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Toyota 4WD systems explained



  • pschreckpschreck Posts: 524
    The V-6 has multi-mode 4WD, similar to the Sequoia, but not in all respects. The V-8 simply eliminates the availability of 2WD mode. Otherwise they are the same.
  • The V8 is working less hard to carry the same weight. Also, there is a slight MPG gain from having all wheels driven. There may also be final drive gear ratio differences due to the V8s available torque that allow the shift schedule to lug the engine under the same conditions the V6 might need to rev to generate power.

  • dl7265dl7265 Posts: 1,381
    Paul and Doug, In TX we dont get the snow you guys do obviously , but plenty of rain and floods. Also the few storms we get tend to be Ice and the lousy TX drivers here. I have a nice lake cabin in Ok and avid mountin Biker. Considering the factors and resale would you recommend getting the V8 4wheel drive ?

  • I have had some concerns over the capability of my 4WD Highlander with VSC/TRAC in a situation where all 4 wheels are losing traction, for example on a deep snow covered or icy road surface. Recently I noticed in the owners manual it says "DO NOT operate your vehicle in a situation where all 4 wheels experience a loss of traction or damage may result!" The manual suggests that in these conditions, tire chains should be used to provide traction and the vehicle should be operated at a very slow speed. How would the 4WD system react in this situation and how could it damage the vehicle? Kind of disappointing since this is exactly the reason I purchased the vehicle in the first place.
  • wwestwwest Posts: 10,706
    The HL and rx ARE ONLY "PRETEND" AWD,

    Read the last twenty or so posts on the highlander thread.
  • The reason for this is that what will happen if for example you are high centered and are not able to get any of the wheels to get traction is that it will spin tires in the front until the viscous coupling kicks in then do the same in the rear and keep cycling. And just like any vehicle that is spinning it's wheels for a prolonged period, the transmission will get overworked and heated and will fail. The number of situations where you may actually find yourself like this is pretty small. If you have 2 feet of packed snow that you plow halfway through at the end of the driveway or something like that. Just like anything else, use some good judgement and prudence. Just because your vehicle is AWD does not mean that it can traverse all situations. The AWD is meant to give you added traction and stability in slippery situations. It is not meant for you to cross antarctica. :) Hope this helps.
  • The torsen is great for situations where there is limited traction, however, it works like a normal, open differential if one wheel has no traction at all. So in a typical torsen set-up, with the torsen in the center, if one wheel is in the air, all of the power would still "leak" out through that wheel. The Torsen actually works opposite the way a normal differential in situations where there is some wheel spin, but not total loss of power. Cliffy's description of the worm gears is right on and what happens is that it actually sends power to the drive shaft that has MORE resistance... The limitation of situations where one wheel has not traction is why they set up the max/min power transfer on the 4-Runner. So no matter what, there is still power being sent to both drive shafts. Along with this, the TRAC system will sense if one wheel of the pair either front or back is spinning and apply the brake to that wheel and thereby transfer power through the differential to the other wheel. So here's an example of these two systems in practice.

    Vehicle is parked. Right front and left rear wheel are on ice. You step on the gas. The normal bias of the open diffs. sends power to the two wheels on ice. Computer senses this and applys brakes to RF and LR wheels, and left front and right rear wheels get power, moving you forward. Now as you're moving forward, the right front comes off the ice along with the left rear, but right rear goes onto ice that the right front was on. Right rear tire starts to spin. Torsen increases power to front drive shaft, TRAC system applys brake to RR tire and so other three wheels get power to move you.

    Sorry for the long-winded example, but hope it helps.
  • wwestwwest Posts: 10,706
    wasn't the question, nor the desire.

    Driving around in a ski area parking lot to find a space may be a more perfect example. Neither the RX nor the HL can get that done. At least not last winter at Snoqualmie Pass.

    If all four wheels have equally low traction then the Trac system has no ABS sensor reference to use to come into play and the VC also doesn't come into play.

    "The" spin one wheel and then the other and then another is a perfect example of what the RX or HL will do on packed snow or ice. Even the Jeep Cherokee Limited's AWD system will do that.

    Viscous couplings MUST have a long time delay before the onset of tight coupling and that leads to the problem of continuously shifting torque back and forth to the wheel not spinning, then this one, now that one, etc, etc.

