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4WD and AWD systems explained



  • Hi Drew,
    I'm cross-shopping the Highlander and the Outback VDC. My primary uses for the vehicle will be:
    1. to drive on beach sand and occasionally through medium-deep snow
    2. enjoy a decent ride before I get to the beach so Part Time AWD is out.
    In the Outback I assume the VDC wouldn't be an issue after reading about Patti of SOA's test-drive in the sand with no problems.
    If I choose the Highlander my best choice would be the V6 with Limited Slip and WITHOUT VSC because of post #477 of 744 Snow and SC by cliffy1 Jan 24, 2001 (10:44 am)in the Highlander Forum, that describes the Highlander stuck in the snow, right?
  • Drew: can you shed any light on the differences/benefits of the 4wd auto system in 2001 Yukons/Suburbans vs. the AWD in the 2001 Denalis? What are the power biases front and rear in the two systems? Can GM's 4wd auto mode be run all the time even in dry conditions? Do both systems have locking rear differentials or limited slip differentials (or are these different terms for the same thing?)

    Other than the lack of a 4wd low mode in the AWD is there any other reason why one system might be considered better than the other? In a towing application would one system be considered preferable to the other?
  • OK, now I'm confused. I'll admit I am pretty ignorant about these things (not a manly thing to admit, I know!). tshaldle provided a reference to a comment by cliffy1 in the HL section in which he believes VSC is not good in the HL 4WD. I have a chance at a HL with FWD with VSC or at a HL with AWD. Which would be better in wet rainy roads in FL?
  • There is a big difference between GM's AWD system and the auto4wd plus true 4WD(hi) and 4WD(lo). Specifically when trying to get unstuck. The Yukon Denali isn't meant to be an off-road vehicle in the true sense of the word, nor is it intended to handle the same towing capacity as its brother vehicles. In reality, the Denali's AWD system might help if you are driving on snowy pavement, but if you get stuck with anywheels either off the ground, or stuck in mud or snow you aren't going anywhere, you'll see the wonderful AWD system spin your wheels, where on a 4WD system when you put your vehicle in 4WD(high or low) all the wheels turn at the same time no matter what. So you'll be able to get unstuck, by rocking the vehicle or just powering out of it. As for better wear on your tires using auto-4WD, sorry this isn't true. Auto-4wd won't engage unless there is a considerable loss of traction while driving, so on turns and during acceleration it won't help. As for gas milage, again the previous statments are wrong. I lose at least 1 mpg when in Auto-4wd vs. 2hi. on my 2001 Yukon. I know because I have very carefully been tracking MPGs. And currently normal driving in 2hi I get 14.2 mpg and in auto-4WD in snowy conditions I get around 12.5-13.2mpg Hope this is helpful. As far a towing, if you notice the 2WD versions of the Tahoe, Yukon, Suburban, and YukonXL all have greater towing capacities than the 4WD counterparts.
  • Drew: This may be a stupid question, but could you explain what low range gearing is? In your excellent post of Feb. 5th you say this is the difference between AWD and 4WD systems. Does it mean that AWD vehicles become 2WD in low gear?

  • paisanpaisan Posts: 21,181
    You are correct in your above post about the AWD on the Denali and Escalade. I wonder why GMC didn't use a beefed up version of the TOD system that is used in the Isuzu Trooper. With this we get the advantages of a true T-case, 4wd Lo, yet we get an initial split of torque of 15%f/85%r which varies up to 50/50 based on weight distribution, wheel spin etc. And we can throw it in 4wd Lo for a locked 50/50 split with a standard LSD in the rear. We also have the fuel saving 2wd mode as well.

  • ateixeiraateixeira Posts: 72,587
    Jim: a simplified example may be best.

    Let's say you have a AWD vehicle with a 4 speed auto tranny. You have 4 forward gears, with both axles receiving power all the time. There is no low range gearing (not to be confused with 1st or 2nd gear, which every transmission has).

    Compare to a traditional 4WD vehicle with a 4 speed auto. It normally operates with 4 forward gears and only one axle getting power. Put it in 4-Hi, and both axles get power. Put it in 4-Lo, and both axles get power, but you use a different set of gears, all much shorter than normal. So there are actually 8 forward gears, if you will, even though you only use 4 at a time (either the Hi or Lo set of gears).

