What was this truck-mounted widget?

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SUMMARY

The discussion centers around a truck-mounted widget observed on a van belonging to Carmichael, which features a distinctive six-inch diameter spinning device resembling a black stereo knob. Participants hypothesize its function, suggesting it may serve as a wind-driven cooling fan for the interior or a GPS tracker. Further investigation revealed that the widget is likely an air vent, potentially functioning as a small vent fan to enhance airflow within the vehicle.

PREREQUISITES
  • Understanding of automotive ventilation systems
  • Familiarity with vehicle-mounted equipment
  • Knowledge of basic vehicle aerodynamics
  • Experience with visual identification of automotive components
NEXT STEPS
  • Research automotive air vent designs and their functions
  • Explore the role of cooling fans in vehicle interiors
  • Investigate GPS tracking systems used in commercial vehicles
  • Learn about the impact of vehicle aerodynamics on component design
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Automotive enthusiasts, vehicle maintenance professionals, and anyone interested in the functionality of commercial vehicle equipment will benefit from this discussion.

DaveC426913
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I saw a van downtown that had a spinny widget on top. (The van belonged to a company called Carmichael. It had extension ladders on its roof.)

Anyway, this widget looked like a black stereo knob but six inches in diameter - a circular base with a bar across it. It spun at several rotations per second, like the short-wave radar bar seen on boats.

Its rotational speed seemed to be correlated with engine speed but I'm not positive.

Anyway, my hypotheses:
- a wind-driven cooling fan for the interior
- a GPS tracker

[ EDIT ] Ah. There it is.
Flettner-2000-white-B.jpg

Ironic. I was actually Googling for pictures of van rooves so I could PhotoShop a pic for clarity. And this pic popped up...
 
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Air vent.
 
I was going to venture a small vent fan.