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Unlocking the mystery of AWD

Discussion in '80-Series Tech' started by LandyLover, Aug 15, 2005.

  1. LandyLover

    LandyLover

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    Ok, I’m a bit confused. I’m sure the cruiser’s AWD principles are archaic to today’s technology, which is probably the source of my confusion. But isn’t AWD supposed to be better on snowy roads than with CDL on because power goes to the wheels that grip and not slip? In any case, with respect to the CDL and high range, I would think you’d get better traction with CDL off because there is no drivetrain windup. In snowy conditions doing 40 MPH on the highway I would think it would be safer to negotiate corners etc in AWD because wheels are free to turn at road speeds (because there is no windup, and therefore no loss of traction in snow). Taking corners (I mean sharp corners) can be freaky even at low speeds with CDL on.

    Secondly, can I assume that our viscous couplers aren’t smart (like newer AWD vehicles)? I’ve read that we should lock the center diff if your front driveshaft is disconnected otherwise you wont be going anywhere (i.e. power goes to the wheels that “slip”), which suggests to me that power is not delivered to wheels with traction.

    Thirdly, I assume that AWD power is not divided evenly front to back because 80 series cruisers have the 8” or whatever front diff while the back is 9.5” or whatever.

    So to recapitulate the questions…

    1) Is there any real benefit to having CDL on in snowy driving conditions or is it in fact a detriment?
    2) Does power go to the wheels that slip or the wheels that grip? What if you’ve got crappy grip on the fronts (ice) and good grip on the backs (no ice), will most of the power still go to the fronts?
    3) What is the power distribution in AWD going down the road with all 4 wheels getting equal traction?

    I’d like to know these answers because I do a lot of highway driving in winter where the roads are ALWAYS icy and snowy.


    Thanks in advance…
    :cheers:
     
  2. skyshark186

    skyshark186

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    Do yourself a favor. Find a dirt road. Take a "normal" 4wd....say a friends pickup. In 2 hi hit a corner nice and easy and throttle hard through the corner. Tell me where your nose generally points and how many times you spin the wheel back and forth.

    Then. Hop in your cruiser. Hit the same corner, in the same fashion without the CDL locked. keep a mental note of the nose position and wheel spins. Do it again with CDL. Draw your own conclusions. I think youll be suprised.

    ETA: Im not being a smartass. I did the same test last night out of curiosity. :D
     
  3. IdahoDoug

    IdahoDoug

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    Landy,

    A couple of these merit more discussion, but here's my opine:

    1 - Yes, there is a benefit to having CDL on in (some) snowy driving conditions, but it can also be a detriment as you noted due to axle windup. Personally, I drive in high range on snow/ice for that reason as your CD is not L'd.
    2 - Power will go to the wheels that slip. With the fronts on ice and the rears on pavement, you'll get front wheel spinnage (technical term I just made up) though the truck will also get some power to the rears even without CDL.
    3- You actually don't have an 'AWD' in the engineering sense that AWD customarily refers to the presence of viscous couplings and a light duty system to drive all four wheels. However, in high range your 80s power distribution is 25% to each corner - no bias.


    To elaborate a bit, you don't have a viscous coupler on the 80. A VC is a unit where power comes in one side of what could be called a 'transfer case' (connecting front and rear axles) and is not put out the other side unless enough wheelspin occurs to heat the fluid and cause the VC unit to deliver power to the other side. Simplified, I know. This happens very quickly in modern VC units and there are slight variations in this process. VC's are built to various torque splits and response rates.

    In the 80 you have a true '4WD' system, which is generally associated with mechanical power delivery via differential gearsets. The center differential (often mistakenly referred to here on the forum by my esteemed colleagues as the transfer case) on the 80 uses steel gears to split power front and rear. However, there is a novel coupling in it to allow for some slippage to occur that we've routinely called a viscous coupling on the list. In fact, it's a device designed to allow some slippage in a mechanical linkage (your center differential) that can allow for the front and rear output shafts to spin at slightly different rates (but I don't think they're at different torque splits - it's just slippage) to take the unsavory aspects of axle windup out of driving a full time 4WD system. Such as turning sharply in a parking lot, making a U turn, etc. Great system, really.

