Re: Torsen bites Performance Car

[Sargent Schutt <sargent@novagate.com>]

Dave Eaton wrote:

>errr, no.  scott states that the torsen bite can affect a car when in high cf
>conditions (dry tarmac), not low cf (snow, ice).  in the pc article, the torsen
>is doing exactly what it is designed to do.  shifting torque at tmax (ie. 70%
>of torque) on dry tarmac is what is called the spider bite.>

Survey says? Bzzzzt. You are definitely confused: Torsen can bite on low cf
all the more. That's what the magazine was commenting on. And if it can happen
while going straight, it can happen in a corner, at which point the
ramifications are a bit more serious. Torsen is doing its thing, true. But
'its thing' is a nefarious force above 7/10's. Same physics apply in low as
high cf, and b/c of the low cf it is even easier for the torsen to be
activated, and per experience: fooled. This is true everywhere from gravel on
a dry corner to ice. BTDT. If Scott ever implied torsen doesn't bite in low
cf, my reading comprehension skills must be in decline...
>
>i don't have any doubt that the locked centre could be better in low cf
>conditions because the tshift is 50% of torque, while with the torsen, this can
>be 70%.

Way off base there: Torsen can only shift 40% of the torque (in your 70/30,
30/70 model: actually 22/78 = 56% max shift), not 70%. Again, you are
confused. A locker effectively shifts from 50/50 to as much as 100/0 or 0/100
(theoretically) as the F/R wheels gain/lose traction. Difference is the locker
isn't a 'thinking' device, so it can't be fooled. The split is constant, only
the road surface is 'unpredictable'. 

Torsen hunts in a straight, and this is good for straightline acceleration.
Torsen also hunts in a corner, and sometimes this is bad. Why? Because things
like slip angle and wheelspeed differences at a given axle lead torsen to
transfer torque in the wrong direction (fooling it). Moreover, it works so
quickly as to be unpredicatable at times. Unlike the locker,  with the torsen,
you might only be applyin of the torque to the tractive axle, as a result of
the torsen having been fooled. Rare that it's that bad, but it can happen, and
usually at the worst moment (above 7/10's). 

One unpredicatble variable (road surface) is enough. Torsen brings two, and
they play off each other. Makes quite a balancing act of things for the driver
at times. I wouldn't mind torsen center if the rear and front were limited
slip-type diffs, or even f/r torsen. Then it would not be fooled as easily.

Obligatory caveat: This is my *experience* based on 9 years in a 5ktq locker,
3 years in a torsen 90 90q20v, and 1+ years in the 91 200q spyder. The 'bite'
is more pronounced in the 91 200q than the 90 by far, I suspect owing to the
higher power output and more cumbersome dynamics of the 200q (even mit tyres
and suspension). 

Sarge 


P.S. - interesting article on Subaru's set-up in the most recent Car and
Driver. Not too much tech detail, but some pertinent info on the diffs. It
might be on their website at www.CarandDriver.com, otherwise I can post if
anyone's interested.

>
>Date: Mon, 20 Apr 1998 12:45:26 -0500
>From: "Matt & Jenai" <matjen@xsite.net>
>
>As one who has been bitten, I offer the February 1998 Performance Car (p.
>50) as another possible reporting of the torsen bite.  In a comparison
>between a fwd A6 and an A6q for 0-30mph acceleration on simulated ice the
>article says  "By contrast, the quattro dragged itself up to 30 mph with
>ease.  There were some unusual sensations as the drivetrain juggled the
>torque, favouring the front and rear wheels in turn..."
>
>It seems to me that if this happens in a straight line, there is no reason
>it couldn't happen during a turn thereby causing US, OS, and US.  I think
>Scott was claiming that lower cf really brings out the bite.  I'll leave it
>to the more technically inclined to explain the cause/physics.
>
>Matt Pfeffer - 89 200TQW
>