UK Speed Limits

[Oldboy]

[PeterWJones]

Peter W Jones AMInstP

Speed limits are usually set by Governments, but in my view the laws of Physics dictate that the speed limits for trailers should be much lower than the norm.
Manufacturers are required to make known the “Stalling air speed” of each aircraft sold and the highest permissible side wind speed for a safe landing (this will produce a range of values depending on loading weights.)
Similarly I think trailer manufacturers should give the air speed at which trailers become liable to snaking and the side wind speed needed to set off the snaking.

For my small Swift Challenger 400 SE (1000kg) Caravan I think the relevant air speed is about 50mph and the side wind speed approx 30mph.

In Cornwall Uk tractor drivers can be fined very heavily if they do not pull over into the first available space to allow any traffic to overtake. A similar law should apply to trailer towers, universally.

I am a retired Head of Science in a Birmingham Comprehensive School who has had 32 years experience (on a recreational basis) of towing large and small caravans, luggage trailers and sailing cruisers.

I was allowed to renew my Associate Membership of the Institute of Physics in 2003, 10 years after I had retired, for the sole purpose of investigating trailer and caravan instability. The IoP do not of course take a view of my publications, but any member of the IoP could complain if they thought I was bringing the IoP into disrepute by the use of unsound physics. I started publishing on the web in early 2007 and had previously had a short paper published in Hansard. This can be seen by visiting www.parliament.uk and entering “Caravan and HGV Trailer Snaking Accidents” in their search engine.

If any complaints had been made to the IoP I would have been informed.

My basic premise is that the Caravan Clubs (the CC and the CCC) have not informed their members of the results of the Bath University (Department of Mechanical Engineering) Research on Trailer Aerodynamics or caravan friction based stabilisers.

A short section of the Fratilla 1994 Phd thesis concluded that friction based stabilisers could not be relied on to prevent caravan/trailer snaking. Calculations had been carried out using a computer model to reach this conclusion. Before I found out about this result I had carried out some rather crude calculations and had concluded that a friction based stabiliser could not possibly have any influence on a snaking trailer. The device is just incapable of producing enough friction to do the job. More recently I have devised and executed a practical test which proves the same thing.

In 2003 Bath University published further results on an ALKO friction based stabiliser. The conclusion was that the ALKO stabiliser did not improve the safety of a caravan under tow, as tested on a private road.

In 1999 Bath University published a further Phd thesis by Standen, “Towed Vehicle Aerodynamics.” At the time the CC magazine gave extensive cover of this event, but wrote virtually nothing concerning the results.

Standens’ wind tunnel tests concluded that an hgv or similar could destabilise a car/caravan combination. We all know this but it was good to have it confirmed.
I have experienced serious snaking (caused by HGV’s ) when towing correctly matched and loaded large and small caravans and a sailing cruiser, but I was lucky and managed to keep control.

In addition Standens’ wind tunnel tests showed that an aerofoil could improve the stability of a car/caravan combination. This result implies that the forward motion of the car/caravan combination must produce some aerodynamic lift which needs to countered by a down force to keep the trailer/caravan in better contact with the road.
Few people will be unaware of the fact that Formula 1 racing cars have been developing this idea for many years.
In addition evidence sent to me by a retired and very experienced Police Accident Investigator shows that when a caravan is snaking it only makes tyre marks on the road at the extremities of each swing confirming Standen’s result on the need for aerofoils.

Bath University were not asked to investigate caravan/trailer brakes ( by the caravan industry or the caravan clubs) so it would appear that the long standing refusal of the caravan/trailer industry in the UK to use electric brakes as in the USA and other countries, has no scientific validity. There have been no answers to the ideas I have advanced concerning the dangers of the over run braking system.

We now have a farcical situation in the UK; we cannot purchase electric brakes but can only buy ( at much greater cost) over run braked trailers which can then be converted to electronic/electric braking by the use of the ALKO ATC device, which has so far not been independently tested.

Notwithstanding all the above I have recently purchased a fairly new 26 foot long caravan to replace the lunar Delta 590 that I have had for 18 years. I feel confident that by having regard to all I have discovered I shall only make my Land Rover Discovery marginally more likely to be involved in an accident when towing as compared with solo use.


I shall be sticking to 50mph air speed with my Swift Challenger touring caravan until I can obtain tested reliable electric/electronic brakes and aerofoils. In addition I will not tow on a motor way if the wind forecast is over Force 6 and not tow at all if wind strength is over Force 8.

All the above and more is covered in detail in my 4 web blogs.
Those who wish to read further should start with
www.20six.co.uk/roadtrafficaccidents
The other 3 blogs can be reached by links from the “20six” site.
In particular some items on stabilisers will be found in
www.caravanaccidents3.wordpress.com
Scroll down to “Para 4. Stabilisers. Bath University.”

[FrequentFlyer]

That is a shame IMHO, sticking to 50mph. That's what causes problems of bunching etc., and holding others up, (And giving the rest of us a bad name.) Hopefully you at least stick to the inside lane, with horse boxes etc., so that I can pass you on the motorway at 60.

