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4WD and AWD systems explained

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Comments

  • ateixeiraateixeira Posts: 72,587
    Didn't some reviews with the HL think the traction control interevened kind of early?

    For sand, it may be better without it.

    -juice
  • drew_drew_ Posts: 3,382
    is starting in 10 minutes time. See you there!
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    Vans, SUVs, and Aftermarket and Accessories message boards
  • Drew - very much appreciate this topic. Given the number of different systems and the use of the same words to mean different things, there is a great lack of clarity leading to mis-understanding.....

    Am interested in your comparison of the xWD systems in the 2002 Bravada and the 2001 MDX. How would you describe them and what do you feel to be the advantages/disadvantages of each?

    Also, for some time Bravada claimed that their xWD system was unique to that model. Is it your understanding that this remains true for the new 2002 models or is the xWD offered on the Envoy the same as the Bravada?
  • paisanpaisan Posts: 21,181
    Unique is always "excludes other GM products"

    The bravada/envoy have a viscous center diffy, similar to the Astro van/safari and the MT versions of the subarus. Constantly tries to split power 50/50 to the front and rear drive-shafts.

    -mike
  • hengheng Posts: 411
    6 inches of granular snow, grades and bumpy terrain. It was the bumpy terrain that did me in. Once 1 or 2 wheels drop into a dip you are stuck. I couldn't go any faster because it was a forest type trail (on my property). We got stuck multiple times. We did get out of there after a lot of work (I'll spare you the details).

    Question: (just for thought) Is it a workable solution to install locking diffs on each end (driver activated)? Does it even have to be both ends?

    I know the units run about $400 apiece so it is a pricey proposition.
  • ateixeiraateixeira Posts: 72,587
    My understanding is that Olds' system is unique, and Chevy and GMC use more traditional 4WD systems.

    heng: what car/truck do you own?

    The differential itself actually sounds cheap - it's putting them in that is the trick. In this case, I'd guess that labor will cost you well more than the parts.

    Think about yanking out the diffy, probably the exhaust for access, disconnecting the drive shaft, the half shafts, etc. You'd need new seals, so even the parts price would skyrocket.

    IMO, trading it in makes more sense.

    -juice
  • tonychrystonychrys Posts: 1,310
    heng Feb 25, 2001 3:43am


    If you want "diffs" between the front and rear axles, and between each wheel on both axels, you are basically talking about a JGCL with Quadra-drive.


    http://www.jeepunpaved.com/gr-cherokee/4wd/feature2.html

  • bblahabblaha Posts: 329
    Were you driving in full time 4wd or part time? In PT, you would have had to lose 2 tires (1 in front and 1 in back to get stuck).

    Locking the differential aids whichever axle has lost traction on one tire.

    One of the more popular lockers is an ARB air locker. Uses compressed air (compressor needed) but a press of a button locks the differential.
  • hengheng Posts: 411
    I was thinking of, but the cost and the amount of work Axteixeira points out doesn't make sense to invest. I'm not a rock crawler so maybe keeping cable chains for each wheel is all its worth.

    Don't laugh guys, its just a 2000 Jimmy with open diffs. I'm calibrated now to its limitations.

    At various times during this episode I was hanging 2 wheels. I was even spinning 2 wheels on the same side. Even got it to go sideways some.

    On non-bumpy ground it did great. The same forest track, without the snow is not a problem. So I'm not considering a more capable 4WD, maybe just cable chains.
  • Does anyone know why Chevy no longer offers traction control as an option on the 2001 Tahoe? I heard someone talk about Traction Assist on a 2000 Tahoe. Is Traction Assist the same as Traction Control or are they two different things? I would like to order a Two wheel drive Tahoe with a no slip differential. Does the new Tahoe come with a traditional free spinning differential or a newer version something similar to the limited no slip differential? The car salesman don't seem to know anything about the differential designs. I'm hoping that something is taking the place of the traditional differential since Chevy is no longer offering traction control as an option. Is traction control worth having when you tow a small trailer and for winter driving? Please inform me if you have any information to share.
  • I'm wondering where the Mazda Tribute / Ford Escape AWD system falls in Drew's categorization system from his post #2 on this board. In particular:

    - The Mazda has a switch on the dashboard that distributes the torque 50/50 between the front and the rear, and apparently can be used at all speeds. Does that mean this switch turns the car from full-time AWD to permanent AWD?

