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4WD and AWD systems explained



  • arriearrie Posts: 312
    To make it clear.


    4WD has transfer case which has two output shafts. One goes to front axle differential and the other to rear axle differential. When both output shafts are coupled to work it means that front and rear axles are turning at the same speed (if tires are designed to be same diameter as usually is the case) and this means that both front and rear tires should be exactly the same diameter under any driving conditions. Well, front and rear tires practically never are the same diameter and that is the problem. Different tire radius on road side of the axle means tire speed against road surface is different. This makes one of the axles pull the car and the other to slow it down.

    If Honda CR-V has a rotary blade coupling in the drive shaft going to rear axle it means it is a 4WD system with an automatically or manually controlled coupling that engages rear wheels when it is necessary. It is not an AWD system as what AWD is originally meant to be.

  • rshollandrsholland Posts: 19,788
    Well, Honda doesn't refer to their CVR [AWD? / 4WD?] system as AWD. They call it "Real-Time 4WD."

  • steverstever Posts: 52,683

    4WD has transfer case

    What about a hybrid with electric motors at each drive wheel? Now that's real four/all wheel drive, complete with lots of torque at any speed.. And you know they are coming.
  • arriearrie Posts: 312

    separate electric motor on each wheel would be the best, just like it is already done using hydraulic motors in some earth moving heavy machines.

    That system would be a really good solution as electric motors waste very little power compared to mechanical gears that lose 1 - 2 % of input power at each gear mesh. This lost power of course is the reason why transmissions get so hot.

    Problem with electric motors is though that for slow speed motor diameter gets very big if it is connected directly to the wheel hub. This means that to keep electric motor physical size small enough a gear reducer has to be fitted in and the mechanical power loss is back.

    AWD name probably originally came from All time 4WD as differential between axles makes it a true 4WD but as that name was already in use something else had to me made to differentiate between the two.

  • steverstever Posts: 52,683
    This means that to keep electric motor physical size small enough a gear reducer has to be fitted in and the mechanical power loss is back.

    There's always a gotcha. :)
  • ateixeiraateixeira Posts: 72,587
    I think your definitions worked ... 15 years ago.

    Now there are many system that do not meet either of your definitions.

    The RX400H is one.

    There is no center diff, so it's not AWD.

    There is no transfer case, so it's not 4WD.

    What is it, then? ;)

    We can no longer adhere to traditional definitions for AWD and 4WD when they just do not fit modern technology.

    A separate electric motor is used for the rear axle. There is no 2nd driveshaft connecting the rear axle to the front axle, hence no center diff is needed.

    This is what I would call a "through the road" AWD system. The only link from the front axle to the rear axle is the fact that the tires are on the ground.

    That's still AWD, though, so your "no center diff" rule fails.
  • arriearrie Posts: 312
    A real AWD with electric motor running rear axle?

    First of all, I was talking about mechanically drive axles. Secondly, you could think of the electrical power output as a differential between axles, i.e. electric control allows for the speed difference between axles without danger of breaking drive system parts when driving on dry pavement.

    What power rating does the electric motor for the rear axle have?

  • ateixeiraateixeira Posts: 72,587
    That's just it, you have to qualify the definitions.

    I'm gonna guess the rear axle gets about 40hp or the equivalent nowadays, probably less when the original Highlander Hybrid came out. Torque is better and peaks just off idle, though.

    Funny thing is it was based on a FWD Highlander, not the AWD model.

    I saw a cut-away at an auto show and studied it closely, it was just .... odd.
  • kdshapirokdshapiro Posts: 5,751
    I don't think electric motors at every wheel could contribute anything more than neglibile torque. There is a reason 28KV are used to run electric locomotives on the nations commuter railroads.
  • steverstever Posts: 52,683
    So, better just to stick it (or a couple of them) in the drivetrain somewhere?

    I'm thinking a fun application would be to reroute the brake lines so you could brake the right side wheels while goosing the left side wheels. Be a bit of tire scrub but think of the turning circle. :)
  • arriearrie Posts: 312
    Transmission that accelerates one side and decelerates the other already exists.

    The way this works is the power from gearbox comes to a drive gear just like it normally comes to the differential, i.e. a pinion gear turning the big ring gear. Normally ring gear turns differential gear housing that connects right and left side wheels with differential gears allowing speed difference between the wheels.

    In this "forced" speed difference system the ring gear does not turn differential housing. It turns the sun gear of a planetary gear for both sides, i.e. there is a planetary gear for both right and left sides and the sun gear for both is turned at the same speed by the ring gear. Then for both sides there of course are planetary gears and the ring gears of those.

    The speed difference between sides is made by turning the ring gears of the planetary gear systems of each side at different speeds and this different speed actually means same speed but in opposite directions.

    The speed control is done by having the ring gears of the planetary gear systems connected to each other by a pinion gear, i.e. the two ring gears on each side of the pinion gear are in mesh with the pinion gear and turning the pinion gear automatically means that one of the ring gears turns ahead while the other turns backward. This means that on the side where ring gear turns ahead the turning of the planetary gears is accelerated and on the other side where ring gear turns backward planetary gears are slowed down and this generates the speed difference between right and left side wheels.

