Today, South Denver Automotive is answering a frequently asked question, what’s the difference between drum and disc brakes? We’ll also explain a little bit about the dynamics of braking. As with anything in the automotive industry, car manufacturers are always seeking ways to improve braking systems and offer greater stopping power and safety in today’s vehicles. Since the 60s, however, there have been two main types of brakes used, drum brakes and disc brakes.
The goal of brake research and development has always been to improve stopping power and make vehicles safer to drive. It has been a constant quest for many years, and the designs have improved by the addition of better performing materials like sintered metal, carbon fiber, and a lightweight version of steel as well as the introduction of systems like ABS (although the effectiveness of that is still up for debate).
Through it all, however, one thing has remained constant. Disc brakes have always proven to be a more effective design than drum brakes. Now, of course, today’s drum brakes have been significantly improved over earlier models and are better than the early disc brake designs. However, the disc brake design has also improved proportionately, so it still comes out as the leader.
Now, if you know anything about cars you probably know that both types of brakes are in use today, which begs the question, if disc brakes are so much better why not just use them exclusively? The answer, quite simply, comes down to cost. Drum brakes are significantly cheaper to manufacture, and that makes it worth it both for manufacturers in their production costs and consumers with the resulting purchase price.
But should cost be such a concern when it comes to safety? Probably not, but everyone knows that in the real world, price still comes into play regardless. Plus, the way that drum brakes are incorporated into modern cars does not diminish their safety or braking ability by much.
What it boils down to is that between 60-90% of your stopping power comes from the front tires of your vehicle. So, it makes sense to put a set of disc brakes on the front wheels, and drum brakes on the back, to create a vehicle that’s both cost-effective and safe. In many high-performance cars, especially the type used for racing and activities of the like, it is worth it to install a four-disc brake system. However, four disc brakes are overkill and add unnecessary cost for the average everyday driver.
The Principles of Friction and Heat
Now that we have established that disc brakes offer more advanced stopping power than drum brakes, let’s talk about why. Both systems rely on friction and heat to slow your vehicle. Both designs use brake pads to apply resistance to the vehicle’s wheels, thereby creating friction that will eventually stop the vehicle. How quickly depends on a variety of factors including the vehicle’s weight, total braking surface area, braking force and heat transfer. The heat transfer part is important for us because that’s where the difference between drum and disc brakes comes in.
Now, as anyone who has taken a high-school science class can tell you, heat is a byproduct of friction. In autos, it’s the reason why oil changes are so important, to keep engine parts lubed and moving freely instead of building up heat that can damage your car engine. So, when you are braking and applying all that resistance to the wheels and causing all that friction, the natural byproduct is heat. If that heat remains trapped near the wheel for long enough, the brakes will become too hot and start to fade. Stopping power will be significantly diminished as a result.
Now that you understand the importance of heat transfer, let’s look at the difference between drum and disc brake design. Even just a quick look will explain why disc brakes do their job so much better than drum brakes do.
Modern braking systems began with drum brakes. The oldest designs (after hand levers) incorporated drum brakes on all four wheels. They get their name from the drum-like housing in which resides the brake shoes. When you press on the brake pedal, hydraulic fluid is used to force the brake shoes against the drum and cause the wheel to slow down.
Of course, the components are made of heat resistant and heat wicking materials that help to move heat away from the drum as quickly as possible. But it’s not perfect, and as we mentioned, once it gets too hot, the shoes begin to slip and your stopping power diminishes.
Under most normal conditions this is never a concern. But if you put your vehicle in a high-stress braking situation such as constantly braking as you drive down a very steep hill, or stopping from a high speed repeatedly, you will notice a marked decrease in braking performance.
Disc brakes operate on the same basic principles as drum brakes, but the structure is different. Instead of a drum, the disc brake has a rotor and a caliper. Two brake pads reside inside the caliper. When the brake pedal is pressed, hydraulic fluid is again used to move the brake pads and force them to clamp together.
This is where the difference comes in. In a drum, the brake pads and resulting heat buildup are all trapped inside the drum, slowing heat transfer dramatically. The rotor design, however, is more open and exposed to the surrounding air. That moving air carries away heat much faster, and therefore disc brakes have a much lower chance of overheating and fading.
Again, this is why it is not considered unsafe to use drum brakes on the rear wheels to cut costs. The brake is still a good design and will only fail in rather extreme conditions. The rear wheels are never subjected to as much force as the front wheels when it comes to stopping power, so it is unlikely they will ever overheat. Even if they do, the front brakes will most likely still be going strong, and you will have the stopping power you need to slow your vehicle safely.
We hope this article has been helpful and you now have a better understanding of what’s the difference between drum and disc brakes. Of course, you can always contact South Denver Automotive with any questions and our experts will be happy to answer them for you! Also, don’t forget to have your brakes checked once in a while to ensure that you are safe out there!