What Are Brake Drums and How do They Work?

The automotive industry is constantly pushing the limits of new technologies. You’ve got the highly efficient GDI engines, EVs, and exciting tech such as fully functioning autopilots slowly becoming the standard. Yet, the automotive industry also likes to hang onto old tech. This dichotomy somehow works, especially when it comes to braking systems. A good example is a brake drum. Brake drums were invented back in 1900 and have remained in use up to this day. We’re literally driving around using the same principal technology as people who lived during the dawn of automotive development. Have you ever wondered why that is the case? Let’s find out.

What Exactly is a Brake Drum?

When you ask an average person to describe how the brakes work on a car, there’s a good chance they’ll throw a brake rotor design your way. Rotors are simple, efficient, and widely used on modern performance cars. However, rotors are not the OG of the braking world. That title belongs to drum brakes.

A drum brake is a system that utilizes a brake drum in combination with a pair of brake shoes to slow the vehicle down. The brake drum is actually just the portion of the drum brake that is visible from the outside. It’s only a single component of an otherwise fairly complex system.

Brake Drum Set - Rear (230mm)
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How Do Drum Brakes Work?

Friction has always been the main show in town as far as braking goes. The only thing that changed over time was how friction was used. Generally speaking, modern braking systems such as massive brake rotors paired with beefy, 4-piston calipers utilize what is essentially the same principle as drum brakes.

To understand how drum brakes work, we need to break it down into its main components:

  • Brake drum
  • Brake shoes
  • Adjuster mechanism
  • Brake cylinder
  • Handbrake mechanism

Here’s how all of these components fit together. Much like modern brake rotor designs, drum brakes use hydraulic fluid to function.

The way they work is simple — By pressing the brake pedal, the driver sends hydraulic pressure back to the brake drums. Within each drum, there’s a secondary brake cylinder that sits near the top of the drum.

Once the hydraulic fluid gets inside this hydraulic cylinder, it expands two pistons, one on each side. Each of these pistons then pushes against a brake shoe, which in turn makes contact with the brake drum. When the brake pedal is released, the hydraulic pressure drops, retracting the two pistons of the hydraulic cylinder, which allows the brake shoes to return to their default position.

It’s worth mentioning that there’s also an adjuster mechanism that you can use to adjust the tension of the brakes manually. That being said, most modern drum brakes are self-adjusting, so you don’t have to worry about that.

The Handbrake Mechanism

Aside from serving as the primary form of braking on vehicles that feature this type of brakes, a drum brake also serves as the handbrake mechanism. If you were to dismantle an average drum brake, you’d find that the hydraulic cylinder only activates the top to mid-portion of the brake shoes.

In fact, a worn set of brake shoes isn’t evenly worn from top to bottom. Instead, it’s worn on top where it’s pressing against the brake drum when you activate the brake pedal. The lower part of the brake shoe usually remains more or less intact. This is because the lower part of the brake shoe is used to activate the handbrake.

Just like there’s a hydraulic cylinder at the top of the drum, there’s also a mechanical device at the bottom of the drum that is connected to your handbrake via cables. When you pull the handbrake, the mechanism is expanded, pushing the shoe’s bottom part into the brake drum.

Why are Drum Brakes Still Around?

Why are Drum Brakes Still Around?

Although fairly effective, the brake drums design has become obsolete. Modern disc brakes are better in every single way — that is, if you limit yourself to performance metrics only. Drum brakes aren’t going anywhere in the foreseeable future because they are cheap, durable, and reliable.

Affordable to Produce

One of the main, if not the main, reasons why drum brakes are still being made as you’re reading this is the cost involved. Money makes the wheels spin in the automotive industry. In fact, many pickup truck manufacturers are straight-up refusing to give up drum brakes, citing serious increases in MSRP should they switch to rotors.

Mind you, you’ll rarely find drum brakes in the front of any vehicle these days. That’s where rotors are dominating. Drum brakes are reserved for rear axles, especially in econobox FWD cars. How much justification is behind this excuse is up for debate.

Most critics argue that brake rotors aren’t only affordable enough to the point where you can use them on anything, but they’re the safest choice. And since we live in a world where safety rules supreme, brake rotors should be everywhere. Alas, it seems that it’s not the brake disc itself that will bump up the MSRP. Rather, it’s upgrading the production line that costs the most.

Durable Design

Another reason that is often cited is durability. There’s some truth to this one as well. The fact that a brake drum is essentially an enclosed system makes it a bit more weather resistant. Brake shoes tend to wear in a linear fashion as well.

Lastly, where brake rotors are susceptible to warping or wearing out prematurely, brake drums don’t have those issues. The enclosure you see when you remove the wheel is what makes contact with the brake shoes when you brake.

The whole durability argument is somewhat sound. However, we have to mention that servicing drum brakes is a job that many mechanics simply live for. If you could taste the sarcasm oozing from that sentence, you know what’s up.

Changing brake shoes on drum brakes is a pain in the rear, even on modern vehicles. It’s much more complicated than changing brake pads. Dealing with the mess of springs, self-adjusting mechanisms, leaky hydraulic cylinders, and all kinds of other fun stuff is quite common for rear drum brake jobs.

Proven Reliability

Reliability is a given with these types of brakes. We’ve gone through so many iterations of drum brake design that we know exactly what works and what doesn’t. The only thing to look out for when using drum brakes is not to fade them too much. Why? Because they will fade, and quick! Unlike brake rotors, brake drums have trouble dissipating the heat effectively.

Drum brakes are reliable in the sense that they won’t fail you mechanically. But, they aren’t exactly the pinnacle of braking technologies.

Why are Drum Brakes Used Almost Exclusively in the Rear?

Today, car manufacturers know that they can get away with only so much when it comes to squeezing drum brakes on newer cars. Japan’s latest trends are showing that even the tiniest of vehicles are now being shipped with brake rotors on all four corners of the car.

The reason why you’ll see drum brakes almost exclusively in the rear of many cars has to do with weight distribution for modern vehicles. Your average sedan is no mid-engined performance car that has the center of mass surgically calculated for performance. On the contrary, you’re often looking at nose-heavy vehicles that are made to be safe and reliable.

Because of that, most cars are relying on front brakes to do most of the heavy lifting. The rear brakes are there to help out, stabilize the car when you step on the brake and give you the ability to lock the rear wheels using the parking brake.

Brake Drum Set - Rear (252mm)
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Diagnosing a Bad Drum Brake

When the brake drums decide to go, they usually announce their departure well in advance. There are a few tell-tale signs that you’ll need to hang out with your toolset in the near future or give your mechanic the good news.

Change in Pedal Response

In most cases, the first symptom you’ll notice is a change in pedal response. As the brake shoes wear out, pressing them against the brake drum will cause vibrations. You’ll actually feel the pedal pulsing along with the noise. If this happens to you, get your car checked out as soon as possible.

Handbrake Won’t Hold

Another good way to tell that something is wrong is if the handbrake won’t hold. This usually means that either the handbrake mechanism is faulty or that you lack friction material on the trailing shies within the drum.
Grinding Noises when Braking

Last but not least, if your car is making grinding noises in the rear when you step on the brake, that’s a good sign that your brake shoes are worn out and in need of replacement.

Use Quality Replacement Parts For Your Drum Brakes

Brakes are nothing to mess around with. It is paramount that you use only quality OEM parts to fix your brakes. Get brake shoes from reputable manufacturers and perform a complete inspection of the brake drum when changing the brake shoes. There is a decent chance you’ll need to replace a few springs or deal with a leaky wheel cylinder.

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