RE: The Bomb

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

... while I am all for making sure that the hydraulic system's pressure
accumulator is in proper service, I wanted to comment about the interaction
between it and the ABS system.  

My response about whether or not someone should immediately replace a bomb
as soon as it is known to be bad is somewhat tempered on cars that have ABS.
I have experienced for myself what can happen in an emergency braking
situation on a car without ABS and a bad bomb; and that is a situation that
is definitely asking for trouble IMO/E.  Upon initial and heavy application
of the brakes the driver notices that there isn't as much brake assist as
was expected so it is necessary to apply additional pedal pressure to get
the car to slow down at the needed rate.  A fraction of a second later the
hydraulic pump begins to catch up and now the assist is causing the brakes
to begin to lock, which then forces the driver to compensate by releasing
the brake (much more than if it were simply modulating the pedal around
impending lockup).  

If the same thing happened on a car with ABS, the modulator would begin to
work as soon as the system detected impending lockup, which would make it
unnecessary to have to modulate the brakes.  AAMOF, that is what people with
ABS-equipped cars are told to do, press the pedal as hard as possible and
let the ABS system take care of modulating the brakes.  This characteristic
makes my response to the immediacy of replacing the bomb lower for a person
who feels that they (and everyone else who will drive the car) have
sufficient leg strength to be able to stop the car effectively in an
unassisted situation to be that they should replace the bomb ASAP, but it
isn't that immediate a need.  

Thinking about it just now, I seem to recall reading about a study that MBZ
did that showed that people tend to not supply enough pedal force in panic
stop situations, which led them to design their braking system to actually
provide extra boost when there appears to be a panic stop in process.  See,
even when Audi isn't trying they are able to be ahead of the competition
with new features ... :-)

... just kidding of course ...

Steve Buchholz
San Jose, CA (USA)