Difference between HF2A & HF2AV transfercase

[Hayes]

RT, that link takes me to the "Airplane on a treadmill" debacle. Did you really mean to send me there? I'm not sure I want to relive that thread.

I re-read the FSM page in your original link. I originally assumed that humping was bad--it burned up your VC or something. After reading it again, I guess it's part of the design--a way to transfer more torque through the VC? When the plates contact each other it's more like a clutch, less like a viscous coupling? Still, I wonder if excessive humping would damage the plates.
I also assume that getting the VC fluid hot enough to cause humping would demand a whole lot of differential torque at the VC--meaning a whole lot of wheel spin? How long do you think it would take (say, spinning the rear drive shaft while stuck in snow or mud) for the VC to get to maximum viscosity and start humping? Could you tell when it had reached this point?
Sorry, just thinking out loud.

Also, I thought I read somewhere about a 40/60 torque bias in the 80. I'll see if I can find it.

Hayes

 

[RavenTai]

oops, too many windows open.

try this

http://webpages.charter.net/raventai/HF2AV TRANSFER.pdf

there is no torque bias in the 80 series

 

[Hayes]

Thanks RT. My bad for not reading the thread thoroughly.

In the meantime, I was busy editing and adding to my previous post why you were busy getting me the right link.

Any thoughts on my newly-added questions in post #41?

Hayes

 

[RavenTai]

I re-read the FSM page in your original link. I originally assumed that humping was bad--it burned up your VC or something. After reading it again, I guess it's part of the design--a way to transfer more torque through the VC?

When the plates contact each other it's more like a clutch, less like a viscous coupling? Still, I wonder if excessive humping would damage the plates.
I also assume that getting the VC fluid hot enough to cause humping would demand a whole lot of differential torque at the VC--meaning a whole lot of wheel spin? How long do you think it would take (say, spinning the rear drive shaft while stuck in snow or mud) for the VC to get to maximum viscosity and start humping? Could you tell when it had reached this point?
Sorry, just thinking out loud.

Also, I thought I read somewhere about a 40/60 torque bias in the 80. I'll see if I can find it.

Hayes

Again the VC is not torque sensitive. It is not a Torsen, Torsens (derived from the words Torque & Sensing) can have torque splits can sense and respond to torque but the VC as it is used in app does not.

The VC acts on RPM, the higher the RPM difference the higher the resistance to difference is placed on the center differential

for example a frozen lake try driving across it with heavy throttle you would likely get the VC to activate quite a bit due to wheel spin, but the torque would be very low as there is almost no traction,

on dry pavement during spirited driving you are unlikely to break free a wheel but the torque applied to the drive train will be high the VC is not likely to activate because all the tires track roughly the same and RPM differences are low.


As I can tell the VC acts very quickly, I know this is not a real world example but it illustrates the action of the VC, on my “drive shaft removed rolling down the hill in park experiment” on the steep parts of the driveway the action of the VC was quite evident, feels like the brakes are partially applied did not get into humping but was in the resistance part of the VC’s action, when I got to the bottom of the hill and it was just a gently slope the truck slowed down and the resistance of the VC instantly disappeared and the truck crept foreword very slowly as if it was never there, I would imagine that had even if the hill was very steep this ramping resistance portion of VC action would have kept the speed of the vehicle speed down and still would not have got into humping,

This transfers to real life in that in most cases the difference in traction front to rear is nt that severe, the resistance phase of the VC should be enough to stop the wheel spin before humping (lockup) is needed.

From that I think actual humping is rare, being bathed in viscous silicon oil wear during humping would be minimal those two together should lead to long life of the VC for most FZJ80's,

Axcess wheel spin is still not a good thing, the resistance the VC generates is turned into heat witch can damage the VC itself and also the transfer case, if you are in conditions where a lot of wheel slip is going to happen the you should lock the CDL, casual thing like occasional dead stop at icy lights should be not problem.

The 80 series has 3 differentials, none of these differential have any bias in one direction or the other, each wheel receives 1/4 of the drive under normal circumstances

 

[Rich]

Raven, just for clarity, when I use the term "torque bias" , I am referring to the ability of the center diff to bias the torque output to the front and rear driveshafts. In this scenario an open diff does not, in any practical sense, bias torque. Practically speaking, an open diff outputs equal torque to both drive shafts.

With the addition of the VC, the center diff is capable of biasing torque, to the extent that the VC "engages" or "locks up".

