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Toyota 4WD systems explained

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Comments

  • wishnhigh1wishnhigh1 Member Posts: 363
    Cliffy...

    Could you explain the differences in the 4WD system in the Land Cruiser and the 2003 4Runner(V6)? For any differences listed, could you mention the benefits/drawbacks?

    Also, if possible, could anybody fill me in on how the Land Rover Discovery system works in comparison to the TLC?
  • cliffy1cliffy1 Member Posts: 3,581
    The only real difference is in the center differential. On the Cruiser, it is open, meaning all power can be routed to the front or rear. The new Runner is a Torsen which will route *some* of the power front to rear but will always leave some power at the other end. Torsen is a better system, but not by much.
  • wwestwwest Member Posts: 10,706
    I was living in north central MT in 1963 when LSD, rear lockers (What were they called back then?), first became available. They were a goodsend to those of who wanted/needed to drive our RWD (only) car during the winter months.

    And yes, you could find yourself spinning both wheels and "out", but you soon learned, if you hadn't already, not to play "boy-racer".

    Provided you're willing to live with conservative wintertime on-road driving, even in Alaska, a RWD w/LSD will be more than satisfactory 99% of the time. That other 1% will require a good set of snowchains but overall it will be worth the bother IMMHO.
  • wishnhigh1wishnhigh1 Member Posts: 363
    clffy...

    Both the 4runner and the TLC have a locking center right?

    Also, what would be the benefits of adding locking front and rear differentials, or possibly torsens? Wouldnt that work like a typical AWD system?

    Also...I know this isnt drivetrain related, but the TLC has torsion front suspension and coil rear suspension right? Is it the same setup as the new 4runner?
  • cliffy1cliffy1 Member Posts: 3,581
    The guy was asking about the Tacoma which does not have a LSD. It is a manual locker that binds the two rear wheels together.
  • cliffy1cliffy1 Member Posts: 3,581
    With the advent of traction control, front and rear lockers become much less important, especially if you have a torsen center. The older Cruisers had lockers but they can only be used in limited situations. It is very easy to damage a vehicle with a locked front and rear end.

    As to the suspension, the Cruiser has torsion bars in front while the Runner has coil springs. Coil springs are better on road but the torsion bars are a bit more rugged.
  • idahodougidahodoug Member Posts: 537
    WWest,

    Wow, you've seen some serious winters if you lived in N. Central Montana - hooeeeey!

    The limited slip rear differential is what you had back then in RWD cars. It was often erroneously called a "locker". A true locker such as what was asked about above locks both rear wheels together and is WAY more dangerous than your limited slip experiences. Just a touch of gas with a locked rear end will cause a spin, where on an LSD rear end, you get some warning wheelspin from one tire first. No such warning on a locker.

    Where did you live in MT? Check the Weather Channel and look a the weather this region is getting at the moment (yowza).

    IdahoDoug
  • wwestwwest Member Posts: 10,706
    Winter of 62-63 -56F
  • idahodougidahodoug Member Posts: 537
  • dl7265dl7265 Member Posts: 1,381
    So , the '03 runner can be had in V6 part time , or V8 Full time only ? The later i asumme would be more beneficial in a area with more rain, slippery conditions and a few days of light snow and ice ?

    The full time differs from AWD by having a low gear avaliable ?

    one more of topic question, if the V8 is turning 4 wheels and the added weight , does anyone know how the fuel rating is only 1 mpg less than the part time V6 ?

    thanks
    DL
  • pschreckpschreck Member 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.
  • idahodougidahodoug Member Posts: 537
    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.

    IdahoDoug
  • dl7265dl7265 Member 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 ?

    thanks,
    DL
  • cotterpincotterpin Member Posts: 1
    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 Member Posts: 10,706
    The HL and rx ARE ONLY "PRETEND" AWD,

    Read the last twenty or so posts on the highlander thread.
  • toyotakentoyotaken Member Posts: 897
    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.
  • toyotakentoyotaken Member Posts: 897
    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 Member 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.
  • wishnhigh1wishnhigh1 Member Posts: 363
    solution...lightly apply the brakes and gas at the same time.
  • idahodougidahodoug Member Posts: 537
    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.

    IdahoDoug
  • wwestwwest Member 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 Member 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?
  • toyotakentoyotaken Member Posts: 897
    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 Member 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.
  • brightnessbrightness Member Posts: 40
    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?
  • idahodougidahodoug Member Posts: 537
    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 Member 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 Member 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 Member 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.
  • wishnhigh1wishnhigh1 Member Posts: 363
    "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 Guest Posts: 52,454
    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
  • wishnhigh1wishnhigh1 Member Posts: 363
    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 Member 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.
  • wishnhigh1wishnhigh1 Member Posts: 363
    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.
  • idahodougidahodoug Member Posts: 537
    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.

