4WD and AWD systems explained

tonychrys · March 2001

People keep getting this stuff confused because of the marketing idiots.

The two most popular configurations on non-truck-frame SUVs today are:

AWD = Automatic Wheel Drive-- one axle normally until slippage occurs. Usually no 4-lo gear

P-4WD = Permanent 4 Wheel Drive-- both axles driven all the time, with a near 50/50 torque split. Vehicles that come close to 80/20 split should really be called AWD. Usually no 4-lo gear.

Of course an exception to both examples above are Jeep Grand Cherokees which can be configured with Select-trac, Quadra-trac and Quadra-drive, all with transfer cases and 4-lo.

paisan · March 2001

The Trooper to your list like the JGC, it has 2wd, fulltime 4wd and 4wd lo range.

-mike

heng · March 2001

A quantitative response is not necessary, would it be closer to open diffs or locked diffs? Or probably somewhere in the middle depending on the setup?

Think of open diffs as the easiest to get the vehicle stuck (low end of scale) and locked diffs as the hardest (high end). I'm not looking for any explanation of how LSDs work, just subjective views on how effective they are, particularly in situations where locked diffs would keep you going and LSDs would get you stuck.

I'm sure any differences would become apparent only on non-paved surfaces, so lets start there.

ateixeira · April 2001

I'm not an expert on the subject, but we could start by listing the types available:

open/unmanaged
limited-slip (viscous)
limited-slip (torsen)
open with traction control
lockable

-juice

bwh · April 2001

"Somewhere in the middle depending on the setup" is pretty close. It really depends on how you intend to use it. For sand or mud a LS will be much better than an open diff and as effective as a locker in all but the deepest stuff. For rock crawling the LS is about worthless, a locker will prevail, the LS is closer to open in this case. To me it kind of depends on the vehicle you are considering. Most do not offer a locker period. Toyota is offering them on the TRD Tacomas, GM offers an auto locking rear in its new pickups, and Jeep offers a great system of auto lockers in the JGC. In cases like that get the locker. Where you have a LS as an option give some consideration to its use. Adding the LS will likely cost ~$400 or so. On a truck expected to go off road I would pass and put the money towards an ARB locker or some type of automatic rear locker, a Detroit Locker possibly.

I guess that I really consider LS diffs to be better suited to powerful road cars rather than off-road machines. I just about all cases it is my opinion that a switchable locker, like an ARB, is the best option. The open diff gives better stability than a LS on slippery highways, and the locker is always there IF you need it. The flip side is a LS making your vehicle squirrley on slippery roads and slipping uselessly itself under duress off-road.

paisan · April 2001

I found the LSD on my trooper to work effectively on and off-road. Off road I actually thought it wasn't working, until I realized I needed to pre-load it a bit to get it to lockup. Onroad, the TOD system meant that the rear LSD didn't come into play often.

-mike

heng · April 2001

That's what I thought. LSDs better than open diffs in the moderate stuff but useless in the tough stuff.

Also came to the same conclusion that open diffs are OK on the open road with an ARB air locker switched on for the tough stuff. But since not having direct experience, I wanted input from others.

Now all I have to do is find the considerable dough to cover the locker, compressor and installation. (compressor alone is $200)

paisan · April 2001

What do you drive again?

-mike

heng · April 2001

Don't laugh guys. I, at least, can say I understand its limitations and that most other SUV owners don't.

paisan · April 2001

They aren't bad. A little narrow for my rather large butt

I just wanted to make sure that you weren't one of the ones with like a CRV or Santa Fe, thinking you were gonna put a locker on it!

-mike

ateixeira · April 2001

The author omits the achilles' heel of a torsen diff: if there is zero traction on one side, no power gets transferred. All power leaks through the slipping wheel.

Audi uses traction control now, so that's not a problem. However, newer Miatas have torsen diffs in the rear, and one tire on ice can get you stuck.

Ironically, older Miatas had a viscous coupling, which doesn't have this set back. The torsen is better for the track, however, so that's why they swapped.

-juice

bwh · April 2001

I wouldn't laugh. I have actually taken two of those off-roading. Both times we were in Colorado, once in the BMW and once with the Motorhome. Niether of which work on Jeep trails. It is kind of ironic to talk about lockers and such when I know from experience that a plain old 4x4 anything with low range will go places that would frighten most folks. I do recall though that a long day of wheeling left one of the Jimmys with a substantial front suspension rattle (darn rental cars). If you are fortunate enough to have the ZR-2 option you are miles ahead. These came with heavier front control arms and a larger ring and pinion in the rear end. If you do not have this you may want to consider trading into one as a better starting point, rather than spend a thousand on a regular Jimmy for the locker. I would also make sure you have a real skid plate under the front of your Jimmy. I know the newer ones have an easily damaged cast aluminum oil pan. On a truck like that with an IFS you stand a very real chance of banging it on a rock, ruining your day. Other than that my advice would be to buy good quality oversize tires and take it slow, you should have no problem, even without a locker.

hung0820 · April 2001

The list below are the explanation of the Santa AWD work! What is your comments about this system. Good or Bad?

