Unlocking the mystery of AWD (1 Viewer)

[SUMOTOY]

Torque v Traction

Sumo, thats interesting info. To be honest I have noticed a SIGNIFICANT improvement in stopping distances in snow with CDL locked vs ABS and CDL off.

But when negotiating a curve at 50 MPH I dont like the "slipping" feeling I have when CDL is on. And I know its the right thing to do to power out of the corner and keep the wheels spinning, but it doesnt make for a relaxing 400km drive to the coast from the Okanagan... I think Im simply gonna have to try a couple hundred clicks with CDL on and off.


Thanks...

Again, a lot of this is personal preference. We are starting to stray from the objective properties of CDL in terms of torque allocation, and getting into the more subjective dynamics of traction on handling and comfort . In terms of physics, the advantages of locked vs unlocked in a straight line are tough to argue, in terms of 3 channel ABS, the advantage goes to the locked center in most conditions. In terms of 'feel', it's completely the skill set. I say try both often. The long wheelbase and VC slip allow some slip between "locked" axles makes turning a bit easier, unlike the air lockers in a Audi Quattro for instance, that are mechanical dog clutch type with short wheelbases. Anytime conditions get slippery in Chicago, I go to locked center diff, 4Runner, 80, or Quattro. Certainly for those less involved in the how and why of diff lock driving, don't do the CDL switch or pin7 mod. Let the Toyota ABS system do it's job. Regardless of which you choose however, tire technology has come a long way in optimizing winter driving in terms of traction.

My advocation in AWD systems is to get folks to understand exactly what and how their vehicle's awd system works, and what are the limitations to it's function in terms of torque allocation>traction vs dynamic characteristics (turns/control). In the world of center diff options/choices for real world on road driving, Toyota's choice of the expensive VCDL is one of the best. Add in the F/R locker option, and there is no better in terms of absolute traction (read any 1 of 4 wheels can support 100% of engine torque). The real issue in day to day driving is indeed 'feel', comfort and control. Darn those turns.

My contrasting experience is that the CDL on has better predictability than a LSD or sensor switched CDL. If you know your machine understeers more, you can predict it's behavior much more consistently, especially in Hhp:cf conditions. I'm not a fan of sensor based lockup, especially in turns. The very last thing I want when possibly dancing on the fine line of control, is something electronic in my Toyota deciding it's time to lock up the center diff.

The ABS/EDL/ESP/traction control systems are getting quite sophisticated these days. The real problem becomes their inability to make a judgement, anticipate conditions. And their biggest liability (IMO) is their tunnel vision towards the primary task, avoiding loss of traction. But loss of traction can happen, and there are some inherent dangers in many of these electronic and LSD devices that could actually conflict with the judgement of the driver at the wheel. There's nothing "relaxing" about that possibility to me. I am quite content with the predictable understeer and tire slip of a locked center diff.

My opinions abound on what works best on and off road and why. That aside, my original purpose was only to present exactly what Toyota gave us in terms of the CDL/ABS equipped 80 AWD truck.

HTH

Scott Justusson
'94 FZJ80
'87 4R turbo

 

[Hooligan]

Look at the very first FAQ in the FAQ list

https://forum.ih8mud.com/showthread.php?t=12828

 

[SUMOTOY]

Hoping for some clarification/correction?

Look at the very first FAQ in the FAQ list

https://forum.ih8mud.com/showthread.php?t=12828

Very general, some incorrect, the above information should prove to be much more specific to the 80. I will also note in that FAQ you reference contains a post on "real time 4wd", which is by definition a part time 4wd system ala audi TT quattro and vw 4motion. It is 'marketed' as full time awd, because by definition some torque *has* to be present in both axles (like 5% or less normally to the rear). There is no question that a coupler (viscous or Haldex type) without a center diff is a part time 4wd system.

Luckily for us 80 owners, Toyota called it right with AWD, and in fact understated it, since a viscous couple differential lock is certainly a system worth bragging about. In fact, add in the optional locker front and rears, top gun in terms of absolute traction. There are only 2 modern awd vehicles that can put 100% of torque to a single wheel in production form, LC80 (locker option) and the Mercedes G-wagon.

HTH

Scott Justusson

 

[Hooligan]

Very general, some incorrect, the above information should prove to be much more specific to the 80. son

Yes, well this is an 80 tech forum.

 

[powderpig]

Well no one has quite explained it like this before. Wonderful to read and understand the system more. Thanks scott. later robbie

 

[Rich]

...The biggest misconception on a locked center diff is that's it's a 50/50 torque split. In fact, in a locked center diff, torque split on acceleration/deceleration follows exactly weight distribution...

Scott, was following along ok until I got to the part above, which I believe to be misleading.

