[torsen] RE: Haldex differential

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Dave
Comments inserted
In a message dated 11/20/02 9:46:02 PM Central Standard Time,
deaton at tranzrail.co.nz writes:


>1) don't use hp in this discussion, you need to be talking torque.  in your
>figures for the rs6 you make the same mistake, but as the numbers for nm and
>hp are near enough, i thought it was a typo.

Ok, I'm adding to my list.  ;)
>2) shift and capacity are one and the same.  if the diff is supporting a
>particular bias ratio, then that is where the torque is going.  in a open
>diff, the bias ratio is 1:1 so that input torque is differentiated evenly,
>with the input torque determined by the traction of the least tractive axle.
>in a locked diff, the bias ratio is 1:infinity, so that there is no limit to
>the ability of the most tractive axle to support torque (100% in other
>words) - to the limit of adhesion.  with the haldex you have the ability,
>courtesy of the throttle valve stepper motor to have a variable bias ratio
>determined by the ecu, based on input from the abs sensors, the throttle and
>the "g" sensor etc - provided there is slip in the 1st place.  what you
>*don't* have is a static torque split (bias ratio).

Dave, please math that statement out in terms of T1/T2.  TBR is speaking of
shift IMO, cuz you can't "solve" for a locked differential.  Remember, the
TBR has specific formulas put forth in Chocholek's paper.  You can't
multiply/divide by zero.  In a torsen for example, in it's 78/22 state, it is
also "locked" by definition, regardless of Trg.  That doesn't at all mean the
TBR is infinity:1

>this is the reason that your "45% torque shift rearwards" scenario makes no
>sense in a haldex.  at various times the ecu will be doing exactly that of
>course, in reaction to road conditions, but at other times, it will be 80%
>rearwards, other times (the "normal case") 0%.  the beauty of the haldex is
>that, provided there is pressure in the system enough to create a desired
>bais ratio, you can infinitely vary the bias ratio *below* this using the
>throttle valve, *regardless* of the slip conditions, or even increase the
>bias ratio if slip is continuing to happen.

That's not changing The Bias Ratio based on the definition of it.  The
capacity of a locked diff creates the "infinite", it's not a function of the
Bias Ratio.

>as i see it, there are 2 weaknesses with a haldex: there is no torque bias
>in the normal case (i.e. 2wd until slip occurs).

Completely depends on how the electronics or the clutches are preloaded Dave.
 Even now, that's not a truism, the TT haldex is 95f/5r.  The haldex is
capable of shifting up to 45% of input shaft torque to the output shaft.
Once in that locked state, either axle can support 100% Trg.

> to solve this you need a
>differential (keith you are right about the visco lok - good and bad - i
>have the sae paper).; and secondly that once slip occurs the haldex is
>working to minimise the slip so your pump has increasingly less ability to
>build pressure.  whether or not this is a real issue, i don't know - it
>certainly appears that way on the surface.

It can retain pressure OR change how it "builds" pressure, either concept can
work, right now it can hold "peak" indefintely thru the throttle valve.
Right now the "designed" slip before pump pressure is 15degrees of wheel
rotation differences.  That can be modified several ways, either by an
external pressure resevoir (bomb) an internal modification to the pump
valves/clearances, or by adding more preload pressure, etc.  Gen II haldex
sure appears to take a more concentrated look at center diff applications vs
hang-on.  Regardless, a great first step in active differential technology,
the electronic component creates a massive jump in how the same coupling
concept can be changed significantly with the addition of electronics.  What
can happen in  the Jeep  Halex type is massively different that what can
happen in the TT, same device with ecu.

>the haldex gives the chassis engineer some interesting tools to play with.
>i wasn't the only person wondering if the tail happy nature of the early tt
>had something to do with the ecu opening the clutch on throttle-off
>conditions where a more prudent approach would be to ensure that there is at
>least a measure of rearwards torque bias, particularly at high speed.  again
>the weakness of the haldex in these situations and a nice summary of the
>above 2 points is that if slip hasn't occurred in a high speed corner, in
>the throttle-lift scenario where you would want to apply rearwards torque
>bias to limit oversteer, you can't do a thing.  with an active clutch you
>could do what you liked.

This is summary is *not* the weakness of the haldex, it's a weakness of the
programming of the haldex.  The device is capable of reducing rear torque
without release, it just needs to be programmed to do so.  I don't agree that
the problem with the TT is the haldex necessarily, as the FWD version of this
vehicle were known to have the same problem.  Thinking in terms of the fixes
involved (no programming changes to haldex, adding rear spoiler and already
added rear weights off the back bumper) this is a chassis dynamics problem
inherent to the chassis design, not the driveline components within it (IMO)

>the 2,000nm is a real figure for the torque limit of the current haldex
>coupling.  i'm sure that it can (and is) modified for other applications.  i
>have no idea what sort of capability the existing housing has to support
>2-3x the torque capacity, but i imagine there would be a fair amount of
>re-engineering involved...

That's 1400lb/ft of torque dave.  Which audi (or any other marque)
application do we need to modify to require a re-engineering?  A further
question, why do we need 3860lb feet of torque in the Torsen?  Shock load?
Remember, if TRG is too high in a Haldex or if a "jump" occurs, the relief
valve allows slip.   The exact same reason you see VC's, they have the
inherent design ability to allow shock loads without failure.

SJ