4[A]WD

"HammerheadFistpunch" (hammerheadfistpunch) 10/24/2013 at 18:38 • Filed to: random information.

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There was a throwback by blackchair yesterday about a FP article on how to drive in snow, and since kinja2.whatever loads the comments from that article in the new posting it was pretty impractical to post a response to him and it seems like most of the comments centered around the difference between AWD and 4WD I though I would bring it up and let us fight about it here.

Let me start by saying that there really isn't a right or wrong here, in that all these terms are marketing terms and have no strict definition, so don't get all in a huff if I call permanent AWD 4wd, or vice versa.

I'm also going to go on record as saying that if you want the dirty details or history or entomology or anything like that, hit up !!!error: Indecipherable SUB-paragraph formatting!!! , this isn't going to be that detailed.

Okay, so lets start with the basics:

You need 3 things for this to work - front differential, rear differential, some way to connect to the two. Ill cover each below briefly, but this discussion is based primarily on getting torque to the front and/or rear axles, not on side to side differentiation. In the sense that an open differential 2wd is really only 1wd (the wheel with least traction gets 100% of the torque) so 4wd with open differentials is only really 2wd (one front and one rear wheel with 50% torque each...least traction). So, here is how to do it

4x4/4wd/part time 4wd - locking the front and rear differentials together with permanent engagement/no slip. Typically done with a transfer case after the transmission in the case of longitudinal setups

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.

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

These system require you to stop or slow and put the transmission into neutral and physically shift it into gear (or have it electrically actuated via button)

There are auto 4wd systems that are this type but can shift into 4wd on demand based on sensor input as well.

So why can't you use it on road? simple, you want to turn, right?

As you can see, and as you probably already know, the inside wheel travels less distance in a turn than the outside and the same goes for the front and rear axles. While the side to side difference is taken up in a differential, locking the front and rear axles prevents the same type of differentiation from occurring. The result is: resistance to turn, wheel binding/hopping and driveline wear or damage. To put it succinctly; its unsafe to drive in part time 4wd for long periods on the road.

AWD/Permanent 4wd, full time 4wd - There are many MANY different forms of AWD, but typically AWD seeks 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.

There are dozens of ways to do this and I will cover each briefly, but it boils down to the same basic principles; instead of locking the front and rear axles together you use friction plates, differential gearing or fluid coupling to take up the differential slack to eliminate binding.

Typical on road based AWD systems simply use a single speed transfer case or PTO (power take off) and let the center differential modulate torque, However there are lots of full time systems with low range gearing and to tackle the duality of on road use a center differential with the ability to lock like a part time system for true 50/50 torque distribution better for off road use.

Examples include: Land cruiser 80 series, the hummer h1, most land rovers/range rovers, Jeep quadradrive systems, etc.

In regards to non-locking differentials, you have lots and lots of choices, here are a few:

Open Center differential

Pros: cheap, easy, purely mechanical, instantaneous transfer, smooth and drivable

cons: like all open diffs, this system transfers 100% of torque down the path of least resistance i.e. the slipping axle. this gives you a minimum of 2 wheel drive (front and rear, 1 wheel each) no torque manipulation

Examples: BMX x3/5, Toyota Sienna, etc.

Limited slip center differential - There are a few ways to do this so Ill dive into each

VC (viscous coupling) -

V VC differential is basically 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 shear fluid that increases its viscosity when subject to high shear load. Most of the times its a 50/50 torque split like an open diff but when the front and rear axles are traveling at different speeds a shear force takes place that increases the viscosity of the fluid which increases the lockup of the input and output shaft while still allowing for differing speeds, as the sheer force increases you approach a lockup state that acts like part time engagement while still allowing some tolerance for different speeds in the axles.

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

cons: reactive, durability can be an issue, No full lockup, no torque vectoring

Examples: most Subaru manual transmission models (excluding STI), vw snychro, Land cruiser 80 series (95-97), 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 (invex here) gears. because of the helical cut nature of these and the spur gears they mesh with, they allow perpendicular motion only 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 so there is a torque bias always to both shafts.

Pros: Fast, variable torque bias, safe (if one shaft breaks it would lock the drive and spin the car), instant torque transfer

Cons: Expensive, fixed torque bias once set, no lockup

Examples: Most audi's with Quattro (most VW 4motion as well), 1st gen hmmvw, continental GT, etc.

