This looks like a good 4WD vs. AWD explanation.

Discussion in 'OT Driven' started by deusexaethera, Jul 29, 2008.

  1. deusexaethera

    deusexaethera OT Supporter

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    One of my coworkers just bought a Honda Element (his choice, not mine), and he started off telling me about its "RealTime 4WD" but eventually ended up asking me about it. To be honest, I don't know much about the various flavors of *WD, so I did the responsible thing and did a little research. I found this on www.rubicon-trail.com:

    http://www.rubicon-trail.com/4WD101/difference_4WD_awd.html

    Is this more/less a good explanation? (Text quoted below.)

    - - -

    What is the difference between Full time Four Wheel Drive (4WD) All Wheel Drive (AWD) and automatic All Wheel Drive (auto AWD)?



    Full time 4WD, also called permanent 4WD, (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times and can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.

    2WD is not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low". Full time 4WD vehicles work very well on-road and are very capable off-road.

    When "4-low" is selected the wheels create substantially more torque (on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).

    Important: "4-low" does not create more traction - it creates more torque and that can be detrimental when traction is marginal. Slipping tires are more likely in "low" than in "high"!

    The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed.

    All wheel drive (AWD) is almost the same thing as full time 4WD - it is a system that powers all four wheels of a vehicle at all times as well. Full time symmetric AWD would be the best term to be used. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than full time 4WD vehicles, but work perfectly well on-road.

    Automatic AWD system is the newest kid on the block. PR agency generated names like "Real Time 4WD", "intelligent AWD" or "active AWD" are hiding the fact that automatic AWD is essentially a sophisticated 2WD system. Automatic asymmetric AWD would be the best term for them.

    Here is how they work: Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes up to 50% of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first and then the other axle will be added and try to keep the car moving and stable. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD. So, for a moment you had AWD.

    Automatic asymmetric AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. However, automatic asymmetrical AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.

    Examples: Honda CRV, (newer) Toyota RAV4, LandRover Freelander, Isuzu Trooper (TOD), Volvo V70, 1999 and later Jeep Grand Cherokee (in high range).

    Recently some magazines have called the automatic AWD system "part time 4WD", since it offers AWD only part of the time. They have a point - however, the term "part time 4WD" has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD . A manual system where the driver had to select 2WD or 4WD. The name coming from the fact that 4WD was designed to be used only part of the time (when off-road), most of the time it had to operated in 2WD (on-road).

    A frequent consumer complaint about some older automatic AWD is, that all 4 tires need to be replaced even if only 1 or 2 are bad.

    [​IMG] Confusing but true: some vehicles have a combination of part time and full time 4WD systems, or even a combination of 2WD, automatic asymmetric AWD and part time 4WD (low range). Impossible to give those custom mixes a name..
    more...
     
  2. borazhasleftthebuilding

    borazhasleftthebuilding Lets Party OT Supporter

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    cant recall tires slipping evar in 4lo
     
  3. deusexaethera

    deusexaethera OT Supporter

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    It could happen, though; if you stomp the gas in 4hi and 4lo, you'll have a lot more torque to break the tires free in 4lo. At the same time though, the wheels would speed up proportionally slower, so unless you're on a sloppy surface the vehicle might accelerate fast enough to keep up with the wheels.
     
  4. mtnbikekid08

    mtnbikekid08 Aime-moi moins, mais aime-moi longtemps

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    that's if u suck at driving
     
  5. Oman4x4

    Oman4x4 OT Supporter

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    Come for a drive in the dunes :big grin:
     
  6. Oman4x4

    Oman4x4 OT Supporter

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    And the original explanation on drive systems is pretty clear...


    Another thing to always keep in mind is... how many wheels need to lose traction before you're stuck - and for that you need to look at the differentials.


    In an AWD car or Full Time SUV you have a Center Differential. This means it can drive on paved roads.

    If you have an open Center Diff and open front / rear diffs, then only one wheel needs to lose traction causing it to spin, getting you stuck. All power gets sent to the spinning wheel.
    1 wheel slips = you stuck.

    Most vehicles with that configuration also have pretty limited suspension travel so when the tire comes off the ground, you're done.



    If you have a part time drive system (like a Wrangler), or a Center Diff Lock Option, then you need to lose traction on one front wheel and one rear wheel in order to get stuck. The Center Diff Lock (or lack of a center Differential in the case of the Wrangler) creates a 50-50 split of power between the front and rear axles. It's the open differentials in each axle that let all power go to the two spinning wheels.
    2 wheels slip = you stuck

    This can happen when crossing a ditch at an angle, so you might have the front left and rear right tires in the ditch at full extension on the suspension, spinning on the loose ground because little weight is on them.



    If you then add a rear differential locker then you need to lose traction on BOTH rear wheels AND one of the front wheels in order to get stuck.

    The Locked center diff is splitting the power 50-50 between front and back, and the rear diff lock again splits that rear power 50-50 between the left and right.

    3 wheels slip = you stuck



    Finally - if you have both front and rear lockers like a Jeep Wrangler Rubicon, or an ARB equipped vehicle then you would need to lose traction on ALL FOUR wheels in order to be stuck. The 3 locked diffs are going to keep all 4 tires spinning at exactly the same speed no matter what.

