Practically, what consequences are there for using the 'wrong' compound pads?
Braking performance slightly impeded?
Just pull a bit harder.
Road grease/oil would have a much bigger impact.
Given that I couldn't wreck a rotor braking using the pads' backing plate, I suspect compound makes no practical difference.
It all depends on how practical you are whether you care about the differences or not and, no-one is going to die from fitting the wrong pads.
I'll explain, by starting with the discs. They are made from two different types of stainless steel which vary in hardness and coefficient of friction.
Unlike what you learnt in school science about the coefficient of friction, it isn't static. It changes with speed and it changes with temperature.
The Resin-only discs provide good friction at low temperature but lower friction at higher temperature. In other words, as the brakes heat up, their bite decreases. Imagine using these under trying conditions such as tandem rear wheel use, enduro or downhill applications. Not only will they perform poorly, but they will start to deteriorate permanently. Contrary to belief that stainless steel is inert, chemical changes do occur, mostly with carbon penetrating the steel and forming spots of hard material called cementite.
The Metal (pad) discs provide reasonable friction at low temperatures but better friction is higher temperatures. Their friction also increases with speed. The faster the speed during braking, the better they perform. They can also withstand higher temperatures before glazing over.
Over to the pads. By now you've guessed that pads meant for Metal Only discs perform better at higher temperatures and don't melt at higher temperatures. That's right. Further, they are made from metal (and some other stuff) that doesn't melt nor just burn away in one hard session. Unfortunately they don't work well at cold disc temperatures and therefore not suitable for pottering about on your sit-up-and-pray with a basket in front.
Resin pads are made from organic material. Organic as in carbon-based, not herbs and stuff. These pads work nicely with colds discs and provide a good and predictable bite without having to be worked red-hot.
I've told you there are two types of kinetic friction. They are called abrasive friction and adherent friction. We say the friction is abrasive when there is a large loss of material from both the disc/rim and the pad and very little transfer of material between the two. With adherent friction there is a to-and-fro transfer of material and less outright loss of material. Resin disc pads are mostly abrasive and sintered metal disc pads mostly adherent. In the rim brake camp the rubber pads on aluminium and carbon rims operate mostly abrasively. However, when abrasive systems overheat they become adherent and deposit uneven layers of material onto the harder of the two surfaces, which causes vibration, roughness and noise.
Note that the terms abrasive and adherent appear to be illogical if you look at a disc pad’s point of view where the metal pads are harder than the resin pads yet the harder pad is the adherent one and softer on the abrasive one. This is counter-intuitive. It makes more sense if you look at lifespan. The abrasive disc suffers complete loss of material and thus doesn’t last as long – resin. The adherent disc recycles material and lasts longer – metal.
Tribologists purposely include some abrasive material in adherent pads so that the pad can help to keep the deposited layer on the disc uniform and clean. The real trick is to control the transfer of material and keep it uniform. If it is not uniform, the brakes will vibrate or pulsate, squeal horrendously and not perform optimally.
Any bicycle mechanic will tell you these problems plague the industry and customer satisfaction concerning disc brakes is often poor. Much of this can be prevented by understanding that material transfers from pad to disc and vice versa and, that the transferred and transient layer must at all times be absolutely uniform.
I recently read up on the woes of Mercedes car owners who complain that their fancy AMG sedans suffer from noisy brakes and poor brake performance at low speeds. What they actually meant was at low temperatures. The reason, if you read what I've written above, is because those brakes are designed to work at very high operating temperatures under trying conditions, as one would expect from a high performance car. Yet, they want soft, sedate, quiet brakes on their 600 horsepower 2-ton monster sedans.