The compact Double Differential Unit serves as both the front differential and the centre differential and is located inside the four-speed automatic transaxle housing. The unique DDU uses two planetary gear sets one for the left side and one for the right side to deliver torque from the engine to the wheels. Power is delivered to the rear wheels via both a
viscous coupling and helical drive gear. In normal on-street situations, the power is delivered to the wheels by the helical gear. The viscous coupling action is bypassed.

In low-traction situations, when sensors detect a difference in wheel speed between the front and rear wheels, driving power is transmitted via the viscous coupling to the wheels with better traction (those determined to be moving more slowly).

The viscous coupling consists of a series of parallel plates riding in thixatropic silicone fluid. A thixatropic fluid is one in which heat causes a reaction. Both are contained in a small welded housing. When all four wheels have similar traction (they are spinning at the same speed) the plates rotate at the same speed and the viscous coupling has no effect. However, if one wheel loses traction and starts to spin faster than the other wheels the coupling locks the front and rear wheels together, channelling more torque to the end of the vehicle (front or rear) with the most traction.

Noise and vibration have been kept to a minimum by using a single serpentine belt system with an auto tensioning mechanism. The Santa Fes plastic engine cover also helps reduces engine noise.

ateixeira · April 2001

The AWD sounds a lot like the one in the RAV4 and manual Foresters. Basically a sealed and silicone-filled viscous coupling.

I guess the traction control manages each differential (side-to-side).

-juice

bwh · April 2001

.... a good system for a car not a truck. There are some SUVs I fail to understand, yet they seem to sell quite well. What you have is an add on system. It is basically a front drive system with a vicous coupling to the rear axle. Fine for snowy roads, not much advantage anywhere else.

ateixeira · April 2001

The Santa Fe is car-based (Sonata platform), and most owners will stick to pavement.

In other words, it's fine for the light duty owners require from it, and helps in snow and ice the rest of the time.

-juice

heng · April 2001

I've looked for skid plates for the Jimmy/Blazer with no luck. My last resort is the factory plates? They don't look very effective.

Maybe I need to buy some steel plate and make one myself.

notdos · April 2001

OK, so I'm an idiot. I still don't understand all the differences. The frustrating part is the car salesmen seem to know less than I do. So here is my bottom line question. What 4wd/awd pickups can be driven and function as 4wd/awd on dry pavement?

thor8 · April 2001

No wonder, the more I read this topic, a miriad of systems appear.

I will try to be simple. The designation 4wd or AWD simply denotes that there is traction in the front and rear axles, they both mean the same thing, traction on both axles as opossed to 2wd or normal.

The means to accomplish 4wd or awd are many and hence the confusion, think of it as several ways to skin a cat. Each manufacturer adopts a system based on several factors, like price, complexity, user friendly, off road agresivness, fuel economy by reducing mechanical drag or friction etc.

All these systems work on dry pavement, except when you have locking differentials and you have them on the locked position.

To answer you question, all pick ups with 4wd or awd can be driven on dry pavement.

paisan · April 2001

You can't have 4wd engaged on dry pavement on most pickups and some SUVs.

-mike

bwh · April 2001

That is not very good advice. There are lots of vehicles out there that are NOT designed to be in 4wd on pavement.

The 4wd definition would be that the vehicle must have a selectable low range. This can be part time or full time 4wd. Part time 4wd is in 2wd normally, in most cases switching to 4wd in this type of system LOCKS the front to the rear. This causes driveline "fight", making it unsuitable for dry road use. Some of the newest systems, like the GM pick-ups, can go from 2wd to full time 4wd and this is ok for dry roads. I think you can lock the center diff (a component unique to full time) invoking the aformentioned "fight".

Now there are many variations on the awd systems as well. The most common is a full time awd like the Audi quattro system or like a modern Subaru. There are also automatic awd systems that are normally front drive and add rear traction when slippage is detected. There are several devices used in these systems. They include Torsen, Viscous couplings, and open diffs with locks. They all function a little differently but produce the same end result of dividing torque amongst four wheels. If it says awd, it IS safe for dry roads.

