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I've started making my own bike luggage. The first is a simple zipped frame bag, unlined. Now that's finished successfully I want to make a full length toptube bag. Because these sit on top of the bike they're usually stiffened with a sort of foam, about 2-3mm thick, to stop them flopping around. The foam is trapped between the outer and a lining. I need to buy some of this foam sheet, but the typical EVA foam sheets in craft shops (and kids' craft kits, so I have plenty in lurid colours) is too soft at about the right thickness.

I'm using waterproof fabric (ripstop polyester) and may seal the seams on this bag, but I need non-absorbent foam. The zips won't be waterproof.

How can I identify a stiffer foam? Densities are given for the better craft foams, but how does that relate to stiffness. I don't want something actually hard, as I'll only just be able to stand over the bag on the bike; it will be packed with soft stuff on top. As a bonus, the foam will provide a bit of insulation for keeping a drink cool.

I've added some links to commercial products for those unfamiliar with the terminology. The main reason for making them is to maximise the amount I can carry without panniers (bike-packing style; cost is also a consideration.

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What you're looking for is foam interfacing (Pellon Flex Foam, Annie's Soft & Stable, Bosal In-r-Form, etc). This is a soft foam material generally about 1/4" (6mm) thick, designed to be sewn into projects like bags, totes, and hats to provide them with a stable structure while still allowing them to flex--it's meant for exactly your intended use. You may feel like it's "too soft" when handled, but when sewn into the shape of your bag, it will hold the form very well (especially once it's filled with other items). As far as your waterproofing concerns, the stitching across the seams should be sufficient to prevent it from getting wet in any weather you'd want to be out in yourself.

The blog Seasoned Homemaker has some information about the three main brands, as well as how to work with this type of interfacing and examples of projects made using foam interfacing for their structure. I've personally used it to make a witch hat (the pattern designer's work is shown here, also using foam interfacing for the structure) and have found it to keep its shape extremely well, even in the flat shape of the hat's brim.

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  • That does sound like the right stuff; the brands I can easily are different but the review is still useful. I've noticed on some early experiments that sewing a pouch and inserting a tight-fitting piece of foam is stiffer than the foam alone. As for the weather, you'd be surprised what I ride in - 17 hours in almost unbroken rain, for example. But the important stuff will be sealed in plastic or in dry-bags
    – Chris H
    Commented May 26, 2021 at 15:38
  • Since it's washable and dryable, I don't think it'll become like a soggy towel, though I admit I've never dunked a piece in water... I suppose I could, I do have some on hand leftover from the hat, but it feels like it would dry fairly quickly and not soak up too much. You might also consider a flap over the zipper to help block water from getting to that area in the first place.
    – Allison C
    Commented May 26, 2021 at 16:19
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    I'll test it first, but I think it will be OK. I'd rather avoid flaps over the zips I want to open while riding in gloves, though I suppose a waterproof zip might be an option for one of them at least;' there will be 2 and the other is going to be tricky enough anyway.
    – Chris H
    Commented May 26, 2021 at 16:25
  • I've ordered some, so we'll soon find out about how it handles water. I haven't quite finalised the bag design yet so actual construction is some time away
    – Chris H
    Commented May 27, 2021 at 21:10
  • Well, the first brand I tried is just about stiff enough, but soaks up water like a sponge. I'll have to think of another use for it
    – Chris H
    Commented Jun 1, 2021 at 13:59
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If the only problem with the craft foam is the floppiness, you may have luck with high-density EVA foam. Depending on where you're located, there are a number of online cosplay suppliers who have it available, and I know that TNT Cosplay, at the very least, provides free samples of their foam.

Speaking from experience, high density 2mm EVA (the thickness of craft foam) is on par with the heaviest weights of interfacing, but is obviously much thinner. It is beautifully waterproof, and holds its shape firmly when the pieces are cut and glued following an appropriate pattern.

(You can generally just use a fabric pattern for this, especially for something structured like a bag, and just glue the edges together with contact cement wherever you would normally sew; for this project, you could easily construct the entire bag from foam before slipping it in between the outer and lining fabric, although this is maybe a bit esoteric for your needs.)

If pressure is applied, 2mm and 4mm thicknesses will bend out of shape, but will generally spring back when the pressure is removed or with a little bit of encouragement. Thicknesses of 6mm and up are quite difficult to bend out of shape.

And, of course, like their low-density cousins, all thicknesses of high-density EVA are extremely water-resistant. However, because EVA lacks any sort of weave, it frequently behaves a little weirdly when punctured. 2mm EVA foam can be sewn with a decently strong sewing machine (my Bernina Artista handles it without too much complaint), but I would only recommend this for sewing the foam into the fabric. If any strain is applied to a sewn seam on EVA, the thread will almost certainly just rip through the pieces.

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    Having only come across craft foam (too soft) & floor mats (too thick), knowing about alternatives is good, thanks. Your link is in the wrong country, but it gave me some useful tips. I've got an Activa, far simpler than your machine, but appearing similar in terms of strength. But I was thinking of sewing each panel as a pocket, then inserting the foam before joining the panels, never sewing through the foam. I want a box-like construction anyway for these bags rather than curved corners, as it should sit better on the bike between my knees and seemed to work on a prototype
    – Chris H
    Commented May 28, 2021 at 7:51
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    @ChrisH That should work just fine! I'm not sure where you are, but if you google cosplay EVA foam + whatever country you're in, there's usually some distributor around!
    – Cooper
    Commented May 28, 2021 at 14:34
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    There's plenty on eBay if nothing else. I'm in the UK, and try to include location when it matters, but not when it doesn't (i.e. very rarely on crafts.se except regarding terminology)
    – Chris H
    Commented May 28, 2021 at 14:59

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