Last post on Aug 21, 2013 at 5:16 PM
You are in the Toyota Sequoia
What is this discussion about?
Toyota Sequoia, Toyota Highlander, Toyota 4Runner, Toyota Land Cruiser, Toyota RAV4, Toyota Tacoma, Truck, SUV
#2193 of 2493 Re: What..WHAT...?? [wwest]
Apr 03, 2008 (4:35 am)
"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.
Apr 03, 2008 (6:51 am)
How about the Audi system's?
#2195 of 2493 Denali AWD
Apr 03, 2008 (8:11 am)
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.
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
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...!!
#2196 of 2493 Re: AWD 4WD [harbo]
Apr 03, 2008 (9:01 am)
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.
#2197 of 2493 Re: Denali AWD [wwest]
Apr 03, 2008 (9:55 am)
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.
#2198 of 2493 Re: What..WHAT...?? [hdfatboy]
Apr 03, 2008 (10:16 am)
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.
SUVs and Smart Shopper
#2199 of 2493 TC, Traction Controlled, AWD
Apr 03, 2008 (11:11 am)
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...?
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"..??
#2200 of 2493 Re: What..WHAT...?? [tidester]
Apr 03, 2008 (3:23 pm)
"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.
#2201 of 2493 Re: TC, Traction Controlled, AWD [wwest]
Apr 03, 2008 (3:36 pm)
"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:
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
#2202 of 2493 "08 Sequoia H4F mode vs H4L mode.
Apr 03, 2008 (6:13 pm)
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.