With the inclusion of the center diff locker, the center diff is capable of practically 100% torque bias when the center diff is locked.

So when I refer to the VC biasing torque, it is in the context of it enabling the center diff to do the same in the absence of the center diff being otherwise locked.

 

[Rich]

Scott, look to your post 16 for the example you gave where the VC can be locked along with the brakes being applied.

In addition, you need to consider the fact that ABS is disabled when transfer case is in low range.

I don't believe that I am confused about the operation of the center diff. It is possible that you are confused by reading more into my posts other than what I have directly stated in those posts.

My direct answer to your question remains yes.

 

[SUMOTOY]

You are right Rich, I AM confused

Scott, look to your post 16 for the example you gave where the VC can be locked along with the brakes being applied.

Rich:
My post 16 says you can have slip angle differences caused by tire traction with NO VC lock under braking, and can have those different slip angles over time without locking. I ask for a specific scenario where the VC is locked under braking.

In addition, you need to consider the fact that ABS is disabled when transfer case is in low range.

Why do we need to consider this Rich? In stock trim ABS is disabled when the transfer is in low range because the differential is locked over the VC, hence the VC is 'disabled' as well. Second, if you do the pin 7 mod and don't push the center diff switch, ABS is indeed active in low range (btdt). I'm confused as to what low range has to do with the VC in either scenario. In stock trim the VC is disabled along with ABS. In the Pin 7 mod/switch trim, the VC/ABS acts exactly the same as in Hi Range.

I don't believe that I am confused about the operation of the center diff. It is possible that you are confused by reading more into my posts other than what I have directly stated in those posts.

I believe you understand the theory of a diff, but not the practical application. If so, you would need to preent the real world example of VC locked under braking.

My direct answer to your question remains yes.

I'm only confused as to how exactly you can claim a HF2AV FZJ80 VC can be locked under braking. Rich, without that chassis dynamic scenario, you have only presnted that in 'theory' the VC can be locked under braking based on the theoretical concept of VC operation. Without giving the specific instance, the answer is NO (a VC will not be locked under braking on a HF2AV FZJ80) based on the practical application.

Scott Justusson

 

[Rich]

Scott, real world example you already posted: Driving with center diff unlocked, say up a steep hill, front end looses traction due to weight transfer, wheels spin, VC locks, in order to modulate speed left foot braker applies brakes while maintaing throttle with right foot.

VC is locked. Brakes are on. You already know all this.

No need for you to rattle on about the appropriateness of somebody braking and applying throttle at the same time. It is merely the example, of which you are already aware, that you are imploring me to provide.

The braking only affects the VC to the extent that braking reduces or eliminates the difference in rotation speed between the front and rear driveshafts. The significant factor is the relative speeds of the driveshafts, not whether or not the brakes are applied.

 

[SUMOTOY]

Open vs VC vs Locked Center diff

Raven, just for clarity, when I use the term "torque bias" , I am referring to the ability of the center diff to bias the torque output to the front and rear driveshafts. In this scenario an open diff does not, in any practical sense, bias torque. Practically speaking, an open diff outputs equal torque to both drive shafts.

Rich:
Let's be clear (maybe you can trip me up here) and say exactly that a open diff will never change toque bias (ratio). An open diff will always (no exceptinos) apply torque to two axles exactly 50/50.

With the inclusion of the center diff locker, the center diff is capable of practically 100% torque bias when the center diff is locked.

Again, let's jump on board with both feet Rich. In a locked center diff, the torque bias is infinite. Which means exactly that either axle is capable of supporting 100% of engine torque (the engineers refer to this as Trg = Torque at ring gear). Further, a locked center diff torque allocation in dynamic straight line acceleration follows exactly weight distribution (good trivia) - same cof and same tire size.

So when I refer to the VC biasing torque, it is in the context of it enabling the center diff to do the same in the absence of the center diff being otherwise locked.

Not true. A VC is a reactive LSD device, that isn't designed to ever fully lock (it can in theory, but RT knows that's a short term phenomenon - humping - that causes LSD failure). Since a LSD VC can never have torque allocation follow weight distribution, and allows speed differences under applied torque, it doesn't and can't ever do the 'same in the absence of the center diff being otherwise locked'.

More for the nerds still here: Is there any other torque allocating device in a center diff that will follow exactly weight distribution up to the maximum Torque Bias Raito?

