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Side view and underside view of a conventional 18-wheeler semi-trailer truck with an enclosed cargo space. The underside view shows the arrangement of the 18 wheels. Shown in blue in the underside view are the axles, drive shaft, and differentials. The legend for labeled parts of the truck is as follows:
1. tractor; tractor unit
2. semi-trailer (detachable)
3. engine compartment
4. cabin
5. sleeper (not present in all trucks)
6. air dam
7. fuel tanks
8. fifth wheel coupling
9. enclosed cargo space
10. landing gear - legs for when semi-trailer is detached

Alternative forms[edit]


From the total number of wheels: two front steering wheels on the tractor, plus dual wheels on each end of two axles (total of 8) at the rear of the tractor and supporting the front of the trailer, plus dual wheels on two axles at the rear of the trailer.


18-wheeler (plural 18-wheelers)

  1. A big rig truck: a semi-trailer plus the truck or tractor pulling it.
    Synonyms: semi-trailer truck, semi truck, semi, articulated lorry, artic, big rig, juggernaut, tractor-trailer, truck and trailer