    The difference is that in the Jeep you can LOCK the center diff'l and you have the option of a rear LSD AND (this is the biggy!) you can put chains on all four wheels.

    With a locked center diff'l one front wheel AND one rear wheel will turn ALWAYS. Add in an LSD in the rear and both rear wheels will turn.

    The RX or HL, dead in the water, HARD water, ICE that is.
  • solution...lightly apply the brakes and gas at the same time.
  • Wwest and Toyotaken,

    The information on the viscous coupling "cycling back and forth" and "MUST have a long time delay" are both incorrect. The transfer of torque is nearly instantaneous as it is caused by the shear pressure of the VC fluid between the plates - no notable delay and potentially faster than an electronic device could react.

    As to the cycling. VC's do not transfer torque back and forth - only 1 direction. There's an input shaft from the front wheels and torque is either sent / not sent to the rear. It all comes from the front, it is not sent back and forth. Further, this is not what Toyota is worried about as VCs are meant to be used as continuous duty (constantly) when needed and the fluid itself acts to prevent overheating.

    What Toyota (and ANY vehicle with electronic traction control -TC) is worried about is overheating the brakes. TC includes a function of applying the brakes to a spinning wheel. If you are in a condition where you're not going anywhere (Toyota's "all wheels spinning"), then the brakes can get overheated. No airflow, plus braking action generates heat load. This takes some doing Cotterpin, so frankly it's not a reason to worry IMO. It is not a fragile system and has plenty of reserve strength and heat capacity in the brakes. They just want to prevent a numbskull from standing on the gas in a sustained manner for 5 minutes without letup being able to sue them for damaged brakes.

  • wwestwwest Posts: 10,706
    The basic princple of a VC coupling or clutch is that the "effective" viscosity of the fluid increases dramatically with relatively small increases in its temperature. Even Toyota/Lexus will advise you that the reaction time will be on the order of hundreds of milliseconds.

    The viscous fluid gets heated via the turbulence created when the two sets of interleaved clutch plates have disparate rotational rates.

    Therefore you should NEVER have substantially differing diameter tire sizes on the front vs the rear. That would create constant "stirring" of the viscous fluid causing it to overheat and fail.

    And yes, once the fluid is heated the increased viscosity will remain until it cools, maintaining, until it cools, a relatively high coupling coefficient from the front driveline to the rear.

    First one tire spins, then another, and then...

    Right front spins, VC stiffens and now the right rear spins along with it and the VC cools.

    Trac is supposed to handle side to side rotational disparaties but doesn't. I can understand why not for the front, jerking the steering wheel out of somebodies hands, etc, but I know not why Trac doesn't act as an LSD in the rear as Lexus says it should.
  • wwestwwest Posts: 10,706
    If all wheels are spinning then the Trac computer has no reference point and the brakes would never be applied at all. Been there, done THAT!

    If all wheels are spinning and there is a substantial variation of the rate at some wheel(s) the Trac will moderately brake the faster wheel(s). But in this case it would also quickly dethrottle the engine.

    Leaving us with the first case, all wheels spinning freely with no substantial disparity. Other than the driver being an idiot what's the harm mechanically?
  • The wheels spinning non-stop with no resistance is a recipe for transmission failure. That is the same reason why it is recommended that you don't spin the wheels in ANY vehicle that is stuck for any extended period of time. The brakes don't build up heat unless they are applied. After they are applied, they will generate heat which is dissipated through the air contacting the drums or disc, but in the situations where we are discussing the brakes aren't being applied. Although light application of the brakes would be helpful as it would help bind the wheel that is spinning.
  • wwestwwest Posts: 10,706
    "Wheels spinning non-stop with no resistance is a recipe for transmission failure".

    REALLY !!

    At something like 90MPH and above there is virtually no wheel "spinning" resistance, almost all resistance is wind drag. Guess I better stop driving my C4 that fast for hours on end.

    In my many years in Seattle I have seen many inexperienced/California BMW drivers spinning out on a snow and ice covered uphill roadbeds with the only result being lots of burned rubber smell.
  • I'm a bit confused about the 4WD in '03 4Runner. The Toyota website says it's a Torsen Differential based system with locking option. That doesn't sound like the ATRAC system in Sequoia or anything close to it, but rather like the system use in Audi's back in the 80's. Please tell me that's not so. Does '03 4Runner run on an open differential like ATRAC or Torsen?
  • I must say, you do speak with passion on this topic. First, let me address the VC issue you raise, then I'll comment on the 90mph comment above.

