Toyota 4WD systems explained

cliffy1 · March 2002

I posted this about 18 months ago in the Sequoia topic but I think it bear repeating again here. Back when the Sequoia was being introduced, salesmen were taken to "ride and drive" events around the country to introduce us to the product. There were various demonstration but one really stood out.

A large tarp was set up in a parking lot with water flowing over it and dishwashing soap applied to it. It was on a slight incline. Being that we were there on a 90 degree day, this was as close an approximation of ice as we could come up with.

First, a Ford Explorer was brought on to the corner. The steering wheel was turned all the way to the right and the driver mashed the throttle to full. The wheels spun a bit and the thing drifted and slid but did manager to get off the tarp with a decent amount of control.

Next, a Chevy Tahoe was brought to the line. We were asked to step back. The rear wheel spun in place for a moment and then it lurched forward and began to spin. It didn't make it off the tarp and ended up facing the place that it started. Had it actually made it to dry pavement, it may have flipped as the slide was VERY violent.

Finally a Sequoia was brought to the line. At first, it was left in 2 wheel drive. The rear wheels slipped for a brief moment and then stopped. I could hear the rev limiter kick in. It made a very slow but controlled turn off the tarp.

The Sequoia was brought back for another pass, but this time in 4WD. It took off like a shot, under complete control. It turned the corner without so much as a hint of a fishtail.

It was an impressive demonstration.

cliffy1 · March 2002

Thanks. I was going to look that up myself, but you saved me the trouble.

wwest · March 2002

Ain't VSC (w/AWD*TRAC***) wonderful...

*** (AWD*TRAC) = "virtual" AWD !

or:

Three open diff'ls for hard/dry surfaces and TRAC to implement (VIRTUAL {UNREAL!!}) locking diff'ls for LTS, low traction surfaces.

cliffy1 · March 2002

You and I disagree from time to time, but I'm with you on this. My only hope is that Toyota will add the Active-Trac system to the Tundra in time for my next lease. Selling aside (and since I don't sell any more, that is easy to say) I think this system ranks right up there with sliced bread in terms of being pretty cool.

pschreck · March 2002

Have you seen the Sequoia transfer case pages? What a system. Multi-Mode Full time 4WD (a step above plain vanilla AWD), VSC, Active TRAC, EBD, locking center differential,etc. YOW!

heatwave3 · March 2002

pschreck and wwest: Sounds like you guys have a strong need for reassurance on your drivetrains. Do you all feel better now that you've patted each other on the back with your "yows" and virtual celebrations?

I certainly wouldn't want to dampen anyone's party with real world comparisons to proven systems used by performance manufacturer's from around the world, so I'll just watch as you all convince yourselves that the Sequoia system is actually a cut above an Abrahms tank when measuring traction.

In the interim, I posed a few questions to Toyota along the line of cliffy's thinking above and upon receiving a response I'll pass it on to everyone.

heatwave3 · March 2002

Cliffy: the demonstration on a soapy tarp you shared, sounds like an interesting comparison however I wonder if it was a truly fair comparison. You described the performance of the Sequoia in 4wd as "The Sequoia was brought back for another pass, but this time in 4WD. It took off like a shot, under complete control. It turned the corner without so much as a hint of a fishtail."

It sounds like the 4wd system was engaged with the center diff locked by the way it accelerated. In fact it sounds like the Sequoia might have been in 4wd lo. If so this type of performance could have been achieved with almost any 4wd system on the market if it were engaged in its part-time 4wd mode ensuring torque and power was delivered to every wheel.

From your description the Explorer and Tahoe were in their full-time 4wd mode and not 4wd hi which engages the center locking diff.

Do you have any direct knowledge of what 4wd mode the Sequoia was actually in?

cliffy1 · March 2002

You're grasping at straws here. At the speed it left the mat, there is no way it was in low and because the center can only be locked in low, it wasn't locked. Is that the best you can come up with to explain the poor performance of the others? Isn't it just possible that you have been wrong and the Sequoia really is superior?

heatwave3 · March 2002

cliffy: its absolutely possible the Sequoia is better. In fact, its my view that the Sequoia as a vehicle and its 4wd system IS better than both of the vehicles it was compared to in the demo you described. I just happen to think the comparison might have been a shade less than fair because it was a dramatization.

Making it more dramatic was in Toyota's (and the dealership running the drama) best interest and it certainly wasn't a scientific comparison. My only point was that both the Explorer and the Tahoe were not engaged in the best 4wd mode for the trial being run. The results might have still showed the Sequoia to be better, however I doubt it would have been as dramatic as you described if the other vehicles were given the opportunity to be engaged in their highest traction mode.

