4WD and AWD systems explained

markobodor

Thanks Paisan, you are right - I did not test both cars on the same day and on the day I got stuck with the Allroad the snow ramp I was trying to go up was full of fresh snow and not hardpack, so you are right, the Porsche would not have made it up the ramp, but then again, I think the rear wheels, with most of the weight of the car on them on an inclined slope, would have pulled me out backwards. Regarding plowing snow, I have done that -- last year, I-80 was at a standstill because of winter conditions, so I turned on to old Highway 40 that runs parallel to it and it was unplowed, but there was a set of wheel tracks left behind by a service vehicle I saw going up the road, so I chose to follow it. The snow got to be up to a foot or more and it was very slick underneath. At times my car did a bit of fishtailing, but it kept going and going and I made it up 6 miles or so till I got to the top of the road. As I was driving, my engine temperature was going way up and when I inspected my car at the top of the road, I saw that I had been plowing snow with my front spoiler and that the snow had clogged up my radiator and it was unable to cool the engine.

nrossi

I am sure that the allroad has a torsion center diff, which means that as soon as one axle spins faster then the other, power is instantly transferred to the other axle. It might very well be the case that the front had better traction then the rear, or just that the rear spun first.

Maybe if you drove all the way into the drift the car would have performed better?

If you start at the beginning of this topic there is a good description on how torsion works.

I prefer to be able to lock diffs as needed. I had a 4runner a year ago, and was in Vermont for the winter. I was able to get out of every mess I got myself into by locking the center diff and on rare occasion, the rear as well.

At any rate, snow tires make huge difference.

As for the false sense of security with 4wd, I agree. I drove up to Vermont one weekend last year in my 4runner through a snow storm. On the shoulder of the freeway, was a tipped over SUV.
4WD does not equate to invulnerability. That said, I would take a 4wd with snow tires over a 2wd with snow tires any day. Theres is no doupt that 4 is better then 2, but that does not make up for reclace driving.

kens

It's actually Torsen, for Torque Sensing.

Ken

ateixeira

You're onto something there. The torsen would be ineffective on ice, so that would leave the job to the 4 wheel traction control. Applying the brakes retards momentum, so it could not climb the steep hill.

-juice

kens

Wasn't that the achillies heel of Torsens? If you have zero traction on one end, it can't transfer power?

Ken

nrossi

I am sure I have read that there must be some resistance for torsen to work. That is most likely why Audi chose to use it as a center diff not an axle diff. The chances are fairly slim that both wheels of an axle would be off the ground at the same time. It is far more likely that one wheel might be in a ditch or off the ground.

I believe that the order of diff locking on the Audi's is torsen first then EDL. But I am not sure. I do recall reading somewhere that EDL is fifty % more effeicent when used in conjunction with a torsen center. I think it was an article about the Mercedes SUV's.

ateixeira

I think Mercedes now uses three open diffs and lets the traction control to the job. No center torsen AFAIK.

-juice

nrossi

The article was commenting on how Mercedes might have cut too many corners on their new SUV's by having three open diffs, and that by adding a torsen center the system would be50% more effective.

Don't quote me on that, it has been a while.
I did a lot of reading back in May/June when I was shopping for a new SUV. I came across this article when investigating the pros and cons of EDL.

I believe the efficiency comes from using the torsen first, thereby saving wear and tear on the brakes.

ateixeira

I get it now, that makes sense. A Torsen does act very quickly, this is why it's so desired by autocrossers.

Coupled with T/C, when the torsen fails you have the brakes to manage wheelspin. A good combo.

In fact, Subaru mentions that in their VDC model they tuned the traction control to allow the VTD AWD systems to act first.

-juice

nrossi

I always thought the Audi AWD system with the Torsen center and EDL was the best. I test drove an Allroad and was very impressed. I guess nothing is infallible.

drmp

I would first check the EDL if it is working properly. You can test each wheel on a roller and see if the Audi moves. I read somewhere about Chrysler's T/C having a "rest" period after a predetermined continuous cycle to cool off the brakes. The Audi may have similar logic.

paisan

You might also want to check the tire pressures. On my trooper I got stuck in deep snow once recently with brand new big AT Pirelli Scorpions. Of course I was running them at max psi, dropped em down to 28psi and they grabbed just fine.

