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

"HammerheadFistpunch" (hammerheadfistpunch) 10/24/2014 at 17:41 • Filed to: Tech, 4wd, HHFP

116

100

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.

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.

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.

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.

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 !!!error: Indecipherable SUB-paragraph formatting!!! 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)

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

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.

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

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.

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.

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.

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.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 17:46

12

The only difference between the hundreds of names for driving all wheels is marketing, so stop the snobbery.

If this is a takeaway that people get, it's really not correct. You've gone to quite a lot of trouble to break down what differences do exist, but when the rubber hits the road (or lack thereof), a FWD-bias AWD will react totally differently than a RWD-based classic 4WD. You could almost say that the fact that they drive all wheels is all they have in common . Anyone who tl;drs your explanation here is really, really, not going to get that.

Tom McParland > HammerheadFistpunch 10/24/2014 at 17:47

17

Excellent post...I get this question from clients all the time. Will save this for future use. :)

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/24/2014 at 17:49

7

I guess my beef is the whole "its not TRUE 4wd man" or "its AWD not 4wd" in the sense that I don't really care what you CALL it, but I do find it fascinating how it works under the skin.

RamblinRover Luxury-Yacht > Tom McParland 10/24/2014 at 17:49

0

Well, the wrap-up, IMHO, is flawed. I seem to recall an older version of this post from months back without that issue.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 17:57

6

There are, to my way of thinking, three loose categories worth putting things in, because it's really necessary to make *some* distinctions. Yes, any of these is probably better than nothing when extra traction is needed. But... is it classic 4wd, (mostly) unmanaged AWD, or modern AWD? Classic 4wd has the distinction of usually shipping with a transfer case, not supposed to be used constantly (with some exceptions: Hummer, etc.), and typically being less immediately useful on the road. Old-school Scooby style AWD is based toward on-road use and needs to be locked up, but isn't *based on* having a solid lockup. Managed modern AWD can run the gamut from "acting like" a classic 4wd to the utter fail of the CRV.

Anybody who buys a CRV based on the ability to be a Real Truck offroad is boned, and that's because real differences can and do exist between "real" (AKA classic) 4wd and AWD. The former is very hard to cock up because there's nothing to it - the latter is variable all the way from stellar to outright shabby - you can see why some would still stress the former with undue snobbery.

SNL-LOL > HammerheadFistpunch 10/24/2014 at 18:06

1

*StandingOvation*

HammerheadFistpunch > SNL-LOL 10/24/2014 at 18:08

1

With a username like that, I'll take it. I wish I could into math so I could make use of my engineering bias.

Crest > HammerheadFistpunch 10/24/2014 at 18:09

0

Cutest shit ever

C-Ki > HammerheadFistpunch 10/24/2014 at 18:10

2

that's all ?? Well this is much simpler to understand than women...

HammerheadFistpunch > C-Ki 10/24/2014 at 18:10

1

Still waiting for that post from Jezebel, come on gals, snap to it!

SNL-LOL > HammerheadFistpunch 10/24/2014 at 18:12

0

As I teach my proteges all the time: as soon as you get the concepts and schematic design right, you're 75% there. Number crunching is the easy part.

TheCrudMan > HammerheadFistpunch 10/24/2014 at 18:12

0

It's my understanding that there isa actually not technical difference, though there are lots of technical differences between lots of different kinds of AWD/4WD systems...and manufacturers just call it whatever they feel like.

HammerheadFistpunch > SNL-LOL 10/24/2014 at 18:18

0

unless your dyscalculic

HammerheadFistpunch > TheCrudMan 10/24/2014 at 18:19

0

Basically. What is "Supermatic wheel control" for one manufacture and "4magic" on another might come from the same supplier.

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/24/2014 at 18:20

0

I changed it a little. Better?

Racescort666 > HammerheadFistpunch 10/24/2014 at 18:20

1

Excellent post. I believe we've even argued about this at one time. At the end of the day, it's cool, it works, enjoy, be happy, nerd out over more engineering stuff.

6cyl > HammerheadFistpunch 10/24/2014 at 18:21

2

I think many (most?) clutch type differentials are not electronically controlled. I am sure on they are on many newer cars but on older cars I think they were often the mechanical standby. Also they can be set up for different acceleration/deceleration lock up profiles.