The traction between the wheel and the ground limits the max torque possible for a given tire/axle shaft. And weight on the tire is one factor, but not the only factor, determing the max traction available.

With the diffs locked, in a situation where the torque being applied to all of the wheels is less than the max traction force available, in other words, even if not locked, no tires would lose traction and start to spin up, then both the power and the torque to each wheel is going to be equal, all of the way around, with the diffs locked, regardless of weight distribution.

With the diffs locked, the only time there will be torque differences to the wheels, is when one or more wheels has insufficient traction to apply that torque to the road, in which case, the lockers will enable more torque, and therefore more power, to be transmitted via the other wheels with more traction.

I do appreciate that weight distribution is one factor that affects how much traction exists at any particular tire, but it is not the only factor, and in any event, is not a factor in the torque or power split until the torque at the tire exceeds the traction available at the tire.

 

[IdahoDoug]

Actually, what Scott said is quite accurate but you have to get fairly esoteric to get into it. Tires with torque applied to them are actually "slipping" all the time. More weight = less slip all else equal. So, on a given surface that's got a uniform coefficient of friction, the tires with more weight on them will slip less in this micro sense, which technically then applies more torque to them as the others have slipped a bit. Being locked together, the non slipping tires are then driving the vehicle more forcefully.

Splitting hairs at this point, but......

DougM

 

[Rich]

Well, I gues Scott would have to define what he meant by "...torque split on acceleration/deceleration follows exactly weight distribution." I presumed that he meant, in the context of locked diffs, the torque is directly proportional to the weight on each wheel. Maybe that is a mistake on my part.

 

[Ocelot]

"real time 4wd", which is by definition a part time 4wd system ala audi TT quattro and vw 4motion.

Part Time 4WD is normal 4WD (ie 2WD, 4 High, & 4 Low). There is no center diff in a part time 4WD system, nor can it be used on road... therefore I doubt an Audi or VW would be equiped with this system

 

[IdahoDoug]

Ocelot,

A "part time 4wd" system would be any system where it is capable of essentially operating in 2wd mode. In some definitions, having a transfer case where you manually engage 2 or 4wd would be the only way this definition fits, but in today's modern world filled with hybrid systems and automated systems it may be best to fall back to that basic definition. There are a lot of AWD systems that operate essentially in 2wd unless slippage occurs and these could rightly be considered "part time 4wd" from a functional view, a view that may be appropriate to adopt in world where the lines are increasingly blurry.

For instance, your statement that a part time 4wd system cannot be used on road is incorrect. I had a 1999 full size Mitsubishi Montero that had the following positions on its transfer case lever:

2wd
4wd unlocked high range
4wd locked high range
4wd locked low range (your traditional 4wd)

It also had a factory rear locker and was quite capable offroad, which is beside the point - which is that there are a lot of variations out there these days.


DougM

 

[fzj80kidpen]

Great thread,
I think you all have covered all the details of these nice HF2AV Transfer systems but I have one loaded questions.
I don't mean to hijack and will be glad to open a seperate thread but it seems to hit here.
What is the average life expectancy of this system?
Anyone out there had to replace theirs?
What year is your rig, when did it die (miles).
I can't be the only one wondering

 

[SUMOTOY]

Slip = Traction

Scott, was following along ok until I got to the part above, which I believe to be misleading.

The traction between the wheel and the ground limits the max torque possible for a given tire/axle shaft. And weight on the tire is one factor, but not the only factor, determing the max traction available.

Ok, let's assume same tire size tread and cf. Slip follows weight transfer, as does torque. Since torque caused the slip...

With the diffs locked, in a situation where the torque being applied to all of the wheels is less than the max traction force available, in other words, even if not locked, no tires would lose traction and start to spin up, then both the power and the torque to each wheel is going to be equal, all of the way around, with the diffs locked, regardless of weight distribution.

Slip and spin are the same concept. Spin is just 100% slip. A tire loses traction as soon as force is applied to it. Torque to the wheel is only equal all the time all the way around when you have an open diff. A LSD or Locked diff really can't send equal torque to each wheel in a dynamic sense. I suppose you could say for a split second torque could be equal, but then it changes again. This is the biggest misunderstanding of locked and gear type LSD devices.

With the diffs locked, the only time there will be torque differences to the wheels, is when one or more wheels has insufficient traction to apply that torque to the road, in which case, the lockers will enable more torque, and therefore more power, to be transmitted via the other wheels with more traction.

Think of traction as wheel slip. When you accelerate a vehicle it puts more weight on the rear wheels, hence the fronts will slip more than the rears by definition. In a center locked diff scenario (straight line acceleration/deceleration, same tires, same cf) torque follows exactly weight distribution, up to and including spin. Think of this as before a spinning tire happens. This is a known and accepted physical property of a center locked diff. It is also a known and accepted physical property of a center locked diff that you have ideal brake force distribution as well.

