Rear disc brakes worth it?

Question: I am puzzled by the fact that, notwithstanding many sophisticated built-in safety features, a number of vehicles above the entry level of the market are sold with drum brakes on the rear wheels. A random sample includes the Honda Jazz, Kia Rio and Toyota Yaris, and most, if not all, double cabs. Disc brakes have been with us for more than 50 years. Surely the research and development costs have long been covered. Are discs brakes at the rear unimportant, or are there other reasons?
ANDRIES VAN ROOYEN, Stellenbosch

Answer: When a car brakes, up to 70 per cent of the braking force comes from the front wheels owing to the transfer of mass. Therefore, there is no need to have excess stopping power on the rear wheels. A quick summary of the types of brakes:
Disc brakes:
• They provide better stopping power and control due to superior heat-dissipation properties;
• Are self-adjusting;
• A small drag force will always be present;
• They need a higher activation force than drum brakes;
• Due to the high activation force needed, the parking-brake mechanism may be tricky to implement. Sometimes, small brake shoes will  t inside a small drum inside the rear discs. Otherwise, a hydraulic/electrical system may be used over a manual cable system to ensure a high activation force.
Drum brakes:
• They are not self-adjusting;
• The spring return ensures no drag force when not activated;
• Drum brakes need less activation force because they are self-acting – the friction force will also increase the shoe-to-drum-contact force depending on the shoe hinge-point location;
• Drum brakes work very well as parking brakes (cable activation). This could be why bakkies tend to use rear drums as they might be carrying a load.

It all revolves around cost. Drum brakes consist of only one casting and all the other parts are cheap stampings. Disc brakes consist of the disc, calliper and calliper bracket that are expensive to manufacture, plus the parking-brake mechanism.

Article written by

CAR’s technical editor was once the senior vehicle integration engineer on Optimal Energy’s Joule electric car and had stints with Ford (UK), Integral Powertrain (UK) and CAE. He completed his MSc in mechanical engineering in 2004, obtained UK Chartered Engineering status in 2009 and is a judge of the International Engine of the Year competition.