It has taken some time to follow up on my proposed method of measuring nib flex as a function of writing pressure. The sample above was done on Clairefontaine paper, using a classic vintage extra-fine flexible dip pen nib, the Spencerian No. 1 Double-Elastic. Once the weighting of the pivot arm was set, using a digital scale, the nib was inked and set down on the paper, which was withdrawn to make the sample line. In this preliminary trial, four weights were used: 15, 50, 150, and 200 grams. Line widths were 0.2, 0.5, 1.4, and 1.85 mm. Since this is a nib prized by calligraphers for its flexibility, it offers us a handy benchmark. We will be posting further test results shortly.
UPDATE: Below are improved test sheets for three different nibs: the Spencerian No. 1 (again); an extremely flexible Ladd & Miller #4 gold nib; and a flexible Fairchild #6, also gold. The ink was blue washable Quink, and the paper was our usual testing standard, Rhodia with a 5 mm grid. Weights used were 10, 25, 50, 100, and 150 grams. In our previous test we pushed the Spencerian No. 1 up to 200 grams, which it can safely handle, but 150 grams was pretty much the safe limit for our two gold nibs. Less flexible gold nibs could easily handle more pressure, of course, and would be tested accordingly.
Note that the Fairchild is very similar to the Spencerian within this range of pressure, with similar snapback as well (not something we are attempting to measure here, however). The Ladd & Miller nib has a larger tip, but it opens up with markedly less pressure than the other two nibs, both of which would be classified as full flex.
UPDATE: Some more nibs now added, updated chart below. Note that the Esterbrook 9788 Renew-Point was an unusual specimen, significantly more flexible than the norm.