Trailer Structural Steel Question

[Dave100456]

Erm, why does the extension have to be unsupported? Why not use the car trailer trick of putting the cantilever on top. The red bit.

Thanks; I had thought of this arrangement, however, the trolley protrudes out from the channel and are likely to foul the cantilever ties.

 

[Dave100456]

That is fine IF al 4 wheels are taking their share of the load however unless teh beds are absolutely planar and the 4 wheels are planar you could find yourself in the situation where only 3 wheels are supporting the load and even worse where only 2 are carrying nearly the full load so I would start at 600kg loading then you need to consider acceleration forces induced while pulling the trailer along a bumpy road (any UK road these days). then a safety factor (2.5) - could be looking at a loading of 2500kg/wheel, not the 400kg you first come up with. I do not know what suspension you intend to have so cannot define the acceleration component of the imposed load. You really should consult some one like indespension. Better still go to an indespension stockist and pick up one of their trailer manuals - they used to have a design section in it which was pretty much dumbed down and allowed the layman to design his own trailer.
Watch out for plod if using it on the road - traler law has changed over the years and is ridiculously tight nowadays and frowns on home made jobs.

Thanks; the trailer is a commercially made one that is within loading for the boat and trolley it is just too small dimensionally hence the need for the trolley to 'overhang' and me to make an 'extension'.

 

[Lakesailor]

Thanks; I had thought of this arrangement, however, the trolley protrudes out from the channel and are likely to foul the cantilever ties.

OK then Spread the load like this with the extra wheels on the trolley which I have suggested.

 

[Avocet]

Can't quite picture how that would get up on to the trailer initially, but I suspect that at some point, pretty much the whole weight of the boat and trolley would be on that middle pair of wheels, wouldn't it?

 

[Lakesailor]

You may have discovered a flaw in my plan. However by replacing the added middle wheels with removable blocks which fit between the channel and the trolley frame, possibly held in position with drop in pegs, the weight would be substantially taken by the supported trailer chassis.

 

[Corribee Boy]

I have a similar trailer set-up. The channels contribute to the structure, but are also welded over a box section. The channel depth is limited by the wheel diameter on the yard trolley, but the box beneath can be any size for structural purposes. Might that design help?