Brake Fade on a 6% Grade
You are descending a 6-mile, 6% grade on I-70 west of Denver in a 5-axle tractor-trailer loaded to 78,000 lb. The posted truck speed is 40 mph. You entered the grade at 45 mph in 9th gear with the engine brake on high. Two miles down, the truck has crept to 58 mph despite the engine brake, so you begin applying the foot brake steadily to hold speed. A mile later you smell hot brake lining, the pedal travels noticeably farther before the truck slows, and the truck is now at 62 mph. The engine rpm is climbing. There is an escape ramp about 1 mile ahead on the right; traffic is moderate. Your air gauges still read 105 and 100 psi.
1. What is happening to the brakes, and what caused it?
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This is brake fade, not an air leak-your gauges still show 105/100 psi, so the problem is heat at the drums, not lost air pressure. You have been riding the brakes: a continuous light application on a 6% grade with 78,000 lb generates heat faster than the drums can shed it. The drums expand away from the linings and the lining surface glazes, so friction drops. That is why the pedal travels farther (the shoes must move further to contact) and why the same pedal effort now produces less slowing. The climbing rpm and speed confirm the brakes are no longer holding.
2. What is the correct braking technique for this grade, and what should you do right now?
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The correct technique is snub braking, not riding. Pick a safe speed at the top-40 mph or lower-be in a gear low enough that the engine brake holds you, and apply the brakes firmly to cut speed by about 5-6 mph, then release completely and let the drums cool between snubs. Right now: make a firm, decisive application to get back to 40 mph, downshift if the speed allows, and keep the engine brake on high. If the truck will not respond to a firm application, do not wait-take the escape ramp ahead. Do not continue snubbing if the pedal is already low; that is your signal to exit.
3. If you miss the ramp and the pedal goes to the floor with no braking, what is your sequence of actions?
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If the pedal goes to the floor and there is no braking, the priority is to shed speed without a jackknife. Keep the steering under control, downshift progressively through the gears to use engine compression, keep the engine brake on, and sound the horn and turn on hazards. Use the escape ramp if one is reachable; if not, look for the least hazardous runoff-a ditch or guardrail-rather than a hard turn. Apply the parking brake control gradually if it is a modulating type; a full spring-brake application can lock the drives and cause a skid. Do not shut off the engine-you will lose power steering and possibly the engine brake.