    Why, then? Because the much shorter gears give the engine more leverage, if you will.

    I took my AWD Forester on the sand last fall, and though it got through just fine (contrary to what many predicted), I could have used lower gearing. The soft sand really eats up whatever torque you have, so a low range would have helped me pull out of deep, soft sand.

  • sidsamson: The Denali AWD is rated to tow more (8500lbs) than any 2wd or 4wd version of a 1500 Yukon. The suspensions are the same. Is it merely due to the extra hp?
  • gpm5gpm5 Posts: 785
    So are sidsamson and paisan saying that on the AWD Yukon, the wheel without traction will spin and this system will not transfer power to the wheels with more traction? If the system is designed to transfer when needed I would think it should work offroad, even if a wheel comes off the ground. However, not having 4WD Lo is a problem. Lo gears give more torque out of slippery spots and more control down steep inclines while offroad. On the trooper, 4WD Lo is very useful offroad.
  • hung0820hung0820 Posts: 426
    Thank you Drew for answering my questions. My Santa Fe is a Full-Time AWD with a viscous coupling centre differential. It sounds good to me? But I still don't know all the benefit of of it?

    That's mean my car will only work 4WD at 3&4 Gear and 2WD on 1&2 Gears? How does the viscous coupling centre differential benefit? Why Did my Manufacture sign said 4WD? Also does my AWD is same as the RAV4? Does my AWD any better or worser compare to the Subaru Vehicles? Is there any after market manufacture build the VSC? Do you think I need the VSC on my car? Please reply Drew. Thank you very much for time and cosideration.
  • actually heatwave you are wrong, the 5.3l V8 on the yukon 2wd is rated to pull 8800, and the 5.3L V8 on the 4WD version is also rated to pull 8800 pounds the same as the 1500 series xl. Thanks for playing though. If you looked up your info on this site I can see how you may have gotten this answer wrong seeing that edmunds only show the info for the 4.8L which shows only 6500lbs. towing capacity.

    For the most power the big 8+L in the 2500 series can pull up to 12,500lbs. just in case you were interested, so If towing is an issue the Denali even though it has a more powerful engine, it isn't rated to pull as much!
  • sidsamson: You need to check out the facts with GMC versus Edmunds as I believe the manufacturer's specifications are the ones to best make purchasing decisions on.

    All of the figures below are for the 2001 model year. There is no Yukon with a tow rating of 8,800lbs except a Yukon XL with the 5.3L & 4.10 and the maximum tow capacity for any configuration of a Yukon XL is 12,000lbs, not 12,500. Not to get to anal about these details, but the facts may be important to those with specific towing needs.

    The highest 2wd Yukon tow rating is 7,900lbs with a 5.3L and 3.73 rear. The other 3 configurations for a 2wd Yukon provide the following tow ratings: 6900lbs with 5.3L & 3.42 rear, 6900lbs with 4.8L & 3.73 rear, 5,900lbs with 4.8L & 3.42 rear.

    The highest 4wd Yukon tow rating is 8,700 lbs with a 5.3L & 4.10 rear (4.10 rear not available in 2wd Yukon). The other 3 configurations for a 4wd Yukon provide the following ratings: 7,700lbs with 5.3L & 3.73 rear, 7,700 lbs with 4.8L & 4.10 rear, 6,700 lbs with 4.8L & 3.73 rear.

    The Denali has a tow rating of 8,500lbs with a 6.0L, 3.73 rear and AWD. (This is the only configuration.) Consequently, my original statement remains accurate that the Denali with AWD surpasses the tow capacity of any 2wd Yukon.

    For those that may be interested, here are the tow ratings for the XLs:

    The highest 2wd 1/2 ton Yukon XL tow rating is 8,800 lbs with a 5.3L & 4.10 rear. The other configuration for a 2wd 1/2 ton Yukon XL is 7,800lbs with 5.3L & 3.73 rear.

    The Denali XL has a tow rating of 8,400lbs with a 6.0L, 3.73 rear and AWD. (This is the only configuration.)

    The highest 4wd 1/2 ton Yukon XL tow rating is 8,600 lbs with a 5.3L & a 4.10 rear. The other configuration for a 4wd 1/2 ton Yukon XL is 7,600lbs with 5.3L & 3.73 rear.

