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4WD and AWD systems explained

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Comments

  • ateixeiraateixeira Member Posts: 72,587
    No fair, you hit a sore spot! ;-)

    Actually, the Aussie low range in Subies is for the 2.0l engine. It's geared only about 20% lower, and our 2.5l has 20% more torque to begin with.

    I'd settle for a 6 speed manual with a lower 1st and a taller overdrive gear! :-)

    -juice
  • drew_drew_ Member Posts: 3,382
    Heh, the last time I used low range was several months ago, and it was only to make sure that the transfer case motor was stll working :-) My low range multiples torque by 2.68 times (max torque in high range is 233 ft-lbs from 3-4.5K rpms), so it's quite a substantial improvement. The traction control system also reacts 50% quicker in low range. I did wonder how it would perform your trip. I'm not willing to make any pinstripes on my paint though ;-)

    The WRX's STi's 6 speed sounds great!


    Drew
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  • ateixeiraateixeira Member Posts: 72,587
    I wonder if the production ST-X will have one. They've officially declared it will be named the BRAT, and the acronym stood for Bi-Drive Recreational All-Terrain. As in low range.

    If so, they'd have to engineer a low range for the US market, and I bet the Forester would get one too.

    -juice
  • drew_drew_ Member Posts: 3,382
    A few video clips for you from the BMW 3-series comparison drive event that I went to last weekend. Click here


    I unfortunately didn't get the camcorder out in time to record the Acura 3.2TL Type-S (with stability control on) having a very hard time understeering and spinning its drive wheels throughout the entire section.

  • ateixeiraateixeira Member Posts: 72,587
    Was your event the same as the Ultimate Driving Experience? I'm signed up for that on 10/11, with a 330xi.

    What systems does BMW use? Haldex? Torsen? VC?

    -juice
  • drew_drew_ Member Posts: 3,382
    Similar to the UDE I believe, but not identical. We were not assigned to a single vehicle and had the opportunity to test each and every car there for 2 hours, after a short 1/2 hour classroom session. Since the groups were very small (20 per session), we didn't have to wait at all (I took my father along). The slick surface is a large polyurethane (I think) sheet soaked with water. Friction co-efficient was similar to ice or packed snow.

    BMW uses something similar to MB cars, an integrated planatery gearset with a fixed torque ratio (38/62). This setup is durable in that it doesn't use any clutches (which can wear out over time), not very complex, effective, and yet light. Because of the rear wheel bias, you'll find that there is actually very little difference between the handling and performance of the 2WD and the AWD models, except for the fact that the AWD model has 100% more traction. All 3 differentials are open with traction control and stability control managing the flow of power to the appropriate wheels as needed. The rearward bias tends to cause the 330xi's rear wheels to spin momentarily on the slick surface prior to the traction control kicking in. With the Audi's Quattro setup, all four wheels spun simultaneously.
  • peaches5peaches5 Member Posts: 91
    kens: I don't know if it was RWD or not - the car was a month or so old, so I'm pretty sure the tires were tread-worthy ;)

    Drew: You're probably right - they didn't know how to use what they had. They mentioned that coming down the hill, whatever kicked in was flashing lights and making a lot of sounds - 3 women in a Benz - I'm thinking I'll probably never get any more of a technical response from them ;)) Oh, and while I DO have 4WD, I don't have LSD :( I didn't realize the mileage info about the Full Time 4WD. I know I can't take corners or spin on a dime when I have 4WL engaged ;))

    Juice: I'm the only one crazy enough to live at the bottom of my hill :) Most people around here have big ol' trucks - that's probably what I'm going to end up with. I have 4WD on my PF now and I usually use it to get out of the driveway and then up the hill when I leave. It really takes just a second! I can remember in college having to get out and lock the hubs on a boyfriend's old Bronco - just moving the lever seems easy! The other cars you've suggested are OK, but I need something larger. When you live in the sticks, not everyone will deliver, so you're on your own to get it. Ground clearance is an issue because most of the roads are gravel with pretty large dips. In fact, my Dad can't (won't??!!) bring his Corvette down it because even my son's Maxima scrapes at the bottom of the hill. Didn't Subaru have a "Brat" small truck many years ago?

