[torsen] SAE885140 vs the Spiderman

QSHIPQ at aol.com QSHIPQ at aol.com
Wed Mar 7 09:52:12 EST 2001


In a message dated 3/6/01 2:15:28 PM Central Standard Time, andre at online.ee 
writes:

> On 6 Mar 2001, at 8:47, QSHIPQ at aol.com <QSHIPQ at aol.com> wrote:

>  > Ok, remember tho, you have chassis O and U *within* the torque split 
> limits 
>  > of the differential.
>  
>   OK, tho you can't have U in turn unless rear looses adhesion.
Sure you can.  Depends on Trg, weight shift, cof, wheelbase, speed, and all 
the rest of the variables in the matrix.

>   Scott, how can we speak about "exactly BEFORE wheels loose all traction"
>   while discussing spider bite? Perhaps I misunderstand you. If wheels have
>   enough adhesion to keep torsen within BR limits, then you don't have a 
bite, 
>   but are cruising with 100% of engine power delivered to the ground.
>   This is what this quoted paragraph is talking about.

no.  The article is speaking of forced slip between axles caused by turning.  
They haven't 'lost all traction' just have forced slip.  Again, think of the 
locked diff while turning to get a concept of this.  If you take a 20m radius 
turn at 2mph, and the axles must rotate at the same speed, what happens?

>  
>   We are talking about exceeding torsen limits, including Trg=0, aren't we?

Not me.  You *can* exceed torsen limits, I'm speaking specifically of *not* 
exceeding torsen limits, and not including Trg = 0.  We can look at any 
quattro and know that if trg = 0, then you have understeer.

>   I guess you mean vehicle without acceleration. In that case torsen behaves
>   like open diff and locked diff would screech. During acceleration I 
believe
>   front wheel would need to slip and would, causing torque to go rear with
>   locked diff. no? OK, not 100%, but still more torque than would go to 
front.

We are getting closer here.  Add more acceleration, then redefine "screech" 
and "slip" in terms of chassis dynamics.

 
>   What I assumed by oscillations you mentioned, is a required slip of rear
>   or front wheels due to overload. Without any slip there can't be any
>   oscillations. Torsen will maintain steady state all the time, just as open
>   diff is doing. With torsen you have O all the way through the turn,
>   _unless_ rear slips.

NO.  Only under the conditions exactly described in the paper, not *all* 
conditions.  The paper only gives a specific set of circumstances where O 
will prevail.  "All the way thru the turn" means what?  If I unwind the 
wheel, I might have O, but the torsen is always looking to return to 50/50 
split which is U.  We also can reasonably conclude that somewhere in that 
turn (before straight ahead U), we will have U as well.  Depends on a bunch 
of variables.

>   You get an O with locked center during front inside wheel lift or slip
>   in a turn. Nothing terrible I guess.

Couple questions:  When do we get front inside wheel lift.  If it's slipping 
isn't that U?

>   Scott, look at it differently (and don't confuse with speed sensing)
>   1) Torque goes to where least resistance is.
Agreed
>   2) Torsen's internal friction resists to speed differences, and axle with
>      less traction force pulls along an axle with more traction. All
>      this with a given split limit. Internal friction is a result of gears
>      being "jammed" between input shaft and output resistance.

Not clear.  What is the "given split limit"?  Isn't indeed a torsen 
constantly changing it's torque allocation while turning, depending on a 
bunch of variables?

>   3) axle rotates slower only if it has more resistance than the other.
>      (otherwise applied torque would spin it up to speed).
OK
>   So, axle cannot rotate slower if it hasn't enough traction. no way.

What about a turn as described in the paper.  Isn't turning causing the front 
wheels to rotate faster and the rears to rotate slower?
>  
>   And I mean it when I say that torque split is tied to axle with _less_
>   traction (or resistance or higher speed, if you like).