-mike

ochssk

My wife and I are planning to buy a Subaru soon and I was wondering about reliability differences and maintenance differences between the electronically managed variable transfer clutch and the viscous-coupling AWD drive systems. We keep a car for 10+ years so reliablility is a big factor with us.

paisan

ARe quite reliable. My family has 10+ year old AT subies with the 4EAT tranny no problems with the 4wd system on them. Same for the VC.

-mike

ateixeira

Browse all the Subaru threads here on Edmunds, as I do, and you'll find not a single AWD system failure, not one. That is impressive.

BTW, if one suddenly appears in the next day or two, you'll know it was a Troll! ;-)

Which models are you looking at? There are several systems:

Manual tranny, VC
Manual tranny, VC + rear LSD
Manual tranny, VTD + rear LSD
Auto tranny, VTD + traction/stability control
Auto tranny, auto AWD
Auto tranny, auto AWD + rear LSD

All are effective, but each works in a different way. Some come only on certain models (VDC, WRX). Which models are you interested in?

-juice

ochssk

We are trying to justify paying extra for H-6 engine which would be the LL Bean edition. The Bean comes with a electronically managed variable transfer clutch and a limited-slip rear differential. The H-4 Wagon with manual tranny has a viscous-coupling center differential and viscous limited-slip differential. I guess if we go with the H-4 with automatic tranny it would all have the electronically managed continously variable transfer clutch.

I know with the viscous-coupling you have to maintain the tire diameters within tolerances and change the fluids regularly.

kens

Actually, you need to maintain the tire diameters with any of the systems. Since they all react to tire spin, different tire diameters would throw off the system, and in worse cases, damage the mechanicals.

Also, there are no fluids to replace with the VC system. In fact, there are no maintenance items for it that I know of.

Ken

ateixeira

The Bean has the last system on my list, auto AWD with a rear viscous LSD. The H4 automatic has the same AWD system, FWIW. The manuals are different.

But both are effective, so let your tranny preference decide. If you do a lot of bumper-to-bumper drives, and auto may be better. I prefer manuals myself.

Ken is right, the VC is sealed and maintenance-free.

Trying to justify the H6? Reasons would include more relaxed power delivery, good torque, still good fuel efficiency, and 3 years of free maintenance (Bean models only). If you like the wood/leather steering wheel and the two-tone leather, even better.

-juice

rswanson

Does the 2002 Toyota Land Cruiser have permanent 4WD or full-time 4WD, using Drew's definitions from Post no. 2?

jd150

Does anyone know the difference between the 4WD Auto in the 2002 Control-Trac and the AWD offered in prior years and in the current Mountaineer? What are the percentages of front/rear power on the AWD? Is it rear only until needed? Does it change as conditions do? Up to what percent? For the 4WD Auto, I know it is rear only until front is needed...up to what percent can go to the front?

oarlox

I am no new comer to the world of 4X4's. I have probable owned 10 of the regular old shift into H/L 4X4 range. But recently we purchased a new Saturn VUE that has AWD. This is our first FWD/AWD. It has a much different feel on the ice and snow. I'm not saying that it handles badly at all, it is very good in fact. I would have to say that the feel I get from it is that there is some what of a delay before the rear wheels engage. Is this my imagination playing games on me or is it for real? I understand that up to 80% of the power will transfer to the rear, does this mean that the rear has no power applied under normal conditions? One of my concerns is: How does this delay effect the car under adverse conditions, say in the snow or in other slippery situations. Do my front tires need to spin before the rears engage?I am comfortable driving in the ice and snow as that's our weather here in Alaska for half the year, so slipping around normally isn't much of a surprise. I read about Viscous coupling and Rotary Blades. What do I have (Own)? If this system is normally FWD untill the fronts slip prior to the rears engaging , is the VUE truely an AWD?

kens

AWD is a marketing term that covers 4WD systems that do not require user intervention. It sometimes refers to systems that do not have a transfer case capable of H/L 4x4 as you mention. In short, AWD describes systems that were meant for on-road use.