Scott Justusson

 

[SUMOTOY]

Scott, real world example you already posted: Driving with center diff unlocked, say up a steep hill, front end looses traction due to weight transfer, wheels spin, VC locks, in order to modulate speed left foot braker applies brakes while maintaing throttle with right foot.

VC is locked. Brakes are on. You already know all this.

I see the problem. Different concept you are presenting, in fact, what you are presenting is a makeshift EDL (no VC required - specifically- VC Assist). What you need to remember is the VC doesn't care about accelerator and brakes applied together. As you clearly state below it *only* and exclusively reacts to driveshaft speed differential. When you go up hill, one axle spins, your foot on the brake is to assure that the driveshaft speeds equal, VC will unlock. What other force are you presenting in this scenario to keep the VC locked? ABS intervention will assure that you can put either your right or left foot as far in as you want, the driveshaft speed will equal = VC unlocks, actually sooner.

No need for you to rattle on about the appropriateness of somebody braking and applying throttle at the same time. It is merely the example, of which you are already aware, that you are imploring me to provide.

You are going after a different principle (total supported torque of 2 axles). Mercedes (2 opend diff) EDL works that way. The spinning axle supports more torque, because it's absorbed by the braking torque. Hence the wheel with traction (slower axle) has more torque available for traction. In your FZJ80 example if you prevent an axle from spinning faster than the other, the VC will unlock. It doesn't care how driveshaft speeds equalized - it's a dumb reactive device.

The problem with VC's and ABS (both are active in your scenario), is the invervention on one in your scenario directly causes the disable of the other - no 'exception' to normal operation of the device.

The braking only affects the VC to the extent that braking reduces or eliminates the difference in rotation speed between the front and rear driveshafts. The significant factor is the relative speeds of the driveshafts, not whether or not the brakes are applied.

Completely agree, and we can now also say the accerator AND brakes applied together will come to the same conclusion - VC unlocked. Brakes applied while under accleration is to exactly allow more total torque to be supported by both axles. In this case you've HELPED reduce axle speed differential causing the VC to unlock faster.

This isn't a good scenario or exception, you are only adding total torque supplied by to axles (T1 - axle 1 + T2 - axle 2 = Trg - torque ring gear).

I'm still confused Rich, you just presented how to unlock the VC sooner under braking.

Scott Justusson

 

[Rich]

Scott, it is entirely possible to apply the brakes and accellerator at the same time, and still have the drive shafts rotating a different rates due to different weight loadings and traction at each wheel.

There is no inherent requirement in the example I provided that the braking equalize the rotational rate of the driveshafts.

With respect to ABS being disabled with transfer case in low range, once 40mph has been achieved with ABS disabled, ABS remains disabled regardless of whether or not the center diff remains locked, regardless of whether or not the transfer case remains in low range, and regardless of speed.

With the ABS disabled, and the center diff unlocked, there are additional scenarios where braking can take place while driveshafts continue to rotate at different rates, including downhill scenarios where throttle is at idle.

 

[SUMOTOY]

VC under braking

Scott, it is entirely possible to apply the brakes and accellerator at the same time, and still have the drive shafts rotating a different rates due to different weight loadings and traction at each wheel.

Not in the scenario you presented in post #48 Rich. See below.

There is no inherent requirement in the example I provided that the braking equalize the rotational rate of the driveshafts.

Rich, there is exactly the requirement in your post 48.

Scott, real world example you already posted: Driving with center diff unlocked, say up a steep hill, front end looses traction due to weight transfer, wheels spin, VC locks, in order to modulate speed left foot braker applies brakes while maintaing throttle with right foot.

Rich, you can't 'modulate speed' left foot braking a VC without first slowing the faster spinning driveshaft. Once both drivshafts are rotating equally, you will 'modulate (vehicle) speed'l, and the VC will unlock.

With respect to ABS being disabled with transfer case in low range, once 40mph has been achieved with ABS disabled, ABS remains disabled regardless of whether or not the center diff remains locked, regardless of whether or not the transfer case remains in low range, and regardless of speed.

And? Are we driving it electrically locked or VC locked? Pick one, right now there is enough confusion just concentrating on the VC. In a stock FJ80 the ABS is disabled all the time in low range.

With the ABS disabled, and the center diff unlocked, there are additional scenarios where braking can take place while driveshafts continue to rotate at different rates, including downhill scenarios where throttle is at idle.

I don't think so. Remember that braking is a component of sustained torque to any axle. It can raise total supported torque to the combined axles, but in a center equipped VC this braking force also speeds up the unlocking of the device.

