The RX/HL AWD setup is actually described by Lim as "the EASIEST HACK".
I would personally classify it as the CHEAPEST HACK that Toyota/Lexus could get away with using and still call it AWD!
But, read it for yourself:
INTRODUCTION TO ALL WHEEL DRIVE SYSTEMS by Eliot Lim
Chapter 4. DIFFERENTIAL LOCKING
Paragraph 10, 11 & 12 are the most applicable.
The closest he comes to describing the T&C AWD setup begins with "Some very crude systems...", but then the system he describes as "crude" does not include the Chrysler's "dog-clutch", used to maintain AWD in reverse gear by locking the over-running clutch.
Yes the RX/HL is a TRUE AWD with rather mild viscous limited slip center diff. But then look at the M class or X5, both doesn't even have a limited slip center diff, they have OPEN center diff.
Regarding the different ratios, I am referring to plain inter axle VC such as the T&C. Please look at Chapter 6, 2nd paragraph of Eliot Lim's website that I originally gave to you.
Anyway, don't believe everything he says. There are also other good sources out there.
Does it really matter where the "final" drive ratio is implemented?
And actually there is an advantage to having a completely "open" center differential, you can then use the brakes, TRAC, to apportion torque F/R, and L/R, and there is NO VC to interfere with the ABS.
Actually Lexus could "empty" the VC fluid, or most of it, and do the same thing, but are the rear drive "mechanicals" up to the additional HP stress?
Didn't you know that the WRX with stick shift (same as the well respected rally car) has the same viscous center diff as the RX/HL? Haven't you wonder what made it so succesful? Well, I'll tell you, it's the AWD. Period
VC fluid can be formulated in an infinite number of viscousities and expansion rates vs heating/cooling. Additionally there is the size of the "air bubble", if indeed there is one in any particular design.
As a matter of fact it is likely that the HL/RX VC could be removed and remanufactured to T&C standards just by changing out the fluid inside, but then it would likely interfere with ABS and if that were disabled, as some 4WDs do, it would still interfere with "normal" braking.
It depends on how the software is programed to make it compatible with the drivetrain. Examples of ABS being compatible are the Subarus and Jeep quadradrive to name a few.
When I said "the same VC", I meant the same drivetrain arrangement with a LIMITED SLIP VISCOUS CENTER DIFFERENTIAL.
"Does it really matter where the "final" drive ratio is implemented?" I pressume you meant torque split. In that case I would say yes.
A RWD has 100% power ro rear and 0% to front. This makes the rear more prone to break loss and gives an oversteer trait. This is nice on dry surface but treacherous on snow.
A FWD has 100% front & 0% rear and gives understeer trait. This has low fun factor.
AWD with 50-50 front to rear split has neutral handling with less tendency to break loss since each tire only receives little amount of power and thus less tendency to spin. Example RX/HL, Subie with stick, Eclipse, A4, 4Runner, Sequia, Jeep selectrac, M class.
BMW's 30-70? front to rear split tries to retain the fun factor of a RWD while increasing traction. However, on snow the 50-50 slpit has the edge since power is more evenly distributed and less tendency to slip.
What about T&C? It is primarily FWD with automatic engaging rear drive. The front mainly receives power. Since the front gets more power (compared to 50-50 split), it tends to break loss earlier just like a regular FWD. However the VC transfer some power to the rear when that happens.
In comparison, a 50-50 split has very little ttendency to break loss (unless that wheel is airborne) since power is evenly distributed on all corners.
80% of the braking is done by the front, so in moderately heavy or severe braking the front wheels turn significantly slower than the rear. If the center differential is "locked" then the slower rotating front wheels will dictate the rotational rate of the rear. If I remember right my 92 Jeep disabled ABS in part-time (locked)mode. The T&C uses the over-running clutch so the rears can still "over-run" the fronts even though the VC may be "locked".
"the same VC" I'm not sure the configuration really matters, if the VC has fluid that does not expand rapidly with heating or has a large air bubble then it will be really difficult to have it "stiffen". That's likely why the HL/RX doesn't need an over-running clutch, it's never "locked" enough that the front wheels can dictate rotational rate to the rear.
"Torque split", No I meant actual mechanical "final drive" gearing, the HL/RX has a 3.xx in the front and a 2.9xx in the rear, about a 12% difference. Why? Dunno.
RWD, lift your foot from the gas, the engine torque "anchors" the rear and the car returns to the line of travel dictated by the original momentum.(Not a "power" skid or oversteer, just hit a slick spot)
FWD, (extremely low fun factor, death is like that) is harder to get into the above circumstance but once you do you'll wish you had a clutch so you could instantly "kill" that engine braking. Or a FWD cadillac with its over-running clutch to eliminate engine braking to the front "drive/drag" wheels.
I think I'll just stick with favoring RWD or RWD biased AWD, then I can dedicate the rear wheels contact patch's roadbed adhesion for driving or "engine" braking and reserve the front wheel's contact patch's roadbed adhesion for lateral manuvers, just in case.
AS I once said, the ideal AWD would be one the moderates drive/drag on the front wheels as a function of increasing lateral/steering forces
Yes, the T&C is predominantly FWD, something like 95/5 though. I have never said I liked FWD, even a little bit. I use the T&C's VC as a standard becuase it is (seemingly) designed to react quickly, so quickly that it could affect front to rear brake proportioning.
