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Imagineer
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5 Posts
Discussion Starter · #1 ·
Hi all!
Im about to start my next plywood recumbent build but im worried about the shape of the finished laminated plywood.

Im going to build a mold, a simple curve with about 2.7 metre radius, and lay steamed plywood ontop of it, building up the frame height with each layer.

But im worried about the rebound, the finished laminate will flex back a bit when taken out of the mold, or jigg, whatever you wish to call it.

Now my question is, is this rebound a known value? can it be calculated?
I need a smaller radius to compensate for this, but how much smaller?

I havent purchased the plywood yet, but im planning on about 5 mm thickness and total frame thickness of about 50mm in height, 10 layers or so.


I know i should build a test-jigg, just to try it out, but im a poor man, with VERY limited resources and plywood is not cheap..

The important dimensions are the height of the bottom bracket (pedals), head tube (front fork) and rear wheel axis centre.
Since the height is important at both ends of the arch, aswell as at one point close to the center, i cannot miss my intended shape by too much, otherwize the bike will be unrideable.

The seat will be built with the same technique, but it will be thinner and smaller than the frame, so its not as expensive to fail with, but still too much money to throw on a fire..


..and yes, i can make YOU a frame kit too! :)
Unemployment isnt exactly filling up my wallet, gotta make some cash somehow.. :)
 

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Banned
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3,533 Posts
I'd build the seat like skateboards. I know there are some folks here who've built them and you can find lots of info on the internet about spring back. Unfortunately, the frame is probably going to have to be trial and error.

As for the frame, I suspect most folks won't see any difference in performance from one to the other anyway. Unless your tolerances at the neck/fork assembly are ridiculously tight for join angle, I wouldn't really worry about it too much.

If you feel like you have to accommodate the spring back, I'd try say 20 mm of tighter radius to start with at a length of about 2 m on the frame. If you have a shorter length frame, you can stay closer to your intended frame radius.
 

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155 Posts
This is a very good question! I want to add to the question a bit: Being the wood used is plywood, shouldn't spring back be quite negligible? With alternating cross grain and no true original orientation amongst all the layers, I would expect there to be very little spring back.
 

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In History is the Future
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6,423 Posts
Have you considered using veneer rather than ply? Basically make the ply. There would be no appreciable spring back.

I helped my brother with a sculpted laminated chair seat when he was in architecture school (some 10 yrs ago) and it turned out really well and with no spring back even in a multi axial form.

Something to consider.
 

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Imagineer
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5 Posts
Discussion Starter · #5 ·
Thank you guys for your replies!

frankp;
i cannot move the rear wheel fix-point, so that would be origo on this design, the only adjustment would be the height of the headtube, the distance between the front wheel axle and centerline of the frame.

The bottom bracket (pedals) need to be a specific distance away from the front wheel, or it would interfere with the riders heels, or worst case, the pedals, when turning.
So the dimensions arent dead serious, a slight miss is neglectible, especially if its merely 20mm over a radius lenght of 2m

And thank you for that skateboard suggestion, i will look into that!


Firemedic;
I have conciderered veneer, but there are two big problems;
its not readily available around town therefore it will cost a pretty penny including shipping.
excluding the financial problem, the big issue is the veneer sheet size. I would have to stitch several sheets together for each layer of the frame and im not confident that i could do that without leaving gaps.

Im pretty certain that the strenght of the laminate would be diminished if there were gaps in the laminate.
The external gaps would be easily fixed with sawdust and woodglue mixed into a filler, but i doubt it would penetrate the gap much below the surface of the frame.

I also concidered strips of wood, i got a stash of beech strips (0.4x4x50cm), intended to stirr paint.
But that fails on the same issue as veneer, i dont know howto get the strips properly bonded with the other strips on the same layer.


chopnhack;
There is bound to be rebound :)
To my understanding, the fibre direction does affect the result, but regardless of direction, it will still rebound.
You must also bear in mind that the plywood sheet is multiple layers glued into a flat surface, and that flat surface, when bent, will always want to go back to its original shape.
 

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Old School
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24,017 Posts
I would use bending ply. You would need to make a two part form (front and back), to sandwich the layers under clamping pressure. Bending ply, in thicknesses under ½" will form to those curves. Once the laminations are glued together, they stay put.

If you can't get bending ply where you are, you could cut the curves out of solid wood in pieces to be edge glued together to ultimately form the seat.







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