There are several ways to categorize AWD systems on the market today. One way is full vs. part time. The other proactive and reactive.

Without argument, full-time proactive systems are the best. One such system is the VTD system found in the Subaru VDC wagon and Impreza WRX (A/T). These systems have the least tendency to slip in the first place since they are continuously managing torque distribution. These systems not only help you in snow and ice, but also improve dry road handling as well.

The least effective systems are part-time reactive such as Honda's Real Time 4WD (CR-V) and the system found in the Saturn VUE. These systems are designed to just give your vehicle a "push" when the front tires get stuck. The downside to these systems is that you can get a herky-jerky motion as the system kicks in and out in slippery conditions. Also, these systems do not let you take advantage of the handling benefits of AWD.

Here are some good links that explain the above in more detail:

http://makeashorterlink.com/?W2A32166

http://www.4x4abc.com/4WD101/need.html

http://makeashorterlink.com/?A2935366

Ken

wwest

You are going to be truly disappointed in your new TOY. Even here in Seattle none of these "new" AWD systems are up to the job in our adverse mountain pass conditions. Like you, I previosuly owned two "true" 4WD Jeeps, my RX300 purchases have been a great mistake.

If you decide to "move", look first at the ML320 or the X5.

canadiancl

I recall someone had explained the delay that you've experienced in the VUE topic. It is not your imagination. It's an intended design. Apparently Saturn wanted the AWD to not react to the slightest slippage at the front so they can use a space-saver spare.

jam1000

What, if anything, is the difference between these two systems? Sales brochure is less than clear on this.

drmperalta

Select Trac:
2 hi, 4 hi (open center diff), 4 hi (locked), 4 lo {also locked)

Quaratrac:
4 hi is AWD or more appropriately termed automatic 4WD. Rear axle is the primary drive. With rear slip, a hydro-mechanical "gerodisc" at the transfer case reactively distributes some of the power to the front. 4lo will have locked front to rear connection (?). New hardware and lots of bugs, lots of unsatisfied owners.

cmack4

I saw this covered in a past post, but not sure if all the differences were hit. The Smarttrak comes standard with the rear locker, but no lo range transfer case. The Autotrac has the lo range as well as an auto mode, but no standard locker (optional). Another difference as far as I can see is that in auto mode, the Autotrac switches to a 50/50 split transferring from rear to front in slippage situations. The Smarttrak has individual wheel sensors which control a variable torque split in a slippage situation, but is still reactive since it is primarily rear wheel drive until the slip occurs. Does that hit all of the major differences?

paisan

Doesn't that use a center viscous coupling that continously varies the torque to each axle? Autotrac just engages the 4wd-Lo when a certain amount of slippage is engaged, from what I've read. The lockers have nothing to do with the 4wd/awd system really.

-mike

cmack4

Yeah, it's a viscous clutch system. The locker as you pointed out isn't really related to 4wd vs AWD, but it does help out in slippery conditions and is a nice extra. Even if it does only kick in once in a blue moon... I had my Bravada at the beach this weekend and was impressed at how well it handled. I let some air out of the tires, and it didn't even feel that sluggish in the loose stuff.

paisan

cmack4, yep airing down is key for any off-road situation, as well as smart placement of your wheels. The locker is "auto-matic" in the bravada? That is kinda sucky IMHO. I have an LSD which is "auto-matic" but I'd rather have a manual locker, like the TRD packages on the toyotas.

-mike

cmack4

yeah, it's auto... I guess the concept is to make it as idiot proof as possible. I'm sure if it were manual, someone would leave it turned on at higher speeds or around turns causing unnecessary wear and tear. It comes on if it detects more than a 100rpm difference in the axles from right to left at speeds under 20mph.

paisan

Probably works marginally better than a tradition clutch style LSD. With my LSD I can "manually" engage it by yanking up a few notches on the E-brake.

