Effects of semi's on caravans

iana · Guru

Effects of semi's on caravans

Swoffer · Senior Member

Without access to a wind tunnel I'd say that's probably a reasonable assessment .

Nuff said , now lock this thread , its getting out of control .

macka17 · Guru

Little mix of a few things.

High GC helps. Weight of both too.

Only Advice I ever give when asked.

Is DO NOT slow down when you see that monster in mirrors.
He'll be straightened up and accelerating, Long b4 he reaches your arse.
Same as when YOU overtaking him.

Check clear road front and rear.
Pull out and straighten up WELL b4 reaching his bum. and accelerate to pass.

Yrs ago we all used to flash lights to each other for clearance to come back in.
Nowadays?.
I always flash TO overtake.Most reciprocate coming back.

AND Get the "white line fever".. IE The INSIDE.... White line.
When you being O\Taken.

Glue your inside tyres to that till he gone past. More airspace the betterer.
UNless a skinny road. In which case.
Move over as far as you can SAFELY..

If not safe.
DO NOT.
Don't let them intimidate.
Just hold your ground.

MOST are experienced drivers and will work their route around you and yours.

This pulling you into side of tray is normally more with the big bin trailers.

It's Vacuum. Air vacuum.
2 slab sides passing . one faster than other. Speed differential.
Creates vacuum. and draws slower LIGHTER one in.

IF... They are close enuff together.

Same thing happens with ships.
with theirs being water drag. (Read it. it's interesting.)
Sideways AND below.
Shallow water. Too fast.
creates a Pushing\suction effect with water under front of ship.
Belly drags on riverbed. I've seen it many times in shipping channels and rivers.
Stop engines. Let water catch up, and off again.
Slower.
Even with 4ft or more under them.
They'll leave a trail of river bed whirling in their wake
from Prop turbulences.

Anyway.
Caravans.

steady. DON'T slow down.
and hug the verge side.
Jeez. WHY didn't I just say that in beginning hey. chuckle.

-- Edited by macka17 on Sunday 4th of June 2017 11:19:14 AM

Swoffer · Senior Member

blankstare Oh no , here we go again disbelief biggrin

The Belmont Bear · Guru

I find if I am overtaken on a freeway by a semi going in the same direction I sometimes feel the van being pulled a little towards it then pushed away by it's wash as it passes, no big deal as long as you're expecting it. Not all semis have the same effect depends on both the size and bulk of the trailer and the speed that they are travelling. I haven't really felt the same thing happen on a 2 lane highway when a semi passes me going in the opposite direction it very seldom causes any movement of the van at all. Our van has been lifted 100mm so maybe iana as you suggested extra ground clearance could be helping but I would have thought that air being pushed under the van would cause it to lift possibly creating even more movement. Be interested to hear what other people think about this topic but I'm a little concerned that by the way you have written your post it sounds like you're expecting there to be some sort of a bun fight.

Cupie · Guru

With my GQ Patrol pulling a 21'6" Jayco Westport, I feel only a slight 'tug' when a roadtrain gets close behind of is just starting to pass. Wakes me up if I have trundling along with mind in neutral and not watching my mirrors as often as I should.

(Tug = 2800KG; Van AGM = 2334Kg; Ball weight = 271Kg; HR 350Kg WDH; Usual speed 85Km/Hr; Standard clearance; Travelled >200,000Km over 20yrs.)

I just talk to them on the CB if I haven't already done so and maintain my speed & line, perhaps a very slight increase in accelerator to 'stretch' my rig.

No effect as they finish overtaking or when passing and certainly no 'sucking' towards them.

Edit ...

Reading a post subsequent to this one of mine, I recalled an incident on a recent trip down to Port Macquarie from Bris. It was raining heavily & extremely blowy. As I topped a hill with a sharpish RH turn at the crest, I was suddenly pushed over into the oncoming RH lane. Perhaps I aquaplaned or it was the wind & poor traction. In any case it gave me a hell of a fright. We were indeed fortunate that there was no oncoming traffic.

-- Edited by Cupie on Sunday 4th of June 2017 11:51:22 AM

Swoffer · Senior Member

The Belmont Bear wrote:
I'm a little concerned that by the way you have written your post it sounds like you're expecting there to be some sort of a bun fight.

Yep nothing to see here ..............................Yet .

I will not be denied today .

rockylizard · Guru

Gday...

Despite the pessimism of some who will reply to the validity of this thread ... and some who will painfully regurgitate the old hackneyed 'rules/advice' of how to be 'prepared' and/or 'correct' the sway you have highlighted .. I offer the following.

