How Modern 4WD Systems Work And Why They Don't Suck as Bad as You Think.

"HammerheadFistpunch" (hammerheadfistpunch) 01/12/2018 at 10:00 • Filed to: Tech, 4WD, AWD, HHFP

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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 !!!error: Indecipherable SUB-paragraph formatting!!! , as well as !!!error: Indecipherable SUB-paragraph formatting!!! , 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

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“That’s not REAL 4WD!”

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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.

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There are, basically, 3 types of 4WD

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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.

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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 shouldn’t be in that locked mode 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

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SUBARU FORESTER

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

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RAV4

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

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HONDA CR-V

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

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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.

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Why do it this way?

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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

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Are they bad?

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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.

Sweet Trav > HammerheadFistpunch 01/12/2018 at 10:15

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Nah, see here’s my problem with part time AWD. Its fucking useless, it upsets the balance of the car and unless I can lock 50/50 it’s not much better than FWD in bad weather.

Plus there has never been a good AWD vehicle with a transverse mount engine. If I was king, I’d banish that layout to hell.

Milky > HammerheadFistpunch 01/12/2018 at 10:15

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Poor little crossover 4wd systems are the ones with the problems, not me.

Party-vi > HammerheadFistpunch 01/12/2018 at 10:19

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MRW I read all of this

Akio Ohtori - RIP Oppo > HammerheadFistpunch 01/12/2018 at 10:22

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Cool write up! Very detailed. I might have another go at it after a second cup of coffee.

For purely selfish reasons, I’d be interested in seeing someone on “how to 4WD,” like how to drive through an obstacle with different systems. Specifically, my Disco’s 4WD (as far as I am aware) basically uses three open diffs to run all four wheels and uses traction control to simulate locking on demand. This appears to necessitate different tactics, as the system needs the wheels to be moving in order to work, but I only have trial and error observations on my side and they seem counter-intuitive to “normal” off-road driving.

adamftw > HammerheadFistpunch 01/12/2018 at 10:25

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This is great! Super informative, and I’m def going to watch that video. I like my GM Autotrac, it seems to work (for now).. but as soon as it gives me issues I’m swapping for a floor shift NP241C. It is definitely shittier than my Selec-Trac XJ was, and nowhere near as confidence inspiring as the Disco’s.

Dr. Zoidberg - RIP Oppo > HammerheadFistpunch 01/12/2018 at 10:26

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Maxima Speed > HammerheadFistpunch 01/12/2018 at 10:27

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AHH its here! OK I’m going to read it I just need some breakfast and coffee in me to get started.

adamftw > Akio Ohtori - RIP Oppo 01/12/2018 at 10:28

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Step one would be to add the shifter so you can lock the center diff, if that year has it available. Otherwise, as soon as you feel slip, give it (a little) more gas. I had a sweet “how-to offroad” Disco 2 DVD with I watched ALL THE TIME before I started driving. I think my dad got it from the dealer in 1999 when they were launched.

adamftw > Sweet Trav 01/12/2018 at 10:31

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No good transverse AWD cars?

cmill189 - sans Volvo > HammerheadFistpunch 01/12/2018 at 10:34

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Great write-up! If I wasn’t at work, I’d save it.

I do have a bone to pick though...

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...and when they do fail you will simply not have 4WD (Hi V-70 owners!). 

How dare you? You’ll be hearing from my lawyer, sir. I won’t tolerate truths being spoken about me.

/s

Akio Ohtori - RIP Oppo > adamftw 01/12/2018 at 10:35

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Yeah... I should have the CDL (built in early 2001), just lacking the linkage. I just can’t get myself to pull the trigger on buying a linkage. The ‘04 linkages are expensive and difficult to find, the Ashcoft seem great, but are more than I can justify at this time, and all the other options aren’t particularly elegant.

But yes, through trial and error it seems like allowing the wheels to slip seems to help the traction control immensely, but that runs counter to more traditional “don’t dig yourself a hole” strategies.

