This diagram is from Wikipedia.

Diagram of an 18-Wheeler Semi

I am interested in what people call the part marked 6 on the diagram. The label suggests it is called an "air dam", but searching around, that term seems to be used for other parts of cars and trucks - even by Wikipedia!

Is there a typically-used name for this part?

3 Answers 3


It's most typically called a wind deflector. You can see it listed as such on this website. It can also be called a cab air deflector or even an air foil.


That part is known as either a wind deflector or a Roof Fairing. I believe the latter is the correct technical term.

To satisfy your curiosity about the other parts as well. I know it is a little more than you asked but what the hell I'm on a roll.

1: Tractor Unit (or Truck)

2: Semi- Trailer.

10: Pedestal (crank unit has 2 speeds)

11: Trailer Dolly( which can slide backwards or forwards and be locked in position). This is done to increase or decrease weight capacity of the payload and also to make backing up to the dock easier by shortening turning radius. Weight is measured on weigh scales between # 8 and #11

8: Tractor side: Fifth Wheel, Trailer side: King Pin

  • Coming from the aerospace world, "fairing" would be the most apt term, as it serves an identical function to similar shapes on rockets and aircraft -- namely to modify airflow around a vehicle to be more favorable.
    – Tristan
    Commented Dec 6, 2016 at 16:35

As others have clearly stated, it is indeed a wind deflector or a roof fairing. As this article states, the purpose of it is to reduce the drag that would be produced by wind hitting the trailer directly.

It is especially useful for highway driving, where the amount of wind encountered could substantially affect gas mileage. The foil cuts through the wind and prevents the sharp fighting against the wind which would otherwise be an issue.

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