Surge brake

A surge brake (overrun or inertia brake, German Auflaufbremse) is a mechanical trailer braking system that uses the trailer's own inertia. As the tow vehicle slows, the drawbar pushes into the tow vehicle through a sprung coupler, and that movement actuates the drum brakes via a linkage. The dominant standard for European caravans and small trailers.

Used on European and UK trailers up to O2 class (750–3,500 kg). Components: 80–110 mm sliding drawbar, a Bowden-cable linkage, and drum brakes at the wheels. Pros: no brake controller needed in the tow vehicle, no power required. Cons: braking effort can't be tuned, brakes don't hold uphill, and a reverse latch is needed to back up. Almost unused in the US, where electric trailer brakes dominate (see electric-trailer-brakes).