SnapUndersteer, Italian Spiderman > HammerheadFistpunch 10/24/2014 at 18:23

0

For a squirrel, you sure know a lot about drivetrains!

HammerheadFistpunch > Racescort666 10/24/2014 at 18:23

2

That's the just of it. The truth is that the details only matter for 3 groups of people

1. on the far left, the people who actually want to off road and need to know that they will be safe.

2. On the far right, the track day folks where traction management is a real issue

3. people who just love some good engineering.

For the rest, you could just as easily call it "the snow package"

HammerheadFistpunch > SnapUndersteer, Italian Spiderman 10/24/2014 at 18:24

1

Just the nuts and bolts stuff.

sumfoo1 > HammerheadFistpunch 10/24/2014 at 18:24

0

Well done!

HammerheadFistpunch > 6cyl 10/24/2014 at 18:24

1

This is true, many are hydraulically controlled, but most these days are electromagnetically actuated.

samssun > HammerheadFistpunch 10/24/2014 at 18:26

0

You missed a big one: lug pulleys

http://kalecoauto.com/index.php?main…

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:27

2

Very good edit. It's important to know what you want more so than what your desired system is called by this or that manufacturer... up until you choose to buy it and need to see it on the options list. The purpose of the tool is, after all, more important than what the manufacturer calls it - I didn't mean to be snippy, I just figured that since not all purposes are the same, it'd be a bad thing to misunderstand from the piece.

RamblinRover Luxury-Yacht > Tom McParland 10/24/2014 at 18:28

0

He fixed my gripe - very well, in fact.

HammerheadFistpunch > samssun 10/24/2014 at 18:30

5

Funny story, there is a REAL product called the bush winch that is pretty damn close to this

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:31

0

Much better, I noticed. It was mostly the fact that your summary and article were at cross-purpose. So many people I've run into... "AWD is just like truck 4WD"...

As long as people understand there are different sets of functions and don't think their X5 is a rock-crawler because they ignored your article's meat, I'm fine. Since you are the standard bearer here for off-road stuff in general, a dullard's excerpt from what you wrote might be hard to unseat.

Wojtek > HammerheadFistpunch 10/24/2014 at 18:31

1

Nice, but not deep enough:
A Haldex system can NOT decouple the front axle in VWs and Audis with transverse engine. This is physically impossible when you have the Haldex coupler fitted to the rear diff and the torque to the front axle is transmitted directly from the gearbox.

The description of the first method needs to be sent to BMW ASAP, since their AWD system from the E46 3-series XI's and 1st-gen X5's works without any center differential at all (the transfer case splits the torque 43-57, front-rear using a couple of crowned wheels in E46 and chains in E70). You only need two diffs (one on each axle) to cope with speed differences of the the axles. Worth noting, the E46 is far superior to the x-Drive in very difficult driving conditions, like "15 inches of snow".

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/24/2014 at 18:32

1

Yeah, Like i said to 666, only 3 people care:

1. off road types

2. track types

3. engineer types.

For the rest of the crowd, you could just as easily take off all the xDrive/AWD/4WD badges and just put on a "snow package" badge, cause they are all going to work within a close enough envelope that the buyers who don't care...don't need to.

HammerheadFistpunch > Wojtek 10/24/2014 at 18:35

1

I didn't say the Haldex could decouple the front, but you are right, it can't. Truth is, it can't even send much power aft either, its sort of a mild awd setup. The BMW did use an open diff but the the transfer case was as you described.

samssun > HammerheadFistpunch 10/24/2014 at 18:40

0

Now that's a riot

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:41

2

...if they read the manual. It's tricky, because the buyers who don't care should really have some minimal threshold of how *their* "snow package" works, but it's hard to set exactly what that level of knowledge will be. I'd say the complexity of possible user input into the system has something to do with it: completely automatic systems, just learn how to drive the system in the wet and dry. Systems with multiple settings a la Range Rover, the winter months are not the best place either to start or to assume that they'll do the same things as the fully automatic one.

Wojtek > HammerheadFistpunch 10/24/2014 at 18:41

2

I was thinking:

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

(...)

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

The BMW's old system is as simple as that. No center diff there.

