Peter W Jones AMInstP

As I have noticed that monitoring for wind speed at critical points on our major roads is only done with pre ww2 wind socks, I will start by trying to establish why it is vital that this facility be improved. Every sailing cruiser only slightly larger than my Jaguar 21 (with 2 crew weight = 1000kg ) seems to be equipped with an anemometer giving a digital read out of wind speed in front of the person steering.

We need these devices on our major roads wired to traffic control centres so that Police can give out on the indicator boards actual wind speeds so that drivers can act on their own initiative as appropriate for their own vehicles and trailers, instead of just being told (for instance) that there is a speed restriction of 40 mph due to strong winds.

My small 4m body length caravan weighing 1000kg is susceptible to snaking when its air speed exceeds 50 mph and the side wind component exceeds 30 mph. I need to know therefore if the side wind component at a particularly exposed section of road exceeds 30 mph. I have already explained in other parts of my blogs how I arrived at these values, partly using my experience gained on open waters with sailing cruisers.

I started looking into trailer safety in 2003 when one of my daughters narrowly escaped death or serious injury when her (brand new) snaking caravan somersaulted over the roof of her car (when she stopped) due in my opinion to the failure of the over run brake actuating mechanism of the caravan. I reactivated my associate membership of the Institute of Physics (AMInstP) for the sole purpose of conducting the enquiry. As I have explained elsewhere in these blogs I soon discovered that the Caravan industry and the caravan clubs have a very flawed policy on caravan safety, but when I also realised that there was a lack of such obvious provision as anemometers to monitor wind speeds on major roads I concluded that a situation of this type could only have arisen if the most Senior Executives who control the Road Haulage Industry were involved. The Industry is supported by extensive lobbying at Westminster and if the Senior Executives really wished wind speeds to be measured accurately at vulnerable positions on our major roads, it would have been done a long time ago.

Similarly with caravans; the two MP’s who are paid to lobby by the Caravan Club could have made an impact if the club had not had, in my opinion, such a flawed safety policy.

In an e mail to myself from the caravan club Technical Manager ( 2003) sent on the instructions of the Executive Secretary after I had spoken about related matters at a large CC meeting, I was told that snaking only takes place if the caravan is incorrectly loaded (for instance).

In the latest 2007/8 CC members’ Handbook on page 622 the club now qualifies this position slightly by saying that a cross wind may affect the situation when one is being overtaken by a coach or large lorry. On page 625 we are told, “If in doubt about the advisability of towing in stormy or windy conditions, let common sense prevail. Telephone a motoring organisation or Weathercall to check.”

As I have previously pointed out for the weather forecast to be of any use one has to know the critical speeds for a particular trailer.

All vehicles are subject to be being blown off course, or even off the road, in certain conditions.

If the weather forecast is for a wind of 60 mph one cannot rely on there not being a Hurricane force gust in a particularly exposed position.

My 2000kg Land Rover Discovery is particularly prone to drifting in gale force cross winds (about 44mph), but when I am not towing the matter is usually easily rectified by steering modifications, followed by a drastic reduction in speed.

As there are notices on the back of a large number of HGV’s inviting the public to phone a particular number to report any drivers seen to be driving badly, I can only assume that the industry thinks bad driving is the main cause of snaking/jack knifing and further wonder if drivers are being given similar advice to that given by the Caravan Club.

I have previously pointed out that vehicle manufacturers ( as with air craft manufacturers) should be required to establish critical wind speeds with some degree of accuracy for each type of vehicle sold.

I have observed that road users are increasingly prepared to pay for extra safety devices such as electronic brakes and air bags, and feel certain that when they understand why knowledge of air speeds and side wind components is important for safety reasons, they will be prepared to pay

extra for vehicles that are sold with the relevant information in the owner’s handbook.

As I have never driven an HGV or sailed a boat larger than my daughter’s 29ft Westerly (about 4000kg), my estimates for HGV critical air speeds and side wind components would not be very accurate. I am therefore pointing out how to make estimates so that those with relevant towing experience can do this for themselves.

I can be quite accurate concerning the fact that in the USA (for instance) when a Hurricane force wind is expected those able to do so move out of the immediate coastal area until the danger has passed. The remainder of the population go into buildings constructed so that they can withstand Hurricane force winds.

According to the Beaufort Wind Scale for Mariners a Hurricane is Force 12 ( about 78 mph) and above. If you are driving at an indicated 55 mph it will only take a head wind of 23 mph to bring the air speed that the vehicle/trailer is subjected to up to the bottom end of the dreaded Hurricane strength. The aerodynamics of trailers in particular needs to be taken very seriously.

