Attaching frame pack — velcro straps or fastex sliders? 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?
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.



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.