March 28, 2008








(4)  WAVES

Peter W Jones    MInstP
The item above is not in the syllabus for GCSE, but  if a school
has a minibus which tows a trailer with “over run” brakes I think
the pupils would be interested in applying some basic Physics
that they have studied to this brake actuating mechanism and
see whether they think it safe.
If the school mini bus trailer quite legally has no brakes at all
(eg a trailer load of stacked up canoes made from glass reinforced
plastic)  pupils should apply the same above mentioned basic
Physics and postulate on what may happen to the trailer if
heavy braking was needed in an emergency at 60 mph.
Over run Brakes
These brakes are the same as ordinary drum brakes, as used on some cars. Very small ones are used on cycles. They are similar inside to the electric drum brake described below.
They are called “over run brakes,” because they only operate when the trailer runs into the back of the car and pushes a lever which operates the brakes.
See  diagram much further down in this blog

Item 7 fits on to the tow car.
Number 1 is for putting on the brakes whilst parked.
3 is the pivot, 4 goes to the brake drums.
6 is the plunger which pushes on no 1 to operate the brakes.
10 is a very strong spring which ensures that the trailer must exert a great force to operate the brakes (the spring has been replaced on modern caravans with a hydraulic damper).
This prevents slight jolts of the trailer on the tow car applying the brakes.
The caravan below had brakes as shown in the diagram. The photograph was taken about 1985 and the van was possibly manufactured before 1960. The over run brake actuating mechanism on modern caravans is the same (except for the hydraulic damper). The only change is that it is covered with plastic, and can hardly be seen.
On a school mini bus trailer the brake actuating mechanism will most probably be visible.
 The over run brake actuating mechanism was approved for use by the Government in 1930. Very few cars could tow caravans in those days and most certainly they would hardly ever reach a dangerous speed for trailers/caravans, which could be as low as 50 mph air speed.  The critical speed at which snaking becomes a possibility is a matter which has not been thoroughly researched.
Look in “Archives for 2007”  and then scroll to  Paragraphs :-  1e, 1f, 1g,1h, 1i, 2c,  3a, 6b    see paragraph 10ci,  12a, 15a
I hope that students will  also be able to find further arguments concerned with over run brakes that I may have over looked. 

Peter W Jones MInstP
GCSE Single Subject Physics

Caravan/trailer Electric Brakes

GCSE Physics   OCR Gateway
Published by Coordination Group Publications
The Revision Guide
Higher Level
The page references below refer to particular items in this Revision Guide for GCSE Single Subject Physics that are “required” knowledge for the exam at this level.
Page 96  Relay switch; Electromagnetic Relay
Page 85  Electromagnets
I have been retired for some years. The last syllabus I used
for teaching purposes at Year 9 to 11 level had the above topics in the Physics Section of “Double Subject Science.”

The electric current is switched on when the tow car driver applies the car brakes. The rear “red” stop lights come on at the same time. Some of the current to the rear lights also goes to the electromagnetic relay in the caravan, which switches on the current to operate the electromagnet inside the brake hubs.

ADDENDUM  December 2009.

I have now added a diagram to illustrate the above but have placed it in  

to minimise problems which arise when posting images in to this type of blog.

   It can be seen from the diagram that the small amount of current taken from the stop lights’ circuit will be sufficient to operate the electromagnetic relay. This switches on the current from the caravan battery which applies the caravan brakes. In the lab demonstration the electromagnet inside the brake hubs is represented by the large wire coil and soft iron core.

Students will be able to appreciate the force exerted by observing the reading on the force meter. It is a comparatively simple procedure (in industry), to fix levers and/or gear wheels to increase the effect of this force to operate the brakes.

The brake away cable required by law to operate the brakes if the caravan is detatched from the car is not needed. A simple electronic device ( covered in a different section of the same syllabus)  will switch on the EM relay if the electrical  connection to the tow car is broken or becomes  “unplugged.”



USA Electric Brakes.

If one inserts “DfT Caravan Brake Regulations” in to Google one item that appears is an official DfT publication.

Paragraph 3 is as follows;

3 Supply of electricity for electrical braking systems

The electrical energy required for the electrical braking system must be supplied to the trailer by the towing vehicle. If there is a battery on the trailer which is fed by the power supply unit of the towing vehicle, the power from the battery must be disconnected during application of the service braking system.


