Which brakes are better: disc or drum brakes?
The main role when braking a vehicle lies with the brake mechanisms. They slow down the rotation of the wheels due to friction forces.
The operating brake system of a passenger car consists of two main components – the actuator, which provides the transmission and increase in force, and actuators installed on each wheel. Their task is to convert the force into friction force.
Types of brake mechanisms and design features
On passenger cars, only one type of actuator is widespread – hydraulic, working mechanisms – two types:
The first type is gradually abandoned in favor of the second due to certain features. On cars can be different arrangement of actuating mechanisms: only disc (occurs more and more often), all drum (remained only on trucks), combined (on the front axle – discs, rear – drums).
The entire structure consists of movable and stationary elements. The main among the moving is a drum, made in the form of a bowl. It is installed on an axle (through bearings), which provides the ease of rotation. Attached to it is a wheel, when moving, they both rotate at the same speed.
The fixed part is the shield fixed to the hub. A hydraulic cylinder with pistons and pad support are bolted to this shield.
The brake shoes are crescent shaped. Friction pads are mounted on the outside to increase friction forces.
The tops of the pads rest on the cylinder pistons and supports. In this position they are fixed by means of clamping springs and clamps. The drum is placed on top of these pads.
The mechanism works simply: when the brake pedal is depressed, the brake fluid flows under pressure into the mechanism cylinder. The pressure created pushes the pistons out of the cylinder. As the tops of the pads rest on them, the movement of the pistons is accompanied by their divergence. This causes the pads to press against the inner working surface of the cup, and friction is created between them, which slows the speed of rotation of the drum with the wheel. When the pedal is released the pressure in the cylinder drops and the springs pull the pads back to their original position. The wheel is braked.
Video: Drum or disc brakes. Which are better? Easy to understand
Disc brakes use a different design. In it, the main working element is a disc mounted on the hub. Brake pads (in the form of plates) with friction pads are located on the sides of the disc.
On top of this is a caliper with a working cylinder. Some cars used a caliper with two pistons, each acting on the pad.
But more often used a single-piston caliper design, but in order to ensure that both pads are pressed, it made movable.
This mechanism works like this: when pressure is applied, the piston comes out and presses one pad against the side working surface of the disc. This creates a counterforce which causes the caliper to move on the guides and starts to press the second pad with its body. This movement ensures an even distribution of the clamping force.
As you can see, both mechanisms use different ways to get friction, in the first case you need to separate the pads to trigger the mechanism, and in the second you need to press down.
So which is better and what are their disadvantages?
Compared to the disc version, the drum version doesn’t have many advantages:
- The closed design eliminates the ingress of dirt between the friction elements, which makes them last longer.
- The drum is a massive element, so it is not “afraid” of sharp temperature fluctuations.
- In the drum mechanisms, it is easier to organize the mechanical locking of the wheels (parking brake). For this purpose, it is enough to install an additional lever, the movement of which will ensure the separation of the pads and their retention.
- Large area of contact with the pads of the disc (due to the increased overall dimensions) provides a high braking force.
As for the disadvantages, drum brakes also have a lot of them, and many of them are intertwined with the advantages:
- Due to the closed construction the products of wear of the working surfaces are not drained off and get between the rubbing surfaces, reducing the friction force;
- Friction is accompanied by intense heat, which is insufficiently dissipated in the drum mechanism. This causes the drum to expand (because it is metallic), and the driver presses down on the brake pedal to create the proper pressure, otherwise the braking efficiency will be reduced;
- uneven wear of the pads. The pistons are not able to press them evenly, so they do not contact the drum with the entire area;
- a number of components are in a closed space, so when they break, the broken off parts have nowhere to go. They can get between the pads and the drum, resulting in a blocked mechanism.
Video: Disc and drum brakes: pros and cons
Positive qualities of disc brakes:
- High efficiency (20% higher than that of drums). Here, too, thermal expansion plays a role. The disc from the heat expands, which increases the frictional forces without additional impact on the pads.
- The open design is well ventilated, allowing heat to escape.
- Wear products are removed and do not fall on rubbing surfaces, adding to the efficiency of the mechanism.
- Moisture that gets on the disc is also removed from the surface.
- The pads are pressed against the disc with their entire working surface.
But also not without disadvantages. Disc brakes have such disadvantages:
- The disc is sensitive to sudden changes in temperature. Intensive braking, followed by a run in a puddle, causes warping (violation of the geometry of the working surfaces);
- wear and tear of pads and discs causes frequent maintenance with replacement of consumables;
- difficulty of using disc mechanisms for organization of parking brakes;
- dirt getting on working surfaces, provides intensive wear.