    The highest 2wd 3/4 ton Yukon XL tow rating is 12,000lbs with the 8.1L & 4.10 rear (sidsamson's figures on maximum tow capacity for 2500 series with 8.1L were also inaccurate). The other 3 configurations for a 2wd 3/4 ton Yukon XL are 10,900lbs with 8.1L & 3.73 rear, 10,300lbs with the 6.0L & 4.10 rear, 8,300lbs with the 6.0L & 3.73 rear.

    The highest 4wd 3/4 ton Yukon XL tow rating is 12,000lbs with the 8.1L & 4.10. The other 3 configurations for a 4wd 3/4 ton Yukon XL are 10,500lbs with 8.1L & 3.73 rear, 10,000lbs with the 6.0L & 4.10 rear, 8,000lbs with the 6.0L & 3.73 rear.

    I hope this info. sets the record straight and assists those shopping for vehicles of this type with tow capacity as an important criteria.
  • thor8thor8 Posts: 303
    It is a case of two words meaning the same thing, just like a pig and hog, referr to the same animal. Now there are several ways to give four wheel traction or all wheel traction, but that is a technical description. I own an ML430 and a Unimog 406 Mercedes, the Ml has a central diferential that transmit power at a ratio of 48% to the front and 52% to the rear by a system that has some give or elastic, since the intended market is as a daily, luxury, uncomplicated transportation, with open differentials, because the differential pinions can orbit and/ or rotate on their own axis, if one wheel loses traction all the torque will flow to that wheel, it is necesary to stop that wheel, to make the torque flow back to the other wheel with the traction, using the ABS. Another method is limited slip differentials, they have disk or cones clucthes to send the torque more evenly on uneven traction. The reason we have differentials is because when turning a vehicle different radiuses will be created for the wheels on the same axis, the inside wheels turn less than the outside, there has to be some give or compensation, another reason is that tires, even if brand new have diferent diameters, if front and rear traction was 50/50 that small variation in tire diameter will cause shruder and tire wear as they try to mate themselves to the same diameter, affecting the quality of ride as in sligth vibration. The Unimog has on command positive mechanical locks in the differentials and 50/50 front to rear power ratio, but it is to be used on soft ground, where the traction loss or friction will compensate for the tire uneveness and turning radius. If I use this mode in asphalt the vehicle will vibrate, don't want to turn and will scrape the tires so badly that it will leave ugly tire marks all over the road, believe me, we don't want that system in our SUV's. The Unimog is a heavy, slow, strong vehicle to operate off road and an SUV is an onroad vehicle with limited offroad capabilities. Which one is the AWD or the 4WD, both, just different aproaches.
  • tincup47tincup47 Posts: 1,508
    Land Rovers are very capable off-road, that is what they are built for. They are used by the English military and the U.S. Rangers because of their abilities. The Range Rover, Discovery, and Defender have their differences, but all are among the best production off-roaders on the market.
  • What are the disadvantages, if any, of traction control with 4WD? I have also heard some complaints about Skid Control with 4WD- disadvantages of it? Thanks.
  • ateixeiraateixeira Posts: 72,587
    I can think of a few - less fun since you can't wag the tail, the possibility of baking your brakes, or loss of momentum in a race situation.

    Still, in normal driving these just aren't much of a factor.

  • I have to say that you all have done an excellent job of defining and explaining the differences of the various systems. Especially Drew, who seems to be a veritable clearing house of power-transfer information. If any of you are interested, here is a link that gives a great description of the various types of 4WD/AWD systems (in fact, I'm a little surprised that it hasn't been posted yet):

    Be forewarned, it gets fairly technical and the author is not a big fan of SUV's. In fact, at the end of that article, he provides a link for another article which is actually a fairly well written analysis of SUV technology. I would reccomend both of these to anybody who has an interest in the topic.

  • gpm5gpm5 Posts: 785
    Split 90:10 and 50:50 on slippage. That's good. What do you mean by no help in understeer conditions? Surely, it must be better than front-WD alone in the snow. I would think it must be as good as the tribute or the subaru. Yes?
  • gpm5gpm5 Posts: 785
    Any other comments on the T&C AWD would be appreciated particularly in terms of reliability. Not many posts in the Chrysler T&C section.
  • ateixeiraateixeira Posts: 72,587
    A friend of mine is a Chrysler mechanic, and he said the only thing to watch out for was the tranny. Just have it serviced every 30k miles - drain and refill the ATF.