In 1999 Bath University published “Towed Vehicle Aerodynamics” by Standen; the wind tunnel tests had proved that aerofoils to create down force as speed increases improved the stability of a caravan with a (approximately) central axle.

Aerodynamically a HGV trailer with approximately central axles/axle is the same as a caravan.

Only a minority of HGV’s tow trailers with central axles, but there will also be the evidence of tyre marks on the road from all snaking accidents which will be similar to those which have been “leaked” to me concerning snaking caravan accidents; I am sure they will show similar results to those I have outlined for caravans.

The above Bath university results clearly stated that some trailer snaking is caused by the wind and explained how this happens.

As I feel that I have established that the maximum safe air speed for my small caravan is about 50 mph, and this is the stalling speed for a typical light aircraft of the same weight, I feel that I am justified in saying that the maximum safe air speed for a central axle HGV trailer is VERY ROUGHLY the same as the stalling speed of an aircraft of the same weight. In the absence at the moment of accurate figures from people with access to instrumentation that can measure the amount of lift created by the air under the vehicles, I suggest that HGV drivers cultivate the acquaintance of an airline pilot and try and obtain some confidential advice concerning stalling speeds to compare with their own experiences of the air speeds and side wind speeds when snaking/jack knifing has taken place. Starting about 30 years ago I always managed to avoid wind induced snaking by obtaining information from Radio 4 shipping forecasts and estimating the likely speeds inland. More recently I have been able to get a very good wind speed forecast from the BBC weather forecast on the web.

As I started towing when I purchased my first sailing cruiser I was always very aware of the power of the wind and those who doubt this should talk to people who sail some of the types of boat I list below. These sailors will be only too pleased to help as the relevant information concerning sailing cruisers is not confidential. I want the sailing community to tell people the area of sail they use to sail their boats in gale force winds (about 44mph) in sheltered waters where the sea is calm. If HGV drivers then compare the relevant sail area with the area of the side of their trailer of the same weight as a sailing cruiser, they will then understand why something needs to be done to improve road safety by using electronic braking systems and aerofoils to create down force on all trailers as well as paying greater attention to weather forecasts.

Aerofoils for HGV trailers would have to be quite large, and as this matter was not allowed for when bridges were built engineers will have a difficult problem to solve when designing the aerofoils for large trailers. For those who can afford the million euro VW sports car this problem has been solved by a computer controlled aerofoil over the rear wheels ( but I would not suggest that the latter is a good tow car!) Formula 1 racing cars, after years of development, now have two fixed aerofoils, one between the front wheels and one over the rear wheels. Rally cars are beginning to use fixed aerofoils and when the latter are above the level of the car roof they must be very effective.

A few Statistics:-

SAILING CRUISERS

Nicholson 55 (Weight 17.3 metric tonnes; area of main sail = 48.3 square metres ); Nicholson 42 (wt 10.16 tonnes)

Bowman 57 ( wt 19.05 tonnes); Ocean 71 (29.06 tonnes);

Source:- Boating World Guide to Sailing Cruisers; 1976

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PS; 25-03-08

"Morning Cloud"

The above was once the best known boat name at Westminster as it belonged to the Prime Minister. If Ted Heath had lived a little longer I may even have managed to secure the interest of a Conservative MP  as I am sure that his "racing yacht" must have been comparable in weight with an HGV trailer!

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AIR LINERS

Boeing 737; weight 45.36 tonnes; take off air speed 150 mph.

My estimate of stalling air speed = 120 mph.

Fairchild Aviation C123B

Weight, 27 tonnes.  Stalling air speed, 96mph

Based on the above I would therefore estimate that a 27 tonne HGV trailer (if such a trailer is legal and exists) with approximately central axles would become liable to snaking when the air speed reached 96mph. The driver would be aware of the air speed if he/she had an appropriate indicator.

At 55mph road speed one only needs a head wind component of 41 mph (gale force is about 44 mph) to reach the estimated critical speed.

Tiger Moth Biplane.

As one of my main recreational activties over the last 32 years has been estuary and coastal sailing, I have never had the usual experience of flying off to wamer climates for my holidays.

I have only taken to the air twice, and that was as a passenger in a Tiger Moth Biplane in 1951. During the Summer holidays HMG "called me up" again to do an additional two weeks National Service in the RAF. During my 18 months national service I was an "Audiometrician," so it was deemed appropriate that I did some flying.

In 2003 when I started enquiring about the reasons for caravans snaking I recalled that the Tiger Moth take off speed was about 60 mph and that it must have weighed about the same as my small caravan.

You will see from my blogs that my memory had not failed as similar sized modern light aircraft have comparable statistics.