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Toyota 4WD systems explained

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Comments

  • hdfatboyhdfatboy Posts: 324
    I've owned 7 vehicles with rear LSD over the past 30 years. I've never done any maintenance at all on the LSDs and most of those vehicles did 100K miles.

    The other advantage is that the LSD works under all conditions at all speeds. The approach of using brakes to transfer torque is generally limited to speeds around 30mph and lower (to avoid rotor warp and really premature wear of brakes, in addition to instablity of braking at higher speeds on slippery surfaces) therefore you have no torque transfer on slippery ramps or other circumstances where slippery conditions might exist at higher speeds.
  • wwestwwest Posts: 10,706
    You don't say what area of the country (world??) you reside. For many/most of us as non-off-roaders the need, requirement, for LSD is rare enough that a failed one might not even be noticed. And if I ever find a circumstance wherein one is needed above ~30 MPH I'll be sure and contact you from my hospital bed.

    But to be totally fair the above statement probably also means an LSD would not incur enough wear in 100,000 miles so as to result in a failure.

    But given that the method of implementing the new "virtual" type of LSD, rear and center, is also integral to ABS/VSC/TC/AWD/EBD/BA (is there more...??) the need for an actual LSD seems more than just redundant, more like in a total waste.
  • wwestwwest Posts: 10,706
    "..is there more....??"

    yes, actually, more to come.....

    Trailing, "drag", braking.

    The brake fluid pressure application/release valving already integral to these systems would allow the rear brakes, ONLY the rear brakes, to be applied first/initially, with "LIGHT" braking. Anything beyond a certain brake pedal pressure point/level would bring in the front brakes also. A second method might be to only use rear braking until ABS activation is "triggered" for the rear brakes indicating that the driver is asking more of the rear brakes than they alone can supply or roadbed conditions are limiting the level of braking the rear can supply.

    On a slippery wintertime roadbed, or even with a fresh rain on an oily roadbed, this would not just result in the desireable effect of stabilizing the direction of the vehicle, it would also result in the dedication of the ENTIRE front tire roadbed adhesion coefficient to lateral, directional control/correction, forces, much like the way drag brakes are often used on a tractor/trailer rig.

    Many vehicles already make use of the above method for directional stabilization/correctional aid, rear braking only, when VSC detects plowing/understearing.

    Delayed ABS activation.

    Unless VSC senses/detects that directional control is being threatened via severe braking there is no reason for ABS activation on the front wheels.
  • hdfatboyhdfatboy Posts: 324
    "You don't say what area of the country (world??) you reside. For many/most of us as non-off-roaders the need, requirement, for LSD is rare enough that a failed one might not even be noticed. And if I ever find a circumstance wherein one is needed above ~30 MPH I'll be sure and contact you from my hospital bed.

    But to be totally fair the above statement probably also means an LSD would not incur enough wear in 100,000 miles so as to result in a failure.

    But given that the method of implementing the new "virtual" type of LSD, rear and center, is also integral to ABS/VSC/TC/AWD/EBD/BA (is there more...??) the need for an actual LSD seems more than just redundant, more like in a total waste."


    I'm from the NorthEast so I think there are enough wet, icy, snowy conditions to fully benefit from a mech LSD. Unless you've driven both types of vehicles I think its hard to evaluate however I've driven enough vehicles with both open and mech LSD (both rear and ctr) to know I would always and I repeat always... take the vehicle with the mech LSD over the brake induced torque transfer vehicle with an open differential.

    But don't take my word for it. Just do some research and you'll find that nearly every single performance vehicle on the road uses a mechanical LSD and that includes vehicles that benefit from its instant torque transfer response that are generally traveling far in excess of 30 mphs. And every single off-road or full time 4wd vehicle on the market includes a mech LSD or locking differential in the rear and center .....other than Toyota.

    So don't take my word for it...just look at what the rest of the marketplace is doing. They're not installing LSDs because the customer demands it since most customer's don't even know what it is. They're installing it because it provides better traction under all conditions, is more reliable and causes less wear and tear on brakes and rotors than a brake driven torque transfer system. It simply distributes torque better than any computer/brake drive system since its based on simple physics vs complex computer algorithyms. Which explains why Toyota has migrated to center LSDs on their drivetrains with higher power. Now they just need to fully match the competition by adding a rear mech LSD to those higher end vehicles that deserve it.

    BTW can you name a high end performance sedan or a serious AWD or 4wd vehicle that doesn't have a mech LSD in the rear (other than Toyota)? I know I can't but maybe you can help identify another manifacturer that views LSDs the same as the views above.
  • wwestwwest Posts: 10,706
    In the early sixties when LSDs were just beginning to become available I happened to live in North Central Montana, so yes I have lots of experience with both mechanical LSDs and virtual ones.

    Going back a few years we lived up in the woods (Seattle eastside) at the end of a two mile dirt and gravel road (mostly dirt [MUD!!] for the last mile or so) with several fairly steep inclines. That is when I began my second experience with LSDs. We purchased a 1985 Jeep Cherokee LTD to be sure we could get in and out of that road in all conditions. It took chains on all four wheels several times but we always made it through. I later traded up to a '92 Jeep Cherokee LTD with the same RWD/AWD/4WD/rear LSD capabilities as the '85.

    In 2000 the '92 was retired to the ranch in MT (still "going" strong even today) in favor of a '00 AWD RX300. The '00 RX was shortly traded in for an '01 AWD RX300 that had HID, GPS/nav, VSC, Trac etc. Like the '00 the center diff'l of the '01 has a VC, viscous coupling, to "stiffen" the otherwise open center diff'l when disparate F/R wheelspin/slip develops.

