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CHAPTER 1

BASIC BRAKE OPERATION

When you come to a stop sign, your vehicle has to be able to stop, and when it comes to safety, brakes are critically important.

Safety

Automobile manufacturers of the 1960s and 1970s boasted seat belts, safety glass, radial tuned suspension, safety cages, radial tires, and crumple zones. Since then, we’ve seen airbags, back-up cameras, and a long list of other safety-related updates. Antilock Brake Systems (ABS) have been around for more than 30 years, and now with the addition of Advanced Driver Assistance Systems (ADAS) and autonomous vehicles with no steering wheels on the horizon, we are moving into a new level of safety. The promise of these systems may completely eliminate vehicle fatalities someday.

Automotive brake systems are obviously important to vehicle safety and must be in excellent working condition, ready to perform in all conditions. Over the years, improvements have been made to greatly enhance driver assistance and comfort. Electronics have made it possible for engineers to blend the brake system into other vehicle systems, but the basic brake system has not changed all that much since its early designs.

Electronics have made this possible and are a big part of our lives and the modern vehicle, but there will always be a need for basic brake maintenance. The friction components are wear items and must be replaced at some point. Unless we jump to hovering vehicles on magnetism, air, or some other means, we will continue to have wheels that must be stopped.

You need to also consider your own safety when working on your vehicle’s brake system. Safety glasses or a face shield, gloves, a dust mask, and ear protection (if rotary cleaning tools, impacts, or air hammers are used) are all important so that you are not injured while doing the work. Doing the job right is also extremely important to your safety and those around you, so be sure to read and fully understand the contents of this book before you begin.

I have tried to cover all the details, but because brake systems vary in design, you should consult your vehicle’s service information or speak to a professional technician for specific procedures and specifications. Your local auto parts store can be a big help too.

Automotive Brake Systems

Believe it or not, the first vehicles had no brakes. Drum brakes were the first brake system used on early vehicles, but they did not use brake shoes like we know today.

History

The first designs used a flexible friction band that clamped around the outside of the drum that the wheel bolted to, like a band in an automatic transmission.

Most vehicles had them only on the rear axle. When the brake pedal was applied, a long metal rod from the brake pedal moved a lever that squeezed the band around the spinning drum, hopefully bringing it to a halt. No hydraulics were used at that time and this design was also subject to water contamination and rust since it was exposed to the elements. Needless to say, the system required a lot of pedal force to stop. You might think vehicles didn’t go fast enough to require much force, but actually vehicles in that era could achieve highway speeds and were extremely heavy, so accidents were pretty common due to inadequate brakes.

The external band (arrow) that applied around the drum was very similar to those used inside of an automatic transmission but were exposed to the elements. As a result, they were greatly affected by rain and dirt contamination. Rust was also a common problem with this design.

Eventually, internal brake shoes replaced the external band on drum brake systems and they were added to all four wheels, but hydraulics were still not used. The systems remained mechanical. This made it difficult for brake engineers because the front wheels must turn, and the mechanical linkages to the brakes must accommodate this. Enter hydraulics.

This 1919 Buick has a basic rear brake–only system that squeezes friction bands around spinning drums with mechanical rods to the brake pedal. Highway speeds were possible with this vehicle, which made braking an interesting experience.

Mechanically applied drum brake systems used long metal rods between a lever at the brake pedal and levers at the rear axle (arrows). Because the axle moves up and down when the suspension moves, brake pulsations could occur when stopping.

Hydraulics provide the same function as mechanical rods but are flexible and can route around obstructions and move with the suspension and steering systems. The term “hydraulic advantage” is used to explain how different-sized pistons can increase force inside of a brake system. If an input piston attached to the brake pedal is smaller than the output piston that applies the brake shoe or pad, the force is increased.

When brake engineers added hydraulics to apply the shoes inside the drums, brake systems became extremely effective and reliable. Hydraulic fluid within a closed system acts like a solid metal rod, but it can be routed around components and pass through flexible hoses so that the front wheels can turn and the vehicle suspension can move freely. Hydraulic principles also allow for different piston diameters to be used between the master cylinder (input at the driver’s foot) and the wheel cylinder (output to the brake shoes) so that pedal force and travel can be varied to best suit the driver and vehicle type.

