can all wrap our heads around.

There are, basically, 3 types of 4WD

Part-time – These are the old “4X4” systems and as the name suggests, cannot be used all the time. Up until the most recent 15 years or so this was by far the most popular type of system. This locks the front and rear axles together mechanically. It’s simply, effective and robust. It’s also the big dumb hammer of 4WD; Great if your problem is a nail. “Real 4 wheel drive” for you crusty recalcitrant types.


Permanent – These are slightly more modern and sophisticated systems that drive all 4 wheels all the time but also solve for the speed differential problem, that is, allowing the front axle to spin at different speeds than the rear allowing their use on a full-time basis. They are basically part-time systems adapted for use on the road. Also real 4WD.

On demand – These systems engage all 4 wheels as needed and while some systems allow for a small amount of power to be constantly sent to all 4 wheels many of them decouple an axle completely when not needed. These are BY FAR the most common systems on sale today and despite their derision, are also by far the most intelligent of the systems and offer the largest cost to benefit ratio for the majority of users. They may not be the perfect tool for the job, but it’s the one you’ll end up using most. These also, wait for it, drive all 4 wheels.

It’s these on demand types that tend to get the most hate, I suspect on account of them being added to so many vehicles that aren’t “fun” that they are guilty by association. But are they bad?


Lets start with why they exist in the first place, then how they work and if you’re still mad we can talk.

Crossovers are hot and that’s not going away. Consumers like that they drive like cars, but have some of the versatility of SUV’s without the high costs of fuel use, handling and comfort of the early SUV’s. To achieve these goals crossovers are basically cars that have expanded to fill additional rolls and as a result are constrained by car engineering limitations; Transverse engines, subframes and maximum space utilization. These hard points of engineering, means using truck based SUV layouts no longer work.

A traditional longitudinal 4wd layout


The first main difficulty is a transverse engine. Traditional 4WD layouts are all based longitudinal layouts with a transmission behind the engine and a transfer case behind that. In a transverse layout the engine is sideways, meaning there is no room for a transmission behind the engine let alone a transfer case. In these cases a transmission has been replaced by a transaxle.

A transaxle is, simply, a transmission and axle put together. The transmissions job is shifting gears and the axle takes that output and applies a final drive ratio before splitting power to the wheels by means of a differential. In a transaxle the final drive and differential are part of the transmission case. This means you can have a compact transmission and final drive that fits the confines of a transverse layout.


You can start to see why this is a problem for sending power to the rear, as the drive force is all heading 90 degrees away from where you want it at the rear axle. The solution is what manufactures call a Power Transfer Unit, Power Take-off Unit or simply transfer unit.

Its added to a conventional FWD transaxle, with minimal modification to serve several functions I’ll cover a little later.


To help illustrate the point I drew up some power flow diagrams for part-time, permanent and on-demand 4WD.

Sweet marker skills in play here

On the top is the power flow for both permanent (left) and part-time systems (right).


For a permanent 4WD system, power comes from the transmission into the transfer case to rotate a differential housing which turns both side gears which are each attached to one axle. This allows for power to flow to both axles and allow for the speed differences that occur, similar to a standard differential in any car.

On part-time system, power goes from the transmission output shaft into a transfer case where a main shaft goes uninterrupted to the rear axle, a gear on a bearing on the main shaft can be engaged with a shift collar similar to a manual transmission to engage the front axle.

This is a general breakdown as there are a lot of fancy ways to do this, however, they mostly just involve replacing collars with clutches or open diffs with limited slips, the power flow is mostly the same.


Can you believe I wasn’t an art major? Me either!

And here is what a PTU system looks like, again some systems vary.

Power from the transmission is put to a final drive reduction which is bolted to the front differential housing which is also attached to a bevel gear and pinion going to the rear.


That means that, in most systems, the rear output shaft is ALWAYS getting direct drive, same as the front differential. I say most systems because some manufactures systems (GNK supplies Jeep and FIAT among others, for example) actually decouple the output for additional fuel savings but the principle is the same.

There is some weapons grade explanation happening on Weber States excellent school of automotive engineering channel if you can stomach 30 minutes of classroom level demonstration.

