Why do passing trucks suck instead of blow?

You're not alone. It is weird but if you're not prepared, it feels like it could pull you in. :)

There is actually an area of lower pressure immediately BEHIND the truck, due to the shape of the airflow over the truck trailer. That's what people who are drafting get in. It sucks a relatively lightweight trailer towards it. Right behind that is an area of turbulence.

Actually, in my experience, they do both.
So does your trailer!

I'm not an expert, but I would guess that fluid dynamics are at play here. The air flowing around the truck/bus is traveling at a much faster speed than the rest of the air. Bernoulli's principle states that the faster a fluid (air) travels, the lower the pressure. This is how old time carburetors worked, and same reason why you have lift on airplane wings.

Same reason an airplane wing provides lift.

The air between the front of the truck and the back of the trailer must travel at higher speed. High speed corresponds to low pressure.

Try this, hang a piece of paper from a pencil and blow hard across the top. If you get the angle right, even though you are blowing, the paper will lift toward the air you are blowing.

Air is traveling around a high profile vehicle as speeds down an interstate at 65mph to the sides as well as above. As the front of two of these vehicles meet, the air is now directed around both vehicles instead of between, acting like a single even wider vehicle. Less air flows between them which acts something like a vacuum.

Ever drive through a dust storm?

Nope, you are all wrong.
The real explanation is that the truck IS pushing air against the sides of the trailer and tow vehicle. BUT the least stable part of that assembly is between the trailer wheels and the hitch. When that part of the assembly is pushed away from the truck it tends to also move the back of the tow vehicle away from the truck which tends to turn the front of the tow vehicle TOWARDS the truck thus giving the impression of the truck sucking in the tow vehicle.
You're welcome.

Same for a train. There have been people pulled from a platform into the passing train.

The blowing across a piece of paper as described in a previous post is the best example I have seen.

I have found motor homes do the most sucking. I suspect it is due to the very low ground clearance they have compared to most semi's.

Watch a Nascar race. They talk about the side draft at the higher speed tracks.

When it is the worst is with a cross wind coming from the side that a truck or motor home passes. First you have a 20-30 mile wind pushing from the left then all of a sudden there is a suction pushing towards the truck then when it clears a bunch of turbulence and a 20-30 mile wind pushing again.

Dick_B wrote:
Nope, you are all wrong.
The real explanation is that the truck IS pushing air against the sides of the trailer and tow vehicle. BUT the least stable part of that assembly is between the trailer wheels and the hitch. When that part of the assembly is pushed away from the truck it tends to also move the back of the tow vehicle away from the truck which tends to turn the front of the tow vehicle TOWARDS the truck thus giving the impression of the truck sucking in the tow vehicle.
You're welcome.

BINGO.

But look at the little picture that the OP provided in his post.

The high pressure wave near the front of the overtaking big truck first pushes against the back of the TT; that tends to push the back of the TV closer to the semi. Just about the time you correct for that, the pressure wave contacts the front of the TT and the back of the TV pushing it in the opposite direction.....which is the same way that your correction was going.

The dynamics of a one piece unit, like an A or C is a bit different but the same basic principle still applies. If your front end is worn or out of alignment, the effect is MUCH worse.

There isn't much you can do about it.......except maybe to minimize the speed difference if you can. This is just one of the reasons that I prefer NON-Interstate/Freeway roads; almost no big trucks.

It's called Bernoulli's principle. It's why airplanes can fly, and why NASCAR drivers and cycle racers can draft. Lots of web pages describing this effect in both layman's terms and in scientific and mathematical terms.

For us towing trailers, it's our rig getting caught in the air flow around the semi truck/trailer that's trying to flow back to where it was.

Hiker_01 wrote:
I'm not an expert, but I would guess that fluid dynamics are at play here. The air flowing around the truck/bus is traveling at a much faster speed than the rest of the air. Bernoulli's principle states that the faster a fluid (air) travels, the lower the pressure. This is how old time carburetors worked, and same reason why you have lift on airplane wings.

Sorry, I didn't your post right away! Didn't mean to repeat you.