    My advice? Get the best tires (true winter) you can afford, and drive like a Grandma (or maybe Grandpa though I've seen some Grandpa's drive like a "bat out of hell", so take this with a grain o' salt...)

    DougM
     
  4. Rich

    Rich

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    Doug, that is not all quite right.

    Landy's transfer case contains both a center diff and a viscous coupler. When there is a difference in rotational speeds of the front and rear drive shafts the viscous coupler starts "coupling", biasing the torque to the axle with better traction and less slippage. By its self, the center diff allows the front and rear axles to rotate at different speeds. The design intent of the viscous coupler is to partially reduce the difference in rotational speeds of the front and rear drive shafts when one or more wheels loses traction.

    When all diffs are unlocked, and the viscous coupler is not yet biasing torque, the torque split is equal to all wheels. The power distribution will be split, perhaps unevenly, according to the rpm of each wheel.
     
  5. RavenTai

    RavenTai

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    From the horses mouth first sentence.

    So everybody is kind of right?


    HF2AV TRANSFER


    Landy, torque split is not my favorite phrase means different things to different people, torque requires traction, and it is easier to follow revolutions/RPM IMO.

    If you have not yet give this a read

    http://www.safari4x4.com.au/80scool/george_couyant/diffs/diffs.html


    The 80’s system was marketed as "full time 4 wheel drive" the marketing phrases are pretty blurry but the 80 system is different from most all wheel drive vehicles, there is a viscous coupling in the transfer case but it is not used like it is in a AWD vehicle, the VC is used for dampening differences in RPM in the center differential (fights the differential) instead of the sole conductor of torque as it would be in an AWD.

    As you figured it has no fancy computer controlled or automatic bits, it is a physical system and I think better and by no means “archaic”, it is very stout versatile system for both on and off road,

    1. I don’t know squat about driving on snow & ice

    2. With the CDL unlocked more revolutions will go to the axle that has no traction but enough will go to the axle that has traction to move the vehicle. If the event is severe or long enough the VC will lock and both drive shafts will spin at the same rate.

    Locking the CDL will make the front and rear driveshaft turn at the same rate same as the max control of the VC but for as long as it is locked instead of by conditions. Front and rear axles will travel at the same speed no matter what making it act just like a part time system in 4 H/L

    3. 25% per tire.


    The 91/92 have the HF2A, same basic T-case but with no VC, any one wheel looses all traction all RPMs go to that tire and it will spin at 4 times the speed it was before loosing traction. For this reason it came with the CDL switch from the factory.
     
  6. Walking Eagle

    Walking Eagle

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    It is a detriment. Locking the center diff will make the rig push in corners. I've seen it plenty of times, guys in 4x4 pick-ups go straight with the front tires comepletely turned. Done it myself in my 40 in highschool. Any verision of AWD will work better in the snow and ice than normal 4WD up to the point that it's 12" deep and you need all the traction you can get just to move. At which point, you should call into work and start up the snowmobile.

    People with 4x4's always used to crack me up when I was in Wisonsin. They'd drive like bats out f hell cause they had 4wd, but forgot that it doesn't help you stop, and hurts your steering, and on any stretch of I94, you'd see more 4x4's in the ditch than cars, even though the # on the road was about even (unlike some western states).
     
  7. powderpig

    powderpig SILVER Star

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    I disagree Heath. With the cdl locked the 80 will have more equal pull and push through a corner as well as on the road. The early ABS system on the 93-94 is not as good as the 95-97, that said it is still is 3 channel ABS and I do not like it in 95% of my snow driving. If the computer senses one of the rear wheel locking up, it will not allow the whole rear axle to lock up. Not a great thing in my book. With the CDL unlocked you can end up with a one wheel drive truck until the center VC lock up (not real quick in my experence), but the VC works better in mud than snow. To get a real good push straight in a corner lock the rear with the center locked(not a great combo, ala lock right). Every one's comfort level is different as well as skill level, with the center lock it is just like landcruisers of old with the 4-high engaged, decent two wheel drive with no traction and 4 wheel drive with traction.
    As for braking, I perfer to brake my self in the snow then the abs, it sometimes gets goofy in conditions, and does not want to stop real well(no it is just not this 93, both the 94 and the 93 act the same in the same type of conditions).
    Any how I have not had a major multi car wreck on snow and ice with any of my toyota's (mini tru8cks, 4/runner, fj62 or my 80's (knock on wood))That has hurt anyone or my trucks in over 30= years of driving on snow and ice. The 80 is more capable on snow and ice than the past model cruisers and the 100 series is even more capable(better ABS and trac control, center diff lock, better computers). This has been my experence over the past 30 years. later robbie
     