HammerheadFistpunch > samssun 10/24/2014 at 18:41

2

I'd totally buy one if it wasn't 90% the cost of a winch.

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/24/2014 at 18:42

1

Truth, but you are coming from set A, so this stuff matters to you (and me) where for most people, it just doesn't.

HammerheadFistpunch > Wojtek 10/24/2014 at 18:44

0

What is that just aft of the chain drive on the lay shaft (top) that is cutaway? It looks like a clutch system or something and there is definitely a disconnect between that and the output shaft

MyPhoresterLovesMeBack > HammerheadFistpunch 10/24/2014 at 18:45

2

I have an engineering degree. The title of this post made me giggle in excitement like a little girl.

clarkwgriswold001 > HammerheadFistpunch 10/24/2014 at 18:45

3

This is great. How about another article covering 4WD? You could include mechanical lockers (Detroit Locker), air lockers, manually locking hubs, electronic locking hubs, semi-automatic locking hubs (i.e.: reverse 12 feet to lock), etc.

HammerheadFistpunch > MyPhoresterLovesMeBack 10/24/2014 at 18:46

2

Making engineers giggle like little girls is what I live for.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:47

1

Right. Let's hope those "most" people buy "modern" AWD systems, stick to sane areas, and don't ever get a 4WD truck without getting put some knowledge. Otherwise, I foresee the suffering of many tires. Many tires...

HammerheadFistpunch > clarkwgriswold001 10/24/2014 at 18:47

1

Thats a great idea. There are some of those discussion happening on a sub blog i belong to, you should check it out because it sounds like its up your alley.

overland.kinja.com

LTIROCKS > HammerheadFistpunch 10/24/2014 at 18:49

3

http://www.carthrottle.com/attention-all-…

jizawiz11 > HammerheadFistpunch 10/24/2014 at 18:49

0

a thermocouple is something used to measure temperature, not hydraulic connection. http://www.omega.com/guides/thermoc… I work in a business where thermocouples are our bread and butter. i don't know how the term was ever included in a description of hydraulically locking differentials, maybe it has just been used wrong over and over and nobody ever knew

HammerheadFistpunch > RamblinRover Luxury-Yacht 10/24/2014 at 18:50

1

man alive, everytime I write something like this i remember hearing horror stories from people who were driving their trucks in 4wd all the time "for the added traction" ......

I guess its kind of the same kind of thing I used to say to people who weren't interesting in buying or wearing bike helmets: Oh, that's fine if you don't want to buy a helmet you obviously don't need one anyway.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:50

1

I'm only not giggling because I'm still at the office and consequently more peevish than I really mean to be.

HammerheadFistpunch > LTIROCKS 10/24/2014 at 18:51

0

exactly what started me on this path (FYI, Honda uses a fully decoupling electronic system from 2012 on.)

Wojtek > HammerheadFistpunch 10/24/2014 at 18:52

3

Yupp, but it is not a differential. It is a coupler that allows torque transferring 100% front, 100% rear or anything in between. This coupler is to a differential as torque vectoring is to "electronic brake-assisted LSD". Both technically and functionally it is not a diff and it works contrary to a diff.

Also, in case of a shot diff you end up with a locked drivetrain and cry for some roadside assistance. In case of a shot coupler, you drive with your default torque split. :)

HammerheadFistpunch > jizawiz11 10/24/2014 at 18:52

3

GM uses a thermocouple, or more accurately a small heating element with a thermocouple to heat a small amount of fluid to push a solenoid hydraulically.

RamblinRover Luxury-Yacht > HammerheadFistpunch 10/24/2014 at 18:53

0

Somebody whose judgment was, shall we say, flawed in some other ways that I knew always put his car in 4wd on our drive because it was gravel. What the hell, man. Moderate slip is what gravel DOES, learn to control your idiot right foot.

HammerheadFistpunch > Wojtek 10/24/2014 at 18:54

0

so its just a coupler, on and off in varying degrees? i guess that would technically go into that category then.

44444444444 > HammerheadFistpunch 10/24/2014 at 18:55

0

HALDEX is Volvo too...

Drakkon- Most Glorious and Upright Person of Genius > HammerheadFistpunch 10/24/2014 at 18:57

2

Fail! Fail I say! You forgot the PTO + hydraulic motors on the front axle.

