[torsen] RE: Torsen differential

Keith Maddock Keith.Maddock at trw.com
Mon Nov 18 06:40:42 EST 2002

Man, this is a rough crowd! :)  While I don't necessarily always agree with Scott J's comments, some of you guys are making similar non-informed BS statements that you accuse him of doing.  And please, lets leave personal attacks off the list.

Warning:  If you are scared of long posts with a lot of information, (perhaps even more info than is needed to answer the question), hit delete now.

Torque capacity and failure rate for a torque-transmission unit has a lot to do with not only the input torque, but also with the load that it is subjected to.  I have had  experience with a non-torsen, non-haldex rear diff that is in production without problems in a high hp European RWD car, which was unable to pass reliability testing in a American RWD truck application with less hp (but about 30% more torque and nearly 100% more vehicle mass).    I'm going to guess that GM truck racing is dealing with both a lot more torque and a lot more load than Audi will ever.    (Case in point, NVG rates their transfer case torque capacity based on a load factor which differs for diesel and gas engines even if the torque rating of the engine is the same)

There are more definitions within the industry for "full time AWD", "full time 4WD", "part time AWD", etc than you can shake a stick at.  Let me clarify a little bit what (I'm pretty certain) Scott meant by his comment about full-time-AWD, and slightly clarify some comments about "haldex lsc-based units for jeep", as well as defend that "torque levels virtually limitless" DOES make sense

Technically the Haldex LSC is not a differential, its a coupling.  (hence it being called a LSC not a LSD) Of course this is all depending on the definitions of these terms, but they are quite different.

Differential: Device for distributing torque to two outputs with one input.  Average speed of two outputs is always constrained to be the same as the input speed.  Max torque bias between two outputs is dependent on amount of available force within differential (lock or LSD) to limit speed difference between two outputs.

Coupling:  Device for distributing torque from one input to one output.  Max torque bias between input and output a direct function of coupling force. Also called "Limited Slip Coupling" or LSC.

There are three main kinds of LSC's in use:

Mechanical/Hydraulic : 
2 Gerotor or Shear pumps, one on the input, one on the output.  Hydraulic pressure is generated when there is a speed differential between the outputs.   The hydraulic pressure activates a wet-pack clutch which then progressively locks the input and the output.   
Current Applications: 
- NVG247 on Jeep, marketed as "Quadra-Trac and Quadra-Drive".  (Located within center transfer case, connecting the front axle to the rear axle when needed.
- Dana #? on Ford Escape, marketed as "Control-Trac II" (Located in "hang-on" unit attached to the rear diff)
- Steyr-GM Versatrak on GM Dustbuster Minivans / Ass-tek / Rendezvous (Located in rear differential unit, connecting rear axle to the front, has electronic check valve to allow vehicle to disable AWD)

Electronic controlled Mechanical Clutch: 
Speed sensors on the input and the output provide input to a ECU.  ECU then calculates (along with other information from the other vehicle sub-systems) a command to a motor or magnetic device which regulates clutch pressure. (there are a number of different ways to do this).  
Current Applications:  
NVG 126/136/226/236/246 on GM trucks marketed as "AutoTrac" (internally known as ATC (Active Transfer Case))
BorgWarner "TOD" on Ford Truck marketed as "Control-Trac" (Explorer/Expedition)

Electronic controlled Hydraulic Clutch:  (Haldex)
2 Gerotor or Shear pumps, one on the input, one on the output.  Hydraulic pressure is generated when there is a speed differential between the outputs.  Speed sensors on the input and the output provide input to a ECU.  ECU then regulates (along with other information from the other vehicle sub-systems) the maximum hydraulic pressure generated by the pumps.  The hydraulic pressure then activates the wet clutch-pack.  A small electric pump is capable of generating small amounts of pressure for quicker response and minimal coupling with no speed difference.  Good link: 
Current applications:  
Haldex in VW/Audi transverse-engine vehicles, newer Volvo AWD, upcoming Saab. (Located in "hang-on" unit attached to the rear diff)
GKN now makes a very very similar unit and are very very aggressively trying to take Haldex's market away, they will be on some future Volvo's.

So now we know the difference between the Jeep LSC unit and the Haldex LSC unit.  The Jeep unit is purely passive/reactive, where the Haldex is actively controlled and also is *capable* of a small amount of pre-emptive coupling.  Either way, we can call both of these "full-time AWD" because they are designed such that the vehicle is "all-the-time" in "AWD mode".  However, in the absence of any speed differential between the front and rear axles, there is no hydraulic pressure, and therefore no coupling, therefore *technically-speaking* these vehicles are **2WD**.  
(As opposed to a full-time AWD vehicle with a center differential (open or LSD) which is ALWAYS distributing torque to both axles regardless of speed difference, therefore NEVER 2WD unless you have a separate disconnect for the front or rear axle  IE  Jeep Selec-Trac, BMW X5).

With the Haldex though, you have the ability to have the pre-emptive coupling pressure with the electric pump.  Also as such, either Haldex or VW has the ability to tune the electronic control to be more or less aggressive.  This gives you the ability to make a Haldex unit "never in 2WD", which is what Scott (most likely) meant by his "full-time AWD mode" statement.    The other main difference of the haldex is the electronic control which allows Haldex to back off coupling in a turn, or when ABS/DSC needs the front and rear axles to not be coupled to prevent unwanted interactions when braking all 4 wheels different amounts.

