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4WD and AWD systems explained

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Comments

  • bblahabblaha Member Posts: 329
    Opinions will vary quite a bit, but I think most would agree that for on the pavement, an LSD works better. Marketing is probably the reason they each choose differently.

    Aftermarket products are widely available, so I don't think is particularly important which is offered. If you don't want what the manufacturer is offering, go with an aftermarket one.
  • hengheng Member Posts: 411
    Anything cheaper out there? I'll be even willing to get out and lock the hubs.
  • bblahabblaha Member Posts: 329
    What vehicle?
  • hengheng Member Posts: 411
    bblaha got something in mind?
  • bblahabblaha Member Posts: 329
    You might consider a Powertrax Lock-Right locker, should cost around $300. They also make a no-slip thats around $400 to $450. The locker should fit both the front and rear differentials.


    http://www.powertrax.com/locker.htm

  • hengheng Member Posts: 411
    The Powertrax look very interesting considering the air compressor alone for the ARB would go for about $300.
  • jimbo48jimbo48 Member Posts: 1
    A sales manager of a local GMC dealership was explaining the 4-wheel drive systems of the 2002 Envoy and Bravada to me yesterday. He claimed that neither system will ever have all four wheels
    driven at the same time regardless of the conditions. He said that with the Envoy, if the Autotrac is called on or if the 4Hi or 4Lo is engaged, you will have the left front and right rear wheels driven only, UNLESS you have a locking rear differential which would engage both rear wheels, giving you 3-wheel drive at best. Furthermore, he added, the SmartTrak is the same except that "there are no buttons".

    Can anyone please clarify this issue? Is this guy correct? If so, I am very confused because what he said contradicts the info in the Bravada brochure that says: "But if conditions require it, all four wheels are driven to make the most out of available traction."
  • paisanpaisan Member Posts: 21,181
    The Bravada IIRC, has no low range, which means that there is a center coupling that distributes power 50% front 50% rear axle. Once that 50% reaches either the front or rear axle, 100% of that power goes to either of the wheels, but not both at the same time, it will then switch that 100% between left and right wheel on an axle. The 50/50 split in the center coupling can vary but always tries to maintain a 50/50 split.

    On the Envoy, the Transfer case, will send 100% to the rear axle, and 100% to either left or right rear wheel, until it senses that the front axle is spinning slower than the rear, then it will send power at a 50/50 split to the front axle, where 100% of it either goes to the front right or the front left. If you have a rear locker, you can lock both rear wheels in, which would send 50% power to the left and 50% to the right. The front 50% would still be sending 100% to the left or right, but not both at the same time.

    Anyone feel free to correct me, I could be off base.

    -mike
  • tincup47tincup47 Member Posts: 1,508
    In an open diff as long as both wheels on an axle have equal traction, power is split 50-50. As one wheel loses traction, power goes to that wheel. A locking diff, posi-traction or ATC all work to prevent this from happening. The one thing to remember is that this isn't like a light switch, it is proportional to available traction. My Mustang has an open diff, and I don't lose all forward motion if one side breaks loose in the rain or snow (or if I'm playing around ;) )
  • paisanpaisan Member Posts: 21,181
    Ok, I knew I was probably off somewhere in there. The open diff though puts more power/all power to the wheel with least resistence (slipping one) while an LSD tries to do the opposite, and a locker locks in the power to both?

    -mike
  • tincup47tincup47 Member Posts: 1,508
    An LSD limits the traction loss to the spinning wheel, A locker does lock the rear, doing away with any differential action.
  • drew_drew_ Member Posts: 3,382
    The sales manager is wrong. Basically both SmartTrac and AutoTrac are the same, that is, both provide power on demand as needed (i.e. when slippage is detected). In normal situations, the vehicles are in RWD mode, which unfortunately means that they are not able to proactively stabilise and improve handling. When slippage is detected power is transfered to the front wheels. All four wheels will have power. The locking rear differential only locks in low speed conditions, when one of the rear wheels is on a more slippery surface than the opposite wheel. By mechanically locking the rear differential, power doesn't get leaked out to the wheel on the slippery surface. The differential unlocks when traction is restored to both rear wheels.

