Doesn't really surprise me. Think about it this way: as a truck or any blunt body moves forward through the air, air "pushes" it back and also "pulls/sucks" it back as well via the area of low pressure behind the body. So the tailing truck fills in the area of low pressure behind the truck. If you've ever seen one of the CFD simulations from Dailmer, Volvo, or any of the manufacturers making aerodynamic skirts, boat tails, etc you'd see that the area of low pressure behind a semi traveling 55mph is enormous.

BTW, it was 30-50 feet, not meters :)

Not sure how I feel about this. When I try to merge onto the highway and 4 trucks riding so close together offer me nowhere to get in. We already have these double tractor trailers where I live. If they are in front of you when you pull off the ramp they accelerate so slowly it is really scary seeing an 18 wheeler crawling up you backside and you have nowhere to go.

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They constantly try to escape from the darkness outside and within
Dreaming of systems so perfect that no one will need to be good T.S. Eliot

https://vimeo.com/50872582

Remember that fuel consumption to overcome air drag is exponential w/speed (and we're talking highway speeds here). Semis (esp.those w/o tail fairings) leave a pretty big wake, so helping to fill some of the gap helps a lot (even at 50m).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Ready for something really wild?

A jet engine or rocket engine/motor throws stuff out the back to create thrust. Since the stuff is coming out as a mass with velocity (momentum), there's a momentum transfer = thrust. But the 'stuff' is coming out at a pressure different than local atmospheric pressure (typically), and that's called pressure drag. The pressure drag is in addition to the base drag (your truck's drag).

While creating thrust, a jet/rocket is creating drag...

I can't see where it mentions the differences in these. I assume this is at various speeds? If so what are the speeds?

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The difference is the fairing on the back of the truck. Top case has none, middle has a short fairing and the bottom has a longer one (which I've seen a handful of times, and I do relatively little highway driving).

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All we have to decide is what to do with the time that is given to us. ~Gandalf

In addition to the tail, look at the x-axis (it's not to scale) to see just how long the low pressure wake is.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

The moral of this story seems to be that you need to get butt cheek implants to make your trailing edge more aero and reduce the low pressure behind your butt. Come to think about it, Thierry Marie used something like that in the TdF Prologue circa mid 1980's

 Same things happens with bikes pace-lining. the front rider gets a benefit as well.

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Brian Stover USAT LII
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