All that said, it seems to do great in everything but very deep snow. It probably would have done better if we’d bothered airing down the tires.

random001 > HammerheadFistpunch 01/12/2018 at 10:35

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No.

adamftw > Akio Ohtori - RIP Oppo 01/12/2018 at 10:38

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Definitely spend the money on the linkage. My Disco 1 was insane in deep snow, it got out of some holes it had no business getting itself unstuck from. You don’t want to have to rely on the traction control ECU in those situation... even though it was amazing in it’s day, it won’t save ya. Adding a TruTrac LSD to one or both diffs will also make that thing a hero.

EL_ULY > HammerheadFistpunch 01/12/2018 at 10:38

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not just “today’s cars”.

Lexus had full electric diffs for over 10 years on the RX400h.

Of course, if Audi had done it, “oh what amazing German engineering and blah blah blah LeMans inspired technology”

It was so rad seeing a diff back there with no propellershaft going to it, only 2 thick orange cables.

HammerheadFistpunch > random001 01/12/2018 at 10:39

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Yes

Future next gen S2000 owner > HammerheadFistpunch 01/12/2018 at 10:39

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If a 4wd/AWD systems doesn’t actually power all 4 wheels at the same time, is it lying?

I think that is the hangup, 4wd sends power to all wheels all the time while AWD systems send power to where it is needed/slipping wheels. This really seems to come down to diff differentials. (hahaha get it?!?! Get it??!!)

More importantly, if you have 4wd don’t you also have AWD as well? Unless you have six wheels, in which case you could have 6WD.

HammerheadFistpunch > Akio Ohtori - RIP Oppo 01/12/2018 at 10:40

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There is a great Andrew St Pierre white video on this with the Disco. I’ll find it here in a bit

wkiernan > HammerheadFistpunch 01/12/2018 at 10:41

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Only question I have that this excellent article left unanswered is, how much additional weight does 4WD/AWD typically add to a car?

HammerheadFistpunch > EL_ULY 01/12/2018 at 10:42

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Yeah electrical drive modules are going to be making a comeback

Takuro Spirit > Milky 01/12/2018 at 10:42

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Should have pre-loaded the system with a little left foot braking. Durp.

adamftw > Future next gen S2000 owner 01/12/2018 at 10:42

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Technically, most part-time 4WD systems are actually two wheel drive... unless you have traction devices in the axles. There are situations where a 1976 Jeep CJ7 will get outclasses by a new Crosstrek.

HammerheadFistpunch > wkiernan 01/12/2018 at 10:43

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138 lbs in the case of the golf, which is representative

Takuro Spirit > HammerheadFistpunch 01/12/2018 at 10:44

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But Does It Show You Da Wae?

cmill189 - sans Volvo > EL_ULY 01/12/2018 at 10:45

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I learned something today.

Milky > Takuro Spirit 01/12/2018 at 10:47

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From a simple perspective, you shouldn’t have to brake to move. I know a lot of systems do that automatically for you though.

shop-teacher > HammerheadFistpunch 01/12/2018 at 10:47

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Excellent article! A few typos, as promised :)

I don’t think the “enthusiasts” will ever lay off the crossover. They can’t accept that the reason they are so successful is that the very compromises that enthusiasts despise, are exactly what the average driver likes about them. These are the same people who refer to all season tires as “no seasons.” They have no understanding of the fact that normal people do not do the kind of driving that sees any benefit from switching between summer and winter tires.

MonkeePuzzle > HammerheadFistpunch 01/12/2018 at 10:49

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MonkeePuzzle > HammerheadFistpunch 01/12/2018 at 10:49

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the extra weight is good for handling :D

HammerheadFistpunch > shop-teacher 01/12/2018 at 10:50

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Let it not be said I don’t deliver on my promises. And yeah I agree

MonkeePuzzle > Party-vi 01/12/2018 at 10:50

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Milky > adamftw 01/12/2018 at 10:50

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I mean, the STI still exists and the EVO doesn’t.

HammerheadFistpunch > MonkeePuzzle 01/12/2018 at 10:51

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Truthfully... Yeah. I mean at least it helps with these front heavy vehicles a little

Sweet Trav > adamftw 01/12/2018 at 10:52

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Yes Evo’s have transmissions that are made of glass. They are garbage.