    The only difference is that SmartTrac comes standard equipment with a locking rear differential for better side to side traction at low speeds for the rear wheels, and AutoTrac comes with low range gearing (4WD Low) whereas SmartTrac is essentially in 4WD Hi at all times.


    Drew
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  • ateixeiraateixeira Member Posts: 72,587
    The sales manager should have said that "at least" one wheel on each axle will get power when 4WD is engaged. If traction is good, all four will.

    -juice
  • longdaylongday Member Posts: 8
    Drew,
    The Pathfinder LE/Infinity QX4 can be had with "Auto" All mode drive and an optional limited slip rear differential. Nissan says it is from the Skyline GT-R race car system. Without input this can put the truck into 50-50 Front to Rear, and LSD can divide the rear wheels. Am I correct in understanding that under poor road conditions at least one of three tires should have grip?
    How would you rate this system compared to the others out there? In particular to winter road conditions?
  • drew_drew_ Member Posts: 3,382
    Yes, the QX4/Pathfinder's system is adapted from the Japan-only Skyline sports car. I'm not sure what you mean by "one of three tires have grip". Would you please care to elaborate?

    The system works by detecting wheel slippage, and then automatically (when in "Auto" mode) transfering power to the front wheels until traction is restored. It then reverts back to RWD. The rear limited slip differential will help to prevent power from leaking out to one rear wheel on a looser surface, but like all limited slip differentials, only to a certain extent. The nice thing about the system is that you can lock the centre differential if needed; that is, in really really poor conditions.

    For winter conditions, I would probably rate this system as good in comparison to other systems out there, but not excellent. The simple reason being that it is mostly reactive and requires the rear wheels to slip before the front wheels activate. Personally, I prefer permanent 4WD or AWD systems which allocate power prior to slippage. This way, when you are cornering, all four wheels are already powered and have traction and you don't need to lose traction to regain it.

    That said though, the Nissan's system is similar to the Ford Explorer's Control Trac, but far more refined, advanced, and faster reacting. Power transfers are seemless, unlike the Ford's which can be felt by very noticable bucking and jolting. The computer can also send some power to the front axle to reduce front end bobbling when braking in the snow, for example.

    If you have any questions, feel free to ask. I hope this helps :-)

    Drew
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  • paisanpaisan Member Posts: 21,181
    You need to order the moonroof on the PF in order to even have the option of getting the LSD. I personally like the full-time, or systems that put some power to the front like Drew mentioned better than the reactive ones. Look at the ML series from MB, or the Trooper for a "next generation" system. Also a lot of LSDs (I know I need to do this on my trooper sometimes) need to be "pre-loaded" by pulling up on the E-brake a few notches in order to get the LSD to activate.

    -mike
  • longdaylongday Member Posts: 8
    I understand from both of you that fulltime 4WD is better than the reactive system, and the pathfinder LE (with sunroof)/QX4 has that ability.
    The three wheel theory comes from my perception from reading past posts. When the truck is put into 4WD, power is only really going to one front and one rear wheel. Add LSD to the rear and power is shared to some extent by both rear wheels. So in my understanding, either of the rear and only one of the front has the ability to grip when the "Auto" kicks in the 4WD.
    I just bought a Pathy LE 2 weeks ago and by what you guys are saying, I'll leave it in 4WD when the snow comes.
    Thanks for your quick responses!
    -steve
  • larry91larry91 Member Posts: 189
    I own a 2001 Denali XL. I have been under the impression that the AWD system is the "Premier" four wheel drive system to own, and is all around the best.
    When I bought the Denali, I was also looking at the Suburban 2500. U see i wanted the 6.0L engine.
    From reading all these postings, it seems, from a "Better" standpoint I made a mistake and should have opted for the 2500 with autoride. I live in Illinois 7 months a year, 4 to 6 weeks of that time will be in the snow and sleet, then it's down South for the winter where the drive system one has is not too important.My last vehicle was a 99
    tahoe 4x4.