    Automatic Foresters have a 90/10 split much like your T&C, except that in 1st, 2nd, and reverse gears it's actually 50/50. Still, in winding turns it still understeers.

    My manual-equipped Forester is more neutral handling. In the snow I can even wag the tail a bit, and that was true even before I added a fatter sway bar.

    So I'd say the 50/50 AWD definitely feels different - in hard turns, all four wheels tend to drift at about the same point (i.e. less understeer).

  • drew_drew_ Posts: 3,382
    Apologies for the lengthy delay in getting back to you. I've been busy and have been putting it off for a while.

    In any case, I think I can appreciate why you are cross shopping the OB and the HL. WRT your question, what kind of sand will you be driving in (i.e. loose, packed?)? The reason I ask is because the Highlander doesn't have an off switch for its VSC (neither does the OB VDC for that matter, but it's activation threshold is much higher) and hence the kind of sand can make a difference.

    When you say medium deep snow, how deep is that? A few inches?

    If you choose the Highlander, I still think the version with VSC would be better because of the stability control system. The advantages far outweigh the disadvantages. BTW, with the way the Highlander's AWD system is set up, it is possible to get stuck even with the rear LSD (if you don't have VSC).
  • ateixeiraateixeira Posts: 72,587
    FWIW, a Subaru rep friend of mine took a VDC onto sand and did all the could to get it stuck, but it kept going. IIRC it was a volleyball court area, which has plenty of deep, soft sand.

    So it seems the VDC's stability/tracion control is not so intrusive, as Drew mentioned.

  • paisanpaisan Posts: 21,181
    Did they come to a complete stop and start again? I think that is where the traction control might cause problems since if you are moving and continually pushing it's ok, but from a dead stop you might tend to dig in a bit causing TC to take over. Just a thought.

  • ateixeiraateixeira Posts: 72,587
    We actually specifically asked her to try that, and she did. She also did sharp turns, turning while pulling away, backing up, etc.

    I think it's capable of more than any sane owners will ask from an Outback.

    Of course, we're not all sane! %*}

  • Does anyone here know anything about the 'Rotary Blade Coupling' AWD system on the Tribute / Escape, and how it compares to other systems (such as Subaru's or Audi's)? The website and brohures don't say talk much about it (which worries me a little), but it sounds like a new kind of locking differential.

  • gpm5gpm5 Posts: 785
    Are you referring to understeers with respect to a 50:50 split on a different vehicle. I would assume it should not understeer with respect to the front-WD version of the same T&C van but give better overall handling. thanks for the info.
  • gpm5gpm5 Posts: 785
    I had also saw that posted on the T&C site in regards to the regular maintenance. I think that's probably true for all transaxle vans. I mean these things when loaded down are really under stress (I have a '98 Ford Windstar--tranny nightmares from day 1) and are not designed like their former van/truck counterparts that had a rear differential.
  • ateixeiraateixeira Posts: 72,587
    Sorry, but I've never hot-lapped a T&C to be able to tell you how it handles at the limit! Sounds like fun though! ;-)

    I meant among Subarus. The automatics, with a FWD bias, seem to understeer more. The manuals, less so. Mine has a bigger rear sway bar so it's right about neutral.

    The AWD van must distribute weight a bit better, since the drivetrain extends to the rear. With some power also going there, reducing the burden on the front wheels, my guess is it would handle better than the FWD version, but I can't verify that.

    If and when Edmunds Live comes around this year, see if you can make it. We had a hoot last year. There were never any lines for the vans (though we didn't try them), and you were basically allowed to drive as fast as you wanted to.

  • Thanks much for the info.
    I'd like to be able to take the Highlander on the beach. Worst case scenario would be loose, deep sand after a long dry spell.
    As far as snow in the Baltimore area, it will vary tremendously. I guess 6 inches is typical.
  • I know you think the VSC is a great safety feature on asphalt but I'm thinking for what I'm planning to use the HL for, I should probably order it without any extra traction features at all.
    I have the limited slip rear on my Pathfinder but as to whether that's helped on the beach in addition to the locking diff. I can't say.
    Maybe in the future Toyota will include the off switch.
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