I agree with Dustydog. Of course, I notice it more because my outfit is over 12m, the caravan is a single axle, and I’m running on a weight ratio of close to 100%.
We were fairly new to caravans when we had a severe snake. We had almost finished overtaking a HGV on the M5 between Jn12 and Jn13 and were about to pull in. In those days no ATC and the car was a lot heavier than the caravan.
Suddenly the outfit started snaking across 3 lanes. I lifted my foot off the accelerator, but did not brake as the outfit slowed to probably about 50mph I gave the accelerator a slight nudge and this pulled the outfit straight again. At the time we had no idea what caused such a bad and unexpected snake.
Subsequent trips on the same stretch showed up bad "tramlines". After reading it on them I came to the conclusion that the snake was a result of pulling in and catching the tramline and also perhaps the effect of the buffer in front of the truck on the rear of the caravan. We have never experienced a snake again as now very vigilant to tramlines etc when overtaking.
Suddenly the outfit started snaking across 3 lanes. I lifted my foot off the accelerator, but did not brake as the outfit slowed to probably about 50mph I gave the accelerator a slight nudge and this pulled the outfit straight again. At the time we had no idea what caused such a bad and unexpected snake.
Subsequent trips on the same stretch showed up bad "tramlines". After reading it on them I came to the conclusion that the snake was a result of pulling in and catching the tramline and also perhaps the effect of the buffer in front of the truck on the rear of the caravan. We have never experienced a snake again as now very vigilant to tramlines etc when overtaking.
The effect of the van running in the surface grooves such as the tram lines created in tarmac, where the van is "held" non aligned to the tow car, then gets suddenly pulled out because the side force is too high, can certainly result in the unit getting an initial knock sideways. That and its direct opposite where the van suddenly "finds" and locks into a tack whilst passing over it can be the disturbance needed to excite a unit's instability.
Also where a passing vehicle's bow wave, the vehicle either being passed or passing you progressively travels along your vans huge side area can cause issues. This bow wave acting on the area of the van clearly creates a deflecting side force and that because of the difference in vehicle speeds transverses along your van's side. Here one critical element is it passes the van's polar rotation centre, be that a single axle or the "centre" of the twin axle's, so the "sign" changes creating a disturbing "yaw". The relative speeds involved affecting the also very critical frequency of that initiating nudge. Even without your unit becoming unstable, its handling is inevitably being influenced while this occurs, again with the dynamic of that influence changing as the side force moves along the van's flanks.
Some of the bigger "HGV rigs" seem to have for whatever reason* done something where they affect us less than a way smaller "Luton" van conversion, which also tend to travel way faster, adding to the hassle they cause us.
* less bow wave equals less drag, less fuel needed, a massive motivating aspect of HGV design.
Also where a passing vehicle's bow wave, the vehicle either being passed or passing you progressively travels along your vans huge side area can cause issues. This bow wave acting on the area of the van clearly creates a deflecting side force and that because of the difference in vehicle speeds transverses along your van's side. Here one critical element is it passes the van's polar rotation centre, be that a single axle or the "centre" of the twin axle's, so the "sign" changes creating a disturbing "yaw". The relative speeds involved affecting the also very critical frequency of that initiating nudge. Even without your unit becoming unstable, its handling is inevitably being influenced while this occurs, again with the dynamic of that influence changing as the side force moves along the van's flanks.
Some of the bigger "HGV rigs" seem to have for whatever reason* done something where they affect us less than a way smaller "Luton" van conversion, which also tend to travel way faster, adding to the hassle they cause us.
* less bow wave equals less drag, less fuel needed, a massive motivating aspect of HGV design.
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The "tramlines" have another unwanted effect - the track width of an artic trailer's tyres is wider than a caravan's tyres' track width and there's a tendency to oscillate from one across to the other - that unwanted oscillation can itself trigger a snake or can contribute to the effects of bow waves from passing trucks/vans/coaches.