MonkeePuzzle > HammerheadFistpunch 01/12/2018 at 10:52

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ok, but why are you not covering the most oppo of 4wd options?

ya weld the front diff, you weld the centre diff, you weld the rear diff, you burn rubber just turning corners. #GetAllTheWomens (or men, or ducks, whatever you’re into they will love your diffs)

Snailkite > HammerheadFistpunch 01/12/2018 at 10:58

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So how soon are these systems replaced by rear electric motors once we have standard 48V/light hybridization?

Takuro Spirit > Milky 01/12/2018 at 10:59

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Correct, but if you know nothing about off-roading, you shouldn’t be driving off-road.

HammerheadFistpunch > Snailkite 01/12/2018 at 11:02

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All the driveline makes have one in the wings

functionoverfashion > HammerheadFistpunch 01/12/2018 at 11:11

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This is great. I have two comments on the “4WD” term, as this bothers me, too - that is, it bothers me when people are overly pedantic about it.

1. I once was talking to a guy about his E46 325xi, joking with him that he should take it off-road (he was borrowing a truck to go up a woods road). “But it’s four wheel drive, it’ll be fine!” I said, jokingly. “Well, it’s all wheel drive , actually,” he said, completely missing the point. At least he didn’t say, “Well, actually it’s x-drive.” Part of me wanted to then follow him up the smooth, grassy woods road in my own E46, just to prove a point.

2. My 1961 Ford F100, which is “REAL 4WD even for the crusty, recalcitrant types,” (to borrow your terms) has a sticker on the visor explaining the function of the transfer case. This sticker refers to the modes where all the wheels are driven as “ALL WHEEL DRIVE,” which is a nice example of why it doesn’t matter what term you use, exactly, unless you’re trying to specifically discuss the differences between competing systems. There are four wheels. Sometimes, all of them, all four of them , are given power.

Great write up, thanks for the detailed info. I need to read it again to digest it all.

Textured Soy Protein > HammerheadFistpunch 01/12/2018 at 11:19

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Can I be that guy?

It’s GKN not GNK.

Fun fact: the Mazdaspeed 6 has a helical LSD in the rear drive unit.

Subarus have regular rear differentials not PTUs, and it’s VTD not VDT.

Everything I’ve seen about the assorted Subaru awd systems including Active Torque Split on the CVT cars has a nominal torque split that at least somewhat resembles 50/50, not 90/10. I’m more inclined to consider them permanent instead of on-demand.

Subaru used to have rear LSDs on several non-STI models but at the moment everything except the STI has open front & rear diffs, while the STI has front and rear LSDs.

Most of the longitudinal-mounted cars nowadays have a system similar to Subaru VTD/DCCD where there’s a transfer case with an open differential supplemented by an electronically controlled clutch. BMW xDrive, which Cadillac also uses (both supplied by Magna), Mercedes 4Matic 3rd gen, Nissan ATTESA E-TS (except the R35 GT-R with its wacky rear transaxle and 2 driveshafts, and older versions engaged the front wheels on demand). WK and WK2 Grand Cherokees are similar.

Audi quattro for a long time was a transfer case with a Torsen center diff but now quattro ultra they’re phasing in on certain models is a longitudinal PTU type deal. Lexus also uses a Torsen center diff in their transfer case.

Mercedes 4Matic 2nd gen (which was the majority of their cars until recently, and ignoring the original W124 4Matic) was an open center diff.

BaconSandwich is tasty. > HammerheadFistpunch 01/12/2018 at 11:33

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One thing I am curious about show these various systems recognize when things are slipping. I’m suspecting most are using a wheel speed center and comparing it to the input power, but that’s just a guess.

HammerheadFistpunch > functionoverfashion 01/12/2018 at 11:43

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Thats exactly my point. Awd/4wd...its all the same. its just 4 wheels being driven. Thanks.

Highlander-Datsuns are Forever > HammerheadFistpunch 01/12/2018 at 11:45

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I emailed this to myself for a later read.

My real world experience:

1989 subaru xt 4-cy, with an auto transmission: fucking hell miracle car can go anywhere even on those little 14" tires amazablalls.