How are you/did you get stuck on the 50/50 thing?
The RX (and presumably the HL) delivers virtually no torque to the rear in the VC's "non-slippage" state, and at the very most about 1/4 to 1/3 if the front wheels begin to slip.
On a four wheel dyno it only took about 5HP of dyno braking to prevent the rear wheels from exceeding 20MPH, we didn't allow the non-braked front wheels to go much above that level but it was quite clear that the VC was not apportioning engine torque on anything close to a 50/50 level.
OH, the two examples you mention have AWD setups that go "slack" absent torque delivered on the engine input end of the "devices".
Is it an option to get the VD-VSC on a Highlander with only FWD? I've been told mixed messages on this question. I am thinking of only a FWD-V6 rather than getting a 4WD that I don't need nor want.
So if I understand you correctly, there is absolutely no difference between the 4WD found in the Land Cruiser/LX470, and that found in the Sequoia, other than the Sequoia offers a 2WD mode, and that it is button-operated. In effect, the Sequoia "does" have a "full-time" 4WD mode. How about low-range? Can that be used on dry non-slip surfaces too, or is that a part-time, slippery surface only mode?
Why doesn't Toyota use this same 4WD on the Tacoma and Tundra too? As you know, GM is offering an on-demand (full-time) mode on the new Chevy and GMC pickups; Insta-Trac. Why doesn't Toyota do the same? I think part-time "only" 4WD's are a thing of the past.
Lastly, I see the new mid-size Lexus GX470 has a 5-speed automatic. Do you expect to see Toyota SUVs and pickups also getting that transmission? Or do think that will be a "Lexus exclusive?"
You are right on the money with the LC and Sequoia systems. The only other difference is the the LC is easier to lock the center differential because you have a button, rather than putting it in the "low-low" mode. Yes, you can drive in the low gear range on dry pavement in either, as long as you don't lock the center differential.
As to the Tacoma and Tundra, I think it is only a matter of time. Traditionalists will have a hard time accepting this, but the Activ-Trac is the future of 4WD systems. My Tundra lease is up in November and I am really hoping the 2003 will have it.
I may be wrong, but I believe the GM system only sends power to the front wheels when the rear wheels lose traction. The Sequoia sends power to the front and rear wheels regardless of traction. In my opinion this is a much better system that the GM product. Why should I have to wait for my rearend to slip while pulling out into traffic, in the rain, to see the benefits of having my ActiveTrac engaged?
I don't know the answer to either one of your questions. It is possible that Toyota uses different transfer cases because of a marketing decision. If may be that Sequoia buyers may assume that full time 4WD translates into substantially reduced fuel economy, where as LC buyers don't care.
GM is the only "pickup" manufacturer to offer On-Demand Insta-Trac. No, it's not full-time 4WD, in the traditional sense, but it's better than part-time 4WD. I too would rather a permanent full-time 4WD, but as of today, GM offers the closest to it—in a pickup.
<< If may be that Sequoia buyers may assume that full time 4WD translates into substantially reduced fuel economy, where as LC buyers don't care. >>
That may be true, but all it does is confuse the customer (for example, I wouldn't be asking this question in the first place). From a production standpoint, it would be simpler just to have 4WD system, whichever is best
but not on pickups, only SUVs. We have Ford Explorer with Control Trac, which similar to GM's Insta Trac, in that they're both on-demand 4WD, with a 4WD Auto mode.
BTW, I forgot: the Dodge Dakota offers full-time (and part-time) 4WD. The full-time unit is essentially the Jeep Selec Trac unit.
I live in Southern California and travel to Mammoth Mountain 3-4 times each year to go skiing with the family. I am looking into the Sequoia but can't decide on the 2WD or 4WD model. We are planning to keep the SUV for 7-8 years if all goes well. I have a few questions/concerns regarding the Sequoia 4WD system:
1) If ran in the 2WD mode, will the SUV act exactly like the 2WD model without moving any of the 4WD components?
2) Does the 4WD system require a significant amount of extra routine maintenance?
3) How reliable are these 4WD systems? The last thing I want is to pay the extra $3K for 4WD, only use this system at most 2x/year for 8 years and have an expensive problem. The system sounds complicated and that there could be many things that could go wrong. Is it safe to say that this should not be an issue with the limited amount I will be using the 4WD?
4) What do you think the difference in resale value will be with between the 2WD & 4WD Sequoia after 8 years/120K miles?
Are you sure that LC and Sequoia (and 4runner) can be driven in LOW range on dry land??? I always thought that when you are in LOW range, the center diff is AUTOMATICALLY locked. Same goes for selecting 4-HI. I think the ONLY time the center diff is NOT lock is when you are in auto mode (full-time) or 2WD. Thanks.
First to Heatmiser. The 4WD system does require a bit more maintenance but only because you have more drive shafts and differentials. I doubt the added cost would be significant over the long term. The beauty of the system in in the fact that it uses open differentials. Those are extremely reliable and don't require rebuilds like limited slips do. Resale will vary by state. Here in VA, there is NO demand for 2 wheel drive SUVs and resale reflects this. In Southern CA, that may not be the case.
Next to Intmed. You should probably read the first few posts in this topic. There is no auto mode on these models. The 4WD system does not use a transfer case but rather, an open differential. It takes a deliberate effort to lock this and therefore, it is fine to drive on dry pavement in these vehicles in the 4WD mode.
