I need to construct a 3D wooden bicycle trailer for Kids. The weight of bicycle trailers must also be light! The Frame should be made of wood. what criteria should I use to construct a Frame,Children's weights?
Thank you
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1$\begingroup$ by tubular, do you mean pipe? $\endgroup$ – Jem Eripol Jan 10 '18 at 10:10
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2$\begingroup$ Are you usng the words chassis and frame to mean the same? $\endgroup$ – Solar Mike Jan 10 '18 at 11:11
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2$\begingroup$ Possible duplicate of Steel selection for building a trailer $\endgroup$ – Solar Mike Jan 10 '18 at 11:47
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2$\begingroup$ this is overly broad and unclear. Please edit the question to clarify what you are actually trying to accomplish. $\endgroup$ – agentp Jan 10 '18 at 15:07
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$\begingroup$ it is edited, sorry i meant Frame. $\endgroup$ – Zouhir Bensebban Jan 11 '18 at 1:50
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I assume that you are talking about a steel frame with a wooden body mounted on it.
In general round tube is more efficient, but if the loading is primarily bending in one direction rectangular section can be better. So for something like a space frame chassis or roll cage where the whole structure can be subject to quite complex loading round tube is typically more weight efficient but a ladder or A-frame chassis tends to act more like a simple beam. So rectangular section may actually be better in terms of structural efficiency as well. Certainly almost all ladder frame chassis are made from RHS.
Rectangular section is also a lot more practical to work with as it just needs to be mitred rather than fish-mouthed for welded joins and you can also make bolted joints with flanges. Similarly having flat surfaces tends to make it a lot easier to attach brackets and drill holes, a major consideration for a trailer frame. It also has the advantage that it can be tapered fairly easily by cutting out elongated triangles and bending and welding it back together.
answered Jan 10 '18 at 14:48
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$\begingroup$ Thank you for your helpful answer. I want to make a wooden frame for children bicycle trailers. I also think that the connection of wood cross-sections with rectangular section is better than round tube. $\endgroup$ – Zouhir Bensebban Jan 11 '18 at 2:25
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$\begingroup$ I've now determined the safety factor at 1.5. what's bothering me, how can I get ahead? what should I put on the chasis? rectangular wooden plate or as in the photo here imgur.com/a/ekvmA. which one is favorable? $\endgroup$ – Zouhir Bensebban Jan 13 '18 at 15:41
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From a structural viewpoint, please let me enumerate the pros and cons of this idea - pipe vs rectangular hollow steel section (HSS).
In terms of strength, circular profiles have the strongest moment of inertia (the one that resists bending) among structural profiles.
Ex. For an HSS of 2'x3'x1.0mm THK, the moment of inertia is $201,744.50 \ mm^4$. For an equivalent section, we take a pipe of 79.59mm diameter, 1mm thick. The moment of inertia is approx. $366,967.62 \ mm^4$. That's roughly 45% more strength than HSS.
In terms of workability, HSS is a lot more easier to work than pipes. Imagine connecting two pipes into a junction. One has to cut the end part of the other pipe in order to fit it into the body of the other pipe. And cutting requires more precision in it. See image below: 
Another thing is that pipes cost more (with reference to our local suppliers) per kilogram of weight than HSS. This is maybe due to the difficulty in making circular profiles.
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1$\begingroup$ As the OP specifically states the chassis is to be made of wood, where are you suggesting hss? $\endgroup$ – Solar Mike Jan 10 '18 at 11:10
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1$\begingroup$ This answer is not for the construction of the chassis but for general engineering viewpoint. $\endgroup$ – Jem Eripol Jan 10 '18 at 11:12
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1$\begingroup$ So you are not answering the question as asked but rambling on ad hoc... $\endgroup$ – Solar Mike Jan 10 '18 at 11:13
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2$\begingroup$ Sir, if you are answering an engineering question, you are playing with very large quantity of uncertainties, therefore for this question you cannot say that "pipes are better because...". There will always be pros and cons to a design and there are no perfect design. That is why I am offering him the chance "to answer his question by himself". $\endgroup$ – Jem Eripol Jan 10 '18 at 11:16
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$\begingroup$ I've now determined the safety factor at 1.5. what's bothering me, how can I get ahead? what should I put on the chasis? rectangular wooden plate or as in the photo here imgur.com/a/ekvmA. which one is favorable? $\endgroup$ – Zouhir Bensebban Jan 13 '18 at 15:41
