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4WD and AWD systems explained

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  • I am looking at a ford explorer (1998) with traction control and four wheel drive or a Mercury Mountoneer(2000) with all wheel drive which is better.
  • drew_drew_ Posts: 3,382
    Hmm, I don't recall the RWD Explorer being available with traction control. In any case, your vehicle will have more traction with AWD, so I would go for that.
  • drew_drew_ Posts: 3,382
    FYI, the Acura MDX has full-time AWD, not permament AWD. The reason is that it is usually a FWD (2WD) vehicle unless you're accelerating (but only up to a certain speed), if slippage is detected, or if the VTM-4 lock button is pressed (up to 18mph, only in 1st, 2nd or Reverse gears). The MDX's AWD system is not always proactive, but rather, reactive.

    Traction control only makes use of whatever traction is available at the front wheels. If those two wheels simply do not have enough grip to pull the vehicle, you will still go nowhere. With AWD on the other hand, you will have an extra set of 2 drive wheels, and hence twice the amount of available traction. Permanent AWD/4WD (ex. Audi Quattro, MB 4-matic) is still the best, but full-time AWD is better than FWD with TRACS. Which vehicles are you looking at specifically?

    Hope this helps!

    Drew
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • wmquanwmquan Posts: 1,817
    FYI, the Acura MDX has full-time AWD, not permament AWD. The reason is that it is usually a FWD (2WD) vehicle unless you're accelerating (but only up to a certain speed), or if slippage is detected. The MDX's AWD system is not always proactive, but rather, reactive.

    Besides engaging the rear wheels during acceleration or when slippage is detected, it will also engage the rear wheels if the manual VTM-4 switch is pushed by the driver. This is only for low speeds and it will progressively disengage as the vehicle approaches 18mph. Thanks.
  • drew_drew_ Posts: 3,382
    Thanks for the addendum, William. I had already added the "VTM-4 button" to my post above. Guess you must have caught it during my editing. The VTM-4 lock is one of the more unique features of the MDX's AWD which sets it apart from many other AWD systems on the market; though the Mazda Tribute/Ford Escape has something somewhat similar with their RBC AWD system, but usable at all speeds on slippery pavement.
  • wmquanwmquan Posts: 1,817
    Drew, since this is a good board for this type of question, I've been told by (surprise!) Audi fans that their Quattro system is "better" than the permanent AWD system in the new BMW 3-series, and the 4Matic in MB's.

    Does Quattro have a number of "enhancements" beyond 4Matic and the BMW system? I know that BMW's split is not as optimal as you'd like, but I'm not sure if Quattro is really "better" than 4Matic (other than the fact that you can buy it in an under $30k vehicle in the form of the current A4 -- in fact, are there many otherPermanent/Full 4WD or Permanent AWD systems under $30k?).

    And any other vehicles out there that have the Audi Allroad's pneumatic suspension system for under $60k?
  • wmquanwmquan Posts: 1,817
    And where does VW's 4Motion system fit into as far as these classifications go?

    Thanks.
  • Host: Is GM's awd in the 2001 GMC Denali's and 2002 Cadillac Escalade's considered permanent or full-time? Their literature says permanent but I am curious how you would categorize based on your definitions in this forum. Also if you do categorize them as permanent, what other SUVs are available in the US with permanent awd?
  • drew_drew_ Posts: 3,382
    The Quattro system (but only with the Torsen - Torque Sensing - diffs) is superior...kind of, sort of, to the BMW/MB systems. The reason for this is that it is almost purely mechanical, and can react almost instantaneously to even a small amount of tire slip.

    As you know, I think, Audi's Quattro is split 50/50. Audi has also started using using traction control, but only to distribute the torque side to side. They used to have 3 Torsen diffies before (front/centre/rear) and that was complex and pricey...which brings us to the next point. The problem with the Torsen based Quattro system is its high cost, as well as fairly high weight. As such, Audi has started using a Haldex clutch based AWD system for their TT and the A3 (not sold in N.America) partly because of this. This system is actually more reactive than proactive, but it's very quick and hence that makes up for some of the differences between it and the Torsen system.

    The Passat's 4-motion system uses the A4's Torsen based Quattro system since it is basically a lengthened, but de-contented version of the A4. It shares all of the benefits, but also the disadvantages (the acceleration times are not very quick).

    Subaru's manual transmission AWD vehicles and the new H6 VDC Outback wagon have permanent AWD (50/50 split and 45/55 split for the VDC OB). Subaru's auto tranny vehicles (excluding the VDC OB) have full-time AWD systems which are partly proactive like the MDX's system. Power is transfered to the rear upon acceleration, as well as in 1st, 2nd, and reverse gears. The Toyota Highlander and the RAV4 both also have a 50/50 split in no-slip conditions. I suspect that the new Toyota Matrix hybrid small SUV will have the same system.

    FWIW, a torque split ratio of about 50/50 produces very neutral handling, but some consider it boring. This is why BMW and MB (for the cars) have reverted to a RWD biased AWD torque split ratio. To emulate some of the RWD feel of their RWD vehicles. Frankly though, I really don't think that MB 4-matic owners care too much about that 2WD feel. I certainly don't.

    As for the suspension system, no, not that I can think of in N.America anyway. The Lexus LX470 and Range Rover both have pneumatic air suspension systems (R.R has had it for decades), as does the '00+ MB S-class.