  8. LandyLover

    LandyLover

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    I love that HF2AV link, I’m gonna read that tonight…

    I understand that the center diff is what allows the “differential rotational speeds” of the driveshafts to take place, but I wasn't sure on how the VC came into the mix. So to be sure the VC’s job is to a) absorb/dissipate energy from windup and b) reduce different front and back driveshaft speeds so they are more alike but it is not "smart" in the sense that it knows to send power to the wheels that grip.

    SO when the CDL is on, the VC cannot/will not work because “the pin” in the CD makes power go 50/50, right?

    So if torque is ordinary split 50/50 (even with CDL off), it seems interesting to me that Toyota would put in a front axle of a mini truck. Say I’m towing a 6000lb boat through the mountains, (which I’ve done) if torque is split evenly front to back then the poor little axle up front, brother of SR5 axle, is doing way more work than his twin in the SR5!

    If the VC doesn’t bias torque can we expect to see premature wear of the front gear set? After all, it seems to be the weakest link.



    Thanks to everyone whos been contributing this thread...
     
  9. Rusty Phillips

    Rusty Phillips

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    except for the fact that on high traction surfaces (on a paved road) you are typically gonna be in high and unlocked...... so no one axle ever gets 100%

    and dont forget that that tiny little axle in the front has a bigger axle in the back helping to push things along
     
  10. powderpig

    powderpig SILVER Star

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    The front diff is a reverse cut, high pinion design that is acutally stronger in the forward driving motion then the older big diff. This is because the pinion gear is driving the ring on the proper side of the gear instead of the reverse side of the gear in the bigger diff. It is no accident that toyota put this in front. later robbie
     
  11. landtank

    landtank SILVER Star

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    I'll through in my opinion here as well. The VC is only present in trucks with ABS. And as Robbie stated the ABS trucks use a 3 channel system, and if one of the rear wheels locks the whole axle is released. At this point you have no braking at the rear and in a dangerous control situation. Toyota calls this unit a Control Coupling and I think that is exactly what it is. When you're starting to lock one of the rear tires it creates a rotational difference between the drive shafts. The coupling then starts to transmit power to the rear shaft loading up that tire that was starting to lock allowing more braking pressure before actually locking.
     
  12. Walking Eagle

    Walking Eagle

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    You're more than welcome to disagree - and if you're more used to driving with a locked center diff from older cruisers and Mini-trucks, that's fine too. But, with the center diff locked, the vehicle will push more in a corner. The front wheels and the rear wheels don't take the same path, just like the lefts and rights in a corner. Easy to tell in the snow, as you see 4 seperate tracks in fresh fluffy stuff. If the fronts aren't going the same speed as the back - which they're not in a corner, they're going to loose some traction. You can't have all wheels turning the same speed going different paths without it. Just like in a locked rear end, if you turn a corner the outer tire is going to slide or skip a bit - that wonderful "erk, erk" sound.

    A locked center diff will make the car push more in corners. That is a fact, weither you agree with it or not. You can look it up many places. Weither you prefure to drive this way in the snow or not is wholely up to you. I'm not trying to come off as a smart arse, though it probably reads that way. This pushing is why with a couple of notable exceptions (VW syncros, the early 325ix), most all AWD street cars do not have normally locked center diffs.

    To test it - go to a gravel lot. Make the sharpest circle you can with the CDL off. Then make the same circle with the CDL on. It won't be as tight, and you'll hear much more gravel getting thrown as the front and the rear fight each other. I'm going to have to stop by the park tonight just to make sure myself :) Course, you through a limited slip and some intelligent traction control in the mix and everything gets more complecated.

    Heath
     
  13. -uhuru-

    -uhuru-

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    I can testify to that 'cause i've had to limp home without my front driveshaft and that was before I got my CDL switch... she would take off slowly at half or less power and after a couple of feet more power would kick in ... I guess this means that after a couple of spins the VC locks.... after I got my CDL switch i tuned my rear driveshaft angle by driving without my front driveshaft... with the CDL locked and ONLY the rear driveshaft the car feels and handles like a regular 2WD ... it did not feel as if i was only getting 50% power.

    On the other hand lets see if you guys agree with my analogies


    1. the behavior of the CD WITHOUT the CDL engaged on a Fulltime 4WD 80 is SIMILAR to the behavior of a LSD differential on a wheel axle... allowing the driveshafts to ''slip'' limitedly

    2. the behavior of the CD WITH the CDL engaged on a Fulltime 4WD 80 is SIMILAR to the behavior of a 100% locked differential on a wheel axle .... allowing no slipage between front and rear driveshafts


    You guys agree?

    Peace
     
  14. re_guderian

    re_guderian SILVER Star

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    What's snow? ;p
     
  15. SUMOTOY

    SUMOTOY

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    Torque, axle/wheel spin and traction - Long

    Hoping to help some of you here with how our 80's AWD work (I'll speak specifically to my 94 FZJ 80 - CDL switch installed, no front/rear lockers, ABS), which should be the same as the 95 80 in the original question. I'm going to go AT the center diff, which has two driveshafts attached to it. I'm gonna leave what happens at the front and rear diffs alone (spinning wheels), since their sum affects what happens at the center diff.

    First, technically the AWD system in the 80 is referred to as a Viscous Coupling Differential Lock. This means that in normal driving (open diff) 50% of the engine torque goes to the front axle, and 50% of the torque goes to the rear axle - ALWAYS. The Torque Bias Ratio is 1:1, which means that the non spinning axle can only support the same amount of torque as the spinning axle. So if a spinning wheel/axle takes 8lb/ft of torque to spin it, the non spinning wheel/axle will only transmit 8lb/ft of torque to the rear wheels.

    Now, let's lock the CDL (switch or low range engagement - stock ABS 80). You have effectively removed the center diff, the front and rear axles are now mechanically locked and neither can spin faster than the other. The biggest misconception on a locked center diff is that's it's a 50/50 torque split. In fact, in a locked center diff, torque split on acceleration/deceleration follows exactly weight distribution. So at any given dynamic state (affected by wheelbase, acceration, static weight distribution, turning radius = sum of all tractive forces), either axle can support 100% of engine torque. This is considered an infinite Torque Bias Ratio, in that either axle can support 100% of engine torque, and any split in between.

    ABS aside. Toyota 3 Channel ABS uses what's called the Select Low Principle in the rear axle. This means that the rear wheel with the highest braking force will only apply the same braking force as the wheel with the lowest braking force. Not ideal, but control is maintained.

    The debate on CDL vs ABS is a complicated one to consider. It is well known that a locked diff will yield shorter stopping distances in gravel, sand and snow, since the buildup under the tire tread actually reduces stopping distances. My personal preference is a locked center in what I call all high horsepower to Cf ratios (Hhp:cf). This includes dry/wet tarmac track conditions as well as snow/ice conditions in winter driving. The locked center creates some handling issues that many find undesireable in comparison to ABS. It requires a higher skill set to master those conditions, which I summarize in terms of predictability. IME, a locked center diff is infinitely more predictable than a open/LSD/torsen/quaiffe. In fact, if you look at AWD racing (WRC in the 80's), Audisport didn't even use a center diff on the winning quattros.

    We can say that a locked diff gives ideal acceleration and brake force distribution in a straight line. In terms of a 3 channel abs system, many times the braking with the CDL will outperform the ABS (see select low principle - it applies always when ABS in 'on').

    My background: Prorally, Solo, SS racing, rallycross and quattro marque driving event and instruction. I also am Eventmaster for a AWD Ice track event at Steamboat Winter Driving Track in Colorado (shameless plug: www.gruppe-q.com). In 9 years of that venue (12in of sheer ice - occasional snow), locked center diff rules at speed driving. In fact, center *and* rear diff locked on sheer ice is usually the preferred method of control. That said, other members in my house are quite content (and, no question safer) to let wheel speed sensors and ABS call the shots.

