Sandwhich on the rails.

From previous posts, it is agreed that it is the composite shell that gives a board its strength. As far as the composite shell goes, it seems to be fairly well agreed that the rails are the most significant area for strength, a rail failure will lead to complete failure. Given that those are two truths you could infer that the most important place to have sandwich construction would be on the rails. There have been a few people that have wrapped their balsa sheets around the rails to make a sandwhich there, but it seems to be the exception to the rule. Mostly I’m seeing perimeters of balsa or d-cell with no sandwhich on the rails where I think it would be the most important. Somebody will surely bring in the flex characeristics of a balsa perimeter stringer, but it makes sense to me that there should be a sandwhich on the rails regardless, you could compensate with lighter glass schedule overall if it affected flex. I’ve done no testing so this is purely theory. What do you think?

I wrap both the top and bottom skin around the rails. It works for me but I notice that Bert wraps the top and doesn’t wrap the bottom. I imagine that its significant, but I can’t figure out why (its hard to wrap the deck and easy to do the bottom so why leave it out unless theres some benifit).

my theory and what i did was to wrap on the bottom and top then add another stringer on the outside … it think it is cosmetic…

on my first board ive got 10mm of striner wrapped and another 5mm on the outside

i left a flat surface when i shaped the blank and finished off when the outside stringer was glued on…

heres a pic of how i make stringers out of sheet of 5mm balsa plywood (made in vac bag) then cut with knife and glued together. there is very little waste this way… i think its pretty quick way to do it

second board . i wrapped the whole lot and sanded so the line isnt neat because balsa didnt wrap that well on the ends…

if you leave a flat you dont have to worry about it because the top sheet doesnt have to wrap right around

cheers



Actually,

I believe the theory is pretty straight forward.

(reality is always the hard part)

the highest stresses are tension and compression in the top and bottom skin.

these would be the fibers the greatest distance from the neutral axis of the board (midplane)

the core is mostly responsible for carrying horizontal shear loads.

the stringers and rails are needed to suppliment the requirements of the core,

which has relatively low stress.

You can certainly use higher strength materials in the rails to carry more of the flexural loads.

but they will be most beneficial when used in the skins (max. distance from the neutral axis)

IMO.

-bill

were not talking , wall panels , or building codes , nor static principals of engineering …

dont forget the ride …

regards

BERT

bert.

yeah, that’s true.

but the rules dont change, just because your objective is not max static strength.

Most folks, myself included, equate strength with stiffness.

and that appeared to be the direction of thre original post.

But it appears that having the strength issue under control,

you are able focus on tailoring the flex or srpring rate to suit conditions (design goals).

I would say it’s still the fibers in the skin, that dominate the return rate for any particular sandwich.

i believe you have suggested as much when declaring balsa much superior to D-cell.

additionally, most folks (myself again) are impressed at the way you can reduce the thickness of your boards.

But, i believe, You actually need to, in order to get the desired deflections.

However,along the lines of the original post, i am curious to know what you consider the weak link when your boards are actually pushed to failure.

regards,

-bill