I can only comment on my experience. Perhaps I am extremely lucky or just am better prepared than others - either by osmosis or care full planning.

From being on the road full-time for the past eight years, I believe the main reason some vans do not get affected by the 'suction/draft' when being overtaken by a big rig is the initial 'basic balance' of a van and the loading of the van. Tyre pressures will contribute - too low or too high can/may affect stability.

You have a 'high ground clearance' van and do not experience the problem when trucks overtake you. I have a 'low clearance' on-road van and do not experience any affect from overtaking trucks.

Indeed, I rarely feel the effects of cross-winds ... unless the winds are constant and then all of a sudden a huge gust - that might upset the balance momentarily. Coming out from behind a row of trees does sometimes cause a bit of 'effect' but rarely does it require me to do anything but 'gently maintain the dead ahead' and the rig 'rights' itself quite quickly.

Even back in the 1980s when I towed a 23ft Viscount with WDH and a 60series Cruiser I didn't have any problems in regard to gusting cross winds and/or overtaking trucks. I travelled full-time for two and a half years back then.

So from all that - a well designed, well balanced, correctly loaded van will quite probably have good 'inbuilt' stability and not suffer being 'sucked into/by the draft'.

OH ... and even though Macka didn't include it in his response, the weight of tug vs Van will be a contributor to 'stability' as well I would suggest.

Cheers - John

{edit: and I really do not want to stir the pot and attract derision but I would be remiss if I didn't include that my vehicle does not use a WDH and there is no additional anti-sway device.}

-- Edited by rockylizard on Sunday 4th of June 2017 11:48:51 AM

Swoffer · Senior Member

It doesnt have to be that complicated tho John .

Eaglemax · Guru

Dont know what problem some of you have with this topic, if you dont like it, ignore it. Topics on caravans are sort of limited anyway. If not then start your own. When I built our tiny full height 11ft van I was worried about semi's coming the other way...after all my vans tare is only 450kg, gross 748. A recipe for being blown off the road. So before I did the lap I installed a friction type stabiliser type unit, the type thats adjustable and runs from the side of the drawbar to beside the towball. $40 off gumyree. No problems. It would not sway more than one inch at the rear of the van even when road trains went close. Hope that helps. Tony

Swoffer · Senior Member

Its gaining momentum now biggrin biggrin

Aus-Kiwi · Guru

Yes talking or listening to them on CB is great . I've been notified when traveling behind one . Said he was turning off a head . Very safe attitude . Plus you'll find windage in most cases is the difference in speed between the two vehicle . Concentrate stay straight . If you have to go off the bitumen to pass ?or let them pass ? As in main roads north WA.. Get set up and straight, settled before passing if you can . There is a vid clip doing the rounds where a van pulls off the road 100m before oncoming semi . The van looses control and spins in front of semi .. He braked also which assisted in jack knife effect . Just lack of experience and confidence!! Thing is these days we don't do paddock bashing when young to Esperance these things . Office workers !! Lol. Ok flame suite on !! Lol ð

Swoffer · Senior Member

Aus-Kiwi wrote:
Office workers !! Lol. Ok flame suite on !! Lol ð

It'll get locked now , the PC police will not allow this .

rockylizard · Guru

Gday...

Smiley Thinking.jpg

WTF .... confuse sometimes this forum AMAZES me .... the absolute SH!T some bung up denigrating a post ..... I must be so far outta touch compared to the rest of ya ..... I found the question from Iana a valid one ... and worthy of valid discussion .. and some actually seem to agree and have provided intelligent, adult comment. hmm

If ya think it is just all BULLSH!T ... then keep ya fingers outta ya brains and get orf the thread.

Cheers - John

Swoffer · Senior Member

rockylizard wrote:
Gday...
WTF .... sometimes this forum AMAZES me .... the absolute SH!T some bung up denigrating a post ..... I must be so far outta touch compared to the rest of ya ..... I found the question from Iana a valid one ... and worthy of valid discussion .. and some actually seem to agree and have provided intelligent, adult comment.
If ya think it is just all BULLSH!T ... then keep ya fingers outta ya brains and get orf the thread.
Cheers - John

Argh , come on John , keep it civil mate . Take the blue pill instead .

Mike Harding · Guru

17 posts from him in two hours and not a useful one among them.

Swoffer · Senior Member

Mike Harding wrote:
17 posts from him in two hours and not a useful one among them.