    -Why wouldn't you just leave this switch turned on all the time? I've read that Mazda does not recommend it be turned on on dry pavement - can anyone confirm this? Does anyone know why?

    - How the technology used (Rotary Blade Coupling) compare to the Torsen differentials and viscous couplings discussed earlier in this board? How does the AWD system in general compare to other systems, such as Audi/VW and Subaru?

    - Will the car be able to move if it loses traction on one side (both wheels)? Will it move if it loses traction on three wheels?

    Thanks,
    Jim
  • ateixeiraateixeira Posts: 72,587
    I believe they lack a center differential, so it's either FWD or locked 4WD. So they are part-time only, with automatic or manual engagement. On dry surfaces you would have binding if it's engaged.

    Anyone want to elaborate?

    -juice

    PS I believe both front and rear axles have open differentials. Car & Driver's got stuck during one of their SUV comparos.
  • drew_drew_ Posts: 3,382
    This week, chat with Edmunds.com's managing editor, Karl Brauer. The chat is tonight at 5-6pm Pacific:
    http://www.edmunds.com/chat/brauerchat022701.html


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  • drew_drew_ Posts: 3,382
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  • londonfinlondonfin Posts: 31
    Drew-
    Wanted your opinion regarding the AWD on GMC Denali vs. the "Auto 4WD" offered on Yukon/Tahoe. I live in Mass. and do need to go out in all kinds of weather (snow/rain) as I work in public safety, but do not plan to go real off-roading other than some beach travel. I'd welcome your thoughts as well as any one elses.
    Thanks
  • paisanpaisan Posts: 21,181
    AWD on Denali/Escalade: This is a viscouse center differential type awd. It tries to achieve a 50/50 split of tourque between front and rear at all times. This system is great for on-road, and is a "no-driver interaction required" type system. For your application I would choose this one over the auto-4wd system.

    Auto-4wd on GM (yukon/tahoe/suburban): This waits until there is significant slipage on the rear wheels and only then will engage the front axle. This system doesn't provide any power to the front axle under normal conditions and is better for off-road use because it can be locked in 4wd lo, 4wd hi, or can use 4wd auto.

    Have you looked at any other vehicles? I know drew is partial to the ML series which are great vehicles and provide a better AWD system IMHO than either of the GM ones. I own an Isuzu Trooper which also has a variation of AWD that under normal conditions splits power @ 15% front and 85% rear, and has a guage that shows how much torque is going to each axle, this system can be left engaged all the time, and uses speed and weight sensors to change the torque up to 50/50 split depending on conditions. The best part of troopers is their price. I paid last May/June for a fully loaded trooper w/moonroof and no leather $27K before TTL. Definitely worth looking at the ML and Trooper as alternatives.

    -mike
  • londonfinlondonfin Posts: 31
    Mike-
    Thanks for the info. I will look at the ML and Troopers as well but I must say I'm smitten by the looks of the Yukon. Who says people have to make rational decisions when it comes to cars!

    -Jack
  • paisanpaisan Posts: 21,181
    Yukon is my 2nd choice actually, since they have 4wheel Disc brakes and rear coil springs now, that make a big difference. I just couldn't justify the 10K+ extra over the price of my Trooper.

