Trailer brakes and braking can confuse. Trailers under 750 kg (1650 lb) laden weight are not legally required to have brakes. It makes sense to do so, however. Even a light trailer may push a light (braking) tow vehicle sideways on a slippery descent. Trailer brakes and braking explains all.
Basic over-run brakes have a tow hitch with limited fore/aft movement constrained by a spring. When the tow vehicle brakes, the trailer pushes against and compresses that spring. This pulls on the trailer brakes via a steel cable. Such brakes work well on-road but cause the rig to ‘jerk’.
How a trailer’s inertia actuates the ‘brake lever’ when the trailer pushes against the braking tow vehicle. Pic: RVBooks.
Overrun brakes may legally be used with trailers up to 2000 kg (4400 lb). They are best confined to trailers under a (laden) 1200 kg (2650 lb) unless the towing vehicle is far heavier.
A drawback is that, whilst reversing, the tow vehicle pushes against the trailer. This may actuate the brakes even on a flat surface. A lock-out catch is provided but if not reset, braking is prevented. Optional devices automatically lock and unlock the catches. It is better to use the electric brakes described below.
Over-run brakes are available. They do, however, cause the hand brake to compress the brake fluid whilst the trailer is parked. This can, over time, cause the hydraulic seals to leak. Or fail completely.
Drum or disk?
Most camper trailers use drum brakes. Thay act by forcing an electromagnet against the rear of the brake drum. This slightly rotates a lever that forces the brake shoes against the revolving drum. Braking is controlled by varying the electromagnet current and thus the grip on the plate. Power assistance is provided by the movement it seeks to prevent.
How electromagnet drum brakes work. Pic. RVBooks.
These brakes work well if set up carefully and well maintained. Any internal loss of friction, however, unbalances braking. Drum brakes, however, have almost no braking when wet. Furthermore, they take 15 minutes or so to dry out. Their main appeal (to trailer makers) is that they are cheap to buy and install. Moreover, they need no external power assistance.
Disk brakes are still rare on camper trailers. These brakes work well on trailers under one tonne or so. They are actuated via a mechanical or hydraulic over-run mechanism. Those for heavier trailers need power assistance. Drawbar mounted devices are made specifically for this. Lack of brake controller standards currently hinders their wider adoption.
I arranged (in 2004) to have our then-owned Tvan and also 750 kg (1650 lb) twin-axle garden trailer to be fitted with overrun disk brakes. The Tvan’s were actuated hydraulically, the garden trailer’s were cable operated via an overrun coupling.
The Tvan’s disk brakes. Pic: RVBooks.
It was originally planned to add electro-hydraulic boosting to the Tvan’s brakes. Experience, however, showed over-run hydraulic operation was adequate. They even worked when the brakes were submerged.
It is recommended to a towing vehicle mechanism that detects that vehicle braking. Earlier types use a pendulum that responds to the tow vehicle’s braking. It generates a proportionate electrical braking signal. Others use a solid-state equivalent. This type of brake controller is obligatory with trailers over two tonnes (laden).
Tekonsha Prodigy P2 brake controller. Pic: Tekonsha.
A manual override provides backup in case the automatic controller fails. It may also be used to operate the trailer brakes alone. This can be handy for stabilising the rig on slippery downgrades.
There are also systems that detect brake pedal pressure. They generate an actuating signal proportionately. These work well.