Converter Dollies
The two-axle dolly that turns a semitrailer into the second unit of a double.
Endorsement: Doubles / Triples (T) · Source: FMCSA CDL Manual (public domain)
A converter dolly is essentially a two-axle (sometimes one-axle) trailer with a fifth wheel mounted on top and a tongue with a coupler that hooks to the lead trailer\'s pintle hook. The dolly converts a standard semitrailer into the second unit of a double or triple by providing a fifth-wheel coupling for the second trailer\'s kingpin. Pre-trip inspection of a dolly looks much like inspection of any trailer\'s rear axle area: check the tires, brake chambers, slack adjusters, suspension, and the fifth wheel itself for proper grease, jaw condition, and locking-lever position.
Dolly inspection adds two unique items. Check the pintle hook on the front of the dolly tongue for cracks, wear, and a properly secured locking pin. Check the safety chains for damage and the proper crossing pattern beneath the tongue (the standard is to cross them so that if the pintle hook fails, the chains catch the tongue rather than letting it drop to the ground). Air and electrical connections from the lead trailer to the dolly include short "glad hand" extensions and a seven-way electrical pigtail, all of which must be in good condition with no visible cracks or chafing.
Uncoupling a dolly is the reverse of coupling, with one critical safety detail. Always lower the dolly\'s landing gear before disconnecting the air lines, because disconnecting air will cause the dolly\'s spring brakes to apply, and the dolly will be locked in place by its own brakes. If you forget this step and disconnect air first, you will need to recharge the dolly air supply just to move it. The exam tests this sequence and tests the placement of safety chains, both of which are easy points if you have memorized the standard procedure.
Key terms to memorize
- converter dolly
- pintle hook
- safety chains
- crack the whip
- rear trailer
- pup trailer
Other Doubles / Triples (T) topics
- Coupling Order for Doubles and Triples — Which trailer goes where, and why the heaviest goes first.
- Inspection of Doubles and Triples — Walking the full length of a double or triple before every trip.
- Handling Doubles and Triples on the Road — Following distance, lane changes, and the crack-the-whip physics.
- Prohibited Cargo and Restrictions — What you cannot legally haul in a double or triple combination.
Test what you learned
Now that you have the Converter Dollies material in your head, drill the Doubles / Triples (T) practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Doubles / Triples (T) test for your jurisdiction.