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Zorluk: ZorMountain Driving and Steep Downgrades

A commercial motor vehicle combination with a gross vehicle weight of 64,000 lbs64,000\text{ lbs} approaches the crest of an unfamiliar 5-mile5\text{-mile}, 7%7\% mountain downgrade on a rainy afternoon where the road surface is wet. The designated safe speed for this truck is posted at 25 mph25\text{ mph}. Which procedure must the driver follow to maintain complete vehicle control and prevent brake failure during the descent?

  1. A
    Maintain the transmission in cruising gear, disable auxiliary retarders, and apply steady, light continuous brake pedal pressure to hold the speed precisely at 25 mph25\text{ mph}.
  2. Downshift into the proper low gear before cresting the downgrade, turn off the engine retarder due to wet road conditions, and use intermittent snub braking down to 20 mph20\text{ mph} whenever road speed reaches 25 mph25\text{ mph}.Cevap
  3. C
    Engage the auxiliary retarder at its maximum setting to minimize service brake wear on the wet pavement, while keeping the transmission in neutral to avoid engine overspeeding.
  4. D
    Begin descending the grade in top gear and execute a double-clutch downshift into a lower gear once vehicle momentum causes the speed to exceed 25 mph25\text{ mph}.

Cevap

Downshift into the proper low gear before cresting the downgrade, turn off the engine retarder due to wet road conditions, and use intermittent snub braking down to 20 mph20\text{ mph} whenever road speed reaches 25 mph25\text{ mph}.
The driver must select the appropriate low gear before starting down the grade to harness engine braking. Because the pavement is wet, the auxiliary retarder must be turned off to avoid drive-wheel lockup and skidding. When service brakes are needed to manage speed, proper snub braking must be utilized by applying firm pressure until speed drops 5 mph5\text{ mph} below the maximum safe speed (25 mph−5 mph=20 mph25\text{ mph} - 5\text{ mph} = 20\text{ mph}), then fully releasing the brakes to allow heat dissipation.

Adım Adım Çözüm

1
Identify the primary source of speed control before starting the descent.
The vehicle must be shifted into the correct low gear before cresting the summit or entering the downgrade.
Engine braking in low gear is the primary holding mechanism; shifting during descent risks getting stuck in neutral.
2
Assess road surface conditions for auxiliary retarder operation.
Deactivate the auxiliary engine retarder because the road is wet.
Retarders apply braking force only to the drive axles, which can cause drive wheels to lock up and cause a severe skid or jackknife on wet pavement.
3
Execute the proper service braking technique (snub braking).
Apply firm brake pressure to reduce speed from 25 mph25\text{ mph} down to 20 mph20\text{ mph} (5 mph below safe speed), then fully release the pedal.
Snub braking dissipates heat during the release interval, whereas continuous light braking overheats the drums and leads to brake fade.

Anahtar Kavram

Safe mountain descent procedures combining pre-crest gear selection, retarder deactivation on wet surfaces, and snub braking
Tahmini Süre:2m 0s
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Mountain Driving and Steep Downgrades

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