What’s Stopping You: RV Brakes
Words and diagrams by Joseph Van Woerkon
Photography by Caravan & Motorhome on Tour
So many caravans are equipped with electric drum brakes that people could be forgiven for thinking that these are the only type that are available. However, there are a number of other braking systems that may be more suited to your circumstances.
THE LEGAL POSITION
The number of braked wheels and type of braking system on any trailer is controlled by regulation as detailed in Vehicles Standards Bulletin #1 “National Code of Practice - Building Small Trailers” (aka VSI01). In summary:
- Trailers below 750kg GTM are not required to have brakes fitted.
- Trailers over 750kg GTM must be equipped with an efficient brake system operating on the wheels of at least one axle that complies with ADR 38.
- Trailers over 2000kg GTM must have brakes operating on all wheels. Over-run brakes are not permitted. In the event of the trailer becoming detached from the towing vehicle, the brake system must immediately apply the trailer brakes for at least 15 minutes.
Note that the law does not dictate what type of braking system must be used. The only requirement is that it must be “efficient”. Also, except for over-run brakes, all trailer brakes must be operable from the driver’s seat of the towing vehicle.
The trailer brakes can be activated by any convenient form of energy including mechanical, electrical, hydraulic or air pressure or a combination of these.
The simplest system is the mechanical type known as over-run or over-ride brakes. In this system, the energy for activating the brakes comes from the inertia of the trailer itself. The key to its operation is a sliding coupling between the trailer and the tow vehicle.
When the brakes on the tow vehicle are applied the vehicle starts to slow down but the trailer continues at the same speed and the sliding coupling allows the trailer to move towards the vehicle. As the trailer moves forward, the coupling moves backwards and pushes a lever, the other end of which is connected to either a hydraulic master cylinder or cables attached to the brakes. The harder the tow vehicle brakes, the harder the trailer brakes are applied.
When the tow vehicle accelerates away, the coupling on the trailer slides forward again to be ready for the next brake application.
Electric brakes rely on a brake controller that is installed in the tow vehicle and is powered by the vehicle’s battery. When the vehicle’s brakes are applied, the brake controller outputs a voltage that causes the trailer brakes to be applied. Most electric brake systems utilise electromagnets within the wheel hubs to activate the brakes but there are also some electric-over-hydraulic brake systems available that convert the electrical signal to hydraulic pressure.
Hydraulic trailer brakes are most often found in older over-ride braking systems but, as previously mentioned, can also be part of an electric-over-hydraulic system.
Air brakes are only type of braking system that you’re likely to find on heavy trailers such as on semi-trailers. However, they’re not very common on light trailers as they are very expensive and need a lot of space for the compressor and air tanks. While commonly known as air brakes, these brakes actually require air pressure to release the brakes and a loss of air pressure will lock them on.
DISC OR DRUM?
There are only two main types of brake mechanisms generally available for vehicles and trailers: disc and drum. While these are vastly different visually, both work by pushing friction material (brake lining) against a cast iron surface which rotates with the wheel. The heat generated in this process is what slows the vehicle down.
In drum brakes, the brake linings are bonded to two curved metal ‘shoes’ and press against the inside of a circular ‘drum’, also known as the brake ‘hub’. In disc brakes, the brake linings are bonded to flat ‘pads’ which press against both sides of the brake ‘disc’ or ‘rotor’. In both cases, the harder the brake linings are pressed against the metal, the higher the stopping force.
THE ELECTRIC DRUM BRAKE IS BY FAR THE MOST COMMON BRAKING SYSTEM FOR CARAVANS
In general, the force required to produce effective braking in disc brakes is significantly higher than that required for drum brakes because most drum brakes are self-energising. That is, the action of braking causes more pressure on the brake linings which increases the amount of braking. As a result, drum brakes require a heavy spring to pull the linings away from the hub when the brakes are released by the driver.
Because they need more application force, disc brakes are usually activated by hydraulic pressure, although some light trailers use an over-ride coupling to activate the brakes by means of a cable and lever mechanism. Also because they aren’t self-energising they also don’t need any springs to retract the pads after the braking event.
Brake fade is the termed used to describe the reduction in stopping power that is caused by brakes overheating. It’s most likely to occur at higher speeds, especially when descending long, steep hills.
Once brake fade occurs, braking performance can only be regained by allowing the brakes to cool. Stepping harder on the brake pedal will have little to no effect.
Disc brakes are much more resistant to brake fade than drum brakes because the heat can be more easily removed away from the rotor and pads. This is why disc brakes have become standard equipment in most passenger vehicles.
Brake fade is worst in self-energising drum brakes as used in caravans since the reduction in friction on the primary shoe also reduces the force applied to the secondary shoe.
TYPICAL CARAVAN BRAKES
While there’s no doubt that disc brakes can produce greater braking force and are less affected by brake fade, electric drum brakes are by far the most common braking system for caravans.
There are two main reasons for this; 1) electric drum brake systems are much cheaper to make and 2) drum brakes can perform adequately at low speeds with light trailer weights.
ELECTRIC-OVER-HYDRAULIC DISC BRAKES OFFER THE ULTIMATE IN CARAVAN BRAKING
In electric drum brakes, the brake shoes are moved by a lever that has an electromagnet attached to one end. During a braking event, the brake controller in the tow vehicle causes current to flow in this electromagnet. The higher the current the stronger the magnet becomes.
When de-energised, the electromagnet rests lightly again a machined face on the brake drum. When energised, it is attracted to the drum and attempts to rotate with it. As it moves, the other end of the attached lever pushes the brake linings onto the braking surface.
Since the brake shoes are positioned very close to the drum surface, they don’t have to move very far before contact is made. The other end, with the electromagnet attached, is free to move a much greater distance. This arrangement provides the mechanical advantage to produce a large force on the brake shoes and therefore significant braking force can be produced.
While electric drum brakes are adequate in most situations, electric-over-hydraulic disc brakes offer the ultimate in trailer braking. These systems work by taking the brake control signal from the electric brake controller mounted in the tow vehicle and converting this to a corresponding hydraulic pressure to move the pads against the disk.
The conversion from electrical to hydraulic can be 1) directly via a small electric motor that pushes the piston in the hydraulic master cylinder or 2) indirectly via compressed air that pushes against a diaphragm which is attached to the hydraulic piston. This second arrangement is similar to power assisted disc brakes found in all modern passenger cars and offers the fastest response.
Another disc brake variant, often found in off road camper trailers and boat trailers, is the mechanical disc brake which is actuated via an over-ride coupling. Instead of using a hydraulic piston to force the pads onto the disc rotor, the cable from the over-ride coupling pulls on the end of a lever, the other end of which contacts the inside pad of the floating caliper.
The main advantage mechanical disc brakes is that they are easy to clean after being submerged in water or mud. However, the lever mechanism is not able to produce the same level of pressure on the brake pads as the electric-over-hydraulic system and so is not recommended for heavy trailers.
So what type of braking system should you have on your caravan? The ubiquitous electric-drum system has proved to be cheap and effective in the past but is it good enough for the modern era in which caravans are generally larger and heavier and travelling at much higher speeds?
Naturally the choice is yours but in my mind, if you’re spending big bucks to get a large luxury caravan, maybe you should spend a few more and get the best braking system that’s available: insist on electric-over-hydraulic discs.