RE: awd vs rwd braking - long

[Eaton Dave <dave.eaton@minedu.govt.nz>]

flame bait to one side.....

"on corner entry" meant just that.  not (power on) cornering.  throttle lift
going into the corner.  in this "mode" the braking torque to the front
wheels is *increased* (rotating at higher speed), and the braking torque to
the rears are decreased (rotating at a lower speed)...

the point is that the quattro with torsen is able to *remove* engine braking
force from the rears.  this has 2 effects: -

1) putting more engine braking through the tyres in the front where the best
adhesion is, and

2) leaving more rear tyre adhesion for braking.

remember that snap oversteer under brakes occurs when the forces applied to
the wheel/axle exceed limit values of friction (friction circle).  these
values are determined of course by the loads applied to the wheels and the
cof between the wheels and the road surface.

now to your other points...
> 
> Date: Mon, 19 Oct 1998 23:25:11 EDT
> From: QSHIPQ@aol.com
> Subject: 
> 
> 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.  
> 

wrong scott.  the locked centre is worse than the torsen under brakes
because of the torque (50%) it forces rearwards  leaving less road/tyre
adhesion left for braking.  we won't mention lack of abs either because you
consider this a "band-aid for bad chassis design" (bafbcd -tm scott).  any
awd setup which is *not* proportioning torque to the front isn't helping
your braking performance over one which does.  see below.

[snip]
> 
> 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.
> 

errr... nope.  in "coast" mode, the torsen apportions torque to the front
and removes torque from the rear.  if the rear goes into oversteer
(increased slip angles), the torsen will remove more torque from the rear
and send more to the front.

so, by definition better awd braking than a locked centre car (forced to
send 50% braking torque rearwards), and less propensity to snap oversteer
than the generation 1 car.

against rwd, better still for obvious reasons (better rear braking).

> 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.
> 

and the reason is because you have less braking performance at the rear.
regardless of your chassis design...

> 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.  
> 
mmm, so abs is not longer a "band aid for bad chassis design" scott?
changed your tune a little?  agree about the abs btw, but to get best
performance you need to reduce the onset of abs front or rear if possible.
by moving engine braking torque around to where the friction circle is
biggest (front) and removing it from where the circle is smallest (rear) you
prolong the onset of abs longer, and get better braking.

btw, the best braking (and circuit) performance in the planet is developed
in formula 1 cars.  wanna know what their weight distribution is scott?
hint.  nowhere near 50:50.

"understeering like a pig" (tm) - scott in his generation 1 quattro.

happy to supply references for any of the points above...

dave
'95 rs2
'90 ur-q