Thanks for the explanation. My knowledge of how 4WD systems (or any car at that) work is very minimal so I have another few questions:
Since I live in San Diego, CA and we hardly get any foul weather, I would like to keep the car in 2WD almost all of the time. With this said, will the 4WD parts (shafts/differentials) still be moving while in 2WD?
If the 4WD parts are not moving in 2WD, am I correct to assume there should be no increased maintenance/repair costs on the 4WD Sequoia since the 4WD system will most likely only be used 16-24 days over the next 8 years of ownership?
You can see where I am going with this. The last thing I want is to purchase the $3K option that I would hardly ever use and end up paying a significant amount more for maintenance.
ONE MORE QUESTION: Do you have any experience/knowledge on how a 2WD Seaquoia will handle in snow with tire chains compared to a 4WD Sequoia without any tire chains?
Cliffy...i am aware of the 4WD system of Toyota SUVs. In the LC, for instance, you have FULL-TIME 4WD that you can use anywhere...yes, i know that. In THIS mode, the center diff is operated by viscous coupling. However, when you MANUALLY shift the TRANSFER CASE to 4-HI, you lock the center diff. When you manually shift to 4-LO, you also lock the center diff (along with multiplying your torque). In 4-HI and 4-LO, you canNOT use it on dry land because the center diff is LOCKED. Is there something i am missing in my understanding??
(In 4Runner and Sequoia, you have an additional option of 2WD mode; when you hit the "4WD Auto" button, you go into full-time 4WD, but NOT 4-HI.)
I think (mind you, this is a statement of opinion and not fact) that the shafts still turn, even when you are in 2WD. Engaging the 4WD system connects the front drive shaft to the transmission.
As for chains on a 2WD versus no chains on a 4WD, again, this will be an opinion. I have tremendous respect for the 4WD system and think it would work much better. Plus, you have the advantage of not worrying about wear when you hit a section of road that is bare.
For you though, you need to honestly asses how often and under what conditions you will be using it. I lived in Southern CA until I was 22 and skied Mammoth. If memory serves me, the roads were not too difficult and well maintained. I am sure there are times where 4Wd would be a big advantage, but unless you are also looking to do some off roading in the desert, you'll probably be fine in a 2WD. Personally, I like the 4WD but it ain't my money were talking about.
Ummmm.... you've got a few faulty premises in your post. The Land Cruiser does not have a viscous coupling. Also, there is nothing to shift to engage 4-HI. It is always in 4-HI. The modes of operation in the LC are: 4-HI, 4-LOW, 4-HI locked and 4-LOW locked. There is a separate button to lock the center differential. There is also a VSC off button which disables the skid control function.
Finally, as I mentioned in my previous post, there is no such thing as a "4WD Auto" button or mode on the Sequoia or Runner. Where are you getting this information?
You seem to be saying that since it takes a "deliberate act" to "lock" the center diff'l then it is okay to drive on dry pavement in this mode.
Is that actually what you intented to say?
Snowchains. Never, NEVER, put snowchains only on the front wheels of ANY vehicle. Putting an extraordinary level of traction on the front vs the rear can lead to disaster, QUICKLY.
I survived a few winters in MT and ND with RWD sedans and snowchains as needed.
How do I respond to this? It is not because it takes a deliberate act to lock the center differential that makes it safe to use the 4WD on dry pavement. It is safe because the center is unlocked.
Also, who said anything about putting chains on the front of a Sequoia?
Cliffy, you are right. I just stopped at a Toyo dealership to get reaquainted with the LC. However, i am still not sure if you can shift into LOW range on dry land. My thinking is that when you shift into 4-LO, the center diff is automatically locked.
I understand why one would assume that the center would be automatically locked in the low gear range, but it isn't. It still take pushing the button on the dash to lock it. I like that feature myself because it expands the possibilities.
It appears there is some confusion in the use of terms and their meaning when describing AWD vs different types of 4wd systems. If you go back and use the definition of awd as designed by Audi and Subaru (the originators of awd) it will help clarify matters somewhat.
AWD is any 4wd system which delivers a minimum amount of torque to each of the 4 wheels under normal roadway conditions and can be operated at any speed on dry roads. These systems are represented by all of Audi's quattro vehicles, Subaru, GMC Denali, Cadillac Escalade, MB AWD, BMW AWD, Volvo AWD and Porsche AWD (there may be others as well). These typically do not have a 4wd lo ratio as they are primarily designed for "on-road" performance.
Next is permanent part-time 4wd. These systems can be run at any speed on dry roadways and under normal roadway conditions deliver 100% of the engine's torque to the rear wheels (or front wheels) until either momentum or wheel slippage sensors determine that power should be transfered to the front (or rear) wheels. There are various technologies for accomplishing the transfer such as limited slip diffs, viscous liquid diffs, electronic sensors that brake wheel slippage forcing torque through open diffs to wheels that aren't slipping, etc.
There are lot's of different technologies however these systems all have one thing in common. They remain 2wd until a "need" is detected and then power is transferred and they can be operated under all conditions. These systems are represented by Toyota, GM, Ford and other manufacturers.
