The over run brake actuating mechanism

Peter W Jones MInstP

 

 

 see also  

    www.schoolminibustrailers.wordpress.com

 SECTION 1  OVER RUN BRAKES

Peter W Jones    MInstP

 

 

 

The item above is not in the syllabus for GCSE, but  is of interest to any one towing a trailer or caravan.

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Over run Brakes

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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.

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See above diagram

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.

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See also  www.caravanaccidents2.wordpress.com  

Look in “Archives for 2007″  and then scroll to  Paragraphs :-  1e, 1f, 1g,1h, 1i, 2c,  3a, 6b

    www.20six.co.uk/roadtrafficaccidents    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. 

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SECTION  2  ELECTRIC BRAKES


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

 

APPLICATION OF THE USE OF ELECTRO MAGNETS AND RELAYS.

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

www.universityminibustrailers.wordpress.com  

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.

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

www.caravanaccidents3.wordpress.com

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

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SEE ALSO

www.caravanaccidents2.wordpress.com

Near the start of this blog will be seen a photo of a luggage trailer likely to be selected as a trailer for a mini bus. This is followed by notes on towing this trailer.

www.schoolminibustrailers.wordpress.com 

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.

 

Section 4

Testing a Caravan/Trailer Stabiliser.

 

see  www.schoolminibustrailers.wordpress.com   

Edit  8/12/09

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