    -mike
  • Still interested in the issues discussed in #95...
  • nrossinrossi Posts: 47
    I feel like I am really beating this to death, but I am interested in buying a Denali and I want to make sure it meets my needs. Could someone explain how the AWD on this SUV works. I understand that this is a permanent AWD system (I think), but from my readings on the GMC site, they claim that it is supposed to transfer torque from the wheel that is slipping to the one that has traction. Mike, are you saying that the way it does this is by transfering torque from the front wheels to the back? Also, GM claims this vehicle has an automatic locking rear differential. Could someone explain the pro/cons and function of this.

    Thanks
    Nick
  • paisanpaisan Posts: 21,181
    The locking rear differential will move power from the left to right rear wheels or visa versa if one is slipping. The AWD system will transfer power from the front to rear axle if either the rear or front axles is spinning faster than the other. For instance:

    Front wheels are on ice, the front axle will spin at say 20mph on the ice, the rear axle is on solid ground only spinning at 5mph, the AWD system (viscous center differential IIRC) will move power from the front axle to the rear axle until the front and rear axles are both spinning @ the same speed, in this case 5mph. The locking rear diffy, will do the same as described above, except that it does it from side to side on the rear axle.

    This AWD system is basically what I call a First generation "reactive" AWD system. It's one of the best ones for your purposes (on-road, wet/snowy conditions) vehicles with similar systems include:

    Subaru (manual transmissions)
    Chevy Astro/Safari
    Caddy Escalade
    Oldsmobile Bravada

    There is no driver input at all on these systems and requires no "thinking" or anticipating. I hope this helps. Post up any other questions you have.

    -mike
  • nrossinrossi Posts: 47
    Mike,
    I am concerned that this type of drive might not be adequate. I do a lot of outdoor type activities. I have a house in Vermont and frequently find myself in snowy conditions, sometimes deep, and fairly steep hills. Mud and sand might also be a concern. I am getting the feeling that AWD is useless for anything other then a wet road.

    I currently have a 4Ruunner and it is almost always in 4hi when I am up there. I have found myself in situations where one side of my car was in deep snow and it was a little tuff getting out even with the 4WD.
  • paisanpaisan Posts: 21,181
    The 4wd Hi will lock in 50/50 split front to rear. If the 4-runner has no rear Limited Slip Differential, then only one wheel on each axle can spin. This IMHO makes that actually give you less grabbing than the Denali, because the Denali has the rear LSD/Locker. I don't know if you need 3 rows of seats of the Denali, but personally I'd look at the Trooper, since it gives you the best of both worlds. 2wd (to save gas) TOD/AWD (15% power to front/ 85% rear as a standard, and will automaticaly shift up to 50/50 with weight and speed sensors), and 4wd Lo (which locks you in at 50/50 with 2x the torque) The trooper is also a very very capable vehicle, comforatble, and 10/120K powertrain warranty. Pricing on a loaded Trooper LS w/o Leather w/moonroof is around $28K before TTL. It tows up to 5000lbs (i've towed 3500lbs 300miles w/o a problem) it has 4wheel disc brakes, anti-lock, heated seats, power retractable mirros, heated mirros, 6 disc changer, etc. etc.

    One of the only AWD vehicles on the market that also gives you 4wd Lo range, and 2wd options.

    -mike
  • nrossinrossi Posts: 47
    Thanks Mike. I was just on the Trooper web site. Its a very nice SUV, but one of the big selling points to the Denali is it's size, the others are it's engine and ride. The 4Runner is coming up short on space for my needs. I am still considering the Yuk or Sequoia, but I am partial to the Denali.
  • paisanpaisan Posts: 21,181
    Don't let the length and wheelbase nor the interior space send you away from it. It is quite boxy, which makes all those inches of space quite a bit more usable than an equivilent rounded vehcile. Power, I'm a pretty heavy lead foot, growing up driving a '74 olds with a 455, and an '83 Dodge Ramcharger with a 318. The Trooper can hold it's own up to about 110mph and when I'm towing the 3500lb boat, with cruise control on, I barely notice it. If there is an isuzu dealer nearby, drive it just for the heck of it. (sorry to sound like a salesman, I'm not)

    -mike
  • nrossinrossi Posts: 47
    Mike,

    Is the 4WD on the Yuk Full or Part time - does it have a locking center diff? Can you drive it in
    4 Hi on a paved road?