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. The Center diff in the subaru STI, for example, is off this type but despite its computer controlled operation, it is still only able to achieve lockup, it can't send more or less torque front to back that is predetermined by the mechanical bias built into the unit. typically these systems are set up with a bias 60/40, 70/30, etc and then lock up or slip in varying degrees to achieve a torque vectoring effect.

These or VC diffs are the types of systems in modern full time 4wd setups so that you can fully lock up, or run in varying degrees of open to allow for speed differences.

Pros: advanced functionality, variable profiles for terrain, can be fully open or even decoupled.

Cons: expensive, require computer control, clutches wear out or can break, can be harsh.

Examples: Subaru Sti, VW/Audi with Haldex AWD, etc.

as a related but separate category there are advanced friction types with center and rear differential functions built into one unit, also there are ways to overdrive wheels be inserting small planetary gearsets in the output shafts that, coupled with variable lockup, provide overdrive to either front or rear axle.

Examples: Acura SH-AWD, Evo Iv and up, etc

So to summarize:

1. there is no technical difference in nomenclature other than "4wd, 4x4,etc" typically means part time, no center differentials and "AWD, Permanent 4wd, full time 4wd, etc" mean there is a center differential of some type involved

2. I have no life

3. Engineering is cool.

Sidenote: there are strange systems that don't conform to these rules (Ferrari FF, Hybrid 4wd, etc) but I didn't feel like including them despite #2.

Textured Soy Protein > HammerheadFistpunch 10/24/2013 at 18:02

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Very good post, but...

Torsen center diff doesn't have a fixed torque split. It varies on the fly, same as if you have a Torsen front or rear diff it's constantly shifting torque left or right.

Unless by "there's always a torque bias to both shafts," you meant that a Torsen can't have 100% torque going to the front or rear.

magman007 > HammerheadFistpunch 10/24/2013 at 18:02

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just a mild correction to the STi.

The 04/05 sti's were able to go completely open, in 06 and up the LSD has about 15% locking capability at all times, and the DCCD system reacts with the mechanical differential to do its magic.

HammerheadFistpunch > Textured Soy Protein 10/24/2013 at 18:12

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torsens have a fixed bias ratio at which point they act like an open, thats why they are safe if a shaft fails.

From Torsen: "

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Textured Soy Protein > HammerheadFistpunch 10/24/2013 at 18:22

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Got it, so talking about the limits of the range of torque split it can vary through, not that it's a fixed 50/50 split.

HammerheadFistpunch > Textured Soy Protein 10/24/2013 at 18:28

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Correct, though most are fixed at a set ratio (50/50 65/35, etc) as a default

MooseKnuckles > HammerheadFistpunch 10/24/2013 at 18:28

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I read through it, but it's been a long day so maybe I missed it.
Let's take a 4x4 truck, it runs rwd typically but I opt to put it in 4hi, what system is this? What about 4auto?

I don't drive a 4x4 or awd vehicle, so when asking this question it isn't because I am guilty of it:
If someone left their truck in 4hi, or even 4auto, what are the consequences since this seems to be a poor choice?

I know someone that leaves their truck in 4hi because they are a bro and bros only do boss things.

HammerheadFistpunch > MooseKnuckles 10/24/2013 at 18:31

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depends on the system, my truck (land cruiser) is always in 4hi because I dont have a choice, its a full time system with a VC center diff that locks. if its just a part time truck, leaving it in 4hi is stupid and dangerous. That being said, there are a lot of part time systems with full time capability...Whats the truck?

HammerheadFistpunch > magman007 10/24/2013 at 18:36

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interesting, I didn't know the 06 and up couldn't go fully open. I used to think the dccd was so pimp...then I realized its just a variable lockup. I guess its better than VC, but its nothing special, I think I'd rather have a torsen in there.

MooseKnuckles > HammerheadFistpunch 10/24/2013 at 18:55

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Silverado

HammerheadFistpunch > MooseKnuckles 10/24/2013 at 19:05

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if it has a 4auto and he leaves it in that...he's fine. if he actually leaves it in 4hi locked. he's wrecking his truck or already has and wont know it until he realizes his front axle isn't actually engaged.

MooseKnuckles > HammerheadFistpunch 10/24/2013 at 19:12

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interesting. he leaves it in 4hi, has the floor shifter always in it.