Next is Full-time 4wd which delivers torque to all 4 wheels however it uses the slippage from a dirt/gravel, wet or snowy roadway to act as differential to compensate for different wheel rotation rates. Consequently, these systems are not designed to be operated on dry roadways and give the driver the "binding" sensation when make slow, tight turns. These systems are also typically accompanied with 4wd lo gear ratios.
While I'm sure some will disagree with my catgorization, I think you'll find that by checking you manual or the manufacturer for two pieces of information every vehicle can be classified into these categories.
First, can the vehicle be operated at any speed on dry pavement in 4wd mode.
Second, is there a guaranteed amount of torque delivered to all 4 wheels at all times, including normal dry roadways.
Answer these two questions and your vehicle will fall into one of the 3 categories above.
dielectric: I agree with all of the definitions provided and in fact my definitions align perfectly with the earlier ones. They just provide greater granularity.
Just being activated all the time is not the same as having power to all the wheels all the time. Until you've driven both types of vehicles I suspect you'd have a hard time understanding why the difference matters.
I've driven all three types I've described and even a novice would be able to sense the difference in a 4wd system that starts with 0/100 split of torque, encounters a slippery road condition and then "feels" the torque being moved from front to rear. There is no such sensation in an AWD vehicle and that is what differentiates AWD from permanent part-time 4wd systems such as the Sequoia and GM's Auto 4wd.
Take an AWD vehicle for a drive on a slippery road and compare it with a 4wd system and then you'll understand.
I understand the differencees between a vehicle which is always delivering power to both axles and one which 0/100 then slips and engages the front. It's just some systems may be at 0/100 when cruising in a straight line, but when you turn or press the accelerator pedal it may engage the front whether or not the rear is slipping. There are just too many systems.
I think it could be bad if a system delivers power to both drive shafts all the time. This means that the drive shafts are always connected in some way. This could make ABS faulty like when part-time (locked) systems are engaged (granted I don't want to get into the good and bad of ABS here).
The Chrysler T&C overcomes that particular problem with an over-running clutch which allows the front wheels to turn slower than the rear during (ABS) braking.
Unlike the RX and the HL which simply doesn't "couple" the rear wheel drive enough to be a problem with ABS.
dielectric: I'm not sure I grasp the concern you suggest in a system that delivers a minimum of torque to both axles all the time. In fact every, hi-performance vehicle with awd has a dedicated amount of power delivered to both ends all the time. These vehicles include the Audi A8 quattro, Porsche 959, BMW 325iX, Subaru WRX, GMC Denali, Volvo X-Country, Cadillac Escalade etc.
There are no issues with ABS brakes or any other systems related to the power being delivered to the road. In fact these vehicles have some of the highest performance specs for any vehicle in their class and all have dedicated awd.
Do you understand in a Part-time system why ABS doesn't work? I am just saying that for the same reason why ABS doesn't work when a part-time system is engaged, these other AWD systems that are always delivering power could encounter a problem. I am not saying they do encounter problems, but I am saying it may not be the best way.
The main problem is that the front and rear axles don't operate independently as they do in a 2WD vehicle (or a vehicle operating in 2WD mode). Take this for an example: Center diff locked, front diff locked, rear diff locked. Now one wheel locks on your 4WD vehicle. Now all the wheels are locked. ABS just can't operate like this. Granted on an AWD system the case will not be nearly so extreme or easy to initiate, but it could be there.
Yeah some of those vehicles you listed could be consider high performance, but others I would never put into that group.
dielectric: your hypothesis is entirely wrong. ABS works no differently in an AWD vs a part-time 4wd system vs a full-time 4wd system. The computer is simply measuring the momentum of the vehicle against the rotation of a wheel. If the tire has stopped rotating and the vehicle is still in motion, than the computer automatically releases the brake to allow the wheel to continue its rotation. It continues this process, maintaining the tires rotation on the edge of locking up but not quite, in order to force the tire to provde its greatest adhesion for braking and giving the added benefit of improving steering while braking.
This process is entirely unrelated to whether the drivetrain is AWD, 2wd, 4wd, 1wd or any other setup you can envision.
I'm sorry, but no. ABS works AFTER the wheel is locked, hence the whole drive-train has become locked, and now you must unlock 4 wheels instead of just 1 wheel.
ABS doesn't maintain you on the edge of lock-up, once the wheel locks up and the system recognizes this, it disengages that one brake. A professional driver will get shorter braking distances in a car that has no ABS compared with the same car without ABS.
A system with the front, center, and rear differentials locked has effectively only one brake as each wheel must turn at the same speed.
A system with a front, center, and rear differentials open has one brake acting upon each wheel independently, so 4 separate brakes.
A system with a mix of locked/open/LSD/VC/etc... will be in between the two extreme cases above. Most will barely be off of the open/open/open case, but they will be off a little!
One of the long standing problems with this discussion is old definitions. Heatwave has correct definitions of the various systems until you get to the Toyota Sequoia and 4Runner. The hang up is the "part time" aspect of the whole thing. In general, Heatwave is correct about how they engage and disengage, but the Sequoia and Runner are different.
How are they different? Think of them this way: When engaged, they are actually "full time" 4WD systems. This is easy to imagine because when engaged, they operate identically to the Land Cruiser, which by all definitions is a "full time 4WD" system.