Also 4wd lo will have the perception of acclerating far faster through the first 5-10mph than a vehicle engaged in 4wd hi due to the low gear ratio. The fact that the Sequoia "took off like a shot" sounds to me like it was engaged in 4wd lo since its doubtful that it exceedded 10 mph in this demo.

pschreck · March 2002

Took off like a shot in 4WD low? I don't think so. Try taking "off like a shot "sometime in 4WD low. Not gonna happen. And to lock the center diff on a Sequoia the transmission also has to be in low. Trust me, I own this truck, and it ain't going anywhere fast in 4WD low and locked.

wwest · March 2002

that the total and complete difference was the VSC + TRAC systems. These would have both been active even in 2WD mode. T/L's new version of TRAC along with VSC deffinately makes a big difference in the LTS performance of my 2000 GS300 vs my 92 LS.

Put a Tahoe on an icy lake surface in 4WD and floor the throttle and try to control its direction. Now do the very same thing with any 2WD, AWD, or 4WD equipped with VSC.

cliffy1 · March 2002

I had a customer looking at a Land Cruiser two winters ago. As you all should know, the LC has the same 4WD Active-Trac system as the Sequoia. We took the vehicle to a parking lot that was covered in ice. I told the customer to stop, turn the wheel and then mash the gas. We actually did this before I saw the Sequoia demonstration. Rather than sliding and slipping across the parking lot, we heard a beeping noise from the dash and a clicking noise from under the hood. The beast made a rapid and uneventful u-turn. The customer's eyes got very wide and he exclaimed, "can we do that again?"

That customer was actually a participant here by the name of McCulloch. He bought the Cruiser and I had to ship it to him in California (he was here on business).

For those of you who doubt the ability of this system, wait until next winter and go to your local dealer on an icy day. This is a VERY easy demonstration to try. Once you do it, you will have no doubts.

jynewf · April 2002

As an owner of a 2000 4Runner 4x4 Limited, I'm agnostic as to the question of whether the Sequoia's or the GMC Envoy's system is better. I have to say, though, that I'm pretty skeptical of these brake-actuated 4x4 systems, although they seem to be proliferating (i.e. the Mercedes M-class, the 2000-2002 Landrover Discovery II (which, to my surprise, is not available with a locking center diff, although it will again be available 2003), and the 2001- 4runners and Sequoia). In fact I purposely purchased a 2000 4runner because it wouldn't have the TRACS system (the 2000 comes with a "multimode system" which is basically a part-time, center-locking diff system, with a rear-locking diff, as well as an AWD mode that simply disengages the diffs while still engaging the drive shaft to both the front and rear axles. There are obvious flaws with this system as well, which I won't go into here).

With respect to the Sequoia's TRACs system, I find the heated discusion re: the torque split curious. Assuming the center diff is not locked, then the system basically consists of three open differentials. All things being equal, torque will (1) try to equalize throughout a system; and (2) travel through the path of least resistance.

For the Sequoia, on dry pavement, torque is sent through both the front and rear drive shafts through the center diff. If the resistance is the same between the front and rear drive shafts (which I imagine it would on dry pavement), then the center diff would equalize the torque between the front and rear drive shafts. This would imply that the torque distribution is roughly 50/50 (give or take frictional losses) between the front and rear drive shafts. In fact, its not clear to me how, on dry pavement, the torque distribution could ever vary from a roughly 50/50 split between the driveshafts. In a system of open differentials, any difference in torque between the front and rear driveshafts would have to be equalized by the differentials, otherwise you would have disequilibrium in the system. A torque split that is not 50/50 could only be explained by the fact that (a) the center diff is not truly "open", or (b) the front and rear drive shafts are meeting different resistances.

As far as the Envoy is concerned, I don't pretend to know how that system works. However, assuming for the moment that the Envoy employs a viscous-coupled center differential, and, like the Sequoia, drives both the front and rear driveshaft through that differential, then I would surmise that the Envoy's system also splits torque 50/50 until a change in resistance occurs between the front and rear driveshafts. Why is that? A viscous-coupled differential acts like an open differential until shearing occurs. Until sufficient shearing occurs, then the encased liquid lacks sufficient viscosity to engage the differential.