    Yes, I live in an adventure, but I LOVE it!! BTW, I'll probably end up with a 4WD F150 SuperCrew - at least that's where I'm pointed now. Plus, all the redneck boys around here will think I'm cool - for an old lady ;))
  • drew_drew_ Member Posts: 3,382
    LOL, great post. I stand corrected about the LSD. Yes, traction + stability control activation will indeed make ABS-type grinding noises and will flash the traction warning triangle in the middle of the speedometer. Perfectly normal operation and what you have to do is to keep your foot with steady (but not excessive) pressure on the throttle pedal to allow the system to do its work.


    Regarding that F150 pickup truck, you will probably find all that you need in Town Hall's pickup truck message board. However, they are a bit more, uh, rambunctious than in SUVs, so be prepared ;-) Consumer Reports has tested the F-150, so you may want to take a look at their review as well. Keep in mind though, that with a really light rear end, pickup trucks tend to lose traction easily, with the tail of the vehicle possibly coming around quickly.

    The IIHS has crash tested the F150 in its 40 mph front offset crash test. You can see the results here:
    http://www.hwysafety.org/vehicle_ratings/ce/html/0110.htm


    All IIHS tested full-size pickup trucks:
    http://www.hwysafety.org/vehicle_ratings/ce/html/0108.htm

    Just can tell you more about the upcoming Subaru BRAT. Who knows, it may be exactly what you are looking for, if you can hang on to your (relatively new) Pathfinder for a bit longer.

    Good luck,

    Drew
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  • ateixeiraateixeira Member Posts: 72,587
    Ha ha, for a minute I thought I read interplanetary gearset. ;-)

    I was laughing at peaches' post too.

    Guess what? The BRAT is coming back. Did you see the ST-X concept? It'll basically be a crew cab with a small bed, based on the Legacy.

    I'm partial to small cars, because handling is my #1 priority and fuel efficiency isn't far behind. Then again, gas is pricey where I live, still about 1.70 for regular.

    If you lived close to DC, I'd bring my Forester by. It's small, but get an $800 5'x8' utility trailer and you can haul 3 yards of mulch, no problem. The AWD works great, L models start under $20k, and I get 25mpg to boot.

    -juice
  • nextmoonnextmoon Member Posts: 386
    Anyone had any experience with GM's version of AWD?
  • ateixeiraateixeira Member Posts: 72,587
    I just test drove a Rendezvous with VersaTrak. It actually feels just like FWD does.

    I stuck to pavement so I don't think the rear axle ever even engaged, but you can expect understeer, as usual.

    Anyone put it to the test in snow or other loose surfaces?

    -juice
  • tonychrystonychrys Member Posts: 1,310
    It is FWD, it only goes in to AWD when it has to react to something. Nothing ground breaking about it whatsoever.

    From personal experience, once you drive a vehicle where there is torque going to all 4 wheels all the time, you'll never go back. Even the everyday safety handling is superior.
  • ateixeiraateixeira Member Posts: 72,587
    I agree with you. My Subie is 50/50 and I far prefer the way it felt.

    -juice
  • rnursesbarnursesba Member Posts: 4
    Hi All,

    I vaguely remember reading a good piece about the confusion between 4WD and AWD. I'm in the market for an SUV (Yukon or Sequoia) and I'd like to read this article again before going for a test drive.

    Thanks.
  • ateixeiraateixeira Member Posts: 72,587
    Generally, 4WD has a low range for crawling, as well as a locking center differential (usually). If so, then they are part-time and meant only for slippery pavement.

    AWD is full-time, and usually incorporates a center differential so both axles can receive power all the time. Most of the time it lacks a low range.

    There are exceptions to both. Many newer 4WD systems operate full-time, or at least engage automatically.

    Meanwhile, some AWD systems, like the ones Subaru markets in Australia, indeed have a low range.