1991 Subaru XT-6 with manual: pretty damn good, a little loose in the rear end. Push center diff lock, CANNOT TURN F-UUU dumb car.

1992 Subaru SVX- go ahead drive 95 mph on icy roads, see if I care. OMG 5" of snow, I can’t move!!!

1991 Isuzu 4WD pick up-manual all the things. 2-feet of snow? No problem, 3-feet of snow? No problem, 4-feet of snow? get a little run at it before you go. Wet train tracks? try 4-low. —By far the most competent 4wd I have ever driven and it has open diffs on all wheels.

2002 Subaru Impreza 4-cy auto trans. FWD most of the time, a little thunk underneath and it will pretty much go until you bottom out on a rock or a log.

2013 Subaru Outback, manual 6-speed with TCS. Where does this magic come from, is it dark magic or light magic? I don’t know but damn you don’t even know how bad it is until you drive something else.

2012 Chevy 2500 duramax 4x4, please shift into 4wd, please god shift into 4wd, I do not want to call a tow truck. Easy on the gas so you don’t bury it up to the axles, OMG turbo!!!! weeeeeee!!!!

2015 ford explorer (XLT, 4wd sticker): nice front wheel drive station wagon ford, nice. Oh what is this nob in the center console with pictures of trees and rocks? Okay now, that is slightly better.

His Stigness > shop-teacher 01/12/2018 at 11:46

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What I find so annoying about “crossovers” is how wasteful they can be compared to a large sedan which has more space, or a wagon, which can be safer due to reduced rollover risk, and more efficient thanks to a lower drag coefficient. But Americans are so incredibly short sighted they don’t see how even a few miles per gallon can add up over the life of a car.

The other thing I find egregious are automakers calling things that are actually cars a crossover. I’ll use the X1 (first gen) as an example. If I, a person of average height, am taller than the car, a crossover it is not. All it is is a slightly lifted, and shorter wagon. It’s also ugly as hell.

The new Honda HRV(?) and the Toyota monstrosity (god that thing is ugly) are just traditional hatchbacks, they’re not crossovers.

But if is quite amazing to see automakers just lie about what a car actually is, and suddenly people flock to it. If VW had called the Golf a crossover it would have sold in the billions here.

HammerheadFistpunch > Textured Soy Protein 01/12/2018 at 11:47

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Have I mentioned that i’m dyslexic? For reals. I see acronyms the right way, even when they are written wrong. I will go in and correct them.

As far a Subaru, their 4EAT was on demand with preload, and as far as I can tell the CVT system still has no center diff which means it would have to allow for slip in the clutches. The 5 speed has the VTD system and maybe the cvt in the WRX too, Im not sure. You are correct about the rest as well, they don’t have to reduce before the final because they don’t have transaxles.

HammerheadFistpunch > BaconSandwich is tasty. 01/12/2018 at 11:51

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thats the gist of it. my guess is that its either a hal effects sensor using the ABS speed ring or using an arrangement similar to a stepper motor. In most cases these days data is also collected from the steering, accelerator and a g-meter in the car. By the time the computer sees the pedal being pressed it can apply clutch pressure to the RDM and prelock the system before slip happens. Since most slip happens from a stop this is an effective way to counter slip delay.

Textured Soy Protein > HammerheadFistpunch 01/12/2018 at 11:53

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You’re right, there’s no center diff in the Subaru ATS system but it doesn’t default to 90/10. Here’s Subaru’s description:

!!! UNKNOWN CONTENT TYPE !!!

The WRX CVT uses VTD instead of ATS.

shop-teacher > His Stigness 01/12/2018 at 11:54

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They really are just wagons and hatchbacks with a slight lift. The increased rollover risk and extra drag are very minimal. Way too much is made of that around these parts. I do agree that a lot of them are hideous, especially that Toyota. I actually think the X1 looks pretty good. Not that I would buy one.