-mike

cmack4

I'm sure the parking brake would give enough resistance to make it engage if I ever wanted to use it in a pinch. A toggle switch would be nice though. I don't plan on doing a lot of off-roading in the Bravada though. I might try out some of the trails I saw on your website for the Pine Barrens here in NJ, but nothing like I used to do in my Old FJ40, http://communities.msn.com/cmack/rc.msnw?action=ShowPhoto&PhotoID=88" I should have never sold that vehicle, but I was young and foolish...

paisan

We were just out there yesterday, found some new trails and a really cool sand pit that had some nice mud areas and some good places to climb up the sand walls. Pictures are on their way onto the server, you should join us sometime.

-mike

cmack4

Mike, I was wrong about the viscous clutch. I stopped by the dealer today and talked to a mechanic. The newer Smarttrak systems (99+) no longer use it! They have an electronic transfer case that uses individual wheel sensors to detect slippage. When slippage is detected, the system sends feedback to an electronic transfer case which in turn electrically engages a multi-disc clutch via a small motor. I'm guessing there's more than one disc assigned for different torque distribution? The sensors tell the system how many rpm's each wheel is traveling, and the system then automatically compensates until the same rpm is achieved for each wheel. This is supposedly better, because it is not only faster (.25sec vs 1-2 sec for viscous) but also requires less maintenence. It's also tied into the rear locker, so that in the event that the rear wheels are traveling with a variance of more than a 100rpms at any speed below 20mph, the rear locker will automatically kick in. Also, the amount of pressure and duration of pulses from the ABS braking system is somehow tied in based on the conditions for front and rear axles. All I gotta say is, hopefully the electrical system doesn't go down!

paisan

Interesting. I wonder who makes the unit, and if it possibly is a variation on the TOD system used in the Trooper which is made by Borg Warner. Sounds similar, except we get low-range that locks in the torque split @ 50/50 and also gives low-range at the same time.

-mike

cmack4

I did a search, and this is what I was able to come up with. Smarttrak was originally designed and built by New Venture Gear, a joint venture between DC and GM (known for Jeep 4wd and Hummer). In January 2001, the venture split when DC took ownership of 64% of the holdings. As part of the split, GM held onto the Muncie Indiana plant, as well as rights to the Smarttrak system... http://detnews.com/2002/autosinsider/0201/10/autos-387376.htm

Since about February 2001, guess who has been working out of the Muncie plant, none other than Borg Warner. I haven't been able to find anything directly linking the Smarttrak to Borg Warner, but since it's still coming from Muncie, and that's who is operating out of that plant, I'm guessing that's who is manufacturing the new system. The sad thing is... since Olds is on the way out the door, We'll most likely never see a lo-range like the TOD on the Trooper.

paisan

I know the feeling... Now that the Trooper is in it's last year of production for NA... Isuzu will likely end up in the GM-defunked graveyard right next to Oldsmobile.

-mike

funits

Is the Autotrac setting on the 2002 Envoy (or TB) that same as the Bravada Smarttrack system? I read that an Envoy/TB with the locking differential option is the same setup as a Bravada with Smarttrack. Is this true? If not, what is the difference.

Thanks in advance.

cmack4

No, it's a common misconception, but they really have very few similarities. The autotrac system still uses the viscous coupling versus an electronically managed transfer case. There also aren't wheel sensors to manage torque shift. When activated, it just splits the torque 50/50. The locking rear on the Bravada is tied into the sensors and automatically activates. I'm not sure on this one, but I think the regular GM locker has a manual switch? The one thing the Autotrac system does have that Smarttrak doesn't is the lo-range transfer case. That makes the Autotrac better suited for more severe off-road situations where high torque is required; The Smarttrak system on the other hand should be better suited for on-road adverse conditions.

paisan

I don't believe it uses a viscous coupling. I think it actually is vacum actuated that basically will "automatically" engage 4wd hi locking it at 50/50 when is senses a difference in spin rpm to each of the 2 driveshafts. Essentially it would be like you flicking the switch from 2wd to 4wd-hi except that instead of you doing it when you sense the wheels slipping, it uses sensors to detect the slippage and engage the shift on the fly for you. The rear Locker has a manual switch as well on the GMC/Chevy version as opposed to automatic on the Olds. I agree the smart-track is more suited for on-road than the auto-track.