I do appreciate that weight distribution is one factor that affects how much traction exists at any particular tire, but it is not the only factor, and in any event, is not a factor in the torque or power split until the torque at the tire exceeds the traction available at the tire.

Ok, I see the issue. Remember that slip happens as soon as you apply torque to a wheel. More slip with a tire with less weight on it, less slip with a tire with more weight on it. So your statement is kinda true I suppose. Physics would dictate that any torque applied to a wheel causes it to exceed the traction available at the tire (compared to a stopped vehicle), it's called tire slip. I'll grab some SAE articles from my office tomorrow that should help you understand this concept better.

Some Audisport trivia: Audisport removed the center diff alltogether in the 80's WRC rally cars. Why? Because Audi found that with a static weight distribution of 55/45 under maximum acceleration the locked diff had a 50/50 torque distribution.

I'll stick with this a while, I'm hoping someone will alert me when it's overextended it's welcome.

HTH

Scott Justusson
QSHIPQ Performance Tuning
Chicago

 

[SUMOTOY]

Part Time 4wd

Idaho Doug:
Thanks on the posts above. Agree in both... In a locked diff, torque follows exactly weight transfer, no splitting hairs. I think you hit the nail on the head with the definition of tire slip.

Ocelot, remember, a Audi TT and a VW 4 motion *have NO center diff*. Hence it is exactly a part time 4wd system. A coupler based 4wd system is not awd, nor traditional quattro. In the TT and 4motion Haldex systems, once the 4wd is enabled, it is a locked (no center diff) 4wd vehicle. "low range" doesn't need to apply here in definining PT4WD, only that it's not full time awd, and the 4WD mode has no center diff. There are more of these systems around than you may think, and more coming.

HTH

Scott Justusson
QSHIPQ Performance Tuning
Chicago
'94 FZJ80

Ocelot,

A "part time 4wd" system would be any system where it is capable of essentially operating in 2wd mode. In some definitions, having a transfer case where you manually engage 2 or 4wd would be the only way this definition fits, but in today's modern world filled with hybrid systems and automated systems it may be best to fall back to that basic definition. There are a lot of AWD systems that operate essentially in 2wd unless slippage occurs and these could rightly be considered "part time 4wd" from a functional view, a view that may be appropriate to adopt in world where the lines are increasingly blurry.
DougM

 

[Ocelot]

I didn't realise that the audi quattro / vw systems were part-time 4WD... sorry. I woulda thought cars would have had full-time/AWD systems

 

[IdahoDoug]

The older Quattro systems were the hot setup, IMHO. The 85-89ish models had manually locking front, center and rear diffs and they were amazing. Stout, too as they were early gen stuff and Audi overbuilt it so they could also race with it. AFAIK, those systems are still running fine though other things on the vehicle die. Mine was a 97 A6 Wagon (best weight dist) and with studded Hakka 1's and its tepid power to weight I could floor it on hard packed road snow and only get a bit of wheelspin before it hooked up and pulled away as if on dry pavement. This was not the better system, either.

As to the above question on the 80s drivetrain components. If you don't overgrease/hydrolock the drive shafts and keep fresh gear oil in them, and don't pull something heavy in reverse, they should run a half million miles or more in road service. My bro in law drove his with no front gear oil for at least 10,000 miles after 180,000 miles of casual maintenance before it gave up. These are the last of the big-parts heavy diffs. For instance, normal transfer cases use a chain drive which dies early if it's used much, where these are direct gear drive center diffs like they used to build 30 years ago (legendary NP205 comes to mind). Ya gotta break these.

DougM

 

[SUMOTOY]

The older Quattro systems were the hot setup, IMHO. The 85-89ish models had manually locking front, center and rear diffs and they were amazing. Stout, too as they were early gen stuff and Audi overbuilt it so they could also race with it. AFAIK, those systems are still running fine though other things on the vehicle die. Mine was a 97 A6 Wagon (best weight dist) and with studded Hakka 1's and its tepid power to weight I could floor it on hard packed road snow and only get a bit of wheelspin before it hooked up and pulled away as if on dry pavement. This was not the better system, either.

DougM

Ok Doug, now you are in my shop
The audi quattro was introduced in 1982 in Gen 1 form with vacuum actuated dog clutch locking center and rear diffs and this system continued thru 1988. Audi never offered a locking front diff in production, but did race with it in World Rally (driver preference, most ran open front or 20% locking VC). Indeed the 016 trans was very bulletproof, though I helped a friend find the limit installing a tweeked LT1 into a quattro. The front ring gear tends to strip over about 450lb/ft of torque.

