[torsen] RE: [V8] Torsen diffs

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Thu Apr 17 15:14:03 EDT 2003

... this topic was posted to the V8 list ... and I thought I'd post a copy
of my response here for comment ... and so people here would have a chance
to comment on Bill's questions ...


On topics such as this there rarely is a simple answer ... ;-)

Be advised that the torsen rear diff can also cause some unfamiliar behavior
in tight turns on slippery surfaces.  You might also want to investigate a
little before installing the torsen ... there may well be unintended
interactions between the operation of the torsen and the ABS system.  If you
wanted to investigate that further I heartily recommend bringing it up on
the torsen list.  

My opinion as to why Audi dropped the rear torsen is cost.  As long as you
have independent control of braking at each wheel you have what you need to
keep wheels from spinning via the ABS system (i.e. "EDL").  While typically
I too ascribe to the KISS principle, using the ABS system to control wheel
slip *allows* a more sophisticated response.  Notice I didn't say whether or
not the developers of the EDL system took full advantage of that.  The
torsen doesn't even keep track of how much wheelspin is occurring, it
operates based on the differences between the shafts.  A fully implemented
ABS-based AWD traction control system can determine whether or not the
vehicle is moving in a straight line ... it can make decisions based on the
rotation sensed at all wheels ... if a full accelerometer is included it can
also get information about the acceleration vector the vehicle is

One of my dream projects is to actually build up my own ABS/EDL system just
to see  what migh be possible ... but the way things are with other things I
need to do I doubt I'll actually ever get to it.  I haven't had that much
luck coming up with references on ABS/EDL systems and algorithms ... 

Steve Buchholz
San Jose, CA (USA)
 >  I am wondering why Audi apparently only installed torsen rear
 >  differentials in the V8's?  Is there a simple answer?
 >  I have recently purchased a used torsen rear differential 
 >  for install on
 >  my urS 6 as I believe it that it be superior to the stock electronic
 >  open diff.  The stock version abs will sometimes produce a 
 >  chattering
 >  /hammering noise in rain or snow.  I believe the torsend 
 >  rear diff will
 >  eliminate this.  So, I am curious why, if a torsen rear 
 >  diff is better,
 >  Audi abandoned using it?  Too heavy?  Too costly? Any theories?

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