I'm still confused Rich. The scenario you presented doesn't show a VC diff locked, it shows you *helping* to unlock it while 'moderating (vehicle) speed'.

Scott Justusson

 

[SUMOTOY]

Center differentials and braking

REFERENCE MATERIALS: CENTER DIFFERENTIALS AND BRAKING
Search Engine success
Rich et. al.
For more on dynamic chassis characteristics of center LSD's, there are several interesting articles worthy of reading (pardon the Audi Quattro slant, but they do go address general LSD properties):
* SAE 880321
"Brakes and ABS on Four-Wheel Drive Vehicles"
* IMechE 1988 C368/88
"The development of a differential for the improvement of traction control"
* AUTOTECH 95 C498/30/14
" Traction and Handling Safety Synergy of Combined Torsen Differential and Traction Control
* SAE 885140
"The influence of a Torsen Center Differential On the Handling of Four-Wheel Drive Vehicles

Rich,
I'm not the enemy at all here, just a very well seasoned AWD guy, looking to go much further into the dyanamics or 'practice' of accepted VC operational theory. I'm relatively new to the Landcruiser's Viscous Coupling Differential Lock in this respect, but so far the HF2AV has been quite predictable in terms of the VC, IMO/E. As I'll be taking my FZJ80 (with Blizzack Ice tires) to the Steamboat Ice Track next week, I'm sure I'll have more to report in this arena. The main purpose of my posts above, was to route out the contradictions I saw in regards to your presentation of the theoretical operation of the device. A lively discussion, if you are a center diff nerd like myself.



Scott Justusson

 

[Rich]

Scott, we are sharing words, but communication is not happening.

I orginally stated: "There is no viscous coupler "unlock when braking" function. By design, it will stop biasing torque after both drive shafts are rotating at the same rate for a period of time."

And you stated, in a different post: "if you prevent an axle from spinning faster than the other, the VC will unlock. It doesn't care how driveshaft speeds equalized - it's a dumb reactive device." Well, thats is the point I was making in my first post, it is the rotational difference in speed, and not whether the brakes are applied, that makes the difference, as to whether or not the VC is locked. You apparently disagreed with my statement in my first post in this thread.

You say: "There is no question that the VC will disengage itself when brakes are applied". I assert, if the braking is insufficent to result in the the driveshafts rotating at the same speed, that the VC will remained locked.

You asked for an example where the VC is locked under braking, and I gave you an example, starting with VC locked, braking is insufficient to overcome throttle and is insufficient to overcome driveshaft rotational speed differences, therefor VC does not unlock. Per your requested example the VC is locked, and remains locked even though the brakes are applied. I included the apparently fatal words "modulate speed", and you chose to focus on that, instead of the, what I thought was my obvious intent to communicate that braking is not sufficient to equalize driveshaft rotation. It is a contrived example, that you yourself first presented. And then you state that I did not show a scenario where the VC is locked - and I am baffled, given that I stated the scenario starts out with the VC locked.

In another post I explain to Raven what I mean by the VC biasing torque, within the context of the entire center diff assembly. I stated "So when I refer to the VC biasing torque, it is in the context of it enabling the center diff to do the same in the absence of the center diff being otherwise locked."

And you say: "Not true ...it doesn't and can't ever do the 'same in the absence of the center diff being otherwise locked'".

What's not true? The VC, in the absence of the center diff being otherwise (electrically) locked, does allow the center diff (the assembly, taken as a whole, the entire system contained within the transfer case) to bias torque between the two driveshafts.

Like I say, it doesn't appear to me that communication is happening. What's the point?

 

[SUMOTOY]

VC misunderstandings

Rich;
We've spent a lot of time on this already, the basics of VC function on the FZJ80 should be a baseline. I believe where "communication" is not happening, is that several of your statements aren't accepted theory or practice in regards to the VC operation.

The problems to the theory come when the practical application is explored without accounting for a lot of variables. If you apply enough left foot braking to 'modulate speed', the VC is already uncoupling or is uncoupled. That's because you have to define 'modulate speed' in regards to driveshaft speed vs traction. If you feel that's NOT the case, then please acquire the references I posted, you might find (like I did over several years researching diff applications) center diff 'practical' application of theory has some well documented contradictions/anomolies. Further without knowing the TBR of the HF2AV and it's internal design, your presentation has no validity, more specifically, it's oversimplified. All VC's are not equal, in design or application.