Ok

>   Point is that torque tries to go out a least resisting axle first, and
>   only then internal friction resists that by holding onto slower axle. And
>   this internal friction is smaller if speeding axle has little resistance.
>   (because the "gear jamming" effect is reduced)

Not clear.  Torque goes to both axles 50/50.  Then as resistance is detected, 
the torque goes to the axle with the *most* resistance, up to 3 times the 
amount of the axle with the *least* resistance.
>  
>   This is the whole reason why torsen looses all torque (Trg=0) when one
>   of its axles looses all traction (resistance=0).

Not really.  The reason torsen looses all Trg is that the axle with the least 
amount of resistance approaches 0 (it can't support *any* Trg).  At 0 x 3 = 
O.  

>   No. there is no such dual shifting in torsen. it's oneway device. I agree
>   that it applies more torque to rear in turns, but not because of speed 
diff
>   per se, but only because it is "easier now" to propel front.
>   You say that torsen is always locked on torque. If so, I can't see how it
>   behaves differently to locked diff during that time. And if its not 
locked,
>   I can't see why we can't compare it to "tight" but still open diff.

Not with you here.  The paper exactly states that it is the forced slip 
between axles that causes slip while turning.  If it's not locked you can 
compare it to an open diff.  The problem is that is a 0 torque argument.  ON 
torque, it's always locked.  Not sure what a "tight" open diff is.

>   I agree that when rear spins, we have O-U with torsen. I also agree that
>   torsen may be the culprit in spider bite. But I can't agree that torsen
>   can cause this _without_ rear spin. _With_ rear spin, we have to compare
>   torsen to locked diff, and we see that there are similarities. Now I
>   propose that with locked diff regaining of rear grip is less noticable
>   than with torsen. Thats ok with me. From here I try to deduce how the
>   bite happens.

You can't deduce it until you understand that when this phenomenon is 
occurring, all 4 wheels are on the ground.  You need to define rear "spin".  
With rear spin a torsen behaves all by itself, it's *not* like a locked diff, 
because a locked diff can't allow 2 axles to spin at different rates, ever.

>  
>   Now as per your other post on 205T vs torsen we realise that rear spin
>   is actually wanted for racing. Torsen with spinning axle is loosing
>   engine power compared to locked diff. Torsen without spinning axle
>   probably has better overall traction, but on average, locked diff has
>   a little edge in racing. IMHO.

Why?  Per my post on the 205T16 we realize that torque split is used to mimic 
the weight transfer of the car under acceleration.  In high grip, it's more 
than low grip conditions.  Again, to understand the torsen limitations, we 
have to get over the idea of a smoking wheel at the back.  Torque shift 
happens before this, way before.  Once a wheel actually starts to 'spin" or 
"lift", the torsen behaves predictably, Trg>0

>   Come on, what are we arguing here? VC is purely speedsensing device with
>   some problems torsen doesn't have. They are similar, yet quite different.

How?   I'm not arguing anything Andres.  I'm trying to get a baseline 
understanding of how a torsen works.  If you look at how a VC works in a turn 
vs how a torsen works in a turn, you might well get my point.

>   Hmmm. Now I'm puzzled. If you don't spin/slip/slide the car, wheres the 
> bite?
>   So you're running with 100% engine power to the ground, and you have a 
bite?
> 
>   Would you care to describe one of the bites once more, in detail, please?

What is .2% forced slip between axles causing 62/38r/f split Andres?  That's 
from a turn with all 4 wheels on pavement.  If I add more power with that 
torque split, eventually the rear could have .2% forced slip that causes the 
torsen to shift to 50/50 which is understeer.  Is .2% forced slip a spinning 
wheel?  It wasn't when you turned.  We are speaking of very small slip angle 
differences causing torque allocation to change.  

>   You can't have torsen providing torque forward in a turn without rears
>   loosing grip. Whats the difference between fast spin and slow spin?

Define .2% slip, it might help you.  

>   Rear wheel on the ground can only loose traction force if it spins. right?
No, it slips first (starts to lose it's tractive ability).  Put your car in 
neutral come up on a 90 right, and the car doesn't make the turn.  Why?