Therefore while driving on dry pavement, the Envoy is splitting torque roughly 50/50 (again ignoring frictional losses). When slippage occurs at the front, torque is lost to the front axle until sufficient shearing occurs. During those split seconds (or however long it takes for the liquid to heat up and viscosity to increase) torque is sent to the front, and taken from the rear, such that the split might be 70 front/30 back (I'm making these numbers up). At maximum viscosity, the split is 38 front/62 rear. The system certainly delivers a minimum of 38% torque to the front drive shaft under adverse conditions, although under dry conditions it, like the Sequoia, should deliver a roughly 50/50 split.

Thus, based on the facts as I understand them, the real difference in the Sequoia and the Envoy is how the systems react to a loss of traction (brakes vs. VCD) Otherwise, on dry pavement, the distribution in torque should be the same.

With regard to how a brake-actuated system works when all 4 wheels are slipping, I'd like to know myself (another reason why I avoided the TRACS system). I've heard the stories about the Mercedes M-class grinding to a momentum sapping, brake-induced halt on snowy inclines. However, it is worth mentioning one thing: (1) how such a system reacts to all 4 wheels slipping depends on how the system defines "slipping". If slipping is defined for each wheel independently, i.e. the wheel must rotate a certain amount of revs before being braked, then it seems to me that a vehicle would get "stuck"; HOWEVER, if slipping is defined vis-vis other wheels, i.e. slipping means a wheel is rotating much faster then an adjacent wheel or wheels, then the vehicle might not necessarily be stuck. If all wheels are slipping at the same time and the same rate, that wouldn't count as slipping, and thus braking won't kick in. I don't know which system is on the Toyotas (or the Mercedes or LR Discos), but it would effect the answer.

Sorry for the long post.

wwest · April 2002

Might be a long post but seemingly quite accurate.

With one caviate.

On a four wheel dyno the RX300 indicates about 75/25 torque split F/R, with three open diff'ls with a VC across the center diff'l. All of my own shade tree testing indicates virtually no drive to the rear wheels if the fronts are allowed to spin freely.

Now...

The final drive ratio to the front is 3.29:1 and to the rear is 2.92:1 indicating to me that for every engine revolution the rear wheel tread surface moves further than the front. Seemingly that would mean the fronts are just idling along but testing seems to indicate otherwise.

And I think you're right about the Sequoia, or any brake implemented "LSD", if all wheels are turning, "slipping" at approximately the same rate then TRAC will not likely work.

BUT...

The RX300 already has a deceleration sensor and so the next step might be using that to tell if the wheel spin rate is a lie.

What is the Sequoia's final drive ratios F/R?

heatwave3 · April 2002

jynewf: Your summary above is for the most part on target when referencing the distinctions of the various systems on dry pavement. An open differential will distribute the power evenly so long as the traction exceeds the power. Once the power exceeds the traction is where the "issues" arise and for the most part the main reason to own 4wd is to provide more traction when power exceeds traction.

The viscous coupling center diff can be pre-set to deliver a fixed amount of torque to one end or the other which cannot be achieved with an open diff. Therefore where an open diff with a traction control system (Sequoia) will transfer all its power to one end or the other when slippage occurs, a VC setup will maintain some baseline of power regardless of traction thereby reducing the need to transfer all of the power back once traction is regained. At higher speeds on highways this type of minimum torque to both ends provides a better arrangement for on-road performance and is why AWD systems employ this approach.

The open diff systems with traction control may offer better flexibility for off-road performance particularly if the setup offers a locking center diff which is typically not offerred in an awd system using a VC center diff.

wwest · April 2002

Most*** VCs are pre-set to be TOTALLY slack until a disparate rotational rate occurs, and persists, between the two outputs of the center open differential.

As long as the disparate rotational rate persists the viscous fluid will continue to heat, growing hotter and hotter until the fluid volume expands enough to create enough pressure to form a firm enough coupling that the slower turning output shaft begins to come up to a speed more closely matching the faster shaft.

Notice that if the coupling coefficient ever becomes firm enough that the two shafts match turning rate the fluid will then rapidly cool and the VC will again go slack.

So, by the very nature of its operation the VC CAN NEVER fully equalize torque distribution, most of the "rotational force" will always go to the wheel(s) with the least traction.

*** Most VCs are mounted across the two output shafts of the center diff't to help eliminate the diff'l's willingness to route all of the force in the direction of the least resistance.

Factory pre-setting of A VC involves controlling the size of a gas bubble inside the sealed VC case and controlling the formulation of the fluid such that its expansion rate due to heating is appropreate to the rate of coupling "onset" desired.

The fluid will begin to expand as soon as it starts being heated due to disparate clutch plate turning rates but the coupling coefficient will not begin to increase until the gas bubble has been completely compressed by the fluid expansion.

