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Toyota 4WD systems explained

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Comments

  • 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: 10,110
    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: 10,110
    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
  • 2toyotas2toyotas Posts: 104
    Techinfo.Toyota.com

    I have a subscription, and it clearly states the RX300 came out in 99 with a VC in the center differential. It was redesigned in 04, and the VC was taken out of the center diff., it was an open bevel gear design with a 50/50 split and TRAC to take care of slip. The RX350 was redesigned for 07 and the VC is now back in the center diff. It handles front to back and TRAC handles side to side. The Highlander has followed that pattern until it's redesign for 08, the VC is not back, it uses the same transfer unit as it did in 04 with an open center diff. I am puzzled as to why, because the V6 engine is the same, and the transmission is also the same. I believe the VC makes it a better AWD vehicle, by always keeping power to both axles, whereas the open center with just TRAC, can brake both wheels on one axle, hurting forward motion especially on inclines.

    As for the Sequoia, Wwest is reading the details from 2001 when it came out. In 2005 it got the same transfer case as the 2003 - current V6 4Runner which has a torsen center diff. 40/60 split normal, Up to 53/47 on rear wheel slip, and 29/71 on front wheel slip.

    Now as for your comment on the Denali being the best, and the Sequoia being second I have to disagree. A torsen diff is proactive, and a VC is reactive as far detecting wheel slip. Which means the torsen is better. The Sequoia has ATRAC, and the Denali has an automatic locking rear diff., and TRAC. The locking rear diff only works until 20 mph, and then it is all up to TRAC. The Denali has NO locking center diff, and no low range. The Sequoia has both, along with one of the best Traction Control systems, in line with Land Rover and Mercedes, the Denali couldn't keep up with the Sequoia's 4WD abilities ever. You keep knocking Toyota and their AWD and 4WD systems, are you forgetting the Land Cruiser? I think Toyota knows a little bit about AWD and 4WD.

    Last, as for the Subaru comparison, the Highlander has a Bevel gear center diff, which unlike a planetary center, can only split power 50/50, so when the front wheels started to spin TRAC should have braked them and power would have went to the rear. They definitely shut off TRAC when doing that test. Now when on the rollers, the open center sent all power to the front and TRAC was off and couldn't step in. TRAC just doesn't allow all that wheel spin.
  • wwestwwest Posts: 10,706
    No, I selected Toyota, Sequoia, 2008 and then searched for:

    4wd

    So the information I "quoted", found, at techinfo.toyota.com is for the 2008 Sequoia.

    2toyotas,

    Go back to the site and search the entire document set for MF2AV for the 2007, '08, and '09 RX350. Then search for MF2A and look at all the hits you get for all years of the RX350.

    For the 2007 introduction year of the RX350 there is documentation indicating that the VC has been re-adopted, changed from the MF2A in the RX330 to the MF2AV, VC "transfer" model for the RX350. When detailed pictorials of both transfers. The pictorial for the MF2AV is an exact replication of the transfer in the original RX300.

    And it isn't just the above. If you read Lexus description of the TC functionality it clearly flys in the face of also having a VC.

    Now I have no idea why GMC would mislead their dealers with regard to the center diff'l of the Denalis, or by default allow them to mislead the public, But it has become pretty clear to me that the direct GMC statements on the matter indicate that the denali series has a simple open center diff'l and uses TC for both LSD and AWD functionality.

    Then there is Lexus. In their case it is clear that Lexus itself is involved in misleading the public, Intentionally or accidentally, who knows at this point. Or maybe they're just being liberal with terms. If they stated that TC was being used to provide the functionality of a VC, quicker responding functionality, that would be much more acceptable in my mind.
  • wwestwwest Posts: 10,706
    2toyotas: "I believe the VC makes it a better AWD vehicle, by always keeping power to both axles, whereas the open center with just TRAC, can can brake both wheels on one axle, hurting forward motion especially on inclines."

    If the RX series were rear torque biased I would totally agree.

    In 2000 the FWD version of the RX300 had Trac, Traction Control, but the AWD version did not. In 2001 the AWD RX300 become equipped with both traction control and VSC. As I have stated before, the RX series is quite heavily front torque biased, ~95/5 F/R and as high as 75/25 F/R once the VC is given time to "stiffen". I don't really have any idea how the biasing is accomplished. I first verified the front biasing for myself, to my own satisfaction, using shade tree mechanic techniques. But so many posters expressed doubt that I had my testing verified on a 4 wheel dyno. The figures above come from that test.

    The fact that the AWD RX series is so heavily front torque biased forced Lexus, for safety reasons, to adopt Trac in the same way, for the same reasons, as existed on the FWD RX's.

