Over taking truck vacuum

Liventhedream · Veteran Member

Over taking truck vacuum

Bryan · Guru

This is normal, some sort of venturi effect. I read a long time ago that train tracks had to be laid a certain minimum distance apart for the same reason that the coaches would lean towards each other in similar circumstances.

Yuglamron · Guru

Brian is correct Venturi effect.

If you notice large trucks generally (Not All) move left on narrow roads to reduce the effect. happens with water too When ships 'RAS' replenish at sea, they have to be a set distance apart when they use cables to transfer good and pipelines at sea otherwise they get drawn together.

Worst time on the road is when two rigs pass on passing places and the lanes are narrow. Just another thing to watch on the highways and byways.

Bruce and Bev · Guru

it also depends on the "type" of truck eg cattle truck, solid side truck etc.

I had ESC fitted to the 'van we have now and you don't get this effect at all......

The Phantom · Guru

You should experience this effect while sitting on the Maglev train in Shanghai and another passes it going in the opposite direction. With both doing 411Km/h.

It is similar to an explosion.

The Phantom

Liventhedream · Veteran Member

Hi thank for replies. ..how much is ESC cost what does it do
Cheers john

Bryan · Guru

Liventhedream wrote:
Hi thank for replies. ..how much is ESC cost what does it do
Cheers john

The ESC Bruce is talking about is an caravan specific Alko product. In your motorhome it's best to just slow down as this will lessen and even eliminate the problem.

Happywanderer · The Master

It happened to me once when I had Myrtle. I was on a bridge when an overtaking truck went flying past.
I could feel the van being pulled in, it was a struggle to keep it straight especially with the concrete of the bridge on the other side.
Frightened the bejesus out of me.

Tony Bev · Guru

Hello liventhedream

What Bryan said is correct, Venturi/Vacuum effect

What you described is a normal situation, between trucks and motorhomes, and you will get use to it

I keep to the left as far as possible, (without slipping off the bitumen, or falling into crumbling edges), and find that there is less Venturi/Vacuum pulling me towards the overtaking truck, as well as less wind buffeting from the approaching trucks

When there are no trucks around, I keep away from the crumbling edges

gumpybsc · Senior Member

I'm agreeing with all of the below. Pressure is reduced in the space between the RV and the truck as the air speeds up in the gap as the truck passes. Big tall solid-sided trucks such as panteknicans (?spelling) are the worst. The Alko ESC on our van really helps, however my simple strategy is to just keep calm, drive straight, and back off the speed a little before the vehicles pass.

Vince · Senior Member

I feel it most when a truck of size is coming along side same direction and passing us

it just draws us back on speed from the air rush hitting the front of the van.

Don't suffer from vacuum passing opposite directions though.

Bruce and Bev · Guru

Hey Phantom - yep - fast trains passing each other can be startling the first time - to say the least.

Have been on several "bullet" type trains in various countries and when your train leans over away from the other passing train is "interesting".

Also standing on a train platform when one of these trains go through the station at full speed and you feel it sucking you towards it is frightening

The Phantom · Guru

The first time I rode the Maglev, we pulled into Longyang Road Station and a Chinese guy, in full uniform, was saluting the train. It made me feel a bit special, but, mind you, I had been in China for a full hour before that ride.

It was so high on my bucket list that I came out of Pudong Airport and straight onto the train. I just had to do it. Edit: The 25km journey took about 8 minutes.

Once I got to Longyang Station, I had to backtrack in a taxi to my hotel.

Sitting on the train watching the speedo is riveting stuff.

The Phantom

-- Edited by The Phantom on Wednesday 25th of May 2016 10:25:54 PM

Hylife · Guru

Actually it is neither venturi nor a vacuum.

The speed difference between you and the semi is little more than jogging pace. You at 90 them at 100 is 10kph and does not create a vacuum or venturi.

The effect you feel is the removal of the headwind drag on YOUR tug/van/motorhome/combination.

When YOU are (excuse the pun) breaking the wind, the headwind drag keeps your vehicle/combination inline. Should your van move sideways the headwind on its side pushes it right back.
Now, during that passing move, when the semi cab is in line with you your van, you will feel a push AWAY from the semi as his headwind flows sideways from his cab. You will naturally self correct on your steering wheel.

When the semi cab has moved in front of your van or vehicle that push is removed, requiring you to remove the correction and you mistake it for a vacuum.
In addition, he is now breaking your headwind and now you have no drag to keep you on the straight and narrow.
As a consequence of you correcting for the removal of the sideways push as it coincides with the drop in the headwind drag, you get that eerie feeling of being sucked towards the semi and an inexperienced driver can easily over-correct and get the wobbles.

It is only a reverse sensation that is no different to getting an unexpected sideways gust. No vacuum, no venturi, just a removal of drag.

Relax, hold the wheel steady, do NOT lift your foot off the accelerator, do not over-correct, do not panic, just drive on straight as normal.

The general term is called slipstreaming and racing cars use it to great effect in order to pass rivals who may in fact be faster than them.
Truckies used to slipstream each other on the Hume Hwy to save on fuel. Two trucks close together use less fuel and can go faster than one. Thankfully the practice has been banned, because when it does go wrong, two trucks make a h3ll of a mess and usually someone dies.

Liventhedream · Veteran Member

Thanks Hylife ....will take your advise ...but No nite driving for me ..cant see who's overtaking ...to prepare for the ...sway .
Cheers john

rockylizard · Guru

Gday...

Here's some information to explain the venturi effect caused by an overtaking/passing truck on a caravan.

https://caravanchronicles.com/guides/understanding-the-dynamics-of-towing-being-overtaken/

Cheers - John