Hit the brakes!

Retro­fit trailer brakes for tow­ing con­fi­dence.

Boating NZ - - Contents - BY BRYCE RAE

Abraked trailer adds a whole new di­men­sion to tow­ing your boat. It means being able to slow down and stop quickly, with­out the trailer jack-knif­ing or shunt­ing your ve­hi­cle off the road.

You can retro­fit brakes to your ex­ist­ing trailer rel­a­tively eas­ily but, as al­ways, there are choices. You need to de­cide which brak­ing sys­tem is best for your trailer and tow ve­hi­cle and, of course, your wal­let. If you’re not fa­mil­iar with brak­ing tech­nol­ogy, sift­ing through the op­tions can be con­fus­ing.


Trail­ers gen­er­ally use one of three types of brak­ing meth­ods – me­chan­i­cal, hy­draulic or elec­tric. Me­chan­i­cal and hy­draulic sys­tems use ei­ther disc or drum brakes – elec­tric sys­tems only use drum brakes.

Because elec­tric brakes con­tain elec­tro-mag­nets within the drum, they are un­suit­able for trail­ers dunked reg­u­larly in sea­wa­ter. Sim­i­larly, me­chan­i­cal and hy­draulic drum sys­tems tend to seize up when sub­mersed in salt­wa­ter. Avoid them. In ef­fect, this means that for your DIY trailer-brake-retrofitting jour­ney you only need to con­cern your­self with two op­tions –

me­chan­i­cal disc brakes or hy­draulic disc brakes.

Us­ing the di­a­gram over­leaf as a ref­er­ence, note that you can se­lect ei­ther a set of me­chan­i­cal disc brakes with a me­chan­i­cal over­ride/surge cou­pling ( the stag­gered black ar­row) or hy­draulic disc brakes with a hy­draulic over­ride/surge cou­pling ( stag­gered red ar­row). Each has ad­van­tages and dis­ad­van­tages.


This is a com­po­nent that’s fit­ted to the trailer’s draw­bar, just be­hind the ball cou­pling, and it’s needed to ac­ti­vate the trailer’s new brakes. Both the me­chan­i­cal and hy­draulic cou­pling mech­a­nism have a spring-damp­ened, mov­able cou­pling head which uses the tow ve­hi­cle’s mo­men­tum and de­cel­er­a­tion (from brak­ing) to op­er­ate.

The me­chan­i­cal over­ride cou­pling uses an at­tached lever con­nected to the trailer’s brakes by ca­bles or solid rods, or a com­bi­na­tion of these. The hy­draulic over­ride cou­pling uses a small hy­draulic master cylin­der, and it’s con­nected to the brakes with hoses car­ry­ing hy­draulic fluid. Me­chan­i­cal over­ride cou­pling – this is a more ba­sic type of sys­tem with a harsher brak­ing abil­ity, and it has a lim­ited amount of sen­si­tiv­ity to the tow ve­hi­cle’s ac­tions. Because the con­nec­tion between cou­pling and brakes is ei­ther ca­ble or rod/ca­ble, both need care­ful ad­just­ment and

reg­u­lar at­ten­tion to en­sure that any wear in the cal­liper brake pads, stretch in the ca­ble, or dam­age to the rod/ca­ble setup is fixed. If you fail to do this the brakes might wear pre­ma­turely or fail to op­er­ate cor­rectly when you need them most!

When tow­ing with a ca­ble-only brake set-up, the trailer hubs will be reg­u­larly mov­ing up and down as the sus­pen­sion moves. This move­ment can cause the ca­bles to slacken and tighten, and if in­cor­rectly set-up, your brakes will ac­ti­vate every time the ca­ble tight­ens.

As well as being un­com­fort­able to tow and in­creas­ing your fuel use, the brakes may over-heat and the cal­lipers will quickly wear out. For this rea­son, ca­ble-op­er­ated brakes are not suit­able for any off-road ac­tiv­ity.

Hy­draulic over­ride cou­pling – a step up from the me­chan­i­cal sys­tem, this sys­tem of­fers an im­prove­ment in both brak­ing ef­fi­ciency and con­trol. The hy­draulic master cylin­der at­tached to the cou­pling soft­ens the im­pact of the cou­pling mo­tion and gives a slightly more pro­por­tional amount of brak­ing con­trol. The sys­tem is easy to in­stall and set up, re­quires less main­te­nance than me­chan­i­cal sys­tems, and spare parts are read­ily avail­able.

Hy­draulic over­ride cou­plings oper­at­ing disc brakes are prob­a­bly the most com­mon type of brak­ing sys­tems on trail­ers up to 2,500kg GVM (GVM is the com­bined trailer tare – trailer and boat weight).


Disc brakes com­prise a hub at­tached to a disc (some­times called a ro­tor), with a sep­a­rate cal­liper unit. The cal­liper bolts onto the trailer axle. Within the caliper, brake pads sit ei­ther side of the disc/ro­tor and squeeze the disc by ei­ther me­chan­i­cal or hy­draulic power.

The greater the di­am­e­ter of the discs, the greater (bet­ter) the brak­ing torque. As a rough guide, for every one per­cent in­crease in disc di­am­e­ter, the brak­ing ef­fi­ciency in­creases by one per­cent due to the lever­age gained as the cal­liper moves away from the axle. Larger di­am­e­ter brakes also al­low bet­ter dis­si­pa­tion of heat gen­er­ated from heavy or con­stant brak­ing.

Ideally, the best brak­ing comes from big­ger ro­tors and the ap­pro­pri­ate cal­liper pis­ton to force it against the ro­tor. Cal­lipers come in many op­tions with sin­gle or mul­ti­ple pis­ton ar­range­ments. They can be made from cast iron, al­loy or stain­less steel, with dif­fer­ing pis­ton ma­te­ri­als

ABOVE Us­ing the di­a­gram, it’s clear that the only op­tions for retrofitting a boat trailer are me­chan­i­cal disc brakes or, even bet­ter, hy­draulic disc brakes

BE­LOW In a panic brak­ing sce­nario things of­ten end un­tidily.

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