HammerheadFistpunch > Akio Ohtori - RIP Oppo 01/12/2018 at 11:54

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this is full length. its dated but still valuable. I scanned ahead to the part i think is relevant

HammerheadFistpunch > Textured Soy Protein 01/12/2018 at 11:59

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In subaru’s case I think they’ve opted to keep more clutch preload at the expense of mileage to keep that “subaruness” The research I did seems to indicate that the solenoid that controls the clutches is inactive in most cases with the 4eat once you are moving. In either case the net effect is that there is always some torque going back in a subaru

HammerheadFistpunch > Milky 01/12/2018 at 12:00

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I think that has less to do with Evo’s and more to do with Galants.

Akio Ohtori - RIP Oppo > HammerheadFistpunch 01/12/2018 at 12:03

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Awesome! Thanks! I’ll take a look this weekend.

HammerheadFistpunch > Milky 01/12/2018 at 12:05

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This is a perfect example of a manufacture calibrating the system wrong or using the wrong sized unit for the size of the vehicle. I was actually going to include this as an example but...it was already too long. Case in point the CR-V

Fails bad here. Passes here

Go into the computer, fiddle with the code, made some small hardware changes maybe. Boom.

Cash Rewards > Sweet Trav 01/12/2018 at 12:05

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In terms of a gross generalization of a comment with little to no supporting evidence in response to an incredibly detailed post, this is pretty good. HHFP basically wrote a dissertation and you said “nah, trash”.

HammerheadFistpunch > Future next gen S2000 owner 01/12/2018 at 12:07

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What adamftw said. Just because it CAN drive all 4 wheels doesn’t mean it always will.

interstate366, now In The Industry > Cash Rewards 01/12/2018 at 12:12

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That’s literally his entire post history.

TheHondaBro > Cash Rewards 01/12/2018 at 12:18

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I thought that had become a universally accepted argument... if your whole ideology is based on blind elitism and nothing else.

HammerheadFistpunch > Sweet Trav 01/12/2018 at 12:19

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What? I can’t hear you?

Cash Rewards > TheHondaBro 01/12/2018 at 12:23

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Yeah, it comes before *wanking motion* and “loud farting noises”

HammerheadFistpunch > Akio Ohtori - RIP Oppo 01/12/2018 at 12:25

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I will add that you really want the linkage.

I have another burner, try to guess it! > HammerheadFistpunch 01/12/2018 at 12:26

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Is driveline stuff sprung or unsprung weight?

Akio Ohtori - RIP Oppo > HammerheadFistpunch 01/12/2018 at 12:27

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I want the linkage! I just can’t bring myself to spend $400 for some bits of metal. You can buy an entire transfer case,  for that much. SHIPPED! Right now I’m hoping we get one at a local pick and pull, but I’m not holding my breath.

HammerheadFistpunch > His Stigness 01/12/2018 at 12:28

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I remember Car and Driver used to say the Forester was the minimal acceptable height to be an SUV. That was with the SG generation at 65 inches. Wanna guess what Car and Drivers best compact Suv was this year?

sigh. How the mighty have fallen. *63.5 inches so you don’t have to look it up. Still, its half an inch taller than the Porsche Macan so...

HammerheadFistpunch > I have another burner, try to guess it! 01/12/2018 at 12:30

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Mostly sprung. Only the driveshafts in the back would be unsprung.

HammerheadFistpunch > EL_ULY 01/12/2018 at 12:31

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Sadly those units aren’t so strong. There is a great video on that weber state channel about those too, btw. The next gen units are going to be 2 speed and able to dish out a lot more power. The highlander hybrid was garbage in the snow, but again that could be down to tuning as much as mechanical limits.

HammerheadFistpunch > Highlander-Datsuns are Forever 01/12/2018 at 12:32

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The safest car I drove in the snow was by far my moms 2nd gen Highlander with snow tires...that is until we got the GX. Man that thing monsters the snow.

vicali > HammerheadFistpunch 01/12/2018 at 12:33

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Real AWD would also drive the spare... just saying.

Scary__goongala! > HammerheadFistpunch 01/12/2018 at 12:37

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Nice write up. Just trying to explain to people that there is a difference between 4WD and AWD produces rather puzzled faces.

vicali > HammerheadFistpunch 01/12/2018 at 12:37

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Super fun article. Ultra-detailed sharpie work. Would read again.

Now do A-TRAC..

