Disc Brakes in the Muck
I recently responded to a question on r/cyclocross on why his brakes just quit working in the middle of a muddy cross race. This has been an off year for most racers, but there's still a lot of good riding to be had in the fall, and mixing it up on trails and dirt roads is probably a better idea than sitting in a paceline on the road. The Hudson Valley is now getting washed over by the remnants of a tropical storm, so we are going into this weekend with some wet, sloppy roads.
Disc brakes can work very well in wet conditions, and can also not work well. They have a lot of mechanical advantage and clamp to a surface that is further from the source of contamination, but the pads are thin, and only retract a very small amount and thus have very little clearance for contamination that does find its way onto the rotor or pad. Unlike rim brakes, which will almost always work if you death-grip the lever, discs work really well all the time, until they don't, and no amount of force at the lever will slow you down.
This can happen for a combination of reasons.
1 - You have mechanical discs (cable operated). These do not automatically advance as pads wear, and you need to get off the bike and manually turn the barrel adjuster so that the pads can reach the rotor (or pad advance screws if you have them, these are better).
2 - You have organic pads. These pads are made from a mixture of different thermoset (epoxy) resins and offer decent and quiet braking in dry to moderately wet conditions and preserve rotor life. However they wear quickly in wet / muddy conditions. Switch to sintered metallic pads, which have a longer wear life, but you will need to replace rotors more often (you also do not want to interchange rotors on organic and metallic pads, disc brakes work best when the pads deposit a thin film of pad material onto the rotor, and you don't want a mix of different materials deposited on the rotor). Organic pads are sometimes called "resin" pads. Metallic pads are sometimes called "sintered" pads.
3 - You have poor pad retraction. Your brakes are otherwise working properly, but they do not retract far enough to allow clearance between the pads and rotor plus a small amount of grit when you are NOT braking. Perform a full bleed of the brake lines from the lever and work the pistons back and forth to make sure that the system is not contaminated and working freely. Note that SRAM brakes tend to have less pad retraction in comparison to Shimano. I experience this on my cyclocross bike which has SRAM force, even when the brakes are working properly, the pads wear just from the rotor picking up a small amount of grit, and that grit grinds against the pads as the wheel turns whether I'm braking or not. My mountain bike uses Shimano hydraulic brakes and they only grind when dirty during braking, and then the pads open up enough to allow clearance. I experience much, much greater pad life on my mountain bike (I use organic pads on both).
4 - Your pads were too worn to start with. Note that when pads wear out completely and the metal pad base contacts the rotor, you have ZERO braking at all and this is nearly indistinguishable from when the pads have no contact with the rim.
5 - You brake too much. In cyclocross you should brake as little as possible. You should only be scrubbing off a small amount of speed before you enter a corner. If you are slamming on the brakes and sprinting out of every turn, you're giving up a lot of free speed and wasting effort.
My recommendations are: Make sure your bike is properly maintained before your race or ride, that includes a close look at your pads. Get a set of metallic pads and a set of "burner" rotors for cyclocross season. For summer riding and earlier fall races install the organic pads and your nice rotors for better, quieter braking and less rotor wear. Improve your turning and handling skills so that you brake less (and so you can get a good result and have fun.)