[torsen] RE: Haldex differential

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In a message dated 11/22/02 2:47:54 AM Central Standard Time,
Dave.Eaton at clear.net.nz writes:


>no, with a locked differential the tbr is infinite (the result of dividing
>by 0) as there is no way of controlling the torque shift (the traction
>available does that for you).  with the torsen, the tbr is a *limit*.
>torque will be shifted around up to that amount - whereupon speed
>differentiation occurs.  in other words, with a torsen the tbr is the limit
>up to which the diff will remain locked.  with the haldex there is no tbr in
>the normal case, as there is no clutch engagement.

Er, it's 95f/5r to 50f/50r..  The haldex in it's current trim is never fully
decoupled, just like most VC and VCT.  You could say it's negligible in
torque at 5%, but Dave it's not 0r.

>there is also no 5% rear as you suggest with the tt haldex lsc - as there is
>no method of initiating the shift (pump) until slip occurs.  technically,
>once slip has occurred, you could initiate rearwards torque shift and hold
>it there i suppose, but i'd be extremely surprised to find that audi do that
>just for the tt.  there is certainly no mention of this as a feature of the
>haldex lsc - i think you are getting confused with a classic vc operating in
>a "hang-on" mode (as opposed to one integrated with a true differential).
>in this sort of application (vw synchro in other words), it is common to
>have a nominal torque bias due purely to the type of silicone used and it's
>shear properties - it is advantageous to reduce the engagement time.

A couple things Dave.  Audi could hold rear lock at any point with the ECU,
where I've been trying to lead you and Keith all along, massively
constrasting the strictly mechanical application in the Jeep for example. The
throttle valve is controlled electronically, which means that it can hold any
lockup or %of lockup as long as the pressure is there.  Put another way, the
throttle valve is a wastegate frequency valve (with a stepper motor no less),
it can add 100% of piston pressure, or ANY % up to piston pressure.  With no
throttle valve intervention the torque is allocated 95f/5r.  Like a turbo
car, the stepper motor can't create it's own pressure yet.  The addition of a
brake bomb or a revision of the piston pressure (make the turbo more
efficient) input could change that.  Changing the turbocharger to a
supercharger if you will.

>fwiw, the porsche 959 system also used a hang-on clutch driven by an axial
>pump powered by the front/rear speed difference - the difference in that
>application was that the difference in tyre size front/rear created the
>pumping pressure to allow clayton's active control.

Which can also be done in the TT.  OR just change the slip to zero, piston
pumps all the time.  With a throttle valve, it doesn't matter that the sytem
is always active, as long as you can bypass 100%.  It might be interesting to
understand why the 15% was chosen for activation.  It might be interesting to
understand why audi chose 95/5 as well.

>lets look at it another way:

>open diff:   normal tbr 1:1,  limit tbr 1:1
>locked diff: normal tbr 1:1*, limit tbr: 1:infinity
>torsen diff: normal tbr 1:1*, limit tbr: 1:3 (depending upon applciation)
>haldex lsc:  normal tbr 0;    limit tbr: 1:infinity

Dave, it might help me and others understand your point if you tried plotting
it ala fig 6 in the chocholek paper.  I don't read a "normal TBR" into the
definition of TBR, that would just be the static torque bias.  For example,
you could put the static torque bias of 60r/40f, and still have a 3.5:1 TBR
(78/22/78).

>once again, with the clutch fully locked, the haldex lsc has no ability to
>control torque shift, and remember that, once engaged, it cannot effectively
>increase lockup until there is further slip.  what it can do though is to
>monitor wheel speeds and reduce clutch engagement if the rear is sliding.

The clutch is only fully locked if the throttle valve locks it.  As such, it
is controlling lockup all the time.  Again, one should think of the throttle
valve as a WGFV.  Your "cannot effectively increase lockup" should be it just
"can't" in current programming.  a series of ck valves could change that, so
could a revision to the pump, or an accumulator.

>the modification to an active clutch would be a good 1st step - but this
>requires a completely different operating principle (why continue to use
>axial pumps?), new software and a powerful external pump.

It IS an active clutch by definition.  It might be a more crude device than
others, but it is active.  Currently, if you have slip, it's fully active in
all senses of the word (like:  if you have boost, the WGFV is *always*
active, without boost -piston pressure, it doesn't matter what you do with
the tvalve).  To make it "better", one could say it needs full lockup
capability all the time.

>however, regardless of the nature of the lsc, i think the biggest
>improvement would be in the incorporation of a differential, then you would
>have a normal static torque split, and use the clutch to supplement control.
>the haldex lsc is too reactionary to my mind.

Gen II will allow 1 input 2 outputs which means it can be used as a
differetial.  You mean to say as a "LSC differential lock".  As a
conventional differential (ala torsen) you haven't done anything to decrease
reaction, you've only moved the location of the device.

>this is where zexel have gone with the t3 - although this is aimed at a
>centre application, it is by definition (it uses an epicyclic gear set)
>aimed at providing more rearwards torque static bias than forwards.

Defined as a Torsen center differential lock.  Different than the current
torsen center differential.

>a true active differential (a la wrc) is simply a clutch with a
>differential.  i hope that this is what haldex are developing now.

Not sure I agree, as you would have a fully active differential with the
current Haldex with only adding the ability to have 100% piston output
available 100% of the time.  It doesn't need to move to the differential to
accomplish this.

HTH,

Scott Justusson