Effects of semi's on caravans

Webmaster · Guru

RE: Effects of semi's on caravans

the rocket · Guru

Fringe Dweller wrote:
No wonder why Good people Leave THIS forum.
Good Bye . PERMANENTLY this time.

Hi fringedweller, hope u do not leave. your contributions r valuable.

This is a very interesting thread.

-- Edited by the rocket on Tuesday 6th of June 2017 08:25:09 AM

iana · Guru

What is interesting about this (my) and nearly all of the other threads, is that those who respond don't read the initial question properly, then it comes down to a "I know more than you" bun fight. My question was " is the clearance between the road and the bottom of the van a reason why I was not feeling the effects of semi's passing". A high ground clearance allowing pressures positive or negative to equalize quickly, and not have any effect on the towing of the van, whereas some imported vans seem to have the ground clearance of an F1 racing car, and so possibly would be sucked about when being passed.
Swoffer was perhaps the only person who read the post correctly. But the conclusion is " Nobody knows", and won't know unless you have a wind tunnel.

Desert Dweller · Guru

iana wrote But the conclusion is " Nobody knows", and won't know unless you have a wind tunnel.

There you go you've answered your own initial question. No-one on this forum seems to have a wind tunnel on hand at the moment.

Aus-Kiwi · Guru

Oh yes some do ! Plenty of wind ! Ahaha The pressure builds up !! Boofff just be prepared !! Lol

spida · Guru

A friend who has a brand new $150k 24' van had the kitchen window sucked out by a truck. To me that is either shoddy workmanship or has something to do with the fact that he pressurises the van inside with a fancy pump every time he plugs in to his rig?

Bryan · Guru

iana wrote:
What is interesting about this (my) and nearly all of the other threads, is that those who respond don't read the initial question properly, then it comes down to a "I know more than you" bun fight. My question was " is the clearance between the road and the bottom of the van a reason why I was not feeling the effects of semi's passing". A high ground clearance allowing pressures positive or negative to equalize quickly, and not have any effect on the towing of the van, whereas some imported vans seem to have the ground clearance of an F1 racing car, and so possibly would be sucked about when being passed.
Swoffer was perhaps the only person who read the post correctly. But the conclusion is " Nobody knows", and won't know unless you have a wind tunnel.

The short answer is no. The side airflow of car, truck, caravan passing each other at speed creates a vacuum between them which "sucks" both objects towards each other. The science has nothing to do with how low slung the object is.

This venturi effect is also felt and is especially dangerous when a truck passes very close to a cyclist. There has been reports of cyclists sucked under the rear wheels of trucks. There's similar reports of people standing too close to railway tracks feeling the effect of being sucked in towards the train.

Aus-Kiwi · Guru

Hight and stability too ? Keep in mind most semi's have flat services from 1.5m and up . So any pressure effect is in the upper section of van . Then there's the push - pull effect with each trailer ??

Bryan · Guru

Aus-Kiwi wrote:
Hight and stability too ? Keep in mind most semi's have flat services from 1.5m and up . So any pressure effect is in the upper section of van . Then there's the push - pull effect with each trailer ??

The higher the object is in overall height has a more pronounced effect within reason even if that object was only a few centimetres off the ground. Ground clearance by itself has little effect. Its a side air flow thing.

A related example are aereoplane wings which are asymmetrical in a typical section view to exacerbate the venturi effect to create lift. Hell even Hobie catamarans make use of the venturi effect in their hull design.

Thats me done

The Belmont Bear · Guru

iana wrote:
What is interesting about this (my) and nearly all of the other threads, is that those who respond don't read the initial question properly, then it comes down to a "I know more than you" bun fight. My question was " is the clearance between the road and the bottom of the van a reason why I was not feeling the effects of semi's passing". A high ground clearance allowing pressures positive or negative to equalize quickly, and not have any effect on the towing of the van, whereas some imported vans seem to have the ground clearance of an F1 racing car, and so possibly would be sucked about when being passed.
Swoffer was perhaps the only person who read the post correctly. But the conclusion is " Nobody knows", and won't know unless you have a wind tunnel.

Iana I said at the start of this thread and again half way through I was concerned about the fact that you (and Swoffer) thought what seemed like a legitimate question was going to be turned into a bun fight. at last you have provided me with the answer i.e. you believed that those of us who responded would not read your initial question properly and a "I know more than you bun fight" would ensue. Sorry I wasn't smart enough to pick up on that earlier or I wouldn't have bothered responding at all confuse .

-- Edited by The Belmont Bear on Tuesday 6th of June 2017 03:19:12 PM

Tony Bev · Guru

iana wrote:
Hmmm here we go! something to liven things up more . Just been reading about what to do to avoid being sucked ---- in by a semi or road train. The thing is, I have a fairly big van, and to date have felt no effects from the passing semi's at all. I feel the effect of wind, you know the blowing type, but nothing from traffic.
Now my van has a high ground clearance, and I am wondering if it is the amount of gap between the van and road which is preventing air pressure from equalizing, so a van with very little clearance would suffer from the effects more, while the larger gap would allow the negative pressures occurring to equalize.
A-n-d off we go!!!!!!

Hello Ian

That is a very highly technical question

Most of us are not related to Adrian Newey, the Formula One aerodynamicist

But...

Peter D, has given a link to a very informative article, (thanks for that article, from me, Peter D)

The rest of us, can only give an answer from our own experience

The consensus seems to be, that the larger the gap between a caravan, and an approaching/overtaking semi, results in less instability of the caravan

Reading the article which Peter D has given, I will put my neck out and say

No the height of a caravan does not affect the forces from the semi, as those forces start from the tray of the semi, which is usually higher than any caravan ground clearance, I have seen

To attempt to prove up my (unproven) opinion

My daily drive, a Ford Falcon sedan, does not suffer any instability from an approaching semi, at any legal road speed, while I keep in the middle of my lane

-- Edited by Tony Bev on Tuesday 6th of June 2017 03:26:48 PM

Hydeeeho · Senior Member

Swoffer wrote:
Aus-Kiwi wrote:
Office workers !! Lol. Ok flame suite on !! Lol ð
It'll get locked now , the PC police will not allow this .

Are you for real ??????? No need for all your bull****, please just take it easy

Hydeeeho · Senior Member

Swoffer wrote:
BB

Sheesh , cheer up man .

Look back at my first comment , first line , Total agreement with Iana's assessment .

Second line (and other comments) in jest , come on lighten up . Ive said nothing nasty , no malice .

You have a strange way to lighten things up, maybe this is not the forum for you

Aus-Kiwi · Guru

To a "point" can tell all the tech on the WWW . It's DRIVING EXPERIENCE!! That matters ! You need koala fixations ( intended lol ) to drive a 6 ton truck ! But just car licence to tow a van up to or over that weight ! Find some swerve away as semi passes which exaggerates any windage !