So the onset of VC coupling capability is delayed by controlling the size of that gas bubble.

heatwave3 · April 2002

wwest: your following statement "Most*** VCs are pre-set to be TOTALLY slack until a disparate rotational rate occurs, and persists, between the two outputs of the center open differential." is only accurate for those "4wd" systems using a center VC.

Every "AWD" system using a center VC (to the best of my knowledge), in fact, IS pre-set to provide a minimum amount of torque that CANNOT be pre-set with a system using an open center diff. Your explanation is a very accurate description of why an AWD system using a center VC is measurably different from a 4wd system using a center VC with no minimum delivered torque or a 4wd system with an open center diff that CANNOT designate a minimum torque to both ends of the vehicle.

wwest · April 2002

1. It's up to the factory to determine the level of VC "tightness" in normal use and personally I see no reason at all why the VC should be anything but SLACK in NORMAL use since the center open diff'l does an absolutely adequate job as long as no wheel is slipping.

2. A VC across the center diff'l cannot be used to bias torque either way, front vs rear or vice versa.

heatwave3 · April 2002

wwest: on point # two almost all VCs in awd setups do, in fact, provide a bias minimum torque towards one end or the other. Check any web site for a hi-perf awd vehicle and they will typically provide the ratio applied to the bias. In most awd Porsches, you'll find a bias typically in the range of 45/55 fr/rr. In the GMC Denali and Escalade its 38/62 fr/rr.

On point one, you appear to be mis-understanding the difference between torque distributed evenly under an open diff (so long as it doesn't exceed traction) versus the benefits of "forcing" torque to a tire. The benefits come from the ability to better accelerate in turns and the ability to hold traction better in turns and upon acceleration with higher hp engines. That's why you'll find some of the highest G's in skid pad tests from hi-perf vehicles with AWD providing minimum torque to the front end. Likewise some the highest slaloom speeds will occur with AWD vehicles utilizing LSDs of some type.

The Porsche for example with high hp, can put more of it on the road by forcing it to multiple tires through an LSD than necessarily restricting available power by only reducing power to a tire with traction control. This explains why every high power awd vehicle in the world will employ LSDs in putting power to the road.

wwest · April 2002

I absolutely agree that a VC (I'm specifically not saying LSD because that would encompass other implementations) can be "manufactured" to have instantaneous "onset", for the racing venue, of one type or another, for instance.

But I see little reason for doing that with a vehicle built for everyday use, and the little testing I have done on the RX300 would indicate that it has very little, if any, AWD functionality on a "normal" roadbed other than that provided by the center "open" diff'l.

And there is yet another reason for most manufacturers not to have an AWD system (outside of the normal one with three open diff'ls) with quick onset or full-time engagement. That would create significant feedback to the steering input and just like the way many people reacted to ABS feedback to the brake pedal most people simply would not know how to react to a jerking steering wheel. Probably turn it loose and let it have its way!

Torque bias...

Now, if you can explain to me how a VC, alone, mounted across the center diff'l as in the RX can create a torque bias to either output shaft I send you a case of champagne.

I suspect that Lexus biases the RX (and HL{?})torque to the front (90/10 normal, 70/30 LTS) by using different final drive ratios front vs rear, but I don't know enough to argue that point one way or another.

wwest · April 2002

I happen to have an 01 911 AWD and I'm pretty sure the VC is INLINE with the front drive shaft and in that way "slack" it only allots 5% to the front. Same method as the T&C except it's VC is inline with the rear drive shaft to get 95/5 "slack".

jynewf · April 2002

Interesting responses;

A few questions and responses.

1. With respect to VC's being "pre-set to deliver a fixed amount of torque," I'm in agreement with wwest. My understanding is that VC's are typically slack (and thus act essentially like open differentials) until disparate rotation occur.

However, as I'm sure everyone is well aware, certain manufacturers advertise various f/r torque biases in their cars (i.e. jaguar x-type, BMW 330xi, BMW X5, etc.). Perhaps the manufacturers are oversimplifying for the sake of their customers, and in truth are stating the minimum torque bias, as opposed the permanent bias?

Otherwise, as stated by wwest, a fixed torque distribution in a VC implies: (a) steering feedback in turns as the wheels, in rotating at rates that differ from the set rotational rates (be it 45/55 or 32/68) fight to maintain its preset torque distribution; and (b) a signficant gas mileage penalty. Obviously, AWD/4WD systems incur a mileage penalty already, given the extra frictional losses of driving the extra drive shafts and differentials. However, a constant torque distribution in which the VC is not slack (and thus not open) would add such a significant constant frictional loss and heat build-up that I would imagine mileage would plummet.

