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  1. #1
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    Need help with some math

    Hello all you DIY'ers out there. I am working on a top secret project that involves sandwiching a 3/8" thick piece of closed cell foam between two sheets of ripstop. The foam pad inside will be expected to be rolled up but not super tight. I anticipate that there will be wrinkling of fabric on the inside of the roll, and the outside will be taut. Maybe too tight! This is where I need some math help. I know when you roll things up the outer side is longer than the inner side due to thickness of material. What I don't know is how to anticipate the extra length required for the sandwich material if I don't want the fabric to be ripped apart or cause buckling or yanking on the stitches prematurely. I need to give extra room I am pretty sure. Can someone help me out with how much?

    thanks everybody! I love this board!
    the meek will inherit the earth!

  2. #2
    Senior Member kayak karl's Avatar
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    i think you are going to find the difference to be 3/4" , but you could put a cloth tape on each side of pad and roll it up to check for sure.
    Last edited by kayak karl; 10-18-2012 at 20:29. Reason: 3/4"
    "Tenting is equivalent to a bum crawling into a cardboard box, hammocking is an art" KK

  3. #3
    Senior Member gargoyle's Avatar
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    I've tried to rip ripstop with a 300 pound butt cheek...I doubt rolling up some ccf will be more stressfull.

    Technically, the inner will wrinkle some and the outer will remain the same. Since ccf doesnt stretch (much), the fabric sleeve can be made the same size (allowing for seam allowances)

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    The circumference of a circle is 3.142 x diameter (pi x D) so...if the diameter of your roll is 12", the inner circumference will be 3.142 x 12" = 37.7" and the outer circumference will be 3.142 x (12"+0.375+0.375) = 40.06"....etc., etc.

    If the roll is like a rolled up sleeping pad instead of a tube, you will need to calculate the circumference for each layer (circle) of the roll.

    Clear as mud?

  5. #5
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    Quote Originally Posted by gargoyle View Post
    I've tried to rip ripstop with a 300 pound butt cheek...I doubt rolling up some ccf will be more stressfull.

    Technically, the inner will wrinkle some and the outer will remain the same. Since ccf doesnt stretch (much), the fabric sleeve can be made the same size (allowing for seam allowances)
    Thanks so much! After I posted I thought about it again and it seemed to me my way of thinking was off and you just confirmed it!
    the meek will inherit the earth!

  6. #6
    Senior Member Klaussinator's Avatar
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    Quote Originally Posted by gargoyle View Post
    Technically, the inner will wrinkle some and the outer will remain the same. Since ccf doesnt stretch (much), the fabric sleeve can be made the same size (allowing for seam allowances)
    I agree with this conclusion.
    However, if you don't like the idea of wrinkles and stress, then how about using a material that will stretch a little?

    -Klauss
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  7. #7
    Senior Member oldgringo's Avatar
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    Quote Originally Posted by gargoyle View Post
    ...a 300 pound butt cheek...
    I...uh...erm...did you really have to say that?
    Dave

    "Loneliness is the poverty of self; solitude is the richness of self."~~~May Sarton

  8. #8
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    Quote Originally Posted by gargoyle View Post
    I've tried to rip ripstop with a 300 pound butt cheek...
    That must have been tricky to keep the other 300 pound butt cheek off...I refuse to think about this any more.

  9. #9
    Senior Member JohnSawyer's Avatar
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    Grizz could write the differential equation that would give you the exact difference, but it would depend on how small your starting roll diameter is..

    Truthfully, the larger you leave the center core open, the less differential there is...
    "Do or do not, there is no try." -- Yoda


  10. #10
    Senior Member stevebo's Avatar
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    Ive made backpack straps before using pieces of a ccf pad and ripstop. ----since ccf is flexible and spongy, maybe just sew the ripstop into an envelope the size you want, (leaving one side open)-----sew it, turn it inside out so the seam is inside, and then fold/bow the ccf pad slightly and slide it inside. The ccf will compress a little bit and give you a nice snug fit with no wrinkles! sew up the open end and that should work for you!
    FYI: If you want to know what type a certain bear is, sneak up behind it and kick it. Then,
    run like crazy and climb up a tree. If the bear climbs the tree and eats you, it's a black
    bear. If the bear just pushes the tree over and eats you, it's a grizzly bear : )


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