Some of these early designs use a wheel cylinder with a single piston pushing on only one shoe. When the shoe touches the spinning drum, rotational forces cause energy to transfer to the other shoe and wedge it into the drum to stop the vehicle. This is known as self-energizing, or servo action.

The typical drum brake system has remained virtually unchanged for many decades. A hydraulic wheel cylinder expands when the brakes are applied, which presses the brake shoes into the inside of the spinning brake drum to stop the vehicle.

Brake Adjustments

Today’s rear drum brake systems look very similar to those on a 1957 Chevy, but early vehicles required routine brake adjustments to maintain pedal height as the brake shoes wore away. As a child, I recall seeing signs at nearly every gas station that read, “Brake adjustments while you wait: $3.” ■

In those early days, drum brakes were used on all four wheels and they really haven’t changed much since then. Early drum brake systems did not automatically adjust, but modern vehicles with drum brakes have automatic adjuster systems that take care of this for you provided everything is assembled properly and not rusted or damaged.

Four-wheel drum power brakes with automatic adjusting capability became commonplace in the late 1960s and early 1970s. A vacuum-controlled booster was added between the brake pedal and the master cylinder that greatly increased the force, allowing virtually anyone to stop a heavy vehicle with minimal pedal effort. This system was a great improvement, but drum brakes are susceptible to water and use many moving parts that can develop problems. If water got in one front brake during a rainstorm or one side was not properly adjusting, the vehicle pulled to one side when stopping. Drum brakes also tend to retain heat and generate a lot of wear material that can get between the friction surfaces, reducing efficiency.

Then along came disc brakes. They were first used in aviation and have many advantages over drum brakes. They can shed dust and squeegee water off as soon as they are applied. They can dissipate heat well and automatically adjust to wear hydraulically, requiring no mechanical methods. They were first used on the front, and drums remained on the rear, but most of today’s vehicles use disc brakes on all four wheels. They are extremely reliable and easier to service.

Some early drum brake systems used a wheel cylinder with only one piston, such as on this 1952 Ford F2 pickup truck. The piston (arrow) pushes the front shoe into the spinning drum. The spinning drum causes force to be applied to the other shoe, which wedges it into the drum and stops the vehicle. This concept is known as self-energizing or servo action. This truck has four-wheel drum brakes with no assist.

A conventional disc brake system uses a brake caliper over a spinning rotor. When the brakes are applied, piston(s) in the caliper (A) press brake pads (B) into the rotor (C) to stop the vehicle. Disc brakes are widely used on today’s cars and light trucks because they are superior to drum brakes in almost every way.

Electronics and Brake Systems

Antilock brakes were first used on trains and airplanes and became mainstream in the United States in the mid-1980s on the rear of many pickup trucks and vans. ABS provided a solution to the problem of rear wheel lockup when the vehicle was unloaded, which could lead to an accident, especially on a curve in the rain. As systems were improved and better sensors and data processing came about, ABS was added to all four wheels. The main goal of ABS is to allow a driver to steer around obstructions during a panic stop. It does this by keeping the wheels from totally locking, which would eliminate the ability to steer.

An antilock braking system (ABS) uses a computer module to control pulsing action, improving steering and control during braking on slippery surfaces or during panic stopping situations. In many cases, stopping distance is also reduced because the brakes are applied to the point just before wheel lockup. The main advantage of ABS is being able to steer during an emergency stop.

If you have ever ridden a personal watercraft and released the throttle while attempting to steer, you know what happens: You can’t steer. ABS is also the basis for other safety systems on modern vehicles, such as stability control and traction control. Some applications use ABS for automatic stopping and during distance-based cruise control operation. This system will be even more important when autonomous vehicles become more common.

