SECTION  3  STABILISERS FOR CARAVANS AND TRAILERS 

Old type of leaf stabiliser

See text below for explanation of above dagram

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Peter W Jones MInstP

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GCE “A” level/GCSE Maths/Physics

A Caravan/Trailer Stabiliser Test

 

 

 

 

Although the position of the centre of mass of a caravan/trailer can be ascertained approximately for towing purposes quite easily, for a Scientific test to have “academic” respectability the CoM position needs to be found by calculation. This procedure would fit very well into the course work for “A” level Maths (Theoretical Mechanics).

Calculation of the distance of the centre of mass of a caravan/ trailer from the tow hitch.

F= maximum allowed weight on tow hitch ( see tow cars’ handbook for drivers)

d= distance of tow hitch from axle ( or the mid point between the axles)

x= distance of centre of mass from axle (or mid point between axles)

W= weight of caravan/trailer

Using the Principle of Moments, for equilibrium :-

Fd = Wx

 

x = Fd/W

ie x = F multiplied by d divided by W

 

It is then necessary to construct a lever of length = d- x

For a small 4m body length caravan this came out to be 3.33 m

The construction of the lever is quite a lengthy procedure, particularly as the old type of “leaf” stabilisers need one type of lever and the more modern ALKO stabilisers need a different type made from a length of tubular steel.

Furthermore a tow car is needed to fix the stabiliser in place and it would be a good idea to have at least one caravan available so that the lever could be seen in place at the side of the caravan.

It can be seen therefore that this exercise needs a joint effort between adjacent schools/colleges with the proceedings being recorded on DVD for later perusal in the lecture room. The costs of this experiment could easily be recouped by the sale of copies of the DVD as long as purchasers agreed that they would only copy the DVD for their own internal school/college use.

 

To test the ALKO stabiliser (fixed permanently to the caravan draw bar,) simply remove same from the caravan draw bar.

It is usually only held in place by two large bolts and nuts.

In the case of the small caravan for which the calculation has been carried out, the tubular steel would have to be 3.33m long.

Whether using wood or steel, the lever weight is considerable and the end away from the stabiliser needs supporting on wheels (furniture castors are ideal if your car park has a good surface).

Fix the stabiliser to the car in the normal way. You should find that you can easily move the stabiliser from side to side by gripping the long lever between finger and thumb 3.33 meters from the tow ball. Even this test over estimates the effectiveness of the stabiliser as there is also the friction between the ground and the castor wheels.

Do you think the stabiliser is a good means of reducing snaking?

 

Before carrying out the test check that the force of friction that the stabiliser can exert is the same as stated in the Haynes Caravan manual.

Alternatively Caravan Club Members can gain access to special “Technical Advice” at www.caravanclub.co.uk

Several pages of advice on stabilisers will be found which includes the specified force needed to turn the stabiliser arm for all types available except the ALKO, which can not in any case be adjusted.

The force meters usually available in science laboratories may be difficult to connect to the stabiliser arm. Caravanners use Bath Room Scales. Students who know their own weight can check the accuracy of the scales used. If a compression type of force meter is available this can also be used to check the bath room scales.

 

The photograph above shows one of the old type of leaf stabilisers.

The friction plate adjusting nut is clearly visible to the right of the tow ball.

Similar stabilisers will still be found for sale in caravan and motor spares shops.

Which type of stabiliser has the greatest amount of frictional force

(or torque) to act against the snaking trailer/caravan?

How can a tow car driver reduce the possibility of an HGV bow wave setting off a caravan/trailer snake?

Why do think that snakes produced when overtaking an HGV are much more severe than those produced when the caravan/trailer is over taken?

It may happen that a slight snake could be set up by one over taking HGV and a following HGV’s bow wave may actually “kill” the first snake. Explain the theory of this event.

How can a wind induced snake be avoided ?

All answers to the above can be found by reading the worksheets on trailer snaking for GCSE Physics.

 

Bath University on Stabilisers

See

www.caravanaccidents3.wordpress.com 

Scroll down to paragraph 36

 

Do you think the Government (The DfT) should warn the public concerning the lack of reliability of these stabilisers?

Explain your reasons for reaching a decision.

Describe how you came to see the lever test demonstrated.

 

For further advice on GCSE Physics see

www.schoolminibusaccidents.wordpress.com

NB

Note that the above work may be covered at GCSE Physics level if triple science is studied.

It may then also appear in an “A” level Maths course with more sophisticated calculations.

All the Physics items covered could be in either GCSE or “A” level depending on whether triple or double Science is studied for GCSE.

 

 

 

 

 

 

SECTION 4  FURTHER ITEMS FOR GCE ‘A’ LEVEL 

Suggested activities for students of GCE “A” level Physics

(1) Trailers up to 750 kg with no brakes

Advanced Level Physics by Nelkon and Parker (1958 )

Page 12 Newton’s First Law of Motion. (1686)

“Every body continues in its state of rest or uniform motion in a straight line, unless impressed forces act on it.”

