A brake drags / the wheel locks or overheats

Warning: braking is a safety-critical system, and a brake that stays applied is dangerous — don't ride until it's fixed. The bike brakes on its own, a wheel is hard to turn or slows quickly when spun, or a disc/drum gets very hot after a short ride. A brake that drags all the time wears the pads, overheats the disc and the brake fluid (risk of vapor lock and loss of braking), wastes energy, and can, in the extreme, lock the wheel while riding — hence the urgency. Main causes: a seized caliper piston that no longer retracts (drag, bluish disc), a master cylinder that doesn't release pressure (blocked compensating port: the brake stays under pressure), on a drum brake a seized cable or cams that keep the shoes extended, or more simply a poorly adjusted brake (insufficient free play, maintaining residual pressure). You tell them apart by the type of brake (disc or drum) and by tests: with the wheel lifted, does it spin freely or brake? Does the disc/drum get abnormally hot? Urgency threshold: a stuck brake that heats the wheel strongly, or a wheel that locks, requires an immediate stop — get it towed or repaired on the spot. Hydraulic causes (caliper, master cylinder) are for a professional (expert-only); free-play adjustment and mechanical drum brakes are more approachable, but always handle this vital component with rigor. Start by identifying the type of brake (disc or drum) and checking, wheel lifted, whether the wheel brakes on its own: these two simple observations quickly point to the right cause.