    Speed difference pinion can be driven by any normally used means but I think it typically is driven by hydraulic motor, which can provide very easy control and high torque.

    This kind of steering system is used in modern advanced military tanks and perhaps in some bull dozers.

    The benefit of this system is that instead of using brakes on each side of the "tank" for steering, which wastes a lot of energy and cause wear and tear in the braking system steering is done by just changing the speed between the right and left side. As one side accelerates the other slows down by the same amount and no energy is wasted at brakes that would turn it to heat that can not be used for anything.

    This steering system is far advanced compared to brake steering and allows the "tank" to literally turn at the spot as one track turns forward while the other turns backward. When on the spot turning happens the main drive gear from main gear box is locked in place and the turning of the vehicle is done by just turning that extra pinion between the left and right side planetary ring gears.

    This might be a bit off of the topic but I though you might be interested of hearing it.

  • steverstever Posts: 52,683
    I once cut a Simca in half with a sabre saw for a guy who said he was going to reroute the brake lines. He never finished anything. This was after tanks, but before skid steers.

    But I can still imagine some appeal for independently driven wheels with the power source at the wheel, and electric would be the way to go for that.

    Want more power? Just swap out your tires.


    Green Car Advisor
  • ateixeiraateixeira Posts: 72,587
    Cool concept but makes me wonder about the unsprung weight. The suspension would be working overtime on that puppy.
  • bielssbielss Posts: 2
    I have a 1989 Cherokee, when I shift it into 4 wheel drive the gears engage (hi & low) but not the front axle. Anyone have a guess???
  • steverstever Posts: 52,683
    "it looks like Toyota has given the Trail Grade 4Runner as much four-wheel-drive capability as anything else sold in the U.S, giving Toyota the widest and deepest range of serious 4x4s (FJ, 4Runner, Land Cruiser) in the industry."

    Is the 2010 4Runner Trail Grade the Best 4x4 Package Sold in the U.S.? (Straightline)

  • ateixeiraateixeira Posts: 72,587
    Not the best, but one of the best.

    I'm impressed that with 270hp on regular octane they still manage a 19mpg EPA combined. With 4WD, no less.

    Looks good. :shades:
  • II am currently reading thru the 1372 comments of this forum & hoped you could point me to or answer my dilemna -

    My previous AWD/4WD experience: 87 Subaru Turbowagon MT, 92 Toyota 4Runner AT, 98 Ford Expediton, 00 Chevy Tahoe, 04 Honda Pilot, 05 Infiniti G35X. In the usual VA, NC, SC, TN driving arena I prefer the additional traction of AWD/4WD systems to handle in order of priority (1) heavy rain/wet roads (2) aggressive dry road driving (3) muddy camp/park roads w/ BSA (4) occasional snow/ice roads (5) occasional over beach trips in the OBX. I need room for 2 adults and 3 growing boys + gear; WITH DECENT MPG as I will use this vehicvle for 30K+ mi/yr business. I realy like the Pilot's AWD system and loved G35X ATESSA AWD with RWD bias. Currently trying to find best bang for buck FWD/AWD system for family "wagon" and am looking at Hyundai Santa Fe ($), Toyota HL ($$), Acura MDX ($$$), amongst others (Subaru Tribeca, but relatively hi $ and low MPG). Cargo area behind front seats matters, but don't necessarily need 3rd row.

    Recent reads on Toy's HL AWD system have me scared of this system and web information is lacking on it. I really like the Borgwarner ITM 3e electromag actuation for responsiveness and decent MPG. What's the difference betw/ 04 Pilot's and 08+ Acura MDX SH-AWD? I think SH-AWD is perfered but its hi $$ and lower MPG is a big trade off. HELP!
  • ateixeiraateixeira Posts: 72,587
    You have some contrasting needs. Aggressive on dry roads, while still ok in mud and on the beach?

    You'll have to compromise somewhere.

    Get a CPO 4WD and a used Miata. :shades:
  • Test drove the 2010 4Runner Limited today (5th generation) w/ X-REAS sport enhancement suspension in the dry, dirty and wet - think I found the answer, albeit not a cheap one, but built like a tank. Sure blows my 2nd gen 92 4Runner away. Thanks for your feedback ;)
  • ateixeiraateixeira Posts: 72,587
    I really liked that new one when it debuted. I will definitely check it out this auto show season, even though I'm not likely to buy a truck.
  • Hi there,

    I was wondering if you had found out the problem with your transfer case / 4WD system. I also have a 96 cherokee with the exact same problem you described in your post. Whining sound and hesitates on the disengage with a "thud."

    Your post is the only thing I have found while spending awhile searching for a possible solution to this problem.

    Were you able to get your Jeep fixed and what was the problem?
  • >> There's no torque vectoring on the rear axle, I think that's pretty clear. If it could Mitsubishi would mention it.