    4 wheels slip = you stuck.



    As you can see it's not just a case of which axles get the power, but what the differentials do with it when they get it.
     
    Last edited: Aug 5, 2008
  7. popsnbeer

    popsnbeer New Member

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    Any more trip pics of you and your jeep in dubai? I remember those like a year ago and there were a bunch of pics.
     
  8. deusexaethera

    deusexaethera OT Supporter

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    Yeah, that's why (despite my various beefs with VW/Audi engineers right now) I really admire Audi's Quattro system. With a couple simple mods, you have a system with Torsen diffs all around (front, rear, and center) and a computer system that applies the brakes to individual wheels when they slip, to keep torque going to the wheels with traction. Much more slick than Subaru's setup, which uses clutch packs that inevitably need servicing.
     
  9. Oman4x4

    Oman4x4 OT Supporter

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    Haven't posted anything recently... I need to get proper OT access back again so I can post up epic threads in forums with traffic :big grin:

    Anyway there's been a lot of modifications after the accident, getting the jeep up on 35" tires with a bodykit to provide the clearance... ARB back bumper... AMP side-steps... a few other toys here and there... the snorkel...

    Trying my hand at video editing - you can check it here - http://www.youtube.com/watch?v=USl6nCPIbg0

    And some recent pics:


    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]

    [​IMG] [​IMG]


    All the mods got it ready just in time for our annual desert challenge... literally 1 am on the day of the challenge.

    Challenge is approximately 40 km in the desert... have to reach about 10 waypoints in the allotted time... take photo proof of being there... shortest distance taken wins.

    [​IMG] [​IMG]
    [​IMG] [​IMG]


    And the results:

    [​IMG] [​IMG]

    :big grin:

    Anyway - winter is approaching and I now have a Safari Snorkel so expect more epicness.
     
  10. Oman4x4

    Oman4x4 OT Supporter

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    Audi's are fucking brilliant machines... I saw one guy hop a pavement in Dubai to beat the traffic and it wasn't a small ledge at all... he had one tire about 10" off the ground and immediately the traction control system locked that tire until it came back down to earth again... no spinning at all.

    If you were to cut the hell out of the fenders, put 18" rims and big mud tires on a Q7... it would be incredible.

    Might wanna do something about the whole fragile plastics thing though :big grin:


    Next time I'm at the Audi dealership I'll take the camera... they have a bunch of old A4 type vehicles in Oman that have been lifted, with wide arch fenders, bash plates, bull bars, spare tire on the back... Looks like a hell of a lot of fun!
     
  11. popsnbeer

    popsnbeer New Member

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    Nice. The Jeep looks a lot better than the last time I saw your pics. That desert event looks pretty sweet!

    I'll watch the video tonight at home.
     
  12. GammaRadiation

    GammaRadiation Active Member

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    Cliffs:

    If you aren't crawling over things 2wd with a locker will do you just fine most of the time.
     
  13. borazhasleftthebuilding

    borazhasleftthebuilding Lets Party OT Supporter

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    waht difference does it make weather its spinning or not?
     
  14. deusexaethera

    deusexaethera OT Supporter

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    Quattro uses torsen differentials to account for a certain amount of slip, but IIRC torsen differentials still require that both wheels have some traction to speak of -- Quattro makes up for that by automagically applying the brakes on wheels that are spinning freely, so that the torsen differential has some resistance to push against to transfer power to the wheels with traction.

    A Subaru, on the other hand, would disengage the clutch to the wheel that's spinning, and clamp down on the clutch on the opposing wheel so all of the torque on that axle can go to the wheel with traction. It works, but the long-term problem with this is the clutches wear out and stop working right, whereas with Audi's Quattro system the only thing that wears out is the brakes, and those get changed out every few years anyway.
     
  15. Oman4x4

    Oman4x4 OT Supporter

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    If a tire spins, all your power is being wasted because a differential will transfer all the power to the path of least resistance, which is the wheel up in the air.

    These traction control systems detect wheel slippage through the ABS sensors and then apply the brakes to that wheel forcing power to be transmitted to the other ones that are on the ground.

    I'm guessing cars with the automatic ride levelling feature will also take into account the suspension compression to know that one wheel is at full-droop, so it knows there is no weight on that tire too.

    If you don't have the ride levelling, you probably won't have sensors on the suspension to know what it's doing so they purely rely on wheel speed information.
     
  16. deusexaethera

    deusexaethera OT Supporter

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    My guess is the ride-level sensors are used more for telling which wheels can support more torque (more compression = more traction), not just for telling if wheels are totally up in the air. That's just a guess, though, I have no idea if they really do it.
     
  17. Oman4x4

    Oman4x4 OT Supporter

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    Same Same but different...
     
  18. dan7532

    dan7532 New Member

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    That is SOOO COOOOOLLL!!!! You are living out my fantasy.
     
  19. BlkDrgnZ28

    BlkDrgnZ28 OT Supporter

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    damn sounds prettys sick you need to post some
     

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