>   So with torsen, for a car to have U in turn, its rear has to spin. ok?
NO
>   At least virtually. If torsen stops the spin as it starts, then so be it,
>   but this doesn't change the fact that spin was about to evolve.
>   If so then we are back to my proposal of bite, built on spin precondition.
The problem is that I don't argue with the fact that when a torsen car has a 
spinning wheel, the torsen behaves predictably.  I fully claim, before this 
happens, the spider bite has either already happened or not.
>  
>   I feel like you have a confusion about how torsen "allocates" torque.
>   It doesn't do anything fancy. It just grips onto an axle with better
>   traction. So does locked diff. no difference.

No.  4 wheels on .9 cof.  2mph torque applied.  It allocates torque rear just 
based on turing radius.  Better traction or forced slip of a locked axle?

>   Yes, but I'm talking more about the 2 points on the graph (fig.2) between
>   which things happen. On that steep line, between torsen giving 100% engine
>   power to the ground, and Trg=0, it behaves weird. If track surface is with
>   uneven grip, you'd feel that alot more with torsen than with any other 
diff.

Let's presume (which I base all my arguments on) that COF is constant with 
all 4 wheels, smooth surface.  I think you are trying to add too many 
variables.  It's not necessary.

> 
>   More because torsen takes rear torque, "multiplies" it by TBR and provides
>   it on front axle. Small variations in rear grip can cause large variations
>   of front torque. I think this is what you are talking about, O-U-O-U
>   oscillations. I merely try to show where and when they occur and are more
>   noticable.

YMMV?  Mine did

>   right, exactly what I'm saying: front "torque can't be more than as much
>   as rear torque". You should note the moment rears regain some grip. What
>   I describe happens, just with open diff it isn't very noticable. With
>   locked diff also less, because it can fluctuate between 100/0 and 50/50
>   split. What I want to say is that during a slip, torsen can go like this,
>   in terms of engine power to the ground: 4/1, 16/4, 60/15, 80/20, 70/30,
>   50/50, 30/70 (if in turn)
>   (given BR of 4 times). Notice how front torque goes multiplied upto 80,
>   and then back to 50 as rear axle regains grip and axle torques equalise.
>   This is O-U-O which is different from other diffs, because in the range
>   of grip loss it changes alot faster.

That describes the initial turn in.  What about when the rears lose traction 
(which means the fronts have more traction), isn't the tractive ability of 
the fronts a multiple of the tractive abilities of the rear (so we have 
30r/70f)?  My claim is that you must continue the split beyond where you 
stopped.  Looking at the above (assuming the variables in 885140) I claim in 
your scenario we have O.  Then, *if* that 70/30 causes the rear tires to slip 
more than the fronts, torque shifts forward, creating U.  

>   I fully agree that this is a weakness of torsen. It should provide 50/50
>   during turns to have better results. But I can't see the bite moment here 
;)

See above,you are only looking at 1/2 the equation.
> 
>   I can imagine it elsewhere, but not here ;) Locked diff also can't imagine
>   its turning.
>   When going from straight to a turn on power, locked diff split is what?

Dave got confused with this too, I encourage you to *not* compare a locked 
diff... Yet.  Understand what a torsen does in and of itself.  It *is* 
unique, and really until you understand what it does, you can't really 
compare them to any other diff.  

>   Fronts have to slip, right? how does their slip change torque split?

Better question:  How does that slip change the chassis dynamics if it can't 
allow any speed differentiation.

>   How predictable it is, based on unknown track surface?

Always U is very predictable.  That's why most audi race cars (and just about 
all audi rally cars) run no center diff at all, cuz if it's locked all the 
time, you don't need a diff at all.


>   Riding an open front locked/c and having a wheel lift in turn.
>   Is this a bite? I guess not, but here we also have clear O.

No it's not bite, I just might ask, if you have 2 rear wheels on the ground 
supplying 50% of the power to the ground, and a single front wheel to the 
ground supplying 50% of the power to the ground *and* steering, what will the 
chassis dynamics be?

>   I can read you. Somehow I feel that you read my wording "slip" as a full
>   loss of any traction (Trg=0). I'm sorry if I cause a confusion here, but
>   I really mean slow slip, while not all traction is lost, but it is reduced
>   alot. In this case Trg is an important factor, as is traction force on
>   slipping axle.