    Mitsubishi says Outlander has Active Front Differential and Active Center Differential, and based on illustrated PDF file I’ve seen, the Active Center Differential is integrated with its rear differential and using electronically controlled coupling to transfer torque to the rear wheels. Outlander AWD system is still as advanced as it gets. Top slalom speed and “ice walk test video” validate that.

    I am not sure about rally/exotic cars, but it appears that no car in $100k price range has both front and rear active differentials, and very few cars have active center differential.

    Both new torque vectoring-capable Audi A4, A5 and Q5 and BMW X6, X5M have only one active differential on rear axle, conventional mechanical center differential, and conventional open front differential:
    * BMW / ZF “Vector Drive"
    * Audi / ZF “Active Sport Differential"

    Since 2009 Mercedes appears to use side-to-side torque transfer based on 4-wheel active braking, which according to online source is limited to 50% of torque transfer.
    Mercedes calls it “Torque Vectoring Brake system"

    Acura’s SH-AWD, and few other manufacturers also utilize single axle active differential.

    In nineties Mitsubishi implemented the Initial version of its torque vectoring S-AWC based on rear axle active differential. In addition to Active Center Differential, Active Rear Differential and Helical Front LSD Differential, Mitsubishi EVO S-AWC uses four-wheel independent Active Braking system on all 4 wheels (not to confuse with a typical ABS/stability control) ; another S-AWC article

  • ateixeiraateixeira Posts: 72,587
    Mitsubishi says Outlander has Active Front Differential and Active Center Differential, and based on illustrated PDF file I’ve seen, the Active Center Differential is integrated with its rear differential and using electronically controlled coupling to transfer torque to the rear wheels

    So ACD works front to rear but not side-to-side on the rear axle, else they'd say it had an Active Rear Differential.

    The ice walk test video is nice but it shows the Outlander can climb smoothly if at least two wheels have traction. Others have shown they can do the same task with just one wheel.

    C&D has an article in this month's issue about Porsche Torque Vectoring, which is on the rear axle.
  • Can someone help me understand the CX-9's supposed fulltime AWD system?

    I have seem a schematic in the service manual which shows the normal front wheel drive etransmisson and a rear differential. I believe it runs normally with a 90-95% front bias and progressively routes up to 50 front / 50 rear during slip conditions. Is this possibly like any of Subaru's systems?

    Some website posts have referenced Borg Warner's ITM 3e as similar, but that doesn't seem to gibe with the service department's schematic I saw.

    I'm looking to purchase for mostly pavement driving very soon - please help!
  • ateixeiraateixeira Posts: 72,587
    A good person to ask would be aviboy97. He works at a Mazda dealership and usually attends dealer training events, so he'd have the inside story as well as some background knowledge.

    I haven't seen him around in a while, however.
  • kdshapirokdshapiro Posts: 5,751
    I've come to the conclusion the type of AWD/4WD drive only really matters in the marketing literature unless the vehicle is faced with extreme racing conditions or extreme trail conditions. For most people slogging through the weather without getting stuck and flipping switches is the most important thing.

    After the recent winter storm and before the snow was way above the bumper, I took the trusty XT out on unplowed roads. XT was as surefooted as mule. What else could one ask for? I didn't care if the front/rear bias was fixed, was 90/10, 80/20, 50/50 or 100/0. I didn't care if it had an active center differential or not, full time/part time, side to side, one wheel, two wheels etc. All I know it worked.

    The small percentage of people who scrutinize the drive train and make a decision on only the drive train before buying a car, have to be in the minority.
  • ateixeiraateixeira Posts: 72,587
    Well said.

    My kid went sledding with a little buddy and his mom got stuck on their way home in an Odyssey. Steep, long hill.

    The next day she asked if I'd pick him up to take them sledding. No problems. Even with uneven surfaces it just went straight up, no hesitation.

    Experiences like mine (and yours) are why the Forester wins in owner loyalty for the category. I'm on my 2nd. :shades:
  • who on here can explain ( viscus couplers) in the transfer cases of the 1999 jeep grand Cherokee Laredo 4x4 and can you drive them with the front drive-shaft removed without damaging the transfer-case?
  • Have a question about the 4X4 system on my 2010 Grand Cherokee Limited 5.7L

    To be exactly it is a 2010 Grand Cherokee Limited Standard plus Customer Preferred Package 25H. This package includes 5.7L Hemi, Quadra-Trac II 4X4 system and several other things.

    - The window sticker says the 4X4 system on this vehical is Quadra-Trac II (which is part of the Customer Preferred Package 25H)

    - However the badge on the center console (between the gear shifter and the cup holders) reads Quadra-Drive II

    Quadra-Trac II and Quadra-Drive II are different 4X4 systems. I contacted the dealer as well as Jeep support but none of them can tell if the 4X4 on my vehical is indeed Quadra-Trac II or Quadra-Drive II.

    Is there an easy way to figure out whether it's Quadra-Trac II or Quadra-Drive II?

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