Conversely with the electronic controlled truck coupling units, like the various NVG units offered by GM on their trucks, they are capable of being operated in several modes, "full-time AWD", "part-time AWD", and "2WD" functionality (marketing descriptions) is capable in the SAME unit.  So Dave, it is **quite possible** to call the Jeep and Haldex units by different descriptions, not to mention these other units which have the capability of operating as multiple descriptions in the same vehicle.

To further that fact, GM will install the same basic ATC transfer case in different vehicles, has between model to model, and between year to year, changed both the software and the clutch mechanical pre-load such that in "Auto 4WD Mode" aka All-Wheel-Drive, the functionality of the same part is quite different.    In one installation, it can be setup to be purely 2-wheel-drive until it engages the clutch to make it AWD in reaction to slip.  In other installations it can be setup to have 5% or more coupling in no-slip conditions such that it is never in 2WD operation.  Or if they want fully-locked 4 wheel drive, they just drive the motor to max clutch engagement force, and then put a brake on the motor shaft to hold it there.  Bingo, now we can call it "Four Wheel Drive" instead of "full time AWD" in the same vehicle.  Amazing!

It really boils down to a difference in "technical descriptions" vs "functional descriptions" vs "marketing descriptions".  This description discrepancy is a source of a lot of controversy in the automotive 4WD OEM/Supplier community, and SAE is having a difficult time establishing a standard.

WRT to  Dave saying " "torque levels virtually limitless" makes no sense":
No, it does make sense.  In multiple ways.

First, I dont think Scott meant to say that the torque levels were virtually limitless in the actual haldex units supplied to VAG for production at the moment.  But that's not to say that they don't have the ability to make a unit, or pull one off the shelf, that *IS* capable of as high of a torque level as they want.

Second,  think in terms of failure mode, not amount of torque transferred.  Think again of the difference between a differential based and a coupling based center torque transferring device. 

In a differential unit (such as the torsen) all of the input torque is *always* going out to one or both outputs. So if you put too much torque in, something will break!   In a coupling unit, if there is more torque than the output can handle, then the clutch just slips.  Most likely, nothing breaks.  Plus , like you said, the Haldex as well as the Steyr-GM unit both have hydraulic pressure releif valves  

So, if you put too much torque in, it just doesnt get transferred, but nothing should break.  (Note I wont say it will NOT break, any automotive part can be broken if somebody wants to).  
500 Nm of torque in a 300 Nm rated torsen = broken
500 Nm of torque in a 300 Nm rated LSC = full amount of torque not transferred.

If you think of it in terms of torque transfer, "torque levels virtually limitless" is possible.  But all you have to do is strengthen the clutch if you want to transfer more torque. (More hydraulic pressure, larger clutch area,  or more clutch plates, etc etc). A lot of the pseudo-SUV LSC systems actually have quite a low amount of maximum torque transferred (Escape, VUE), but the limit is well within the indended needs / usage of these vehicles.



Dissenting statements welcome as long as they are fact-based and not personal attacks.
I'll never claim to be right 100% of the time, and welcome the chance to learn more when I'm incorrect.

Keith Maddock, TRW Automotive,  Koblenz, Germany
Slip Control Systems, Systems Design, Traction Control
+49 (0)261/ 895 2474     -    -    keith.maddock at trw.com

>>> "Dave.Eaton" <Dave.Eaton at clear.net.nz> 09:59:09 17.11.2002 >>>
the audi torsen can't handle high torque applications?  it handles the rs6,
the largest torque application audi has, ditto the w12 in the old a8...

haldex?  this is the name of a company, not a differential.  haldex makes a
range of (mainly truck-based) equipment - brakes, abs systems, hydraulic
systems etc.

haldex also makes an awd differential, called the lsc.  interested to
understand how audi could have installed it in "full-time awd" mode?
certainly the jeep application you mention is the same lsc unit, in the same
application as audi use.  nvg started using their haldex lsc-based unit for
jeep at about the same time as it debuted in vwag applications.  you can't
call it full-time awd in one, and not the other - because it isn't.

there is no doubt that an electronically controlled lock-up clutch (which is
what the haldex lsc is), it the future of awd systems - cheap, and fully
controllable.  "torque levels virtually limitless" makes no sense.  the
haldex lsc has an overload protection valve - but is not operational at that

'95 rs2
'90 ur-q
-----Original Message-----
From: QSHIPQ at aol.com 
Date: Thu, 14 Nov 2002 10:21:19 EST
Subject: Re: Torsen differential

[ Picked text/plain from multipart/alternative ]
A couple points.  First, the Torsen isn't bulletproof, it wears or breaks,lt t
that's all.  It's not meant to handle large torque applications in the audi
application.  Heavy equipment is actually going towards the Haldex systems,
not the torsen systems.  The main reason you see torsen centers is the
ability to use ABS AND EDL with no human interaction.  Current audi applic
ation torsens are 75/25/75 or 78/22/78 in the pre nue S4 cars, depending on
which audi literature you read.


What's "new" in terms of technology?  The haldex.  The Haldex engineers
that audi could use it in "fulltime" awd mode (actually *they* wonder why
audi doesn't do this), and that the torque levels are virtually limitless
(haldex roots is as heavy equip supplier).  Audi is actually late in the
haldex game, since Jeep has been using the haldex type center diff in their
SUV's for a couple years now.

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