Drum brakes were the first type of speed reduction on an automobile (apart from engine braking). The design came from the clasp brake that railway locomotives used – the difference being that a clasp brake applies pressure on the outside of the wheel and the drum brake applies it on the inside of the mechanism. It is a very simple design and concept, to reduce speed, the brake applies friction to a surface that then converts the velocity to heat.
Wilhelm Maybach used drum brakes on one of his first cars in 1900 and Louis Renault took the drum brake a step further in efficiency in 1902 by using asbestos linings to remove as much heat as possible. Renault managed to patent his design.
The brake unit consists of a drum housing (hence the name), two “shoes” to create friction, a master cylinder that applies the force to create the friction and a back plate that gives structure to the device and helps to keep the inside of the drum dry. Initially a series of mechanical components were used to push the shoes against the inner wall of the drum. This would have been OK for lower speed driving, however as cars progressed to higher speeds, it was necessary to redesign this to provide greater force and therefore greater friction to reduce the velocity. Remember, cars quite quickly were able to hit 100mph and you need a braking system to cope with that!
Hydraulic systems were developed using fluid to provide greater force through the master cylinder to pistons that pushed the shoes outwards into the drum. This basic concept is still in play today – even though braking systems have evolved substantially using discs instead of drums.
For the geeks in us, the shoe at the front of the assembly is the Primary and the rear shoe is the Secondary one. There is also the concept of having a twin-leading system. This is where there are two primary shoes and one secondary. This allows for a greater amount of friction and therefore a faster reduction in velocity.
This leads on to one of the dis-advantages of the overall system: heat dissipation. As the design is an enclosed drum, heat is retained in the components far longer than a disc which is exposed to the atmosphere. This can lead to brake fade (although is not the only reason for this) where the braking performance is reduced due to the heat changing the properties of the shoes and drum linings. Heat has a tendency (thanks to physics) to expand materials, so as the drum gets hotter it gets slightly bigger, thus more pressure is needed to get the shoes in contact with the drum. Also the heat generated can cause the brake fluid to change properties and be less efficient!
The other disadvantage is the dust created by the friction – when most shoes were made of asbestos, this was a serious issue as the dust can cause fatal illnesses in humans. The drum collects most of the dust, so when it came to be removed for servicing, the dust would then become airborne. Asbestos has been banned for use in all automotive applications in many countries including the US, Australia, Japan and the EU.
Most modern cars have disc brakes to slow them down, however they might have a mini drum fitted for the parking brake – this function is particularly suited to a drum as it doesn’t get used in the same way as a disc. Heavy trucks, some motorcycles and delivery vehicles still use drums on the back where the load needs to be controlled – the front brakes typically do most of the work, the back ones help balance the vehicle. Some hybrid cars use drums as well as part of the KERS (Kinetic Energy Recovery Systems) to generate power under braking.
Like many technologies, drum brakes have never really become extinct as they are a good fit for some functions and so today, 117 years later, we are still using them!