    Correct me if I am wrong, the Toyota Seq has a locking center diff, so this is a full time 4WD.

    As long as the center diff is not locked, you can drive in 4 hi on a paved road, is that right?

    If a full Time 4WD Vehicle is in 4 Hi, is it basically like a Perm 4WD vehicle?

    What is Active TRAC 4-wheel Traction Control on the Toyota is that similar to the ESP system on the ML? Do any of the GM products have similar functionality?

    Thanks
    Nick
  • paisanpaisan Posts: 21,181
    The Yukon (not denali) has the following settings:

    2wd
    Auto-4wd
    4wd Hi
    4wd Lo

    You can use 2wd and auto-4wd on dry pavement. The Auto-4wd is a reactive system, basically once the rear wheels are spinning, it will push power to the front. Not a bad system, but not one of the better ones for on-road traction. 4wd Hi cannot be used on dry pavement because it locks in 50/50 power.

    Toyo:
    The Sequioa can be driven IIRC in the following settings:

    2wd
    4wd Auto/AWD
    4wd Hi
    4wd Lo

    Basically you can run on dry pavement with the 2wd and 4wd auto/awd setting. I'm not sure of the torque spilt but it's probably at least 10%Fwd/90%rwd to start with, it may be 50/50. 4wd Hi and Lo are off-pavement only settings.

    You can lock the center Diffy which can only be used off-road. The Active trac 4wheel tractions is similar to the ESP on the ML, Good system to have. The only GM product I know that has anything remotely similar would be the Escalade "stabilitrac" but that has the same AWD system as the denali. Hope this helps.

    -mike
  • nrossinrossi Posts: 47
    Mike,

    I have been reading up on differentials I think this is the way they are supposed to work - correct me if I am wrong.

    In a standard Perm 4WD vehicle, there are 3 diff's - front center and rear. The center diff regulates torque to the front and rear axles and the front and rear diff regulate torque to the left and right wheels.

    Differentials allow the wheels to spin at different rates.

    If the center diff is locked, the power is split 50/50 front to rear.

    If say 1 of the rear wheels is slipping, the rear diff will send all the rear power to that wheel. But because the center diff is locked the front wheels can pull.

    If the rear diff is locked, the the rear wheels are forced to spin at the same rate an the wheel with traction gets used.

    If no diff is locked, the only wheel that is spinning is the one with no traction.

    The viscous center differential IIRC works by sensing slippage. The slippage causes the viscosity of the fluid in the c diff to change an that acts like a center diff lock.

    The active trac on the Toyota and the ESP system act like front and rear differental locks. By applying breaking to the wheel that is spinning, it simulates traction and the causes the differential to send power to the wheel that has traction.

    Is that right?

    I have a question, when say the toyota is in 4 hi, does that cause the center differential to lock? If the center diff is not locked, why is it not recomended to drive on pavement?
  • paisanpaisan Posts: 21,181
    I'm not positive on the Toyota systems. They may not have the 4Hi, 4Lo, AWD, 2wd Options. They may have 4 Hi and 4Lo and 2wd with a button for center locking differential. The active trac also will apply brakes when sliding as well, not just when gassing it.

    The Sequoia to my knowledge does have a dry version of the 4wd system that provides power to both axles normally, whereas the GM systems on the Yukon do not normally provide any power to the front axles when in "auto-4wd"

    -mike
  • nrossinrossi Posts: 47
    Mike,

    Do you know what kind of diffs the Denali has on the rear and front? Also, is the center a Viscous Coupling Locking Center Differential or an LSD?

    Thanks
    Nick
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