The Combination Vehicle

What are the components of a combination vehicle?

Truck drivers may drive many types of Commercial Vehicles. A combination vehicle is formed when a truck tractor or straight truck has a trailer added to it.

Class B CDL Straight Truck

What are the basic terms that describe a combination vehicle?

Truck Tractor (Power Unit)

Truck Tractor (Power Unit)

Truck tractor is a power unit (with an engine) that is self-propelled and pulls one or more trailers.

Steering Axle

Steering Axle

Steering axle is the front axle that controls vehicle direction 


Drive Axle

Drive axle is the axle (or axles) in the back of the tractor that are powered by the engine and transmission to pull the load.

Single vs Tandem Axle

Tandem Axle

Tandem axle is when there are two axles on the tractor or the trailer (or both).  Some tractors are single axle, and some are tandem axle.

Dual Tires

Dual Tires

Dual Tires are when there is a pair of wheels and tires that are located on one side of one axle.  A tandem axle tractor will have 4 sets of dual tires.

5th Wheel Top View
5th Wheel Side View

Fifth Wheel

Fifth wheel is a coupling device located on the tractor’s rear frame mounted over the tractor axles. The fifth wheel is how the tractor is connected to a trailer.

Converter Dolly

Converter Dolly

Converter dolly is a device that has an axle with a set of wheels, and a fifth wheel on top that is typically used to connect one semi-trailer to another trailer. The dolly “converts” a semi-trailer to a full trailer.

What are the different types of trucks or tractors?

Conventional Tractor

Conventional Tractor

Conventional tractor: typical U.S. tractor, with the engine and front axle placed in front of the driver, usually with a longer hood

Straight Truck

Straight Truck

Straight truck: Has one continuous frame and does not “articulate” (or bend) as a combination vehicle would.

Cabover Tractor

Cabover Tractor

Cabover tractor: typically seen in Europe, these tractors are configured so the driver sits over the engine and front axle (“cabover”)

Single Drive Axle Tractor

Single Drive Axle Tractor

Tandem Axle Tractor

Tandem Axle Tractor

Day Cab vs Sleeper Cab

Day Cab vs. Sleeper Tractor

A day cab has no sleeping area, whereas a sleeper has room for the driver to rest.

Single Wide Tires

Dual Wheels vs. Super Singles

Dual wheels vs. super singles:  uses large and wider tires rather than dual tires

What are the different types of trailers?

A trailer is a vehicle designed to haul cargo.

Semi Trailer

Semi Trailer

A “semi-trailer” connects to a tractor by the fifth wheel that is mounted over the tractor axle. A semi-trailer only has axles in the rear, not the front.

Converter Dolly to Full Trailer

Full Trailer

A “full trailer” has axles in the front and rear. A full trailer is made by attaching a “converter dolly” to a semi-trailer

There are many different types of trailers with various uses:

Dry Van

Van: hauls general freight in a dry environment

Liquid Tanker

Liquid Tanker: hauls liquids such as milk or fuel 


Flatbed: hauls large and bulky objects like steel or lumber

Drop Deck

Lowboy or Drop-Deck: has various configurations usually to haul heavy equipment for construction or farms

Bulk Tanker

Dry Bulk: used to carry bulk material like powder or pellets


Reefer: Refrigerated van that hauls food that is kept cold


Container Chassis: hauls containers that are used in “intermodal” transport (more than one mode of transport – for example ship, rail and truck)

What are the different arrangements of wheels and axles for combination vehicles?

Combination rigs have many different configurations.

Combination Vehicles

There are several axle arrangements on a standard tractor and semi-trailer combination:

There are numerous trailer combinations on U.S. highways.  Some combinations are only allowed on certain roads

18 wheel combination
Double Trailer (5 axles)
Rocky Mountain Double (7 axles)
B Train (8 axles)
Turnpike Double (9 axles)
Triple Trailer (7 axles)

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