    With such severe front torque biasing initial wheelspin/slip due to drive torque will normally develop at the front wheels FIRST. As we all already know, if wheelspin/slip at the front "stearing" wheels cannot be quickly abated, alleviated, loss of directional control quickly becomes a distinct threat. Since the VC is reactive, only responds AFTER a sustained period of F/R driveline rotation differences, VC CANNOT be depended on to provide this functionality.

    So, in effect, the AWD RX is really mostly FWD. So just as they did with the actual FWD for SAFETY reasons, Lexus is forced to adopt Trac for AWD and rear LSD fucntionality, engine dethrottling included.

    Side note: Keep in mind that the above is NOT a problem with R/AWD since it is the rear wheels that will normally develop spin/slip initially and that does not represent a direct threat to directional control. So the use of a VC to implement a center LSD is perfectly acceptable for R/AWD vehicles.

    So, as of 2001 Toyota/Lexus had themselves a real connubdrum, what was the VC to be used for, of what purpose did it now serve.

    NONE....!!

    Toyota, in fact, had no problem, no equal problem anyway. Neither the AWD Highlander nor the AWD Sienna had ever been advertised as having a VC. So those vehicles had a virtual duplicate of the RX's F/AWD system, but no VC, EVER.

    As the introduction of the RX330 approached, with the engineers having REMOVED the now uselss and otherwise non-functional VC, I can almost visualize the (heads down) non-argument amongst Lexus's japanese upper management personel.

    So, when the RX330 came to market without the VC, the marketing literature said otherwise. Now, big surprise, here we are again.
  • hdfatboyhdfatboy Posts: 324
    "Now as for your comment on the Denali being the best, and the Sequoia being second I have to disagree. A torsen diff is proactive, and a VC is reactive as far detecting wheel slip. Which means the torsen is better. The Sequoia has ATRAC, and the Denali has an automatic locking rear diff., and TRAC. The locking rear diff only works until 20 mph, and then it is all up to TRAC. The Denali has NO locking center diff, and no low range. The Sequoia has both, along with one of the best Traction Control systems, in line with Land Rover and Mercedes, the Denali couldn't keep up with the Sequoia's 4WD abilities ever. You keep knocking Toyota and their AWD and 4WD systems, are you forgetting the Land Cruiser? I think Toyota knows a little bit about AWD and 4WD"

    Let me clarify. I'm not knocking Toyota's 4wd drivetrain. In fact I just bought a new Sequoia with the new drivetrain as one of the features that made it attractive. That does not mean I view the Sequoia Drivetrain as the best on the market for primary on-road conditions and light duty off-roading. I do feel the Denali has a superior drietrain when operating on-road in 4wd (which of course is the only operating mode for the Denali.) The Sequoia has the significant advantage in that it has more flexibilitywith various other modes for different conditions.

    I also agree with you that a mechanical clutch type differential is superior to a viscous LSD. I think the GM engineers also agree with you because as I stated earlier, the Denali transitioned to a clutch type ctr differential in 2004 (I believe). The Denali had the viscous Ctr LSD from 2001 until they changed over. They've had a mechanical rear LSD or locking diff all along. They also now have much of the same stability software used by most SUV manufacturers to use the ABS brakes in reverse to make suvs more stable (GM's is called Stabilitrak). Alot of this stablility software was added as a result of the gov. applying rollover ratings to suvs.

    Since the COG of these vehicles wasn't likely to be lowered much (although ground clearances are definitely lower than they were a decade ago with the Sequoia being one of the highest in the industry at 10"), many manufacturers looked to the software engineers to see if the braking system and engine management computer could be used to keep drivers out of rollover situations..... ie VSC, e-LSD, Stabilitrak and a half dozen other aphabet soup names for this software.

    Does it help traction...yes to a point ...but then instead of improving torque transfer, it moves into "nanny" mode to keep the driver out of a bad situation. My only point is that by placing mech LSDs into the drivetrain (in addition to the software which almost all suvs manufacturers are now using), a vehicle can be designed to have better traction before the "nanny controls" take over.

    Open diffs are likely to have the nanny controls take over sooner which can be a PIA to the driver. I believe that's one of the reasons that Toyota included a Torsen ctr diff to the Sequoia, LC & LX. With the 401 ft lbs torque in the new 5.7 combined with open diffs, you'd have had a recipe for the nanny controls kicking in nearly everytime the driver hit the throttle hard.

    My only knock on Toyota is that with all the available power of the 5.7, they could have put more of it on the road with a mech LSD in the rear than giving it up to the "nanny controls". Which explains why the Sequoia has its best 0-60 times when all the controls are turned off and the vehicle is operated in 4wd hi mode. My guess is that if the Sequoia had a mech rear LSD its fastest 0-60 time would have been in 2wd with all the controls turned off.

