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4WD and AWD systems explained



  • ateixeiraateixeira Posts: 72,587
    Subaru's VTD/VDC (Variable Torque Distribution with Vehicle Dynamics Control) is very impressive because it really meets every criteria you could possibly want from an AWD system:

    * it's full-time
    * it's pro-active
    * it can send 100% of torque to either axle
    * the front axle is managed by the VDC
    * the rear axle is managed by the VDC

    The only thing missing is a low range, or the ability to increase the bias side-to-side proactively like Acura's VTM-4.

    VTM-4 can't do the 100% part, so it's not perfect either.

    IMHO these two are about as close as you get to ideal AWD.

    If you think about it, the RAV4 can only meet 2 of those 5 criteria. Same with the Highlander.

    4WD have different criteria, of course. paisan could probably tell you more about that.
  • agreed that Subaru's AWD system is very nice, but as you said, apparently the side-side bias is fixed to whatever the limited slip diffs decide to do.

    I do not know if the limited slip diffs in Subarus are torsen or (I assume) clutched bevel/ring design.
  • ateixeiraateixeira Posts: 72,587
    The only model that gets a Torsen diff is the Legacy spec.B, on the rear axle.

    The others that have a rear limited-slip use a viscous coupling. Simple but effective.
  • Is the Subie's _front_ differential limited slip as well?
    Via viscous coupling?
  • naatz1naatz1 Posts: 188
    The Subaru has a good AWD system however their vehicles (Forester, Legacy, and now new 08 Tribeca) are just too small for a 6ft3 driver with 6ft+ backseat passengers. I have seriously considered each in the last 7 years of new car buying and they always lose.

    The bad press and a few other features caused us to drop the RAV4 and Highlander for country MN winter driving considerations and boat towing.

    I think the new roomy Saturn Outlook(Acadia/Enclave) has a fairly "tough" AWD system w/4500lbs of towing capacity but there's no way to lock it in < 25 mph. Even the Hyundai Veracruz had that, however like most imports only 3500 lbs towing max. So you have a Grand Cherokee with solid (but heavy) 4WD/AWD and a true low range, 6500 lbs towing with the "small" 4.7L torquey V8 and semi-crummy MPG. A go through anything AWD, just pay for it in cost of ownership.

    One of the above may fit everyones requirements, it depends ....
  • paisanpaisan Posts: 21,181
    No, all the subarus have an open front diffy, except the STi which has a viscous front diffy.

    On VDC models you get an ABLS on the front and rear where it will lock a faster moving wheel to create a limited slip. I have this on my Armada and it works well in all but full-out track conditions where you can overheat the brake fluid (saw this on an E500 AMG a few years back)

  • Sounds like the archilles heel of Subies (save STI) would be if both rears and one front wheel had no traction, but other front wheel did. Not likely but in winter, anything is possible :surprise:

    That is, unless the Subie has VDC, which would lock that spinning wheel and you'd be good to go.
  • ateixeiraateixeira Posts: 72,587
    The Outlook is big, I'd call it full-sized.

    We test drove one, but ended up buying a Sienna minivan. Just to give you an idea of the size class it's in.

    The Forester is a good 3 sizes smaller. Sportier and more fun to drive, but a lot smaller, naturally. I'd say a RAV4 is 2 sizes smaller, and the new Highlander is still one size class down from the Outlook.

    It all depends on what you want.
  • paisanpaisan Posts: 21,181
    Sounds like the archilles heel of Subies (save STI) would be if both rears and one front wheel had no traction, but other front wheel did. Not likely but in winter, anything is possible

    That is, unless the Subie has VDC, which would lock that spinning wheel and you'd be good to go.

    The system is still better than all the other AWD systems out there though No other AWD system out there has an LSD front axle w/o traction control.

    The advantage of the Subaru system is that because it has power going to both axles at all times you are less likely to get into a situation where you are stuck/spun/etc. than if you are driving one of the systems with a reactionary AWD system.

  • I think you mean Acura's SuperHandling-AWD. VTM-4 can't bias side to side like SH-AWD. With VSA,VTM-4 could brake one wheel slipping to give the other more traction and acting as a LSD.

    I'm waiting for the Super Terrific Awesome Handling All Wheel Drive that can send all torque to one wheel. :D
  • ateixeiraateixeira Posts: 72,587
    I stand corrected, got the names mixed up.


    I'd like STAHAWD too. :D
  • Agreed that active power management sounds better (if more expensive) than braking wheels with no traction.

    Seems like on the Impreza and newer AWD cars, the designers are relying on braking to keep the spinning wheels under control.

    So if the Subaru anti-brakelock system can manage 4 wheels independently, and, in the possibly ridiculous situation where 3 wheels are on ice/slippery stuff and 1 wheel has traction, it brakes the 3 spinning wheels to send power to the 1 wheel with grip, it sounds worthwhile.
  • paisanpaisan Posts: 21,181
    Yup, that's why the VDC is a great on-road system. In a track situation you would eventually overheat the brake fluid after doing lap after lap pushing the car to the edge (saw this done on an E55 AMG at Pocono) but for on-road it's pretty much the best, most cost effective way to get front and rear LSDs.