Any explanation as to how a VC could implement constant torque distribution without the above problems would be greatly appreciated.

2. As far as the RX300, my guess is that the 25% rear torque bias just isn't enough to get the car moving from a stand still, whereas on a dyno, since the wheels meet no resistance, the rear wheels rotate as designed.

3. Aren't there road going implementations of brake-actuated traction control/AWD systems? I thought the audi quattro system was essentially a brake actuated system coupled with a center torsen differential. Likewise, I thought the BMW 330xi/X5 also used brake actuated systems (although I don't know whether they also use an LSD or other type of VC). Any input from BMW/Audi technophiles? The Mercedes ML, a road-biased SUV, also uses open differentials with a braking system.

Why is a brake-actuated system inherently inferior to a VC implementation on pavement? Is it because VC's allow for a putative f/r constant torque bias, and thus allows for slight oversteer?

Although handling considerations such as oversteer are paramount in a sports car/sports sedan, why is having such handling characteristics important in a lumbering 2 1/2 ton SUV? Why wouldn't the Sequoia's implementation work as well on a boat ramp (where theoretically, all power could be diverted to the wheels with grip, as opposed to a fixed distribution of torque as per mechanical lockers and/or a VC diff. with a fixed torque distribution)?

wwest · April 2002

The four wheel dyno I tested the Rx on had roadbed frictional drive resistance throughout the testing, for both front and rear drive wheels.

IMMHO the Sequoia system is far superior to a VC implemented AWD system. Constant "use" would wear out the brakes sooner but who needs constant use.

heatwave3 · April 2002

wwest: I don't want to suggest I am the absolute authority or for that matter a design engineer on these systems that would qualify me as an expert. I am familiar with the technologies involved an do have my opinions based on various 4wd system experience as well as examining the systems employed by some of the premier drivetrain manufacturers in the world.

Here are a few thoughts:

wwest: Regarding the Porsche 911 AWD. I find it surprising as an owner of this fine vehicle that you wouldn't agree that an awd system incorporating LSDs, center VC combined with traction control is superior to a system of traction control and 3 open diffs such as in the Sequoia. Do you think that your Porsche's performance would be superior if they eliminated the center VC and LSDs?

Here are two good sites on the subject:

The first <http://www.austinmonthly.com/articles/default.asp?ArticleID=54&mode=detail> shares the following views on the advantage of AWD in your Porsche, which I'm sure you agree with.

"With the AWD, you can actually feel the changing amounts of grip on the front tires as the car glides through an expanding corner.

The AWD offers exceptional traction on slippery surfaces, even though it was designed for performance on mainly dry surfaces, not as an all-weather traction control system. From a stop, you can crank the wheel 90-degrees and stand on it without making any steering correction. The rear end wont slide (power oversteer) and the front end wont wash (understeer). The Carerra 4 just accelerates awayand quite quickly at that."

For road performance this is the exact type of traction sensation (albeit at much slower speeds) that I have experienced in a 2.5 ton SUV with AWD, a center VC with 38/62 torque distribution and a rear LSD. In poor road conditions it is unflappable and superior to traditional 4wd systems, IMO. The question is whether its superior to a 4wd system utilizing 3 open diffs and traction control.

Theoretically, I believe it is, although I have little experience behind the wheel of a 4wd system like the Sequoia's other than a short test drive on dry pavement. I would think if it were superior that Porsche (and all the other world-wide manufacturers of awd systems) would have dropped the expense and weight of LSDs and VCs in favor of open diffs and trac control. The reason they don't, IMO, is the diminished traction of an open diff system, even though the trac control will reduce slippage it will not force a minimum amount of torque to a wheel.

The second site further explains the Porsche system of AWD, which is very similar to the GMC setup without the electronic traction control.

<http://www.fortunecity.com/silverstone/lancia/58/911/911_9.htm>

The following is an exerpt "To make the viscous-coupling always engaged the front wheels, the rear tyres were made marginally smaller in diameter, enhance established a small speed difference between the drive shafts to front and rear. With the speed difference, the viscous liquid normally transferred 5-15% torque to the front axle, which was much less than the 964s system. In abnormal conditions, that is, whenever one axle lost grip, the viscous-coupling LSD may send up to almost 100% torque to the other axle.

Both the center LSD and rear LSD were now pure mechanical, but clever electronics was used in the newly-added ABD (Automatic Brake Differential). Again, ABD was simple yet effective. It was just a program, sharing all the hardware with ABS. Whenever rear wheels spin, it braked the spinning wheel thus the rear differential would send more torque to the other wheel. It was particularly useful for extreme conditions such as on snow, while LSD covered most normal conditions."

