Re: Boating with an S4

["Avram Dorfman" <dorfman@est.org>]

Can you offer us links to these references or some other way of actually
locating them?

-Avram
-----Original Message-----
From: Dave Eaton <Dave.Eaton@clear.net.nz>
To: quattro@audifans.com <quattro@audifans.com>
Cc: dorfman@est.org <dorfman@est.org>
Date: Saturday, August 14, 1999 3:54 AM
Subject: RE: Boating with an S4


>interesting case.  the behaviour you describe is normal for a torsen.
quite
>simply, with a wheel in the air, a % of torque will go to the axle without
>grip, while the remainder goes to the axle with grip, but because the
amount
>of torque supported by the spinning wheel is low, so is the amount sent to
>the "high" side.  in other words, your car should be able to drive off a
>jack with a wheel in the air -folks, don't do this at home unless your
>parents are with you at the time :-), but not proceed up a hill (not enough
>torque).  this is the reason for the diff lock on the torsen audis.
>
>the issue is that *any* limiting differential is only able to support a
>particular (designed) torque "bias ratio" across the diff.  our house has a
>driveway where this can be the case, the rr wheel hangs in the air, and the
>vehicle will stop, until the rear diff lock is pressed.
>
>manual traction control (rear diff lock) and you're away.  the latest
>technology is to replace the manual lock with a brake-based traction
control
>system (tcs).  without tcs, or rear limited slip differential, a locker
>centre or a centre with a very high tbr would be required.  then you would
>have to live with the low speed manoeuvrability issues.
>
>as another way of illustrating this issue, with a limited slip diff, if you
>can *increase* the torque being sent to the "low" side of the diff by
>braking that side (remember that a spinning wheel consumes little torque),
>then the torque sent to the "high" side (where the traction is) is
increased
>and all is well.  this is of course, what a brake-based traction control
>system (tcs) is for.  audi's 'ebd' exactly.  the handbrake does similar of
>course.
>
>the other way around the problem is to have a limited slip rear
>differential.  the integrale had a vc centre, and a torsen rear for this
>reason.
>
>there are of course philosophical differences between the various methods
of
>centre limited slip differentials.  the vc is speed sensitive (ie requires
>wheel spin before it operates), while the torsen seeks to eliminate wheel
>spin in the 1st place.  this, btw, is why the amalgamation of a tcs and a
vc
>is a little difficult.
>
>the other way of "solving" this issue, and by far the most elegant, is
using
>"active" differentials where the torque shift is controlled by computer and
>based on the input of various sensors (wheel speed, accelerometer, g sensor
>etc).  the haldex differential goes someway down this track.
>
>however, before we all get carried away, the major concern of manufacturers
>is of course for dynamic (rather than static) behaviour.  to illustrate,
>take the case of a vehicle constantly accelerating in a straight line with
>either an open or a torsen centre.
>
>in the open centre diff, you have equal torque to either axle (by
>definition), while the dynamic weight distribution moves rearwards (as the
>vehicle accelerates).  this results in increased front slip (less weight at
>the front therefore less traction but equal torque).  when this eventually
>results in front wheel spin (increased longitudinal slip overcoming
>traction), the torque able to be distributed by the open centre to both
>axles decreases to the level able to be supported by the spinning *front*.
>
>the difficulty with this of course is that for optimum performance, torque
>should be redistributed in proportion to the wheel traction/loading (ie..
>longitudinal slip), which of course varies according to the dynamic weight
>distribution.
>
>for the torsen centre diff, the front and rear wheels continue to run at
the
>same *speed* of rotation, and the same longitudinal slip (by definition) as
>torque is re-distributed rearwards until either the limits of adhesion or
>the differentials torque split range (bias ratio) are encountered.  so the
>fronts don't spin.  even on wet roads, studies have shown that the torque
>redistribution is well within the bias range, and goes some way towards
>matching the dynamic weight distribution of the vehicle.  at the same time,
>because of less torque going through the fronts, steering performance is
>obviously improved over the open diff case.
>
>this issue has caused more than a little mis-understanding.  the fact is
>that the torsen is "locked" until the torque bias ratio is exceeded,
>whereupon it will allow differing output shaft speeds.  this is why, for
>example, you get axle windup with torsens on full lock car-park manoeuvres
>(as a point of reference, with the audi torsen in the 80q, the bias ratio
is
>exceeded at under about 15m turning circle on a high grip surface).
>
>the difference between the torsen and the locked centre in the
straight-line
>case is very little as both are governed by the keeping the same output
>shaft speeds of rotation, and so both will re-distribute torque rearwards.
>the difference occurs in the case where the torsen diff's bias ratio is
>encountered whereupon it will limit the torque shift, and at the bias
ratio,
>and allow *different* output shaft speeds, while the locked centre diff
will
>(theoretically) shift 100% of torque, while keeping the same output shaft
>speeds (by definition).
>
>references for your further reading:
>the "torsen white paper" you quote from is actually a relatively poor
>reference to understand the device.  the 1995 "torsen and tcs" white paper
>which i scanned and distributed last year spends much more time on the
>behaviour of a torsen in a centre application, while the 1988 audi sae
paper
>on the influence on vehicle handling of the torsen centre is specifically
on
>this issue.  the sae paper "analysis of tcs augmented by lsd"  looks at
>precisely the case when one side of the torsen has no traction (split-u),
>and the other does, although this is also discussed in detail in the
"torsen
>and tcs" paper above.
>
>hth,
>dave
>'95 rs2
>'90 ur-q
>'88 mb 2.3-16
>
>
>-----Original Message-----
>
>Date: Wed, 11 Aug 1999 14:42:38 -0400
>From: "Avram Dorfman" <dorfman@est.org>
>Subject: Re: Boating with an S4
>
>Sorry for this long post - I have included excerpts from the Gleason Torsen
>white paper which I feel explains this dilemma clearly; no matter how much
>torque you engine can produce, the most you will ever get out of the
>driveline is determine by the amount of resistance provided by the lowest
>traction wheel and the bias ratio of the diff, and if it's not enough to
>move you, you don't move. Open diffs have a bias ratio of 1:1. Torsens are
>more than 1:1, e.g. 4:1.
>
>No, it wasn't your post that explained it. As a refresher, my brother
(sorry
>for those of you who've read this before) and I took his 80q to a funny
>shaped little hill that causes 1 rear wheel to go airborne when you climb
>it. The torsen didn't get us anywhere - once we ran out of momentum, the
car
>rolled backwards with the airborne wheel spinning at 60mph until it hit the
>ground again - it didn't matter how much gas we gave it. The analysis at
the
>time was that a torsen doesn't do squat for you unless you have *some*
>resistance against the slipping wheel. Regardless of how much torque the
>engine "can" generate, the entire driveline only "is" generating however
>much it takes to do what you're doing. It just doesn't take much torque to
>spin a wheel in the air.
>
>