[torsen] Re: [s-cars] static torque bias

Theodore Chen tedebearp at yahoo.com
Thu Dec 7 20:31:15 EST 2000

--- qshipq at aol.com wrote:
> The Helix gears are not available separately from audi.

try torsen.

they're the ones who made the differential, and in fact, their
"University Special" sold to Formula SAE teams is very similar to
the audi center differentials (main difference is that the audi unit
is driven by a splined shaft rather than a ring gear).  torsen 1 was
used until 1995, and then the torsen 2 became the OEM center differential.

i'm not sure if torsen will sell you the gears separately.  although
they do list prices for them, they also claim to sell only complete

it would be simpler to change the torque bias by changing the gearing
in the rear differential.  i don't know what the rear differential looks
like, but assuming it's a ring and pinion, you could change those to
change the front to rear bias.

as for torque biasing ratio, torsen says it's about 3.2:1 initially,
and after the unit is broken in, about 2.6:1.  you can change the TBR
by changing the viscosity of the lubricant (higher viscosity lowers TBR,
lower viscosity raises TBR), by changing the washers/bearings in the 
side gears to increase friction inside the differential, installing
clutches (this is what the torsen 2-R does).

FYI, torsen sells the university special at $360 to FSAE teams, compared
to the $1600 that the audi dealer would charge.

> My .02 on this reason for change, is that the S4tt has 
> the absolute worst quattro f/r static balance, of 60f/40r.

that is the basic problem - weight distribution.  the engine 
sits in front of the front axle, which is a big hose from a
handling standpoint.

as to the dynamic characteristics of the torsen, i'll leave it up
to you guys to argue about it.  i have yet to drive the S4 (or any
other AWD car) at speed on a racetrack.

scott, is it possible that a race team might weld the torsen
(thus making it into a spool) or remove the torsen, more
for reliability considerations than for performance reasons?

helical gear differentials generate a lot of heat when differentiating,
particularly under load.  i haven't instrumented the torsen 2 in my
mustang yet, but other people have managed to destroy their gear
differentials in road racing behind high-powered engines without 
a differential oil cooler.  wheelspin really heats up the gear oil.

> I also might note that Dave Lawson and I both have tried to find more data on
> the new S4 center diff in terms of published bias ratio.  Anyone know for 
> sure what the F&R torque shift limits really are?

the biturbo S4 uses a torsen 2.  biasing capability for the torsen 2 in
center differential applications is in the range of 1.5:1 to 2.5:1.  
that depends on the lubricant used, and the specific application.
i don't know what audi specified, but i'd guess the higher end of
the range.  maybe you should email torsen and ask.

FYI, the torsen 2 has lower max TBR than the torsen 1, though it has
a wider range of possible TBRs.

> > this can be done with a torsen.  the s4tt has a static 46:54 rear torque
> >  distribution.  it is accomplished through different angles on the helix
> >  gears.

dave, are you sure about this?  it seems to me that it'd be simpler to
accomplish this by changing the rear final drive gearing.


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