    To your comment that VC's have a delay in the "hundreds of milliseconds", which you also consider "long". To my mind, that's pretty fast. In addition, the VC does not have to heat up to operate. The fluid in a typical VC (they vary somewhat) is the consistency of honey when cold, so you can see that if the plates of one shaft are spun, the spinning action of the thick fluid will immiately exert pressure on the other plates to transfer torque.

    To your comment on the VC overheating. VC's are easily the most durable component of a VC based drivetrain, with no heat issues, the ability to operate on a continuouos duty cycle, and no maintenance required for the entire life of the vehicle. They are simply a liquid filled canister and are designed to handle enormous heat.

    To your comment on the danger of different tire circumferences F/R. A VC is able to handle this relatively small difference in rotation with ease. It is systems like Subaru's which use an electrohydraulic clutch that have problems with this as the clutch is not designed for continuous duty and can overheat. Yes, it is always best to have the same diameter tires on ANY vehicle, but this is not an achilles heel on the VC system.

    To your comment (again) about the "cycling back and forth" brought forward again above in your scenario about the RF spinning, which causes the RR to spin. You imply the spinning RF causes the RR to spin, then the VC cools because the shafts are spinning together, which in turn causes the RF to spin again in a cycle ad nauseum. This is not true. If the RF spins, the average speed of the two fronts increases, which sends torque to the rear shaft via the VC. When the spinning stops, the sending of torque stops - it's that simple. The sending of torque does not wait for the VC to "cool" before stopping its delivery, it waits for the spinning. There is no mechanism to send torque back to the fronts as you imply, and there is therefore no constant cycling back and forth. This is a much simpler system than you seem to think it is.

    Now, to the 90mph comment. You state that there is "no wheel spinning resistance". Mechanically, all resistance to forward travel is registered by trying to spin the wheels, and this resistance is huge at 90mph due to aerodynamic drag. To state that there is "no" resistance is simply to operate in ignorance of simple physics.

    The warning is for the following reason. Spinning the tires while stuck will do all of the following:

    -Damage the tires.
    -Break an axle/differential if a spinning tire suddenly finds grippy pavement.
    -Overheat the tranny (high fluid heat from internal shaft speeds with zero cooling airflow).
    -Overheat the brakes if the system has TC (yes the brakes will apply despite all wheels spinning unless you have the incredible happenstance of all of them breaking loose at the same instance and maintaining the same rotational speed during the entire wheelspin. A situation that is impossible with a VC system, by the way. I'd be more than happy to elaborate on this, BTW)
    -Create a system that is ripe for loss of vehicle control.

    All of these scenarios are bad for the manufacturer of the vehicle, and they'd like you to avoid them. Thus the warnings.
  • wwestwwest Posts: 10,706
    Please go educate yourself on the operation of viscous fluids as used to implement viscous clutches or couplings.

    When I started out I was quite ignorant on the subject. The catch phase was "a fluid in which viscosity increases with temperature". I don't have much of an education but I do know that most fluids' viscosity DECREASES with rising temperature.

    So I went looking to find the truth, or something to convince me that the "catch phrase" was true.
  • wwestwwest Posts: 10,706
    The 03 4runner, and apparently the new GX470, has a torsen center diff'l with three modes. For the moment I am assuming the modes are selected electronically by the A-Trac system. The brochure wording seems to indicate that the torsen switches modes, reducing the torque to the front, 29/71 when turning.

    It seems the A-Trac will be dedicated to managing the torque distribution side to side rather than having mechanical LSDs.

    At first glance I too liked the Sequoia system. But owning an 01 AWD RX300 I have taken notice that its Trac system doesn't seem to be aggressive enough to get the job done.

    Think of this. With Trac, or A-Trac, if the left front wheel is spinning freely then Trac would need to apply the brakes to that wheel to "force" engine torque to the other wheels. "Hard" braking application to only one front wheel would provide horrid "jerking" feedback to the steering wheel and might even result in loss of control entirely.