Tom McParland > HammerheadFistpunch 10/24/2013 at 20:19

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Great post, actually I got a few questions about this from my article earlier. I linked a TTAC article, but now I will change that link to yours.

magman007 > HammerheadFistpunch 10/25/2013 at 10:56

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Its a pretty cool system, its not spaceship shit, but you notice every new high dollar german performance car is coming with an electronically controlled rear differential, basically doing what subaru has been doing for years, and touting it as new cutting edge technology. I am sure those systems are more advanced, but its the same principal. The subaru unit is simple and robust, like the whole car.

On 04's and 05's you can pull the e brake up one click and run full open/rwd mode.

I really liked this article, I have a thing for differentials. I wont buy an rwd car without an LSD, and wouldn't consider the WRX for its open diffs. Evo 8 and 9 had open front differentials IIRC as well.

HammerheadFistpunch > magman007 10/25/2013 at 12:06

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Its a nice system, but certainly not revolutionary. After all, the Vehicross used a similar system in 1997 but had low range too. regarding the handbrake to rwd trick...That's not quite right, you could force FWD, but not RWD since the output shaft of the transmission has a ring and pinion directly to the front, before the center diff. so you will always have at least 50% torque to the front wheels.

Which is why the DCCD only ever gets to full lock and nothing more, because it only has 50% of the torque to send to the back wheels at most.

magman007 > HammerheadFistpunch 10/25/2013 at 12:08

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Its limited to the pre determined torque split correct? Full lock is effectively 41/59.

magman007 > magman007 10/25/2013 at 12:10

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while in the open state, the way I understand it, is it can transfer 60% to the rear, and nothing to the front, as the differential is open and taking the path of least resistance. never can it achieve full rwd torque, but it can allow the fronts to sit stationary and the rears to roast, albeit with 60% of the engines total torque output?

HammerheadFistpunch > magman007 10/25/2013 at 12:16

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Even in full open mode, you have to remember that the output shaft being directly connected to the front diff means that, essentially, the center diff plays no part on the front diff. Its basically fwd with rear toque bias on demand. i.e., if you disconnected the rear shaft entirely you would still get a fwd STI. I'm not sure what the static torque split is, but locking means 50/50 split no matter what the diff is biased towards, since you are locking the two shafts together and they have to spin at the same speed. Its why the front LSD on the sti's is so important to their performance.

For Sweden > HammerheadFistpunch 11/02/2013 at 01:04

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God Damn this is a good write-up. I'm crying tears of joy.

BigBlock440 > HammerheadFistpunch 06/04/2014 at 12:58

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Reading this article, a little tidbit hit me about my parent's 1st gen Durango. It had a few options for 4wd, part time, full time and low, as well as 2wd. Being a kid, naturally I assumed full time was better and didn't know why they'd even bother with part time. I assumed it engaged only part time, like a AWD system that's based on slip. On a snowy hill one day where 2wd didn't cut it, my mom put it into 4wd full time, also assuming it was better (maybe even at my suggestion, I don't remember). Made some progress but still slipped all over. Tried part time 4wd, and holy shit what a difference. Up the hill no problem. It all makes sense now, part time means you should only use it part time, and full time means you could leave the lever in all the time. Damn I miss when SUVs were actually SUVs.

HammerheadFistpunch > BigBlock440 06/04/2014 at 13:02

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The Jeep system is the same one, you can do 2wd, a fully open diff full time 4wd (which as you found out is little help most of the time) and then true 4wd lock with lo range. I've always been jealous of the Jeep system since it lets you go to 2wd, but I have a VC center diff in the cruiser which the jeep systems don't, so there's that. Traction is hardly ever a problem for me in 4hi and I've actually made the mistake of tackling some pretty big challenges in 4lo unlocked (a mod I did allows for this) because I forgot to lock the center diff and I didn't even notice until it was over.

BigBlock440 > HammerheadFistpunch 06/04/2014 at 13:38

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That confused me for so long, how did the part time work better than the full time? Even after that incident (and getting my license) I still used full time whenever I needed 4wd thinking it was better. After all this time, I finally get it (though I guess I could have figured it out if had it mattered, still had it, etc.).

webmonkees > HammerheadFistpunch 06/04/2014 at 15:06

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Excellent.

I had the previous owner go through the 4wd system again for my reminder of the binder.