Difference between HF2A & HF2AV transfercase (1 Viewer)

[Tapage]

maybe just wondering .. but the HF2AV can share half and half the torque .. in any specific moment .. ?

Maybe not the same .. but for example the Honda CR-V have a torsen ( yap is diferent ) VC to share under normal conditions 100% front and nothing rear. Under slippery road or traction loose in the front tires it star sharing some % to rear tires ..

So the maximos % that can sent to the rear wheels are 40% keeping 60% front.

 

[Rich]

Scott, your statement "The HF2AV adds a VC LSD to the center diff that will disengage itself when brakes are applied" could easily mislead the casual reader. It implies that there is a mechanism, that automatically disengages the VC when the brake pedal is touched, similar to how cruise control disengages whenn brake is applied.

Do you not agree that it is only the difference in rotational speed between the two driveshafts that determines whether or not the VC is engaged, regardless of what drive forces cause that difference in rotational speed? That is what I stated in my previous post.

 

[SUMOTOY]

VC and Braking

Rich:
VC LSD changes torque Bias based on the differences in input shaft speed (specific to the HF2AV the front and rear drivshafts exiting the transfer case). That said, we don't know how the center VC is designed on the HF2AV (I'm trying to find that information). I suspect the locking characteristics of the Viscous Coupling Center differential lock are similar to other awd VCDL systems, in that they are very slow to lock with torque applied, and very quick to unlock with engine overrun or any braking activation.

There is no question that the VC will disengage itself when brakes are applied, because tire slip can't cause enough driveshaft speed differential to engage the VC. The VC will disengage itself with just the act of lifting the gas pedal (0 torque x torque bias ratio = 0), and the concept of axle speed differentiation under any ABS equipped braking or engine overrun situation is tough to present, since neither scenario can cause axle speed differences. Hence my statement at the VC will uncouple when brakes are applied. The event that triggers that disengagement is lack of axle speed differentiation.

There are several design revisions in VC that can make that happen sooner or later, but certainy less than 90degrees of axle rotation is in most modern VC's. I'd also argue that driveshaft angle is within the feedback loop of the FZJ80 ABS. So, with the brakes on and ABS enabled and/or activated, it is not possible to have any sustained axle speed differentiation. Read: A VC is always uncoupled during braking on an ABS equipped Landcruiser.

Working these concepts with traction vs torque arguments is where a lot of the confusion gets started. Without torque applied to the driveshafts, there is no LSD. Reverse torque (traction or tire slip the input instead or engine overrun torque) is a much tougher argument to present in real terms. I say nary impossible on an automatic equipped ABS Landcruiser.

Scott Justusson

Scott, your statement "The HF2AV adds a VC LSD to the center diff that will disengage itself when brakes are applied" could easily mislead the casual reader. It implies that there is a mechanism, that automatically disengages the VC when the brake pedal is touched, similar to how cruise control disengages whenn brake is applied.

Do you not agree that it is only the difference in rotational speed between the two driveshafts that determines whether or not the VC is engaged, regardless of what drive forces cause that difference in rotational speed? That is what I stated in my previous post.

 

[RavenTai]

Rick, this one does have plates and does hump, it is described in the 96 LX "new car features manual"

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

Scott, yes this is a best of both worlds setup, a lot of people think it is just viscous coupler differential like some land rovers and many AWD cars. I thought you had fallen into this trap but reading you reply and the posts above again I see that is not the case.

I guess the only place Rich and I disagree with you is that the VC in this app is never aware of or directly exposed to engine torque nor direction, it is aware only of the rotational difference of the front and rear drive shafts, I can think of only a few instances where the VC would come into play during braking, they would be very rare and in these time I think stabilizing front to rear rotational differences could only be helpful.

This one is slow to lock up, a description of my experience with that in this thread about half way down

https://forum.ih8mud.com/showthread.php?t=38483

One thing to take away from that is the transmission was in park, the engine was completely out of the picture yet the VC was still doing its thing.

 

[RavenTai]

The event that triggers that disengagement is lack of axle speed differentiation.

OK were are on the same page.

