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DOWNHILL BRAKING: PARK’S VIEW

Posted: August 1, 2014 by Jim Park

From the May 2001 issue.

There’s no smell like the stench of overheated brakes. It’s a sickening smell, like
the feeling you get from standing hard on the brake pedal while watching the
speedometer wind ever higher. Almost every driver has had this experience once in his career, and once is usually enough.

So how should you or your drivers approach the challenge of a long grade?

Good downhill driving technique demands two key things.

First, don’t take your engine brake for granted. We’re hooked on the technology,
which has grown increasingly better over the years. Nearly all heavy trucks are
equipped with engine brakes these days, some of them extremely powerful, and
few drivers ever have to go down a hill without one. But these “helper” brakes are

no replacement for good technique. And the technique of downhill braking just isn’t practiced much any more, which is troubling, because it’s a skill best mastered before you need to use it.

Second, brakes are a non-renewable resource-once they’re gone, they’re gone.
As the brake components heat up, their ability to slow the vehicle is compromised-a phenomenon called brake fade. You’ll know you’re experiencing it when you feel the need to press harder on the pedal just to maintain the same sense of deceleration. It gets progressively worse until there’s virtually nothing left under the pedal.

Why? When a brake drum heats up, the metal begins to expand, actually causing
the drum’s diameter to increase by as much as 50 or 60 one-thousandths of an
inch as temperatures approach 600 degrees F. As the diameter of the drum widens, pushrod travel must also increase in order to push the lining firmly against it.

Think of it this way. If you stand in a doorway that’s slightly narrower than the reach of your outstretched arms, you’ll be able to exert some force against the doorjamb.

Try that in a wider doorway, and you’ll see that the force you can exert is much less. To make matters worse, this intense heat can cause the resin used on the linings as a binding agent to burn, hence the smoke. The charred layer rubs off in tiny particles that actually act as a lubricant, reducing the friction between the
surface of the lining and the drum. Talk about pouring oil on troubled waters.

SCHOOLS OF THOUGHT
There are two schools of thought on how to brake on a long downhill grade. The
first, and once the preferred method, calls for a light but steady application pressure (about 10 psi) to all the wheels all the way down the hill. The other suggests hitting or “snubbing” the brakes periodically with a moderate 20-psi application to control the speed. Both methods work, but “four out of five brake experts” today recommend the snubbing method.

“If there’s any inconsistency in the pre-set crack pressures, a light application may not be sufficient to open all three valves,” explains Al Wright, a Hope, B.C.-based consultant who drafted British Columbia’s first air-brake instructor’s course. “This
may result in one group of axles doing all the work, but you’d never know it by the
feel of the foot pedal.”

In other words, all the brakes may not be doing an even share of the work. And
that’s not the only reason to snub the brakes.

“A pretty stiff application is required to activate the adjusting mechanism,” says
Wright. “With a prolonged light-but-steady application, the drum will begin
expanding away from the lining, with no opportunity for the slack adjuster to
compensate.”

They need to be applied and released in order to readjust themselves, sometimes
requiring up to 30 application cycles for the adjustment to become effective. This is a key point we’d all do well to remember.

In any case, we must keep the engine brake in perspective here. It’s a helper, not
the primary source of your stopping power. Consider where you’d be without it. If
you can maintain a given speed with the retarder on and little or no use of the
service brakes, you’re using it correctly. The oldest rule of thumb in the book still
applies: descend the hill in the same gear you’d use to climb it. Consider the
horsepower required to climb the hill at that speed, and then decide if you have
that kind of horsepower under the other pedal.

Once you’ve picked a gear, stay with it. It’s dangerous to attempt to downshift on a
grade. Upshift only as the grade lessens, but don’t get carried away. Free rolling
allows the brakes to cool, so don’t get into a big hurry to use them before you need them.

As you descend, allow the engine to run up to its maximum governed speed, then
make a 20-psi application to decelerate the truck to the lower end of the engine’s
operating range. Allow it to run up to the governor again and repeat as required all
the way down. If the application pressure required to decelerate the truck as just described begins to increase to 30 psi or beyond, the brakes are beginning to
fade, possibly dangerously.

Consider the remaining descent. If you’re near the bottom and the remainder of
the hill can be safely driven at a higher speed, upshift and give the brakes an
opportunity to cool. But each time you do this, you’re one step closer to the point
of no return. If there’s any possibility of having to make an emergency stop during
what’s left of the hill, stop it while you can-while there’s still some reserve left.

Once stopped, put the truck into its lowest gear, turn the wheels towards the
shoulder and chock them if possible. Avoid setting the parking brakes. Allow at
least 30 minutes for the brakes to cool before proceeding.

A FORMULA
Brake gurus like Jim Clark have braking down to a science. Clark is Dana Corp.’s chief engineer for foundation brakes and wheel equipment. In the simplest of
terms, Clark says, the same amount of power is required to climb the hill as is
needed to maintain a steady speed on the way down. He has a mathematical formula to prove it. It calculates the horsepower you need to maintain a truck at a steady speed while descending a given grade. Let’s try the example of an 80,000-lb rig wanting to maintain 45 mph on a 4% grade. The formula looks like this (0.002667 is a constant):

0.002667 x 80,000 lbs. = 213.36
213.36 x 45 mph = 9601.2
9601.2 x 0.4 (4%) = 384

So, the braking horsepower required in our example is 384, which seems about
right for a climb as well.

Let’s look at a heavier and faster vehicle on a steeper decline-a 138,000-pound
B-train descending a 14% grade at 50 mph:

0.002667 x 138,000 = 368.05 368.05 x 50 = 18402.5
18402.5 x .14 (14%) = 2576

Yikes! The formula shows we’d need 2576 horses of retarding power in the combination of natural resistance, foundation brakes, plus any engine brake or retarder to maintain that speed. (Flip it around, and the engine powering our B-train would have to churn out 2576 hp to drag it up that 14% grade at 50 mph.)

Trucks do have some natural resistance to movement, which engineers measure in retardation horsepower. That’s stuff like aerodynamic drag and friction between
gears. Clark points out that an average 80,000-lb truck has a natural retardation of
about 110 hp at 30 mph, and close to 200 hp at 50 mph. This may surprise you,
but an average 16.5-by-7-inch truck brake — a single brake — produces only 10 hp over a sustained period. Add to the 200 hp of natural retardation 10 hp for each of
the eight dual wheels and you’ll see that-at a total of 280 hp — you’re still well short of the 384 hp required to maintain 45 mph, as in our example. The situation is
downright grave for our B-train, with a braking deficit of something like 2236 hp.
These numbers illustrate just how much work we often demand of our brakes.

They also should tell you that it’s physically impossible to maintain 45 mph on a long 4% grade with a gross weight of 80,000 lb with just your foundation brakes.

So there you have it. Just get your slide rule out at the top of the hill and your
descent won’t be any problem at all. Actually, all the theory in the world won’t help you if your brakes burn up underneath you, but at least now you’ll know why it’s happening. Better to slow down and leave nothing to chance. You’ll never have to worry about what might have been.

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