What is your specific model? There are a number of people here who could help explain just how it works.

bwh · April 2001

What year is the Jimmy? 3 door or 5 door? I'd be happy to offer some suggestions. Didn't the body style change in '95? From then on you should be able to get a set of factory plates from a ZR2 pick-up or SUV. Maybe scour the junkyards, I bet the internet may provide a source. Many salvage yards price at half of new cost. This easily saves enough to pay for shipping and then some. Next time you need tires spring for BFG A/T tires either 31x10.5-15 or 265/75-16 depending on your rim size 15 or 16. Get a high lift jack ($50), a 30' minimum recovery strap, no metal hooks, a portable air compressor and some hand tools(securely fastened). Oh, and a full size spare, the right size. I really think this is all you need for some pretty good wheeling. I did it with less for years.

OOps I guess you got my $.02 anyway.

thor8 · April 2001

Everybody has enough sense not to engage whatever he has or is not supoused to, in dry pavement.

There is no vehicle that states, do not drive on dry pavement.

ateixeira · April 2001

notdos: given the huge selection, it would help if you narrowed it down to a size and price class, maybe even listing the ones you are considering now.

I remember at Edmunds Live they had a Durango, and we're talking about a quick test drive here. So people would put it in 4WD for the dirt hill and then forget to put it back in 2WD for the dry portion.

So eventually they just told people not to use 4WD for that event.

It can happen - valets, friend/relative borrows it, who knows!

-juice

bwh · April 2001

I am trying not to nit pick here thor, because I respect your opinions (and Unimogs). There are plenty of trucks out there with just such a warning. Just look on the sun visor of a domestic pick-up. It should say something to the effect of not intended for use on dry roads.

thor8 · April 2001

I understand your concern and thanks, but what I do not understand is this, domestic pick-up, do not drive at all on dry pavement? or do not drive with the 4wd engaged on dry pavement? I assume you mean with the the 4wd engaged.

notdos · April 2001

Let me give a few more details as to what I'm looking for. I have a 95 Toyota 4wd that states it can't be driven on dry pavement while engaged in 4wd because of the previously mentioned "fight". I want a pickup truck that can be driven in an awd/4wd mode on a lot of varying surfaces from dry pavement (at hwy speeds) for added stability to the worst kind of construction site muck you can imagine. To me Awd sounds more like what I need but with a low range. All comments appreciated.

paisan · April 2001

The only pickups with AWD IIRC are the new GMCs, but they don't have low range. Then you have the pickups with auto-engaging 4wd, but those are only marginally better than part-time because they will not engage until there is significant slippage (reactionary system)

SUVs on the other hand have a few choices w/low range + full-time 4wd/4wd systems that can be engaged in dry conditions.

-mike

drew_ · April 2001

Try one of the GM/Chevy pickup trucks. They can be had with auto 4WD (Autotrac) while they do activate the front wheels only after rear wheel slippage, it's not too bad and the power transfer is seemless and actually not very perceptible. I don't believe there is any other pickup truck on the market right now other than GM's that offers this sort of system.

Hope this helps!

drew_ · April 2001

Actually the transfer is not too bad (we're talking milliseconds here) and it's about the same as a viscous coupling.

Drew
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paisan · April 2001

But, is there any power to the front until slippage? Just curious, not trying to be a PITA. I figure if there is at least some power going to the fronts before slippage, you are better off than if there is 0 power (less likely to slide) especially in ahighway/high speed situation.

-mike

drew_ · April 2001

I know you're not trying to be a PITA, LOL...No, AFAIK, the AutoTrac equipped GM pickups and SUVs are basically rear wheel drive rigs. When the rear wheels slip, the AutoTrac activates an electronic control module, which uses a wet clutch pack, activated by an electric stepper motor, to send torque to the front wheels that still have traction. It is a "4WD on demand", or auto 4WD system, activating 4WD when needed. It has part-time 4WD and 2WD options, as well as a low range in the T-case. There is some advantage to having a variable torque split between the front and rear. As the surface mu increases, higher peak acceleration occurs when the torque split is weighted toward the rear wheels.

Although I still prefer permanent AWD/4WD equipped vehicles, I will say that AutoTrac performs okayl. There was a demo a couple of years back where they had a Chevy Silverado pickup accelerate on a piece of soapy vinyl while in auto mode. The system transfered power to the front a brief hesitation after the rear wheels spun. Yes, it's a reactive system that cannot help all of the time, but it sure is better than a part-time only system. Of course if I had my choice, I'd go for the GMC C3 truck with the permanent AWD system :-)

Drew
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heng · April 2001

Skid plates off the ZR2s is a good idea. (I've got a 2000 4Dr)

I've got to pick the upgrades carefully since I'm not planning to do much off roading (and don't want the wife on my case). Mostly getting in and out of my own property. The AT tread tires would be too aggressive for 99% of my driving.