    Oh, whereas the '00 RX had a mechanical rear LSD the '01 RX uses TC to implement a virtual rear LSD. As a matter of fact the center VC turns out to be dead weight since TC also implements a center virtual LSD. The VC was dropped from the RX series beginning with the 04 model year.

    And...

    My '01 911/996 C4, which I drove home in 4" of snow just the other night, has a TC (PSM) implemented virtual rear LSD of which I am very pleased. Currently there seems to be a lot of 911/996 owners who have added a mechanical LSD without realizing they need to find a way to disable the virtual one otherwise the mechanical one is just dead weight.

    And I would be VERY surprised if the ML320 or BMW X3/5 has anything but a
    virtual LSD.

    "Name a high end performance sedan or a serious AWD or 4WD vehicle that doesn't..."

    Frankly I don't know of any high end performance sedan that I can say for sure either way...except for Lexus using the virtual one but that's just another Toyota. But I serious doubt that it matters to those buyers one way or another, or if they even "know".

    IMMHO there is NO SUCH thing as a "serious" SUV with AWD and not 4WD or 4X4 mode also. No self-respecting "serious" SUV would stoop to that level. But I will grant you that if it has a 4WD or 4X4 mode then it is marketed, at least partially so, to the off-road crowd and therefore it very likely has a mechanical rear LSD.

    Be advised that I am in no way denigrating mechanical LSDs, just that they have their place and that is NOT in a general public market vehicle.

    Hmmm...

    I wonder if the PSM equipped Porsche Cayenne has a mechanical LSD.....I think probably not.

    Now, there is ONE SERIOUS 4WD SUV...!!

    But useless, overall, as teats on a bull.
  • hdfatboyhdfatboy Posts: 324
    Good discussion to better understand LSDs and their value .

    "In 2000 the '92 was retired to the ranch in MT (still "going" strong even today) in favor of a '00 AWD RX300. The '00 RX was shortly traded in for an '01 AWD RX300 that had HID, GPS/nav, VSC, Trac etc. Like the '00 the center diff'l of the '01 has a VC, viscous coupling, to "stiffen" the otherwise open center diff'l when disparate F/R wheelspin/slip develops.

    Oh, whereas the '00 RX had a mechanical rear LSD the '01 RX uses TC to implement a virtual rear LSD. As a matter of fact the center VC turns out to be dead weight since TC also implements a center virtual LSD. The VC was dropped from the RX series beginning with the 04 model year."


    The 2008 Lexus RX AWD has a center LSD (VC) and an open rear differential. (Its a Toyota and I don't believe Toyota uses a rear LSD in any of their vehicles including trucks, which I believe is a competitive mistake.)

    "My '01 911/996 C4, which I drove home in 4" of snow just the other night, has a TC (PSM) implemented virtual rear LSD of which I am very pleased. Currently there seems to be a lot of 911/996 owners who have added a mechanical LSD without realizing they need to find a way to disable the virtual one otherwise the mechanical one is just dead weight."

    All versions of the 2008 Porsche 911 (Turbo, Carrera, GT3, Cabriolet) have a mech rear LSD. Various models can be upgraded to include an auto-locking rear diff.

    "And I would be VERY surprised if the ML320 or BMW X3/5 has anything but a
    virtual LSD."


    "IMMHO there is NO SUCH thing as a "serious" SUV with AWD and not 4WD or 4X4 mode also. No self-respecting "serious" SUV would stoop to that level. But I will grant you that if it has a 4WD or 4X4 mode then it is marketed, at least partially so, to the off-road crowd and therefore it very likely has a mechanical rear LSD."

    By "serious" AWD I simply meant with sophisticated awd designs meant to manage all available power to avoid slippage. Toyota's approach is to generally reduce the power through engine management to compensate for slippage (in addition to transferring some power by brake management).

    Audi, Subaru, GM and others take the approach of more agressively transferring torque through the use of mech LSDs to find a non-slipping tire with a less agressive engine management to reduce power. I prefer to manage the power as a driver and want the vehicle to figure out which wheel can best utilize that available power. Toyota assumes the driver doesn't know what's best and decides for them through more agressive engine management.

    "Be advised that I am in no way denigrating mechanical LSDs, just that they have their place and that is NOT in a general public market vehicle. "
    "I wonder if the PSM equipped Porsche Cayenne has a mechanical LSD.....I think probably not.
    Now, there is ONE SERIOUS 4WD SUV...!!"


    I'm not sure I would consider the Cayenne a serious suv however it is certainly a serious performance vehicle. The 08 Cayenne has a center LSD and a rear mechanical auto-locking differential.

    Here's a few vehicles and what their designers thought were the best designs for traction and power distribution:
    2008 MB ML & GL uses open diff. designs plus the ABS brakes to transfer torque. They can be upgraded with locking rear differentials. If you want the AMG versions, both models will be upgraded with limited slip rear differentials.
    2008 BMW X5 comes with a ctr LSD and an open rear diff. You can upgrade to a rear LSD for $625 and all M versions of any BMW come standard with a rear LSD.
    2008 Subaru Forester, Outback, Tribeca, Impreza all have mech rear LSDs and a ctr LSD (VC).
    2008 Audi Quattro's R8, S8, A4, A6 have mech rear LSDs.
    All of GM's Full size suvs either have a rear mech LSD or offer it as an option including the new Tahoe Hybrid which comes std with a rear mech LSD.