    My question is, Did I make a mistake in going for the Denali? ? ? ?
    Thanks
  • drew_drew_ Member Posts: 3,382
    I'm curious, why do you say that from a "better standpoint" you made a mistake and should gone for the Suburban's AutoTrac auto 4WD system? I actually have the exact opposite impression for the environmental conditions that you mentioned.


    Drew
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  • larry91larry91 Member Posts: 189
    Better in the respect that the autotrac is a More Positive system. That what i am getting from many of the postings, that when the going gets tuff, autotrac is the system to have. Also, I am surprised to hear the center differential is not a locking unit, but a viscous coupling. Would u consider this a GM "Shortcut"? I also believe I read that the Land Cruiser has AWD and a 4 low setting..
    Do u like the Denali's AWD System?
  • paisanpaisan Member Posts: 21,181
    For the situation you describe larry, the Denali's viscous coupling is a better choice. It gives aprox 50/50 torque distribution to the front and rear axles, which is what you want for higher speed control, such as snow, ice, sleet, etc. Whereas the 2500 would do better off-road. The auto-track system only engages the front after slippage has occurred which in some cases is too late to prevent a skid. I'd personally have opted for the TLC or the ML or the Trooper all of which have full-time awd, and low range, kinda the best of both worlds, but no 6.0l engine in them.

    -mike
  • drew_drew_ Member Posts: 3,382
    There you have it :-) Paisan's post pretty much says it all. Pro-active with the Denali XL vs. reactive with the AutoTrac system.
  • drew_drew_ Member Posts: 3,382
    The Denali and Escalade's AWD system uses a 38/62% split :-)


    Drew
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  • paisanpaisan Member Posts: 21,181
    Note to self, 38/62% split on the GM viscous coupling.

    -mike

    PS: is that the same split as the Safari? (I think it's a similar unit that is used)
  • drew_drew_ Member Posts: 3,382
    No, completely different. The Safari/Astro's unit is a significantly older design which hasn't proven itself to be reliable, from what I have read from Consumer Reports and from Town Hall van owners. The Safari's system is also 0/100% until slippage.
  • paisanpaisan Member Posts: 21,181
    Thanks, always trying to build the knowledge base.
    Towed my 2500lb boat today on the newly re-build boat trailer. The Trooper did great, even with t paisan-sized adults in it plus all our gear, and about 700lbs of gear in the boat.

    -mike
  • larry91larry91 Member Posts: 189
    for your input. It is always nice to know, when one spends that amount of money on a vehicle, they have made a good choice. Just to recap, please bear with me, I am not technical when it comes to drive systems: The AWD is better for general, on road driving conditions because it is " ALWAYS" engaged in the 38/62 confiquration, and reacts more quickly to road conditions, but that can change when necessary. The autotrac is better for "OFf" road driving, or in areas with poor road conditions, and very deep snow. Is this correct?
    Thanks
  • paisanpaisan Member Posts: 21,181
    Larry,
    Generally that's the idea. If there is some power going to the front axle, it's more pro-active than a reactive system such as in Fords and GM auto-trac vehicles in on-road, higher speed situations. With my TOD engaged on my Trooper which puts 15% power to the front as a std. with up to 50/50 split, I was able to drive up a 45% angle 1.5-2' deep snow covered dirt driveway about 200' long. I didn't need to engage 4wd lo, and I've off-roaded a # of times w/o having to use 4wd lo either.

    -mike
  • drew_drew_ Member Posts: 3,382
    I think the only reason why AutoTrac would be (a lot) better off-roading is because it has a low range. It probably shouldn't make any difference in deep snow or with poor road conditions compared to your vehicle unless you decide to use low range.