 

[Tapage]

Raven Tai .. thinking in the same situation in a AWD car as subarus, with auto tranny .. what do you think gonna happen ... ?

 

[RavenTai]

The situation being removing a drive shaft and rolling down a hill in park?

many AWD cars have a directly driven axle and one that is driven through a viscous coupler, I do not know specifically if this is how a Subaru is set up but assuming it is, and assuming it has a in line mount engine with a front and rear drive shafts: if you remove the directly driven axle's drive shaft the vehicle will slowly roll down the hill in park, if you removed the driveshaft that was not directly driven then the vehicle would stay put kept in place by the parking pawl in the transmission

Many of these AWD cars are transverse mount where the front CV's are driven directly and the rear axle is driven through a viscous coupler

 

[SUMOTOY]

Braking VC's

RT:
I can't think of any situations where the ABS Cruiser VC is operating in a braking condition. From an engine torque perspective, the VC is a known entity. Where we find ourselves differing, is that you and Rich claim it's possible in theory to have VC locked in braking, my claim is in practice on an ABS equipped FZJ80, it's not possible.

I've written thousands of posts on the subject of center differentials (mostly due to the misunderstandings of torsens), and am currently using a variety of differential applications on street and race cars (audi quattros) I service in my shop. I also (insert shameless plug) am an eventmaster at the Steamboat Ice Track in Colorado, see www.gruppe-q.com where I've tested varying differential setups every year (it's a close choice between open front, center locked, rear locked vs open front, center locked and rear Torsen). I'm intrigued to read about misunderstandings of the devices, and prod most folks to go beyond the theory and into specific application: into the dynamics of handling, engine torque, and braking. Right now, that's where I'm playing. I applaud Toyota for spending the bucks to put in the VCDL, as I believe this center diff is one of the best setups possible (well the new active diffs are kinida cool).

Here, I believe completely that the *theory* of VC application in the Landcruisers is understood by you Rich and myself. My challenges to you two come from the application in terms of tractive force scenarios or chassis dynamics where application follows exactly the theory. In a VCDL with ABS active, I can't see any practical or real world situation where we would see axle speed differences under braking. The VC doesn't magically unlock under braking, any and all braking causes a VC to unlock. ABS just means you can't fool the VC, no matter how hard you hit the brakes. An autotrans makes that exception even harder to present, becuase generally a torque converter absorbs more overrun torque than a VC will.

Disconnect a driveshaft and roll a car downhill in park? Ok, now that's stretching reality a bit.



Scott Justusson

Rick, this one does have plates and does hump, it is described in the 96 LX "new car features manual"

http://webpages.charter.net/raventai/HF2AV%20TRANSFER.pdf

Scott, yes this is a best of both worlds setup, a lot of people think it is just viscous coupler differential like some land rovers and many AWD cars. I thought you had fallen into this trap but reading you reply and the posts above again I see that is not the case.

I guess the only place Rich and I disagree with you is that the VC in this app is never aware of or directly exposed to engine torque nor direction, it is aware only of the rotational difference of the front and rear drive shafts, I can think of only a few instances where the VC would come into play during braking, they would be very rare and in these time I think stabilizing front to rear rotational differences could only be helpful.

This one is slow to lock up, a description of my experience with that in this thread about half way down

https://forum.ih8mud.com/showthread.php?t=38483

One thing to take away from that is the transmission was in park, the engine was completely out of the picture yet the VC was still doing its thing.

 

[semlin]

this is interesting reading. Just to throw a spanner in the works, I am pretty sure I recall reading somewhere that the viscous coupling t-case was introduced because of the addition of ABS. I am also pretty sure it was a toyota write up but I can't recall where now.

 

[Hayes]

RT:I applaud Toyota for spending the bucks to put in the VCDL, as I believe this center diff is one of the best setups possible....

Could you elaborate?

Just spectating.

Hayes

 

[landtank]

Rick, this one does have plates and does hump

Thanks RT, good to know something in the truck is getting some.

 

[curious1]

O.K. I never had any idea that this thread would get so in depth. It is great to have all this knowledge.

