Brakes, unlike most things, tend to come in two main configurations. This is fortunate for the layperson because it’s much easier to learn (or make educated guesses) about what may be happening when something goes wrong. Brake drums are a part of one of these configurations.
So, what is a brake drum? A brake drum is a hollow, cylindrical, cast iron container that is attached to the inside of some car’s wheels. The brake drum always rotates with the wheel and never moves independently. The inside of the brake drum contains several basic components that make up the brake configuration called a “Drum Brake”.
Note: A brake drum is the metal container. A drum brake is the entire brake system that uses this style.
These may be less common than disc brakes, but they’re still in cars and large vehicles everywhere – and you may be likely to use them if you park on a hill. The interesting thing about these is that their basic design is several decades old, but they’re still used today in certain situations.
What Is A Brake Drum?
A brake drum is a hollow, cylindrical container that is attached to the inside of your car’s wheel and rotates with it. There should never be a time when your brake drum and your wheel don’t rotate together.
This is because inside of the brake drum contains two brake pads – usually called brake shoes – that extend to push against the inside of the drum and slow your car down.
These drums are typically made from cast iron, though some are composed of aluminum, which is a better heat conductor. Good heat conduction helps with improving heat dissipation and resisting brake fade, but cast iron works well enough in most cases.
As we mentioned before, brake drums are actually part of a brake style called the drum brake.
The first modern drum brake for vehicle use was recorded in 1900, but not patented until two years later. They were regularly used until the 1960’s when disc brakes began to take over. This was the beginning of the fade out process, and by the mid 1980’s, disc brakes were the dominant design.
How Does The Drum Brake System Work?
Brake Drums, as we’ve already mentioned, are cast iron containers that rotate with your wheels – in modern cars, this will almost always be the back wheels, if the car has drum brakes at all.
Brake Shoes are the equivalent of brake pads in this system. When the brake pedal is used, they press against the inside of the rotating brake drum and create enough friction to slow your car to a stop.
Wheel Cylinders are the small components that push the brake shoes towards the wall of the drums.
Return Springs are stretched when the wheel cylinder extends, then pull the brake shoes away from the drum wall when the brake pedal is released. Without this, your brakes would drag along the inside of your drum after you use them once.
The Self-Adjusting System is a mechanism that readjusts the resting position of the brake shoes whenever necessary. Over time, the thickness of the brake shoes wears down, making the travel distance to the drum wall noticeably larger.
The adjustments made to the position of the brake shoes keep the travel distance the same. This is why you don’t have to push your pedal any further than usual.
In either drum brakes or disc brakes, the movement will be actuated by the brake pedal and transferred to the brakes through a hydraulic fluid called brake fluid. The only exception to this is in very large vehicles, which will use compressed air.
Issues
Brake drums can occasionally have issues that are characteristic of their hollow, enclosed design.
- “Long Pedal” happens when abundant amounts of heat can’t be dissipated. This is likely to happen after excessive braking while going down a hill, for example. The heat from the friction will make the drum expand – something that almost all heated materials will do.As the walls of the drum will expand away from the brake shoes, the driver will have to push the pedal noticeably further to make the same amount of contact to slow the car.This will reverse itself when the brake drum slows down, but it’s good practice to avoid prolonged braking when going down hills. This advice applies to both drum brakes and disc brakes.
- Water can sometimes get trapped on the inside of the drum. While the drum is spinning, the liquid will spin on the inside and work its way between the brake shoes and the drum walls.This will decrease the car’s braking ability by reducing friction, until the water is released or vaporized by the heat.This is why anyone who had spent more of their time driving older cars may advise you to use your brakes for a few seconds after driving through a puddle. Fortunately, modern cars use disc brakes in the front wheels, and the majority of braking power comes from the front.
- Design of the brake drum is not exactly an issue, but it may take more time to service or replace than a typical disc brake. .
- Health Hazards of working on drum brakes aren’t as bad as they used to be, but they are worth noting. The brakes on very old cars used asbestos in the brake shoes as part of the friction material.This is nonexistent now, but may still come up when work is done on a significantly older car. Avoid this whenever possible.
Modern Use
Though drum brakes aren’t used often in modern consumer vehicles, they are still installed under certain conditions. The majority of modern cars use disc brakes in the front and many have discs in the rear.
Some lower end cars have drums for the back wheels instead. Brake drums are much more effective as parking brakes than their disc relatives, and can be more cost effective.
Large vehicles – think tractors, buses, and 18 wheelers – also use drum brakes, and often for all of the wheels. There is some variability to this with other other countries, as air disc brakes are becoming more common, but large vehicles in America have historically stayed with the drum style.
Manufacturing
Drum brakes are less expensive to manufacture. Drum brakes are often used in lower end cars primarily as parking brakes in the rear wheels, while disc brakes are used in the front. In many cases, this helps with the bottom line.
Safety
Safety is a huge one. Drum brakes being better parking brakes is another reason why you may still see them on the back wheels of some less expensive cars.
The shape of the brake drum and brake shoes creates something similar to a “self-locking” wedge – meaning that trying to turn the wheels after the brakes are engaged will only make them hold tighter.
Drum brakes can also provide more stopping power than a disc brake of an equal diameter.
Aren’t you glad that we have these on larger vehicles?
Longevity
Drum brakes, with their enclosed environment, are less likely to corrode or contact with damaging material and may end up lasting longer than disc brakes used in a similar environment.
Keep in mind, though, that that majority of braking power comes from the disc brakes in the front, so this also plays a factor in a disc brake’s potentially shorter life span.