    I think I've shared that there are many on-road vehicles with mech LSDs and in fact most of the higher end vehicles either come with mech rear LSDs std or offer them as options.
  • wwestwwest Posts: 10,706
    "The 2008 Lexus RX AWD has a center LSD (VC)...."

    NOT...!!!!!

    When the RX330 first came out all of the maketing materials, printed and inline, indicated it had, like the previous RX300, a VC, Viscous Coupling, across the center open diff'l.

    It did not and I have an email from Lexus corporate agreeing with me and apologizing for the miss information.

    Now all of the maketing materials for the RX350 indicates it uses the VC.

    It does NOT...!

    I have opened email discussions with Lexus about this falsification on three separate occassions. Each time I am asked to convert the discussion to verbal via telephone and each time I have ended up being told that I will recieve a retrun call with a definite answer with 48 hours. No such return call has ever been made.

    The NCF, New Car Features, at techinfo.toyota.com, for the first model year of the RX350 indicates a return to the MF2AV transfer as was used in the RX300. But all of the other documentation, repair manual, etc, for that same initial model year and all following years indicate that the MF2A transfer is used. A search for the term MF2AV for any RX350 model year yields no results whereas a search for MF2A
    in those years yeilds dozens.

    In addition I recently tried to buy a new VC "canister", from an online source I often use for Lexus parts. I gave a valid VIN number for a 2008 AWD RX350 but got no response. On the same day I ordered parts, same online source, for my 1992 Lexus LS400 and got a response within 24 hours.

    I don't have a clue as to why Lexus might or maybe would be falsifying, continuing to falsify, marketing information about the AWD RX series in this way.
  • wwestwwest Posts: 10,706
    Given what I see out in on the public streets when the weather turns adverse, snow covered, icy roadbeds, I'm not so sure anymore that quick dethrottling isn't a good idea, a DAMN good one regardless of the drive method.

    Look at it this way, even if you are driving a 4X4 what would any experienced or knowledgeable driver do if all four wheels began spinning on ice or packed snow due to the lack of enough traction to move up an incline.

    You'd STOP what you're doing, scratch your head and say "what can I do to add more traction?"

    But how do we begin to get the general public to understand that, absent it being FORCED on them via engine dethrottling..??

    Yes, spinning your wheels will often help to get you up and moving initially, but what I seem to be noticeing is far too many drivers simply "burning rubber" with little or no hope of moving up even the smallest of slippery roadbed inclines.

    So, what I see coming, soon, is that all vehicles will have a dethrotting "feature" to prevent such actions by unknowledgeable and/or inexperienced drivers.

    I often see circumstances wherein NOTHING will get a vehicle moving forward, not even a 4X4 drive system, other than when equipped tire studs or tire chains. And in that case, studs or chains, RWD will often easily suffice.

    I am totally opposed to tire studs as a result of the damage they do to our highways when they have no use anyway, so I always carry a set of tire chains, two during the winter months.
  • hdfatboyhdfatboy Posts: 324
    All I can do is share the same resources that are available to everyone that clearly state the 2008 RX AWD has an "all-new viscous liquid limited slip center differential".

    http://autos.yahoo.com/lexus_rx_350_awd-features/?p=brak
    http://consumerguideauto.howstuffworks.com/2008-lexus-rx-5.htm
    http://www.autobeat.com/index.cfm?page=4&INVENTORY_ID=2074143
    http://www.carsoup.com/new/detail.asp?year=2008&seriesid=2008600903&styleid=2008- 600903&vehicletypeid=1&makeid=24&modelid=20086009&mode=make&cont=1&redirtype=InB- ound&delta=1
    http://www.conceptcarz.com/vehicle/z14609/Lexus_RX_350.aspx

    I kinda doubt that all these publications got it wrong when they published that the 2008 RX has a viscous limited slip center differential. However if that's not enough evidence for you here's the formal press release from Toyota announcing the all-new center LSD in the 08 RX that was published to the entire media. http://pressroom.toyota.com/Releases/View?id=TYT2007082898955

    Lastly its rather hard to refute the clear evidence from the Lexus website that highlights the viscous center LSD as an important "Performance" feature of the 2008 RX. I kinda doubt the lawyers would let the marketing team publicly market a prominent feature that is not a part of the vehicle. http://www.lexus.com/models/RX/features/performance.html

    BTW, based on March 08 sales it looks like the sales of the RX and the GX are struggling abit http://pressroom.toyota.com/Releases/View?id=TYT2008040198482

    I think before making the claim that Toyota is falsifying their marketing of the 08 RX you might consider publishing a single thread of evidence from a reputable source or photos of an 08 ctr LSD that shows it to be an open design. My money is with the lawyers that would prevent Toyota from falsely publishing a major feature in press releases and their website (for any lengthy period of time) before taking it down quickly or sending out a correction.
  • wwestwwest Posts: 10,706
    Well, first of all they quite clearly falsified this claim for the RX330 series and I can provide you with a copy of the email making the admission.

    There is little need for referencing publications other than Lexus themselves as it seems quite clear to me that Lexus would be the base source of such information.

    http://www.lexus.com/models/RX/features/performance.html

    Now go back and read the very first paragraph under the bold title:

    "Full-Time All Wheel Drive with Vehicle Stability Control."

    But first, let me tell you that lots of Porsche 911/996 owners have gone out and installed mechanical rear LSDs onlyto discover them to be totally non-functional usless a way can be found to disable the factory supplied "virtaul" LSD using traction control firmware.

