The tale of drum and disc brakes

By Barry Lake

TODAY, drum brakes are mostly found only on the rear of a new car, usually on budget or modestly priced ones at that.

High-priced cars and high-performance cars all have disc brakes on all four wheels.

Disc brakes have replaced drum brakes as the norm on cars for many reasons. They are more consistent in their operation, are self-adjusting and it is easier to fit new pads to disc brakes than new linings to drum brakes.

So why weren't drums made obsolete decades ago?

One reason was cost.

Disc-brake systems are more expensive to produce than drums. This is partly because discs require power assistance for optimum performance, while drum brakes could get by without such costly and complicated help.

It is far easier and cheaper to fit an effective hand brake to a drum rear brake system than to rear discs.

Internal expanding drum-type brakes replaced more primitive systems around 1900 and -- with continual development -- were to dominate the scene for more than half a century. It took about another 50 years for drums to give way to discs.

Obviously, the drum system did its job well. The most common system used a pair of metal "shoes" with friction linings on them.

They sat inside the drum, had fixed pivots at opposite ends and were operated by a mechanism that pushed the free ends of the shoes away from each other, into contact with the drum.

This meant that, whichever direction the wheel was rotating, one of the shoes was a "leading" shoe and as such had a "self- energizing" effect.

As the free end would come into contact with the drum, the friction would tend to pull that shoe tighter against the drum.

Disc brakes were first used regularly in motor racing in the 1950s, notably on Jaguars, and began to appear more and more on road cars from a decade or so after that.

Although power-assistance (via a vacuum servo) preceded the introduction of disc brakes, it wasn't really necessary for drum brakes.

But power-assistance was almost an essential ingredient for the success of disc-brake systems because they do not have a self-energizing effect like drums.

A disc brake consists of a thick disc, machined on both sides, rotating in place of the drum. It has a caliper straddling it, with metal pads faced with a friction material adjacent to the disc.

Foot pressure, aided by hydraulics and power assistance, clamp these pads on to the sides of the disc and thus the braking friction is created.

Among the benefits of disc brakes is that they are self- adjusting, so that brake-pedal height is not lost throughout the life of the pads, the exposed disc cools far more efficiently than the enclosed drums and worn pads are easier to replace than the linings in drum brakes.

Also, after driving through water, the water flies off the disc instantly, while water held within a drum brake can seriously reduce braking efficiency over a much longer period.

One deficiency is that because disc brakes require higher operating pressures than drums, it is more difficult to equip them with an efficient, yet easily applied hand-operated parking brake.

This, as well as the added cost of manufacturing a disc system, is one reason why some small cars retain drum brakes.

Possibly the main advantage with disc brakes is that they work exceedingly well with antilock brake systems, far better than drum systems.

The cost of disc brakes might be higher than for drums, but the advantages far outweigh this cost.