If we consider the arrangement of mechanisms on the car, the combined one is still the most optimal. The disc ones have a high efficiency, and the drum ones do not require frequent maintenance. Therefore, many manufacturers use this arrangement for budget and average price category cars. But drum mechanisms are gradually being superseded by disc mechanisms.
Disks vs. drums
Why do some cars have all disc brakes, while others have front disc brakes combined with drum rear brakes? Which system is better? What are ventilated disc brakes? And what are tuning brake discs with holes for? These questions are often asked along with others related to brakes. Let’s find the answers to them.
Drum brakes for all four wheels have been the standard for many years. In this type of brake system, the piston is subjected to hydraulic pressure and, in turn, pushes a curved shoe outward. In doing so, the friction material riveted or glued to the pad contacts the inside of the brake drum, slowing it down as the axle rotates. As a result, the car stops – that is, we hope it stops.
In fact, drum brakes have worked well in the past and still do, but anyone who has tried to stop a car traveling at high speed using only drum-type brakes quickly realizes that they have their limits. They “fade,” meaning they lose effectiveness when they get hot. Heat causes expansion, and the pad needs to travel a longer distance to make contact with the drum, which means the brake pedal needs to be pressed harder. In addition, as the shoe and drum rub against each other, gases are released that become “trapped” between the two parts, also reducing braking efficiency. You might be able to brake well once at high speed, but try it twice and you’ll have to test your luck.
Manufacturers added cooling fins or riveted aluminum to the drums to better dissipate heat, but it still didn’t do much good. That’s when discs came along.
Disc brakes had been used before in aviation and industrial vehicles. In them, the friction material (brake pad) is pressed against the rotating disc by the same hydraulic pressure applied to the piston. Discs, unlike drums, have no tendency to “stick”, so they allow the vehicle to better maintain trajectory stability when braking. They are also much more open to access, which has its pros and cons.
Disc brakes cool better because air can circulate freely between the disc and the pad surface. There are also vented discs, which have two friction surfaces. They are separated by jumpers that allow air to get inside the disc and even better dissipate heat from the brakes. Most front disc brakes on modern cars are ventilated, because they do most of the work when stopping the car. At the same time, most of the rear brakes are not ventilated. They have a solid disc, because the rear brakes simply do not generate much heat.
Another benefit of disc brakes is that dirt and gases are easily discharged from the disc as it rotates, unlike a drum, which easily collects dust from the pads, for example. Water, oil, and friction gases are all quickly diverted away from the working surfaces without impairing braking.
Brake discs with perforations (holes drilled into the discs) are partly just decoration, but not entirely pointless: the holes allow the water and gases between the pad surface and the disc surface to “plug” into them, and the brakes thus act faster, without waiting for an extra turn of the disc to clean it. This can be important in situations encountered in motorsports, but in everyday urban driving is usually uncritical. Also, the holes reduce the rubbing surface area of the disc, and small stones can get stuck in them, which will require extra work to remove them.
The biggest disadvantage of disc brakes is that they are prone to dirt because of their openness. Dirt and dust that gets between the disc and the pad can quickly render the disc unusable. If it is too thin, it is not able to dissipate heat and in extreme situations can simply crack. Therefore, disc wear should be monitored and replaced if necessary.
The front brake discs are in relatively favorable conditions, but the rear ones take up all the dirt that is thrown back by the front wheels. That’s why the rear discs often wear faster than the front discs, even though they take a much smaller share of the work during braking.
The anti-lock braking system (ABS) works best with disc brakes because the discs detach quickly and smoothly from the pads. Of course, after some modifications and changes in the shape of the piston and its location, the drum brakes began to separate better. Disc brakes, however, are still ahead of the curve. For this reason, and to ensure maximum braking efficiency, you can now find disc brakes both front and rear on many cars.
Personally, I prefer a combination of discs at the front and drums at the rear. That way I get all the benefits of disc brakes on the front axle and can stop quickly and easily where I need to. At the same time, the drum brakes in the rear are less affected by dirt and wear slower. And that saves me money. And don’t ask me to switch to disc brakes on all wheels – they scare me!
About the author: Jim Kerr is an automotive mechanical designer and automotive technology instructor. He has written automotive-related articles for newspapers and magazines in Canada and the United States for 15 years. He is also a member of the Automotive Journalists Association of Canada (AJAC).