§393.70 Coupling Devices and Towing Methods, Except for Driveaway-Towaway Operations

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 1/2 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: §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 drawbar 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.

If the drawbar design is such that bolts, connecting pins, etc., are used to connect structural members of the drawbar, and are located at or near the midpoint of the drawbar (beyond the attachment points for the safety chain at the ends of the draw bar) the safety devices would have to extend from either the frame of the towed or towing vehicle to a point beyond the bolts, connecting pins or similar devices.

In the case of an extendible draw bar or reach, if a separate safety device(s) is used for the front and rear of the drawbar, a means must be provided to ensure that the drawbar will not separate at the movable portion of the drawbar. The use of welded tube stops would satisfy the intent of §393.70(d) if the ultimate strength of the welds exceeds the impact forces associated with the drawbar extending suddenly with a fully loaded trailer attached.