u/CuriousAndBusy

The front brake was pulling the rotor to one side (outwards), and this would often cause a strong vibration and loud noise. If I didn't back off on the brakes I could see myself potentially losing control of the bike. I tried re-centering the brake calipers, and maybe there would be temporary relief, but soon enough the rotor would once again be pulled to one side and the loud, embarrassing, potentially dangerous resonance would return. I most hated how pedestrians would get startled and look up from their phones at me with rage, thinking that I didn't see them, and had just panic braked to avoid hitting them. I have SRAM AXS hydraulic disc brakes.

I finally solved the issue with the following process.

• Put the bike in a repair stand.
• Remove bicycle wheel.
• Remove the brake pads.
• Clean the calipers and pistons with IPA followed by wiping down with a clean microfiber cloth.
• Insert SRAM ultimate piston press tool into the brake caliper with the smallest width setting.
• Pump
• the brake levers to protrude both pistons.
• Wipe the newly exposed piston edges with IPA and a Q-tip.
• Apply SRAM DOT fluid to the sides of the exposed pistons.
• Press the pistons all the way back into the caliper with the piston press tool.
• Return the piston press tool to its narrowest setting, and pump
• the brake levers until the pistons protrude all the way against the piston press sides.
• Install new brake pads (SRAM organic Steel Back).
• Clean the wheel rotor with IPA and a microfiber cloth, and reinstall the wheel.
• Loosen the brake caliper mounting bolts all the way, and pump the brake levers a few times to ensure the pistons and brake pad backs have a solid connection.
• Tighten the mounting bolts a little, leaving them loose enough to reposition the caliper by hand.
• With visual inspection against a white background, adjust the caliper position by hand until the two brake pads are centered with respect to the rotor, exhibiting a uniform gap between rotor and brake pad.

Now that the problem is solved, I am reflecting on the cause. I think the weird resonance issue was caused by some combination of:

• Stuck brake piston that needed massaging
• Cheaper grade rotor that has perfect periodic cutouts, making it more prone to unexpected resonances; I notice that nicer rotors, such as shimano ultegra, do not have a single spatially periodic cutout shape on the rotor disc.
• Not having the calipers centered originally
• Wearing in a slight angle into brake pads because of uncentered calipers

Since there were multiple variables in the solution --- brake pad change, caliper repositioning, piston massage --- I cannot say for certain the true cause.