Thread: welding across RHS steel

Originally Posted by Legion

... my limited space frame understanding suggests a proper design only has tension and compression at each node.
... But you couldn't just put a jerry can holder or a spare wheel or a jockey wheel in the middle of a span, they'd all have to be carefully placed. And the undercarriage stuff too - spring hangers or links or shocks or whatever all having to be at nodes.

The space frame design distributes the load itself. Provided the loading is not applied directly to the braces, they will only ever be in tension and compression. They shouldn't need to be the same size as the main member either, so you could save weight there

You might have to be a little more careful about where you put things but if a jerry can holder is going to bend your space frame you are in trouble anyway. Large loads like suspension points should be located at nodes, but the nodes don't have to be equally spaced either.

I once built a 4m long space frame structure (triangular section) with 12mm dowels for bracing. When I supported it at the ends and stood on the middle, it deflected around 5 to 10mm. The strength of a truss/ space frame is that it puts material far out from the neutral bending axis where it does most good.

Michael

Remember there are some stereotypical forms of trusses and space frames.....but the concept is not limited to those forms.

A truss can be any flat structure that gains strength from multiple spaced members and a space frame is likewise any three dimension structure that gains strength from multiple spaced members.

Many of the higher order truss and space frame designs gain massivly over plain beam structures..and yes all the forms have advantages and disadvantages.

The particular form of space frame in the examples by Leigon have specific advanges in building roof structures.....the big advantage is that they are made up of multiples of the same diagonal element that can be pumped out on a production line, they are then combined with light flat beams top and bottom.

That design is very cheap to ship, fast to assemble and covers huge spans for its weight...it is a very high strength gain structure......and is mostly bolted.....it realy got some exposure in the mid to late eighties thru into the nineties.....there where several structures if its type at brisbane expo88 and Corbould Park race track at Caloundra shows good examples.

BUT I don't believe its a form of space frame that is suited to trailer construction.

The sorts of trusses and space frames that would be appropriate and advantageous in trailer construction would be simpler and lower order forms.

The higher order trusses and space frames yeild strength for weight gains upward of 10 times over plain beam alternatives....but modest strength gains of the order of 50 to 100% achieved by very simple and robust truss or space frame forms I believe would be well worth having....a modest 100% strength gain could reduce the weight of a trailer by 50%

and the lower order forms would be fairly easy to construct and less dependent on weld strength.

for example.....in a simple two cord truss with plain upright spacers.....if the top and bottom cord where braught together by bending.....there are only two critical welds....the ones at each end..all the welds in the upright spacers will be in compression.

Good welds all the way thru will increase the reliability and the rigidity..as they do in any steel structure

cheers

Originally Posted by soundman

The sorts of trusses and space frames that would be appropriate and advantageous in trailer construction would be simpler and lower order forms.

Yeah, this whole topic has got my gears spinning and I've investigated a few build concepts. Latest idea is vaguely inspired by the Crowswing teardrop. With a design like this, at the 300mm high junction line between the lower diamond plate compartment and upper black ply living quarters you could have a truss rail with uprights down to the chassis and diagonals down to the outer step. I could probably get away with 75 x 50 x 2.0mm chassis rails and light pipe trusses. I quite like their concept, storage underneath and low profile sleeping on top. Plus there's so much information on the internet on building teardrops to guide a build. Combined with a regular 150 x 50 x 3.0mm A-frame drawbar and the reasonably light trussed chassis rails I think it would work out well. I'd even be tempted to go with lighter rails - 75 x 50 x 1.6mm or 50 x 50 x 2.0/1.6 with careful design of suspension points and truss nodes.

Incidentally that Crowswing teardrop is awesome. Here's another thread on their build if you have a spare hour to read it.

100 x 50 drawbar...hell what are you building a car trailer.

when we start looking at caravans they are a dead sitter for space frame construction with the whole side of the van being a truss...start using structrual sheeting and you add a diaphram to a truss and things become very strong.

remember too that a truss does not need to be straight.

It would be entirely reasonable for the second cord of the truss to be below the main cord fore and aft of the suspension and above the main cord as it passes over the suspension........or being radical it is not unreasonsble for the axle and suspesion to project thru the space in the truss.
This could be very much more feesable with rubber suspension units or any cantilever independednt suspension system.

Remember on a conventional leaf spring suspension there are two more or less equal loading points on each side.....on a short trailer this dramaticly reduces the overhang length.

sounds like some of us need to build some bits of truss and do some "Rocker tests".

( Rocker was a forum member, who was an ex acedemic...he got into chair building and one of his things was this zig zag chair with a composite joint in the corners of the zig zag.....he would turn up at forum gatherings with sample joints and we would break them.....by clamping a board to the joint and loading the joint up with members who where game.....somewhere there is a picture of me and several other members standing on a board supported by one of Rocker's joints)

I am pretty sure with only a little bit of thaught and not a lot of extra effort very much lighter and stronger trailers could be built.

cheers

Originally Posted by soundman

100 x 50 drawbar...hell what are you building a car trailer.

150 x 50. There's the backyard overengineerer in me. I could probably go smaller but the place I get all my steel only has 100 x 50 x 3mm as import steel and for a trailer drawbar I'd prefer Australian with the right certifications etc.

Other options:

(i) 100 x 50 x 4.0mm painted
(ii) 150 x 50 x 2.0mm OzTube Kleer or Duragal

(i) is almost the same weight as the 150 x 50 x 3.0mm Duragal I'm thinking of. I'd prefer the taller section in that case. Price is almost the same. (ii) is an option, but I think I'd prefer 3.0mm thickness. 2.0mm just makes me nervous for that application. I could be convinced to go with 2.5mm but that's not available.

Drawbar would be for a ~1 tonne or maybe 1.5 tonne small caravanny thing to go down some typically corrugated bush dirt tracks with light to moderate 4wding thrown in. Behind a Land Cruiser touring WA on short holidays. Gibb River Road kind of thing.

Originally Posted by soundman

sounds like some of us need to build some bits of truss and do some "Rocker tests".

While reading up on space frames I came across a cool website with a guy building ally bikes. He soldered together what looked like tig filler rods to test prototypes. It would be fun to bend up filler rods and tig weld them in place. Make a 1:10 scale model and load it up with 1:10 weights etc. More practical than welding together coke cans too.

Here it is. Using copper wire and soldering.

While looking for it I came across another cool bike space frame. Now that's a time consuming, intricate build.

I bought a second hand box trailer that had a 100x50 chassis and a V 800mm long draw bar also 100x50.
Tired of its limitations and my inability to reverse it properly, I extended the drawbar to 1.8m with 150x50 from the center of the box trailer through the V old drawbar ending in the tow hitch or whatever the name of the thing is. The old V towbar is welded to the sides of the new towbar with the aid of a small flat bar.

I also changed the shaft and springs to take 1000 kilos. Did this 20 years ago and have carried bricks and sand and other building materials a ton at the time since then with not one problem.