    Drew
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • Drew/William: thanks for the clarification (I went and re-read post #2 - duh!).
    I was looking at MDX vs. Highlander FWD with traction control or Highlander with AWD (I understand the Highlander AWD is permanent unlike MDX). No one has the Highlander AWD with traction. This is one of the factors I am considering between these two. I likely won't be driving in snow (unless I take it north for the holidays) but we have lots of hard rain with slick roads in FL.
  • drew_drew_ Posts: 3,382
    We get a lot of rain in Vancouver as well and if you choose the Highlander, I would recommend that you get the AWD/VSC (which is stability & traction control) version of the HL. With the wet roads, the FWD with TRACS may cut (especially when you're starting out and in intersections) and that can be annoying. I know because just a couple of years ago I was also occasionally driving a '99 Toyota Camry XLE V6 with TRACS. Being a long time RWD/AWD/4WD driver, I was constantly frustrated by the lack of traction in many rainy/snowy situations. It was enough for us to get rid of the car after 1 year of ownership and 6600kms.


    Drew
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • wmquanwmquan Posts: 1,817
    Thanks for the great explanation about Quattro. I do remember a long time ago you mentioning that it was excellent but expensive. Wow, three Torsen differentials in past vehicles. I'm also more impressed with the 4-motion now too, I had always (and wrongly) assumed it was a cheaper implementation.

    As far as mechanical reliability, which is sometimes a function of complexity, is today's Quattro any more difficult / expensive / problem-prone than other systems?

    And is it issues (cost?) in integrating automatic transmissions that causes Subaru to not have permanent AWD in their automatic tranny vehicles, except for the H6 VDC?
  • Thanks for your response. Unfortunately, we haven't been able to find an AWD with the VSC option. Have found FWD with VSC or AWD without it.
  • drew_drew_ Posts: 3,382
    Your dealer should be able to order the vehicle to your specifications. They don't like to do this, but it is definitely possible. Have you considered purchasing out of state? Read through the Toyota Highlander topic, and you will find customer recommedations for dealerships.

    To see a major reason why VSC is beneficial, watch this video clip. FYI, skids usually occur before rollovers, so if you can prevent a skid from occuring and going out of control, the probability of rolling over will be significantly reduced as well.


    Drew
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • Nice video. Thanks. I am surprised MDX doesn't offer VSC.
  • drew_drew_ Posts: 3,382
    Yup, 3 torsens. Overkill for a street vehicle, don't you think? :-) Audi prides itself on having state-of-the-art AWD technology though, so in the past, just about anything went. Did I mention that you could even lock the front, middle and rear differentials via dashboard switches in some of the older Audis (1980's era)? Wow!

    MB's and BMW's AWD systems are actually relatively simple compared to Audi's system (hence the significant cost and weight savings). As you know, much of the work is electronically controlled. I would think that they would be cheaper to maintain in the long run. Then again, Audi has been in the AWD business for many years as well.

    VW did initially want a different (called Syncro) AWD system for the Passat. However, it wouldn't be very cost effective to design a whole new system when the sister company has a (proven) system right there and then. FYI, Skoda and Seat (also VW owned, I think) use the Torsen & Haldex Quattro systems as well.

    I'm currently watching a segment on the restyled VW Passat and I have to admit that it looks pretty good. Those new HID Xenon headlamps are impressive, and no doubt borrowed from the Audi division. Like just about all factory Xenon headlamps, lighting performance is improved by 250%. Now, if only the standard warranty was longer than 2 years...the price is also pretty close to a basic ML320. Wow...the new C230K coupe is also being covered in this MotorVision segment.


    Drew
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • drew_drew_ Posts: 3,382
    Acura may choose to add their version of ESP/VSC, called VSA - Vehicle Stability Assist, in future MDX's. The MDX's wider track (vs other mid-size SUVs) and VTM-4 AWD does somewhat make up for the lack of a stability control system. The AWD cannot assist in a skid the same way that stability control can though, simply because it does not have any sensors/computer controls that tell it that the vehicle is not going the direction that the driver is pointing the steering wheel.

    As I mentioned, perhaps Acura will decide to add VSA in future versions of the MDX.
  • wmquanwmquan Posts: 1,817
    What type of AWD/4WD system do the Chrysler minivans have?

    By the way, you said with authority that the next-gen M-class won't arrive until 2004 (2005 model year). Any news on when the MB 4Matic minivans will be on the scene?
  • drew_drew_ Posts: 3,382
    The Chrysler minivans have a full-time AWD system with a viscous coupling limited slip centre differential. Power is split 90/10 in no-slip conditions, and up to 50/50 when slippage is detected. It actually works quite well, but is absolutely no help in understeer conditions. Understeer is not much fun at all in the snow since you can't steer the vehicle with your right foot. :-p

    The next generation V-class (if they decide to carry over the name) based on the W164 next generation M-class platform is supposed to arrive - based on what I've read (unconfirmed officially with MB, of course)- when baby Justin is about 10 years old...in other words 2009.
  • gpm5gpm5 Posts: 785
    "in "Auto" mode, it is active at all times and monitoring for slippage. It's also part-time because it doesn't have a centre differential and hence cannot be used continuously on dry pavement (hence the reason why the auto mode only sends power to the opposite axle when slippage occurs)."

    What does the above mean Drew? can it or can it NOT be used on dry conditions where slippage could occur, i.e. dry with occasional ice spots for example?

    How reactive is the auto mode. Is it reactive under acceleration or only from a stop. Any help here would be appreciated. Thanks.
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