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Toyota 4WD systems explained

2487 messages,  Last post on Apr 28, 2012 at 12:48 PM

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#2207 of 2487 RX series front torque biasing. by wwest

Apr 04, 2008 (1:05 am)

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.

#2208 of 2487 Re: "08 Sequoia H4F mode vs H4L mode. [2toyotas] by hdfatboy

Apr 04, 2008 (8:54 am)

Replying to: 2toyotas (Apr 03, 2008 10:25 pm)
"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.

#2209 of 2487 Re: "08 Sequoia H4F mode vs H4L mode. [2toyotas] by hdfatboy

Apr 04, 2008 (9:05 am)

Replying to: 2toyotas (Apr 03, 2008 10:25 pm)
"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.

#2210 of 2487 Re: RX series front torque biasing. [wwest] by hdfatboy

Apr 04, 2008 (9:34 am)

Replying to: wwest (Apr 04, 2008 1:05 am)
"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

#2211 of 2487 Maybe... by wwest

Apr 04, 2008 (10:12 am)

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.

#2212 of 2487 2007 RX350 Traction Control by wwest

Apr 04, 2008 (11:07 am)

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.

#2213 of 2487 2004 RX330 4WD System by wwest

Apr 04, 2008 (11:35 am)

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.

#2214 of 2487 Does anyone know... by wwest

Apr 04, 2008 (11:41 am)

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.

#2215 of 2487 Re: Does anyone know... [wwest] by hdfatboy

Apr 04, 2008 (12:47 pm)

Replying to: wwest (Apr 04, 2008 11:41 am)
"No input is needed/required if you believe the torque biasing is 50/50, we already understand the reasons for your position, understanding."
 
And what would you say are the "reasons for my position"? Seems like you are really struggling with Toyota, not me. I don't have a position. To be honest, I really don't care whether a Highlander or RX, does or doesn't have a viscous ctr LSD. I have no interest in either of these vehicles from a personal perspective.
 
I'm simply sharing the readily accessible information that Toyota makes available to the public. The readers can decide for themselves based on the sources I shared as to whether Toyota's formal press releases and product information brochures are more or less accurate than the source of the information you're basing your view on.... that's not available to the public (without a fee).
 
For my money, I would go with the publicly available information from Toyota because I am very confident that these documents (particularly press releases) go through an extensive copy control process and legal review to avoid lawsuits. I also think that the tech files are likely to be somewhat behind the launch of a new product as all these vehicles are.
 
If you're frustrated, and it sounds like you are, its not with me. It must be with Toyota. You seem to keep referring back to documents that concern 7 & 8 year old Toyota vehicles. All of my posts have been about 2008 Toyota and Lexus vehicles.
 
Based on the information from Toyota's own publicly accessible website here's the facts as I read them. These facts apply to 2008 vehicles and 2008 only (although they may also be true for previous model years as well).
 
  • 08 RX AWD has a mechanical viscous center LSD combined with TRAC that delivers 50/50 fr/rr torque distribution that moves torque from this starting point to wheels with greater traction as needed.
     
  • 08 Sequoia, Land Cruiser and LX570 have a mechanical Torsen LSD differential that maintains a minimum of 40fr/60rr torque distribution until slippage occurs at which point power can be moved from the rear wheels to the front "instantaneously" (Toyota's words, not mine). Each of these vehicles also has a mechanically open rear differential.
     
  • Toyota engineers view a mechanical Torsen LSD as superior to a "speed sensing" LSD (ie. e-LSD using electronic engine and brake management to simulate a mechanical LSD).
     
    These are simply statements of fact from Toyota's own publicly available and highly promoted documents (since much of the same information is in the promotional brochures for these vehicles). To the extent you disagree with these facts you're not disagreeing with me.... but with Toyota.

    • #2216 of 2487 Re: Maybe... [wwest] by hdfatboy

    Apr 04, 2008 (12:58 pm)

    Replying to: wwest (Apr 04, 2008 10:12 am)
    "Why would any manufacturer put a Torsen, Torque Sensing Differential, in what is basically a FWD vehicle to begin with?? "
     
    I'm not aware of any manufacturer that has applied a Torsen mech LSD to an AWD drivetrain for a vehicle that would otherwise have been a FWD vehicle. What manufacturer and vehicle are you talking about?
     
    "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."
     
    What "segment" are you talking about? SUVs in general? Many of the GM full-size suvs have locking rear differentials standard (including the new Hybrid Tahoe) or offer them as an optional upgrade. If you're talking about smaller suvs, then the Porsche Cayenne is worth noting since it has a auto-locking rear differential.
     
    If you're referring to lower and mid-level SUVs (both size and cost) I'm not aware of any suvs that would have a locking rear differential since they aren't likely to have the power and torque to need or benefit from such a differential.

    Messages Page 222 of 249

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