General Air Brake Requirements - BENDIX A-18 TRAILER ABS Manual

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.
Vehicle Classification
Tractors, Trailers and Buses
Towing Trailer
Converter Dolly
Single Trailer
Note: A 50 cubic inch reservoir is used to simulate the towed trailer volume at the gladhands of towing units.
FIGURE 2 - Air Timing Requirements Chart
Application Time (seconds)
Pedal movement
From pedal
to reach 60 PSI
movement
at 50 cu. Res
for chambers
at gladhand
to reach 60 PSI
.45
.50
.55
.60
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.
From pedal
movement to
reach 5 PSI
(w/95 PSI initial
chamber pressure)
.35 .55
.50 1.00
.55 1.10
— 1.20
Release Time (seconds)
From movement
of the pedal until
50 cu. in. reservoir
reaches 5 PSI
(With 95 PSI initial
chamber pressure)
.75
1.00
1.10
—
3