    So I would suspect that neither A-Trac or Trac could provide aggressive side to side torque distribution to the front absent a method for "locking" the steering wheel into position during differential braking.
  • cliffy1cliffy1 Posts: 3,581
    The '03 Runners do use a torsen center differential with the ability to lock it as well. That is the main difference between this model and the previous year and with the Sequoia.
  • "Please go educate yourself on the operation of viscous fluids as used to implement viscous clutches or couplings.

    When I started out I was quite ignorant on the subject. The catch phase was "a fluid in which viscosity increases with temperature". I don't have much of an education but I do know that most fluids' viscosity DECREASES with rising temperature."

    Wrong, fluids heat/viscosity properties vary greatly depending on the intermolecular forces of the fluid. It is too big of a generalization to say that most fluids viscosity decrease with temperature.

    Also, VC does NOT operate on the principle that the fluids viscosity increases with temperature. In fact, most VC systems use fluids where temperature has little to no effect on viscosity. They do however use fluids based on their specific shear thickening properties. Doug had a great example: Honey. Go take a jar of honey, and stick it in a blender. Turn on the blender and listen closely to the motor. The motor strains, and lowers its speed, because the Honey thickens with shearing.

    I dont know where you have been doing your research, but it is wrong. I would advise you to not take everything you read on the internet as gospel.
  • steverstever Posts: 52,572
    I guess I need to stop microwaving my honey 20 seconds in the morning before squeezing it on my toast? Or turn up the thermostat in my house.

    Steve, Host
  • I didnt say that it doesnt happen, I said it is too big of a generalization to say that most fluids decrease in viscosity when they are heated.

    For example, corn starch and water in solution will thicken when heated. Honey, like you mention, will thin when heated. Water will not experience any viscosity change when heated.

    There is absolutely no way to say that most fluids behave one way or another, because every fluid is different, and every fluid will react to heat differently.
  • wwestwwest Posts: 10,706
    Cornstarch mixed in water ain't no fluid!

    Viscous clutches and couplings work on the principle of thermal expansion of fluids when heated.

    Look it up.
  • cornstarch in water IS a fluid. It is called a solution, which is a fluid, and a solution in aqueous form IS a fluid.

    Believe what you want.
  • I'm not sure I want to continue this debate because basically I'm constantly stuck in the role of saying "wwest, you are simply wrong" and frankly I'm not comfortable with that in a public forum. If you and I were sitting at Starbuck's havin' a coffee this would be much easier and come off less harsh. But here goes.

    I went through several of your comments in my last post basically saying you were wrong in the interests of having this board be a source of accurate information. Your next post did not address any of these comments in active debate, but simply brought up a new topic, which is the viscosity of fluids. OK, now I guess we're on the subject of viscosity and you're startled to find that VC's are full of a fluid that thickens with heat.

    As a result of your startling discovery, you contend that most fluids thin as they get thicker. Since we're talking about automotive fluids, here's my response - you're wrong. Gear oil, and motor oil have HIGHER viscosity at warmer operating temperatures. Why was it a surprise to you that the fluid used in viscous couplings behaves the same way when it is actually quite common? And why did you assume I did not know this, and that therefore I am somehow in error? I truly don't understand the relevance, but felt the need to clear it up anyhow.

    As to your comment about "horrid" jerking on traction control systems acting on the front wheels, again you are sensationalizing and completely inaccurately portraying such systems. Traction control systems can provide quite aggressive torque distribution using the brakes without any undue steering input. The secret is good software that measures the rate of difference, and ramps up the braking force. It is not rocket science and has been around since the mid 80's. In 1986, I drove a Mercedes 4Matic Wagon flown over from Germany for our research which used wheel braking quite smoothly. 16 years later, this technique has become quite common. My '97 Audi A6 Quattro Wagon had front-wheel-only braking action and even with one wheel on glare ice and the other on dry pavement the only way I knew it was working was the sound - nary a wiggle in the steering wheel. So, I disagree with your characterization of TC-type systems, which are quite mild and act to stabilize the vehicle rather than reduce control as you imply.

    It is plain to see that you are unhappy with your RX's system BTW, and I have a suggestion for you. Put winter or studded tires on it and you'll be amazed at its mobility on both the slick stuff and on the deep stuff. Better yet, try Michelin's Arctic Alpin 4X4 or another new generation studless winter specific tire. With all season tires, the system's potential is barely tapped. If you choose not to, then you'll save $$ but must recognize that you're operating on a lower level in terms of traction.

    And, yes cornstarch and water is a fluid.

