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I am trying to build a frame pack similar to Apidura and came to a question — why do they use Fastex sliders only at the bottom of the bag and attach top of it (which receives most part of the load) with Velcro straps.

In my experience straps are not very reliable and can unfasten accidentially; in humid and dirt conditions their fixing power drops — does not look like very good choice for attaching things to bicycle frame, but still they use it instead of more trustworthy latches. And not only they — small bags from noname vendors, lamp batteries are attached the same way.

Is there something to be concerned about using fastexes? E.g. did you have any negative experience with them?

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    The sliding buckles work loose over time. Velcro, on the other hand, is either stuck or it isn't -- it doesn't tend to "slip". Apr 3, 2016 at 18:45
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    Velcro can be done/undone wearing gloves. When looped around, It has higher holding power than a slider, and there are no critical plastic bits to break. It also works fine when wet whereas sliders will slide. On the downside, velcro's hooks can gum up with hair, grass, or mud and then they don't work so well.
    – Criggie
    Apr 3, 2016 at 20:11

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I personally prefer as little plastic as possible on my rigs. Plastic gets brittle when it gets cold. All of the frame bags I have ever owned had full velcro all way around the bag. I've ridden them in snow, ice, mud, rain and dust and never had an issue with the velcro. Velcro rarely "fails". If it gets gummed up, you can clean it (usually on the spot). Plastic buckles require an extra stitch area (where the buckle connects to the strap) and the plastic itself can fail.

All that being said, sliders are a good choice for often on/off straps or frequently adjusted straps (the sleep systems I have carried on my bike have had sliders). A frame bag shouldn't fall into this category.

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    In addition, velcro is far less likely to mar the paintwork like a hard plastic clip.
    – Criggie
    Apr 4, 2016 at 5:47
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I would be more tempted to use metal slot-rings to let you double the velcro back on itself because the rings reduce the force on the velcro at a small cost in weight and complexity. Friction fasteners tend to slip when used on tubes. That is probably why they're not used for the load-bearing straps.

Those fasteners rely on friction caused in part by the pressure of the fixed end lifting the non-release side out so the other end presses down on the looped-back portion. When you wrap one round a tube they often slip because that pressure is missing.

strap fastener

In the diagram the orange is the strap which comes in, goes under the release tab, under the blue "wrap bar" that's actually part of the one-piece fastener, then comes back over that bar and under the tab. Pressure down where the red arrow is results in friction where the red oval is. That's what holds the strap. But if there's no matching support/pressure underneath, there's not much friction and the strap slips.

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