For those of you who have never worked in the trades, this is supremely bad ass

Wojtek > HammerheadFistpunch 10/24/2014 at 18:59

1

AFAIK it has infinite adjustment, but hey, I am just hooning it (and it won't break).

David Tracy > HammerheadFistpunch 10/24/2014 at 19:02

0

You're right, there's really no distinction between AWD and 4wd. The old-school thought was that 4wd had a low range option, and AWD didn't. But cars like the new Dodge Durango, which have AWD systems with low range capability, kinda threw a wrench in that notion.

One thing that I think would be cool to show is an image of a traditional 4wd on a north-south engine configuration and a 4wd system on an east-west engine layout. I think the difference in the package is pretty cool stuff. On one hand, you've got a trans bolted to a t-case, which has two driveshafts coming out of it- one to each diff. In the case of the east-west engine config, you've got your PTU (Power Transfer Unit) bolted to the trans. Often times, there's the rear driveshaft and one of the halfshafts coming out of the PTU housing, and the other halfshaft plugs into the trans housing, so you wind up with different length halfshafts. The rear driveshaft then goes to the Rear Drive Module (RDM- basically a rear diff). So, basically, the PTU has the front diff integrated, and the RDM contains the rear diff.

OhAlbanianKar > HammerheadFistpunch 10/24/2014 at 19:03

0

superdave847 > HammerheadFistpunch 10/24/2014 at 19:24

0

Ok, I've got it—wait, I need to read that again.

707Miata > HammerheadFistpunch 10/24/2014 at 19:30

0

Marvelous! Brilliant article. Addresses the qualms that I have with how some people (incorrectly) refer to some systems. Also clears up some things I wasn't 100% sure about. Definietly sharing this with friends/family so they can learn as well

ly2v8-Brian > HammerheadFistpunch 10/24/2014 at 19:35

0

Good write up. All semantics really, thanks for pointing that out.

High Road > HammerheadFistpunch 10/24/2014 at 19:40

2

The best system I've ever had was the Borg Warner Quadra Trac in a '77 J10. It was unstoppable. The truck was full time 4 wheel drive, but when things got desperate there was an vacuum activated switch in the back of the glove box to engage the quadra trac. The pssssh sound, followed by a heavy chain clunk, meant shit was getting real. It never failed to get me unstuck.

mtdrift > HammerheadFistpunch 10/24/2014 at 19:50

0

Good stuff HammerheadFistpunch. It was a Fistpunch to my brain!

iridiumisred > HammerheadFistpunch 10/24/2014 at 20:05

0

I applaud you for taking the time and effort to put this together. As an engineer myself, I am deeply interested and intrigued. Keep these types of posts coming... thanks!!

Scaggnetti > HammerheadFistpunch 10/24/2014 at 20:07

0

THANK YOU FOR POSTING THIS!

I'm Abe Froman > HammerheadFistpunch 10/24/2014 at 20:13

0

Great article. The 4WD vs. AWD has ways CJ fused me since my parents had an AMC Eagle (faux wood paneling!) when I was a kid.

J. S. > HammerheadFistpunch 10/24/2014 at 20:29

0

Then there's this hub mounted hillbilly zombie spooky wood splitter "The Stickler".

macanamera > HammerheadFistpunch 10/24/2014 at 20:30

0

Great post dude! Very enlightening.

datsteve > HammerheadFistpunch 10/24/2014 at 20:36

4

Are there different ways to drive all 4 wheels? Yes. Do I care what they are called? no, and neither should you.

Oh?

Ramblin Rover - The Vivisector of Solihull > David Tracy 10/24/2014 at 20:40

2

There needs to be a good term for AWD systems with full lock, so you can classify classic 4wd, that type, and non-locking/torque distributing systems of the two kinds: self-balanced/torque balanced or unmanaged and electronic/TCS-managed. Maybe call it dual-mode 4wd or something. HHFP glosses over that a bit, but in a sense, the classic AWD setups with an open-or-locked center diff have more in common on some points with a "standard" 4WD than something more modern - when in locked mode, there is a mechanical fix between front and rear axles that will behave a certain way that anything "managed" will not do.