  • cliffy1cliffy1 Posts: 3,581
    When are we all going to realize that having this discussion with wwest is futile. I too wish he would pipe down, but I know there is no way to accomplish this. I just resign myself to a lack of coherent information whenever he posts. This is not to say is isn't right about some things, but the style gives me a headache.
  • So did the Car & Driver article misrepresent the Sequoia's capabilities? Between the Sequoia and the new Runner, which is more sophisticated? Can the Sequoia be driven at highway speed in 4WD with no adverse effects? Also, I noticed 3 button in a line on the center console. One looks like it pertains to the Hill Decent Control; does anyone know what the other 2 are for.
  • pschreckpschreck Posts: 524
    Yes, C&D did misrepresent the Sequoia. They couldn't figure out how to lock the center diff.

    Yes the Sequoia can be driven at highway speeds with no problems whatsoever. Mine spend about 95% of it's time in 4WD.

    Sophistication? I give the nod to the 4Runner. Ask cliffy for sure.

    Not sure what the other buttons are for. The Sport that I drove had those buttons covered with plugs.
  • cliffy1cliffy1 Posts: 3,581
    Because of the torsen center differential, the Runner gets the nod in terms of sophistication. That doesn't mean the Sequoia isn't a very capable off road vehicle but it just isn't quite up to the Runners level. The Sequoia also only has a center differential that can be locked in the low gear range.

    The other buttons you saw on the Runner are for the heated seats on the Limited and the optional height adjustable air bag suspension.
  • wwestwwest Posts: 10,706
    Typo? "thin as they get thicker"

    I'll assume that was a typo.

    Several instances come to mind.

    Years ago in the wintertime in Anchorage I went out early in the morning to drive my rental car away that had been plugged into a block heater all night so the engine oil didn't become so thick and sluggish that the starter couldn't turn the engine over. The engine started just fine and as I backed out of my parking spot and attempted to steer I discovered that I couldn't turn the steering wheel.

    After a little bit of checking I discovered that the power steering fluid was thicker than a good batch of sorghum molasses. Don't remember for sure but I wouldn't doubt that the outside temperatrue had been as low as -50 during the night. I left the car running and went back in and had another cup of coffee while the steering fluid warmed up to the point wherein its viscosity was low enough for it to "flow".

    It is my understanding that petroleum engineers have spent years and years trying to find the correct additives, formulations, to keep engine oil from thickening as the temperature declines.

    Back on the farm in the wintertime 60 years ago we had to put a light weight oil in our equipment simply because engine oils with wide viscosity ranges, 5-30W, simply didn't exist then.

    VCs operate because the engineers use a formulation that has a high degree, rate, of thermal expansion with a small change in temperature. It's the increasing PRESSURE of the fluid inside the SEALED chamber that raises the EFFECTIVE viscosity. That why many manufacturers that use an adequate fluid formulation can, and do, use a gas bubble inside the sealed case to delay the onset of coupling action.

    I have never said VCs cannot work, and work VERY effectively. What I have said is that seemingly the formulation of the viscous fluid in the RX and HL is such that the coupling coefficient to the rear is initially VERY low and does not rise as rapidly as one might want on a roadbed environment wherein the need for chains is just a tad away.

    The same being true for VSC and Trac. I know that the PSM firmware in my C4 is specifically "composed" to delay its onset to the point wherein my seat of the pants sensor has a chance to warn me to react. If I don't it steps in with a very agressive manner.

    Since you're in Idaho I would suspect that you know how SUVs with intentional offroad capability solve the problem of front LSD steering feedback. They have huge shock absorbers mounted between the frame and the steering mechanism.

    My 92 LS400 even has one and I know not why. Luxury and softness of the driving experience, I suppose.

    As you are already aware, VSC and Trac systems can be VERY aggressive, as they tend to be in most vehicles of european origin.

    But the RX300 is of Japanese origin. Watch those ads on TV, even see a man driving an RX? My point is that the RX, and by default the HL, are about "softness" of the driving experience.

    The VC fluid is formulated for a "soft" non-aggressive nature, as is the composition of the VSC/Trac firmware.

    Oh, last I checked the components of a solution don't separate over time. Last night I mixed cornstarch and water, thoroughly, and this morning I had two separate levels of substance in the glass. Sounds more like a "suspension" to me.
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