    I'm not saying that 0-60 time is the best measure of performance for an suv but simply to demostrate the advanatges of mech LSDs with electronics versus open diffs with engine management and brake application to control wheel slippage.
  • hdfatboyhdfatboy Posts: 324
    "I have a subscription, and it clearly states the RX300 came out in 99 with a VC in the center differential. It was redesigned in 04, and the VC was taken out of the center diff., it was an open bevel gear design with a 50/50 split and TRAC to take care of slip. The RX350 was redesigned for 07 and the VC is now back in the center diff. It handles front to back and TRAC handles side to side. The Highlander has followed that pattern until it's redesign for 08, the VC is not back, it uses the same transfer unit as it did in 04 with an open center diff. I am puzzled as to why, because the V6 engine is the same, and the transmission is also the same. I believe the VC makes it a better AWD vehicle, by always keeping power to both axles, whereas the open center with just TRAC, can brake both wheels on one axle, hurting forward motion especially on inclines."

    I have no facts to backup the following guess on why the RX would have the viscous ctr LSD and the Highlander wouldn't...for what its worth.

    The RX is made in the US. The manufacturing costs are measured in $s. The Highlander is made in Japan. The manufacturing costs are measured in Yen. Toyota is probably losing their shirt on the vehicles made in Japan and sold in the US due to the $ - Yen exchange rate. Keeping costs as low as possible on the Highlander given the current economic cycle is key to profitability.

    The RX OTOH has more room to work with and a higher end image to maintain. Maintaining or improving the RX drivetrain is "cost effective" because its made in the US.

    Just one opinion on why the RX might have a better drivetrain than the Highlander.
  • hdfatboyhdfatboy Posts: 324
    "The fact that the AWD RX series is so heavily front torque biased forced Lexus, for safety reasons, to adopt Trac in the same way, for the same reasons, as existed on the FWD RX's."

    The 08 RX AWD is not fwd biased...at least not as Toyota describes it or promotes it. In fact Toyota describes the Torque distrbution in the 08 RX AWD as split 50/50 until wheel slippage occurs

    "The all-wheel drive RX 350 uses a viscous limited-slip center differential to enhance drivability in all types of driving conditions. The full-time AWD system works with TRAC to evenly distribute power to both axles with a constant 50/50 front-to-rear power split. If slippage occurs, the viscous coupling differential directs torque to the wheels with the most traction."
    http://pressroom.toyota.com/Releases/View?id=TYT2007082898955

    BTW here's the Toyota official Product features sheet for the 08 RX 350. You'll note it states " Center Differential (AWD)....Viscous coupling type limited slip differential"
    http://pressroom.toyota.com/presstxt/2008lexuskit/2008RX350_sf.pdf

    I thought you all might find this quote from the "Lexus Technology round-up" of interest. You'll note that it specifically references the torque transfer advantages of a mech LSD over "speed-sensing" e-LSDs (I interpret that to be electronic or "virtual" LSDs). This is exactly what I have been saying all along...that mech LSDs are superior in torque transfer over any type of software approach, as evidenced by their statement below about the LS600h Torsen ctr differential.

    "All-Wheel-Drive Passenger Cars

    The 2008 GS 350 and IS 250 models offer all-wheel-drive as an option. A planetary-gear center differential and a wet-type multi-disc clutch control power distribution. The system normally sends 70 percent of the power to the rear wheels to provide the traditional performance advantages of a rear-drive vehicle, but it will vary the torque split ratio from 30:70 to 50:50 in response to driving conditions and driver input. The system's electronic control strategy takes inputs from steering and throttle angle, combined with vehicle signals from wheel speed and yaw rate sensors.

    Power in the all-new LS 600h L flagship model is distributed by a newly developed full-time AWD system that delivers secure handling and traction in various driving and road conditions. A Torsen® planetary gear-type limited-slip differential (LSD) distributes torque 40:60 under most straight-line driving situations. The compact new Torsen® differential is 30 percent smaller and 11 lb lighter than previous Torsen® systems.

    Unlike speed-sensing LSDs used in some AWD vehicles, the TORSEN unit in the LS 600h L L is a full-time torque sensing, torque biasing system. Torque and differentiation are continuously managed between the front and rear wheels and biased instantaneously according to varying road conditions. As a result, power is automatically shifted to the wheel or wheels with the most traction even before wheel slip can"

    http://pressroom.toyota.com/Releases/View?id=TYT2007082869358
  • wwestwwest Posts: 10,706
    Here's the thing...

    Anyone buying a FWD or F/AWD vehicle, especially an SUV, very likely isn't wise enough, hasn't bothered to do enough research to fully understand just what capabilities, poor safety factors, are being purchased.

    So why shouldn't the manufacturers take advantage of this customer "class", haven't they been doing exactly that for YEARS..??

    Buyer Beware...!!