    IdahoDoug
  • cliffy1cliffy1 Member 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.
  • topgun10topgun10 Member Posts: 11
    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 Member 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 Member 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 Member 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.
  • wishnhigh1wishnhigh1 Member Posts: 363
    I am going to gloss over the whole temperature viscosity thing, because it is a dumb argument, and only partially relevant to the true discussion at hand.


    "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."


    First of all, you are talking about a mechanical device that, when spun rapidly, raises the temperature of a fluid in the case. In other words, you are using mechanical work to raise the thermal energy of the fluid. While this can happen, the amount of mechanical work needed to raise ANY fluids thermal energy level a significant amount is astronomical.


    Assuming you stick by that logic, you must also consider that in order for the VC to release its "locking" of the two shafts together, you must lower the temperature of the fluid, which means giving off the same amount of energy as you put in, which is a long and slow process...which could mean that long after you leave your slipping wheel situation, your axle is still locked together, which is very unsafe and no manufacturer would ever be stupid enough to put that on a production car.


    Also, according to you, the fluid only needs a small increase in temperature to provide enough pressure to lock up. Considering that you think that a mechanical device is being used to raise the temperature of the fluid, you must mean that this fluid has substancially altered its volume through thermal expansion, to couple the plates together, with only a few degrees of difference in temperature. But how do you account for climatic temperature changes? If this fluid expands significantly with only a few degrees of temperature change, then how does the unit not explode from pressure in 100 degree heat? How does it couple the shafts together in -40 degree weather?


    The answer is simple. VC DOES NOT USE THERMAL EXPANSION TO COUPLE THE SHAFTS TOGETHER.


    Summary: A bunch of VC discs spinning can only raise the temperature of a fluid by a few degrees in a few seconds. If that fluid has thermal expansion properties that allow a significant pressure change when the temperature changes by only a few degrees, then that little VC case would explode in summer, and not work at all in winter.


    In reality, VCs use a very simple fluid. They do not need expensive special formulations to operate, like you suggest. If they did, all manufacturers would be using TORSEN instead. VCs actually operate on a very simple principle.


    Read away:

    http://www.howstuffworks.com/differential7.htm

  • idahodougidahodoug Member Posts: 537
    Yes, typo - thanks. Meant "thin as they get warmer" of course.

    I haven't checked the link above, but it might be useful to read a summary of how a VC works if they have one. It is much more simple than you surmise - no clutch being activated in there, no pressure function, etc. Just a bunch of spinning discs with holes in them alternating "input shaft, output shaft". When one set spins, the thick fluid causes the other to spin through simple rotational friction just like the propeller of a boat spins when underway and you put it in neutral while still moving - residual drag, not pressure.

    As to your LSD comment, I'm not aware of any SUV available with a front LSD. The number of them taken offroad where such a feature would be marketable is well below the level it makes sense to offer it.

    The feature on your LS400 is a steering damper. It prevents abrupt steering inputs from reaching the driver and I honestly cannot recall it had one - good for me!! I was the Product Planner for that vehicle, as well as the Lexus ES250, the SC300/400 and the ES300. Following that, I was a District Manager and if you bought it in the Seattle area, I helped set up the dealer you purchased from. Small world, eh? :-)

    I think you have a good grasp of what's going on under there, but at a certain point the subtleties of how these things work require fairly specific knowledge. You are knocking at this door and clearly have the capacity to grasp it. The "how stuff works" site has been pointed out by people who's opinions I respect as a good starter place to learn things like this. In some areas it's a bit dated, but overall a good site to check out.

    As to your RX, consider the tires. I recall the LS400s with TC also not having an aggressive enough schedule and their snow capacity was dramatically improved with tires. The minimum available traction determines your mobility after all - increase this with tires and everything gets better.

    IdahoDoug
  • wwestwwest Member Posts: 10,706
    I removed the damper from my LS400 after I upgraded to 225/55/16 Turanzas w/BBS-RS wheels. Picture at clublexus LS gallery under wwest.


    I am of the understanding that the hummer has a locking front diff'l. But what is more to the point is that your statement seems to imply that you are in agreement that the side to side Trac activity on the front cannot be too aggressive unless something is there to absorb the shocks resulting.


    With reference to the VC:


    4th paragraph...



    http://www.mmae.ucf.edu/~jtt/differential.htm

    "...flat viscosity slope from -40F to +400F...."

  • wishnhigh1wishnhigh1 Member Posts: 363
    the link doesnt work
  • wwestwwest Member Posts: 10,706
    Doug,

    Are you currently in Id?

    Wife and I are driving to central MT, Lewistown, on the 20th.

    She wants to take the RX.

    I want to take the C4.

    My other choices are a 98 AWD T&C or a 92 AWD Aerostar.

    I was just kidding about the C4.

    The RX can't have chains on the rear unless I find time to switch the rear struts to the front type and collapse the springs a few inches.

