Bendix Commercial Vehicle Systems A-18 TRAILER ABS 5/04 User Manual
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FIGURE 2 - Air Timing Requirements Chart
General Air Brake Requirements
Basic design requirements for trailer air brake systems are
set forth in FMVSS-121. FMVSS-121 covers requirements
for new construction. Once put into operation, the trailer’s
brake systems must be maintained in accord with the
following FHWA/OMC standards.
•
FMCSR 393 - Covers required equipment
•
FMCSR 396 - Covers inspection and repair
Air Timing Requirements
FMVSS-121 specifies the maximum times that are
permitted for application and release of brake chamber
pressure. Refer to Figure 2. ABS equipped trailers must
meet the same air timing requirements as prior, non-ABS
equipped trailers.
Reservoirs
Trailers must be equipped with air reservoirs that provide
a volume of air eight times that of the service brake
chambers.
For example: a type 30 air chamber has an effective surface
area of 30 sq. in. For short stroke type 30 air chambers,
the volume is typically 89 CID. For a typical two axle trailer,
the minimum required volume is therefore 2848 CID.
Reservoir size requirements for non-ABS and ABS
equipped systems are the same.
Air Consumption
During ABS activation there is a loss of reservoir pressure.
There are no specific requirements limiting air consumption
in the U.S. (In Europe Regulation R13 states specific
limitations). ABS manufacturers take air consumption into
account when developing and evaluating ABS control
algorithms. There has been no need to change reservoir
size requirements as a result of the ABS mandate.
Vehicle Classification
Application Time (seconds)
Release Time (seconds)
Tractors, Trailers and Buses
.45
.35
.75
.55
Towing Trailer
.50
.50
1.00
1.00
Converter Dolly
.55
.55
1.10
1.10
Single Trailer
.60
—
1.20
—
Note: A 50 cubic inch reservoir is used to simulate the towed trailer volume at the gladhands of towing units.
From pedal
movement
for chambers
to reach 60 PSI
Pedal movement
to reach 60 PSI
at 50 cu. Res
at gladhand
From pedal
movement to
reach 5 PSI
(w/95 PSI initial
chamber pressure)
From movement
of the pedal until
50 cu. in. reservoir
reaches 5 PSI
(With 95 PSI initial
chamber pressure)