    Don't worry so much and enjoy the drive :-)

    Drew
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  • larry91larry91 Member Posts: 189
    for your input, I really appreciate the feedback. I have had 4x4's since 1990, and all of them had a 4 low, which I have never had the need to use anyway..Thanks again guys..
  • paisanpaisan Member Posts: 21,181
    You have some much torque with the 6.0, that you probably don't need to worry about the low range, especially if you don't do a lot of off-roading.

    -mike
  • larry91larry91 Member Posts: 189
    I do no offroading, here in the Chicago area there are no places to legally off-road.

    I had read an earlier post, don't have the patience to try to find it again, which had some negative comments about the GM AWD... I just wanted to get some other opinions. This vehicle only gets about 4 or 5 weeks of potential snowy conditions before we migrate to south Florida for the winter. Hopefully, it won't start snowing there anytime soon..ha, ha

    Is the 6.0 really considered that " Great"? I would have LOVED to have gotten it with the 8.1L, perhaps the drivetrain is not up to the torque produced by that engine...
    larry
  • paisanpaisan Member Posts: 21,181
    The only difference between the denali and the caddy are the exhaust manifolds IIRC. I think the downside to the GM AWD that was reffered to was the lack of Lo-Range. I wouldn't worry at all if you are only around for a month of potentially snowy weather. From what I hear though chicago roads are like NYC roads, in that they *are* an off-road trail!

    -mike
  • larry91larry91 Member Posts: 189
    Well, let me try this post again, the site quit on me and i couldn't copy and paste in this site???

    So the 4 low was the problem for some readers, I have had 3 vehicles with the 4 low option and Never had the need to use it, or even try it out. I am not concerned.

    Yes, u r correct, Illinois had the second worst roads in the country, right behind our neighbor, Iowa. Chicago is the "pothole" capital of the world. The state should give drivers here an allowance for damage to their vehicles. I always have squeeks and rattles in the winter(on my Tahoe) and lo and behold, when I got to Florida, they all went away.

    I fiqured the difference in horsepower on the Escalade was the intake manifold, an easy modification to garner a little more advertisable horsepower. Oh, the games they play with or on us?
    larry
  • ateixeiraateixeira Member Posts: 72,587
    Despite the lack of a low range or lockable 4WD, I once again managed to have a nice time on the sand just south of Salvo on the Outer Banks. I'll admit it was a little intimidating to climb over the 4x4 access ramp to get to the beach, but airing down to 18psi and keeping my momentum meant I got through it all without a hitch.

    Mine's a manual tranny Forester with 225/60R16 tires, a tad wider than stock. The VC splits power 50/50 to begin with, but the suspension did a good job keeping all tires in contact and the system never seemed to have to interfere.

    I posted some photos of my drive here.

    Ironically the only one stuck was a new Chevy Crew Cab 4x4, but he'd forgotten to air down so the tires dug in. Once we adjusted the pressure, he drove right off.

    -juice

    PS The surf fishing was great! Too bad the shark attach in Avon kept us out of the water, though.
  • peaches5peaches5 Member Posts: 91
    Everytime I read a subject in Edmunds I get more confused ;) Here's the situation: I have a 2000 Nissan PF with part time 4WD. Yes, I'm one of those soccer moms that actually uses the 4WD SUV! I live at the bottom of a VERY steep hill and most of the roads are gravel. My road (the hill) is "rough paved" - the cement is poured with large amounts of gravel so the surface isn't smooth so the traction is better. Unfortunately, gravel from the main road gets on it and therefore it can be tricky getting up and down. Some car's have drivers that think too much of them, and they get stuck and have to be towed - most recently a 4cyl minivan. Right now, in the summer, I usually have to put it in 4WH in order to get out of my driveway and up the hill to keep the wheels from slipping all over. Granted, I can go do it w/o the 4WD and slip and slide, but it's there, why not use it ;) We don't get TONS of bad weather up here, but the roads were covered in ice 7 days in a row last year - this was before I moved up here. Probably around 12-14 icy days a year on average. If possible, I don't WANT to leave the house in snow/ice, but if I do, it's my understanding that I will probably only be able to get up and down by putting the car into 4WL. To confuse the issue, I really want a new SUV or truck. Should I be looking ONLY at part time 4WD vehicles? From reading the articles here and the links, it SEEMS to me that if I get an AWD, I'm losing the ability to use 4WL which might just keep me here longer than I really want to be ;) Thanks in advance for any insight!
  • larry91larry91 Member Posts: 189
    I believe the reference to four wheel low refers to getting out of "Deep Stuff" and I mean deep. I don't believe the low range would be of any advantage to you, provided u had a full time all wheel drive with two differentials, when trying to negotiate ICE. If I am wrong, I am sure someone will correct me, gratefully so... The proper winter tires might be more important.
  • paisanpaisan Member Posts: 21,181
    Full-time 4wd capable, and Lo range. The Troopers can be had for $22-$26K nicely equipped.

