Thread: Wall thickness on A frame draw bars

HI GRAHAME,
I am a boat builder who likes to build custom trailers for the boats I build or restore.

For me the minimum thickness of any section is 3mm.
That is my way of ensuring the strenth and longevity of the trailer.

For boat trailers, the boat is always worth way more than the trailer. Therefore, in my opinion the trailer should not only be custom made to the boat, but it should be the equal quality of the load it is carrying.

Paul.

Hi Graham,
I have never heard of using 1.6mm for a draw bar. I always thought the minimum was 2.5mm and that would be for a small box trailer.
Just goes to show the company is cutting corners.
I could only imagine what sort of accident it would cause going down the road when it cracked all the way through. They would then blame it on the age of the trailer and rust.
Now that you have strengthened it, it should out last the trailer.
Dave

Thanks for the replies
I was concerned enough to tell the guy to keep a watch on it.
My guess is it that started life as a box trailer that the average bloke would find good enough for trips to the tip and so forth.
Apparently this fellas Dad, also a builder had started the canopy on the trailer and gave it the young fella when he retired.

It was a tipping trailer and had a huge NO NO .a pair of welds staright across the draw bar to prevent an accidental tip on a fully loaded canopy trailer.They were cut and dressed and the gussets fitted else where that did not create a stress fault during a bending moment.

The young bloke has done an excellent job on that trailer and has everything that opens and shuts on it.
Drawers with rollers on them open to reveal his levels and screeds all set in fixed clips and holder.There is even an on board compressor.

I am surprised that any Australian maker would spec metal that thin on the drawbar.It is begging for problems in my opinion.

Grahame

Hello Graham
Whilst researching my trailer build I looked at many websites for ideas and steel sizes. I was amazed at what steel sizes and wall thickness was specified for larger trailers that I thought might be right on the minimum. This minimum standard of course is right through a lot of industries. If a Engineer specifies a minimum size then that's how it's built. No safety margin seems to be built in. If you look a Marshall Engineering's steel sizes for their tandem car carrier you might think -GEE IS THAT A BIT SMALL? Yet I bet it's been OK'd by an Engineer. The only size of steel I liked on the trailer was the drawbar which was 100 x 50 x 5.
Cheers.
PS. I'm not saying anything against Marshall. They have a great website full of good information and are super helpful.

Broken


Comment by Yonnee, added here because Aussie got in before my next post:
Two things about that photo, which I seen previously, is if you look at the size of the H/R weight distributing hitch, that's a reasonable sized Caravan with a fair weight on the towing vehicle (the trend in Australia is to place the wheels further back for minimal chance of sway). That drawbar I'm guessing would be 100 x 50 RHS, but I couldn't tell you the wall thickness.

BUT... If you have a close look at where the break originated from, and you can clearly see the "Strengthening Rib" leg, that was weld straight across the bottom of the drawbar. This poor Caravan's drawbar had no chance.

Yonnee

Originally Posted by Grahame Collins

I am surprised that any Australian maker would spec metal that thin on the drawbar.It is begging for problems in my opinion.

Grahame

Don't be surprised. There are manufacturers out there that will cut as many corners as possible. Particularly some of the biggest 'Australian trailer'(s) manufacturers out there are some of the worst. (See how I didn't name any names...) They are having made direct Chinese copies of the AL-KO and Tregg couplings, which I would wonder whether have actually tested to the Australian Standard they're supposed to be. And they will use as light a guage of steel as possible, because it's cheaper.

Originally Posted by buildspacetrain

If you look at Marshall Engineering's steel sizes for their tandem car carrier you might think -GEE IS THAT A BIT SMALL? Yet I bet it's been OK'd by an Engineer.

Here's the thing. There is no requirement for a trailer built in Australia to have an engineer's report, or even be looked at by an engineer. You can even start up your own trailer building company, with no experience, and little more than a business name. There is however, a calculation required for the drawbar itself that states that it must withstand particular forces, and yet there is no proof required when actually registering the trailer.

My mate who bought a expensive tradesmans trailer from awell known maker here in Adelaide, basicly it destroyed itself becuase of the light gauge material. I don't know if he still has it if so I will take some pics of the tares and stress, fractures.

Wow!!! And here I was thinking that my 1.6mm wall RHS boat trailer I built was pushing it for lightness... At least my trailer will never carry more than 100kg or so over the tare!

Hi graham,about three months ago i did a drawbar repair on a six by four box trailer that had a drawbar ex 50x50x1.6 rhs,to make matters worse it was actually 1.4 when i miked it out of curiosity.the owner had pulled into a roof tile yard and a forkie had loaded a full pallet of terracotta tiles into the front of the trailer and it died on the spot,luckily,he wasnt on the road at the time.His son paid $350 new for the trailer in melb northern suburbs and gave it to his father for a present.No idea wether it was made in aus or china.regards danny

With a wall thickness of 1.6 I would avoid extending the drawbar big time. I would either replace the brawbar completely with 3mm wall (min) or reinforce it with some 3mm angle line on the underneath side with a few drilled hols for water to escape. This will not stop the rust completely but will slow it down. It is no worth the risk of just extending in its current form.

Then you see it at the other extreem where materials that are far too heavy are used, simply because they are easy to weld.....particularly sheet parts.

I have seen quite a few trailers that are near their maximum legal towing weight...and not a thing in them.

I know of one big green mowing trailer that has no brakes.....I reccon ot weighs over 750Kg empty.

I recon that it would be a valuable public service for someone to spec a few reasonsble trailer designs and construction guidelines..... simply so the self builder ( and the small time manufacturer) has some guidance.

something like the "framing code" the building industry has.

cheers