 

[Rich]

Scott, you still haven't direcetly answered my very simple question: "Do you not agree that it is only the difference in rotational speed between the two driveshafts that determines whether or not the VC is engaged, regardless of what drive forces cause that difference in rotational speed?"

 

[SUMOTOY]

VC vs Speed

01-27-06, 11:27 AM #23 SUMOTOY vbmenu_register("postmenu_906644", true);
IH8MUD Rookie

Join Date: Jun 2005
Location: Chicago
Posts: 47


VC and Braking
Rich:
VC LSD changes torque Bias based on the differences in input shaft speed (specific to the HF2AV the front and rear drivshafts exiting the transfer case).

Scott, you still haven't direcetly answered my very simple question: "Do you not agree that it is only the difference in rotational speed between the two driveshafts that determines whether or not the VC is engaged, regardless of what drive forces cause that difference in rotational speed?"

Rich:
I directly conveyed that already above, there is NO misunderstanding of the theory Rich. We could ignore the dynamics and design of the VC vs theory, or study it to see that the theory has a bunch of variables that affect it's practical application. Simply stated, in theory you are right, in practical application it appears to be N/A. Why can't we concentrate there? It directly applies to the abs 80, and is quite unique to it's very chassis.

Here's one that bucks the "simple" question... Scenario: Turning radius in LC. The rotational difference is tire slip caused by turning radius (front axle rotating faster than rear). No VC lockup. The real question: How much tire slip is allowed? Isn't the theory really dependent on the variables of and placement (attached to rear axle or front axle) of the VC, wheelbase, traction, slip angle, Torque Ring Gear, road speed?

I went thru this already with the Torsen application with the audi quattro. A lot of dynamic variables affect LSD Rich. Other than turning radius, are there any other dynamic scenarios you can think of where driveshaft speeds will differ causing VC locking? Can you think of any Braking Scenarios where the VC will be locked because of driveshaft rotation differences?

Maybe the theory and question are too simple?



Scott Justusson
1994 FZJ80 SC2B

 

[Rich]

Scott, the question I posed is straight forward, the context of the question is specific to the US model FZJ80 Landcruiser with a center diff containing the VC, and can easily be answered with a simple yes or no. I appreciate that you are not inclined to answer in that manner.

 

[SUMOTOY]

P-VC's

Rich:
Reread my first line of post number 23, I addressed your only concern 3 days ago. Now, a straightforward question for you: Can the VC be locked under braking on an FZJ80 with a HF2AV?

Rich, if you want to focus strictly on theory and nothing else, then reread line one of post 23 and let's drop this thread.

Thanks

Scott Justusson

Scott, the question I posed is straight forward, the context of the question is specific to the US model FZJ80 Landcruiser with a center diff containing the VC, and can easily be answered with a simple yes or no. I appreciate that you are not inclined to answer in that manner.

 

[Rich]

...Can the VC be locked under braking on an FZJ80 with a HF2AV?

The answer to your question is yes. You have already provided an example where this is true.

 

[SUMOTOY]

HF2AV and Braking

The answer to your question is yes. You have already provided an example where this is true.

Rich:
I believe the answer to my question is NO, you can't have the VC locked under braking on an ABS equipped 80. The example I provided is only in reference to speed differential (by turning radius) without VC locking.

If you believe a VC can be locked under braking, a brief description of the actual chassis dynamic scenario might be helpful. I believe you partially understand the accepted theory of a VC device, but you might misunderstand the practical application of the HF2AV in the FZJ80 chassis.

Thanks

Scott Justusson

 

[Hayes]

This is an interesting and informative thread. Could you folks who know a little about VC answer a few more questions?

What causes "humping?"

What exactly is happening with the 40/60 torque bias?

Does the viscous coupler itself ever need to be serviced?

Thanks.

Hayes

 

[RavenTai]

for humping read tis https://forum.ih8mud.com/showthread.php?t=77282, it was linked above

there is no torque bias in the 80 series, it is an RPM based system.

the VC cannot be serviced, should it become damaged such as by running diffrnt sized tires then it will have to be replaced, the transfer case has to be disasembled to do so.