"Care to Comment" -> differential t...

Rob Andrews randrews at sbcglobal.net
Tue Apr 9 18:27:59 EDT 2002

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Here's one response to that Scott...I'd like to see what you have to say...
"First of all, you've got a couple things wrong. You are assuming the
traction of the rear gets exceeded first....which isn't hardly true. I will
agree that when you transition from trail-braking turn-in, to on-throttle,
the lightest corner of the car is the inside rear, and there a chance that
you can spool up that wheel because of the lack of grip. That can be
overcome with technique of throttle application, and with suspension tuning.

Once you get past that point, however, you begin to transfer weight back to
the rear of the car, therefore making the tractibility of the rear higher
than the front. Especially when you get to full throttle, trying to get to
the apex, then tracking out past the apex, the tractibility of the rear is
much greater than the front.

How do I know? How about countless amounts of data and seat time from 42
autocrosses in 2001? When we finally got a high bias Torsen to put in the
car, the benefit was immediately noticeable. Now that the power-on
understeer has been greatly reduced, we can apex earlier and get on the
throttle earlier, which mean higher corner exit speeds. How about 4 wheel
speed sensors on the Stasis A4 touring car? I've seen their data, and the
advantages of the higher bias Torsen were very apparent. You could see the
high bias Torsen doing its work, transfering power back and forth as
traction levels were changing (which including heavy throttle at apex,
bouncing over berms, and heavy throttle at track out). With just the 2:1
Torsen in their car, the car always had some amount of wheelspin in the
FRONT of the car, which resulted in on-throttle understeer.

You bring up a good point about locking the diffs, but if you really think
about it, you'd be limiting yourself. A biasing system is dynamic, and can
handle changing situations. Also, the "championship" winning S4's (the
Champion S4's) are running a viscous center (which is also very expensive!).
Although a viscous center doesn't seem like the best solution for road
racing (widely used for rally), it is working for them because they've made
the car bias most of the power to the rear of the car at all times.

Corey S"

  -----Original Message-----
  From: QSHIPQ at aol.com [mailto:QSHIPQ at aol.com]
  Sent: Monday, April 08, 2002 10:47 PM
  To: mlped at qwest.net; quattro at audifans.com; randrews at sbcglobal.net
  Cc: 200q20v at audifans.com; s-car-list at yahoogroups.com; torsen at audifans.com;
SlvrBulletS4 at aol.com
  Subject: Re: "Care to Comment" -> differential t...

  WHOA up boys...  Torsen nerd hat on....  What Stasis is doing directly
contradicts what all the S4tt boys do in the motorola cup,  more
specifically, they weld the baskets solid for faster laps.  Extending the
S4tt bias ratio *can* put UP TO 80% rear bias, but the problem with that is
that once traction of the rear wheels has been exceeded, then even more
torque shift will occur to the front.  This is also specific to the S4tt
which has a static 55/45 split, NOT the URS4's which have a static 50/50
split with a 3:1 bias ratio.  Further research would show that the torque
split of the urs4 is 75/25, or 78/22 (whomever you want to believe) which
equates to 3-3.5:1.  500 bux to go to 4:1 sounds expensive, and exactly the
wrong way to track any understeering quattro.

  I've been on the forefront of the torsen center (in)capabilities for quite
some time.  I read the below, and I wonder how exactly shifting more torque
front to rear at the limit of adhesion can yield better lap times.
Especially when the S4tt race cars that win *championship* races aren't
extending the BR to 4:1, they just lock the diff solid.

  Someone hasn't done their homework, and is selling hard.  I'd be happy to
have a on line discussion with any Stasis representitive.  Or they could sub
to the torsen list, but it would be a tough road, and I'm biassed (pun
intended).  I'd say the BS words but I'd sure like to hear the logic.  I'm
intrigued too at the formula for sec>HP conversions, I haven't seen that one
yet.  I'd add the 60hp before I'd mess with the torsen in any S4tt.

  Scott Justusson

  In a message dated 4/8/02 2:53:00 AM Central Daylight Time,
mlped at qwest.net writes:

  Sorry Rob, I should have read your post more carefully all the way to the

  So I take it Stassis Engineering does exactly that, offer "blue printed"
  center differentials etc.?


  ~-----Original Message-----
  ~From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
  ~Behalf Of quattro-request at audifans.com

  ~   1. Care to comment? (Rob Andrews)
  ~From: "Rob Andrews" <randrews at sbcglobal.net>

  ~From stassis engineering....
  ~Differential Tuning
  ~Tuning the center differential provides one of the most significant
  ~performance gains available to the Quattro owner. The standard
  ~torsen center
  ~differential in the S4 and A4 provides a 2:1 torque distribution
  ~This enables the end of the car with traction (generally the rear) to
  ~deliver twice the torque being delivered by the least tractive end of the
  ~vehicle (generally the front). For high performance street cars, we
  ~recommend a bias ratio of 4:1. This is the same bias ratio that Audi has
  ~provided in the stock RS4 driveline.
  ~In professional competition a high bias center differential provided a
  ~performance gain of almost 2 seconds a lap. This is the equivalent
  ~of adding
  ~60hp to an S4. This is our first step in almost every tuning project. It
  ~provides a measurable performance gain for track-oriented drivers. For
  ~enthusiast street driver in good weather conditions, it can provide the
  ~balance and control of a rear wheel drive vehicle, as 80% of the cars
  ~will be applied to the rear wheels upon corner exit.
  ~The center differential in an S4 is a slip in cartridge accessible from
  ~rear of the transmission. The A4 center differential is a self-contained
  ~bolt on unit. Install time is approximately 2 hours.
  ~Center Differential rebuild A4/S4 - $500
  ~Rob Andrews
  ~99 A4 2.8QS   83 Ur-Q


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