Overrun brake

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An overrun brake—also called an inertia brake or surge brake (in North America for hydraulic versions)—is a brake system that allows a trailer or caravan to brake proportionally to the deceleration of its towing vehicle. The system uses the trailer’s own momentum acting through the coupling to operate its brakes automatically, without a separate electrical or pneumatic control line. Overrun braking has been widely used in light trailers across Europe, Australia, and New Zealand since the mid-twentieth century, where full air-brake systems are impractical on small trailers. In the United States, surge-type hydraulic actuators serve a similar function for boat and utility trailers. Early mechanical trailer brakes appeared in the 1920s, using crude spring linkages that relied on drawbar compression to pull a rod connected to drum brake levers. By the 1950s, purpose-built “overrun couplings” with sliding drawtubes and integrated dampers were common in European caravans. German manufacturers such as AL-KO Kober and Winterhoff refined these systems into modular products combining coupling head, handbrake lever and damper in one unit. By the late twentieth century, European construction and safety standards for overrun brakes were codified under ECE Regulation 13 and EU Directive 71/320/EEC, defining performance limits, response times and compatibility with reversing functions. The overrun system consists of: When the towing vehicle decelerates, the trailer’s inertia compresses the drawtube inside the coupling housing. This movement rotates or pulls a lever connected to either a brake rod/cable or a hydraulic piston.