    The Aerostar is at 125K miles but I know I can implicitly trust the AWD system, not so sure about other mechanical parts.

    The T&C, 57k miles, can also handle chains all around and the AWD system uses a much "stiffer" VC than the RX.

    What would you recommend?
  • idahodougidahodoug Member Posts: 537
    I think you can get too aggressive with TC, but this would take some truly and incredulously poor engineering to actually sell it to the public. I've driven systems in prototype that were rough around the edges, but something like that would/should never get to the public and it's not difficult to eliminate.

    Neither the Hummer nor the H2 have or ever had a front locker. A lot of people assume this, but in fact the Hummer had a fine and quite unusual system. You guys are going to start thinking I'm BS-ing with all these coincidences, but I also do offroad events for Isuzu and have also driven the old and new Hummers on the AM General factory offroad course. The older system (ended 99ish) used novel locking hubs at each wheel, which locked up when you applied both brake and gas. After this and still today, it has a 4 wheel TC system that is still quite effective offroad. I don't know much about the H2. I've trounced a military Hummer in an impromptu hillclimb challenge in the snow with my Montero (factory rear locker), so they're not invincible though obviously quite a serious machine.

    For the trip you describe, I'd take the T&C as it will be quite capable, but have the smallest 'wince factor' while getting rained on with the gravel chips you'll get on that trip. I suspect any of them would make the trip in fine style, but I'm reluctant to take a nice car on the freeway in winter. In many ways though, I'd refer back to my prior statement about tires. When it's slick out, tires provide a far larger margin of safety than anything else, though CU recently found an AWD vehicle on all seasons will still out accelerate a FWD vehicle on snows when it's slippery out. Your mileage may vary...

    Of course, the C4 would do it all, but it will also have the highest wince factor. Magnificent automobile, BTW.

    IdahoDoug
  • wwestwwest Member Posts: 10,706
    howstuffworks also has a dessertation on viscous clutches as once used to engage radiator cooling fans. If the air flowing through the radiator was too hot then the viscous fluid was heated to the point that the fan would then be driven with the engine.


    My 68 Ford country squire station wagon had one of those.


    BUT.

    http://www.colt.demon.co.uk/CavTurbo/4wd_transfer_box.html




    says it ALL.

  • wwestwwest Member Posts: 10,706
    I tend to run summer tires on all my vehicles year 'round. More comfort and quietness. But I ALWAYS have a set of chains nearby in the wintertime and do not hesitate to put them to use if the need arises.

    But the RX does have Michelin cross terrains.
  • wishnhigh1wishnhigh1 Member Posts: 363
    The first link says:

    "Most of the viscous couplings already in use in many all-wheel drive vehicles have an assembly which is made up of a die cast aluminum with forty four steel plates. The unit contains a heavy, heat stabilized, viscous silicone fluid sealed in the assembly with a relatively flat viscosity temperature slope ranging from –40oF to over 400oF. The conventional interaxle differential is encompassed by the viscous assembly with the outer housing connected to the rear output shaft and the inner member to the front wheel driving sprocket (1)."

    A flat viscosity temperature slope means that viscosity does not change significantly with a change in temperature.

    The second link says:

    "The properties of the silicone fluid are such that as the temperature increases so does the viscosity or thickness of the fluid. This results in the previously freely spinning plates to become increasingly locked together allowing more of the power to be transferred to the rear axle via the external discs."

    Which implies that the viscosity does change significantly with a change in temp.

    neither of which agree with your theory.

    You state that fluid is chosen by the thermal expansion properties, because the fluid expands increasing pressure in the case, which locks the plates together.

    The two sources you have given say that it uses a simple, non pressure operated design. One says that viscosity increases with temperature, which locks the two together, and one says it doesnt.

    The funny thing is, one of the sources is from a guy who drives a cavalier and took apart his AWD system and took some pictures of his project. Another was written by a college student with fully referenced sources, who has some very significant research experience.
  • wwestwwest Member Posts: 10,706
    The viscosity doesn't change, but the fluid is instantly heated...

    "...resulting in the silicone fluid heating up instantly..."

    If the viscosity doesn't change as the fluid is heated then what characteristic is left?

    Thermal expansion.

    "If" the fluid is heated and "if" it expands accordingly, and considering that it is encased in a hermetically sealed container, haven't we just "compressed" a fluid into a smaller volume?

    If so then the fluid is now a lot "thicker" than it was at the lower temperature and the "effective" viscosity might just be dramatically higher.

    I seem to remember a "child's" trick of putting a piece of a matchstick in a coke bottle filled with water and then using my thumb or finger in the neck of the bottle to "compress" the water such that the matchstick piece would sink to the bottom.

    What was obviously happening was the "pockets" of air in the wood were being compressed and displaced by the pressurized water.

    Didn't someone once tell me that liquids could not be compressed?
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