Why do passing trucks suck instead of blow?

There are some ways to mitigate this problem,such as Air Tabs

Bernoulli's principle as stated is the REAL reason. Faster moving air passing between both rigs creates a low pressure area. Same principle that creates lift on an airplane wing where the shape makes air flow faster over the top than the bottom.

Always drive within you capabilities, at a speed that is comfortable for you.

However......

The pressure around your vehicle is equal...until a FASTER moving vehicle "lightens" the air on one side.

According to Bernoulli, you are not really sucked in the direction...you actually DRIVE in that direction. It is YOUR vehicle that is basically creating lift (albeit sideways----air does not know up/down, left/right) because the pressure on the side not being "passed" is higher than on the side being "passed".

SORRY all you get a BIGGER truck guys, but a tractor-trailer passing another at roughly the same speed, is hardly noticeable....they creep on by. That same truck passing an equally sized, heavier, slow moving truck at 10,15,20 mph faster will cause the SLOWER moving vehicle to "lift" in the direction of the faster moving vehicle (lighter air).

That is right...I have been the heavier, longer, taller truck....yet it was I who was sucked (as you guys say) toward a smaller, lighter faster moving vehicle. Think straight truck passing tractor-trailer. That straight truck can move that tractor-trailer...even though it is, smaller and lighter

Does MASS help? Of course it does. Larger, heavier is harder to move, whether it is being sucked or lifted. It is however the difference in speed that is causing the airflow difference.

Maybe some of the engineers can explain faster moving air causing lift (better than I) and how your vehicle is floating toward that lighter FASTER moving air.

Can the "shockwave" of a larger truck push a smaller vehicle out of the way....Yes, but that is different than that sucking feeling. A bigger heavier truck may help you from being pushed out of the way, Or as I like to say.....ROCKED.

There is a lot more going on than...get a bigger truck.


OH....by the way. When you RVers pass my slower moving, much heavier truck at higher speeds....you scare me cause it pushes me around.

Plus I never know if you drive that thing more that twice a year, which is really scary. :E

Not trying to cause an argument, not going to argue science that I really do not understand. Just trying to point out real world experience. Proper set up will go much further than....get a bigger truck.

Both push and pull...

The front of the 'other' vehicle has a high pressure area in front
of it, as it is moving/pushing ahead. It will be in the form of
a 'V', or a wake of a boat going through the water

Behind that 'other' vehicle will be a vacuum pressure area in back
of it, as it is moving ahead and the air will be backfilling the
void. Vortex at all of it's trailing edges.

The faster the 'other' vehicle is going, will have both these
effects become ever greater

Now your vehicle is also doing the exact same thing...

As the high pressure wave touches your vehicle front *AND* then
ride down the side of your vehicle(s)...it will push on the side
and your vehicle will try to pivot on an axis...the tire in this
case

As the high pressure wave continues down the side of your vehicle,
it will continue to push and try to push your vehicle on a 'pivot'

This is why 'P' or soft side walled tires will allow the vehicle
to 'wiggle' more than a higher strength sidewall. In the case of a
dually, 8 side walls and a much wider stance (track)

Then it gets to the trailer's front area.

Now there is a *new* pivot point and is the connection or ball
Both Fiver and tongued, but the Fiver's pivot point is on top of
the TV's rear axle. So the moment (engine speak for lever arm)
is very small to nothing if dead centered on the rear axle center
line

This is why folks recommend or talk about the over hang of the rear
from the rear axle to the ball...much longer moment

The trailer also has a moment from the ball/coupler to it's tires
center line (if dual axle, centered between'm, etc)

This will have the trailer potentially sway if there are no anti-sway
measures

Then the 'other' vehicles rear end vacuum comes along to do the
exact same thing, but opposite...vacuum...so pulls on the side
of your setup

Ditto a building you pass at high speed...just that it will be
a 'wall' that your high pressure bow wave and low pressure wave
will have that high pressure against that building wall and your
vehicle side...

JD and Beastlet wrote:
Nope, it's not the first line of a joke or a rhetorical question.

I would expect a passing truck or bus to be pushing a wave of high pressure air in front and beside, but I've noticed time and again that passing trucks and buses tend to pull our trailer in towards them rather than push it out. It's a point of curiosity for me and I'm interested in the explanation.

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.

Major difference between a shockwave in front of an object, the low pressure area created by the faster moving object, and the buffering when the trailing edge of that shock wave gets you (or even the draft created).

ALL of which tend to have YOU moving toward the passing vehicle whether you are causing it, or "lift" is causing it.

BUT.....to suggest that the wiggle point in the middle has that much affect.....would also suggest that a class A/B or even c has nothing to be concerned with.

Ask one of those guys.....even with no wiggle point....bet they get moved.

cbshoestring wrote:

BUT.....to suggest that the wiggle point in the middle has that much affect.....would also suggest that a class A/B or even c has nothing to be concerned with.

You are contradicting yourself.

LESS effect does NOT mean NO effect.

When there is a pivot point in the middle, the effect is slightly different and more pronounced.

If there is no pivot point, there is a fairly large area square in the middle where the extra pressure or suction or whatever is roughly equal on both front and rear wheels and unless your steering gear or an axle is loose, little or no correction is needed with a 90 degree side wind.....up to a point.

The big problem occurs when the side vector is applied ONLY to the front or back....or to the middle if there is a pivot point there.

Thanks for the responses, folks. As an aviation enthusiast and RC airplane pilot I am familiar with Bernoulli's principle; I just didn't make the leap to its application here.

Vulcan Rider wrote:

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.

In my case I'm afraid you're both wrong. My ProPride 3P hitch eliminates sway, so the entire rig gets pulled in towards passing trucks.

PR and HA hitches project the pivot point towards the TV's rear axle

The 'effect' of the push pull is still there

As for Bernoulli's principle, yes has some bearing IMHO, but there are too
many airfoil spoilers (vortexes) and the air does *NOT* rejoin cleaning (stall
for you airplane folks) and is where the vacuum comes from

'Shock Wave' was mentioned...to me, that is more attune to an explosion than
the bow wave in discussion here. 'Shock Wave' as in sonic boom or explosion
where some force is moving the air molecules together to form a semi-solid
to a solid (explosion) that will/can cause damage

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.

...but what about the fact that the air is already traveling faster around the trailer just like the passing vehicle? There shouldn't be any significant change.

JD and Beastlet wrote:

In my case I'm afraid you're both wrong. My ProPride 3P hitch eliminates sway, so the entire rig gets pulled in towards passing trucks.

And I'm afraid NOT.
What we are talking about here is NOT "sway".
It is a tiny but perceptible TURN in response to external forces.