    Why would any manufacturer put a Torsen, Torque Sensing Differential, in what is basically a FWD vehicle to begin with??

    No, these customers are much better served via the use of TC for implementing AWD and rear LSD functionality. Absent the quick dethrottling of FWD and F/AWD modern day vehicles our insurance rates would likely be extraordinary.

    Oh, is there any such thing as a rear locking diff'l for this market segment. Drag racing or maybe other racing venues of maybe even serious off-road, but road going.

    NOT...!!!

    And anyone who believes the RX330 or RX350 F/AWD drive systems differ in any way other than firmware with those in teh Highlander and Sienna should go to the dealer's parts department and buy a spare VC "canister" immediately as I understand they are scarce as hen's teeth.

    Otherwise when yours fail Lexus will be scouring the earth trying to find a replacement.
  • wwestwwest Posts: 10,706
    From techinfo.toyota.com:

    2.Traction Control Operation
    The traction control system controls the engine torque, the hydraulic pressure of the driving wheel cylinders, slipping of the wheels which may occur at start or acceleration of the vehicle, to ensure an optimal driving power and vehicle stability corresponding to the road conditions.


    Anyone knowing, understanding, the operation of a viscous clutch or coupling would immediately see the conflict in the above operation of the RX350's Traction Control system were it to have a VC as described. Were the RX350 to actually have a VC mounted "across" the otherwise fully open center diff'l (as was the VC in the RX300) it could NEVER be functional given the apparently INSTANT intervention of TC (as SAFE operation of any FWD or F/awd REQUIRES) at any initial detection of wheelspin/slip due to engine torque.

    Now, when I purchased the 2000 AWD RX300, and then subsequently the 2001 AWD RX300 in order to get HID & VSC. Actually having read the 2001's shop/repair manual prior to purchase I was fairly certain TC was to be used to implement AWD and rear LSD, and quite possibly front LSD.

    At the time the Lexus sales persons seemed to be well aware that these RX300s were heavily front biased, and often touted same thinking it was an asset, a good sales point.

    With regards to the validity, trustworthiness, and truthfulness, of the documentation found at techinfo.toyota.com.

    I have the COMPLETE hardcopy versions of the shop/repair manuals for the '00 RX300, the '01 RX300, the 90, 91, & 92 LS400, the 2003 Prius, the RX400h, and the 05 RX330. These are the exact same manuals you will find at any Toyota or Lexus dealer for reference use of the shop technicians/mechanics.

    To my knowledge and experience the documentation at techinfo,toyota.com is exacting in replication of the actual hardcopy manuals. So, who, which do I trust to me the more truthful, Toyota/Lexus Press Releases or these documents.

    THE DOCUMENTS.
  • wwestwwest Posts: 10,706
    From techinfo.toyota.com:

    4WD SYSTEM

    DESCRIPTION

    * The 4WD model of the '04 RX330 has adopted a full-time system that constantly distributes torque at a ratio of 50:50 to the front and rear axles.

    * This system, which has adopted front and center differentials with bevel gears, employs TRAC control in order to ensure the proper drive when a wheel slips, with the use of an LSD (Limited Slip Differential) mechanism in the center differential. Thus, a lightweight system that offers high levels of driving stability and drivability. has been realized.


    I think the above is where the Lexus dealer sales persons got the idea that the RX330 had 50/50 F/R torque distribution vs 70/30 F/R for the RX300 series. In thoroughly reviewing all of the pertinent documentation for the center & front diff'l and the "transfer" across the RX300 product series vs the RX300 there is NO different other than the VC is not used in the RX330 transfer.

    And I certainly do not disagree that a simple open diff'l, as seems to be used throughout the years, all RX F/AWD models, will ALWAYS distribute torque equally to the two output drives just as long as both represent roughly the same level of torque loading, Large or small.

    I have no idea how the 95/5 F/R rear biasing ratio is accomplished other than the possibility of differing final drive ratios for the front axle vs the rear as is done on teh Honda/Acura SH-AWD system. On the SH-AWD system the rear driveline is overdriven (13%?/) (analogy: rear in 3rd gear vs 2nd in front). Until the rear left and right clutches are engaged the rear is just free-wheeling, with them engaged at a moderate level the SH-AWD system most definitely goes into rear torque biasing.

    If the RX's rear driveline is overdriven vs teh front then the front will receive the majority of the engine torque absent VC stiffening ('99-'00), or ('01-XX) if TC activates to prevent front wheelspin/slip.

    Oh, it appears that in order to prevent a severe level of torque stear with only left or right TC activation due to front wheelspin/slip BOTH front brakes are activated.
  • wwestwwest Posts: 10,706
    How front torque biasing is accomplished on the RX, HL, and Sienna.

    No input is needed/required if you believe the torque biasing is 50/50, we already understand the reasons for your position, understanding.
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