Flex and stiffness
Quotes are verbatim and attributed to their original authors.
Flex is as important to how a ski skis as its shape, and much harder to communicate. The forum's builders measure it, calculate it, and above all build-and-test it — treating numbers as relative guides, not gospel.
What "good flex" means
Beyond soft-versus-stiff, experienced builders care that the ski bends into a smooth, balanced arc — equal in front of and behind the boot — so the tip and tail track to the same place in a turn:
Measuring it
A simple, repeatable rig is to support the ski at its contact points, hang a series of weights (say 50/100/150 lb) at the centre, and record the deflection each time — a light-then-progressively-stiffer curve tells you more than a single number. Measuring during the build lets you correct before it is too late: "In this way I can measure the stiffness during the fabrication steps, and adjust the construction to get the right flex" (davide).
Calculating it
You do not need finite-element software. A structural engineer on the forum was blunt:
The community's go-to tool is the free junksupply ski calculator, which lets you see how core thickness, wood type, and composites change the flex. But treat outputs as relative — one builder copied a bamboo ski's stiffness into a foam core "and they ended up being about 30% stiffer" (twizzstyle). Handbuilt results vary with your tools, resin, and method.
Build a book of data
The most-endorsed method is empirical:
One caution when scaling: "the shorter the ski the more stiff it becomes" for a given core thickness, so thin the core or drop glass weight on shorter skis (vinman).