Again, the confusion is related to the fact that the Sequoia and Runner are selectable systems. This would seem to imply "part time" status as they are not engaged all the time. They do not fall under a conventional "part time" system though.
do you have a link to audi/subaru's "definitions" of 4wd/AWD? some of the wording just doesn't seem to make sense. Full-time 4wd doesn't "bind" when you turn a corner, and "permanent part-time 4wd" sounds like an oxymoron.
First, read and understand the very good article on 4x4s found here on the ineternet written by Eliot Lim. It's a little out-dated, changes in 4x4s arriving faster than new Pentiums, but a damn good read.
The Sequoia is implemented using 3 open diff'l and 3 "dog-clutches". In 2WD mode it is RWD. If the FWD dog-clutch(1) is engaged it allocates torque amongst the four wheels by modulating the brakes, braking a(ny) wheel that slips or the computer deems is beginning to slip.
A dog-clutch(2) is also used to engage the low range.
If the transmission shifter is placed in "low" while in "low-range" then a third dog-clutch is brought into play, locking the center diff'l.
ABS: Jeep disables ABS when the center diff'l is locked.
Becuase your front brakes do about 80% of the job of stopping the vehicle the front wheels almost always at some point will turn slower than the rear wheels. If the center "coupling" is too "tight", then having the ability for the ABS to release a rear wheel brake of a wheel that is approaching "locked" may not be effective.
That's basically how in the beginning I guessed that the RX AWD system wasn't.
AND:
The processors on board most new vehicles are now fast enough that they can "predict" impending wheel lock-up and release and re-apply (repeat as necessary) that wheel's brake before it actually stops turning, at least in most cases.
Bottom line is it is now as practically impossible to "categorize" AWD/4WD system as it is Pentiums and Athlons.
I wonder if one could convert an RX or HL to "true" AWD by transplanting the Sequoia's VSC/TRAC/ABS ecu?
Hey Cliffy, Im new to the group and not sure if you covered this already but....I have a 2000 RX300 AWD, I notice that when going from a complete stop up through first gear the transmission sounds like it winds up. Once its through first gear into second the winding sound goes away. I only appears in 1st gear. Is this a common sound associated with AWD vehicles or possibly a potential problem with my transmission? Its my first AWD vehicle so I am not familiar with its characteristics. Thanks for any comments!
Drove a 2000 AWD RX for about 12K and now have an 01 AWD RX with about the same mileage and I haven't noticed any whine in either, not even from my wife's side of the car.
At the very least, you should have your Lexus dealer take a look at it. Without hearing it myself, I really can't comment on whether it is normal or not.
cliffy: There's no hang up nor any confusion with past definitions. I think the past definitions still apply. Think of it this way.
If you could place a particular vehicle on a low friction dynomometer with each of the four tires of a vehicle on a set of individual rollers and then slowly increase power, how would the tires respond?
1) In a vehicle with the typically "wet-road only" 4wd system engaged, all 4 wheels will turn on the rollers. If 4wd is disengaged, then only the rear tires will rotate.
2) In a vehicle with auto 4wd (for lack of a more universal term), its my contention that because all of the torque is directed to the rear tires until slippage is detected, only the rear rollers will rotate in this experiment. These systems can be run at all speeds, on all road conditions. Examples, IMO are the GM Auto 4wd, Ford Auto 4wd, Jeep's higher end 4wd system (I forget its trade name), the Sequoia, the 4runner and many other new 4wd systems using every improving technologies to increase the stage of detection and the speed of torque transfer to the best corner of the vehicle.
In a vehicle with awd, its also my contention that all 4 wheels will rotate on the separate rollers and no slippage is required to initiate any tire rotation. For it to qualify as AWD it must be capable of operating on all surface types at any speed the vehicle is capable of. Examples are Audi quattro, GM AWD, Subaru AWD, Volvo AWD, BMW AWD, PORSCHE AWD, MB AWD.
If the Sequoia system could rotate the front wheels on these rollers, it would in fact have to have a dedicated torque split %, just as all the awd systems I've listed have. I have not seen a breakdown of any such torque splits from any Toyota or independent lab on the Sequoia. Since this is routinely provided by anyone making a awd system, it is not unreasonable to conclude that the Sequoia and the 4runner do not provide dedicate torque to the front wheels until their system detects the need for power to be directed to the front tires.
This detection system may very well be better than other "auto 4wd" systems (or worse as I've not seen an lab tests measuring the ability to detect and transfer power of any of these systems).
The point is that AWD does not need to detect nor transfer power up to the minimums that have been pre-established and that results in a "seat of the pants" and on-road performance distinctions that will always favor an awd system when measuring on-road performance.
Comments
The RX/HL AWD setup is actually described by Lim as "the EASIEST HACK".
I would personally classify it as the CHEAPEST HACK that Toyota/Lexus could get away with using and still call it AWD!
But, read it for yourself:
INTRODUCTION TO ALL WHEEL DRIVE SYSTEMS
by Eliot Lim
Chapter 4.
DIFFERENTIAL LOCKING
Paragraph 10, 11 & 12 are the most applicable.
The closest he comes to describing the T&C AWD setup begins with "Some very crude systems...", but then the system he describes as "crude" does not include the Chrysler's "dog-clutch", used to maintain AWD in reverse gear by locking the over-running clutch.