If any full time official at the DfT had a rudimentary knowledge of GCSE Physics and caravans I doubt that the above item would have been allowed. It in effect makes electric trailer brakes in the UK/EU illegal.

I can not think of any Scientific reason why the large 12 volt lead/acid car battery in the caravan or trailer is unsuitable for use as a supplier of electrical energy to the trailer brakes.

USA Electric Brakes.

Put “Dexter Electric Brakes” into the Google Search Engine.

You will see that this company offers an electric brake suitable for canoe trailers, and all sizes of caravans.

Further searching will reveal that other USA companies make similar products . 

As can be seen from

Para 42 on USA Electric Brakes, I did this some time ago.

Near the start of this blog will be seen a photo of a luggage trailer likely to be selected as a trailer for a school mini bus. This is followed by notes on towing this trailer.
for further advice on GCSE/GCE “A” LEVEL Physics/Maths
NB. Some items listed may be in either “A” level or GCSE courses. This depends on whether Double or Triple Science is studied for GCSE.

Edit  8/12/09

I have had to edit this section considerably. It seems to have become corrupted  since I last checked.



The over run brake actuating mechanism

The over run brake actuating mechanism

Peter W Jones MInstP


February 28, 2010


December 23, 2008

Peter W Jones MInstP

See also :- 

December 19, 2008
Peter W Jones MInstP
Advice for young Physics Teachers.
Nature of Waves.
See OCR Gateway Science for OCR GCSE, Biology, Chemistry and Physics B, published by Collins.
Pages 196 and 197
Explaining to students the connection between this theory and the waves in sheltered inland  water that can capsize boats, and HGV bow waves that can cause caravan/trailer accidents, will greatly increase interest in the subject. It will only take a few minutes of teaching time to do this, but it will take much longer to decide how you will present the material. However, you will only have to do it once. I would have introduced this at least 15 years before I retired had I known about the Bath University research on caravans. !  15 years after I retired wave theory is still in the syllabus.
(Bath University Results: See  paragraphs 9b and 9d. These are in “Archives for 2007”)
Figure 1  in the GCSE book showing interference patterns in water waves in a ripple tank is quite clear as it stands, but you will sink this topic before you have almost started if you do not demonstrate this in the laboratory.
Today, even in Birmingham, most pupils will have seen a boat proceeding in smooth water and making waves as depicted in Figure 5 right hand diagram.
If it is a very large boat it will make very large waves. A much.smaller boat nearby, if caught broadside on to these waves, is likely to be rolled right over ( ie capsized). If the occupants cast into the water are elderly there will be a considerable number of fatalities. This is a rare happening but always makes the news papers and TV.

The effect described takes place in comparatively shallow water  where adjacent land provides protection against the wind so that the water is quite flat. A related effect takes place as a result of a  tsunami. The earthquake that sets off the tsunami can be hundreds of miles from the place where the disaster is experienced. Ships and boats in the path of the resulting waves will hardly notice their passing, but as the waves approach shallower water the constriction of the waves into a smaller space results in the enormous and damaging waves.

The owner of the sailing cruiser shown below (photo “V”) came very close to disaster as on  rare occasions some very large motor yatchts coming into shallow water from the open sea will not realise that their bow wave is potentially dangerous to smaller boats. In this instance, even though the boat was under sail at the time, he managed to turn the boat through about 135 degrees so that it struck the waves at about 45 degrees to the crests and rode harmlessly over them. This option may not be open to the skipper of a pleasure boat carying around 100 passengers as it would need much more space and time to turn through 135 degrees.

Viewed in simple terms something similar may happen when an HGV/coach over takes a caravan/large high aspect trailer, but even if you could see the waves you could hardly take avoiding action on a motor way .

    Figure 5  (and the diagram below) shows constructive and destructive interference. If the top wave represents the caravan wave (if it is oscillating slightly) and the bottom wave the HGV wave, it can be seen that at any one time you may get slight snaking stopped by an over taking HGV, but on another occasion if the waves were in a different phase you could get the one wave reinforcing the other and finish up with a potentially dangerous snake.


 see also 










Hello world!

March 28, 2008