Brakes: discs or drums?
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Brakes: discs or drums?

Both disc and drum brakes work on the same principle. They stop the vehicle due to the friction force that occurs. However, the technical implementation of the idea is different for both systems.


Despite the fact that many today consider drum brakes to be something archaic and “unfashionable,” they coexist peacefully with disc brakes. Just look at the ones produced cars. Drums are usually installed on the rear axle of small budget cars and expensive crossovers. And disks cannot completely replace them. To understand why, it is enough to understand the structure of both systems.

prehistory


Drum brakes were also used on horse-drawn carriages and coaches. Officially, this system is believed to have been invented in 1902 by Louis Renault. Although, according to other sources, a simplified version was used by Maybach in 1901. Whether this is true or not, we can’t know for sure now, but drum brakes were initially installed everywhere, although in the same 1902, William Lanchester received a patent in Great Britain for an invention that allowed a vehicle to be stopped abruptly by pressing the pads against the disc.

Drum brake design


It has survived to this day practically unchanged since the time of Louis Renault. Except that they began to use two cylinders instead of one, and also installed spacer bars that automatically set the gap as the pads wear out. Until recently, they were pressed manually: our grandfathers regularly performed this procedure.

Brakes: discs or drums?View of the drum system from the inside. Photo: youtube.com

A drum brake is a cup made of metal (cast iron, aluminum, steel or a combination of these) and fixed to the wheel hub. Inside is a hydraulic cylinder (or two) with a pair of pistons that, when pushed forward by the liquid, press semicircular pads with friction linings to the inner surface of the drum. The latter were once replaced by riveting new ones. Today, the linings are glued with special compounds and no one would think of attaching these parts manually today - it is easier to buy them.

Advantages of the drum system


First of all, it is closed and protected from the influence of the external environment. Foreign particles and sand, which contribute to rapid wear of parts, do not get inside. Due to this advantage, drum systems are still installed on the rear axle of frame SUVs and similar equipment, “doomed” to “dig” in the mud.

Real SUVs have drum brakes at the rear, protected from dirt. Photo: youtube.com

The next advantage is that the braking efficiency increases with an increase in the contact area between the pads and the "cup". Therefore, for a very long time, even after the advent of disc systems, drums were installed on heavy-duty vehicles and buses there is no alternative. After all, the size of the mechanism parts does not matter to them: it is impossible to significantly increase the dimensions of the brake components on a passenger car - there is simply no space.

Sports cars have very hot wheels. Photo: youtube.com

And one more advantage is a long service life. But it is associated with lower efficiency of drums, if compared with disks. In the first case, the pads are not pressed as hard, therefore they wear out longer. Someone will say that the drum system is cheaper. If we take today's realities, this question is debatable. For example, it is much more difficult to connect the handbrake to disk brakes and it is more expensive. Nevertheless, the latter are still more widespread, so let's move on to the disadvantages of drums.

Cons


The first of them appeared long ago, and at the legislative level! The fact is that asbestos, a material harmful to human health, was initially used to make brake pad linings. The dust formed from it got into the lungs of mechanics and fitters, causing serious illnesses. As a result, the US government, concerned about the health of citizens, prescribed the use of alternative materials. However, such were not found at that time, and self-adjusting gaps were invented only in the 1950s, when brake discs had already gone on the "offensive".

Inside the closed drum system there is a lot of dust from the worn out pads. Photo: youtube.com

Another disadvantage, which follows from the advantage, is the closed nature of the system. It does not allow wear products to evaporate and, when the brakes are pressed frequently, causes the temperature of the pads to increase, in some cases reaching 500°C.

Environmentalists will be "for" an isolated cavity: then you can dispose of the products of wear so that they do not pollute the environment. But whether anyone does something like this is a big question.

If you get into a puddle melted by reagents in winter, you may not be able to move to the places after parking. The pads freeze to the drums very strongly due to the larger contact area, compared to the disks. The relatively low efficiency of the described system is associated with the work of the pistons in the cylinder (even if there are two of them). They are not able to evenly press the semicircular pad to the entire surface of the drum. Therefore, the car slows down poorly.

Disc brake system


It wins for a simple reason – better efficiency, which is achieved due to the fact that the contact surfaces are absolutely flat, and not convex-concave, as in drum brakes. The second advantage is better cooling due to the openness of the system.

An experiment showing how brakes heat up. Photo: youtube.com

The pioneer among cars with discs was the Jaguar C-Type, a sports car released in 1953. In America, it was the Studebaker Avanti of 1962. Since then, disc brakes have begun their "victorious march" across the planet.

Jaguar C-Type – the first passenger car with disc brakes. Photo: youtube.com

The entire system has fewer parts and fits practically into the caliper. It is fixed motionless. Inside is a brake cylinder (there can be several of them) with pistons that extend and press the pads to the disk. The latter (usually on sports cars) can be perforated (with holes) - ventilated. All the advantages (disadvantages) of disc brakes follow from the disadvantages (advantages) of drum brakes.

The structure and operation of disc brakes. Photo: youtube.com

Perhaps one point can be added: the system is afraid of sudden temperature changes, which may occur, for example, after a series of active braking, when the parts heat up, and rapid cooling (driving into a puddle). In this case, there is a risk of disk warping.

Perforated discs. Photo: youtube.com

A reasonable question arises: why don’t they create a universal system that would combine all the advantages and eliminate the disadvantages?

The disk system is open, so the stationary parts rust quickly. Photo: youtube.com

They tried. Chrysler was the first to do this, back in the late 40s and early 1950s. But it was unsuccessful. However, Audi "dusted off the archives" and made its attempt in the late 1980s. The company installed universal brakes, which were discs in drums, on heavy-duty cars with V-shaped eight-cylinder engines.

Mechanics sincerely hated the universal system because of the complexity of maintenance and the high cost of spare parts. In short, it did not catch on: it turned out that the "improved" mechanism, instead of using the advantages of drums and disks, collected all their disadvantages into a single whole.

Verdict


If someone thinks that the age of "archaic" brakes is over, then you can look at the most modern cars that use battery power. Even they have drum brakes at the rear. This means that their capabilities are far from exhausted.

Replacing rear brake pads on Daewoo Matiz drum brakes

There is another point: today most cars are concentrated in cities, where you can’t really speed up. If you introduce disc brakes on every car produced “in a circle”, you will have to change the production chain. And this is uneconomical. In addition, installing disc brakes on the rear axle will inevitably increase the cost of the car, at least at first. And highly efficient brakes are not needed by owners of, for example, Matiz, Smart, Peugeot 107, etc. So it is possible that drum brakes will outlive the internal combustion engine.
What kind of brakes do you prefer?
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