Regarding the different ratios, I am referring to plain inter axle VC such as the T&C. Please look at Chapter 6, 2nd paragraph of Eliot Lim's website that I originally gave to you.
Anyway, don't believe everything he says. There are also other good sources out there.
And actually there is an advantage to having a completely "open" center differential, you can then use the brakes, TRAC, to apportion torque F/R, and L/R, and there is NO VC to interfere with the ABS.
Actually Lexus could "empty" the VC fluid, or most of it, and do the same thing, but are the rear drive "mechanicals" up to the additional HP stress?
As a matter of fact it is likely that the HL/RX VC could be removed and remanufactured to T&C standards just by changing out the fluid inside, but then it would likely interfere with ABS and if that were disabled, as some 4WDs do, it would still interfere with "normal" braking.
Never an easy answer...
When I said "the same VC", I meant the same drivetrain arrangement with a LIMITED SLIP VISCOUS CENTER DIFFERENTIAL.
"Does it really matter where the "final" drive ratio is implemented?" I pressume you meant torque split. In that case I would say yes.
A RWD has 100% power ro rear and 0% to front. This makes the rear more prone to break loss and gives an oversteer trait. This is nice on dry surface but treacherous on snow.
A FWD has 100% front & 0% rear and gives understeer trait. This has low fun factor.
AWD with 50-50 front to rear split has neutral handling with less tendency to break loss since each tire only receives little amount of power and thus less tendency to spin. Example RX/HL, Subie with stick, Eclipse, A4, 4Runner, Sequia, Jeep selectrac, M class.
BMW's 30-70? front to rear split tries to retain the fun factor of a RWD while increasing traction. However, on snow the 50-50 slpit has the edge since power is more evenly distributed and less tendency to slip.
What about T&C? It is primarily FWD with automatic engaging rear drive. The front mainly receives power. Since the front gets more power (compared to 50-50 split), it tends to break loss earlier just like a regular FWD. However the VC transfer some power to the rear when that happens.
In comparison, a 50-50 split has very little ttendency to break loss (unless that wheel is airborne) since power is evenly distributed on all corners.
"the same VC" I'm not sure the configuration really matters, if the VC has fluid that does not expand rapidly with heating or has a large air bubble then it will be really difficult to have it "stiffen". That's likely why the HL/RX doesn't need an over-running clutch, it's never "locked" enough that the front wheels can dictate rotational rate to the rear.
"Torque split", No I meant actual mechanical "final drive" gearing, the HL/RX has a 3.xx in the front and a 2.9xx in the rear, about a 12% difference. Why? Dunno.
RWD, lift your foot from the gas, the engine torque "anchors" the rear and the car returns to the line of travel dictated by the original momentum.(Not a "power" skid or oversteer, just hit a slick spot)
FWD, (extremely low fun factor, death is like that) is harder to get into the above circumstance but once you do you'll wish you had a clutch so you could instantly "kill" that engine braking. Or a FWD cadillac with its over-running clutch to eliminate engine braking to the front "drive/drag" wheels.
I think I'll just stick with favoring RWD or RWD biased AWD, then I can dedicate the rear wheels contact patch's roadbed adhesion for driving or "engine" braking and reserve the front wheel's contact patch's roadbed adhesion for lateral manuvers, just in case.
AS I once said, the ideal AWD would be one the moderates drive/drag on the front wheels as a function of increasing lateral/steering forces
Yes, the T&C is predominantly FWD, something like 95/5 though. I have never said I liked FWD, even a little bit. I use the T&C's VC as a standard becuase it is (seemingly) designed to react quickly, so quickly that it could affect front to rear brake proportioning.
How are you/did you get stuck on the 50/50 thing?
The RX (and presumably the HL) delivers virtually no torque to the rear in the VC's "non-slippage" state, and at the very most about 1/4 to 1/3 if the front wheels begin to slip.
On a four wheel dyno it only took about 5HP of dyno braking to prevent the rear wheels from exceeding 20MPH, we didn't allow the non-braked front wheels to go much above that level but it was quite clear that the VC was not apportioning engine torque on anything close to a 50/50 level.
OH, the two examples you mention have AWD setups that go "slack" absent torque delivered on the engine input end of the "devices".
Thanks!
Why doesn't Toyota use this same 4WD on the Tacoma and Tundra too? As you know, GM is offering an on-demand (full-time) mode on the new Chevy and GMC pickups; Insta-Trac. Why doesn't Toyota do the same? I think part-time "only" 4WD's are a thing of the past.
Lastly, I see the new mid-size Lexus GX470 has a 5-speed automatic. Do you expect to see Toyota SUVs and pickups also getting that transmission? Or do think that will be a "Lexus exclusive?"
Bob
As to the Tacoma and Tundra, I think it is only a matter of time. Traditionalists will have a hard time accepting this, but the Activ-Trac is the future of 4WD systems. My Tundra lease is up in November and I am really hoping the 2003 will have it.
Bob
Bob
SUVs. that's a different story.
Bob
That may be true, but all it does is confuse the customer (for example, I wouldn't be asking this question in the first place). From a production standpoint, it would be simpler just to have 4WD system, whichever is best
Bob
Paul
BTW, I forgot: the Dodge Dakota offers full-time (and part-time) 4WD. The full-time unit is essentially the Jeep Selec Trac unit.
