['CMV Parts and Maintenance']
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01/03/2025
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Interpretations to Part 393 — Parts and accessories necessary for safe operation
Question 1: Is there a minimum number of fasteners required to fasten the upper fifth wheel plate to the frame of a trailer?
Guidance: The FMCSRs do not specify a minimum number of fasteners. However, the industry recommends that a minimum of ten 5/8 inch bolts be used. If ½ inch bolts are used, the industry recommends at least 14 bolts. The CVSA has adopted these industry standards as a part of its vehicle out-of-service criteria.
Question 2: When two safety chains are used, must the ultimate combined breaking strength of each chain be equal to the gross weight of the towed vehicle(s) or would the requirements be met if the combined breaking strength of the two chains is equal to the gross weight of the towed vehicle(s)?
Guidance: If the ultimate combined breaking strength of the two chains is equal to the gross weight of the towed vehicle(s), the requirements of §393.70(d) are satisfied. It should be noted that some States may have more stringent requirements for safety chains.
Question 3: Section 393.70(d) requires that every full trailer must be coupled to the frame, or an extension of the frame, of the motor vehicle which tows it with one or more safety devices to prevent the towed vehicle from breaking loose in the event the tow-bar fails or becomes disconnected. The safety device must be connected to the towed and towing vehicles and to the tow-bar in a manner which prevents the tow-bar from dropping to the ground in the event it fails or becomes disconnected. Would the use of a pair of safety chains/cables between the towing vehicle and the front of a fixed-length draw bar, or an extendible draw bar, with a separate pair of safety chains/cables between the end of the draw bar and the front of the towed vehicle meet the requirements of §393.70(d)?
Guidance: Generally, separate safety devices at the front and rear of the draw bar could be used to satisfy the requirements of §393.70(d) provided the safety devices are attached to the draw-bar and the vehicles in a manner that prevents the drawbar from dropping to the ground in the event that it fails or becomes disconnected. Also, the arrangement of the safety device(s) must be such that the vehicles will not separate if the draw bar fails or becomes disconnected.
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