    VCs, couplings or clutches, are not instant acting. The VC in my '01 AWD RX300 takes tens of seconds to stiffen enogh with sustained wheelspin/slip before it will convey even 20% of the engine torque to the rear driveline. But the reality of the matter is that even the VC in my RX300 is completely non-function for real world aspects.

    The TC, Traction Control, system in my '01 will instantly apply braking to any wheel or wheels that slip due to engine torque and just as quickly dethrottle the engine. Apparently teh RX330 and RX350 will do the same thing. The only real difference being my TC dethrottles the engine via EFI control, fuel starvation, whereas the others have DBW, e-throttles.

    But read the paragraph that I referenced and see if you can make any sense of the wording. For me, the described operation of Trac makes the viability of having a functional VC non-functional.
  • hdfatboyhdfatboy Posts: 324
    I'm not following your logic. First... do you agree that the 08 RX has a viscous center LSD? You can't really believe that a mistated email that WWest received is somehow more reliable and accurate than the Lexus website AND the formal launch press release which went through significant copy control and legal review before publication?

    Of course mistakes do happen, however even if the website was mistaken at first the lawyers would have required a correction immediately. Since the Lexus website clearly states that the new RX has a ctr LSD and it has been on the website since launch (nearly 6 months), I think most reasonable people would agree that the new RX includes a viscous ctr LSD in the absence of physical proof or a reputable source that suggests otherwise..

    I see no conflict between the software advantages of "selective braking" and LSDs in a drivetrain. In fact, its really the best of both worlds. By incorporating the LSDs you get real time torque transfer at the moment its needed without any speed limitations, wearing of brake pads, potential warping of rotors or overly agressive engine power reductions.

    The physics of an LSD manage the torque transfer seemlessly across the various corners. By having an LSD in the center and the rear, it enables the software developers to dial back the VSC and TC to levels that would only be activated if the LSDs were overwhelmed by speed, traction, torque transfer, driver error or some combination. This would make the VSC and TC the safety features they were originally intended to be instead of a cheap man's version of a mech. LSD.

    The very best high performance vehicles and the best AWD systems layer the mechanical approach with the computer approach to maximize torque from the engine being distributed to the tires that can best use them. If the power is more than can be distributed than the safety features of VSC and TC step in to protect the driver.

    That's why you are probably correct that the guy's adding LSDs to vehicles like 911s that had open diffs with VSC and TC may be wasting their time unless they disable the VSC or modify the programming to acommodate the LSD. That's why its best to have the LSD from the factory if the vehicle has VSC and TC.

    That is my personal preference and the best vehicles in the world take that approach to torque management. Toyota attempts to jump straight from "torque management" to "driver management" by skipping the physics of an LSD (as well as the cost advantage) and instead use software to manage the driver.

    Toyota did make some progress in their thinking by inserting ctr LSDs back into their drivetrain lineups for many of their models in 2008. Now they just need to catch up to the best manfuacturers of 4wd and AWD vehicles by adding rear mech LSDs, at least for their premium products.
  • wwestwwest Posts: 10,706
    First, again, if you are able to get your hands on an early, first year, RX330 sales brochure you will find that it "touts" the fact that the RX330 has a VC. Not so with the brochures for the following years of the RX330. And as for this matter, no VC for the entire RX330 product run, is quite well documented at techinfo.toyota.com.

    "no conflict.."

    I really can't speak, directly, for an RX model but my own. On a packed snow or icy parking lot my RX's Trac system will quickly abort even the slightest level of wheelspin/slip due to engine torque. The brakes are almost instantly applied and the engine is dethrottled via fuel starvation. MY '01 does not have DBW so at first I didn't know how the dethrottling was accomlished but the shop/repair manuals for my '01 indicate it is via fuel starvation.

    Viscous clutches and couplings do not react anywhere near instantly to desparate rotational rates at the two inputs. That's actually their greatest advantage in this particular application. There is very little VC functionality to interfere with the need for the front wheels to be free to rotate independently of the rear when turning. The onset of VC "stiffening" depends on the formulation of the viscous fluid, basically how fast its volume (tries too) expand due to the heat of "stirring".

    If these is no ability for sustaining an extended period of disparate rotation of the two inputs to the VC then the VC will never become functional.

    This, the above, is the basis for my firm belief about why the VC was dropped for the entire RX series with the introduction of RX330 model and is also not used on the AWD versions of the Highlander and the Sienna, although otherwise those AWD drive trains are an exacting duplication of the RX.

    But, I remain quite willing to be proved wrong.

    But the evidence available to me so far indicates, weights heavily, on the side that Lexus is intentionally misleading the buying public.

    As to "why", my only answer would be for you to study the Japanese culture.
  • hdfatboyhdfatboy Posts: 324
    I'm still perplexed by your references to past RX models. My original comment was specifically focused on the 08 RX which I stated has a viscous center limited slip differential. Your view was that I was incorrect and that the 08 RX 350 has an open open ctr differential. Do you now agree that the 08 RX AWD has has a ctr mech viscous LSD?

    My experience with viscous ctr LSDs is far different from yours. My recent experience is with GM's viscous ctr LSD and Toyota's Torsen ctr LSD. The response time of the viscous LSD in the GM Denali AWD is literally measured in fractions of a second as stated in their literature. You can actually feel the difference in a vehicle with a dedicated power distribution compared to a vehicle without a dedicated ctr LSD. For example, when accelerating through an off-on ramp you can actually feel the Denali "pulling" itself in addition to "pushing" itself through the curve.