The Porsche's have now increased the front bias range to 5-40%, instead of 5-15%.

heatwave3 · April 2002

jnewf: regarding the torque feedback of the front wheels. I have never experienced it in the Denali XL and since I own several fwd vehicles now and others in the past, I would readily recognize the sensation.

Regarding the explanation of how the VC designates a minimum torque to one end or the other, it is beyond my technical expertise. I can say that all manufacturers of AWD systems are quite consistent in the description of the distribution as the "minimuM" torque delivered.

Regarding your question "Aren't there road going implementations of brake-actuated traction control/AWD systems? ", I'm not aware of any AWD systems that employ brake actuated traction control, all open diffs and AWD. I'd be most interest in any example you can find.

On your next question "Why is a brake-actuated system inherently inferior to a VC implementation on pavement? Is it because VC's allow for a putative f/r constant torque bias, and thus allows for slight oversteer?", I do not think you'll find torque steer in any center VC
design, mainly because they are generally accompanied by an open front diff.

The following are excellent questions "Although handling considerations such as oversteer are paramount in a sports car/sports sedan, why is having such handling characteristics important in a lumbering 2 1/2 ton SUV? Why wouldn't the Sequoia's implementation work as well on a boat ramp (where theoretically, all power could be diverted to the wheels with grip, as opposed to a fixed distribution of torque as per mechanical lockers and/or a VC diff. with a fixed torque distribution)?"

Here's my shot at answering. Before owning the Denali XL I would have agreed with the premise of your question and in fact its why I was skeptical at first and nearly went with the more traditional setup in the 3/4 ton Yukon XL.

I'm glad I didn't. I can honestly state the traction is remarkable and the sensation even on dry pavement has a sense of both pulling and pushing the vehicle forward at the same time. Hard to explain but can definitely be felt in the seat of the pants and substantially different from my previous experience in traditional 4wd systems. I, for one, will never go back unless I really felt a need for off-roading.

Having towed boats for 20 years, wet boat ramps are always a challenge. My experience tells me I want the power at both wheels to START with, not going through an open diff, slipping and then transferring to the other side. Even if the transfer is measured in fractions of a second. My first Sub had an open diff rear and I swore I'd never do it again. Invariably I'd start slipping, have to stop and shift into 4wd to get out. Would the trac control have prevented it? Possibly, however with the LSD it never slips therefore the need for the trac control will be remote.

I think your last question is the heart of the engineer's design decision of an open diff setup with traction control. A center VC with LSDs or auto lockers will definitely be less fuel efficiently. Likewise it will rob some hp during normal operation.

In the case of the Sequoia, it starts with 240hp and gets 14/17mpg. It doesn't have the hp to "throw away" on the drivetrain and the fuel economy is already as low as I'm sure Toyota wants to be publicly associated with.

The Denali XL starts with 325hp and gets 12/15mpg. It has more than enough hp to spare some on the drivetrain and most owners of this vehicle aren't that focussed on the fuel economy.

pschreck · April 2002

If the Denali is geared for towing, doesn't it already throw away most of its horsepower under normal driving conditions? Just asking because it would seem that low gearing would require a lot more horsepower to get the vehicle up to 65mph in a reasonable amount of time and then maintaining that speed.

wwest · April 2002

My 911 IS NOT an SUV. As a matter of fact except for maybe a few sunny days it stays in the garage all winter.

If I ever decide to drive my RX300 the way I like to drive my 911 then you can just go ahead and bury me since I'd be ready soon anyway.

Clarifications...

Torque steer is caused by more "wind-up" on the longer half-shaft of front wheel drive. Both my 00 and my 01 AWD RX300s exhibited significant torque steer when cornering under moderate to hard acceleration.

PSM, Porsche Stability Management.

PSM....OFF

Hard left turn, engine rpm UP, pop clutch, NEVER-ENDING oversteer!

PSM....ON

Same procedure..then almost instant PSM rear brake application followed swiftly by dethrottling unless you get there first.

Straight line acceleration in the wet, rear wheel slips, PSM brake application....if slippage continues PSM dethrottles.

Sounds sorta like the Sequoia system as backup for the VC doesn't it?

01 911 at:

http://www.hobbystage.net/porsche/whatever

Really, the PSM is a wonder, it seems to detect an out of whack motion long before my seat of the pants motion sensor,and its inputs are so unobtrusive you wouldn't know it was helping at times except for the indicator.

