RE: Torsen for Gary

[QSHIPQ@aol.com]

Orin writes:
>True.  Since this hasn't been quoted yet, here is an extract from
>the infamous SAE 885140:

Thanks, I expect that SAE has a few orders pending, they should.  I just got 
my hard copy.

>[40m radius turn, acceleration of 4 m/s^2, high-grip (uG approx 0.9)]
>"...the front wheels follow a wider radius than the rear wheels on the
>circular course, so that 0.2% of forced slip occurs _between_ the
>two axles, which reduces the wheel slip under traction at the front
>wheels, and reduces the slip at the rear wheels.  This results in the
>tractive forces being redistributed towards the rear wheels, so that
>the tractive force distribution is 38/62%.  The extent of this
>redistribution decreases with increasing cornering radii and road speeds.
>With small cornering radii and low speeds the extent of torque
>redistribution can increase to the torque split limit of 25/75%.
>[Orin: that shudder from the rear you feel in parking lot turns.]
>On a high grip surface, the torque limit split is reached at
>a radius of about 15 meters."
>So, yes, absolutely, you can get 75% torque rear due to cornering alone.
> (2) Assuming #1 is true, how much does the rearward torque shift 
>     affect the attitude of the car?  (i.e. large or only small affect)

>>Good question.  This is the question that needs addressing.
>>Phil says aligment is crucial.   Tires matter too.

So do a lot of factors, for the GIVEN conditions you are trying to optimize 
the device for.  But given all those factors, what is constant, is that 
torque shift across the center axle occurs with steering alone.  The question 
on how high that threshold is in relative slip angle vs traction input given 
a bunch of variables, this would take more 885140 type tests.  

What is significant to this whole issue (and the reason Dave's locker posts 
don't add up to real world experience by all racers and reviews) is you have 
to make two steps in the torsen allocation of torque in the real world. The 
two INPUTS to allocation are: 1) relative slip angles front/rear and 2) 
traction.  The two OUTPUT issues to deal with:  1) the nature of the torque 
shift for given conditions, and 2) the chassis dynamics (U or O) resulting 
from the nature of the torque shift for given conditions.

Look, what we have here is torque shift.  Let's say Dave E is driving his 
RS2.  That torque shift at a given radius and speed (etc) can be 225r/75 
front.  Oversteer?  And opposingly, 225f75r is Understeer.  One could be 
fairly certain of that even on dry pavement.  However, you lower cf, it's the 
same thing as raising engine torque (they are effectively the same variable). 
 So, one could argue this shift becomes significant to any chassis at some 
cf.  On a car with snows, in the snow, the significance is relevent at a 
higher threshold than on the same car with radials in snow.  

The same could be argued with alignment.  A car out of alignment might reduce 
the crossover threshold of slip angles vs traction, but it *can't* by 
definition, eliminate it.  A 44 chassis with a rear toe problem is the same 
as changing relative slip angles (orin's point below).  If you have slight 
toe out on the left rear (in spec on the right rear), one could easily argue 
that in a right turn the torsen will allocate a slightly different torque 
bias (less than the stock side) for a given set of conditions.  However, I 
can still replicate that given torque bias with radius, accel, steering, etc. 
 So, I can agree with Phil that alignments *might* affect allocation, but 
really it's irrelevent, because by the time slip angle difference threshold 
is crossed by engine torque (causing a sudden shift from O to U), the 
effective torque bias is the same as the side with the in spec alignment of 
toe.  

>What does happen when torque goes to the rear?
>Weight transfers to the rear, reducing grip available and
>increasing slip angle at the rear.  This would appear to
>magnify the effect, but it all started when you turned
>in and would have reached some balance, maybe at the 75% rear max.
>In addition, the weight transfer has caused suspension movement,
>changing both toe and camber.  Whatever the camber change,
>it too is likely to reduce grip and increase slip angles
>since maximum grip is at 0 deg camber and you are unlikely
>to be moving towards 0 deg at this stage.

One of the main reasons that the 80q in 885140 had to be tested fully loaded. 
 It takes suspension out of the equation (constant).  However, remember, 
changing any of the chassis variables only maximizes a torsen for a given set 
of variables.  For instance, a stiffer suspension and wider tires would 
affect when slip angle differences become a traction issue on dry cf.  When 
it gets wet, that changes.  On snow, you've screwed yourself.

>Toe is the unknown at the moment.  It directly affects slip
>angle - what is the static toe setting but an initial slip angle?
>Any toe change due to the suspension movement is going to add to
>or subtract from the slip angle changes due to the weight transfer
>and that I believe is where we should be looking.

BINGO.  See above.


>Scott, what are the slip angles?  Is a 1/6 deg difference
>significant?  The SAE paper claimed slip angles of 2 deg.
>Initially I thought the static toe setting would be insignificant...
>but 1/6 out of 2 is about 8%.
>Just more questions...  let me know if I screwed up on thegrip analyses.Orin.

Good post Orin.  Look, you made the point really clear, toe is the same as 
initial relative slip angles.  Is it significant?  It *can* be, but don't you 
change suspension settings anyway, with just car loading (remember it's fixed 
at maximum for the 885140 test)?.  I would say a bunch of variables would 
need to be constant for that to be a measureable test.   

Phil's comments that both the Urq and the 44 chassis handle the same up to 
the limit is interesting.  Looking at a huge steering arm on the Urq vs a 
multilink (and less steering affected rear) of the 44, I would expect toe in 
both cars to be radically different under load and turning.  The devices are 
the same (constant), I argue they shouldn't be.  However, if what we are 
speaking of is crossing the threshold from slip angle to traction, it's 
really irrelevent.

Bottom Line:  The bite, IMO, happens from the sudden shift of inputs to the 
torsen, from relative slip angle to traction.  Predicting both is impossible, 
especially considering engine torque, cf, and radius play major roles at that 
threshold.

HTH

Scott Justusson