The Traffic Accident Reconstruction Origin -ARnews-

Vacuum-Assisted Brake Lag

Albert T. Baxter ([email protected])
Fri, 18 Sep 1998 19:55:55

While I could not locate an exact reference to your request, the following
is submitted for your information. Research of my documents found this
information and definitions regarding brake applications-

>From Automotive Brake Systems by Bosch (1995-SAE) page 18:

Based on EEC Regulation 71/320
Brake response/Pressure build-up times - .36 seconds for M1 class (pass.
veh) and .54 seconds for M2/M3 class (bus/motor coach), N1-N3 class (comm
veh).

Definitions contained in the Automotive Handbook by Bosch (1986-SAE) on pg.
508 are as follows:

Initial Response: The time between the moment when the component of the
control device on which the control force acts starts to move and the
moment when the braking force takes effect. (i.e. brake pedal movement)

Build-Up Time: the elapsed time between the moment the braking force takes
effect and the moment when this force reaches a certain value. (nominally
75% of the full braking potential of the brake system).

Response Time: The sum of the initial response and build-up times is used
to assess how the brake system behaves in time until the moment at which
the full braking effect is reached.

Rudolph Limpert in two books indicates the following:

in Analysis & Design of Automotive Brake Systems (1976) pp 1-2 he indicates
that the initial response is a function of different velocities, (not
stated) at .2, .4 and .54 seconds. On page 8-9 he indicates the build-up
time can range between .5 to 1.0 seconds.

In Motor Vehicle Accident Reconstruction and Cause Analysis 3rd ed.
(Michie) in section 20-3 on page 307 he states, the application time
[Initial response?] for hydraulic brake systems is very small and usually
less than .1 seconds. He also references a chart (fig. 20-3) in which a
measured delay between brake line pressure build-up and pedal force for a
vacuum assisted brake system is shown in this figure. For
'fast-application' the figure illustrates between >50 lbs pedal force at 0
brake line pressure to <250 lbs pedal force at about 1000 psi brake line
pressure, the time is annotated at .32 seconds.

In December 1988 the SAE-AIP Committee did ABS testing at the CHP Facility
at Sacramento California. Although the paper is a report on the results of
that testing, one of the instruments used to record data was a device
called the Motometer. Portions of the recorded data containing the
deceleration traces were printed in this paper. The deceleration trace for
7 tests has an initial spike within the 1st .1 second [trigger
machanisim?], then an average of .4 seconds elapses until the trace shows
the deceleration build up time to the full brake application period. This
value is consistent with the EEC/Limpert MVCA values. The majority of these
ABS tests were run at 60 mph.

A quick review of FMVSS#105 (Hydraulic Brake Systems) section 571.105 of
Title 49 requires some vehicle brake testing involving what is defined as a
'spike stop'. This is further define as requiring the application of 200
lbs of brake pedal force in .08 seconds to meet a maximum braking distance
at defined speeds for the vehicle to receive certification. A technical way
of defining 'panic' brake application by a driver. Perhaps this is the time
you were thinking of in your post.

I hope this information will be of help.

Regards:

Al Baxter

----------------------------
Albert T. Baxter
ACTAR Registration #116
Suncoast Collision Analysis
14840 Atlantic Avenue
Hudson Florida 34667-1006
E-mail: [email protected]
----------------------------

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