The Gen II system introduced in 1989>present (excluding Haldex), including your ex A6 wagon, uses a gear type Torsen Center differential. It's main advantage (really the only reason Audi committed to it), was it's ability to give non driver activated LSD properties while retaining ABS. My quest into on/off road and track handling of awd differentials started at this stage around 1992, after a couple of track incidents where the torsen did unexpected things while I was at the wheel (very nerdy discussion I won't detail). I eventually learned from the engineers at Torsen (and the inventor Chocholek himself) that indeed the torsen could be fooled into strange behaviors at the limits of adhesion. After a couple hundred posts on the subject and mucho research into diffs, I acquired my 80, which I truly feel has the best center diff setup available (with minimal non driver intervention) for a production AWD vehicle, next to my quattros of course. And find the mud forum, now here I go again on the diff discussion...

For offroad, there is no question that a locking front diff is top gun (100% of torque to any wheel), however, for on road dynamics, it's more than a handful. I drove a couple hundred miles in a S2 quattro Rally car over the last 6 years, with locked F/C/R, and really it's more concentration than enjoyment in anything other than snow or gravel.

It's always fascinating to me to enter discussions on differentials where many misunderstandings of them abound, even from the engineers themselves (recalling a discussion with a Torsen Engineer a year ago). It's also interesting to note that many of the recent winning race cars with awd (including audi) ran with center diffs locked and LSD rears. The newest technology is adjustable differentials (ala subaru STI), but even today, just a locking center, on or off road is a tough application to beat.

Thanks for allowing my drivel folks

Scott Justusson
QSHIPQ Performance Tuning
www.qshipq.com
Chicago IL
'97 FZJ80
'87 4 Runner turbo A/T
'91 v8 quattro (CDL center/torsen rear)
'84 Turbo Quattro Coupe (locking center, locking rear)

 

[SUMOTOY]

... And back to the original post

Thirdly, I assume that AWD power is not divided evenly front to back because 80 series cruisers have the 8” or whatever front diff while the back is 9.5” or whatever.

Size doesn't matter? A 4.11 is a 4.11 whatever size the housing is isn't relevent to power distribution. Normally fixed split (other than 50/50) diffs do so before the diffs (read in the trans).

So to recapitulate the questions…

1) Is there any real benefit to having CDL on in snowy driving conditions or is it in fact a detriment?

see posts above

2) Does power go to the wheels that slip or the wheels that grip?

in and open diff each wheel supports the same amount of engine torque. The wheels that slip dictate how much power goes to the wheels that grip. If the slipping (spinning) wheel takes 8lb/ft, the non spinning wheels will only put down 8lb/ft regardless of *potential traction*. With the center diff locked, the wheels with grip (assuming same cf side to side) can tractively transfer 100% of engine torque.

What if you’ve got crappy grip on the fronts (ice) and good grip on the backs (no ice), will most of the power still go to the fronts?

Again in an open diff, the power is identical. In a locked center most/all of the power will go to the rears.

3) What is the power distribution in AWD going down the road with all 4 wheels getting equal traction?

open diff the power is ALWAYS 50f/50r, anytime, any scenario. In a locked diff it varies based on wheel slip (read amount of torque applied, weight distribution and/or turning radius = F/R tire slip)

HTH
Scott Justusson

 

[GXO]

We had a nice ice storm in Atlanta this past winter (3 inches of slush with 1 inch of ice under it) and I noticed significant improvements in handling with the CDL off. I got squirrely VERY quickly with the CDL on.

I could think about all that mechanical stuff, but why? I drove it. I proved it. I don't have that same hardware as most of you all, but I do have AWD with the old school system.

3FE Powa!

 

[SUMOTOY]

We had a nice ice storm in Atlanta this past winter (3 inches of slush with 1 inch of ice under it) and I noticed significant improvements in handling with the CDL off. I got squirrely VERY quickly with the CDL on.

I could think about all that mechanical stuff, but why? I drove it. I proved it. I don't have that same hardware as most of you all, but I do have AWD with the old school system.

3FE Powa!

Could be a miriad of explanations here, my bet is you had exactly the wrong tires for the conditions. As such, it could well be that the open diff gave you better *control*, for it allows speed differentiation of the tires while applying 50/50 torque distribution all the time. With a locked diff, your torque will vary based on slip conditions of the f/r tires (the slush did it?), which means that the locked diff will hunt for traction and will always prevent wheel speed differential, and in turns, it allocates torque to the rear based on turning radius = sometimes a bad thing in ice driving... I'd also venture an opinion based on my ice driving experiences, that a proper set of tires for the conditions would have changed your thinking in a big way.

HTH

Scott Justusson

 

[LandyLover]

Scott,

I guess what Im saying is if the front gear is smaller and the power split is 50/50 front/back then presumably the front gearset will wear faster than the rear. I would see this to be especially true when all diff locks are on and you wheel hard in the rocks... no?