  • Lets see if I have this right, given the changes to Impreza/Forester lines:

    '08 Impreza/ '09 Forester 4-speed Auto lines do not have center diffs, nor LSD's.
    F/R balance is 80/20; Clutch packs control amount of balance. Balance can be adjusted by varying amount of clutch engagement (handled by Vehicle Dynamic Control, I assume). To avoid wheel spin due to traction loss, they rely on braking spinning wheel (also from VDC) and/or cutting engine power.

    '08 Outbacks w/ 5speed Auto have center diff, full time LSD in rear. Balance is 45/55; unknown system (center diff??) controls balance. Wheel spin/traction loss is handled by LSD in rear, VDC for front, and VDC in rear if LSD can't do job by itself.

    Mistakes? Comments?
  • paisanpaisan Posts: 21,181
    More or less. The 09 Foresters can also shift the power to 20/80 or anywhere in between. I used this same system on a 96 Impreza Racecar that I drove for a season. It had the 4-speed auto w/80/20 initial split and it ran wonderfully. So don't count that system out just yet.

    I also have an 05 Legacy GT w/ 5MT 50/50 split w/rear LSD. If I had a choice of rear LSD or VDC all around, I'd take the VDC any and every day over the rear LSD.

    Motorsports and Modifications Host
  • xwesxxwesx Fairbanks, AlaskaPosts: 10,858
    Yeah, the LSD certainly does not do the same job as VDC. Compared to my '96 Outback, which had open differentials on both ends, the LSD makes the rear end of the car far more responsive to throttle inputs, but it is not going to help you recover from or manage a slide any better unless you give it the right input at the right time. VDC has the advantage of independent wheel braking and doing it without driver input. That second part is a major consideration, especially if the driver is in a panic... :surprise:
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • since the Forester and Impreza use clutch packs to redistribute torque...

    Are these wet clutches?
    I can't imagine dry clutches holding up to constant slippage required for such use.
  • paisanpaisan Posts: 21,181
    They are wet electromagnetic IIRC clutches internal to the AT. I have similar ones on my Trooper w/AWD and my Armada with AWD.

    Motorsports and Modifications Host
  • steverstever Posts: 52,572
    Which is better - the '09 Forester AWD system or the Mitsu Outlander FWD/AWD/4WD system?

    And why - in 500 words or less, LOL.
  • kdshapirokdshapiro Posts: 5,751
    10 words or less. Actually a description of how they operate would be interesting. Similiarities and differences. Grammar, punctuation, sentence structure count. The eyes of the world are watching. :shades
  • dodo2dodo2 Posts: 496
    I think that would broaden the appeal, just don't put hokey switches into the thing like the Outlander. Subaru can build an efficient AWD system that's kicks the competitors butts without having the driver look out the window to determine if it is raining.

    Leave it in 4WD Auto and the Outlander behaves like the Forester. Sure, each system may have its specifics, but I think it is really hard to determine from your armchair which one really works better under given conditions. If you believe that the Forester performs better, it's your opinion and it's perfectly fine. You just don't have any data to back that up other than your personal beliefs. Have you seen any rigorous 4WD system tests between the 2009 Forester and the 2008 Outlander?

    Now here is the difference with the Outlander:
    1. If you want to "tell" the car "I don't need 4WD today because it's dry and sunny and I want to save 1 mpg", you could switch to 2WD mode.
    2. If you want to "tell" the car "Hey, I want you to send more power (50% more) to the rear because I'm driving through some deep snow today", you flip the switch to "4WD Lock".
    If you see don't see any value in having a little bit of control over the system, fine, but some do.

    Steve: I promise this is my last post on the topic. I guess all the information on the Outlander&#146;s 4WD system is there for those really interested to learn about it.
  • kdshapirokdshapiro Posts: 5,751
    Mike, the host states the 09 Foresters can shift 20/80 to 80/20 if I'm reading the post correctly. If that's correct the Outlander can't touch it.

    link title
  • dodo2dodo2 Posts: 496
    Yeah, but Joe is saying that the 2009 Forester can only go 55/45. If that's correct, the Forester can't touch the Outlander (40/60). ;)

    Seriously, does anyone have any Subaru OFFICIAL description of how the AWD system works in the 2009 Forester? I mention the car since not all Subaru cars, from all generations have identical AWD systems.
  • paisanpaisan Posts: 21,181
    The Forester can do 80/20 to 20/80 depending on the situation. This is for all 4EAT transmissions in the Subarus that do not have VTD. These also have a 80/20 torque split as the "default", older 4EAT transmissions (Pre 98ish) have a 90/10 split as "default" and can go up to 20/80 if need be.

    On the 5EAT and 4EAT w/VTD (These include but may not be limited to: 02-07 WRX, 05-09 Legacy GTs, 05-09 Outbacks the 5EAT models, Tribecca) With these the default torque split is 45/55 with a slightly rear bias. They can however shift power from 80/20 to 20/80 if the need arises.

