Reference

Quick lookup material for when you need a specific answer: when your chain, brake pads and rotors are worn out and how to measure that.

Chain wear limits: when to replace your chain

When should you replace your bike chain? Wear limits by drivetrain from Park Tool and SRAM, and how to measure with a chain wear checker.
Disc brake pad and rotor wear limits: when to replace

SRAM and Shimano wear limits for disc brake pads and rotors, plus how to measure them — so you replace parts before braking power suffers.