Floor flex

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Floor flex

Postby woodie72 » Thu Feb 13, 2020 4:12 am

Hello everyone. This is my first post here. I have been lurking quite some time, and am happy to have found such a great resource of information and what seems a fairly jolly crowd :)

I have started my first build, which will not be a teardrop, have a second trailer which is waiting for that :D

Looking to build a sort of small shed style trailer foam woodie hybrid, 11.5 x 6,1 x 7 LxWxH for the cabin with a sloping roof lenghtwise, like so

Image


Will be using mainly epoxy for gluing, coating the ply wich is poplar ply, tested in water for a couple of days with drying cycles in between before any coating was done.

I bought an old caravan, yes that is what it is called over here in Belgium :eyebrows:
and stripped it to the bare trailer. Glued up a test board for a making the new floor.

I took measures of the old floor, they used 5mm ply (3/16), 40mm (25/16) wood core and styrofoam board. In an attempt to make a lighter trailer, so i can add bit of length i have made a test beam 50mm wide using 30mm (19/16) thick wood core and 4mm (5/32) plywood skins, glued with thickened epoxy. Have already bought the 30mm xps foam board for insulation .

The glued up beam is supprisingly strong, but has a litte bit more flex i feel, over the original. Over a length of 1230cm, about 48,5 inch, there is a deflexion of about half an inch for 175 pounds. Is this acceptable ?

The original floor was 198cm or 6,5 feet wide

Original trailer frame :

Image

Thanks for any insights.
Last edited by woodie72 on Thu Feb 13, 2020 3:04 pm, edited 1 time in total.
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Re: Floor flex

Postby woodie72 » Thu Feb 13, 2020 4:16 am

Oops, just now noticed i have put this in the wrong subforum :whistle:

Can this be moderated to the proper one ? Sorry.
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Re: Floor flex

Postby edgeau » Thu Feb 13, 2020 5:02 am

Remember it is not any one panel that provides it's own rigidity or strength. It is the completed structure. When the walls and roof are on the whole is greater that the sum of the parts.

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Re: Floor flex

Postby greygoos » Thu Feb 13, 2020 8:44 am

Good luck with your build. Keep us posted.
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Re: Floor flex

Postby Tomterrific » Thu Feb 13, 2020 9:30 am

The floor on that frame provides a bunch of strength. I would suggest using 19mm cross boards and around the perimeter. This will stiffen the floor and provide a stronger attachment for the sides. The reinforcement of the floor adds little weight plus it is low.

That's a good looking caravan in the picture. A nice buildable shape.

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Re: Floor flex

Postby woodie72 » Thu Feb 13, 2020 10:23 am

Thanks.

Forgive me. What are cross boards ? :scratchthinking:

My floor would look something like this :

Image

Don't look at the numbers, as they have changed a couple of times already :lol:

With the different colors being differents widths of wood. Around the perimeter of the whole floor i was thinking of putting in 25mm wide as i was thinking about using 30mm thick walls. Although last couple of days i am asking myself if 20mm thick walls would be fine :thinking:

Sorry for the imperially inclined :lol: :R
Last edited by woodie72 on Thu Feb 13, 2020 2:59 pm, edited 1 time in total.
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Re: Floor flex

Postby OP827 » Thu Feb 13, 2020 11:43 am

woodie72 wrote:Thanks.

Forgive me. What are cross boards ? :scratchthinking:

My floor would look something like this :

Image

Don't look at the numbers, as they have changed a couple of times already

With the different colors being differents widths of wood. Around the perimeter of the whole floor i was thinking of putting in 25mm wide as i was thinking about using 30mm wide walls. Although last couple of days i am asking myself if 20mm thick walls would be fine :thinking:

Sorry for the imperially inclined :R
I would recommend to maintain same floor sandwich thickness as original caravan design at 40+mm. Reducing the thickness even by a few mm will substantially reduce rigidity. You can try saving weight by reducing crossmembers width as their main role is to attach the floor to the frame and upper structure. You can have pieces of hardwood placed specifically at attachment points for frame and internal furniture. Main structural element in this sandwich is skins. Crossmembers also keep the skins where they should be structurally. Covering plywood skin with a layer of epoxy with glass cloth will increase rigidity as well. 6oz glass cloth increases thickness by about 0.6mm from my experience. I wish I could find such trailer frames here in Canada, it must be quite light for the size.. will be following your build.

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Re: Floor flex

Postby Tomterrific » Thu Feb 13, 2020 2:12 pm

By cross boards I mean from one side of the frame to the other. I was thinking you already had the floor made and it was too weak.. I would stick to the original floor build or even sturdier. Consider the floor as the foundation of the box that will be your caravan. A strong wall attachment at the floor and roof is critical.

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Re: Floor flex

Postby woodie72 » Thu Feb 13, 2020 3:27 pm

At the same time i also tried a 40mm thick by 50mm wide board without plywood skins, this flexes about 3/8 of an inch over the same distance.

Would a newly epoxied and fiberglassed 40mm floor (.6/4/30/4/.6) not get in the same range of a non epoxied and fiberglassed floor in terms of ridgidity ?

Reading up on torsion boxes, i wonder if adding a couple longitudinal boards would add the necessary ridgity. :thinking:

Trailer weighs 134kg or 295 pounds.
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Re: Floor flex

Postby OP827 » Thu Feb 13, 2020 4:35 pm

Fiberglassed floor will be more rigid because of fiberglass young module is higher than wood's.

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Re: Floor flex

Postby noseoil » Wed Feb 26, 2020 8:09 am

A 19mm deck frame & side frames will be fine. The "box" you build will be a strong structure if the walls, roof & floor are all glued & screwed in place & the walls are sheared with a thin plywood which is glued in place. We did a 19mm floor frame, skinned with 3mm Baltic Birch plywood top & bottom. The walls were the same (skeletonized 19mm plywood) & skinned inside & outside with 3mm plywood also. Skinned walls make things much stronger & lighter as well.

Remember, the twisting, flexing & sagging loads will be redistributed into the skins & back to the frames continuously & wood is strong. If you build with a sandwiched panel construction (floor, walls & roof) then have good fastening at all the corners, seams & joints, it will be surprisingly light & pretty rigid. A little flex isn't bad, that's why airplane wings are designed to flap not stay rigid. Most people tend to over-build with thick framing & side skins, when a light frame on a small structure will suffice & be strong.
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