Now, because the transaxle final drive is bolted to the differential housing and the housing is directly connected to the PTU output the rear output shaft has been slowed down to wheel speeds. This isn’t an issue here but since we’ll need another ring and pinion on the rear differential it presents a problem: You’ll need to transmit that power through a ring and pinion at the rear. A 1:1 ring and pinion ratio doesn’t work for several reasons and so the solution is to overdrive the PTU gear to speed up the driveshaft then reduced it back again at the rear differential which will have a final drive that is the reciprocal of the overdrive ratio at the PTU. This is an important feature of these systems because it means that the torque of the engine has already been through a reduction meaning the rear drive unit doesn’t need to be as large or strong. This compact, weight saving “feature”, if you will, is a major reason for this type of 4WD adoption for packaging and fuel saving considerations.


Once the torque leaves the PTU it is then sent to a rear drive module (RDM) where its connected to...nothing but a series of clutches. Because there is no speed biasing available in the PTU you can’t just hook up a rear diff and call it good. This is the problem part-time 4WD deals with, the reason it’s called part-time 4WD is because you should do that all the time. As a car corners the inside wheel will travel less distance than the outside wheel, this is why all cars have differentials, well the axles do the same thing; in a corner the rear axle travels a smaller arc than does the front.

On a loose surface like snow or dirt this isn’t an issue because the difference can be made up when the front tires slip a little. On a hard surface this causes binding in the driveline that causes all sorts of undesirable handling and wear problems. This is solved in permanent 4WD system by means of a center differential that can accommodate the speed differences while maintaining drive to both axles. In on demand systems that use a PTU this is (typically) solved by the rear drive module.


The rear drive module is the rear differential, ring and pinion and a coupler to connect the front drive to the rear. These couplers work in a variety of ways but the principles are common: connect the front to the rear as on demand.

During the birth of the modern crossover in the 90s there were pretty much just 3 and each tackled this problem a little differently and I think they are all 3 worth talking about as many of these techniques are still in use or illustrate the maturing tech.


  • Subaru Forester
  • RAV4
  • CR-V



The Subaru Forester was, in the manual version at least, basically full time 4WD. It has a longitudinal engine, a center differential in a “transfer case” to deal with speed differences and a regular rear differential. It also has regular ring and pinion final drives in the axles.

The Automatic, however, acts very much like a modern PTO system where drive is geared to the front wheels and the rear drive shaft is decoupled from the output except by a series of wet clutches. As the system detects slip it binds these clutches and sends power to the rear as needed, then unbinds to a nominal slip of around 90% front 10% rear. This is how all Automatic and CVT Subarus today work, with varying degrees of sophistication. The exception being the addition of the VTD system which brings a center differential back.

Subaru 5 speed manual cutaway, still used today




The RAV4, initially, took a very simple and conservative approach; add a center differential. It’s basically the same powerflow for the on demand system, with the addition of a 2nd differential to the mix to account for the speed differences. This system was passive like the Subaru manual system and if you got a manual it was open with a locking feature, or open with a viscous coupler like the Subaru in the automatic. The rear drive was still geared to the transaxle final drive so the same under/overdrive arrangement was used.

Like colors rotate together


Sweet press photo Honda! A reminder of a time when people still thought these would go off-road


The CR-V was most like modern PTU systems, it followed the flow diagram perfectly but at the rear it decoupled the driveshaft from the rear ring and pinion through a hydraulic, passive system called a gerotor loop. A loop of hydraulic fluid coupled to small pumps, similar to an oil pump, which were fitted to both the PTU and the rear drive unit (effectively). The rear driveshaft wasn’t physically connected to the rear ring and pinion but each side got a set of clutches or plates. If the front and rear axles were going the same speed the pumps would cycle fluid in a loop and the only torque transmitted to the rear would be from a small amount of preload on the clutches. As the the front and rear axles started to get out of sync the difference in pumps speeds generated pressure which was routed to the clutches that would bind and lock with respect to the amount of pressure generated. It was a fully passive, fully automatic and closed loop, requiring no external triggers.

CR-V Gerotor flow and operation diagram

Its similar in concept to what VW did when it made the Syncro Vanagons except in lieu of a pump system, it used a viscous coupler which used a special fluid that would cause it to thicken under shear and bind internal plates together to create a locking effect.


