How to get the most out of drum brakes

The drum brake is still the measure of all things for caravans and light commercial trailers. Several million are in use every day worldwide. Some essential information from AL-KO experts will help caravan beginners in particular to get the best out of them for thousands of miles, both on- and off-road.

Drum brakes can take a lot of rough and tumble. Due to their largely closed housing, practically no dirt gets onto the brake shoes, even when driving on difficult terrain. As Florian Vorreiter from the AL-KO Academy at the AL-KO Vehicle Technology Group knows: “Salt water, falling rocks, gravel, grit, dust: none of this will affect the solid metal drum. The brake shoes themselves are well protected inside the drum.” That's why they are unaffected by the long parking times typical of caravans as well.

Properly maintained brakes have just 0.5 to 1 mm distance, called ‘clearance’, between the brake pad and the drum wall. If the driver in the towing vehicle activates the brake with their foot, the trailer pushes up and thereby presses into the drawbar tube located in the overrun device. This force is transmitted to the drum brakes in the caravan via a relay lever, brake rod and Bowden cables. The brake shoes are pressed outwards, overcome the clearance and rub against the inside of the drum. Vorreiter says: "The relay lever ensures that the Bowden cables have to move three to four millimetres to overcome the actual clearance." Caravan drum brakes usually brake about half of the towed unit's weight and thus relieve the towing vehicle's brakes considerably. "Properly maintained AL-KO brakes absorb almost 70 percent of the attached load," says Vorreiter. If the driver releases the brake pedal at the front, the drawbar tube is pulled out of the overrun device and the Bowden cables are released. In addition, return springs bring the brake shoes back to their original position.

Drum brakes resist adverse environmental conditions just as robustly as they cope with travelling many thousands of miles. They are considered low-wear. “The entire technology is simple, safe and easy to maintain,” says Vorreiter. “If you haven't made it to the workshop before your trip, practically any motor vehicle specialist at home or abroad will be able to help – and when you’re far from home, that's a big advantage.” Adjusting brakes, removing wear and tear, changing pads, checking and renewing lubrication points: none of this poses a challenge for workshop professionals.

Wer den Trommelbremsen eines Caravans etwas Gutes tun will, der nimmt die vorgeschriebenen Wartungsintervalle ernst. Alle 5.000 Kilometer oder einmal im Jahr sollte Fachpersonal die Bremsen durchschauen. Der wichtigste Teil der normalen Wartung besteht darin, die Komponenten zu säubern, den Bremsstaub in der Trommel fachgerecht abzusaugen und zu entsorgen, bevor er bewegliche Teile zusetzt. Ebenso sind bewegliche Teile gängig zu machen und vor allem die Bremsen richtig einzustellen. „Das alles ist in der Werkstatt schnell erledigt und verhindert zuverlässig Folgeschäden“, sagt Vorreiter. „Fällt die Wartung aus, kann statt einfachem Reinigen schnell der Wechsel von Komponenten nötig werden – was dann leicht deutlich teurer wird. Grundsätzlich können zu hohe Temperaturen dazu führen, dass die im Belag gebundenen Harze ausgasen und die Beläge ‚verglasen‘. Wenn die Beläge beschädigt sind oder mit weniger als zwei Millimeter Belagstärke an der dünnsten Stelle die Verschleißgrenze erreicht ist, sollten sie achsweise getauscht werden.“

Drum brakes work best at a brake temperature of 100 degrees Celsius. Vorreiter says: ”200 to 300 degrees is quite acceptable on mountain routes and after several successive braking actions. Anything above 400 degrees is too much.” Heavy grinding – caused by a broken return spring, heavy abrasion not eliminated because of missed maintenance, or vitrified, damaged pads – can lead to overheating. Continuous light braking on mountain routes can have a very similar effect. “Grinding heats up the brake,” says Vorreiter. “With such continuous stress, the airstream and radiation can no longer dissipate the heat quickly enough.” At extreme temperatures, pads can vitrify and even break. Vorreiter says: “In extreme cases, braking performance can be reduced. And at very high temperatures, the grease in the wheel bearing can also lose its effectiveness.”

Skilful driving, clever maintenance

If you are wondering whether one of your brakes actually did overheat on its last drive, it is almost impossible to tell afterwards. Indications are dark spots on the rim, white annealed drums and brake back plates or even the small plastic covers on the back of the brake plate, which melt at around 300 degrees. Vorreiter says: “Temperatures of 300 or 400 degrees are very theoretical values. In practice, very few people have a suitable measuring device with them, especially since it depends on the temperature inside the brake and how long it stays at that temperature.” To reliably avoid overheating, drivers should brake harder and less often, especially when driving downhill. If the route allows it, it can also be useful to accelerate in between to stretch out the vehicle combination again. This prevents so-called dragging, i.e. the permanent grinding and overheating of the pads. In any case, it is important to adjust your speed by thinking ahead.

Drivers should bear in mind that many modern towing vehicles activate an engine brake automatically. “This means that the vehicle brakes without you putting your foot on the brake yourself. But for the trailer, every deceleration of the towing vehicle means more braking.” This can put the brakes in the trailer under stress. So on longer downhill routes, a cooling break can be quite useful – and is a good excuse to treat yourself to a coffee every now and then. The caravan should definitely be given regular maintenance appointments in the run up to the big trip. Vorreiter says: “The drum brake technology is simple and cost-effective. And with properly maintained drum brakes, the next holiday is sure to be a safe and enjoyable experience.”