Viscous Coupler removal and AWD

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Sorry I missed this....

landtank said:
these threads aren't the same without Sumotoy.
Rich has this right, the VC is a VC, it's design is so that the ABS can function in low tractive conditions. The HF2AV is referred to as a viscous coupling differential lock. It is a driveshaft speed differential device (turning can fool it, but that's not it's intended lockup design). It can vary torque between axles at ratios up to lockup (called humping) but is designed to be around 4:1 in most applications.

It's removal won't yield you 1 mpg, and taking it out only makes non locked diff mode less effective under low-cf braking.

You can convert the rotation difference to degrees, the tight VC's (as the 80 is) usually has begun lockup within 45degrees of driveshaft rotational differences.

These HF2AV type differential locks are found on less than a handful of production vehicles, and is a credit to toyota for actually having 4 differentials on a 3 diff awd system.

ID, I suppose under turning more torque is sent to the rear wheels allieviating some of the understeer, but I don't believe that was the intended design. Audi went from lockers to torsen for the same reason, the market demanded passive AWD LSD *and* ABS function, and both the VC and the Torsen deliver that.

HTH

Scott Justusson
 
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Actually, the 98-99 has single ABS rear as the 80s do. BUT the 91-92 FJ80s have single rear braking (obviously sans ABS) with no VC.

Forethought to braking control seams to be the only reasonable reason. However, Toyota's not always known for giving the US anything we really want anyway like a 105, so I can only give 'em so much credit.
 
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Pskhaat said:
Actually, the 98-99 has single ABS rear as the 80s do. BUT the 91-92 FJ80s have single rear braking (obviously sans ABS) with no VC.

Forethought to braking control seams to be the only reasonable reason. However, Toyota's not always known for giving the US anything we really want anyway like a 105, so I can only give 'em so much credit.
The single line to the rear with ABS uses what's referred to as "Select Low" principle for ABS activation. The maximum braking force to the rear under ABS activation is equal to the maximum braking force supported by the lowest tractive wheel. This means if one rear is on ice and the other on pavement, braking force to both rear wheels will be zip.

Most folks believe that 4 channel ABS solved this problem. In fact, most ABS systems use Select Low in the rear to avoid loss of control.

Circa 1987 there was a massive effort by audis and others to reduce the driver involvement and dissatisfaction from active driver involvement to differential locking. Specifically the lack of active driver involvement to unlocking differentials after using them. The late 1980's and early 1990's found a much higher count of VC's and torsens, the side benefit of both being the driver has ABS function and LSD.

I give Toyota a *lot* of credit for leaving the electric locker and *adding* the VC to address the optimal and the compromise in the changing market demand.

Scott Justusson
 
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More diff-conceptions

pulse98 said:
Rookie2,
A limited slip or locker DOES NOT attempt to equalize torque to each shaft. They DO try to equalize shaft speed.
To be clear this is correct except to torsen type (torsen, detroit locker, quaife) gear LSD devices. They are torque sensing devices by definiition, and always attempt to equalize torque to each shaft (or to a preset split). Best to think of these gear type diffs as rubber bands, stretch torque one way, and torque allocation will try to bring it back towards equalized. By the time there is a difference in shaft *speeds*, the gear LSD has already maxed the torque bias ratio away from the axle with the lower supported torque.

This torsen type center diff also has the advantage under acceleration that torque follows weight distribution up to wheel slip or maximum bias ratio, exactly like a locked center diff does.

Scott Justusson
 
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Yeah, Torsen/Gleason limited slips, I did not take into account in my comment, those are whole other animals with torque multiplication factors...I think they call it "bias ratios". An elegant solution.


SUMOTOY said:
To be clear this is correct except to torsen type (torsen, detroit locker, quaife) gear LSD devices. They are torque sensing devices by definiition, and always attempt to equalize torque to each shaft equally (or to a preset split). Best to think of these gear type diffs as rubber bands, stretch torque one way, and torque allocation will try to bring it back towards equalized. By the time there is a difference in shaft *speeds*, the gear LSD has already maxed the torque bias ratio away from the axle with the lower supported torque.

This torsen type center diff also has the advantage under acceleration that torque follows weight distribution up to wheel slip or maximum bias ratio, exactly like a locked center diff does.

Scott Justusson
 
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Torque Bias Ratio

pulse98 said:
Yeah, Torsen/Gleason limited slips, I did not take into account in my comment, those are whole other animals with torque multiplication factors...I think they call it "bias ratios". An elegant solution.
All LSD devices have Torque Bias Ratios or Bias Ratios. This is the accepted engineering term for the ratio of torque transfer from the axle with the lowest supported torque. IOW a traditional "75% locker" would have a TBR of 3:1. Or conversely, a 4:1 TBR Torsen is a "80% locker".

The issues with torsens are they can't jump well (the sudden torque load explodes em, btdt), they can cause some crazy chassis dynamics in center applications during turns, and they tend to hunt in very low cf conditions.

Gear type limited slips do have the advantage in brake traction control systems, and in fact, will outperform a locker in terms of absolute traction in a strait line. They also can reduce turn in steering effects of awd. I personally find them to be less predictable in performance or low cf driving. I'd take a locked center or VC center over a torsen center diff any day. Terminal understeer is very predictable.

HTH

Scott Justusson
 
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Sumotoy,
Since I see you are an Audi tech (I was going to get an Audi A3 until I found out about the FZJ80)...how do you like the torsen solutions in the old Audis? I think every year, Audi is using them less, maybe not at all any more.

