JULY 2017 | VehicleServicePros.com 19
“Split system specifi cations do not reduce overall
performance for the vehicle. However, it is
important to assure all tractor and trailer brakes
are serviced to the original OEM specifi cations.”
Stopping power is noticeably greater when both
the tractor and trailer are outfi tted with air disc
brakes, especially when the rig is traveling at
highway speeds.
Drum and disc brakes employ the same principles
of friction to slow a vehicle down.
Basically, drum brakes – sometime referred
to as self-energizing brakes – are designed so
that a round drum rotates along with the wheel.
Friction material is attached to brake shoes. When
the brake pedal is pressed, the brake shoes push
against the inside of the drum to slow the wheel.
With use, the friction material wears away.
Disc brakes work by compressing two friction
pads against a fl at rotor. Instead of a brake drum
attached to the wheel hub, a rotor is attached to
the hub. A caliper is contained on the brake carrier
with its own brake chamber. When the brake
pedal is pressed, the caliper squeezes the brake
pads against the rotating rotor to slow the wheel.
Because the rotor is exposed to outside air, it is
able to cool rapidly, which decreases its tendency
to overheat or cause fading.
Depending on how a fl eet uses its tractors and
trailers, Meritor’s Watterson says it is commonly
accepted that disc brakes have overall better operating
performance and shorter stopping distances
than drum brakes in optimal conditions. Disc
brakes dissipate heat better than a drum – keeping
brake pads from overheating and causing breakdown.
Serviceability is easier on disc brakes as there
are less components and less labor time needed.
“When combining the benefi ts of better friction
wear, decreased service intervals, less maintenance
time and increased uptime, the total cost
of ownership for disc brakes versus drum brakes
can be lower,” he continues. “Th is applies to both
tractors and trailers.”
Adoption Rates
Disc brake penetration is lower for trailers than it
is for tractors, notes Watterson. Traditionally, an
average fl eet purchases and operates more trailers
than tractors each year, Watterson says. Because
trailers typically run fewer miles per year than
tractors, fl eets have been slower to see the advantages
of disc brakes on trailers.
However, as the price of disc brakes continues
to become cost competitive, and the awareness
of the technology and benefi ts of disc brakes is
more widely shared, Meritor expects more fl eets
to spec disc brakes on both tractors and trailers.
Adoption rates of air disc brakes have climbed
from about 10 percent in 2011 to nearly 20 percent
in 2016, adds Paola Carmona, product manager,
Meritor. It is a trend she expects “to carry forward
to upwards of 30 percent by 2020, mainly because
big fl eets are starting to move into that direction.”
Segments like construction applications,
logging and off -highway – where there is a lot
of dirt and debris – will help keep drum brakes
viable, Carmona says, but she anticipates that air
disc brakes could eventually overtake the market.
“We believe drum brakes will not go away,” she
says. “Th ere are places where they do make sense
and are the most cost eff ective option.”
Carmona expects 6x4 rear tandem confi gurations
that spec disc brakes on the front with
drum brakes toward the rear will rise most in
popularity. Th is is because of the advantages disc
brakes off er, including their resistance to fade and
a maintenance cost reduction that can in some
cases exceed the lifecycle of the unit.
In a 6x4, both rear axles are powered.
“Fleets on a short trade cycle buy the truck
and never touch the brakes, without having to do
anything above standard inspection and maintenance,”
she says. “A lot of the market is very
cost sensitive.”
Preventive Maintenance
When doing preventive maintenance for disc
brakes, it is important to pay close attention to brake
pad wear, Watterson of Meritor says. Also, ensure
all boots and seals are in good condition to protect
the internal mechanism from contaminants. For
drum brakes, he off ers these recommendations:
• Make sure brake linings are operating at the legal
thickness limit. If not, they should be changed.
• If changing the brake linings, also inspect the
cam bushings and change them if needed.
• Be sure to regularly lubricate cam bushings and
slack adjusters.
• Brake drums should be measured to assure they
have not reached wear limit, and inspected for
signs of abnormal conditions such as cracks or
heat-checking.
• When replacing parts, make sure to always use
factory service parts.
• As always, refer to the manufacturer’s maintenance
manuals for proper procedures.
» The importance of a vehicle’s braking
system cannot be overemphasized. This
system will provide the best performance,
safety and return on investment when
properly specified and maintained.
Photo from iStock
Air Disc Brakes Have
Their Advantages
Conversion to air disc brakes (ADBs) is increasing because
they provide some advantages over drum brakes, including
shorter stopping distances. Plus, there is virtually no
brake fade with ADBs, so torque stays more consistent
through the entire stop, says John Thompson, OE sales
manager, TMD Friction (www.tmdfriction.com), one of the
world´s largest manufacturers of brake friction materials.
Another advantage, he adds, is that “because the braking
force of an ADB is simpler than a drum brake – squeezing
a flat rotor, compared to the complexity of brake
shoes against a drum – the possibility of brake pull on
the steer axle is greatly reduced for better straight line
stability. Other factors contribute to side-to-side brake
pull, but brake geometry is no longer part of it.”
Moreover, ABDs can help shorten vehicle downtime
because they are easier to maintain. Depending on
such things as the friction level of the material, the
truck’s vocation, driver habits, etc., Thompson notes
disc pads can last twice as long as drum brakes.
PREVENTATIVE MAINTENANCE
Disc brakes, as with drum brakes, require proper preventive
maintenance (PM) to get the most life and reliability.
“A good PM schedule and inspection procedure are critical
to optimizing safety, brake life and cost,” Thompson
of TMD Friction says. He offers this guidance:
Calipers have wear indicators to give an idea of remaining
pad life. The caliper moves to compensate for pad wear.
Alignment marks, or a wear indicator pin, will give the
approximate pad wear and remaining life. The maintenance
manual for the caliper will describe the method
used for indicating pad wear and how to read it.
• Caliper assemblies are non-serviceable
and should be changed as a unit.
• Technicians need to be well trained
when working on brakes.
• Check the rotor for scoring, cracking or abnormal wear.
Light heat checking, similar to drum brakes, is considered
normal. Cracks are not. Refer to the maintenance
manual for determining normal and abnormal.
A pad change only requires about 25 percent
of the time required for a shoe change, he says,
which saves time and money. Once the wheel is
removed, new pads can be installed quickly.
PADS
Thompson of TMD Friction says the pad brand purchased
and installed is important, and advises purchasing the
same lining material the OEM installed on the vehicle
when it was new; replace like for like. The reason being:
OE brake manufacturers produce the appropriate
brake linings for specific vehicle applications to get the
best stopping power and wear. Improper replacement
friction could compromise performance and safety.