SUV.

I owned two Jeeps prior to the RX, an 85 and a 92. Both had full-time AWD(VC) and part-time 4WD(locked center diff'l).

There were many times when I have to go to the part-time mode to get moving in LTS conditions.

Both Jeeps would provide quite serious "nutating" feedback to the steering wheel when turning and accelerating even in full-time, VC AWD mode, and in part-time mode it would bust your knuckles.

heatwave3 · April 2002

pschreck: the Denali and XL uses the middle of the road rear differential ratio (3.73). For heavier towing you would want the 4.10 rear with the power of the 8.1L engine. For lighter duty service in the Yukon you can order it with a 3.42 rear for better fuel economy.

The Denali uses the 3.73 offerring the best of towing and fuel economy.

cliffy1 · April 2002

Good description of the 911.

heatwave3 · April 2002

wwest: given your great description of the Porsche performance, why do you think they put an LSD in the rear coupled with a center VC AND the PSM system? What advantage do you believe the LSD in the rear provides that could not be accomplished by the PSM alone?

The answer to this question is why I believe an AWD system using a center VC and a rear LSD is a superior drivetrain setup for on-road traction than three open diffs with traction control. Although I readily admit that the AWD setup in the Porsche is even better by layering on the traction control to the mechanical awd system.

I believe this is exactly what Ford is planning for their upcoming awd system destined for their F/S suvs. I also believe that Cadillac will layer traction control to their Stabilitrak on top of their AWD system and likewise offer a similar drivetrain in the GMC models.

pschreck · April 2002

Where does it say that the '01 911 has a LSD in the rear. Just wondering. I did see that the '89 had a LSD. I'm not much of a Porsche guy, but I'd like to be. ;-)

wwest · April 2002

Can't say about earlier models, but my 911 uses TRAC(?porsche?) to supplant the need for a mechanical LSD.

heatwave3 · April 2002

wwest: while I'll take you word for it, I could find nothing on the net sugesting what type of diff is in the rear of an awd 911. PMS is an option in all Porsche's except the carrera 4 and in all other Porsche's they have an LSD in the rear. While its possible the AWD doesn't have an LSD due to the PMS, its my guess that its the same rear diff as other 911's. If you can point to any info outlining that the awd 911 has an open rear diff I'd be most interested in reading it.

wwest · April 2002

Go to rennlist...forums...996 and search for PSM & LSD

muskadine · April 2002

First of all I'd like to thank everyone who has contributed to this thread; I've really learned a lot.

My wife and I are thinking about getting a medium-sized SUV, and we're looking at the Toyota 4Runner (the frontrunner) and the Nissan Pathfinder (a distant second). Are there any interesting, notable, or important differences between the 4WD systems on these two vehicles?

Any advice, feedback, or comments would be greatly appreciated.

muskadine

wwest · April 2002

Isn't the Sequoia and the 4runner basically the same vehicle?

corsicachevy · April 2002

"Isn't the Sequoia and the 4runner basically the same vehicle?"

Not even close. The two are manufactured by Toyota, but that is about where the similarities end.

loma1 · April 2002

Their four wheel drive systems are similar in that they use the active trac. The main difference is that you have a switch on the 4Runner to lock the center diff and you can do that in hi or low and in any transmission gear. Youn also have a traditional shift lever on the 4Runner for hi and low.

cliffy1 · April 2002

I've been wrong once or twice before in my life, but I believe the Pathfinder uses a basic part time system still. If you go back to the beginning of this discussion, read about the 4Runner 4WD system and then look at the Tacoma/Tundra 4WD system and compare the two. I think the Pathfinder uses the same center transfer case as the Tacoma.

walkercm · April 2002

I recently purchased a 2002 4runner, and considered a patherfinder too. The "SE" model comes with a traditional part-time system, but the more expensive "LE" model comes standard with the "Active Trac" (all mode 4wd) system which is basically a button or dial to goto 4wd. Overall, I liked the patherfinder better, but couldn't get over putting the high-octane fuel in it as it requires - so I wend with the 4runner.

pschreck · April 2002

We had looked at the Pathfinder. Too cramped. I think the 4WD on the LE works like the auto 4WD on the GM trucks. That is it doesn't send any power to the front wheels unless the rears slip. That was the impression that the dealer gave me anyhow.

muskadine · April 2002

Hmm...