-mike

cmack4

Thanks for the correction Mike. So, basically it's faster than a viscous coupling, but it still doesn't manage the torque split, so you're still not guaranteed to get the power delivered to where it's needed most.

paisan

I think it's slower IIRC than a VC. Basically there has to be a big difference in wheel-spin in order to coax the system engage the 50/50 split. If I had to rate the ones in the GM family for fastest/best/on-road:

Smarttrack (Oldsmobile)
VC (Denali/Escalade)
Autotrack (Yukon/Envoy/TB/Taho/Suburban/Pickups)

-mike

heatwave3

paisan: I think the order of speed for the different GM 4wd systems should be

VC (Denali/Escalade)
Smarttrack (Oldsmobile)
Autotrack (Yukon/Envoy/TB/Taho/Suburban/Pickups)

in order of fastest/best/on-road.

I state that because the system in the Denali/Escalade provides a minimum 38/62% split in torque between the front and rear axles all of the time, therefore traction from the front tires is instaneous since power is provided permanently to the front end.

Power is then transferred as needed through the center viscous coupling between the front and rear above the minimum as needed based on rear slippage. Likewise this system provides a limited slip rear differential which also transfers power from left to right in the rear based on whichever tire has better traction.

IMO this is a substantially better 4wd system for on-road purposes than either Smarttrack or auto-trac, however the absence of a 4wd-lo would make the Auto-trak setup preferable for off-road purposes.

paisan

Sorry heatwave a VC is REACTIONARY whereas the Smarttrack actually reacts in advance of a skid. Basically the ECU uses weight sensor to sens shifts in weight and puts power to the axle with more weight on it to provide more traction. Also it pre-emtively shifts power in repeat situations such as always peeling out after a complete stop on the rear wheels, after a few times, it will automatically put more power to the front BEFORE you even hit the gas after a complete stop. Also I believe the smart-track puts some power to the front axle at all times so it has torque going to it as well, like the VC.

-mike

heatwave3

paisan: I agree that the computerized sensors measuring weight shift are faster than a VC center diff for transferring power from rear to front. I was really comparing the on-road rating of the vehicles you posted and would still have to state that the VC setup of the Denali/Escalade is preferable and faster to Smarttrack since no transfer at all needs to occur upon rear traction loss in the Denali AWD system.

Even the Smarttrack system has some minimal delay in transfer since it starts with 0 power to the front axle. An AWD system such as the Denali's will be preferable in adverse on-road conditions. Its my belief that eventually most suvs purchased with minimal intention of off-roading will likely come with systems more like the Denali and Audi Quattro in the future.

cmack4

I think Paisan rated the Smarttrak as faster, because it will react faster to changing conditions than the Denali/Escalade. Granted they are both designed to be Proactive versus Reactive like the Smarttrak, so the reaction time isn't supposed to be as good. Smarttrak actually used to be a proactive system as well, but I suppose it took too big of a hit at the pumps on the MPG...

paisan

Oh I didn't realize that the smarttrack had a 0/100 torque split. My favorite of the lot is the Borg Warner TOD which put 15/85 as a std split and works up to 50/50 using the weight and speed sensors. Now if they put a beefed up version of the TOD system on the Denali (it has low-range locked at 50/50) that would totally be the goods!

-mike

cmack4

Yep, when they did a way with the VC, they made it reactive (Rear Wheel Drive). I would prefer they did something like a 15/85 split, since you know it's working all the time, even if it only does take a quarter of a second to kick in. I think the original was 35/65... they should've just lowered the full time ratio! They did do a good job in implementation though, because you don't notice any jerkiness at all when it kicks in. To give you an idea... If you floor it from a dead stop, even in dry conditions, it will kick in even before you have a chance to squeal the tires! I think the torque transfer goes all the way up to 80/20, but I'll have to check.

heatwave3

I agree that when slippage occurs the reaction time of the ECU in Smarttrak will respond faster than a Center VC Diff. My point was that an AWD system with a 38/62% minimum torque split with a VC center diff to transfer even more torque to the front when needed, is a better on-road system than a an admittedly faster system such as Smarttrak that starts with a 0/100% torque split before the need for front end traction occurs.

I think most would agree that having the traction to start with is better than waiting for it to be transferred regardless of how fast it can be moved up front.