How do boat trailer brakes work?
I've just bought a used 20ft Ribcraft on a twin-axle trailer, and I need to understand the braking system before I tow it. It has a Knott Avonride hitch. How exactly do these trailer brakes activate when I slow down, and what maintenance should I be looking at?
Answer
Boat trailers in the UK and Europe rely on a mechanical overrun braking system, where the forward momentum of the trailer physically compresses the tow hitch as the tow vehicle slows down. This compression pulls a steel rod and cables connected to drum brakes on the trailer wheels, slowing your boat safely without requiring electronic connections to the car.
The Overrun Hitch Mechanism
The process starts at the coupling head, which sits on your car's 50mm tow ball. The hitch assembly (typically made by Knott, AL-KO, or Bradley) mounts on a sliding shaft called a drawtube. Apply the brakes in your tow vehicle, and the trailer attempts to keep moving forward due to inertia. This forces the drawtube to slide backward into the hitch body.
Inside the hitch sits a hydraulic damper. It acts exactly like a miniature shock absorber. Its job is to provide resistance so the brakes apply smoothly rather than slamming on. If you feel a harsh clunk from the trailer every time you touch the car's brakes or pull away, this damper has usually failed. Expect to pay £40 to £80 for a replacement part.
Rods, Cables and Compensators
As the drawtube slides backward, it pushes against a lever mechanism at the rear of the hitch. This lever pulls a long, threaded steel brake rod (usually an M10 thread) running underneath the trailer chassis.
A single rod cannot pull the wheel brakes directly. It needs to distribute the force evenly. It connects to a balance bar known as a compensator. The compensator does the following:
- Splits the mechanical pull equally between the two or four wheels.
- Connects to individual Bowden cables running to each wheel hub.
- Allows for minor adjustments in cable tension to keep braking balanced from side to side.
Standard brake cables feature a mild steel inner wire. This wire rusts solid after a few dunks in the sea. Upgrading to stainless steel brake cables (around £25 to £35 per cable) is the first job any boat owner should tackle before launching in saltwater.
Brake Drums and Auto-Reverse
At the wheel, the brake cable enters the backplate of the hub and attaches to an expander. Pull the cable tight, and this expander wedges two curved, friction-lined brake shoes outward against the inside of the spinning cast-iron brake drum. This creates the friction needed to stop.
Reversing the trailer naturally compresses the hitch and applies the brakes. To solve this problem, modern European trailers use an auto-reverse mechanism built into the brake shoes. When the wheel begins to rotate backward, the friction drags the trailing brake shoe backwards on a sliding carrier. This slight movement collapses the expander geometry. It instantly releases the pressure on the drum and allows you to reverse freely. Pull forward again, and the shoe shifts back into its normal braking position.