    I'll let someone more familiar with the specific 4WD systems on the Seqouia and Yukon address those.

    -juice
  • paisanpaisan Member Posts: 21,181
    The Yukon (non-denali) has auto-4wd, gives you 4 settings:
    auto-4wd, 4wd hi, 4wd lo, 2wd

    auto: will engage the front drive after the rear is slipping
    4wd hi: only engageable on non-dry surfaces, 50/50 split of torque
    4wd lo: only engageable on non-dry surfaces, 50/50 split of torque + lower gearing
    2wd: Rear wheel drive only

    Seqouia:
    IIRC, this puts some power to the front axle when in "full-time mode" which is engageable on dry surfaces. Also has low range as well.

    My personal preference for on-road is the full-time systems that put some power to the front axle at all times, this makes it a more pro-active rather than re-active system.

    -mike
  • exzurexzur Member Posts: 166
    My Pontiac Aztek GT AWD is riding on 235/60-17 Goodyear Eagle LS tires. I am considering replacing the rear wheels with the 255-55-17 Bfgoodrich Scorcher/TA tires. The spec for the diameter of both tires is 28.1 inches.

    I heard that mixing tires of different sizes is not good on an AWD setup. In this case, the diameter of both tires are the same. The section width and sidewall length are different.

    Anybody knows why? Is it ok to mix tires of equal diameter but different section width and sidewall length in an all wheel drive vehicle?
  • ateixeiraateixeira Member Posts: 72,587
    There will be a tiny difference, effectively, because a 55 series tire will have a stiffer sidewall and less flex. But not enough to be a big deal.

    Versatrak is primarily FWD anyway, with the rear wheels kicking in only when necessary. So the AWD is engaged only on slippery surfaces, not all the time, and you'll likely not have any problems.

    Still, you will probably increase understeer, because your rear tires will have better traction and the fronts will have effectively less traction. Also, if you're after the colorful burned rubber effect, you won't get it from the rears.

    -juice
  • paisanpaisan Member Posts: 21,181
    On the AZtec you basically have a FWD vehicle and the rear only kicks in after significant wheel spin, so you should be able to put any size on and not hurt the driveline. On other systems that constantly put torque to all wheels (awd) you can't have different size tires on them because it will cause excessive wear and eventually break the 4wd system. With that said, I'd put 4 of the same size, wear etc. on any car that has awd. In fact I'd make sure all 4 of them are bought at the same time. I had 4 tires on my Trooper from the factory, one blew and I put on an equally worn, same size, tread, make, model, brand tire from my older rodeo. There was a nasty whining coming from the awd system. Sure enough the culprit was that the old tire had a 1/4 of an inch difference in height. Generally the rule of thumb is not to be more than a 1/4 of an inch different in circumference on all 4 tires at any given time on an AWD vehicle.

    -mike

    PS: That is actual circumference, not the theoretical one set by the "specs" of the tire.
  • ateixeiraateixeira Member Posts: 72,587
    Good point. There are variances by manufacturer.

    I did a Plus One and found that in theory the new tires should have been only a tiny amount bigger in radius, about 1/4". The actual tires turned out to be bigger than expected, by about 3/4"!

    Part of this was because the new tire is a 60 series, while the old was 70, with much more flex. The rest is manufacturing variance, I guess.

    It was fine because I swapped all 4. Actually, I recommend you do the same. It may look odd to have different wheels front/rear.

    Here is my photo album for that Plus One swap.

    -juice
  • drew_drew_ Member Posts: 3,382
    You can't just put on any tire just because it's FWD biased. Same reason why you can't put on any size tire on the auto tranny Subarus just because they're mostly FWD biased). Most AWD systems rely on speed rotation differences between the wheels to detect slippage, and adding larger tires might confuse it.


    Drew
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  • larry91larry91 Member Posts: 189
    Perhaps this is the right forum to get an answer to this question....