Thanks for the info on these two vehicles. For me, a big selling point is a more "usable" full time 4WD system on a vehicle. I live in Northern California, where it can rain a lot, and I do go to the Sierras in the winter. The 4Runner seems like the more sensible system for mild to moderate inclement weather, but with the balls to deal with the heavy stuff, albeit rare as it may come. I'd rather have an AWD system that could prevent me from needing to pull a partially wrecked vehicle from a ditch after I've slipped off the highway. No matter what, this is an interesting way to market the Pathfinder versus the 4Runner.

muskadine

pschreck · April 2002

I agree. The Active TRAC system is just one reason why we bought a Sequoia. The 4WD system is just so much more useful than a part time system or a straight AWD system. I hope the 4Runner works out as well for you as the Sequoia has for us.

sunshine60105 · April 2002

What is the differnce between AWD and 4 wheel drive? Which is better? Could someone explain this to be easy? Thanks in advance.....
Sunshine60105!

cliffy1 · April 2002

Did you start at the beginning of this topic? If not, you should go back and read the first 5 posts. That should answer your questions about the difference.

The real bottom line is, your question is often a matter of semantics. In general, an AWD uses a light duty viscous coupling center differential that will handle most soccer mom duties. 4WD (either part time or full time) tends to be more robust and able to handle more treacherous situations. The Active Trac system described in post #3 is really the best of all worlds by combining the seamlessness of an AWD with the ruggedness and utility of a 4WD.

heatwave3 · April 2002

sunshine: cliffy's response above is a brief but accurate summary of the different systems. One point to note is that the Active Trac system he refers to has open differentials at both the rear and center of the system deployed on the Sequoia.

Its worth considering that an all open diff system will have some unique disadvantages that won't occur in vehicles with a limited slip center and rear differentials. Some of the disadvantage is compensated for by the electronic traction control utilizing the brakes to reduce slippage, howver it does not completely compensate in the same manner as a Limited Slip Differential (LSD) will.

pschreck · April 2002

Actually all of the disadvantages of open diffs are compensated for with the Active Trac system. I own a Sequoia and a Subaru Outback. Trust me, you'll be VERY happy with an Active Trac equipped vehicle. The real plus is that it will be a Toyota so you can rest assured that the reliability will outshine any American engineered vehicle. And it will most likely be built in the USA to boot.

heatwave3 · April 2002

sunshine: while pschreck provides a valid perspective of his personal experience with his Sequoia, you would be doing yourself a favor to check out the Sequoia forum. You'll find a fair amount of issues and a number of owners that are disatisfied enough to want to sell their Sequoia.

They are clearly in the minority as most owners appear satisfied with their Sequoia, however there appears to be enough disatisfaction and issues with the quality and performance of the 4wd system to warrant a close examination of the vehicle and an even closer comparison to other alternatives on the market. Even those from GM and Ford.

pschreck · April 2002

Of course you should check out the Sequoia forum. While you're at it, be sure to check out the Tahoe/Yukon forum (talk about problems) and any other forum that might provide you with the info you need to make a decision. Whatever you decide, I wish you good luck and enjoyment with your multi thousand dollar purchase.

makakio · May 2002

Hi all -

Just wanted to know if something like this has happened to anyone else and if I'm retarded or what but this past winter I parked my SR5 (read: not active trak but 'normal' Toyo 4WD) after a day of skiing in 2WD (roads were dry) and it snowed about two feet that night. Next morning, I tried to pull up the oh-so-slight incline of my buddy's driveway and VSC/Traction kicks in, retarding power to where I make no progress because of the wheelspin. Can't 'rock' it loose and MORE IMPORTANTLY a push of the button to 4WD only causes dash lights to blink and it won't engage in 4WD (it seems to take a long time - like 250 feet of pavement when moving under normal conditions anyway) - so effectively I'm stuck. Pressing diff lock button does NOT cancel VSC/Traction and though I moved the lever around into 4LO (trans in Neutral, of course) it still was no use as 4WD would not engage. Bottom line: I had to have two friends push me up and out of the driveway, until I could go the usual block or so under 2WD power and 4WD finally set in.

SO WHAT GOOD is the "new" 4WD system if you can't use it to get un-stuck (read: you have to be IN it to use it)? My crappy old Explorer would at least sit and spin wheels until the 4WD locked in...

Very disappointed that VSC and Trac cannot be switched off at the dash - I could've rocked it loose if that were the case. What a stupid oversight on a truck made for heavy-duty offroad use.

That said - am I missing something here? It's my wife's car so I'm happy to have VSC and Traction control for her but I feel that Toyo neutered this truck in a bad way.

Comments/suggestions/etc greatly appreciated.
Matt

Toyota 4WD systems explained

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