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Keep Your Cool
by Jim Park
There's no smell quite like it. The reek of over-heated brakes burns your nostrils and clings like a burr to your clothes. It's a sickening smell, like the feeling you get from standing ever harder on the brake pedal while watching the speedometer wind ever higher. Almost every driver has had this experience at least once in his career, and once is usually enough.
"An engine brake should never take the place of training and proper planning," says Dennis Hutton, a driving instructor with Transport Safety in Burnaby, B.C. His 28 years of mountain driving have taught him never to rely completely on technology. "Too many drivers take their helper brakes for granted, which isn't good considering a 50-cent fuse can shut one down at the worst possible time." We're hooked on the technology, which has grown increasingly better over the years. Nearly all 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 the technique of downhill braking isn't practised much any more, and it's definitely a skill best mastered before you need to use it. Good downhill driving technique acknowledges that 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. It gets progressively worse until there's virtually nothing left under the pedal. That unfortunate but predictable phenomenon is called brake fade. Brake FadeYou'll know you're experiencing it because you'll need to press harder on the pedal just to feel the same deceleration. It's an awful feeling. Brake fade occurs to a worsening degree as lining and drum temperatures increase. As the brake drum heats up the metal begins to expand, actually causing the drum's diameter to increase by as much as 50- or 60-thousandths of an inch by the time it reaches 600 degrees F. If the brakes are smoking, they're getting close to that temperature. As the diameter of the drum widens, pushrod travel must also increase to push the lining firmly against it. The chart below shows how the power output of the brake chamber diminishes as the length of the slack adjuster stroke reaches the two-inch point and beyond. 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 door jamb. Try that in a wider doorway, and you'll see that the force you can exert is much, much less. To make matters worse, the intense heat the linings are subject to under these conditions can cause the resin used as a binding agent to begin to burn away - hence the smoke. Clark says 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. So much for what happens to brakes when they get hot. The bigger question is why.
Not that you'll ever go through this exercise, but this is how Clark sees it. It's the formula for calculating the horsepower required to maintain a truck at a steady speed while descending a given grade, or climbing it for that matter. Required hp = (0.002667 x gvw) x (mph) x percentage of the grade. Right! 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 x 80,000 = 213.36 213.36 x 45 = 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. That's a lot, but lets look at heavier, faster, and steeper - a 138,000-lb B-train descending a 14% B.C. grade at 50 mph. The formula shows we'd need a whopping 2576 horses of retarding power in the combination of natural resistance, foundation brakes plus any engine brake or retarder to maintain that speed: 0.002667 x 138,000 = 368.05 368.05 x 50 = 18402.5 18402.5 x .14 (14%) = 2576 Yikes! Flip it around, and the engine in our B-train would have to churn out 2576 hp to drag it up that 14% grade at 50 mph. Whoa PowerTrucks 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 16.5x7-in. 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 aren't terribly relevant, except to illustrate what kind of work we often demand of our brakes. This 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. The chart below shows the relationship between weight, speed and grade. Increase any one of them, and you'll need more reserve braking power to compensate. Increase all three on a long grade, and you're asking for trouble.
Clark says that of the three factors at play in the equation, speed is really the only one the driver can influence. Keeping the speed to a minimum will extend brake performance, but keeping the truck under control is one thing; being able to bring it to an emergency stop is quite another. Dennis Hutton points out that in heavy traffic, the kind you often find during the tourist season or in a hilly urban area such as North Vancouver, a quick stop may be required at any time. "Too many drivers don't operate within the limits of their equipment," he says. "Hot brakes reduce the stopping capability of the truck, but some guys just plow right on through as if nothing's changed." Proper TechniqueThere 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," he says. "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, 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. So there you have it. Nothing to it, really. 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 much if your brakes burn up underneath you, but at least now you'll know why it's happening. Leave nothing to chance and you'll never have to worry about what might have been.
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So how should you approach the challenge of a long grade? Can't we depend on the good old 'Jake'?
Engineers have formulas for everything. And Dana Corporation's brake guru, Jim Clark, has a formula for this one too. Clark is Dana'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.
Al Wright, the fellow who drafted British Columbia's first air brake instructor's course, and who has appeared as an expert witness at many accident inquiries, explains:
