Braking 101

[QSHIPQ@aol.com]

>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)...

Reread that statement.  Are you sure you have this correct?  If the front
wheels are rotating faster than the rears, where does engine braking Torsen
Trg go?   

>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.

1)  Sounds good, would you want that if the maximum braking weight
distribution is 50/50 in the quattro?   What would you need to do to get a
better actual braking distribution?   What does this mean to those considering
springs and braking upgrades to their quattros?  Those with proportioning
valves?  Those with ABS?  Those with ABS and Torsens?  What does an open diff
do with engine Trg and braking?  2)  Given that total braking in the back is
-/= 22% in a quattro, you need ALL the "more rear tyre adhesion for braking"
you can get.  I'm with you all the way here.  

However Mr. Ilkays' point remains.  Mass of quattro (all other things being
equal), and where that mass is.  Translate this to a street quattro, it can
only send 22% of it's braking force to the rear of the car, this point becomes
lost in the mass of the car (and the corresponding lack of mass in the rear
brakes).  Add to this, a street car has ABS, both the rwd and the awd car will
brake as close to the ideal as the hardware and weight distribution permits.
That's a loss to a front heavy abs car with tiny rear brakes when compared to
a more evenly distributed rwd car.  

>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.

Ok, add ABS into your equation.  How would one snap oversteer on braking?  

>now to your other points...
>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.

Dave, the above is incorrect.  There are SAE articles published on this
subject.  A locked center doesn't force torque anywhere, it's always 50/50
given the same cf.  Some actually written about quattros by the audi boys.
Read them before you claim I'm wrong.  Let me repeat the statement:  A locked
center differential gives the ideal brake force distribution.   You are
arguing a physics issue Dave.  Forget torque "shift" for now.    In a locked
diff:  If one of the front tires locks up, what happens to the rear tires?  If
one of the rear tires locks up, what happens to the front?  What is the point
just short of that?  

>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.

No, in "coast" mode there is no differentiation, that's 50f/50 r, that's U.
If the rear goes into O the torsen will not "remove" (sic) torque from the
rear until the rear DRIVESHAFT (the ones between the f & r diffs) is spinning
as or faster than 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.

Incorrect statement.  See above.  What happens when the fr/r braking
distribution exceeds 22%r?  Like in a 90q under braking.  Dave, not sure this
is worth discussing yet.  You don't understand chassis dynamics and braking.
No big deal, it just makes for a tough discussion.   You don't have the basics
yet.

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

NO.  Explain to us how a car with more weight on it's front wheels, can
outbrake a car with less weight on it's front wheels (all other things equal).
Think ideal vehicle concept.  An ideal vehicle that can maintain 50%f50%r
chassis dynamics under  braking, will have the same size brakes front and
rear, with no proportioning valve.  Why?  Because both front and rear can
apply the same amount of braking force for equal weight.  Will the torsen
quattro outbrake that ideal vehicle dynamic?   Regadless, a moot discussion.
Cars with 4 wheel abs are closest to the ideal braking for a given car (save
the locked diff).  So, Mr. Ilkays point on the weight of the quattro
drivetrain itself becomes very valid.

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

Yes, but the why, and how much might help you more.  You have less braking
performance at the rear because the rear is lifting under braking, increasing
slip, reducing contact patch + weight on it, specifically, decreasing it's
ability to brake.  This happens when you hang a quattro motor out in front of
the centerline.  This is done in the following equation:

Wheel weight increase = ((from wheels/wheelbase) x weight) + weight.

So, your A/C Urq "should" have stiffer front springs, and the fulcrum effect
of weight and it's location is reduced from the rear.  Under braking this
raises the rear of the car, reducing OVERALL braking effectiveness, because
the total braking acheived by the front and the rears can't exceed the sum of
each individual tires' respective slip abilities.  Quattros don't win here
Dave.  It's a chassis dynamics problem.  You shift weight forward, you will
overload the front tires ability to brake.  Again, the ideal vehicle...  If
you keep equal weight on the front and rear axles during braking, you increase
the stopping ability, specifically, to the maximum slip force of all four
tires equally.  Any thing less is a proportioning valve issue, and a little
bit more weight  on the front (not to say a lot more) will reduce the rears
braking ability.  When a quattro is using 80% of it's braking force on the
front wheels, you only need a couple of jokers in the back (ta da, take a look
at a quattro).  Interestingly enough, if you look at your covetted RS2, we
could easily calculate what the rear brake size "should" be.  It certainly
doesn't "need" to be what it is.  Sure looks good tho.  

>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.

Wow, not quite Dave.  ABS is a 'bandaid' for poor chassis design.  If we can
get active suspensions sophisticated enough to control squat and dive, we
could get 50%f/50% rear weight distribution under all power, turning and
braking dynamics.  When we do this, we have done more to win the chassis
dynamics problem, than "delaying" abs function or 'Prolonging onset of abs' -
the problem isn't in the ABS or the hardware.  It's more basic than that.  ABS
senses impending lockup that's all.  A car with more weight on it's rear
brakes will lockup later than one with less.  That's a basic weight
distribution problem, not a braking, torsen or abs one.  With an abs car, the
torsen shift is offset by a very poor weight distribution under braking.  In
fact, with abs the torsen shift is really irrelevent.  Unless you are a no abs
to a no abs comparo.  And there the locked diff wins, not the torsen. 

>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.

What does that mean Dave?  I wouldn't expect them to.  Bottom line is this.
The ideal vehicle concept is 50/50 weight distribution all the time, under all
circumstances.  The physics of that doesn't change.  However, we accept
compromises to braking or acceleration and cornering performance by changing
chassis dynamics.  Last time I checked, no competition cars can run abs.  If
they could, you would see another shift in the weight balance.  If they could
run active suspensions described above, you would see the shift back to 50/50.
Does your quattro, or any quattro for that matter run different front and rear
tire sizes?

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

"Flame bait to one side"...  
And all other quattro cars with more weight up front than me, you included
sir.

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

Maybe some more understanding of braking and brake force distribution is
necessary Dave.  You have an idea in your head on "what" it is.  You are
making brake arguments that aren't valid.  The following needs to be in your
baseline understanding:  Ideal Vehicle Concept, Brake Force Distribution
(Ideal vs Actual), torsen allocation of overrun torque (what are you figuring
is the front and rear Trg are under braking?  Would pushing in the clutch
help?), Locked differentials and Ideal braking.  

Dave, you have not adequately demonstrated that a torsen is better under
braking.  Your extension of your arguments, and your statements are not backed
by documentation.  This isn't some big mystery here.   The difference between
comparing an abs car vs non abs cars is significant.  ABS allows closer to the
ideal brake distribution vs the actual.  That is not an immediate win to a
locked diff.  On "corner entry" as you defined it, the locked diff will give
the ideal brake distribution.  The torsen doesn't, neither does a torsen with
abs.  Even with the ideal brake distribution, remember that is for a given
chassis with the given hardware. Change the chassis (to rwd with 50/50) you
get closer to the ideal brake distribution, by definition.

BMW knows the advantages of 50/50 weight distribution in a road car.  They
have proven it.  Until quattros become better weight distributed, they need a
lot of "extra's" to overcome the inherent disadvantage of a front heavy car.
It's not just about braking either, IMO...

Scott Justusson
87 5ktqwRS2
'84 Urq