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The Science of Towbar Design

The Towbar is now recognised as a safety critical piece of equipment...

In 1950 when Colin Preston Witter established Witter Towbars, the science or art of Towbar design was in its infancy. The main design criteria was that the towing bracket should attach to the best strong points on the now chassisless vehicle, be of neat appearance, easy to fit and comply with the standard height for caravan towing. Witter quickly developed a reputation for Towbars that encapsulated safety and ease of fit, which remains at the heart of every Towbar’s design today.

During those pioneering days when European defined standards for Towbars did not exist, Colin Witter led the way by focusing on creating a safe and reliable industry. It was a steep uphill learning curve, especially as at the time some car manufacturers, including Ford, publicly stated that their mainstream vehicles were not suitable for towing!

Towbar design today is a quantum leap forward from 1950. The Towbar is now recognised as a safety critical piece of equipment and quite rightly, its design and strength are now closely controlled by European legislation.

Science of Towbars

So what is the design process today?

The start of the process is to collect and collate the necessary design information. This core data is the vehicle manufacturer’s specified Towbar mounting points together with the maximum permissible vehicle and Towbar masses. Although towing capacities can vary between models, Witter will always design and test the Towbar to the worst case conditions. It is necessary to obtain as much supporting and quantitative data as possible for example:

  • Does the vehicle have a low rear registration plate?
  • Is there a rear door mounted spare wheel?
  • Are there different bumpers on different model variants?
  • What engine options are available and what is the location and size of the exhaust for the different engines?
  • Are there different ride heights?
  • Is there a model with self levelling suspension?

The latest Computer Aided Design tools and systems are utilised to optimise the development of each Towbar. Three dimensional modelling enables the Engineer to view every part of the Towbar, ensuring that it is capable of exceeding the high loads it will endure when towing.

The next step

The next step is to obtain a vehicle to confirm that the vehicle manufacturer’s information we have received is accurate. On rare occasions Witter has identified that an error has been made and assisted the car manufacturer in re-specifying the mounting points. This may involve the vehicle coming to our Deeside Design Centre, one of our Engineers travelling to the vehicle in the mobile workshop or using our development workshops overseas. At this stage one or more design concepts will be evolved. Important considerations now are aesthetics on the vehicle, the avoidance of bumper cuts, ease of fit and fitness for manufacturing. Not surprisingly these criteria are not always compatible.

A key design tool is the CMM (coordinate measuring machine) which is a portable articulated arm that is used to reverse engineer essential vehicle body and component data. The CMM probe is run over and around surfaces such as the bumper inner/outer faces, the spare wheel well, the rearmost exhaust box and tailpipe etc, with the data directly creating a framework for a CAD model using powerful CAD enabled PC notebooks. The raw data collected enables the Design Engineer to create a 3D model of the concept Towbar structure that links the vehicle's Towbar mounting points to the Towbar, by using and building upon his or her existing knowledge and experience.

This concept design 3D CAD model is then refined until it is considered compatible with the vehicle and of sufficient structural strength to meet and exceed the EC test criteria, before being passed to the design workshop where the technicians will develop a precise prototype sample. From here the Towbar will go to our testing facility where it is subjected to a 2 million cycle fatigue test on one of our purpose built NC controlled servohydraulic test machines. The test can take anything from 2 to 5 days during which the Towbar is repeatedly loaded with the maximum loads it will endure.

European Type Approval of Towbars manufactured by Witter is carried out by the UK Vehicle Certification Agency. They are responsible for agreeing the test criteria, together with the witnessing of the test. Following the successful completion of the 2 million cycle test the VCA Engineer then uses non-destructive test equipment to examine the Towbar for any signs of failure or damage. Once we have been notified that the Towbar has passed, the design is then made ready for manufacturing by producing precise welding jigs in the development workshops.

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