HammerheadFistpunch > Snailkite 01/12/2018 at 12:39

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My other comment being said, you need more than 48 volts to make these systems work, so it will need to be a true hybrid. a 60 hp rear drive module would need about 44 kw to power, at 48 volts thats 916 amps. Thats a lot of current to generate. 350 volts (the bolts battery voltage) on the other hand only needs 125 amp.

HammerheadFistpunch > vicali 01/12/2018 at 12:40

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too true.

HammerheadFistpunch > Scary__goongala! 01/12/2018 at 12:43

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I argue there isn’t...aside from marketing. I mean, my Land Cruiser is “Full-Time 4WD” (so it says on the back anyway) but when you drive it around daily it looks exactly like a manual Subaru for power flow. Transmission > Transfer case > open center diff > VC > front and rear axles.

In daily driving, its awd? Nah, they are both just 4wd. But I wont get bent out of shape about which one is used where. As I said, its all just a marketing exercise.

HammerheadFistpunch > vicali 01/12/2018 at 12:48

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There isn’t much to say I didn’t cover in that differential one I did aside from how wheels slip is measured, but I do want to look into that.

DC3 LS, will be perpetually replacing cars until the end of time > HammerheadFistpunch 01/12/2018 at 12:57

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I guess I’m wrong. But I was under the impression that 4x4 sent 100% torque to both axles (when engaged) and AWD would split the engine torque 50/50, 60/40 etc.

DC3 LS, will be perpetually replacing cars until the end of time > EL_ULY 01/12/2018 at 13:00

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That’s awesome. Living in Texas, like you, I naturally think AWD is too much complexity for not enough benefit, but something like this would awesome. Simple, yet probably performs better than the systems HHFP talked about.

HammerheadFistpunch > DC3 LS, will be perpetually replacing cars until the end of time 01/12/2018 at 13:02

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The trouble with torque split is that it doesn’t really tell the story. “%” versus the confusing “/” (ratio). An example; a part time system sends torque 50/50 but it also has the ability to bias 100% torque to either axle. Comparing the 2 doesn’t really work. You have to think of it in terms of torque transmitted. An on demand system than can lock has the ability to bias all the engine torque to either axle the same as a part time or full time system thats locked.

Nothing > HammerheadFistpunch 01/12/2018 at 13:10

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I’m impressed. Both by the article, and the number of comments, not to mention the art work. I think this goes to show that good content can go beyond the standard attention span of 1 or 2 paragraphs.

HammerheadFistpunch > Nothing 01/12/2018 at 13:12

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Im actually a little shocked at the response. Personally my attention span is poor and I was worried I wouldn’t read it.

His Stigness > HammerheadFistpunch 01/12/2018 at 13:15

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I wonder when they sold their soul? I think it had to have been pretty early on.

I stopped reading reviews of anything close to mainstream anymore, instead, I’ll read them for fun cars like Porsche, or something. But, even then, I haven’t picked up a car magazine in months.

Before I was about to submit this I realized I made a pun on accident. HA.

His Stigness > HammerheadFistpunch 01/12/2018 at 13:16

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And I’m going to spare you from the screaming I’m currently doing in my head. I can’t fathom how someone would call that a crossover. It’s the same fucking size as a Golf or Focus hatch. I hate the world.

His Stigness > shop-teacher 01/12/2018 at 13:18

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True, the disadvantages are very slight. But when automakers are supposed to be eeking every last percentage point of efficiency they become even dumber.

Do you think the first gen X1 is fine, or the second gen? I think the second gen is fine. I wouldn’t buy one, the proportions are way better.

shop-teacher > His Stigness 01/12/2018 at 13:23

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I honestly didn’t even know there was a 2nd gen X1.

random001 > HammerheadFistpunch 01/12/2018 at 13:49

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No. Now if you’ll excuse me, I have a cloud to yell at.

adamftw > HammerheadFistpunch 01/12/2018 at 13:52

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Don’t forget my fave..

AMGtech - now with more recalls! > Sweet Trav 01/12/2018 at 14:22

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CLA/GLA with AWD are a massive improvement over 2wd versions, and the transition is seamless and fast. Having driven a CLA45 with a bad rear diff, essentially fwd only, in the rain, in the winter, on summer tires, I can tell you that its AWD system is excellent and does not upset balance, it feels completely natural. Also have track time in these cars by the way. Can’t say anything for other makes or systems though.