    By having a ctr mech LSD the manufacturer can dedicate a certain % of power to the front wheels without having to worry abound binding in sharp turns. That's why the Tundra's 4wd system is parttime and the Sequoia's is full-time (Tundra has no ctr diff and the Sequoia has a ctr torsen LSD). A 4wd system or AWD system that does not have a ctr LSD cannot distribute torque with minimum power requirements which is a significant disadvantage for 4 wheel drivetrains with open ctr differentials.

    You will not enjoy this satisfying feeling in a 4wd vehicle that does not have a mechanical LSD. This same feeling can be felt with the Sequoia when 4wd is engaged since its power has a dedicated distribution of around 40Fr/60Rr in 4wd with the ctr diff unlocked.

    This same design philosphy equally applies to a rear mech LSD where transferring torque from one side to the other is the priority unlike a ctr diff which is addressing torque transfer from front to rear.

    The new 08 RX330 with a viscous ctr LSD has this advantage of "guaranteed" power to both ends of the vehicle when in 4wd. This article describes the RX's new viscous ctr LSD as delivering a balanced 50/50 power distribution until slippage occurs.
    http://www.conceptcarz.com/vehicle/z14609/Lexus_RX_350.aspx
  • wwestwwest Posts: 10,706
    I suspect that you would acknowledge that a simple center open diff'l will most adequately distribute torque 50/50 until slippage occurs. Just as long as both front and rear drivelines represent roughly equal resistance to the engine's applied turning force a simple OPEN diff'l will work perfectly well to distribute torque 50/50.

    But no, my stated position is that I DO NOT believe ANY RX350 has a VC involved with the operation of the center open diff'l, including the 2008 model. And for that matter not even the new 2009 model.

    Viscous clutch and coupling fluids can be formulated in an infinite number of ways. Some manufacturers even include a small amount of gas, a "gas" bubble, within the hermetically sealed fluid container, so as to delay the onset of the coupling coeffficient as the temperature of the fluid rises due to the "stirring" of the fluid by the clutch plates.

    Suppose, for whatever reason, I drive my VC equipped '01 AWD RX300 in a very tight circle, as fast as is reasonable, continously for say, three revolutions. At what level might the coupling coefficient of the VC fluid rise to in that instance?

    Frankly, I do not, cannot, believe that any Full-Time AWD system can be constructed using a center diff'l VC technique and have coupling coefficient responses on the order of anything less than multiples of seconds. And frankly, what is, where is the NEED..??

    Think of the hazard that might result from driveline "binding" due to a quick and tight VC coupling coefficient as I enter a tight turn, even an accelerating tight turn, on a slightly slippery surface.
  • wwestwwest Posts: 10,706
    "For example, when accelerating through an off-ramp you can actually feel the Denali "pulling" itself in addition to "pushing" itself through the curve."

    That's nice...

    I have flown Cessna 337s, that's the in line thrust push-me-pull-ya Cessna that has a rear engine and prop and a front engine and prop. Not only could I not ever get a "feel" for which engine might be pushing or pulling but the airplane had a serious flaw in the early days because pilots could not readily tell, sense, that the rear engine was not even turning. Later models had a torque sensor at the rear and a light on the instrument panel to indicate to the pilot that the rear engine was not operational.

    But I admit readily that I might be unique in that I could not sense, as you seemingly can, if the vehicle, or airplane, is being pushed or pulled, regardless of attitude, turning, climbing, whatever.

    But even more interestingly, it turns out that the GMC Denali, like the 2008 AWD RX300, has a full-time AWD system which consists of a simple open center differential and a TC, Traction Control system that is used to implement a "virtual" center LSD.

    So maybe you don't have the "seat-of-the-pants" sensor feel of pushing and pulling of the Denali that you thought you had/have, maybe it was your imagination, and a bit of wishful thinking, all along.
  • tidestertidester Posts: 10,109
    "For example, when accelerating through an off-ramp ..."

    I think that is unwise! You would normally be decelerating on an off ramp. An on ramp is the place to accelerate. :P

    The OP referred to an "off-on ramp" so he has himself covered so we should probably take care to quote people accurately. :)

    But seriously, I'm not sure there would be any detectable or discernable difference between a push and a pull. I'm just wondering whether a curve or change in elevation on the ramp may be misinterpreted as a push-pull effect.

    tidester, host
    SUVs and Smart Shopper
  • hdfatboyhdfatboy Posts: 324
    I am quite confident in the sense of "pull" vs a normal 2wd vehicle when accelerating through a curve. Having come from previous 2wd (PT 4wd) Subs to the same vehicle in AWD (GMC Denali XL), I am very confident in my ability to distinguish handling characteristics that are different and noteworthy.

    I believe this to be a "seat of the pants" difference that anyone would notice coming from a rwd vehicle to an awd vehicle that has a dedicated minimum power distribution from front to rear. The only way to have such a dedicated power distribution is to have some type of mech ctr LSD that can be setup to share power continuously between the front and rear drive wheels. The Sequoia uses a Torsen ctr LSD and the GMC Denali uses viscous ctr LSD.

    The Sequoia has a minimum power distribution of 30Fr/70Rr when operated in 4wd Hi with the Torsen ctr LSD unlocked. There is no question of a difference in handling in a curve when operating the Sequoia in 4wd (AWD) vs 2wd. Others might describe the feeling differently but the best way for me to describe it is to refer to the sense of the vehicle "pulling" itself as well as "pushing" through the curve. Perhaps professional drives use more accurate terms but even the most novice of drivers would notice the handling difference when a Sequoia is operated in 4WD Hi (AWD) vs 2wd.

