RE: torsen driveshafts

[David Eaton <david.eaton@computerland.co.nz>]

scott, the bias ratio is the "quotient of the torque in the higher
torque axle divided by the torque in the lower axle in proporation to
unity" (page 3), nce in the audi centre inplementation, this is 70/30:1
or 2.3:1.  6.66:1 is way out.  you're multiplying not dividing.

you are also incorrect in suggesting that *all* the torque will go to
one or other shaft with the torsen.  this is the agreed behaviour for
the generation 1 diff (in open mode), but the torsen *cannot* do this.
it must operate within the parameters of it's bias ratio.  the audi
implementation most of us have is that no less than 30% of torque will
be sent to the shaft with the least grip.  not 0% to one, and 100% to
another...

by definition with the torsen, it is excedingly unlikely that one shaft
will "spin up" relative to the other shaft.  the torsen will resist
'spin up' of either shaft by instantly divind torque between driveshafts
in proportions up to the bias ratio.

contrast this to an open (generation 1) centre diff which automatically
transmits torque to the shaft with the *least* traction, resulting in an
incresed likliehood that that shaft will spin up relative to the other
(in the same way as the wheel with the least traction will tend to spin
up relative to the other with the greater traction.  put thisanother
way, the shaft with the greatest resistance to motion slows with regard
to the other shaft, and transfers torque to the other shaft with faster
rotation.  the torsen resists this, which results in a greater
proportion of torque going to the slower shaft.  much better
behaviour....

re: abs
this is a *significant disadvantage with the generation 1 setup.  with
the locked centre, under hard braking, you are much more likely to have
a slipping (oversteering) rear due to the increased liklihood of wheel
lock.  this will not occur with abs.  so you chassis setup into the turn
will be more neutral under abs, than with the generation 1 setup.

re: the new s4
the new s4 has a static ratio of 47:53 f:r, so is designed more around
rear drive, not front drive.