'same in the absence of the center diff being otherwise locked'".

What's not true? The VC, in the absence of the center diff being otherwise (electrically) locked, does allow the center diff (the assembly, taken as a whole, the entire system contained within the transfer case) to bias torque between the two driveshafts.

The disagreement is with the statement. Care to rephrase? The exact statement that "a VC is the same in the absence of the ctner diff otherwise being locked" is just not true. A limited slip device is not the same as a center diff being otherwise locked, ever, by definition. Limited slip means driveshafts can slip. Locked means driveshafts exactly can't slip. Limited slip means that Torque bias ratio isn't infinite, and is a delayed reactive to driveshaft input forces (either speed OR torque). A locked diff is immediately reactive to ALL input forces (both speed AND torque). So your conclusion is oversimplified and technically not correct = not true.

Like I say, it doesn't appear to me that communication is happening. What's the point?

I've had years of experience dealing with misconceptions of center differential applications. Most of them come from oversimplifying theory, and technically incorrect statements in regards to theoretical operation and practical application. I enjoy discussions of the practical application of various diffs in reference to theory. That said, I proceed with extreme caution regarding making blanket statements of practical operation. If you really want to explore this further, I'm happy to ablige. We really need to get a baseline first. As I explored the depth of your understanding of the VC specific to the FZJ80, I find some contradictions to our simple agreement of driveshaft speed rotation being the cause for VC unlock. Trying to communicate those effectively really means we have to baseline the rules specific to the chassis. Finding dynamic exceptions is really tough because you must assume to many variables that quite possibly can't exist in real world driving.

Thanks for your continued patience Rich. I'm trying hard to find agreement with you.



Scott Justusson

 

[Rich]

Scott, it is clear to me, that when you reply to me, quoteing every other statment I make, including mistating what I said, just so you can then voice an objection, it is obvious to me your intent is not to commnunicate or share information. It appears to me you are just trying to be a pest.

 

[SUMOTOY]

Thanks for the chuckle Rich

Scott, it is clear to me, that when you reply to me, quoteing every other statment I make, including mistating what I said, just so you can then voice an objection, it is obvious to me your intent is not to commnunicate or share information. It appears to me you are just trying to be a pest.

I've been called *much*worse. Rich, you have mistated how VC's work, specifically, you present contradictions to accepted theory of torque bias in a LSD and a locker.

Maybe it would help if I give you my background (though it shouldn't matter). I work on awd audis for a living, and have extensively modified the operation of the differentials in them. This experience includes modifications to full race cars, street cars and offroad (rally and rallycross) prep. I evaluate modifications in the real world, on tracks and for the last 11 years at Steamboat Ice Track venue. This included modifications to Lockers, clutchpacks, VC, and Torsens differentials. I have been 4wheeling my 4 Runner for the last 10 years until I acquired my FZJ80.

This passion for awd chassis optimization has led me to find anomolies and practical exceptions to many of the theoretical approaches to center differential applications. After reviewing the literature and extensive experience with the torsen I proposed the Torsen 'spider bite' phenomenon where under the right conditions the torque bias will oscillate fore and aft... Confirming the presence of this anomoly with Zexel Gleason and the original torsen designer. None of this can be done without a firm grasp on general theory of center differential applications of all types. After my experience with locking differentials nuances, I borrowed a stock (air lockers center & rear) audi quattro in 1997 and won outright SCCA's first Rallycross at the Steamboat Ice Track.

Does it make me an expert on anything? Not at all, I prefer to say a very well seasoned awd guy, and leave it there.

Think I'm a pest now? Hey, I'm bolting on a kazuma and a set of Blizzacks and driving the beast at Steamboat on ice next week!



Scott Justusson
QSHIPQ Performance Tuning
'94 FZJ80

 

[70sguy]

Ok.. I thought I would dredge this thread since I just completed my 1HD-T
A442F HF2A conversion on my 93 FZJ80 (formerly A442F HF2AV). The A442F was a post 95 ECT unit with the HF2A attached. I can't add comment to all the technical banter that has gone on about how and when the VC works, just some real world non technical seat of the pants feedback.
It could be my imagination but it seems that the truck now has a lighter feel in the steering and rolling resistance if you are coasting and turn the wheel. It has so far not affected the ABS sensors, the light goes on as it should when I put in in Low. I have yet to find some slick surface to test it on so research continues. I now have to get the CDL button to activate the CDL while in high range, it currently only activates through the low lever.