Center diffs that are limited slip also blur things like the diff+low-range quandary, but whether or not it has a full manual lock mode is one of the more clear distinctions IMO:

Classic 4WD, dual-mode AWD/4WD, mechanical AWD, and managed AWD - there's your four "mostly covers it" categories. Dual-mode by its nature would include all the combinations of classic and either of the other two, so you can bundle in the high/low from classic style along with the hard link.

damnthisburnershitsux > HammerheadFistpunch 10/24/2014 at 20:46

1

Now this is interesting but I have to ask a question from the cheap side of me;

I've always understood that AWD drive systems of any type chew through tires quicker than part time 4wd systems, because the AWD drive system no matter how well tuned will always increase wear on the tires because they are all driven to some extent, increasing friction, whereas the part 4wd is only increasing wear on the non-driven tires when engaged. In the case of RWD part time 4wd drive systems this means the front tires are wearing slower as they can spin independently of each other with out need for a differential.

Is this true?

redmouseball > Tom McParland 10/24/2014 at 20:47

6

Technically an open different situation send EQUAL torque to all axles, regardless of slip (common misconception). it's just that the system as a whole can withstand less torque and the slipping wheel is doing the lease work (again, depending on your definition of work, i.e F*D) whereas the locked setup varies axle torque by inherent design.

And yeah, engineering is cool

4muddyfeet - bare knuckle with an EZ30 > HammerheadFistpunch 10/24/2014 at 20:48

1

Excellent article, really kept me reading! And I now know that my Defender is full time AWD with a centre locking diff rather than full time 4WD etc etc. All those spare wheel covers are wrong!

deekster_caddy > HammerheadFistpunch 10/24/2014 at 21:09

1

One thing worth expanding on a little is the "Auto-4WD" used by some full size GM trucks (quickly breezed over in your first section). It is basically a full 4WD transfer case, but the output from the transfer case to the front shaft has a clutch pack driven by an adjustable hydraulic pressure. Rear output is 100% at all times, front output has a variable pressure clutch that can be engaged when wheel slip is detected, or the "4WD" button is pressed on the dash. So the "2WD" button means no clutch pressure, "Auto" means light clutch pressure with more available on demand, and "4WD" means full clutch pressure.

Having had that system in my '05 Yukon XL 8.1, I can say that it works amazingly well. In the rain and snow using Auto, I could not spin the tires. 2WD? spin away... (well, except in fully snow covered roads) This system probably saved my dad's life, helping him avoid a major accident while he was towing a large trailer.

J. S. > HammerheadFistpunch 10/24/2014 at 21:31

0

Maybe you can further explain this to me. Our 2010 Sport Trac Adrenalin has the 4.6 V8, 6 speed auto and AWD. The AWD is (supposedly) Ford's in house version of the Volvo Haldex system. The Adrenalin also has advanced traction control (ATC) and roll stability control (RSC). I'd guess it has open rear & front diffs with a center clutch pack that works in conjunction with the ATC and RSC.

When I accelerate hard out of a corner I get the distinct impression it's front wheel drive. I can tow a 3000 lb trailer though the transfer station mud with not a hint of wheel slip (maybe it's the Pirelli Scorpions?) and the only time I felt the clutch pack engage was once when new and starting out quickly on wet grass.

I've concluded that the computer that runs all of this can regulate the clutch pack and wheel brakes to shut down a slipping wheel and allow power to any wheel with traction. Normally in the dry is power 50:50 front & back and it just feels like front wheel drive? But I really have no idea what is going on and I've been unsucceful googlin' the internets.

DeliciousBabyArm > HammerheadFistpunch 10/24/2014 at 21:35

0

Well no, but its probably just as confusing if not as popularly debated, if for nothing else than the determination of marketers 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.

seriously? 3 sentences? wtf hammerheadfistpunch. love it.

madtube > HammerheadFistpunch 10/24/2014 at 21:57

1

I have an Acura TL with what it calls "SH-AWD" (Super handling all wheel drive). That system uses the friction plate/clutch setup with the over-driving planetary gear set. The system uses 2 modes to achieve what they do. Regular operation makes use of a duty cycle solenoid that allows varying amounts of power to be sent to each side. But in a handling situation that calls for it, the over-driving clutch is engaged to make one wheel turn faster than the other. This is usually during a hard cornering instance. The net result is the outside wheel has most of the power being routed to it as well as the over-driving clutch engaged actively trying to make that outside wheel spin faster. I am trying to explain this in the most layman's terms that I can. What is surprising in this is how effective it is. Acura's torque vectoring setup used in their SH-AWD vehicles makes it handle stupidly well for a FWD-based vehicle. My TL is big, heavy, and a 6-speed manual. It may not be fast in a straight line, but that thing can corner! Acura these days get a bad (and deserved) rap. But they were thinking in the right direction with the setup in my car. There are many cars that do things better, but mine will surprise you. It did with me.