Fortunately the various rebuilds of our motorways have put more substantial foundations in, lengthening the roadworks time, and reduced the tramline issue but not eliminated it altogether.
It's dependent on length. )ver 12 m they are limited to 60mph ( unless a lower limit applies) .
So the same as any HGV....
Not sure about that as travelling at about 58mph I have been over taken by coaches that have quickly disappeared into the distance. Definitely over 12m in length.
Me too - however ....
https://www.gov.uk/speed-limits
| Buses, coaches and minibuses (more than 12 metres overall length) | 50 (80) | 60 (96) | 60 (96) |
|---|
Legally their speed limit is 60 mph - but they're fitted with a 90 kph (56 mph) limiter.
Probably related to the 90 km/h limit in some parts of the EU, where they might visit?
HGV been limited to 56 for years on the cruise control. But when Paddy or Mick is driving that's a different story. They used to pull the fuse on the cruise and could fairly motor on. Only downside to that is it disconnects the speedometer as well.
I realise now that it came into being in 2015 when the A road limit was raised from 40mph to 50mph. But at that time Scotland remained at 56 mph. Where they are now is anyone’s guess. 🤔
AFAIK HGVs have been able to do 60 legally on motorways for much longer than that - when the A-road limit for HGVs was raised from 40 to 50 in England and Wales, it remained at 40 in Scotland - but they trialled 50 on the A9 between Perth and Inverness which I believe is now a permanent exception.
Somewhere deep in the legislation, UK speed limits have to be in multiples of 10 on public roads - but physical limiters are usually set to the next lowest kph multiple of 10 - private roads and works sites often have speed limits ending in 5.
I seem to recall that quite long sections of the A9 were 50mph, and latterly had speed cameras and unmarked “Polis” cars in service. Made for much nicer towing.
The Perth-Inverness part of the A9 is about 120 miles with about 90 miles of single carriageway - this is where the trial and subsequent exception are located - the speed cameras were an integral part of the trial and retained afterwards.
It does seem to have reduced casualties by increasing the "convoy" speed from 45 to 55 mph and reducing the temptation for stupid overtakes - although horrific crashes still occur at the flat junctions, hence the political wish to dual the whole section.
It's an aerodynamic effect know as the Bernoulli effect. You can demonstrate it at home. Take two pieces of A4 paper. Hold one in each hand, gripped between thumb and forefinger in the centre of the short edge.My thoughts are that as HGV is travelling it is creating a "buffer" zone at the front of it. This "buffer" pushes out a vehicle overtaking. The driver of the over taking vehicle feels the vehicle being "pushed" and countermands this by pushing against it using the steering wheel. A natural reflex and not really noticeable.
When they pass the "buffer" zone area created by the front of the HGV, the driver is still "pushing" against the steering wheel which then causes the over taking vehicle to feel as if it is being sucked in when in reality they are not being sucked in.
Just my thoughts and probably wrong.
Hold them so they dangle downwards about 5cm to 10cm apart. place your lips between your thumbs, and blow down between them steadily but firmly. You will notice the paper draws together at the bottom.
When you stop blowing, the paper will separate.
This is the same effect you find on the road. The side of your caravan and the HGV trailer are the bits of paper, the air passing between them, your breath...
Regardless of why and how it happens, it happens!
What is important is how to best avoid or mitigate it, and that is to first expect the effect to occur, and to perhaps recognise the vehicle with that have th e biggest effect. Then to avoid it if possible by not attempting to overtake unless its necessary, or to mitigate the effect by maximising the distance between the vehicles before the overtake occurs. and during the manoeuvre to avoid taking action that might make th e situation worse.
What is important is how to best avoid or mitigate it, and that is to first expect the effect to occur, and to perhaps recognise the vehicle with that have th e biggest effect. Then to avoid it if possible by not attempting to overtake unless its necessary, or to mitigate the effect by maximising the distance between the vehicles before the overtake occurs. and during the manoeuvre to avoid taking action that might make th e situation worse.
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