None nasty or derogatory either mike , all in jest . (and HIM has a name to Mike )

You stalking me Mike ?

Is there a Sunday morning post limit ?

Vince · Senior Member

rockylizard wrote:
Gday...
WTF .... sometimes this forum AMAZES me .... the absolute SH!T some bung up denigrating a post ..... I must be so far outta touch compared to the rest of ya ..... I found the question from Iana a valid one ... and worthy of valid discussion .. and some actually seem to agree and have provided intelligent, adult comment.
If ya think it is just all BULLSH!T ... then keep ya fingers outta ya brains and get orf the thread.
Cheers - John

Well written John

Can,t believe the negativity in some threads.

As to iana's OP - I have high ground clearance on my van and yes get a slight draw to passing trucks in the same direction. My thoughts are that the gap between the cab and trailer cause this "suck" effect as the truck passes. My thoughts on it.

Cheers

Vince

Yuglamron · Guru

My two Bobs worth. If you are out in the playground surely you see oncoming trucks move a little left coming towards you and hug the white line to minimise the "Venturi" effect when passing. Sometimes they don't if they will hit those whiteline potholes. Macka17 is correct, Hug the white line when a semi especially the big slabsided rigs come towards you. When they are overtaking the overtaking vehicle tries to stay as far away from you as possible for the same reason.

The Belmont Bear · Guru

I tend to agree John as I stated in my first post I was concerned that what I thought to be a legitimate question was expected to turn into a bun fight. I have theories on why certain things happen or don't happen, and on what I can do to improve the result. If someone with more experience or knowledge than I have can offer me advice on a better way to go then that's exactly the reason I joined the forum. Swoffer can I ask why have you have contributed 7 times to this post just to stir the pot - why are you so keen on seeing the thread closed ???

Swoffer · Senior Member

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 . hmm hmm

Fringe Dweller · Senior Member

No wonder why Good people Leave THIS forum.

Good Bye . PERMANENTLY this time.

iana · Guru

It was a legitimate question, but have been a member of this forum long enough to know what happens with questions of this type, I am surprised it has remained on track so far. A bit of humor mixed in can only make for interesting reading. Making jovial comments is far better than having the thread hi-jacked.
But I was surprised at the lack of effect by semis, but I have to be on my toes when there is a sideways strong wind, and I am cruising with trees along the road and a gap in the trees comes up. I imagined it all to be a lot worse.

Bryan · Guru

I think its called the venturi effect. I read somewhere a long time ago that the distance between train tracks couldn't be less than a certain distance apart as the coaches leaned in towards each other when they passed each other from the opposite direction.

Eaglemax · Guru

Glad this thread is still productive. Tugs and caravans have come a long way in most areas. Since the 1970's vans have become aerodynamic, much lighter and better balanced. Tugs have more rubber on the road, stiifer 4WD suspension and more power/torque to accelerate if a sway begins. But....all this has produced greater confidence in many vanners and their holiday has turned into a fast lane mentality they are trying to escape from. . It amazed us on our "lap" how we cruised at 90kph only to be overtaken by large caravan rigs at much higher speeds....maybe 110 or more. We just wondered, upon seeing an upturned van if the driver was driving as if he wasnt towing at all? . Just my thoughts.

Aus-Kiwi · Guru

Yes it's the pressure at first then the ventury effect when opposit then the turbulence after passing . The thing is not to exagerate any movement with your steering . If you swerve around the same time a semi or wind for that matter ! Chances are it's going to have twice the effect !! Plenty of wind gusts out in open territory too ! Like wtf was that !!

PeterD · Guru

There is an interesting paper written by Rob Caldwell MITE (Life) MAITPM (of Caldwell Consulting) - Truck wind forces on caravans - Read it and you will get a bit of an inkling on the forces involved.

RosieW · Guru

Not sure about vans, but when we added the canopy to the Triton, we noticed a difference - although not always.

macka17 · Guru

John.

Sorry mate.
Lengths . Weights and balance were a Taken.

IE. Safe. rig to start with.
Otherwise they shouldn't be there anyway hey.

Desert Dweller · Guru

We've travelled around 70,000kms to many areas in the last 5 years with our 17ft van with no WDH. We've met heaps of road trains along our way overtaking & coming at us with no worries. Our van has an underslung axle for slightly higher clearance. It could be argued that with a higher centre of gravity a higher clearance van could become unstable more easily.
There aren't many low slung vans around these days except for the lightweight Euro style vans.