In Gear - Today's Trucking

The job of any brake, remember, is to
convert motion into heat and then shed
that heat away from the vehicle. Increasing
the area of the lining that contacts the
drum gives you the ability to generate more
braking torque with less in-stop fade, and
the most logical place for a bigger brake is
up front. NHTSA set the current stoppingdistance
standard (mirrored in Canadian
regulations) using 15-x-4-in S-cam brakes
and Type 20 air chambers on the front
axle. In writing its proposed new rule,
NHTSA tested two beefier steer-axle brake
packages you could spec today if you wanted
to: 16.5-x-6-in and 16.5-x-5-in sizes. In
concert with standard S-cam brakes on the
drive axles, the bigger steer-axle brakes
helped produce an average stopping distance
of 269 feet, a 25-percent improvement
on the existing standard.
It’s important to keep in mind that
these bigger brakes not only are wider,
they’re larger in diameter than the 15-in
brakes they would replace.
“Wide brake packages tend to shed heat
better and wear longer; today’s wider
shoes give you almost a 25-percent
increase in the wearable lining life,” says
Paul Johnston, senior director of the North
American foundation brake business unit
at ArvinMeritor. “But wider brakes alone
won’t necessarily put out enough torque
to stop the vehicle in a shorter distance.”
For that, you need a shoe and lining with
a greater diameter. A 16.5-x-5-in drum brake
provides 12 percent more wearable lining
volume than a 15-x-4-in brake. There’s more
lining for the drum to grab, giving a boost to
the brake’s coefficient of friction.
“No one is certain what the final stopping
distance requirement will be,”
Johnston says, “but I’m confident that
when all is said and done a larger-diameter
front brake with a more aggressive lining
and larger brake chamber will allow
most three-axle tractors to comply using
an all-drum solution.”
HEAVY CHALLENGES
The shorter stopping distance requirements
in NHTSA’s proposal would not be a
snap to achieve for everyone. They raise
important questions for brake engineers,
vehicle manufacturers, and certain segments
of the trucking industry to tackle in
the coming months:
In Gear
1What about trucks that already use
16.5-x-5-in brakes on the steer axle?
Roughly 10 percent of air-braked tractors
made today have high-output drum
brakes on the steer axle, most of them for
heavy hauling, refuse fleets, and other
severe-service applications.
“A 4x2 tractor or a tractor involved in
pulling very heavy weights probably
already has a larger drum brake on the
steer axle,” Johnston says. “Vehicles in
these types of service may need disc
brakes either at the steer position or at all
positions to meet the standards in a final
rulemaking.”
2Will larger brakes require a more
robust suspension and other components?
“When the steer axle has to accommodate
more brake torque during a hard
stop, what you can get is tremendous wrapup
on what is typically a light-duty spring,”
says Clark. “To prevent that, what you’ll see
is an anti-wrap up type of suspension—
something that’s better equipped to manage
the stresses that may come with a more
aggressive brake.”
3Should truck owners expect longer
service life from larger brakes? That
depends. All the basic principles of brake
maintenance apply to bigger drum
brakes on the steer axle, says Tom
Golden, manager of technical services for
BrakePro, which supplies a broad range
of both OE and aftermarket linings for a
range of applications.
“The effectiveness and longevity of any
drum brake depends on one thing: lining
contact with the drum when the brakes
are applied,” Golden says. “A bigger, wider
brake still has to be properly maintained
in order to ensure adequate contact, and
the friction material still should meet the
load and axle requirements and the application
or use of the truck.” If one wheel is
doing too much work, or not enough, the
result will be premature wear and poor
performance no matter what size brake
you use.
4Will NHTSA put forth a firm stopping
distance benchmark, instead of the
range of improved braking performance in
its proposal? Johnston says yes.
“I think it’s clear there will be one minimum
performance standard, one stopping-distance
number instead of a range,”
Johnston says. Imagine a market where
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