KusabiSensei - Captain of the Toronto Maple Leafs > HammerheadFistpunch 10/24/2014 at 22:09

0

The real fun is if you have an Audi with the T1 Torsen center diff. With a 4:1 torque bias ratio (up to 80% power delivery fore or aft) and a default 50:50 fore:aft split, you get an interesting side effect...

The torque delivery is adjusted by the relative normal force placed on each axle. More normal force, more power in that direction.

However, unless you have a Torsen T1 rear (with or without the locker), side to side transfer is via the ABS system braking a spinning wheel, to cause the other wheel to be the path of least resistance (VW calls this EDL. Electronic Differential Lock).

Its decent on road, but if you have one wheel on each axle in the air, then you are le screwed.

I haven't looked in detail at the later crown gear systems, but they seem very interesting due to the more compact size of the unit.

HammerheadFistpunch > madtube 10/24/2014 at 22:13

0

the picture above us the super handling awd unit, what i think is amazing is that the center diff is in that same unit. for heavens sake don't hit that on a rock!

KusabiSensei - Captain of the Toronto Maple Leafs > RamblinRover Luxury-Yacht 10/24/2014 at 22:14

0

At least the S4 is a completely mechanical three differential based AWD. All I have to hate on is VW's EDL programming in the ABS unit...

At least the Torsen T1 unit still shuttles power fore and aft.

ToastedTires > HammerheadFistpunch 10/24/2014 at 22:15

0

AWD: self explanatory

4WD: low range transfer case

MikeP3 > HammerheadFistpunch 10/24/2014 at 22:26

0

Haldex can send 100% of the torque to the rear if programmed to do so. I don't know if the typically thin rear half-shafts will survive long though. The original explorer had a button that would lock the Haldex hard enough that you could not steer without major driveline binding.

ToastedTires > High Road 10/24/2014 at 22:36

1

So it just had a vacuum actuated low range?

Ramblin Rover - The Vivisector of Solihull > KusabiSensei - Captain of the Toronto Maple Leafs 10/24/2014 at 22:40

1

I wasn't talking about you - I was talking about someone who literally thought 4wd in a Suburban was SMRT to get out of our oh-so-awful driveway. No, really.

nmalinoski > HammerheadFistpunch 10/24/2014 at 22:41

1

Is this an A or a 4?!

nmalinoski > HammerheadFistpunch 10/24/2014 at 22:43

0

You look like you could really use an editor.

HammerheadFistpunch > redmouseball 10/24/2014 at 22:46

2

think you responded to the wrong person but you are right about the open diff, though not complete with your info. an open diff transmits power equally but that means that it will only transmit as much power as the surface will allow to the wheel with the least traction. I.e. If there is one wheel on ice and the other on pavement you can only apply 5 ft lbs on the ice before you slip, therefor the ice side gets 5 ft lbs and the traction wheel only gets 5 ft lbs as well, effectively zero in terms of moving you forward.

Ramblin Rover - The Vivisector of Solihull > damnthisburnershitsux 10/24/2014 at 22:47

0

Pretty much all true -*if* you have front locking hubs. either automatic or manual. A really old primitive part-time 4wd actually sometimes doesn't have a way to disconnect the front wheels from the front diff, which makes the front diff always turn - just disconnected from the main driveline as if it were being towed. Great for mileage, you can imagine. Most vehicles that came this way eventually ended up with locking hubs to disengage the diff after a while... as an option.

Ramblin Rover - The Vivisector of Solihull > datsteve 10/24/2014 at 22:49

6

Discrete drive systems is the best way, because fuck logic.

