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Toyota 4WD systems explained



  • steverstever YooperlandPosts: 39,971
    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

    Need help navigating? - or send a private message by clicking on my name.

  • 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.
  • 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?


    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:

  • 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.

  • wwestwwest 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...

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

  • the link doesnt work
  • wwestwwest Posts: 10,706

    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?
  • 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.

  • wwestwwest 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.


    says it ALL.

  • wwestwwest 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.
  • 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 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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