Many websites recommend replacing rim brake cartridges (pads) when the grooves on them have worn off. Why would this be reasonable, other than for the hypothetical reasons listed below? I am looking for a scientific/engineering perspective, not personal stories on when you replace your pads.
I understand that once the cartridge wears down to its metal holder, then the braking performance is poor and the metal wears out the rim. The rim is much more expensive, so the brake pad should be changed before metal hits metal. But new Shimano brake pads are 10 mm thick (I just measured), of which 6 mm is outside the metal holder. The grooves are less than 2 mm. Why not use say 3 out of the 4 mm remaining after the grooves have worn out?
I understand that cartridges are cheap, might as well change them every week. I am not looking for a financial or preference-based explanation, only performance-based.
Another reason to change cartridges early may be that the worn cartridges would be too far from the rim. Brakes would have to be moved closer to the rim, which takes a little labour. With new cartridges, the brakes would have to be moved out again. Some people may prefer to change cartridges instead.
It may be that the material of the cartridge is not uniform, e.g. softer rubber on top, harder beneath. When the soft rubber is worn off, the cartridge starts wearing the rim out fast and offers poor braking. I cannot detect any difference of rubber across a Shimano brake pad. It would be more expensive to manufacture 2-compound cartridges than uniform-composition. Is the rubber of a cartridge uniform?
Grit and metal shavings stick in a brake pad, making it wear out the rim. But this seems to happen right from the start, no difference whether the grooves are there or not. Presumably grit sticks in softer rubber better, so if softer rubber is on top, then the brakes gather more grit before the grooves are worn out than after.