The rim is obviously concave, to the eye and touch. Concavity is probably the best measure of about-to-failness.
As to replacing it at the first sign of a bulge: that's sensible. You won't catch me being sensible. I got 2 more years out of this rim past the first bulge.
I'm trying an Alex Adventurer rim this time; it has 3.5mm thick walls. The Sun CR18 has 2.5 mm thick walls. Will they both fail at the same thickness? If they both last to 1mm the Alex should last 40% longer. It has a wear indicator. Alex's docs read:
Our 0.3mm deep circumference line appears on both sidewall
surfaces. When the sidewall becomes thinner the color of indicator
become lighter and hard to tell the difference between the CNC
machining and the indicator , it's recommended to replace the rim.
.3 mm ?! They expect me to discard a rim with 3.5mm walls when they're 3.2mm thick? Do I misunderstand or do they make money by selling rims?