Cowboy behind the wheel > HammerheadFistpunch 10/24/2014 at 22:49

0

Here's how I view it. Can I put front and rear lockers in it without breaking parts? Then it's 4wd. If it's not built for 100% power to each wheel its Awd. Nothing beats a 4wd with low range and front and rear positive lockers.

KusabiSensei - Captain of the Toronto Maple Leafs > Ramblin Rover - The Vivisector of Solihull 10/24/2014 at 22:50

0

Then you have to claim that they really ought not to be driving something with either a center diff or a transfer case.

At least I know how I can get down your driveway. In 2WD. ;)

madtube > HammerheadFistpunch 10/24/2014 at 22:55

0

I am an Acura Master Technician and have been for years. In that time I have only ever had to replace one of those units. The first generation (starting with the 2005 RL and 2007 MDX) units were just massive! The second generation (2009 TL) units were much smaller but could handle far more power through them. Fun fact: 2010-2014 TL SH-AWD cars were outfitted with the largest and most powerful engines ever put in to a Honda or Acura vehicle. The 3.7 V6 also was in the RL, MDX, and ZDX, but has just a couple more HP and torques in the TL setup.

YumYumOfTheYum > HammerheadFistpunch 10/24/2014 at 22:56

0

Umm.. Which one is the Evo's?

redmouseball > HammerheadFistpunch 10/24/2014 at 23:06

0

we're on the same page buddy, and to be fair the wheel on ice may have the most traction... If you've got steel spiked wheels 

datsteve > Ramblin Rover - The Vivisector of Solihull 10/24/2014 at 23:08

2

when the two independent systems are hefty internal combustion engines, sure, it makes little sense.

but when the two drive systems are small electrical motors with one moving part each, it makes a whole lot more sense. the low-inertia motors can adjust output on-the-fly, and are controlled digitally, which responds much faster to conditions and input than mechanical linkages. additionally, with no mechanical connection from one single motor to all four wheels, there is no driveline power loss.

feather-throttle-not-hair > HammerheadFistpunch 10/24/2014 at 23:09

0

This is rad. Man i want a helical LSD on my car. Viscous LSD is weak sauce.

burpbeepburp > Wojtek 10/24/2014 at 23:12

1

The Chevy Trailblazers.....used a similar system with an NVG226, This shows 4WD/Automatic biasing 4WD mode:

kyle242gt > HammerheadFistpunch 10/24/2014 at 23:25

0

All my Torsens are belong to you. From my first spin (!) in a Miata to the high-buck Quaiffe in my 240Z, you have my heart forever and always.

Ramblin Rover - The Vivisector of Solihull > datsteve 10/24/2014 at 23:26

2

I was pretty much funning with you and the OP - just saying "yeah, more than one motor for 4WD isn't even a new thing! Screw you, HHFP!" It's still true that with the power limits we have, that that extra efficiency with an electric has some hefty things to offset. Efficiency gains requiring extra vehicle weight... aren't. Yet. Any serious off-road or difficult surface bill for multiple wheels driving is going to be typically either remote access (no power sources) or in weather conditions ill-conducive to battery performance and motor life. There's also the bit where you find real mechanical losses with anything short of an actual wheel motor, and a wheel-motor's unsprung weight and grit exposure is death offroad. Some real potential in road applications, though, if we limit to just a pair. Low-floor motorhomes and kneeling buses, new and interesting ways to rework sports car balance...

Now if we can just make offroad courses with in-track power sourcing like a slot car, we'll be past the energy storage Achilles heel. That'd be pretty neat, actually - a rally vehicle with capacitor banks allowing near-instant charge, and freestyle course traversing tied to main course line charge pickup. It's just crazy enough to work.

HammerheadFistpunch > Cowboy behind the wheel 10/24/2014 at 23:31

0

oh you can certainly break parts putting in a front locker, or rear for that matter.

HammerheadFistpunch > nmalinoski 10/24/2014 at 23:32

0

You're hired, the pay is nothing and I will need about 3 times a year when I get published. When can you start?

HammerheadFistpunch > nmalinoski 10/24/2014 at 23:32

3

BWHAHAHAHA you will never know!

nmalinoski > HammerheadFistpunch 10/24/2014 at 23:33

0

I can start tomorrow. Where do I send edits?