Getting closer.  In the case of 885140 the resulting rear slip only has to be 
.2% rear to make a torque shift to 50f/50r

>   Hey, where did I say torsen rulez in racing? I think on contrary. But 
also,
>   for street driving, it is "sticking" better to the ground than open diff
>   or locked diff would, so it is a good "dumb" device in the center, mostly.

Never disagreed with that.  Some of us more hotdog types have realized that 
that predictability limit is reached at the .7th driving level.  So, just 
like honda, the device is good for 70% of drivers.


>   You have to have *less* traction in rear than in front. During a tight
>   turn when torsen already "sees" less traction in front, this means quite
>   a bit of lost rear traction to get U. I'm at loss how this can happen
>   without a loss of traction in rear...

Turning radius of front is not equal to the turning radius in the back?  
Which is spinning faster in terms of axles?
>  
>   I don't know. imho we don't miss anything in relation to how torsen works,
>   maybe we need to know more on how specific Audi's torsen is constructed?

I don't think so.  For chassis dynamics we only need what torque split causes 
O and what torque split causes U.  If that is within the BR, then we don't 
need a spinning wheel to get O or U, we only need some specifics to happen 
within the operating parameters of the differential.

>   I perfectly agree with this, except with "Ad infinitum" part.
>   If that is the case, then you have varying adhesion on track surface.
>   Then we should talk about how this is noticed differently vs other diffs.

Andres, remember why the diff is shifting torque in this case, because the 
rears have reached the limit of adhesion, so the shifting of torque regains 
adhesion rears.  However, this could also mean that the consequence of 
shifting that torque, overloads the fronts, making them reach the limit of 
adhesion, which means torque is again shifted rear.  Same cof.  This means 
something else has to happen, like a lift, a steering correction for 
instance.  But my claim is, when you are in the middle of the bite is when 
the driver realizes that something else needs to happen.  There's another SAE 
paper on that and what is considered 'reasonable" and "likely", but the risk 
exists that lift can cause an LTO spin.  Or that a steering input can cause 
LTO spin or an U off into the weeds.  When you've experienced the bite, I can 
only say that the comfort level in either scenario is about nil.  Why?  
Because whatever the driver does (to change an input), may or may not 
positively affect what the torsen will do in return. 

  
>   Take VC, rwd vehicle. Same turn. O most of the time, after a rear spin
>   you have U, rears regain traction, you get O again. Ad infinitum? Not
>   that different, only that with torsen fast spin is not needed in rear.

Throttle on oversteer?  More givens need to be present to accept this 
argument.  But, the biggest difference is that the center diff is what causes 
the problem, not necessarily the rear.  For fwd and rwd vehicles, remember 
that 'bites' don't happen as I describe, because the drive wheels attached to 
the differential has a constant slip angle.  We are talking about drive 
wheels attached to the differential with different slip angles.  Why torsens 
in fwd and rwd are widely used.  A huge constant is there.

>   But we are searching for something inherently specific to torsen here.
>   So that isn't quite what I hoped.

Specifically it's inherent to a torsen in a center axle <only>
 
>   And if you insist that torsen is unable to achieve a stable state, then
>   we need to think more on why this can at all happen.

A torsen can't be in a stable state in a turn.  We can identify, just like 
885140 what exact points we can *declare* a 62/38 split, but you change any 
of the variables, the torsen has a different torque split.  Unstable = 
unpredictable.

>   ok, its just simpler to say "spin" than "reaching the limit of adhesion".
>   Torque can't go to the front unless rear is providing less "support" to
>   torsen's rear output. This is "spin", be it even as slow as front axle
>   rotation speed. Isn't it?

How bout we speak of slip, instead of spin.  That's what the paper is 
describing as well.  .2% slip isn't that much.
  
Andres, I encourage you to get 885140 in complete context.  It appears pretty 
clear to me that a torsen that turns is reallocating torque based on only 
turning, being fooled into a traction argument as it were.  When that 
allocation causes slip, the torsen goes back to it's primary objective as a 
traction device.

not cool always.

HTH

SJ



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