Todays cars forgo these mechanical solutions for less complex, and more scalable electronic solutions but the principles are the same: Disconnect the front drive from the rear and add as needed. Some use electrically actuated ball ramps to pressure the clutches, others use hydraulic pressure accumulated through a small pump and all are externally triggered by sensors in the wheels at a minimum and many more as the systems sophistication increases. This allows for axle decoupling for fuel savings but it also allows for the removal of a center differential to allow for speed bias, so how does it work to avoid those problems? Simple, the clutches slip as necessary to allow for the relatively small differences in speed. This is very similar in effect to a clutch based LSD differential.

Why do it this way?

There are a lot of compelling reasons for a manufacturer to do it this way. Some of the advantages include:

  • Mechanically simplicity. Remove the center differential means a simplified layout and reduced cost.
  • Easily adaptable to a variety of vehicles. PTU’s can be adapted to existing transaxles with modest changes and rear drive modules are small and self contained.
  • Light and compact. Minimal weight and drag penalties.
  • Highly controllable via programming, from open to locked and anywhere in between as deemed necessary.
  • Major reductions on drag and mileage losses. Single point or dual point decoupling solutions reduce drag a claimed 80-90% compared to permanent 4WD solutions.


So what are the downsides of these systems?

  • There is some delay between slip and engagement. This depends a lot on the design and programing as some designs are preloaded to have some amount of torque going to the rear wheels all the time. Some systems claim to be “proactive” applying clutch pressure before power is applied based on a variety sensor inputs such as gas pedal position, steering angle and absolute vehicle angle, to name a few, which minimizes this delay considerably in many situations.
  • They typically aren’t designed to handle more than, say, half of the engine torque and the clutches will either slip at a predetermined limit, or they simply won’t apply full clamping load sufficient to lock fully. Some systems WILL lock enough to allow fully inter axle engagement automatically or on demand. An example would be a current RAV4 or the first generation Ridgeline which both feature “lock” buttons that force the clutches to full pressure up to a certain speed (usually around 20 mph) and excluding situations like large steering angle inputs. The limitation here is either clutch material, case material and size (for weight/packaging) or heat.
  • Because they rely on friction to both transmit torque and bias speed they can heat up necessitating either cooling systems or cool down modes where drive is unavailable.
  • It’s much harder to accommodate a set of low range gears for true off road work. Harder, but not impossible. AAM ECOTRAC used on the KL Cherokee is a PTU type system with low range. This is accomplished by means of a 2 speed PTU coupled with a 2 speed RDM (rear drive module). It’s only used by one vehicle currently.
  • Depending on the calibration and type, they may only be active to certain speeds, or when applying torque (not under deceleration). The effectiveness of the system is therefor due largely in part to the calibrations as opposed to purely mechanical means.


As you can see they certainly aren’t a silver bullet for the 4 wheel traction problem but they do represent the current attitudes buyers have in the market today where the confidence of 4WD can be had for a much smaller price than before.

You may recognize some of these systems by name: Twinster, SH-AWD, Haldex to name a few.


Now I know people, VW and Volvo people mostly, who consider Haldex a bit of a hiss and a byword but Haldex is just a company name like GKN, Borg Warner, AAM or any other. These systems have evolved over time and have generally improved a great deal over earlier attempts. Are they still subject to failure? Of course they are, they are electronic/electro-hydraulic devices that do battle right down in the trenches of water, grim and vibration and when they do fail you will simply not have 4WD (Hi V-70 owners!).

So now you know how they work and what their pros and cons are we can get back to the question

Are they bad?

No, not really. Sure they have their faults, hell, there are some really crappy Part-time and Full time systems too (GM Autotrac comes to mind) but the truth is that many or even most of these systems are fully capable of doing what their buyers expect them to do. These systems deliver the bulk of the promise of 4WD with a lower cost to the consumer, in price and convenience. And while their performance may be underwhelming based on engineering choices to calibration, in some cases they hold extreme performance promise. Systems like Acura’s SH-AWD with active torque vectoring or the similar GNK Twinster in the Ford Focus RS now with drift mode™ are both derivations of this technology. It also means more vehicles can be had optionally with 4WD and with a much smaller penalty than in the past.


So can you lay off the poor little crossover 4WD systems? No? you got problems man.