July_2021 - 35

representative in stop-and-go traffic, downhill
descents, and/or repeated higher-pressure brake
applies. Then, as the brake drum absorbs more and
more thermal energy, it expands away from the
brake friction. ADBs do not display this condition,
since brake rotors dissipate heat more efficiently
and/or when they do heat up, they expand toward
the friction rather than away. "
The upshot here, McComsey said, is that maintenance
managers need to understand that replacing
the original friction with non-OEM friction can
lead to different performance, including increased
stopping distances, reduced pad wear life, accelerated
rotor wear, and other issues. These changes
in brake performance will then be experienced by
drivers who had grown accustomed to the vehicle's
original stopping performance.
Risks of unbalanced brakes
" Brake balancing includes, but is not limited to,
the even brake force generation within a vehicle
axle combination or tractor-trailer combination
to ensure an equal and evenly distributed
braking throughout the vehicle or combination, "
said Dirk Wohltmann, director of engineering,
Americas, Commercial Vehicle, ZF Group. He
noted that maintenance managers should also
consider other factors when running both brake
types, such as the equal apply-and-release timing
and pressure balance (axle-to-axle or vehicle
combination), as well as the hysteresis between
brake apply and release.
" At the end, " Wohltmann said, " there should
be an evenly balanced brake torque throughout
all involved braked axles (including trailer) in all
possible braking situations: snub braking with low
brake force, up to emergency braking with full
torque in extreme short apply times, as well as
reducing braking activity (release but hold brake).
An evenly distributed brake force and timing
behavior is important for comfortable vehicle
driving, best braking performance, vehicle stability,
even brake wear, and to prevent brake damage
due to overload of individual brakes. "
And, Wohltmann added, fleets should also
consider the safety risks that can arise from
unbalanced brake systems.
" Unbalanced brakes may lead to uneven brake
wear and temperature impact, early fade-out of the
drum brake with the full brake performance transferred
to the remaining brakes (mostly the ADB),
which would lead to lower overall stopping performance, "
he said. " Other risk factors include overheating
and disc rotor crack damage in ADB that
could affect vehicle driving comfort and instability. "
According to Wohltmann, drivers can tell if a
combination brake system is out of balance by
looking for these problems:
ÜEarly heat-related brake fade-out
ÜTrailer push or tractor over-braking (instability
tendency)
ÜIndividual brake overheating
ÜUneven brake pad wear
Likewise, he noted, maintenance professionals
can look for telltale issues such as:
ÜUneven brake pad wear
ÜDamage to ADB or surrounding area
ÜADB disc rotor cracking
» Air disc brakes essentially
don't fade until the
system reaches extremely
high temperatures, which
most drivers will never
experience, unlike drum
brakes which fade regularly at
much lower temperatures.
Photo courtesy of SAF-Holland
disc brake will absorb additional energy that the
drum brake is unable to (as a result of brake fade).
This can lead to a perception of accelerated wear
rates of the more effective [disc] brake, because the
ADBs are carrying a larger share of the braking. "
The act of " balancing " brakes in a historical
» Air disc brakes today are found on about
45% of all new Class 8 tractors and will
likely be near 60% by 2025. Balancing their
performance with drum brakes will be a
maintenance issue for fleets for years to come.
Photo courtesy of Bendix
Optimal semi-trailer brake balance is a complicated
mix of five factors, explained Tony Ryan,
technical services and training manager for
SAF-Holland. Those include mechanical brake
force, pneumatic brake force, pneumatic balance
between the truck and trailer, weight transference,
and brake fade.
" Three of these factors are set before the driver
takes off, " Ryan noted. " Weight transference can
roughly be accounted for, but brake fade is the
big unknown variable. If you don't start with the
first three correct, then you are fighting an uphill
battle for safe braking of your vehicle. "
Ryan advised it takes extremely high temperatures,
which drivers will never experience, to
cause brake fade with air disc brakes. This is
unlike drum brakes, which can more readily fade
at much lower temperatures.
Maintenance considerations
Before deciding to begin a brake-balancing maintenance
program, fleets first need to set some basic
goals, said Randy Salvatora, manager, engineering
systems at Bendix Commercial Vehicle Systems.
He noted that " balancing " can mean different
things to different people, and that at a fundamental
level the ability of a brake to absorb and dissipate
energy can be described as thermal balance.
" Drum brakes tend to fade and reduce their
torque output as they heat up, while disc brakes
provide a more consistent torque output across a
wider temperature range, " he explained. " When
disc brakes are combined with drum brakes, the
context meant a routine maintenance procedure
of inspection, lubrication, and adjustment of the
foundation brake hardware, Salvatora noted. " The
initial pressure at which a brake starts to develop
torque (described as the threshold pressure) is
critical to a balanced brake system. This can be
influenced by many factors, including design as
well as maintenance. "
If an issue is suspected, Salvatora advised that
fleets reference the Bendix BW1555 brake balance
procedure guidelines or the SAE J1505 Brake
Force Distribution Test Procedure to quantify and
measure brake threshold and pneumatic balance
of the braking system.
At Meritor, Kay said, brake balancing refers to
brakes equipped on an axle-to-axle configuration,
adding that brakes are sized and specified by
gross axle weight rating for commercial vehicles.
Additionally, he noted, FMVSS121 requirements
will need to be met for commercial size vehicles
using air brakes.
" All in all, it is important to remember that
tractor and trailer brakes must always work as a
system, " Kay stressed.
" Air disc brake and air drum brake balancing
ensures that each brake is able to apply and
release at the same time and is able to build
torque to convert energy effectively, " he added.
" The brakes need to work together when applied,
and we want each brake to contribute its proper
amount of work-with 'work' defined as the
amount of energy converted during the stop. "
Optimizing brake torque output for the tractor-trailer
combination helps to avoid stability
concerns like pushing, pulling, or jackknifing the
vehicle, Kay noted.
" Universal brake compatibility for existing tractor-trailer
combinations has been a desire for our
entire industry for a long time, " Salvatora said.
" And to that end, brake compatibility has been
examined and refined during each incremental
progression of commercial vehicle technology. "
He suggested fleets communicate with the
vehicle manufacturer and the brake system
manufacturer for guidance on servicing and
trouble shooting brake system issues.
July 2021 | FleetMaintenance.com
35
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July_2021

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