What's the detail on "no metal hooks" on the tow strap? They crack or something?

paisan · April 2001

I wasn't sure how the auto-engaging worked on the GMs. I still am in wonder why GM hasn't used the TOD system from the Trooper which is made by Borg-Warner (and GM owns 1/2 of Isuzu) on it's trucks. Its a great 2wd/AWD/4wd Lo T-case that puts a min of 15% power to the front axle. Maybe it would kill the fuel economy too much for their liking? (although I only take a 1mpg hit on my trooper)

-mike

ateixeira · April 2001

notdos: if you want AWD full time, Subaru is coming out with a production version of the ST-X concept. The concept, at least, had a low range, but we'll see if that carries into production.

It's a 4 door crew-cab with a small bed, but it's car-based so probably meant for light duty (which should be OK for a construction site).

They are supposedly working on a 2.5l turbo, so it would have more than 200hp, full time AWD, and about 8" clearance.

-juice

bwh · April 2001

I have found the BFG ATs to be fine for daily use, I can't hear them at all on the road. They have also proven to be excellent in the snow.

The metal tow hooks or a chain are dangerous tools for vehicle recovery. You have it right, they break and send parts flying, not a good deal. Chain is definately the worst choice. Good forged hooks would be OK but most come with cast ones that crack easily.

heatwave3 · April 2001

I believe the only AWD pickup on the market is the new GMC C3 with the same AWD system on a 1500 chassis that is used in the new Denali, Denali XL and Escalade. It provides a minimum 38%/62% split of the available power continuously to the front and rear wheels respectively. The percentage increases to the front as rear traction is lost. It includes a limited slip rear transfer case.

paisan · April 2001

I believe heatwave is correct. The only other AWD pickup I've seen was the Syclone of the early 90s which was a performance truck, with almost no payload.

-mike

ateixeira · April 2001

But does the C3 have a low range? I didn't think it did.

-juice

heatwave3 · April 2001

The C3 does not have a lowrange on the AWD setup.

mattbart · April 2001

I thought I understood the 4WD system on my SUV, now I'm confused. What would the Ford Explorer's Control-Trac 4WD system be defined as? I have a knob on my dashboard with three positions: "Auto," "4 High," "4 Low." I understand how the system functions in 4 High and 4 Low modes, but what is Auto? More power to the rears or a 50/50 split?

paisan · April 2001

is 100% rwd until you are spinning the rears, then it will engage the fronts. It's a reactionary system but it's better than only having a part time system.

-mike

nrossi · April 2001

What are the Pros and Cons of Traction Control sytems that use ABS braking on open diffs?

Do these systems reduce acceleration? Are they only applicable at low speeds?

Thanks
Nick

gpm5 · May 2001

I saw at the Borg-Warner site that they are making a TOD unit for the explorer. I don't know if this is a new design or on the older AUTO vehicles.

vin_weasel · May 2001

Can someone give me a breakdown of how the Selec-trac works as opposed to the Command-Trac (standard part time 4WD)?

From what I can find it allows you to have a 48/52 split and use it on dry surfaces but I don't know if that is locked, variable, front/rear, rear/front or what.

drew_ · May 2001

Please refer to my post here:
drew_ Feb 5, 2001 8:24pm

SelecTrac is a full-time 4WD system. In 4WD High Full-time mode, the torque split ratio is split at 50/50 front and rear; it's not variable, so all 4 wheels are powered and helping to provide traction at all times. You can leave the system in this mode on all surfaces (since it has an open centre differential) without any decrease in fuel economy.

CommandTrac is a part-time system and lacks a centre differential. It can only be activated on slippery surfaces such as snow/sand/mud.

Drew
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brunna · May 2001

I looked at both used '99 Explorers with the "auto" system and a '99 Mountaineer with the AWD (permanent?) system. After reading through this thread it would seem that most would agree the AWD system suits my needs better. My driving will be mostly commuting on winding backroads in northern MD. - with an occasional heavy snowfall in the winter, lots of rainy spring days and some light hauling. Can anyone please offer an opinion to help me decide between the 2? Thanks!

lurker01 · May 2001

Can someone explain the difference between SmartTrac on the 2002 Bravada and AutoTrac on the 2002 Envoy? If this is in another message, please refer me to it. I looked but did not see one.
I think the AutoTrac is 2WD Rear untill slip occurs, then it brings in some of the front wheels.
If this is true, what does SmartTrac do differently?

paisan · May 2001

IIRC Smart Track is an AWD system in that you can't have 2wd, and you can't have a low-range. Basically it's a center viscous coupling that tries to attain a 50/50 split. Auto-trac is as you describe 2wd RWD til slippage then it applies some power to the front. You can put it in 2wd mode, and also have a 4wd lo range as well.

-mike

hiker1 · June 2001

Any thoughts on why GM has an optional locking differential for their trucks/suvs while a limited slip differential is optional for the Jeep Liberty?

4WD and AWD systems explained

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