    For those that have never driven an awd vehicle, you really don't know what you're missing. Check it out by test driving a real awd. IMO if a vehicle is marketed as awd but doesn't have a dedicated minimum power to both ends of the vehicle, it doesn't really have awd. Since the only way to have awd with dedicated power to both ends is to have a mech ctr LSD, all vehicles with an open ctr differential (regardless of their use of brakes to transfer torque) are lesser in their handling and traction abilities.
  • hdfatboyhdfatboy Posts: 324
    "I suspect that you would acknowledge that a simple center open diff'l will most adequately distribute torque 50/50 until slippage occurs. Just as long as both front and rear drivelines represent roughly equal resistance to the engine's applied turning force a simple OPEN diff'l will work perfectly well to distribute torque 50/50.".

    I do not believe this is true. Its not just a matter of slippage to change the torque distribution but load as well. Going through a turn will "unload" the weight on a particular corner of a vehicle and change the available traction on thate unsprung wheel vs the loaded wheel ina curve. That is one of the reason the better cars have a LSD to start with.

    "But no, my stated position is that I DO NOT believe ANY RX350 has a VC involved with the operation of the center open diff'l, including the 2008 model. And for that matter not even the new 2009 model. "

    There's really not much that can be said when someone has blinders on and refuses to believe the hard evidence from the actually manufacturer claiming that the 08 RX350 has a viscous ctr LSD. I suppose I should use my garden hose to fill my car's gas take because I believe it should run on water even though the manufacturer says to use gas. Can't help you with your refusal to believe the manufacturer's posted information in publications and their own website and press releases.

    "Frankly, I do not, cannot, believe that any Full-Time AWD system can be constructed using a center diff'l VC technique and have coupling coefficient responses on the order of anything less than multiples of seconds. And frankly, what is, where is the NEED..??"

    Once again, given your inability to believe any reputable resources that might suggest your view is incorrect, its not worth trying to prove that a viscous LSD transfers torque nearly instantly (which it does).

    "Think of the hazard that might result from driveline "binding" due to a quick and tight VC coupling coefficient as I enter a tight turn, even an accelerating tight turn, on a slightly slippery surface."

    That's why its called an LSD. Both Torsen and a viscous LSD do not "lock" and therefore cannot bind since they do not transfer all available torque from one side to the other. That's why they are able to operate in dry conditions where a PT 4wd system cannot.
  • hdfatboyhdfatboy Posts: 324
    "But even more interestingly, it turns out that the GMC Denali, like the 2008 AWD RX300, has a full-time AWD system which consists of a simple open center differential and a TC, Traction Control system that is used to implement a "virtual" center LSD.

    So maybe you don't have the "seat-of-the-pants" sensor feel of pushing and pulling of the Denali that you thought you had/have, maybe it was your imagination, and a bit of wishful thinking, all along. "


    The only flying by the "seat of the pants" appears to be in the claims you make. You really don't appear to like facts.... do you?

    The 08 Lexus RX350 clearly has a viscous center LSD as described in technical detail in the many sources I shared earlier including the Lexus website and formal launch press releases. But hey...you seem to want to be a disbeliever in the facts.

    Here's another fact for you to disbelieve. The 2001 GMC Denali XL has an AWD drivetrain that uses a viscous center LSD that has a minimum power distribution of 38Fr/62Rr and can transfer power upto a 50/50 distribution as needed. The Denali XL also has a mechanical auto-locking rear differential to ensure maximum torque is distributed across the rear wheels to the tire with the greatest traction. http://www.edmunds.com/used/2001/gmc/yukonxl/100001202/standard.htm

    IMO, for on-road use, this is the absolutely best available drivetrain design for maximizing traction at all 4 corners of a vehicle. The new Sequoia gets a close second place and would have been even better had Toyota elected to include a locking or mech LSD for the rear to go with the center Torsen LSD.
  • harboharbo Posts: 136
    How about the Audi system's?
  • wwestwwest Posts: 10,706
    This was, is, a bit puzzling. Every Google response I got from:

    GMC denali AWD viscous

    Indicated the Denali's, Envoy, SUV, PU, had a VC type center LSD, dealers, automotive publications, ALL.

    Then I dropped "viscous" from the search and discovered that there is NO official GMC site that even mentions the word viscous with regards to the Denalis' AWD system. Instead some of them, GMC itself especially, go into great detail in describing how the traction control system is used to allocate, apportion, engine torque in accordance with traction at individual corners.

    Professional drivers....

    About two years ago now I had the opportunity to drive my '01 911/996 C4 on the track at SIR, now known as PIR, Pacific International Raceway. The C4 uses a VC mounted in series with the driveline to the front wheel to allocate up to 45% of the engine torque to the front with a sustained period, even discontinuous, of wheelspin/slip.

    Instead of driving myself I opted to be a passenger while the "track master", Don Kitch, drove my 911. During the initial two loops around the track at what I considered sub-standard speeds, not pushing it, I begin to think Don was being overly considered of my "nerves". Then I noticed, a couple of times, a slight look of confusion on Don's face. After the second loop I pointed out that my 911 was AWD and believe me, the following loops around the course had a completely different "tenor"(??)

    When we finally parked the 911 you could smell the brakes still "cooking" from at least 100 yards away. I subsequently had the opportunity to drive my C4 on the track at Daytona. But with no previous experience one way or another with AWD all I can tell you is that the drive was a complete and total THRILL and if the front wheels were ever really "pulling" me through the turns I couldn't tell.

