I have a suspicion that I have ignored a specific but essential part of the formula for easy use of flex pens -- which ink works?
The variation of line width in flex writings depends on the ink flow creating a consistent connection and fillng of the line being developed between the tines.
When the flow of ink only flows off the tines, not remaining connected across the gap between them, you get railroading. When the ink stretch between the tines is not broken as the pen moves a clear, solid line develops as the ink transfers to the paper. Often wide stretch between the tine breaks the ink bridge by not feeding the ink to it quickly enough -- or ink running out on the tines not transferring to the nib from the feed quickly enough.
So far this newbie (me) has found a plethora of articles and videos on fitting nibs and feeds, modifying feeds and writing slowly -- but so far no real analysis or comparative testing of fountain pen inks for their efficacy of use or appropriateness for flex use. Also noted are the notes that India ink has the proper viscosity for flex use in dip pens, but is rotten for use in fountain pens clogging them, corroding them and doing bad things.
Also articles indicate that fountain pen inks vary viscosity and "stretchability" between the tines not only by brand, but often by color within brand. And sometimes, batch to batch, an ink will limit or open the width of line being attempted at a normal speed before it breaks the bridge or runs out of the nib/feed arrangement.
So, the question then becomes what inks work or do not work for fountain pen flex work and within what parameters of width, speed of movement, color, feed type and feed modification, and which do not work?
Is there a brand that consistently, across colors or partly in XXX colors that will work well in flex? Is there an additive that can be put in to an ink that normally would not flex which would increase the ink's range AND not damage the fountain pen. Dip pen users use gum arabic for this addtive, but none recommend using that substance in fountain pens citing severe gumming and plugging issues.
Maybe this is the sort of challenge Nathan at Noodler's could take on to release a line of designated flex friendly inks in various colors or somehow mark current inks as to "flexworthiness." This might not be a profitable set of tests/developments at first, but I imagine it as a fun challenge.
Lastly, if you got this far -- what has your experience shown so far as the best ink to use for flexing -- and does that vary drastically with the nib metal, feed configuration.
and cleaning regime required for your flex pen?