How do you like torsen compared to something more computer controlled like Toyota's new ATRAC?
 
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Sumotoy,
how do you like the torsen solutions in the old Audis? I think every year, Audi is using them less, maybe not at all any more.

How do you like torsen compared to something more computer controlled like Toyota's new ATRAC?
Audi still has them in the mainstay, though the TT and the 4motion Haldex units will probably be the future of every awd system, IMO. Currently they are used in 'coupler' trim (read fwd until slip, then awd), but I have had several conversations with the Haldex boys, and full time awd was it's original design.
I have extensive writings on Torsens about the inet, mostly due to some of the engineering omissions as to it's possible faults. In straight line acceleration, it's tough to beat a torsen, and currently, combined with electronic traction control and engine control, it yields the best straight line traction. It's inherent downside is turning and traciton.

Since the torsen is a gear LSD by definition, it doesn't know care or differentiate between a tractive input and a turning input. So, in a turn, torque is sent to the axle with the least amount of slip ("slower axle"), which would be the rears in a turn. That torque is allocated to the rear up to wheel spin. Then the torque will shift foward immediately, which can cause some unstable chassis behaviors in low cf or at the traction limit. I have experienced this behavior and it can create an oscillating oversteer understeer oversteer condition that can be a handful.

A race buddy and I bounced this theory off the designer of the torsen (chocholek), and 'though unlikely', this phenomenon is possible in the design of the device.

Regarding torsen vs atrac, my understanding is that atrac is a 2 circuit traction control system, 1 for the front wheels, 1 for the rears. Several folks have modded the new FJ to run atrac with the rear diff locked, basically giving rear locker and front LSD. It's probably better to have Atrac and center + rear lockers for offroad. But, a center torsen/locker with torsen rear + traction control will put more traction to the ground.

The other downside to traction control (including Atrac) is that they are not 'over the axle' LSD devices, IOW, the front and the rears operate as separate pairs. Only a handful of Electronic Traction Control systems (mercedes) offer combined over the axle traction control, it's expensive.

IMO, once Haldex comes full on with the refinement of Gen II Haldex (Gen II Haldex with center diff vs Gen 1 = Haldex coupler), they will go after the front and rear axles. When this happens, I doubt any vehicle will run a traditional locker in any axle.

All that said, my personal preference is a locker, because it's predictable, isn't fooled by turns, isn't actively participating in driver mistakes, gives close to ideal tracion, and gives ideal brake force distribution. Simple dumb devices are tough to beat with electronic controls. I'm currently starting work in a project to modify Haldex for a performance offroad environment. They are most receptive to these tweeks, and I suspect this won't hurt their demand any either.

Sorry for taking the nerd platform...

SJ
 
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I kow this is a dead thread, but I'm doing a search on details of running without the VC.

From what I've read, it seems to me that the VC is a "plug and play" traction assist for people who want the vehicle, not themselves, to control the traction.

I conclude that Mr T installed the VC system in the 93-97 80s because he knew that only 7% of the trucks produced yould have the FF/FR e-locker option. The VC is essentially redundant in an e-locker truck, except in extreme uneven-traction situations as mentioned above (and only since the 93-97 US models don't have the manual CDL switch, which presumably you'd use in less-than-perfect road conditions). In much the same way all the 80s are wired for the centre console fridge option, it was more cost-effective for Toyota to produce "X" number of VC drivetrains than 93% "X" and 7% "Y" drivetrains.

So my question is this: What, exactly, is the vicious coupler? Is it a manufactured modular unit, or is it just a chamber of the transfer case that is packed with a heavier than normal grade of lubricant? Forgive me if this is a dumb question; I'm totally new to VC technology.

Can you simply "remove" the VC?

I ask because mine is buggered, and if it is supposed to do what many of you say, I don't particularly need it. I've already disabled my ABS; might as well ditch the VC, too, if possible (I'd rather do this than fix it).
 

CJF

 
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Ok, ok....

I crack me up...:D

MikePL did a nice write-up a few weeks ago. And...

He did it in Poland, with no manual and no back-up parts available.:eek:

have fun :),

Curtis
 

CJF

 
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So my question is this: What, exactly, is the vicious coupler?
This one I ain't touchin'.

Plenty of info around here on this. Plus, it seems to be one of those issues that generates a nice long thread every time it comes up. Sort of like, "Hey, what oil do ya'll run?"

Enjoy the ride,

Curtis
 
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Hi all

I have read this thread right through and have a couple of questions:


  • Why does the LC have a VC installed in order for the ABS system to work? Why can't it work without the VC?
  • If a CDL button was installed later into a LC with a VC, why does this disable the ABS? Why does it need to?
  • Is a CDL button omitted from a LC with a VC by default as the LC is fairly capable in high range in boggy situations as is with the VC without locking the CDL.
There seems to be mixed opinion around as to whether the VC was installed for the ABS, or whether it was to reduce drivetrain slop. What is the general consensus regarding this?

Cheers.
 
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I've always had a feeling that it was put in there partially to keep up with the Range Rover which had a similar device fitted in the early 90's. The major difference being, Land Rover removed the centre diff lock when they fitted the VC. :)

I think the abs is disabled along with the CDL mostly because ABS is generally seen as a hinderance off road. The ABS is likely to be poor at dealing with situations that require Low Range.

Jamie
 
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