    -mike
  • drew_drew_ Member Posts: 3,382
    My personal opinion is that what you might want to consider is an SUV with permanent 4WD or full-time 4WD (that is, a system that has the capability to send power to all four wheels at all times) and stability control. That way, you will have less of a chance of the vehicle's tailend sliding out or the front end pushing at all in snow and especially ice. Low range is probably not necessary since it can provide too much torque causing the wheels to break loose and spin. However, it can be a benefit if you have to plow through deep and thick snow.

    Two SUVs that come to mind immediately are the Toyota Sequoia and the MB M-class, both with low range. BMW X5 can be had with permanent AWD (so no low range) All three of these vehicles have stability control + traction control.

    Good luck,

    Drew
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  • paisanpaisan Member Posts: 21,181
    Forgot the more expensive Sequioa/TLC/M-Class also good choices in the upper price range.

    -mike
  • peaches5peaches5 Member Posts: 91
    Snow will be minimal. I'm in GA, and even WHEN we get snow, the very most I've seen (excluding the 1993 blizzard) has been less than a foot - and a foot is VERY, VERY rare. It's usually closer to 6 inches or less. The problem is the ice. Since we never really stay below freezing, whatever snow IS on the ground melts, then freezes again overnight. I guess I was looking at the low for keeping the traction going up AND down the hill. There are 4-6 inch drop offs once you leave the cement (unless I can talk them into filling them in with gravel between now and then), so once you're off, you're in "I need a tow" land.

    Here's what scares me: recently, my brother, with his FW2WD Infinity just zoomed up the roard in the rain. Not a problem. Next to go up was my SIL in a new MB sedan with all the fancy smancy traction control (not AWD though, that I know of). The MB was having a heck of a time. You could hear the wheels slipping and sliding and not able to gain traction. They did get up the hill, but I'd imagine that after a few months of doing that daily, you'd become the tire salesman's best friend ;)

    My Pathy has never slipped or slid - the only time I have problems is on gravel, and as soon as I put it into 4WL, it's fine. There may still be a slip or two, but nothing like it is when I'm in 2WD.

    To be honest, I really don't want to spend $40K plus for a Seq/MB/TLC (though I'd really LOVE the TLC..). Also, since I do a lot of highway driving (once I get off the mountian ;)), wouldn't the permanent 4WD kill me in mileage? I'm thinking that for what I want, a full sized 4WD on demand truck would offer me the best option at a lower price?
  • kenskens Member Posts: 5,869
    Out of curiosity, was the MB sedan RWD? That, plus tires not up to the job can render any fancy traction control system useless.

    Ken
  • ateixeiraateixeira Member Posts: 72,587
    In my humble opinion, a lot of the vehicle mentioned are overkill for your needs. I bet any AWD vehicle would work, even an Outback or Highlander.

    The big SUVs are heavy and wide, and weight + size work against you in climbing that steep gravel driveway. They can get very pricey and mileage will probably be in the teens.

    Another thing is that many full-time 4WD setups are still reactive. They act like front wheel drive (cars) or rear wheel drive (trucks) and then send power to the other axle after you start slipping, at least in most cases.

