[s-cars] Brake Fluid Equalizer, a.k.a. the Thingy

[DeWitt Harrison]

On Mon, 18 Jun 2007 17:04:00 -0400
     "Max Hoepli" <mhoepli at vif.com> wrote:

> Just had the original brake fluid equalizer/proportioner [C$355] changed.
Found in front of the left rear wheel.
> Anyone know what the equalizer has inside? Mechanic says equalizer was
seized, practically only the front
> brakes were working. Anyone can give some knowledge of the function of the
equalizer? The car with the
> old equalizer was making boing-boing spring noises at the back, most
noticeable in an underground garage.
> With the new equalizer very smooth, quiet boingless braking.
> The rotors in the back were rough, rusty on the inner surfaces. only the
outer surfaces were touched by
> disc pads.
>
> Should the mechanic 3 weeks earlier who changed the front brake shoes
C$135 and both front rotors,
> each C$140 have noticed that the equalizer was defective? The car was
making rubbling noises when
> the brakes were applied after this service, with time I was aware that
this noise was coming from the
> back, which prompted me to see my other mechanic.
>
> There has never been discussion of the brake fluid equalizer as far as I
know in this group.
> It seems to me it might be a good idea to grease the shoulder joint of an
equalizer to prolong its life.
>
> Max
> 1986 Quattro type 44

You're right. This is an overlooked item on the list. The following is a
discussion on
the topic which, I think, pretty well sorts out what's going on. Tell me
what you think.

----------------------------------------------------------------------------
---

By George, I think we've got it! I would explain it this way,
short story made long:

The first priority of the engineers was to adjust fore/aft
brake bias by sensing rear cargo loading. It is always the case
that fore/aft bias means reducing the pressure to the rear wheels
since the rear tires become unloaded during normal braking
maneuvers. As the rear trunk is loaded up with cargo, it is
desirable to have less pressure reduction to the rear brakes
since there will then be less forward weight transfer during a
stop and the rear brakes become more effective.

How to adjust for rear cargo loading? Put in a brake pressure
proportioning valve that senses rear suspension displacement.
Where? On the left trailing arm. Oh, but wait. The right
and left rear suspension deflections also respond oppositely
to cornering forces. Now what? Put in a second pressure
reducer that can distinguish between left and right turns.
(Let's call this valve the Thingy.) Apparently, with the load
sensing reducer looking only at the left rear corner, we need
to put this second reducer to work at regulating the other corner.
(Maybe this problem could have been solved with independent
rear brake line pressure sensing reducers. Or -oh I know! -
independent ABS circuits for the rear wheels a la ur-S4.)

In a right hand turn, there will be weight transfer onto the left
suspension members both front and rear. From the perspective
of the left side mounted load sensitive proportioning valve, the
rear will appear to have become more heavily loaded, and it
will increase the pressure in the rear brake circuit. But at this
moment, the right tire has become unloaded and it will
lose traction and lock up, especially with the surge in rear
brake line pressure. What to do? What to do? Well,
why not just but a pressure reducer Thingy in front of the right
rear caliper that is sensitive to that right turn? Now, if it is a
right turn, reduce pressure to the right caliper. Clever.
Meanwhile, in a left turn, the fore/aft sensitive pressure reducer
thinks a sky hook is lifting the back of the car off the ground and
reduces the overall rear pressure accordingly, trying avoid
rear lockup on both sides. However, this action will fail to
take advantage of the braking forces that would be available
at the now heavily loaded right rear wheel.

In an asymmetrical system like this, barring the intervention of
some pretty darn smart software and electronic brake actuators,
it will never be possible to tweak it enough to realize optimum
performance. If I was willing to give up the rear cargo loading
input to the rear brake bias, I would install either a fixed
proportioning valve to regulate the rear or a cockpit
adjustable rear proportioning valve and remove the Thingy.

DeWitt

 ----- Original Message ----- 
From: "Scruggs, Gross Jr." <gscruggs at nmic.navy.mil>
To: "'DeWitt Harrison'" <six-rs at comcast.net>
Sent: Wednesday, November 16, 2005 9:05 AM
Subject: RE: rear brake circuit on a 5ktq


> >
> >
> > I have a Thingy Theory. It came to me in the dead of night.
> >
> > The rear, load sensitive proportioning valve senses rear ride
> > height. But WHERE? ON THE LEFT REAR TRAILING ARM!
> >
> > Is that a large enough clue? Would you care to take a stab
> > at completing the description of my theory?
>
> Well... lessee now...
>
> If load valve sees increase in ride height then it must conclude that car
is
> braking in either straight line or left turn... and ABS keeps left rear
from
> locking.  Right rear not an issue because: if straight line braking then
ABS
> reduces pressure (actually pulses) and if in left turn then increased
normal
> force keeps RR from locking.
>
> If valve sees decrease in ride height and incipient loss of traction
> (through ABS) then car is braking in right turn and Thingie comes into
play
> by reducing pressure to right rear brake.  Left side not an issue as
> increased normal force keeps from locking.
>
> Agree with yours?
>
> Gross
>
>