When the DIN or ISO demanded rim wear indicators some years back, the rim makers initial response was to turn in concentric grooves while they were machining the brake track. This was easy to implement and didn't call for an added manufacturing step or new machinery.
Later on, I suspect, either the grooves were acting as stress risers and leading to failures of their own, or makers realized that by cutting in the groove they were weakening a new rim to the strength of a worn rim, which doesn't make a world of sense.
Or, possibly, after initially adapting to the rule, the makers had time to create and implement a batter solution, so the grooves gave way to the dimples.
One of the ironies of the grooved rims is that they actually get better with wear, which mitigates the stress riser effect of the grooves until such time as it eliminates it entirely.
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FB
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Last edited by FBinNY; 02-13-16 at 12:21 PM.