Bob
1) If ran in the 2WD mode, will the SUV act exactly like the 2WD model without moving any of the 4WD components?
2) Does the 4WD system require a significant amount of extra routine maintenance?
3) How reliable are these 4WD systems? The last thing I want is to pay the extra $3K for 4WD, only use this system at most 2x/year for 8 years and have an expensive problem. The system sounds complicated and that there could be many things that could go wrong. Is it safe to say that this should not be an issue with the limited amount I will be using the 4WD?
4) What do you think the difference in resale value will be with between the 2WD & 4WD Sequoia after 8 years/120K miles?
Next to Intmed. You should probably read the first few posts in this topic. There is no auto mode on these models. The 4WD system does not use a transfer case but rather, an open differential. It takes a deliberate effort to lock this and therefore, it is fine to drive on dry pavement in these vehicles in the 4WD mode.
Since I live in San Diego, CA and we hardly get any foul weather, I would like to keep the car in 2WD almost all of the time. With this said, will the 4WD parts (shafts/differentials) still be moving while in 2WD?
If the 4WD parts are not moving in 2WD, am I correct to assume there should be no increased maintenance/repair costs on the 4WD Sequoia since the 4WD system will most likely only be used 16-24 days over the next 8 years of ownership?
You can see where I am going with this. The last thing I want is to purchase the $3K option that I would hardly ever use and end up paying a significant amount more for maintenance.
ONE MORE QUESTION: Do you have any experience/knowledge on how a 2WD Seaquoia will handle in snow with tire chains compared to a 4WD Sequoia without any tire chains?
(In 4Runner and Sequoia, you have an additional option of 2WD mode; when you hit the "4WD Auto" button, you go into full-time 4WD, but NOT 4-HI.)
As for chains on a 2WD versus no chains on a 4WD, again, this will be an opinion. I have tremendous respect for the 4WD system and think it would work much better. Plus, you have the advantage of not worrying about wear when you hit a section of road that is bare.
For you though, you need to honestly asses how often and under what conditions you will be using it. I lived in Southern CA until I was 22 and skied Mammoth. If memory serves me, the roads were not too difficult and well maintained. I am sure there are times where 4Wd would be a big advantage, but unless you are also looking to do some off roading in the desert, you'll probably be fine in a 2WD. Personally, I like the 4WD but it ain't my money were talking about.
Finally, as I mentioned in my previous post, there is no such thing as a "4WD Auto" button or mode on the Sequoia or Runner. Where are you getting this information?
You seem to be saying that since it takes a "deliberate act" to "lock" the center diff'l then it is okay to drive on dry pavement in this mode.
Is that actually what you intented to say?
Snowchains. Never, NEVER, put snowchains only on the front wheels of ANY vehicle. Putting an extraordinary level of traction on the front vs the rear can lead to disaster, QUICKLY.
I survived a few winters in MT and ND with RWD sedans and snowchains as needed.
Also, who said anything about putting chains on the front of a Sequoia?
AWD is any 4wd system which delivers a minimum amount of torque to each of the 4 wheels under normal roadway conditions and can be operated at any speed on dry roads. These systems are represented by all of Audi's quattro vehicles, Subaru, GMC Denali, Cadillac Escalade, MB AWD, BMW AWD, Volvo AWD and Porsche AWD (there may be others as well). These typically do not have a 4wd lo ratio as they are primarily designed for "on-road" performance.
Next is permanent part-time 4wd. These systems can be run at any speed on dry roadways and under normal roadway conditions deliver 100% of the engine's torque to the rear wheels (or front wheels) until either momentum or wheel slippage sensors determine that power should be transfered to the front (or rear) wheels. There are various technologies for accomplishing the transfer such as limited slip diffs, viscous liquid diffs, electronic sensors that brake wheel slippage forcing torque through open diffs to wheels that aren't slipping, etc.
There are lot's of different technologies however these systems all have one thing in common. They remain 2wd until a "need" is detected and then power is transferred and they can be operated under all conditions. These systems are represented by Toyota, GM, Ford and other manufacturers.
Next is Full-time 4wd which delivers torque to all 4 wheels however it uses the slippage from a dirt/gravel, wet or snowy roadway to act as differential to compensate for different wheel rotation rates. Consequently, these systems are not designed to be operated on dry roadways and give the driver the "binding" sensation when make slow, tight turns. These systems are also typically accompanied with 4wd lo gear ratios.
While I'm sure some will disagree with my catgorization, I think you'll find that by checking you manual or the manufacturer for two pieces of information every vehicle can be classified into these categories.
First, can the vehicle be operated at any speed on dry pavement in 4wd mode.
Second, is there a guaranteed amount of torque delivered to all 4 wheels at all times, including normal dry roadways.
Answer these two questions and your vehicle will fall into one of the 3 categories above.
Just being activated all the time is not the same as having power to all the wheels all the time. Until you've driven both types of vehicles I suspect you'd have a hard time understanding why the difference matters.
I've driven all three types I've described and even a novice would be able to sense the difference in a 4wd system that starts with 0/100 split of torque, encounters a slippery road condition and then "feels" the torque being moved from front to rear. There is no such sensation in an AWD vehicle and that is what differentiates AWD from permanent part-time 4wd systems such as the Sequoia and GM's Auto 4wd.
Take an AWD vehicle for a drive on a slippery road and compare it with a 4wd system and then you'll understand.