Many high-end and hybrid vehicles use electronic braking. Most of them do not use a traditional master cylinder to push the brake fluid to the wheels. A pedal travel sensor attached to the brake pedal monitors the driver’s intentions and an electronic pressure modulator sends the fluid to the wheels. This sensor may be a chamber of brake fluid with transducers that convert pressure into electronic signals. The system can send different amounts of pressure to each wheel depending on varying driving conditions such as cornering or panic stopping. This allows for more effective control and a greater level of safety.

The mechanical unit that pulses the brakes during ABS operation is called the modulator. It contains two solenoids in series with the affected wheel. Early systems only controlled the rear wheels. One valve blocked the fluid and the other vented the high-pressure fluid to an accumulator, which released the rear brakes. The system was known as rear wheel antilock (RWAL) or rear antilock brake system (RABS), referring to the fact that only the rear wheels had ABS.

Some of these systems have had issues but most use some form of backup system in the event of system failure. Valves may open in the pressure transducers, allowing fluid to flow to the front calipers for example. The major difference between this system and a conventional brake system is the fact that there is no mechanical connection between the brake pedal and the wheels. The brake pedal input is monitored and sent by electrical signals to the modulator, which usually uses a pump and high-pressure accumulator (HPA) to send the fluid to the wheel using several solenoids.

Some late-model, high-end, and hybrid vehicles use electronic brake systems. These systems generally use an electronic modulator to apply the brakes instead of a conventional master cylinder. This design allows for various pressures to be applied to each wheel relative to operating conditions.

Regenerative braking, also known as “regen” brakes, use magnetism to help slow a vehicle. A motor/generator is typically mounted around the flywheel. It can start the vehicle and also provide braking action while it helps recharge the battery.

When in brake mode, the operation could be described as the opposite of a motor. When the brakes are applied on a vehicle with regen brakes, the drive motor is used as a generator and the magnetism causes the shaft to be more difficult to turn, resulting in braking action. This magnetism is used to recharge the vehicle’s drive battery pack. Vehicles such as the Toyota Prius have used this concept for many years. Because of the lightweight construction of this vehicle, the regen brake system is capable of supplying most of the brake force, so the base brakes often last a long time if the vehicle is driven normally. Some have reported that the brakes lasted the life of the vehicle.

Regenerative braking is used on many hybrid and electric vehicles. Most still use a downsized conventional brake system, but the bulk of the braking is accomplished by using the vehicle’s propulsion motor as a generator during stopping. The generator recharges the batteries during normal stopping. If additional braking is required, the conventional brake system is also applied.

The Possibility of Mandatory Maintenance

As vehicles have progressed and more electronics have been added to replace mechanical functions, they have become more like the systems used in aviation. There, mandatory maintenance is required, even on private aircraft.

When autonomous vehicles are common on the roads, it may be necessary for mandatory maintenance to become a reality in the automotive world. For example, a wire harness leading to a caliper may need to be replaced at a specified time or mileage interval. The decision may not be left up to the owner of the vehicle. ■

A new type of brake system is currently being tested in Europe. Known as Electronic Wedge Braking (EWB), the system uses disc brakes with a sliding caliper that is actuated by electric motors rather than a hydraulic piston. A sliding plate with raised humps on it is moved back and forth by two electric motors. The inboard brake pad also has raised humps, but the valleys of the humps of the pads are in contact with the high spots on the sliding plate. When braking is needed, one of the motors slides the plate, which causes the humps to apply the inboard pad to the spinning rotor. As contact is made, the pad can move with the rotor, which increases brake force by wedging action against the plate. The outboard pad is applied in the usual manner when the caliper slides on its pins or guides.

This design eliminates many expensive, heavy components such as the brake booster, master cylinder, ABS unit, lines, and hoses. The calipers must have a wire harness instead of a brake hose, which could be subject to wear when the suspension moves and steering takes place. It may also become brittle over time or due to cold temperatures and require replacement more frequently than a rubber hose.

This is the main drive battery from a Tesla. Most hybrid and electric vehicles use regenerative braking to slow the vehicle and recharge the battery when the brakes are applied normally. Heavy braking causes the conventional brakes at the wheels to help stop the vehicle.