Wout keeps winning on rim brakes, and we've seen a bit of a surge in rim brake builds -- due either to a scarcity squeeze in many alloy rims or the popularity of the Rail 55 and 25, or both -- but there's no doubt that disc brakes continue to gain market share. Contrary to a lot of headlines, when you do them right they can be very low maintenance and low noise, as well as being powerful.
Just like we talked about in the previous rim brake piece, there are two components to a disc brake system - the part doing the slowing (the pad) and the part being slowed (in this case, the rotor). Rotors are in effect just steel plates that have some important facets, but the surface is really just steel and doesn't vary too much that I've seen. An asterisk to that is that there are some budget rotors out there (Shimano makes most of them that I've seen) that are marked "for resin pads only." I'm not a metallurgist and Shimano doesn't offer a lot of info on why this is or what the metal differences are with these, but I assume that they're softer metal and I've avoided them.
As to the "important facets," obviously you need to match the rotor type (6 bolt or center lock) to your hubs, although 6 bolt rotors can be used with center lock hubs by way of an adapter. We also very much prefer 2 piece rotors. These have the steel braking surface, and then an aluminum inner spider that attaches to the hub. Center lock rotors have to be two piece, just to make the center lock connection possible, but 6 bolt rotors can be either one or two piece as shown below.
Perhaps counterintuitively, the 2 piece rotor is lighter, by 12g (102g vs 114g), but that's not why we prefer them. We prefer them because the aluminum is a good heat sink (it essentially cools the rotor) and because they've seemed less prone to warping than 1 piece rotors. Excessive heat can warp rotors, so the heat sink role of the aluminum spider helps with that, but it seems to us that there's also better support and rigidity with the aluminum spider.
There's more to rotors than that, but it takes you deep into the weeds and doesn't contribute to this conversation so we'll leave rotors there.
Pads play a big role in disc brakes, just like they do in rim brakes. There are two major categories of disc pads - sintered (aka metallic) and organic (aka resin). The differences are easy to sum up.
Sintered/metallic pads have a harsher more abrasive compound. This gives them stronger initial bite, more overall power, and better performance in wet and nasty conditions. It also makes them more prone to squealing.
Resin/organic pads have a less abrasive compound. This gives them a more even application of initial braking power, less overall power, and very good performance in feathering. Going back to our rim brake tech spot, a resin pad would be more like my cork pad in a crit use case - you can modulate speed really really well with resin pads. But unlike cork pads on carbon rims, you're able to stop rather quickly on resin pads with a hard pull on the brakes.
I've always preferred metallic pads for mountain biking, simply because they're more powerful and even when it's dry there seems to be some wetness where I ride. No one accused me of being the world's most elegant mountain biker, and your mileage may vary.
For cross I think it's metal because when you want metal pads (which is kind of often in cross) they're a lot better than resin, and when resin would be ideal the difference between metal and resin is smaller, and while I suppose you could change pads depending on the day, who wants to do that?
For road, I've also tended toward metallic but I'm evolving toward resin. With road disc, there's so much braking power relative to braking demands that you just don't need that extra power. Road braking is much more a modulation deal and less a power need. Plus if your brakes do a bit of singing on a mountain or cross bike kind of who cares, but quieter brakes are really nice on the road.
Somewhat painfully, pads are very specific to brakes in terms of fit. There's no "it's either Campy fit or Shimano fit," there are about a dozen different "current" pad fitments so it's critical to get the right pad shape for your brakes.
Bedding in your pads and rotors is quite key. To do this, find a hill that takes about a minute to go down, start with some speed, and do gradually more powerful brake pulls over the course of your descent, with the last one being almost a skid stop. That will "mate" the pad compound to your rotors and make them brake better and quieter.
After a nasty or particularly demanding ride, you can fold over a piece of 120 grit sand paper and gently "floss" the brake pads with it, this will both deglaze the pads and get rid of most contaminants.
Some general tips on keeping discs quiet are here.
Have a nice Monday, enjoy the Tour.