IVA DRAWBAR* STRENGTH

 

Note to American visitors - this page is about British/European trailer rules.

 

There are three ways to get the drawbar through the IVA test:

 

1.  physical test of drawbar;

 

2.  buy and fit an approved drawbar;

 

3.  calculation to international standard ISO 7641-1.

 

The physical test might seem attractive but it is a fatigue test over 2 million cycles that will need to be done in a testing laboratory, so will be very expensive.

 

Using an approved drawbar is an attractive option – see the sidebar on the right.

 

This web page provides a spreadsheet that enables the third option to be used:

- you input the trailer maximum weight and certain dimensions;

- the spreadsheet tells you how strong the drawbar needs to be;

- it also suggests some steel sections that will be suitable.

 

The spreadsheet can produce a printed output to demonstrate compliance with ISO 7641.

 

Although the spreadsheet has been written for teardrop builders, ISO 7641 applies to all trailers up to 3500kg.

 

There are two parts needed to use the spreadsheet:

 

- the drawbar strength spreadsheet itself, which requires Excel to run (and here is a Excel 97/03 version);

 

- the accompanying notes that explains how to use the spreadsheet and its limitations.

 

The spreadsheet and notes are now in Revision C, to correct errors that observant users found that affected drawbars over 2.5m long or with high eccentricity (May 2014).

  

I would welcome any feedback, good bad or indifferent, on the use of this spreadsheet in the IVA test.

 

* the IVA documentation and the ISO standard uses the British term ‘drawbar’ rather than the American term ‘tongue’ which means the same thing.

 

‘Separate Technical Unit’

 

One thing that may appear to offer a way out of needing to prove the strength of the drawbar is that ISO standard applies only to drawbars that it describes as ‘separate technical units’.  The diagram below right show some examples of trailer frames where the drawbar is a separate technical unit (‘Yes’) and ones where the drawbar is just part of the frame (‘No’).

 

Note that a drawbar can be welded on and still be a separate technical unit – the distinction seems to be that if drawbar can be removed without the frame collapsing, it is a separate technical unit.

 

But before thinking this is a ‘get out of jail free’ card, pay attention to this little clause in the IVA handbook:

 

“The trailer structure and all components including their attachment must be suitable and of adequate strength.”

 

And the VCA, who supervise the IVA test, say:

 

“Regardless of drawbar classification, we would like to see all trailers be put through one of the options above.”

 

The trailer manufacturer (yes, that’s you) is responsible for making the frame strong enough and the examiner might well expect that the frame, if it doesn’t contain a ‘separate technical unit’ drawbar, will be as strong as a drawbar would need to be.

 

You also want to make sure the frame is strong enough, and the ISO standard in the spreadsheet seems a reasonable level of strength.  

 

Back

Approved Drawbars

 

At present (July 2012) there is a very limited choice of approved drawbars, but presumably the choice may improve after the IVA test takes effect.

 

Knott offers the following selection in Germany for up to 750kg trailers which may be available (if unenthusiastically) from Knott-Avonride in the UK.

 

 

Straight and cranked single drawbars attach by a bolted connection at the back end and by a saddle clamp at the front of the trailer frame – a good example the non-welded drawbar manufacture mentioned in the spreadsheet.

 

 

A clamp is available that would connect the back end of a single drawbar to a 60mm square axle tube - making almost a complete light trailer chassis.

 

 

A light double drawbar is available that would make an A-frame, again with all-bolted connections and again with an option of clamps to attach to an axle tube to make a complete chassis.

 

Is the drawbar a ‘Separate Technical Unit’?

Hannu of Finland wins the prize of being the first person to use the spreadsheet and then submit a homebuilt drawbar for official inspection.  The trailer passed with flying colours, though Hannu says the inspector didn’t look at the spreadsheet!