Doubles/Triples · Coupling and Uncoupling

You are backing a converter dolly under the rear trailer of a set of doubles. What must be true of that trailer's height before the dolly goes under it?

1.It has to rest a bit below the dolly's fifth-wheel center, so the dolly's push lifts the trailer slightly as it slides underneathCorrect answer

2.It must sit slightly higher than the center of the dolly's fifth wheel, so the dolly slides underneath without any contact at all

3.It must be exactly level with the center of the dolly's fifth wheel, so that neither unit lifts the other during the approach

4.Height does not matter at all, because the dolly's own landing gear sets the final coupled height after the two units meet

Explanation

The trailer needs to rest a bit below the dolly's fifth-wheel center, so that pushing the dolly under it lifts the trailer slightly as the two come together. That slight lift is the point of the check: it is what shows the fifth wheel has actually taken the trailer's weight rather than passing beneath it, and it lets the locking jaws close properly around the kingpin. 49 CFR 383.115(a) names procedures for assembly and hookup of the units as a required knowledge area for this endorsement, but it states no height rule, so this check is attributed to the manual. The wrong options all remove that lift or defer it: a trailer set too high lets the dolly pass under without taking the load, a trailer set exactly level leaves the coupling unseated, and the dolly's landing gear is not a way to correct the height afterward, since the relationship has to be right before the dolly goes under.

Source: 49 CFR 383.115(a)

Drill Coupling and Uncoupling questions

You are backing a converter dolly under the rear trailer of a set of doubles. What must be true of that trailer's height before the dolly goes under it?

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