Recently, drum brakes were almost exclusively found in small cars. Electromobility is now making a comeback for this robust and inexpensive technology.
An electric car doesn’t just brake, it recuperates. Instead of the classic friction brake, the electric motor decelerates most of the time and also generates electricity at the same time. The mechanical brake is downgraded to a part-time worker – and falls back on new old concepts.
The electric car brakes in everyday life mainly with the electric motor. When decelerating, it acts as a generator and converts the kinetic energy into electrical energy, which is then stored in the battery. Electricity for up to 17 percent additional range comes together on average. The friction brake does not offer such a yield – it lets the kinetic energy dissipate in the form of heat. Accordingly, it should be used seldom.
“We are currently experiencing a comeback of drum brakes,” explains Manfred Meyer, head of development for brake and steering systems at the ZF technology group. For many years, this type of design was mainly found in inexpensive micro and small cars. In recent years, however, it has also moved back into larger models – at least if they are electrically powered. Why this is so is explained by the new role played by the so-called friction brake in electric vehicles.
“We only use the friction brake for the last few moments before the vehicle comes to a standstill, when there is a great need for deceleration and when the battery is full,” explains Meyer, also naming the resulting problem. “If the brake is not used, it will corrode over time.” A phenomenon that is also known in the form of rust film from the brake discs of cars that have been parked for a long time. Above all, this is not nice to look at, but in the long run it can also lead to poorer braking performance.
The drum brake on the less loaded rear axle can be a solution. The parts that are at risk of corrosion are packed in a housing that keeps dirt and moisture away and at the same time keeps its own abrasion at bay. In addition, drum brakes are slightly cheaper than disc brakes – this is an important advantage for electric cars, which are already rather expensive. A classic weakness also plays a minor role in electric vehicles: the higher noise level. Since the friction brake is less stressed thanks to the recuperation, the unpleasant effect, the so-called juddering, usually does not occur in the first place.
However, ZF expert Meyer does not see the drum as a solution for all cases. “The disc brake has a number of fundamental advantages that make it interesting for e-cars as well,” explains the developer. It is thermally more resilient and can dissipate the resulting heat better thanks to its open design, which is of particular interest in very heavy cars or when driving in a sporty manner. In the latter case, it can also score with its greater agility – it simply decelerates faster than a drum brake.
To protect the panes from corrosion, car manufacturers and suppliers are breaking new ground. Meyer, for example, relies on the so-called “windshield wiper”. The software function allows the brake pads to be touched briefly at regular intervals in order to rub the disc free. “The driver doesn’t notice anything because we usually hide it in an acceleration process,” says Meyer. Other approaches are new, less rust-prone materials. But stainless steel or carbon are significantly more expensive than normal gray cast iron – and probably only imaginable for very high-quality vehicles.
Disc brakes will therefore continue to play an important role. Albeit in a variant optimized for e-mobility. The supplier Continental, which wants to increase the range of the vehicles through lightweight construction of the brakes, has a special approach.
The models called “Green Calliper” are said to have a lower residual grinding torque than conventional brake calipers and are also lighter. This is possible, among other things, due to the fact that weight could be saved with the heat dissipation technology, because the electric motor does most of the deceleration work in the electric car. The bottom line is that the supplier promises a range gain of around one percent. With the usual 400 kilometers, this corresponds to an increase of 4 kilometers per battery charge.