dave
'95 rs2
'90 ur-q

>-----Original Message-----
>
>Date: Sun, 1 Mar 1998 08:30:17 EST
>From: QSHIPQ <QSHIPQ@aol.com>
>Subject: Torque Sensing Differentials
>
>Specific to the Torsen posts:
>(Please bear with me, for length, I have jumped already, and extrapolated
>'wheel/axle' references to driveshaft - i.e the article is written assuming a
>side to side Torsen, not front to rear)
>
>Dave E:
>Look again at that web site on the Torsen, specifically, page 8.  Locking
>effect vs. Bias Ratio.  How did you get 2.31?  A 75/25/75 Torsen is a 75%
>locking effect (or your claim 70% locking effect), a 75% LE has a 6.66 BR, a
>70% LE has a ~5.7 BR.  For my post, I will go with audis claim (and that must
>be accurate:) of 75/25/75 Torsen Center (6.66 BR).  For the purposes of this
>post you can insert the 5.7, it doesn't really change things.
>
>Your bias ratio changes from 2.31 TO 6.66.  Remember, we are talking
>driveshafts here, so you go from 75/25/75, and anywhere in between.  So the
>BR
>is 2.31 TO 6.66 depending on a whole bunch of chassis and throttle inputs.
>As
>a general rule, though, the BR is thought of in terms of the ratio to the
>SLOWER turning axle.  So the number you should use is 6.66:1.  From an audi
>center torsen chassis perspective, that means you can have 6.66 times the
>torque of the front going to the rear OR 6.66 times the torque of the rear
>going to the front, ALL IN THE SAME TURN.  Why I claim the bite is there.
>For
>a rwd or fwd torsen (this articles example) you don't change the inherent
>chassis character, you're just increasing the traction of that rwd/fwd
>character.
>
>You can't talk of a static split or Locking Effect without talking about Bias
>Ratio.  If you reduce the Locking Effect (to say 60/40/60), that means you
>REDUCE your Bias Ratio, look on page 8 (Bias Ratio = the amount of torque of
>the h
>igher traction wheel supports *  torque supported by the lower traction
>wheel.  Reference, a open diff has a 1:1 bias ratio).  So, let's compare a
>75/25/75 torsen to a 60/40/60 torsen.  The bias ratio of the 75 is 6.66:1,
>the
>bias ratio of the 60 is 4:1.  What does that mean?  The 75 torsen is able to
>give 6.6 times the torque to the slower driveshaft (vs. the slipping one),
>the
>60 torsen is only able to give 4 times the torque (vs the slipping
>driveshaft).  So, as you narrow the torsen scope, so goes your traction
>benefits of that change.  For reference, a 50/50 split center is a 3:1 bias
>ratio.  
>
>For maximum Torsen Locking in terms of absolute traction, you want the widest
>range.  Why?  Cuz it gives the highest available Collective torque transfer
>to
>the driveshaft with the most traction (that is the SLOWEST rotation, btw). As
>you get higher in locking effect, the gains in Bias Ratio are much higher for
>smaller gains in Locking Effect.  So you have 4:1 BR at 50% locking, 6.6:1 at
>75% lock, you have 9:1 at 80%.  This can be simply stated in terms of
>absolute
>traction:  A higher Locking Effect Range gives maximum traction ability in a
>given center differential application.
>
>So, audi marketing went for traction, NOT handling.  Reread that statement.
>For those Gen I folks that never locked the center, audi increased your
>TRACTION 'potential' 6.66 fold on the Gen II Torsen Center.
>
>Regarding your abs argument.  That is true to a certain extent, but it is
>less
>than a VC, and many other types, because these others freewheel (go to open
>diff), not a percentage as the Torsen does.   However, until we gain some
>consensus on acceleration, I'd rather leave the braking part out of the
>torsen
>argument.  Let's talk about torque split on acceleration, not braking or
>coasting for now.
>
>Looking at a 57/43 static split, I ask, ok what is the "range".  As long as
>you have a range, my claim is A) you have a potential bite if you have
>57/43/xx (where xx is larger than 51) and B) you lose the "traction" gains vs
>other lockers as you reduce the range, and C) you reduce the ability to rely
>on a "simple" abs system as you decrease the range (read, changing bias on an
>older car, may or may not enable you do use your abs).  So did audi in the S4
>use 57/43 to 75/25?  Or did they use 57/43/57?  (I argue the former) Either
>way, you assign that to the Bias Ratio, it DECREASES, and you have added only
>another variable to your torsen equation, and you really haven't reduced the
>chassis dynamics while turning.  And as you narrow the range, the gains over
>a
>fixed diff become minimal, and BR Decreases in terms of Traction. In terms of
>chassis dynamics, I say that becomes nil.  Does EDL change absolute Traction?
>I bet a bunch of BR is does.  So did audi.
>
>Or, do we find that the 57/43/xx keeps the chassis dynamics at understeer all
>the time?  I don't know, but my optimism wanes some thinking of the setup.  I
>guess it would depend on whether we are 'shimming' or 'helixing' all
>available
>differentials.
>
>In terms of slip angle what is happening?  Well, a car that tracks
>differently
>in a turn needs a certain amount of driveshaft freedom, specifically to audis
>awd, a car in a turn needs to have one driveshaft spin faster than the other.
>A main reason why you don't have a 100% locking differential in a Torsen.  At
>that 25% speed differential, the torsen will become, well, a torsen.  That is
>to say, on <acceleration> (don't confuse this with engine coasting or
>braking)
>the driveshafts can be moving at a different speed to each other, beyond 25%
>they will lock.  The definition of a 75% locker, either driveshaft can spin
>25% faster than the other.  But if you get enough wheel slip, the torsen will
>lock the slipping driveshaft to the non slipping one.  When that wheel slip
>is
>really high, i.e. a WOT spool up, the rears lose traction the rear driveshaft
>spins faster than the front, and torque transfers forward.  You can feel this
>effect in a straight line, the Torsen hunt, just pull away from a stop in a
>straight line in the snow, with just medium gas.  You can ac
>tually feel the
>torsen working back and forth.  In a turn it does this too, only now, that
>hunt is more dramatic.  Why?  Because the slip angle of the tires in terms of
>track +/- traction exceed the 25% slip of the torsen.  So ALL the torque goes
>front to rear, immediately.
>
>So, I look at fixed split torsens and ask, "What makes them better than a
>50/50 fixed diff?"  In terms of chassis, what gains are made with a 60/40/xx
>Torsen vs. a 60/40 fixed diff, torsen or otherwise?  I see why torsen centers
>have limited uses in racing.   Look at the Bias Ratio.   Any gains in using
>them?  I suppose a 70/30 to 51/49 might have use, wonder what all the elusive
>racing torsens used? For any split higher than that 50/50, my argument is for
>that small gain in BR, you lose big in terms of chassis dynamics,
>specifically, going from pushing RWD style (oversteer) to pulling FWD style
>(understeer) in the same turn.   
>
>Scott Justusson
>QSHIPQ@aol.com
>'87 5ktqwRS2
>'86 5ktqw
>'84 Urq
>
>