    I have a 2001 Denali XL. On about 4 or 5 occasions in the 7 months I have had the vehicle, I have noticed a quirk that has me concerned.

    When stopped for a light, when I accelerated, I noticed a hesitation and then what felt like an engagement of the drive train and I was on my way. This doesn't happen often, but i am wondering if anyone else has had the same problem and what was wrong, if anything?

    At first i thought the system sensed a slippage and transfered more power to the front wheels. I called my dealer and of course, he had not heard of the problem
    Thanks
  • ateixeiraateixeira Member Posts: 72,587
    Is that how VersaTrak works? Differences in axle rotation? Hadn't thought of that.

    Larry: is the tranny slipping? It's new so I'd have it looked at by the dealer.

    -juice
  • drew_drew_ Member Posts: 3,382
    Sort of...it works much like the CR-V's system actually, using twin gerator pumps to build up the pressure to activate the rear wheels if the fronts are spinning faster than the rears.
  • drew_drew_ Member Posts: 3,382
    Sounds like normal 4WD operation to me! Your truck's AutoTrac system automatically transfers power to the front wheels when rear wheel slippage is detected. As a result (especially uphills from a dead stop) you may feel a slight vibration from the front tires initially grabbing. Check your owner's manual for more information.


    Drew
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  • larry91larry91 Member Posts: 189
    I have the AWD on the Denali, is this also called autotrac, I thought that was the sub's 4 wheel dr system.

    Thanks
  • paisanpaisan Member Posts: 21,181
    Denali has AWD (65/35 split IIRC) Proactive
    Yukon/Sub has autotrac (0/100 until slippage) Reactive

    -mike
  • drew_drew_ Member Posts: 3,382
    Ah, I didn't realise that you had the Denali. The split on the Denali (which has a permanent AWD system) is 35% front, 68% rear. While it is a proactive system, perhaps partly because of the rear wheel torque bias, which isn't a bad thing, it is possible that in certain circumstances that you will feel the torque transfer to the front wheels.

    I hope this helps!
  • paisanpaisan Member Posts: 21,181
    I bet it's a similar lag to the one found on a lot of AT subies. If you mash the pedal on them, the TC and AWD Diffy absorbs a lot of energy and then puts it to the ground. I notice it on my dad's legacy. I'd guess that it's not a malfunction of the AWD or anything like that.

    -mike
  • drew_drew_ Member Posts: 3,382
    Oops, that should be 35% front, 65% rear.
  • ateixeiraateixeira Member Posts: 72,587
    I was gonna ask, can you eek out that extra 3% on my Forester for me? ;-)

    -juice
  • larry91larry91 Member Posts: 189
    Thanks for the input, however I don't accelerate quickly, this is just the slow, normal acceleration, on dry pavement. As i said, it doesn't happen often.
  • drew_drew_ Member Posts: 3,382
    Yeah, it doesn't have to be hard acceleration. As long as the system detects sufficient rear wheel slippage, more power will be shifted to the front wheels. Personally, I wouldn't worry too much about it :-)

    Good luck,

    Drew
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  • larry91larry91 Member Posts: 189
    Thank you, I won't...I have at least 3 years for something to go wrong... LOL :.)
  • drew_drew_ Member Posts: 3,382
    robertsmx "Honda CR-V" Oct 8, 2001 12:33pm

    "Wouldn't that be something similar to a stability control system? I.e. taking power away from a wheel ( a stability system would do so by applying brakes).

    Yes, somewhat similar except for the fact that it doesn't do this in that context because it has no idea when the vehicle is about to slide. As mentioned, it has no yaw sensors. Additionally, unlike a stability control system which can apply the brakes whether or not the driver is touching the brake or throttle pedal, an active differential-based stability assistance system (which is what the Mitsubishi Lancer Evolution has) needs engine torque to work; basically this means that your foot has to be pressing down on the throttle pedal for it to be active. In an emergency manuever, most drivers don't step harder on the throttle pedal.

    As I understand it, as you get faster, the VTM-4 system sends less and less torque to the rear wheels until it is a 100% FWD'er.