    There are full time setups that send power to both axles to begin with, so you are less likely to even begin slipping. You have lots of choices: Toyota RAV4 and Highlander, Subaru Forester, Legacy, and Outback, Volvo XC, even the Hyundai Santa Fe.

    In up to 12" of snow, my Forester is fine, even though I have touring tires not meant for the white stuff, and two open differentials (S models have a limited slip diff for even more traction). That plus I average over 25mpg.

    There are lots of options out there, have fun shopping. Ask the salesman if they're willing to let you take it home to try your hill. Or look at what friends are driving and see who has the least amount of trouble negotiating that hill.

    IMO it would be a pain to stop at the bottom of that hill every day to manually engage 4WD.

    -juice
  • drew_drew_ Member Posts: 3,382
    As Ken said, if the tires are the regular high performance all-season tires that MB uses, then traction control can only make use of whatever traction is available from those tires. Also, many people don't know how to use traction control; in the MB's case, they should've turned off the torque reduction feature so that the traction control is still active but engine power is not reduced so as to maintain your momentum uphill.


    Since permanent 4WD is running all of the time (and designed to do so), there should be no difference in fuel consumption. The four powered wheels are there to help you handling-wise should you have to take evasive actions.


    Since ice is a big factor, stability control will really help you to keep the vehicle under control within the limits of your tires. You may find this video clip interesting:

    http://www.theautochannel.com/media/netshow/misc/esp-consumer-video.asx


    For more on stability control, have a look at this article:

    http://www.edmunds.com/ownership/safety/articles/45992/article.html

    You may also want to consider dedicated snow/ice tires for even better handling.
  • drew_drew_ Member Posts: 3,382
    Peaches already has a Nissan Pathfinder with the part-time 4WD system + rear LSD ;-)


    Drew
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  • drew_drew_ Member Posts: 3,382
    demonstrating a vehicle with and without stability control on a simulated icy surface. Vehicles used in this case were Subaru Legacy AWD wagons:


    http://www.imagine-xt.com/snt/VDC1s.mov


    http://www.imagine-xt.com/snt/VDC2s.mov


    http://www.imagine-xt.com/snt/VDC3s.mov


    http://www.imagine-xt.com/snt/VDC4s.mov

  • ateixeiraateixeira Member Posts: 72,587
    Edmunds partial list left out those Subaru VDC models. Contrary to media reports, they don't cost $33 grand. Street prices are closer to $29-30k.

    I would even add the Passat 4Motion to my list, since ground clearance is not a big issue with the little snow you get.

    Heck, I think the Ford Focus offers stability control as an option now. Doesn't the Jetta, also?

    Still, I'd get something that was using all four wheels to climb up that slippery hill.

    -juice
  • drew_drew_ Member Posts: 3,382
    I think the Focus' stability control system is only ava ilable next year. The Passat 4-motion, while nice and has the superb Quattro system, doesn't have stability control or traction control.


    Drew
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  • ateixeiraateixeira Member Posts: 72,587
    That's right, they save that for the Audis.

    I got to drive on some unimproved roads this Sat, mostly loose sand, and the Forester did great. The ground was really loose and I could feel it drifting around corners. Loads of fun. Never left me wanting for a low range nor did it lack traction at any point.

    An interesting lesson was in vehicle width. The trail has low branches scraping the sides of the wider SUVs, even a bit on my Forester. The Impreza RS and Outback Sport in our group were actually better in this regard, because their tidy size allowed them to negotiate the narrow trails without suffering any whacks from errant tree brances.

    -juice
  • drew_drew_ Member Posts: 3,382
    Even on the Audis, ESP is optional equipment (standard on the higher end models). As a result, the vehicle can be a handful on snowy/icy surfaces. Mostly fun for those of us who know how to control the vehicle, but most drivers are not trained to handle oversteer.

    Yes, 4WD Low is overrated in some cases, which is probably why Subaru doesn't see a need for it over here ;-)
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