I think it could be bad if a system delivers power to both drive shafts all the time. This means that the drive shafts are always connected in some way. This could make ABS faulty like when part-time (locked) systems are engaged (granted I don't want to get into the good and bad of ABS here).
Unlike the RX and the HL which simply doesn't "couple" the rear wheel drive enough to be a problem with ABS.
There are no issues with ABS brakes or any other systems related to the power being delivered to the road. In fact these vehicles have some of the highest performance specs for any vehicle in their class and all have dedicated awd.
The main problem is that the front and rear axles don't operate independently as they do in a 2WD vehicle (or a vehicle operating in 2WD mode). Take this for an example: Center diff locked, front diff locked, rear diff locked. Now one wheel locks on your 4WD vehicle. Now all the wheels are locked. ABS just can't operate like this. Granted on an AWD system the case will not be nearly so extreme or easy to initiate, but it could be there.
Yeah some of those vehicles you listed could be consider high performance, but others I would never put into that group.
This process is entirely unrelated to whether the drivetrain is AWD, 2wd, 4wd, 1wd or any other setup you can envision.
ABS doesn't maintain you on the edge of lock-up, once the wheel locks up and the system recognizes this, it disengages that one brake. A professional driver will get shorter braking distances in a car that has no ABS compared with the same car without ABS.
A system with the front, center, and rear differentials locked has effectively only one brake as each wheel must turn at the same speed.
A system with a front, center, and rear differentials open has one brake acting upon each wheel independently, so 4 separate brakes.
A system with a mix of locked/open/LSD/VC/etc... will be in between the two extreme cases above. Most will barely be off of the open/open/open case, but they will be off a little!
How are they different? Think of them this way: When engaged, they are actually "full time" 4WD systems. This is easy to imagine because when engaged, they operate identically to the Land Cruiser, which by all definitions is a "full time 4WD" system.
Again, the confusion is related to the fact that the Sequoia and Runner are selectable systems. This would seem to imply "part time" status as they are not engaged all the time. They do not fall under a conventional "part time" system though.
The Sequoia is implemented using 3 open diff'l and 3 "dog-clutches". In 2WD mode it is RWD. If the FWD dog-clutch(1) is engaged it allocates torque amongst the four wheels by modulating the brakes, braking a(ny) wheel that slips or the computer deems is beginning to slip.
A dog-clutch(2) is also used to engage the low range.
If the transmission shifter is placed in "low" while in "low-range" then a third dog-clutch is brought into play, locking the center diff'l.
ABS: Jeep disables ABS when the center diff'l is locked.
Becuase your front brakes do about 80% of the job of stopping the vehicle the front wheels almost always at some point will turn slower than the rear wheels. If the center "coupling" is too "tight", then having the ability for the ABS to release a rear wheel brake of a wheel that is approaching "locked" may not be effective.
That's basically how in the beginning I guessed that the RX AWD system wasn't.
AND:
The processors on board most new vehicles are now fast enough that they can "predict" impending wheel lock-up and release and re-apply (repeat as necessary) that wheel's brake before it actually stops turning, at least in most cases.
Bottom line is it is now as practically impossible to "categorize" AWD/4WD system as it is Pentiums and Athlons.
I wonder if one could convert an RX or HL to "true" AWD by transplanting the Sequoia's VSC/TRAC/ABS ecu?
If you could place a particular vehicle on a low friction dynomometer with each of the four tires of a vehicle on a set of individual rollers and then slowly increase power, how would the tires respond?
1) In a vehicle with the typically "wet-road only" 4wd system engaged, all 4 wheels will turn on the rollers. If 4wd is disengaged, then only the rear tires will rotate.
2) In a vehicle with auto 4wd (for lack of a more universal term), its my contention that because all of the torque is directed to the rear tires until slippage is detected, only the rear rollers will rotate in this experiment. These systems can be run at all speeds, on all road conditions. Examples, IMO are the GM Auto 4wd, Ford Auto 4wd, Jeep's higher end 4wd system (I forget its trade name), the Sequoia, the 4runner and many other new 4wd systems using every improving technologies to increase the stage of detection and the speed of torque transfer to the best corner of the vehicle.
In a vehicle with awd, its also my contention that all 4 wheels will rotate on the separate rollers and no slippage is required to initiate any tire rotation. For it to qualify as AWD it must be capable of operating on all surface types at any speed the vehicle is capable of. Examples are Audi quattro, GM AWD, Subaru AWD, Volvo AWD, BMW AWD, PORSCHE AWD, MB AWD.
If the Sequoia system could rotate the front wheels on these rollers, it would in fact have to have a dedicated torque split %, just as all the awd systems I've listed have. I have not seen a breakdown of any such torque splits from any Toyota or independent lab on the Sequoia. Since this is routinely provided by anyone making a awd system, it is not unreasonable to conclude that the Sequoia and the 4runner do not provide dedicate torque to the front wheels until their system detects the need for power to be directed to the front tires.
This detection system may very well be better than other "auto 4wd" systems (or worse as I've not seen an lab tests measuring the ability to detect and transfer power of any of these systems).
The point is that AWD does not need to detect nor transfer power up to the minimums that have been pre-established and that results in a "seat of the pants" and on-road performance distinctions that will always favor an awd system when measuring on-road performance.