Auto safety questions: how brakes work

Detailed information on the inner workings of brakes for cars and trucks.

Not everyone thinks about them but you have used them if you're a vehicle driver.

Automotive brakes, or binders as the old folks call them, are ultimately the most important part of your car. Without them, we may as well not even have stop signs, traffic lights and - in the most extreme - school cross walks.

The braking system can be divided into 2 sections: Hydraulic and Friction.

The hydraulic system is the first step and the most overlooked. Essentially, your foot presses the brake pedal which in turn forces a rod/piston into a compression chamber (master cylinder) filled with a special liquid (brake fluid). Brake fluid is a noncompressable liquid, so it has to go somewhere to make up for the displacement of the piston. The fluid travels from the master cylinder to the steel lines and continues from there to each of the four corners of the vehicle.

The friction part of the braking system varies with year, make and model of the car. The most common setups today are Four Wheel Disc brakes, but the older cars generally have a disc/drum combination with the disc brakes at the front wheels and drum brakes at the rear.

Disc brakes are simply a heavier duty version of the brakes you'd find on a mountain bike or a childs' BMX bike. They can also be related to a CD or DVD disc that is a common item in most homes.

The disc part of the brake is similar to a CD/DVD disc because they are round, flat and have a hole in the middle. Some rotors are bigger than others, but it's only a matter of the size of the vehicle.

As an example of how these brakes work put your finger into the small hole in the middle of the disc. This simulates the axle from the transmission (front wheel/4 wheel drive) or an axle facsimilie if you drive a rear wheel car/truck. Now rotate your wrist to make the disc spin in a verticle plane.

Now you understand how the rotor/wheel spins by force of the axle, but how do you get it to stop? Simple. Brake pads are mounted on a caliper around the outside of the rotor like a hand ready to grasp it. Add this to our demonstration from earlier. Spin the rotor with one finger and place your hand around the top of the disc and clamp down on it, fingers on the shiney side of the disc, your palm clamping the label side.

The clamping force by your hand is made by the hydraulic fluid running down from the master cylinder instead of the muscles in your forearm.

Drum brakes are a little different. And by 'little', I mean completely.

The term 'drum' comes from the appearance of the housing that covers the rear braking assembly and does the actual stopping similiar to the rotor in the front.

Hydraulic fluid forces a set of shoes outward and against the drum to create the friction needed to slow/stop the wheels in the rear. In a demonstration of this you can take a small mixing bowl and place your hands inside, palms together and fingers following the curve of the bowl. At your fingertips, have someone else force your hands apart - keeping your wrists together. The backside of your hands come into contact with the bowl and stops it from moving. A set of springs would pull your hands back together after your buddy removes his "foot pressure" from your fingertips and the drum would be free to spin again.

It should be obvious to you now that stopping the rotor was easier than stopping a moving drum. This is why car manufacturers have moved on to using 4-wheel disc brakes. Cars in the 60s used drum brakes at all 4 wheels and this is why they take so much longer to come to a stop--so don't pull out in front of an older car and expect it to stop on a dime.

Disc brakes are also lighter, easier to maintain, and are cleaner than drum brakes so it's a win-win-win situation for everyone.

As cars age maintaining your braking system becomes more important. How are your brake pads/shoes? Any squeaking or grinding? Any leaks from rused steel lines? Does the car pull to one side when you hit the brake pedal?

Don't wait until little Johnny or Sally is in front of you in a cross walk to find out that your brakes don't work. Have your brake system checked over at every oil change or if something is awry.

When a repair facility tells you that you need to have "Pads and Rotors" they are telling you that your CD and hand are worn out and need to be replaced. Not the complete hand but just your fingertips where the brake pads would be.

It's the same thing with rear brakes needing the shoes and drums replaced. They will replace the pads/shoes and rotors/drums and any springs or rubber parts that apply with new aftermarket parts.