Sweet Trav > Cash Rewards 01/12/2018 at 14:32

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In the highest forms of racing, show me one that uses a transverse mounted engine.

Cash Rewards > Sweet Trav 01/12/2018 at 14:58

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Maybe not everything needs to be judged to that standard. My wife’s pilot certainly doesn’t. None of the examples used in this article do. Don’t get me wrong, I love motor sports, and love autocrossing, but that’s not how I judge every vehicle I own. Everything should be engineered to it’s own purpose. Transverse AWD works great in it’s intended applications. All? No, probably not. But that doesn’t make it worthless trash.

TheBloody, Oppositelock lives on in our shitposts. > Akio Ohtori - RIP Oppo 01/12/2018 at 15:13

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Linkage from a D1 should also work IIRC, hell one of the guys I know drilled a hole and stuck on a ratchet wrench with a socket welded to the (#10 socket) and used that to lock the T-Case.

But what everyone else said, the lack of center locking in the D2 was not overcome by the TC system (as seen by the fact they brought it back in 04).

Sweet Trav > Cash Rewards 01/12/2018 at 15:22

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To me a Traverse is the epitome of worthless trash, it isn’t fun, it doesn’t tow anything that is fun. It holds 7 people. If you buy a Traverse you should have got a Minivan.

Spanfeller is a twat > HammerheadFistpunch 01/12/2018 at 15:24

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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?

1: Any and every wagon worth over 130,000 dollars and/or made in a country that produces good wine and has a latin-based language.

2: No.

3: Yes, But depends on the brand: you can always trust Windex being worth it, but I´d need to consult my washer fluid sommelier...

Did I pass the quiz?

HammerheadFistpunch > Spanfeller is a twat 01/12/2018 at 15:27

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Cash Rewards > Sweet Trav 01/12/2018 at 15:31

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I meant transverse layout, not an actual traverse. Those are kinda crappy. But I hold no grudge against the big crossovers, as long as they’re smaller than a minivan. Our pilot is shorter than a minivan, and my wife didn’t want to deal with the extra length every day for the 6 days a year it would be useful.

interstate366, now In The Industry > Sweet Trav 01/12/2018 at 15:34

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whoarder is tellurium > Sweet Trav 01/12/2018 at 15:40

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Akio Ohtori - RIP Oppo > TheBloody, Oppositelock lives on in our shitposts. 01/12/2018 at 15:43

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Haha yeah the D1 linkage and the socket method fall into the “not particularly elegant” category. Once I get the head gaskets sorted I’ll probably devote the time and effort needed to get the CDL reinstated. There is so much info on these and almost all of it is incomplete or outdated, it is hard to figure out what needs to be done. (Except the Ashcroft kit, which is straightforward but $$)

2Fast2Furious: Rotary Powered > Sweet Trav 01/12/2018 at 15:56

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I don’t get why people can’t buy awd cars as runabouts because they are so called “inferior” to 4wd

I mean have an opinion but a minivan and a traverse are separate vehicles because of the consumers

People who buy minivans are not the same people that buy traverse’s. That’s why minivans are dying and crossovers are successful

TheBloody, Oppositelock lives on in our shitposts. > Akio Ohtori - RIP Oppo 01/12/2018 at 16:59

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For the linkage? A bit of blind work under the truck connecting the linkage or you could drop the T-case (which might be easier).

https://discoweb.org has a lot of info on this and pointers on what to do with what options.

Chariotoflove > HammerheadFistpunch 01/15/2018 at 00:01

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Thank you for writing this. This is not a read at one sitting kind of thing for me, but worth the effort.

random001 > HammerheadFistpunch 01/16/2018 at 07:04

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Dear HHFP, I want to know what you think about the FJ Cruiser.

HammerheadFistpunch > random001 01/16/2018 at 14:00

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here is my short take:

Bless their hearts for trying. They surely meant well.

random001 > HammerheadFistpunch 01/16/2018 at 14:07

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Well played...