Discuss the advisability of repealing the current UK law allowing trailers up to 750 kg to travel at up to 60 mph without brakes. Is it advisable to incorporate Newton’s First Law into UK law?

To be less obtuse; does it make sense to put belts round driver and passengers and tow a trailer with no brakes?

(2)

See

www.20six.co.uk/roadtrafficaccidents

 

 

 Paragraphs 11d to 12c

Pick one or two of the following items and use your knowledge of Physics/Maths to explain whether you support or oppose the writers’ conclusions.

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(a) Resonance and Snaking Caravans.

(b) Longer trailers are more prone to snaking.

(C) There is no snaking effect on a caravan/trailer when it passes an HGV going in the opposite direction on a single carriageway road.

(d) Sudden deflation of a caravan/trailer tyre.

(e) Kinetic Energy and Gravity.

(f) Over run Brakes

Note Paragraphs 10cii and 12a of “20six” “The Case Against Over Run Brakes.”

     See also www.caravanaccidents2.wordpress.com

See the photograph of a typical luggage trailer and notes on use of same.

(g) Snaking and “centrifugal force.”

(h) See para 10ci of “20six”

“Evidence from snaking tyre marks”

Do you think that the fact that there are only tyre marks made on the road during snaking at the outside of each swing is evidence pointing towards “aerodynamic lift ” being a factor in caravan snaking?

(i) See para 12c of “20six” above for an alternative approach to investigating over run brakes and stabilisers. I used these approximations  before I became aware of the Bath University research or I devised my simple stabiliser test. It would be helpful if some one else examined this method and expressed an opinion as to whether it has any validity.

(j)  See para 17b of “20six” on overloading of caravans (and trailers).

 

 (k) The Vetrui Effect.

        This can, in certain circumstances, over ride the bow wave  effect of an overtaking coach/HGV.

See www.20six.co.uk/roadtrafficaccidents     Paragraph 11b

 

 

 

 

(3) Elementary Theory of Flight.

Although I have never seen the above topic included in studies for either GCSE, GCE “O” or “A” level Physics, it often appears in the more advanced textbooks used by students. In addition, this topic could be treated as a Maths problem only and would then possibly provide additional follow up work to items covered in either GCSE or “A” level Maths.

I have a copy of “Higher Physics” by E Nightingale  MSc (Vict),  FInstP, ARCO. (1959) but I am certain similar work will be found in current books and in reliable sites “on line” which I am sure current studens will very rapidly find and put to good use.

Due to the number of erroneous formula and accounts quoted by caravan industry executives, employees and ordinary caravanners (all writing under pseudonyms) on the internet, I must set the record straight by quoting from Nightingale.

Page 46.

“Application of Mechanical Principles to Flight. ( by E Nightingale)

In order to understand, in a simple way, the forces involved in flight, let us consider AB ( see diagram below) to be the section of a plane surface travelling through the air towards the left. The pulling force T, called the thrust, has to overcome the resistance or drag D caused by the air. The force of the air against AB also has a vertical component which is responsible for part at least of the lift, L, which, if sufficiently strong to overpower the weight, W, of AB, causes the plane surface to rise.

In level flight at constant speed it is evident that (a) L=W, (b) T=D.

If the resultant pulling force T-D > 0, acceleration will result.

If L-W > 0 the plane will rise.”

END OF QUOTATION

As we are interested in staying in touch with the ground to stay safe on the road L – W must be < 0.

 

 Elementary Theory of Flight (cont)

Nightingale then goes on to describe how he used apparatus supplied by Philip Harris and Co Ltd to deduce experimentally that the drag and lift were proportional to the square of the wind speed and also point out that this result had been confirmed by extensive investigations.

Furthermore Nightingale also showed that when the angle of the wing to the horizontal was about 15 degrees the streamline flow over the upper wing surface was changed to turbulence and the lift due to reduction in air pressure became non existent.

Whilst it is self evident that formula 1 racing cars have made good use of aerofoils to create down force and keep themselves in better contact with the road, it seems to me that it may be preferable that all large road vehicles, particularly trailers, could be made safer if turbulence could be induced over their upper surfaces to reduce the aerodynamic lift. This may be preferable to creating down force using aerofoils.

Reducing pressure underneath vehicles by the use of plastic skirts to increase air speed has been carried out successfully in some cases, but it is a non starter for trailers with central axles as they can not be relied on to stay horizontal.

The issue of speed on our roads has become  very controversial  as many claim that they can drive safely at high speeds.

I pointed out some time ago that the amount of energy that a moving vehicle has by virtue of its speed is proportional to the square of the speed. This energy has to be dissipated as heat energy via the brakes if one is to stop and if we now add that the aerodynamic lift is also proportional to the square of the air speed and remember that this lift will reduce the effectiveness of the brakes, we must urgently address these problems to make our roads safer.

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see also

  www.schoolminibusaccidents.wordpress.com