Understanding over-run brake systems

Over-Run brake systems are fitted to all caravans in the UK and all trailers that are rated above 750Kg weight capacity. So what are over-run brakes?

When you apply the brakes in your towing vehicle, there has to be some way in applying the brakes on the trailer or caravan you are towing. The brakes on a lorry trailer are activated by compressed air. Obviously this system cannot usually be fitted to cars (some Land Rovers can have an air system for towing) so a different solution was sought. As the possible combination of tow vehicle to trailer or caravan was huge, a simple system that required no modification to the tow vehicle was looked for. The over-run system was originally called the ‘surge brake’ when it was developed.

How does it work?

OK lets go through how they operate. In the drawing below shows the major components of an AL-KO over-run braking system. The tow ball (in white) exerts all the mechanical force from the towing vehicle onto the draw bar. When pulling the trailer or caravan forward, the force is opposite to the arrows shown. However when you apply your brakes, the trailer or caravan tries to continue at the same speed (inertia) and travels forward in relation to the towing vehicle and compresses the drawbar and spring. This compression causes the lever to rotate pulling on the brake rod and operating the brakes. (Red arrows)

image (c) Al-KO

The operation of the over-run system is simple and effective, the harder you brake, the more force that is transferred to the drawbar and the further the drawbar compresses transferring more force to the brake rod, therefore applying more braking effort. As you ease up on the brake pedal, the force being transferred eases, so releasing some of the braking effort. This cycle continues through out the braking phase… any increase in the braking force by the tow vehicle causes the trailer or caravan to try to “over run” the towing vehicle and reduction in braking force causes a reduction in braking effort by the trailer or caravan.

The system is self-regulating to a certain extent, but there are problems with this system, and knowing these will help you to operate your trailer or caravan safely.

The cycle of braking and releasing above is reliant on a sprung damper system. If it was just an un-dampened spring, what would happen is when you braked, the trailer would push into the back of the towing vehicle compressing the spring, activating the brakes,    then as the brakes took effect, the trailer would slow quicker than the towing vehicle, releasing the brakes…. and then catch the towing vehicle up and apply the brakes… the cycle would go on and all you would feel is a series of thumps to the back of the towing  vehicle.

So how can we stop this? Well the easiest way it to put a damper, or shock absorber (exactly the same way your vehicles suspension has springs and shock absorbers) to absorb and reduce the “bouncing” of the spring. Getting the spring and damper rate right for a given trailer or caravan weight is important. If the spring is too strong and the damper too stiff for the weight of the trailer, then the brakes will not operate with enough force or worse case, not operate at all. However, if we go to the other extreme and the spring is too weak and the damper too free for the weight being towed, the brakes will operate too easily and could start being applied even when you just lifted off the throttle slightly or at simple bumps in the road.

Having the spring and damper rate too low for the weight being towed will result in vibration when braking and compromising the effectiveness of the trailer brake efficiency. Having the spring and damper rate too high, will cause the towing vehicle to have to do more braking for the trailer, the tow vehicle then is not braking efficiently. In either case, the stability of the vehicle and trailer are compromised.

Some myths explored…

“Going down hill my caravan slows me down as the brakes come on when it pushes into the back of the car”

“In a long decent, my caravans brakes are always on”

A caravan or trailer cannot slow the towing vehicle down on its own. OK, if the caravan or trailer did push into the back of your car…. the brakes would apply, the caravan would slow, the car would pull away slightly releasing the brakes of the caravan. If the caravan was slower than the car, the draw bar would never compress activating the brakes. For safety anyway, the threshold of the brake activation is set so that going down hill, even if you are in low gear to slow you down, in a correctly set up and maintained over-run system the trailer or caravan’s brakes should not activate on moderate hills… if they did, there is a chance on a long decent on a motorway they could overheat. (Think Shap on the M6).

It would be impossible to design a system that would cover all hill descent’s. So a compromise has to be made. So for example a 10% incline (1 in 10) the brakes won’t activate until the towing vehicle slows and the caravan inertia ‘overcomes’ the resistance of the hitch drawbar spring/damper assembly, but on a 20% incline (1 in 5) there is enough weight component from the caravan even when stationary pushing the back of the tow vehicle to apply the brakes. Search “Inclined Planes” for further info.

How can we improve things?

One of the first things to do is make sure the over-run brake system is serviced by trained personnel every year. It’s a hassle, it’s a cost…. it’s potentially a life saver! Don’t skimp out on getting it serviced.

When descending steep hills, be aware that the caravan brakes may have been on while you hold the outfit back using a low gear. If it’s a long mountain pass type descent, if there is somewhere to stop part way down, pull in and give your brakes a chance to cool. A heavier caravan is more likely to activate its brakes on any given descent than a lightly loaded caravan.  It’s always sensible to consider planning a break at the bottom of any long descents and just give your brakes the once-over.

Don’t overload your caravan or trailer, not only is it illegal, but the threshold point at which you over-run braking system is set to operate is calculated by the chassis and caravan manufacturer’s on the maximum mass the trailer or caravan is allowed to carry.

The over-run system is most efficient when you brake in a straight line. If you brake going round a corner, the force transmitted to the over-run system from the towing vehicle is not in a straight line and is actually in two directions (remember the vector representation of forces diagrams from school?) If you want to know a bit more on this read “Understanding the dynamics of towing

When you apply the brakes in the towing vehicle, there will be a delay in the over-run system activating. It takes a few tenths of a second for the vehicle to start to slow, it takes a few further tenths for the trailer or caravan to compress the spring against the resistance of the damper…. and another tenth or two for all the “slack” in the system to be taken up before the brakes operate. So knowing this, you should try to read further ahead and brake earlier.

Some trailers are more inclined to have braking problems. Flatbed car transporters for example are quite light when empty, but can have up to 2500Kg of vehicle on them. The braking system has to be designed to be effective at maximum weight as well as minimum i.e. empty weight too. The threshold set on a twin axle car transporter, can for example compromise its braking efficiency, especially in the wet, and it’s not uncommon to find that a 1000Kg’s of empty car trailer can push the tow vehicle about when braking in the wet, yet fully loaded its stable.

Knowing how your braking system works and some of its limitations will hopefully make your towing experience safer.

The Future

As towing vehicles become lighter and trailers and caravans become heavier, there is increased likelihood the towing to the limit of any tow vehicle will become more frequent. Already in Australia and New Zealand, it is now mandatory to have power assisted systems for trailers above certain weights… the same in America and Canada. There are a number of options on the market… AL-KO has had its “Sensabrake” system since 2009 in Australia and New Zealand and in the USA and Canada, there are dozens of  companies that offer electronic braking systems.

In Europe, currently there is the AL-KO ATC system, which is more an electronic stability aid than a full-fledged braking system.

Further information:

Al-KO  |  Sensabrake

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