    "If Honda had used VSA on MDX (it did offer VSA with RT4WD on CRV in Japan), it would have yaw sensors. But that would be more about stability control than transfer of power to the other set of wheels. During cornering, the wheel speeds aren't the same anyway, and that would be enough to trigger a transfer of power, however small or large, electronically (instead of mechanically, as with a central differential)."

    Since a stability control also incorporates 4 wheel traction control, the traction control feature can be programmed to distribute power side-to-side by applying resistance to selective wheels. It doesn't matter if the wheel speeds of the inside and outside wheels aren't the same. That is what the front and rear differentials are for basically.

    "'I'd like to see where you got the information that lifting off the gas pedal would not 'initiate' rear wheel engagement even upon detecting slippage (loss of traction, not stability). As far as I understand (from what I have read thus far), the on-demand refers to proactive engagement (during acceleration/cornering) and reactive engagement (upon detection of slippage). With the LOCK mode, it doesn't need the driver's foot on the pedal anyway (upto 18 mph, after which it becomes a reactive system)."

    This information can be located in the Acura MDX discussion topic, where it was discussed some time ago. All torque-on-demand systems require torque to work. In order for a useable amount of torque to be sent through the drivetrain, the driver's foot has to be on the throttle pedal. Without this, the system can't redistribute torque since insufficient amounts of it are being produced by the engine at almost idle speed. With the VTM-4 lock activated, the torque split ratio is indeed allocated so that more of it is to the rear wheels. However again, if your foot is not on the throttle pedal, even though the split may be 50/50, the vehicle will not move.

    "BTW, do you remember my post about a trip to Padre islands, and driving the Taurus on sand? I didn't need a lot of 'momentum' to get going, just enough, and it definitely wasn't more than 10-15 mph (on a crowded beach). But I did use the steering to grab as much traction as possible, without having to slow down, and avoiding excessive use of the gas pedal. So I wonder why 18 mph would be 'too slow' to get out of snow/sand banks (the purpose of having LOCK on the MDX)."

    I do remember, but it all depends on the type of sand, etc. All I can tell you is what the owner wrote. In this situation, 18 mph was too slow and whenever it reverted back to FWD mode, the MDX owner got stuck, He couldn't spin all four wheels continuously either (to gain momentum) since the system would start reverting back to 2WD mode.


    Drew
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  • kenskens Member Posts: 5,869
    Drew,

    Lots of great info on AWD systems -- you surely have been doing your homework!

    On the M-Class, how is torque distributed between the front and rear axles? Is there a VC? Just curious.

    Ken
  • drew_drew_ Member Posts: 3,382
    No, there is no viscous coupling in any of the MB vehicles. According to the MB engineers, for durability reasons (off-roading, etc.), the transfer case utilises planetary gears that provide a permanent 48/52% torque split ratio fore and aft. The 4 wheel traction control and stability systems manage the torque to each wheel as needed. The 4-matic sedans and wagons also utilise a similar planetary gear system as the M-class but without the low range gearing, and with a 38/62% split for a more RWD feel.

    Hope this helps!
  • kenskens Member Posts: 5,869
    Drew,

    Got it. The use of planetary gears with a traction and stability system overlay sounds very much like the design Subaru put into the VDC.

    Now, does this set up behave similarly to a Torsen set up with TCS?

    Ken
  • ateixeiraateixeira Member Posts: 72,587
    If the M-B system has a permanent torque split, then no. The Audi could in theory send more than half of the torque to the front axle.

    -juice
  • 7tcn7tcn Member Posts: 4
    I know this question must be answered already somewhere in this board--so I apologize in advance. I have found the explanation of the Pathfinder "auto" plus LSD. I'm trying to determine how the MDX full-time AWD with front wheel drive compares to the Pathfinder LE "Auto" mode with LSD or without the LSD and also whether there is a recommendation based on good weather handling of the front wheel drive versus the rear wheel drive.