[mikealder]

Just out of interest (and you obviously know what you are doing with respect to this problem) - How do you actually measure 50MPH Wind Speed which will obviously be considerably different to wheel speed dependent upon the wind velocity and relative direction of the wind to your vehicle.
I am quite happy with the pitot/ static measurement system an aircraft uses but at 50MPH it is nigh on useless, and as road direction changes on a regular basis this will also affect measurement? - Mike

[DennisN]

[Mullet]

In a seated position through the window at speed? is that possible?

[DennisN]

Depends how good you were in the bike sheds at school.
_________________
Dennis

If it tastes good - it's fattening.

Two of them are obesiting!!

[Mullet]

..err, about your van Dennis. What colour did you say it was? and is it the same on both sides?
Just enquiring so I can keep my distance

[mikealder]

BEHAVE, I would genuinely like to know how you are measuring a vehicles speed accurately at 50MPH relative to wind speed, if nothing else than to satisfy my natural curiosity - I could build a system to do this but I wouldn't trust it at such low velocity - Mike

[Mullet]

Apologies.
A rig employing a doppler type anemometer? I think the airflow monitoring used at my workplace uses this type.
Occasionally I use a windmill type anemometer to record the varying low velocity "wind speeds" accurately to just a few m/s.

[DennisN]

I think there might be a clue starting at post #7 in THIS THREAD. Post #32 seems quite informative.
_________________
Dennis

If it tastes good - it's fattening.

Two of them are obesiting!!

[PeterWJones]

From Peter W Jones

UK Speed limits

My safe air speed for a caravan of 4m body length weighing 1000kg is an estimate.
I can only estimate wind speed , but the estimate is based on the forecasts.
The wind speed is then added to the road speed to give the air speed.

I am only certain that my air speed is 50mph when there is zero wind.

We must take this matter more seriously and use air speed indicators in towing vehicles, whether they are cars, hgv’s or railway engines.

You will see in my blogs that I have written about the 5 fatalities and 35 serious injuries in a Japanese rail crash ( at only 110 kph) where it has been accepted that a strong side wind blew it off the track. The rail company concerned are in trouble over the insufficient number of wind speed sensors at the side of the track. In the UK, as far as I know, we have none and on motorways we have “wind socks” to indicate the wind strength!!
It would be fairly cheap, if mass produced, to have electronic air speed indicators in cars which would give drivers their forward air speed as well as the effective side wind speed. I have discussed this with a Physicist who is actively engaged in designing and manufacturing electronic devices for the military, so please do not dismiss this suggestion lightly if you wish to tow at over 50mph air speed in some degree of safety.

Safety would be improved if drivers pulling trailers not only had advance warning of actual wind speeds ( from sensors at the side of the road) but an accurate indication of the true wind speed acting on their trailers. I do not think this precaution necessary if driving without a trailer because when you are sitting in the vehicle you should be able to sense the effect of the wind and take appropriate action. When towing on the road one only finds out about the effect of the wind on the trailer when it starts to snake, and this may be too late.

The function of an aerofoil is to produce down force and so increase friction between tyres and the road. This will make it harder for the wind to push the van/trailer sideways and set up a snake, but there will always be a speed limit over which you should not tow. Similar “theories” have been turned into “law” for the aircraft industry. This is one reason why flying is safer than road travel. There are regulations against landing an aircraft on a certain runway if the side wind component for a particular aircraft exceeds the manufacturers’ stated value. This is one of the reasons for many diversions to air ports you may not wish to land at, but I doubt whether the public are always told the true reason for the expensive change of route.



Peter W Jones
www.caravanaccidents.wordpress.com

[GPS_fan]

Apart from anything else, surely the 'safe speed' for a given caravan will not only change with each bend in the road, but will also differ greatly depending upon the vehicle behind which it is being towed and the change in aerodynamics caused thereby.

The aerodynamics of a caravan will be completely different if it is being towed by a Volvo estate or a Land Rover Discovery, for example - so much so that the characteristics will surely be like two totally different caravans being towed.

Are you saying that a safe speed must be calculated for each individual caravan and that this has to be determined for every vehicle likely to be towing it and the different air flow across a caravan as the vehicle immediately in front of the caravan changes?
_________________
Andy
PocketGPSWorld.com supports Help for Heroes - Read here

[PeterWJones]

UK Speed Limits

Peter W Jones

The conditions on the road change considerably and often. This is one of the reasons we need air speed indicators in the tow vehicles.

I have emphasised in my blogs that I have had my Land Rover Discovery for the last 12 years and my estimate of safe air speed for a trailer is based on that experience.

As Andy correctly points out different types of vehicle have a different aerodynamic influence on the towed vehicle and there would be a need to have safe air speeds for the various types of tow vehicle suitable for pulling a particular high aspect trailer.

I will answer any further questions ASAP, but there will be a delay as for a considerable time I will be returning to “retirement” mode and will have great difficulty obtaining access to the internet whilst caravanning or sailing.

[M8TJT]

I suspect that we will be talking about Mach numbers, track vs. heading, relative humidity, air pressure, air temperature, the coeficient of friction between various road surfaces and tyres etc. soon. As it seem far too hazadous to tow a caravan, this all sounds far too complicated for me. The easy answer is 'don't tow a caravan' and keep a lot of other road users happy.

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