    These figures were actually conveyed to me from an engineer at Subaru, so I'm gonna trust they are quite accurite. On the Outlander, I'd imagine the "lock" is only good to a certain MPH, like the Pilot where the "lock" unlocks at 19mph. But I'm not familar with the Outlander's AWD system so can't comment on torque splits for it etc.

  • paisanpaisan Posts: 21,181
    Seriously, does anyone have any Subaru OFFICIAL description of how the AWD system works in the 2009 Forester? I mention the car since not all Subaru cars, from all generations have identical AWD systems.

    Yup it's a 4EAT system with 80/20 torque split as a default and can shift up to 20/80 if need be. It has open front and rear differentials and has VDC w/ABLS. The ABLS will apply light braking to a wheel spinning faster than the other wheel on the same axle, thus forcing the power to the wheel not spinning. This is on both front and rear. This system is slightly slower than a tranditional rear LSD, however it will have a longer longevity (traditional LSDs wear out in under 100k miles, or become less effective). The advantage over a traditional LSD is that it acts on both front and rear axles, whereas a tradtional LSD only operates on the rear axle. In a track/road racing situation the ABLS may cause the brake fluid to overheat (I saw this happen on a MB E55 AMG at Pocono that MB was testing a few years back) but in regular driving situations the ABLS is better than a single rear LSD. The VDC system will apply a brake if the yaw sensors detect that you are about to spin and will also cut the fuel for a second if it detects a spin or other out of control manuver.

    Hope this helps.

  • rcpaxrcpax Posts: 580
    OFFICIAL links?
  • kdshapirokdshapiro Posts: 5,751
    Why don't ya find 'em and post 'em instead of asking for 'em?
  • paisanpaisan Posts: 21,181
    You probably won't find any links, most companies don't specify the details of their systems. Like I said, I had a sitdown with a Subaru Engineer and got that info out of em.

  • chelentanochelentano Posts: 634
    So it appears we have no official torque split numbers from Subaru about the &#147;Active AWD&#148; system which equips 4-speed auto Subaru: obviously they have nothing impressive to tell.

    Subaru utilizes mainly 3 types of AWD systems:
    1. Continues AWD (5-speed manual cars)
    2. Active AWD (4-speed auto: 4EAT)
    3. VTD AWD (VRX and H6).

    Today for marketing purposes Subaru has combined all of them under the &#147;Symmetrical AWD&#148; to avoid the inconvenient torque split questions. does say that &#147;Continuous All-Wheel Drive: Models equipped with 5-speed manual transmission utilize a viscous-type locking center differential and limited-slip rear differential with torque distribution normally configured at a 50/50-split front-to-rear.&#148; does say that &#147;Variable Torque Distribution (VTD) All-Wheel Drive: Models equipped with 5-speed automatic transmission utilize an electronically controlled variable transfer clutch in conjunction with a planetary-type center differential, and a viscous-type limited-slip rear differential. Torque distribution is normally configured at a performance-oriented rear-wheel-biased 45/55 split front-to-rear.&#148;

    So publishes official numbers about the Continues AWD, and about the VTD AWD system, but again still there is no official numbers available for the 4-speed auto Subaru cars equipped with &#147;Active AWD&#148;. The Active AWD/4-speed auto combination was utilized first time at least in 1992 on Subaru SVX and it has not changed that much since.

    If Subaru is shy to disclose the Active AWD split numbers, let&#146;s use these independent 3-d party publications:

    1. The automatic transmission used on AWD equipped vehicles would normally send 90% of the engines torque to the front wheels and 10% to the rear wheels… When the front wheels began to experience a loss of grip, the transmission automatically sent available torque to the rear wheels, up to 50-50 split:

    2. Active AWD: 90/10 split under normal condition to 50/50 in extreme conditions: NY Times

    I have more links available, but enough said. Based on the independent info we currently have, the 4-speed Subaru are nearly FWD cars at 90/10 default split and they can do on-demand up to 50/50 under extreme road conditions, which is a typical setup in many cars to deliver decent gas mileage. The "20/80-80/20" theory has no any evidence. The Outlander's new AWD system cannot be touched here by Subaru 4EAT cars: Outlander&#146;s 4WD Lock mode defaults to 50/50, while still delivering a decent V6 gas mileage. The 4WD Auto economy mode available on the Outlander as well, providing greater versatility.

    My neighbor is not a Subaru engineer, but he said that he is NASA rocket scientist and that no real Apollo mission to the Moon was accomplished. People say things…
  • dodo2dodo2 Posts: 496
    On the Outlander, I'd imagine the "lock" is only good to a certain MPH, like the Pilot where the "lock" unlocks at 19mph. But I'm not familar with the Outlander's AWD system so can't comment on torque splits for it etc.

    No. On the Outlander, the 4WD Lock remains engaged regardless of the speed. However, at higher speeds, with no traction problems, the power sent to the rear decreases but it's always more (by 50%) compared to the 4WD Auto. At the end of the day, it's a fully automated system with a little more control for the driver.
    This is one of the differences compared to some other 4WD systems with a "lock" position which disengages above 20-25 mph. You could also switch between the settings (2WD, 4WD Auto and 4WD Lock) at any speed.
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