    I live in New England and will use the vehicle for moderate dirt road use during hiking seasons (not winter hiking) but mostly for daily commuting including heavy traffic and slick wet roads.

    Can you help?
  • 7tcn7tcn Member Posts: 4
    I've done my homework and read every post on this bb, recent posts on the pathfinder board, and recent posts on the MDX board. I seem to be the only person who is comparing the 2002 Pathfinder LE to the MDX. If anyone is interested, I'd be happy to explain :-)

    I now think I understand the difference between the drive systems. The pathfinder has available full-time 4WD (which includes the low gear option) and an Auto-4WD mode, as well as a full-time 2WD mode. The Auto4WD mode seems like the MDX full-time AWD. The MDX never has full-time 4WD. The MDX is reactive instead of proactive (which is the Pathfinder in fulltime 4WD). Ok.

    So, how do I measure the relevance of these differences? As I stated in my earlier post, I'll be using the SUV primarily for commuting everyday on pavement, but we make frequent visits to the backroads of New Hampshire and most of our summer is spent visiting remote (but not technically off-road) hiking sites. We'd like the ability to access sites located farther off the beaten path, so want an SUV that can handle mud, deep ruts and heavy gravel. We are not likely to be driving on beaches. We will definitely be driving in heavy snow and ice. We'd also like a car that will last over 100,000 miles--we keep our cars as long as possible. Finally, I've just heard a rumour (totally unsubstantiated) that Nissan may be in financial trouble.

    Can anyone offer opinions on how to weigh these factors?

    Thanks in advance!
  • paisanpaisan Member Posts: 21,181
    Does the auto4wd in the PF put any power to the front wheels?

    From your description of what you want to do, I'd stick with the PF over the MDX. The MDX isn't meant for off-road stuff IMHO.

    -mike
  • tincup47tincup47 Member Posts: 1,508
    I wouldn't recommend any of the car or minivan based SUV's for the off-road scenario you describe. The 4WD system is only one factor to look at, chassis stiffness, suspension ruggedness are equally important. The PF, even with unibody chassis, will be superior to the MDX, along with many others.
  • paisanpaisan Member Posts: 21,181
    That was going to be my followup post. The MDX = Oddesy chassis, whereas the PF is uni-body, but specifically built for off-road purposes.

    -mike
  • tincup47tincup47 Member Posts: 1,508
    We finally agree on something ;)
  • paisanpaisan Member Posts: 21,181
    We just look at things in different lights :) I believe we also agree on the WRX moonroof as well (aftermarket moonroof would destroy the integrity of the car)

    -mike
  • 7tcn7tcn Member Posts: 4
    That was extremely helpful. I believe that when the Pathfinder is in Auto mode, there is no power sent to the front wheels until there's slippage in the rear--but I'm sure Drew can answer that question better than I can.

    One followup question: It seems that for the use I describe, I will also want LSD. Do you agree? Some of the posts seem to imply that in some situations LSD is a disadvantage, but I don't think my use falls in those categories.
  • paisanpaisan Member Posts: 21,181
    An LSD is always better IMHO than an open differential. Better yet for off-roading, would be an aftermarket locker, but they are expensive and not really necessary.

    If the PF AWD system is 100% RWD til slippage, then it's basically similar to the MDX system, in reverse. A better system would be one such as the ML series or the Trooper that puts some power to both axles at all times. The TLC also has a system that applies some power at all times to both axles.

    -mike
  • 7tcn7tcn Member Posts: 4
    Thanks again. For various reasons that I won't bore you with, our extensive search has come down to the MDX and Pathfinder. It looks like Pathfinder is the way to go. This board and your help has been great!
  • allroyallroy Member Posts: 58
    I have a question for you all, I am in the military and reside in Florida. I will be moving back north during the winter and have decided to purchase an SUV. But since I am stuck in Florida, 4x4s are tough to get (and I will pay a big premium)! How bad does the rear-wheel drive 4x2 SUVs (Expedition, Tahoe, Yukon, etc.) perform in the snow? Thanks for your help!!
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