    For true winter driving (on road) I advocate equipment and education as your two best allies. First, the LTX and standard M/S tires aren't winter tires, they are great 'all season' tires that can get AWD vehicles in trouble quickly. Get a set of dedicated winter tires (doubling as offroad tires is a plus), and use the LTX/AS stuff for the summer cruises. In terms of education, offroading is a good start, but for on road driving, some of the Hhp:cf schools will teach you more than 10 years of driving your 80. Steamboat Ice Track offers them (www.winterdrive.com - no affiliation, just rent the track), and you use their new Toyotas to boot, including some of the big trucks.

    Sorry for the length here boys. I've tried to summarize a couple hundred posts on the subject of torque, traction and differentials I've typed over the years. Happy to get really get nerdy with an extensive library including SAE articles, discussions with engineers, and awd race drivers alike.

    Scott Justusson
    QSHIPQ Performance Tuning
    Chicago IL
    '94 FZJ80 (CDL, 7pin, ABS, open f/r)
    '91 audi V8 (CDL, ABS, rear torsen)
    '84 audi Turbo Quattro Coupe (factory pneumatic CDL & rear locker, no abs)
     
    Last edited: Aug 19, 2005
  16. Hoosier Daddy

    Hoosier Daddy

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    What a fascinating thread. :)

    I've been wondering about some of these questions myself. Seems to be a variety of opinions, but interesting reading nonetheless.

    :cheers:
     
  17. LandyLover

    LandyLover

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    Sumo, thats interesting info. To be honest I have noticed a SIGNIFICANT improvement in stopping distances in snow with CDL locked vs ABS and CDL off.

    You say, "The locked center creates some handling issues that many find undesireable in comparison to ABS. It requires a higher skill set to master those conditions, which I summarize in terms of predictability. IME, a locked center diff is infinitely more predictable than a open/LSD/torsen/quaiffe..."

    But when negotiating a curve at 50 MPH I dont like the "slipping" feeling I have when CDL is on. And I know its the right thing to do to power out of the corner and keep the wheels spinning, but it doesnt make for a relaxing 400km drive to the coast from the Okanagan... I think Im simply gonna have to try a couple hundred clicks with CDL on and off.


    Thanks...
     
  18. MTNRAT

    MTNRAT

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    Scott hit the nail on the head when he refers to "skill sets". Everyone has different skill sets with regards to driving. Especially in marginal conditions. What one finds totally acceptable conditions another may find terrifying. IMO the best thing you can do is get dedicated ice radials for winter on road driving. The CDL on or off debate "depends".

    Cheers,
    Sean
     
  19. IdahoDoug

    IdahoDoug

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    Scott,

    Nice post. I've been up to Steamboat Springs - agree that's 10 years of winter driving experience in a single day. I've lost track of him, but Paul Gerrard was there at the time and I've spoken with him a couple times since. Audi enthusiast random event - got to drive Michelle Mouton's race Quattro on public streets for about 18 hours back in the mid/late 80s. Great fun and great stories.

    On the CDL engaged while driving in winter on public roads. I'll agree with Heath it's best unlocked for 'safe' driving. Maximum speed is quite another thing and best left to tracks. The simple reason is as he states in #12 above the four wheels follow slightly different curves in a turn or curve. This means different radii and different distances over the same time which means slippage must occur. Best traction would be internal slippage such as in the open center diff, worst traction would be external slippage at the contact patches. If it were my wife and kids, I'd have the center diff open for safe normal driving on slippery roads.

    The comment on winter tires is important, and why I switched from traditional siped, studded tires to Michelin Alpin 4X4s. Better traction through technology!!


    DougM
     
  20. sonoranfun

    sonoranfun

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    I'll agree on the snow tires.. I gre up in Wisconsin and durring our 6 months of winter and 47 inches of snow a year I learned quickly the value in having two sets of tires. I always ran Hakkapilita 10's and I SWORE by those things. I would honestly say they made all the difference in the world. While I can't say they drove 50% better in the snow I would say 20% better where it was needed most. Never had a 4x4 there so I can't comment on that but I think those tires drove better then a truck would have with all seasons..