Peter W Jones  MInstP 

 

The above left photo and large item below depicts a media ad for an electric caravan mover.

I show this because I did not have a photograph of my other current caravan.

My caravan  is on the right above. I have owned it  for about 16 years.

It is a Lunar Delta 590 Twin Axle.

I had one serious snake with this van when I was travelling at about 50 mph in a 50 mph road works on the M5. I was caught unawares when a large HGV passed at a highly illegal speeed. ie the bow wave caused the snaking.

It is a terrible situation to face as the slightest application of the car brakes may have a dangerous effect on the over run caravan brakes that only work if the van catches up with the  car.

As I have explained at length elsewhere in these blogs if the van catches up with the tow car when the former is not in a dead straight line with the latter, the tow car could be spun round.

  I have my car radio on for “traffic announcements” when on holiday in the West Country and hardly a day goes by without a report of an over turned caravan holding up traffic. These only count for “accident statistics” if a person is killed or seriously injured.  As other road users can view the build up of a serious snake and take avoiding action the incidence of injuries will be much lower than for most other types of accident.

Edited 30-09-09

 

 

This is a media ad for an ALKO stabiliser. I show the above because it depicts the spare wheel in place just to the rear of the axle. 

My Swift touring van has a recessed position in the front locker compartment for the spare wheel, but to get the weight on the towing hitch down to the max 75kg permitted by Land Rover for my Discovery I had to relocate my spare wheel as shown above. This is a common problem, even with some current new caravans.

 A great many people will be sceptical concerning the use of trailer brakes to conrol snaking. However, I did a similar thing in the mid 1950’s.

The above BSA 650cc twin cylinder motor cycle and side car had an independently operated cable  control to the drum brake on each wheel.

To negotiate a right hand bend I often had to apply the front brake only, and for a left hand bend, particularly with a well loaded side car, it was often necessary to brake the side car only.

For normal straight line braking all three brakes could be applied at the same time.

It is now not legal for the driver to brake a trailer independently of the tow car, but the law does allow this to be done  electronically. This cannot be done with over run brakes (without an electric/electronic modification), but it is possible if the trailer brakes have  pneumatic, hydraulic or electric control.

Bath University have shown in wind tunnel tests that aerofoils producing downforce improve caravan stability, and if independent trailer braking is electronically  used as well, the maximum safe speeds I have estimated can be increased.

PARA 601; DIAGRAMS; SNAKING CARAVANS

These diagrams are intended to amplify my explanation of the marks made on the road in a typical caravan snaking accident.  See  www.20six.co.uk/roadtrafficaccidents  (paragraph 17a).

I would postulate that something very similar will be found after snaking/jack knifing accidents with HGV trailers with approximately central axles.

PARA 602; VECTOR DIAGRAM; EFFECT OF WIND ON CARAVAN

I read in Standen’s 1999 thesis that when the wind is at a slight angle to the direction of travel a trailer will try to “line up with the wind” as is the case with a weather  vane. However, in actual practise he pointed out  that the trailer will swing further than this only stopping when the force of the wind is insufficient to maintain the position; the trailer will then swing back so setting up “snaking.”

It can be seen from my diagram that a side wind of 30mph and a head wind of 40 mph (due to the forward motion of the tow car/caravan)  will give a resultant “apparent” wind speed of 50mph in the direction “R” shown in the digram.

ANEMOMETERS

I can use the above diagram to explain how my suggestion for revised software for the gill/biral anemometers will operate.

If R represents the apparent wind direction and speed of 50 mph as recorded by the anemometer fixed to a tow vehicle it can be seen from the diagram that the air speed of the vehicle is 40mph and the side wind component is 30 mph. The diagram above is drawn to scale using graph paper.

This method cannot be used for automatic calculation by a computer but the values obtained from the scale drawing could be calculated using mathematical formula and these could be programmed in so that the air speed and side wind component would appear on the anemometer display.

It must be remembered that a wind speed of 50 mph is fairly common. The met office will certainly not be putting out any warnings of buildings being damaged in these circumstances, but if your tow vehicle was equipped with an anemometer you can see from the above that you would have an air speed of 40 mph whilst parked.

It will be some time before the manufacturers of trailers will admit that there is a serious problem and follow the practise of the air craft industry, so in the meantime we can only judge for ourselves the critical air speed at which our trailer is likely to snake, and the minimum side wind component that will cause snaking.

 

 

 

PARA 603; OVER RUN BRAKES

The top diagram above shows the result of an over run brake telescopic section having had a sideways knock in a collision with  a large vehicle when the trailer was stored.

The arm has been stretched and compressed on opposite sides and the elastic deformation is permanent. In the second diagram above the tow car is out  of line with the trailer. If it is necessary to apply the tow car brakes in this situation the trailer will try to continue in the direction shown and in so doing exert a force on the telescopic arm which will tend to temporarily deform its shape very slightly ( as shown in the extreme case in the first diagram), with the result that the arm will not slide into the housing to apply the brakes. I have also given additional reasons (in  other paragraphs) which also prevent the brakes being applied in this situation.

With electric pneumatic or hydraulic actuating mechanisms the brakes come on when the tow car driver depresses the brake pedal and so very much reduce the possability of the trailer jack knifing.

The above diagram shows the detail of the over run brake actuating mechanism of the pre 1960 caravan shown below.

2 = the hand brake (but the ratchet is not shown).

3=  the hand brake pivot.

4= the rod leading to the brakes.

5 = the telescopic housing.

6 = telescopic arm.

7  = the housing which fits over the tow car tow ball.

8 =  release handle for above housing.

9 = washer.

10 = spring to absorb shock of trailer running into tow car.

11 = collar

In the 1960’s  the mechanism was clearly visible but although on the latest caravans  in the class 750kg to 3500kg the mechanism has been covered up the essential details have remained unchanged, except that a hydraulic damper has replaced the spring.

see

www.20six.co.uk/roadtrafficaccidents

para 10ci

for further details of the case against over run brakes.

PARA 604; A 1950’s DONNINGTON CASTLE CARAVAN

 

 The above photograph shows our family holiday caravan for the period 1965 to 1985. (the car belonged to a visitor). The caravan was a single axle Donnington Castle of body size about 28ft x 8ft.  It was delivered by the dealer using the only 4×4 made at Longbridge (an Austin Gypsy). This was quite illegal. I only had this van moved with a transporter.

Compared with this caravan the only technological advance concerned with towing  that we now have (apart from the hydraulic damper for the over run brakes) is that the tyre inner tubes have been replaced and we now have tubeless tyres. (ADDENDUM August 2007; ALKO have announced that they will start fitting their new electronic/electric stabiliser to complement the over run brake system on new caravans. However they could import the electric brakes that have been in use for over 30 years in the USA, and at current rates of exchange the electric brake system would be cheaper than the over run system plus stabiliser.