Why replace brake cartridge when grooves wear off 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.
This is a good question. The short answer is that the design of modern rim brake pads is greedy and cynical. 

This is a Mathauser pad, and is representative of how a lot of old brake pads were, and some still are.



The design differences with modern pads are obvious. You get a huge simple block of rubber with no grooves. Nothing about the design of the pad is coaxing you to replace it any time before the metal part starts contacting the rim, a point that most riders will be able to see coming and all but the least observant will hear about the second it starts actually happening, before any serious damage is done.

Pads like these are cheap to produce and last a very, very long time. I'm a heavy rider, and I have a set of these I got NOS on a bike that gets moderate use in all conditions that I've had for maybe 8 years now and the pads are about 1/3 worn. There are some theoretical arguments why grooves might help braking or prolong rim life, but I can endo the brake just fine and my rims aren't getting trashed. Many millions of pads of this design have given totally satisfactory performance.

The reason brake pad design moved away from being cheap, thick blocks of rubber is 100% that you can't make any but the most trivial amount of money producing and selling them. They last too long and are too effective for how cheap they are. Only the most hardcore of cyclists become repeat buyers.

Modern cartridge inserts are dainty and full of conservatively sized wear grooves because it creates a situation that gets more pads sold. Nothing bad happens if you wear them all that way down as long as you stop before you get to the holder, at least as long as you can be bothered to keep the brake adjusted along with the pad wear, and watch out for the worn pad overhanging the brake track. The compound doesn't change, the geometry of the brake doesn't change substantively in most cases, and the grooves don't do anything of note in either direction to braking power or rim wear.

A core question about grooves is whether their presence helps or hinders the pad to clear water and debris from the rim, potentially increasing braking power even as they inherently decrease contact area. Personally I think they don't and because they in effect feed water in to the pad/rim contact area they probably are a detriment when it's wet out, but I don't have any data. I believe the contrary idea would be that as the pad touches down on the wet rim, the leading edge has the job of squeegeeing off water to get more friction between the pad and the brake track, and having grooves essentially gives you another leading edge, getting more area clear of water faster after the pad touches down.