con gear 

Short for converter gear. A set of wheels, dolly or a bogie, that connects and supports two semi trailers (three in some states) as doubles or triples. The word people commonly use to describe big rigs, semi, actually means semi trailer. A semi trailer is supported by it's back wheels, with a truck tractor (the technical for a truck cab) a full trailer is supported by its own wheels, front and rear, being towed or connected to a truck cab. People misuse the word semi, thinking it describes a tractor trailer when in fact the semi is only a detachable cargo carrier, not connected to the power unit/tractor. That's why its called a semi trailer. A full trailer only needs to be pulled behind a vehicle, it supports itself. With two semi trailers they need a set of wheels between them, an axle for the back trailer. A con gear is an independent set of wheels, when detached, interchangeable if it needs repairs. A con gear can weigh as much as a car and sometimes needs to be moved or positioned by the driver.

To connect a set of doubles the driver needs a con gear.

con gear by Faultliner June 13, 2019