    Don's C.V. can be found at teamseattle.com

    hdfatboy:

    But the bottom line is that your apparent lack of understanding of my use of "seat-of-the-pants" sensor, and a few other matters, indicates quite strongly that we are holding this dissertation on two different, ENTIRELY different, levels, so I leave it to you alone to continue this particular RX350/Denali AWD discussion.

    Go for it...!!
  • wwestwwest Posts: 10,706
    With the sole exception of the SH-AWD system I believe that ALL modern day AWD systems derived from an original FWD platform remain, primarily, FWD, or most certainly front torque biased under normal operating conditions.

    It has not been that many years ago that the general public began to wise up to the fact that the only advantages of FWD were on the side of the manufacturers, costs. As a result while some manufacturers decided to abandon the FWD venue entirely, Cadillac, many others stuck a compromise and found an easy and inexpensive way to re label these patently unsafe FWD vehicles as FWD.

    So far the public, in general, seems to be none the wiser.

    Not speaking directly about Audi, but many of these, say the Lexus RX series, along with their brotheren (sisters??), the Highlander and Sienna, are AWD in name only, only for purposes of marketing to a general public that has not yet fully "wised up" to the industry's deceptive marketing practices.

    If you want, and/or need, a fully functional, TRUE, AWD vehicle, a truly SAFE AWD vehicle, then look for one that "boosts" of rear torque biasing in normal operating mode. For many years I was perfectly satisfied to accomplish my need for adverse traction, "AWD" with the part-time models, 4WD/4X4 Jeeps. But since there is no real detriment today to having a full-time AWD system, a TRUE and SAFE AWD system, I opted to move on.
  • hdfatboyhdfatboy Posts: 324
    The GMC Denali XL was launched in 2001 with a viscous ctr lsd combined with a locking mechanical rear differential. This is the vehicle I still own (although currently selling). I believe it was the 2004 model year of the Denali that they moved from the viscous ctr lsd to a mech clutch-type ctr LSD.

    I agree with you that most AWD vehicles (although defintely not all) are using awd as a marketing tool versus an engineering advantage. Most lower end unibody awd vehicles are conversions from fwd and have an open ctr diff. In my opinion these vehicles are not really awd. I would classify these vehicles as fwd vehicle with a TC system that transfers some torque to the rear wheels but only after the front wheels slip.

    I disagree that most awd vehicles are derived from fwd vehicles. The better vehicles are actually rwd vehicles with awd added. This is true for Porsche and many other higher end vehicles

    Real AWD (IMO) is a vehicle with a torque bias to the rear and a minimum % of torque to the front wheels at all times with an lsd in between. The only way these vehicles can have power to all 4 wheels all the time is to have a mech ctr lsd. Vehicles that generally meet this standard are Subaru, Audi and GM vehicles labeled as AWD, although there may be other specific models by other manufacturers.

    Many of the newer awd vehicles have used the credibility of these awd manufacturers to call their vehicles awd while really only offering fwd (or rwd) with limited or no power going to the other end unless there's slippage. The result is a less then capable (but less expensive to make) awd vehicle than one that has a mech LSD in the ctr and the rear.

    I think it was you in fact that shared an excellent video which demonstrates the real world outcome difference between awd vehicles with a mech LSD vs an open ctr diff that tries to distribute torque through engine management and wheel braking. An open system that only distrubutes torque through brakemanagement is clearly not as effective as a system that combines these electronic approaches with mech lsds in the drivetrain.

    http://youtube.com/watch?v=t09ExAUgtyE&feature=related
  • tidestertidester Posts: 10,109
    anyone would notice coming from a rwd vehicle to an awd

    It's been a really long time since I've driven a rwd vehicle. I do remember that 4wd and rwd do feel different so we would only differ in the words we would use to describe it.

    tidester, host
    SUVs and Smart Shopper
  • wwestwwest Posts: 10,706
    I really don't see any real problem with using the brakes to implement "virtual" LSD, rear, center, and even a "light/soft" front LSD.

    IMMHO the problem arises mostly with front torque biased AWD systems, or even AWD systems that are really only FWD with maybe a modicum of rear drive torque ability.

    Think about it, do you really want a high level of engine drive torque on those front wheels when they are already "loaded up" with lateral forces..?? Accelerating into a turn with the surface adhesion questionable..?? Or even a low level of engine torque, leading OR lagging(engine compression braking) on a slippery roadbed...?

    NOT...!!

    Basically that's why TC is so very quick acting on modern day FWD vehicles. It can grow quickly dangerous with torque driven wheelspin/slip on a FWD or F/AWD due to the fact that with the onset of wheelspin/slip also comes the fact that the driver has now also lost all directional control of the vehicle.

    Toyota and Lexus have just recently announced a new TC feature for their R/AWD vehicles that points this out. The Toyota and Lexus R/AWD, rear torque biased AWD, vehicles now come with a TC "sub-mode". When the driver switches into the new sub-mode of TC an extended period of wheelspin/slip with virtual LSD functionality still fully active, will be allowed without engine dethrottling. The only detriment to this new feature, technique, might be brake rotor overheating and subsequent warping. The upper limit of this new operational mode might be the shutoff of the ABS' fractional HP pumpmotor due to the potential for it to overheat and fail if allowed, required to operate continuously for more than a minute or so.

    I haven't yet seen any indication that this new TC sub-mode will be made available on any Toyota or Lexus FWD or F/AWD vehicle. It's likely that the resulting liability would simply be too great.

