Re: Soft brakes? (was: Brake booster diagnostic)

[Scott Fisher <sfisher71@domain.elided>]

--- John Hertzman <johnhertzman@domain.elided> wrote:
> 
> Soft compared to what? 

The only comparison that isn't pointless would be to
my '74 Spider -- and John, please note that I'm using
my Spider as the "good" example solely because it was
maintained better than my Berlina before I acquired
either car, not because I think the Spider is the Good
Alfa and the Berlina is the Bad Alfa, on any scale
other than the relative conditions of the two
individual vehicles in my driveway -- and the purpose
of this whole exercise is to put the Berlina right. 
I'm just trying to get an idea of what "right" means.

On careful analysis, I think that the difference is
more psychological than mechanical, more due to my
confusing "effort" with "effect."

When I first drove my Spider three years ago, my
primary Alfa experience was with the '67 GT Junior --
floor-mounted pedals, no power assist.  Once I rebuilt
the front calipers, that car's brakes had a marvelous
feel: requiring a lot of leg effort but VERY
responsive, easy to modulate, and a joy to use.

The first time I drove my Spider, there was so much
less force required to operate the brakes (and so much
more pedal travel) that, at first, I thought something
was wrong with them.  It became apparent in a block or
so of driving that there was nothing wrong: they WORK
very well indeed, they just FEEL very different from
the unassisted '67.  

How they feel: the pedal in my Spider offers very
little resistance and more travel than the '67, at
which point the brake pads begin to contact the rotors
and friction sets in fairly rapidly.  Pressing harder
on the pedal in the '67 results in a more rapid
slowing without much more pedal travel; pressing
harder in the Spider results in more rapid slowing and
ALSO more pedal travel.  I have assumed that this
difference is due to the vacuum assist.  It's AS IF
the vacuum assist mechanism was designed to keep pedal
EFFORT relatively constant throughout the travel, and
the vacuum booster converts increased travel at the
pedal into increased pressure at the pads, without
requiring more effort by the driver's foot, but simply
more distance.  

Enter my Berlina.  There was very little brake-pedal
resistance when I bought it, as in my Spider, but
there was also significantly less braking effect at
about the same point (to my not-always-attentive foot)
as the Spider.  Pressing harder on the pedal in my
Berlina produced little in the way of additional
braking force (and about the same additional travel as
in my Spider, which should have identical brakes), but
I now believe this was due to worn pads in my Berlina.
 Here's why I believe this.

My Spider, with good pads and solid hydraulics, has a
light pedal effort (vacuum assist) and requires a
certain amount of pedal travel before the pads begin
to generate friction, and the car slows down rapidly
from that point.  There is, however, very little
additional pedal EFFORT required to increase braking
force (due to the vacuum assist) -- a small amount of
increased effort produces a large amount of increased
effect.  (If I remember my math, it feels as if the
vacuum assist increases on a curve with a positive
exponent -- a "hockey-stick" curve.)

My Berlina, with very worn pads when I bought it
(confirmed by inspection on removal and replacement)
but solid hydraulics (by test), also has a light pedal
effort (vacuum assist working properly).  HOWEVER,
with these worn pads, when I pushed the pedal about as
far as I did in my Spider, there was much less
noticeable slowing.  And here's where I got confused:
I pressed harder on the Berlina's pedal and still got
less braking effect than in my Spider at the same
pedal level, pressed still HARDER and still got less
braking effect -- not because of any undocumented
differences in the SYSTEMS, but because the pads in my
Berlina were glazed to the point that NO effort was
going to produce equal braking results to those in my
Spider.

As I continue to bed in the pads on the Berlina (and
now that I have new rear pads as well I'll tackle the
rest of the system in the next few days), I have
noticed that the pedal EFFORT hasn't changed, but the
braking EFFECT has.  And I think this is what I've
been sensing all along -- that the worn pads in *my*
Berlina caused me (unconsciously) to push harder on
the pedal, which the vacuum assist system translated
into pedal TRAVEL (instead of pedal EFFORT).

And... I'm about this close to saying "Nevermind,"
because the solution doesn't lie in writing and
guessing and wondering.  The solution, for me, was in
getting out and replacing known worn parts of my car
and having it work properly.

> Why on earth would there be softer brakes on the
> Berlina, and if there were, how would it have been 
> done? The differences in the brake systems of the 
> three body-styles in a given year were in the 
> length of the handbrake cable and the
> corresponding total lengths of two pieces of tubing,
> to allow for the variations in wheelbase. Nothing 
> else.

The distinction between picking nits and clutching at
straws may be an interesting one, but I suspect that
either phrase would apply to one final observation and
question: 

Brake pedal effort is defined in part by the a
mechanical relationship between the volume of the
master cylinder and the volume of the rest of the
system, expressed as a ratio of pressures per square
inch.  This is one reason why engineers choose master
cylinders of different diameter, as a means of tuning
the proportion between the master cylinder and the
rest of the system; your foot provides the pounds, the
master cylinder provides the square inches, and the
proportion between the master and the rest of the
system gives you the pedal effort.  

There may -- MAY -- be a minuscule difference in
required pedal travel to achieve equal PSI values for
a system with longer tubes, but I suspect you'd need
to use a micrometer to comb the nits off those straws
to detect it.  A knowledgable braking engineer is
invited to clarify the figures involved in computing
this by adding the difference in tubing length for the
different wheelbases, and then calculating the ratio
of master cylinder interior surface area versus
downstream interior surface area, and finally
translating that to the travel required of the Berlina
driver's foot versus that of the Spider driver.  I am
not that engineer, and I can't imagine that the
difference would be more than a few square inches of
volume and perhaps a few millimeters of foot-travel.

On the other hand, if the difference is significant
(on the order of 10% or more), that might actually
make a difference that even an uneducated foot could
perceive.  But again, that falls somewhere between
clutching at straws and picking nits.

Either way, the right thing for me to do is get the
new pads in my Berlina, get them bedded in, and enjoy
it.  

--Scott Fisher
  Tualatin, Oregon
.
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