awd vs rwd braking - long

[QSHIPQ@aol.com]

> CLIFFORD ILKAY wrote:
> 
> If Quattros stop faster than BMWs or anything else, it has nothing to do
> with being all wheel drive. In other words, all things being equal, an all
> wheel drive car will not stop any faster than a two wheel drive car. There
> may be some negligible amount of engine braking on the other two extra
> driven wheels in a quattro but I would think that would be negated by the
> extra weight of the quattro pieces. Force = mass x acceleration squared,
> even for quattros.

Dave E then writes:
>>no, this is not strictly correct....

>>on corner entry for a quattro, the torsen diff will proportion torque to the
>>front meaning that the rears have less drive torque to worry about, leaving
>>more traction (friction circle) for the brakes to use.

>>a rwd cannot, of course, do this and will use a proportioning valve to bias
>>more braking effort to the front.

>>however, clearly the quattro will have better braking at the rear, and less
>>tendancy to snap oversteer on corner entry.

>>the newer computer-controlled centre diffs will actively proportion torque
>>forward on braking to achieve the same effect...

>>this is not the same as braking performance though, which obviously has to
>>do with the specs of the respective braking systems.  what it does say
>>though, is that for the same setup, the quattro will outbrake the rwd.
>>imho, this was one of the significant advantages of the btcc and gtcc audi
>>quattros in the 1996 championship year.  they were clearly the class of the
>>field under brakes, despite their higher weight...

>>dave
>>'95 rs2
>>'90 ur-q

Actually Mr. Ilkay is more correct in his statement.  First, we must consider
the argument.  Mr. Ilkay is not correct ONLY if you are speaking of a locked
center differential.  Why?  Cuz when a center diff is locked, you have the the
ideal braking distribution, regardless of weight distribution.  All other
cases Mr. Ilkay is correct.

For a torsen car, the ideal brake force is no different than that of a rwd car
(assuming the same brakes, same chassis, same weight dist).  Amazingly enough,
the ideal brake force in an unladen in a 200tq, the worst case weight
distribution  is 78:22 f/r (gee, does that number sounds really close to
another, remember Chocholeks Torsen comments).  So, from an abs and torsen
perspective, the torsen only needs to go 78:22 f/r to allow full abs
operation.  However, it should be noted that, this has more to do with the
front/rear (and ideal) weight distribution, than it does with awd vs rwd.  

Specifically, a 50/50 rwd car with abs will have a better "worst case" brake
dist than the quattros, ALL of them.  What does this mean?  Well, the less
weight forward of the centerline of the car, the less the car will dive,
bringing up the rear, reducing it's braking effectiveness.  One of the main
reasons why quattros have very tiny rear brakes, they don't need or use them.
So, quattros don't inherently outbrake rwd cars at all.  UNLESS it's
mechanically linked either with a locked diff, or no diff at all in the
center.  But, since a locked diff no longer allows abs function, the ideal is
somewhat less so.  Rwd wins.  When a quattro can only apply a maximum of 22%
of it's braking force to the rear wheels, it ain't 'better' than a lot of cars
around.  

Claiming better braking with a quattro, "strictly" isn't inherent in the
quattro design at all.  In fact, when speaking of braking, specifically of abs
braking in awd vs rwd cars, very few quattros win here.  The static and worst
case weight distribution has more to do with braking performance than quattro
or rwd.  Yes, the quattros did well in the btcc and gtcc, but they ran HUGE
brakes (not to mention the first to use 19in wheels to accomodate them), and
really high spring rates, negating the weight transfer forward under braking.
Add to that the fact that Audi managed to squeak a respectable 55/45 f/r
static weight dist, something the rest of us audi owners don't enjoy (Urq
excepted), certainly no A4 owner.  

Regarding your torsen appropriating torque to the front on turn corner entry,
I think you need to clarify your two "corner entry statements..  On turn in,
the rears are traveling slower  than the fronts, so how could a torsen
appropriate more torque forward?  I believe you to mean on "braking entry",
then "corner entry" when turning, and there a torsen sends power rearward, not
clear as you defined it.   I smile some at the "snap oversteer" comment, sure
is tough to do that when you are understeering like a pig.

Regarding your rwd car using a proportioning valve, a proportioning valve
proportions a percentage braking hydraulic pressure to ther rear.  The less
restriction, the more the rear brake hydraulic pressure = more rear braking
force.  "Strictly" speaking,  proportioning valves don't ADD pressure to the
front brakes, they reduce available pressure to the rear.

Bottom line is this:  4 wheel abs has all but negated the ideal braking force
concept.  For a given chassis, in fact for a given car (tires, suspesion,
torsen, shox, etc) 4 wheel abs will always maintain braking really close to
the ideal braking force of all 4 wheels and all 4 wheels' slip angles.   AWD,
fwd, rwd, laden or unladen, it could care less.  So, then the challenge
becomes the ideal weight distribution under all these conditions, that is
specifically, how close to 50/50 can you get at all operating parameters of
braking, acceleration and turning.  A win to AWD?  Nope, there the Bimmer boys
have the edge, right up to the monstrous 740i.  ALL things being equal then,
Clifford made a correct summary.  AWD has little, really nothing, to do with
Braking Performance.  


Scott Justusson
QSHIPQ@aol.com
'87 5ktqwRS2
'84 Urq