    Oh, my 1994 AWD Ford Aerostar has normal operational torque distribution of 30/70 F/R and then switches to 50/50 with detection of disparate F/R driveline rotational rates. I'm pretty sure this is accomplished by having slightly different F/R drive ratios in the 30/70 mode, the rear driveline is lower "geared".

    A good analogy is to think about how the open diff'l of a RWD vehicle would work, operate, with one tire's circumference being smaller than it's opposite. Which tire would bring the most "rubber to the road"..??
  • hdfatboyhdfatboy Posts: 324
    "It's been a really long time since I've driven a rwd vehicle. I do remember that 4wd and rwd do feel different so we would only differ in the words we would use to describe it."

    Fair enough...I'm not sure what the right words are for describing the difference between how an awd vehicle with "forced" torque distribution (35/65 for example) feels in a curve vs how a rwd vehicle feels but its definitely noticeable and gives the driver a greater sense of confidence in the vehicle's handling.

    I described it as a sense of "pulling" + "pushing" through the curve, but I'm sure there are better terms. In any case, its a desirable feeling of control that is greater in a "real " awd vehicle vs a rwd or open ctr diff 4wd vehicle. For those that haven't experienced it, its worth the test drive in a Subaru, Quattro or GM AWD to feel the difference. You won't want to go back to a non-awd vehicle after experiencing it.

    In fact, I think given my driving experience in the AWD Denali, I probably would not have settled for the Sequoia if it had not been designed with a Torsen LSD for operation when in 4wd hi.
  • hdfatboyhdfatboy Posts: 324
    "I really don't see any real problem with using the brakes to implement "virtual" LSD, rear, center, and even a "light/soft" front LSD."

    I absolutely agree. I just don't think its a replacement for a mech LSD and its a definite advantage to have it in addition to a mech LSD. Order of on-road benefit to the driver from lowest to highest for maximum traction with mid to high engine power:
    fwd
    rwd
    rwd + rear e-lsd (open rear diff)
    rwd + rear mech lsd
    awd front bias + e-lsd (open ctr and rear diff)
    awd rear bias + e-lsd (open ctr and rear diff)
    awd forced minimum torque 35/65 + mech ctr lsd (open rear diff) + e-lsd
    awd forced minimum torque 35/65 + mech ctr lsd + mech lsd or locking rear diff. +e-lsd
  • wwestwwest Posts: 10,706
    From techinfo.toyota.com.

    The Sequoia's H4F mode, High Speed/ 4WD / "Free" (Diff'l NOT locked), can be used Full-time, regardless of road conditions, and the center LSD function (AWD functionality) is accomplished via TC, Traction Control braking.

    The center diff'l used in H4F mode is a standard, simple, open type wherein engine torque distribution, absent other means/methods, will ALWAYS be 50/50 as long as the loading of both front and rear drivelines are roughly equal.

    The TC firmware is specifically designed to take into account the relative slight front/rear driveline rotational difference due to turning and accordingly will not activate braking for torque apportioning, even with an extremely tight turning radius (probably uses the VSC's stearing wheel rotational position sensor as input for this control.).

    In H4L and L4L modes the center diff'l is locked and therefore these modes should NEVER be used for a prolonged period on a predominantly high traction surface.
  • hdfatboyhdfatboy Posts: 324
    Your source for this info would appear to be as out- of-date as it was for the information it provided you on the 08 RX. The 08 Sequoia has a Torsen center differential with the power distrubuted 40/60. This minimum torque distribution can only be attained with a mechanical ctr LSD as noted earlier. The official product literature and the official Toyota Sequoia Press Release clearly announce and promote the Torsen ctr differential in the 08 Sequoia (it was also announced as newly added to the Land Cruiser and the LX570 which I would also guess your source shows as having an open diff since the LC, LX and Sequoia have nearly identical drivetrains).

    08 Sequoia Launch Press Release quote "On four-wheel-drive models, a two-speed transfer case that contains a lockable Torsen limited-slip differential is used to transmit power to both the front and rear wheels." http://pressroom.toyota.com/Releases/View?id=TYT2007110908452

    08 LC Launch Press Release quote "The Land Cruiser is the first Toyota vehicle to use a newly-developed JF2A transfer case to provide full-time four-wheel drive. This lightweight, compact, chain-driven unit offers a standard 1:1 high ratio for highway travel and a low-range 2.618:1 ratio for traversing challenging driving surfaces. The transfer case employs a Torsen limited-slip locking center differential. The locking function is actuated by a push-button switch. High and low ranges are selected with a rotary dial, located next to the HVAC and audio control panels. Indicator lights in the right-hand combination meter on the Land Cruiser's dash panel indicate when low range and/or center lock is selected." My note: the LC was first because it was launched 1 month before the LX and 2 months before the Sequoia. http://pressroom.toyota.com/Releases/View?id=TYT2007091764006

    08 LX570 Toyota Launch Press Release Quote "A TORSEN® limited-slip locking center differential distributes the power 40:60 front-to-rear, directing more power to the wheels with the best grip should slippage occur." http://pressroom.toyota.com/Releases/View?id=TYT2007121777191

    My guess is that the Toyota technical site you are checking is just out-of-date by 6-9 months. Check the ctr diffs for the LC and LX. If the site is wrong about the RX, Sequoia, LC & LX I think you would have to agree that its not a reliable source of technical data on very recently launched products with substantial engineering changes such as those on the RX, Sequoia, LC & LX (at least when considering drivetrain changes).
  • tidestertidester Posts: 10,109
    but its definitely noticeable and gives the driver a greater sense of confidence in the vehicle's handling.

    I will agree with that!

    tidester, host
    SUVs and Smart Shopper
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