[s-cars] Torsen and EDL (was: EDL Vs. Manual Diff Lock)

[Keith Maddock]

--
Wayne,

Good thinking but you are missing a few non-obvious items.  Your items #1 a=
nd #2 are reversed, which is a big difference.

1) With the torsen, you'll get 3.5 to 4 time more torque to the high-tracti=
on wheel before the low-traction wheel even slips, compared to a open -diff

2)  At the point of slip, yes the EDL will add brake torque, which may conf=
use the torsen a little bit temporarily, and cause it to reduce the bias ra=
tio a little bit.  However since the total torque at the low-traction wheel=
 is increasing, the total torque at the high-traction wheel will also incre=
ase.  Depending on the surface, the bias ratio may not even decrease, as th=
e traction potential of the high-traction wheel may still be higher than th=
e low-traction wheel + brake applied.  You can see this in the graphs in th=
e paper.  However, in the end, the EDL and torsen will give you more torque=
 to the high-mu wheel.

Basically, the torsen prevents the EDL from needing to be used at all in mo=
st normal driving circumstances.
In extreme mu-split conditions, the EDL will kick in, and offer more tracti=
ve torque to the high-traction wheel than torsen could do by itself.

f you haven't read the attached paper, please do so, section 4, especially =
4=2E3 are the relevant sections. (attachment stripped for List, sorry, emai=
l me off-list for copies)

Consider the following examples:

RWD car (for simplicity)  with 3000 ft-lbs of torque at the rear axle in fi=
rst gear..

Extreme split-mu surface such that the Left wheel will slip at 100 ft-lbs o=
f torque (ice), and the Right wheel will slip at 1000. (pavement)
EDL limited to 500 ft-lbs of brake apply (by software to prevent brake over=
heat, pad wear)

Open Diff Case:
Fixed Bias Ratio of 1:1
Wheelslip obtained at 7% engine output.
Left Wheel Max Tractive Torque: 100 ft-lbs
Right Wheel Max Tractive Torque: 100 ft-lbs
Total acceleration torque: 200 ft-lbs

Torsen Case:
Max Bias Ratio 4:1
Wheelslip obtained at 17% engine output
Left Wheel Max Tractive Torque: 100 ft-lbs
Right Wheel Max Tractive Torque: 400 ft-lbs
Total acceleration torque: 500 ft-lbs

EDL Case
Fixed Bias Ratio of 1:1
EDL kicks in nearly immediately at light throttle (at 7% of engine output) =
as left wheel spins.
EDL applies left brake up to 500 ft-lbs as driver increases throttle
Left Wheel Max Tractive Torque: 100 ft-lbs  (500 ft-lbs going into brake du=
st and heat)
Right Wheel Max Tractive Torque: 600 ft-lbs
Total acceleration torque: 700 ft-lbs

EDL+Torsen Case
Max Bias Ratio 4:1
EDL doesnt kick in until 17% engine output when left wheel spins.
EDL applies left brake up to 500 ft-lbs as driver increases throttle
Left Wheel Max Tractive Torque: 100 ft-lbs  (500 ft-lbs going into brake du=
st and heat)
Right Wheel Max Tractive Torque: 1000 ft-lbs (capable of up to 2400, surfac=
e limited at 1000)
Total acceleration torque: 1100 ft-lbs

(please excuse any typos, I had to rush this out)


Make sense?

Cheers,
Keith


****************************************************************
Keith Maddock, TRW Automotive,  Koblenz, Germany
Slip Control Systems, Systems Design, Traction Control
+49 (0)261/ 895 2474     -    -    keith.maddock at trw.com


>>> "Wayne Dohnal" <dohnal at hevanet.com> 03:16:13 17.01.2003 >>>
<snip>
>Jeremy, my personal suggestion is to buy a 95.5 with EDL and then install a
Torsen.
<snip>
>I have a good technical paper from the ZexelTorsen boys that describes well
the benefits of Torsen with EDL, I can forward to anyone interested.
<snip>

After applying a little knowledge and logic, which can of course be
dangerous, this doesn't make sense to me.  Wouldn't the EDL and Torsen work
against each other?  Here's what I figure: (1) A rear wheel slips.  (2)
Torsen wants to transfer torque to the wheel with traction. (3) EDL detects
the slip and starts braking the slipping wheel. (4) Torsen doesn't know the
difference between traction and EDL braking, so it transfers more torque to
the slipping wheel, fighting the EDL. (5) EDL sees the wheel slipping more
and brakes it harder, encouraging the Torsen to send more torque, and so
forth.

Am I missing something obvious here?

Wayne Dohnal
1994 S4


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