While you don't have to be an Einstein to work on brake systems, physics is arguably the most applicable science when it comes to understanding stopping power.
From the day mankind first made something move forward and then had to find a way to make it stop, physics has played a major role in braking. Horse-drawn wagons used a lever that pushed an iron lining. This lining was mounted on a wooden shoe against an iron tire that was mounted on a wooden wheel.
Early motor vehicles used mechanical brakes with steel and, later, rawhide linings. With the development of better friction materials, which were mostly asbestos-based compounds, and the advent of the hydraulic brake system, which was developed by Scottish engineer Malcolm Loughead (which later became Lockheed and then Lockheed Martin) in 1924, you were actually able to stop your vehicle when necessary. As a side note, the patent rights for the hydraulic brake system were sold to Bendix and to two European firms, one of which was Ate (Alfred Teves).
Friction has always been the stopping force behind braking. Even today's regenerative braking systems still rely on friction for most of their stopping power. A day may come when friction will no longer be used and electromagnetic force will take over completely. But, for now and the near future, hydraulics and friction are the order of the day.
The laws of physics are what allow hydraulic brakes to work. The laws of physics are also what allow electromagnetic brakes to work, but that type of braking uses physics to put electrical energy back into the vehicle's electrical system. Having a grasp of physics will also help in diagnosing brake problems.
Basic definitions and principles
If you have never studied physics, there are several basic definitions, principles and laws that you need to understand first. Knowing these will help immensely in understanding how and why hydraulic brakes work.
ENERGY: Energy is a measure of availability to do work. There are many types of energy, but in this article we will basically only deal with three types: potential, kinetic and heat.
- Potential energy is exactly what it says: the potential to do work. A gallon of gasoline contains quite a lot of potential energy.
- Kinetic energy is the energy that is in a body in motion. A 2,000 pound vehicle moving at 70 miles per hour has quite a lot of kinetic energy.
- Heat is the energy that is transferred between a body and its surroundings due to their temperature differential. Heat always flows to less heat. In other words, from hot to cold.
There are three types of heat transfer: conduction, convection and radiation.
- Conduction is the type of heat transfer that causes the handles on that pair of locking pliers to get too hot to hold if they're left locked onto that rusted and rounded off exhaust flange nut while heat is applied to the nut with an acetylene torch.
- Convection is the transfer of heat through motion, such as when air flows through a radiator's cooling fins and the heat from the coolant that has been transferred to the fins by conduction is transferred to the air passing through the fins.
- Radiation is the type of heat transfer that causes the air surrounding the exhaust manifold on a running engine to get hot even when there is no air movement.
FRICTION: Friction is the force that opposes movement between two surfaces in contact with each other. That force depends on the nature of the surfaces that are in contact and the amount of pressure exerted on those surfaces perpendicular to each other. Friction is the force that actually causes a vehicle to stop.
PRESSURE: Pressure is defined as force per area, as in lb./in.
FLUIDS: Gases and liquids are fluids. They conform to the shape of their containers. There are several differences between them. A gas fills the container and a liquid forms a definite interface, such as when brake fluid half fills a master cylinder to create a liquid-air interface.
A gas can be compressed whereas a liquid is virtually incompressible. This difference in compressibility is an extremely important principle in hydraulic brake operation. As you undoubtedly know, if there is any gas (normally air) present in a brake hydraulic system, the gas will compress when the brake pedal is depressed and the brakes won't operate as designed, if they operate at all.
The Law of Conservation of Energy: The Law of Conservation of Energy states that energy can neither be created nor destroyed. It is merely transformed from one type of energy into another.
Newton's First Law of Motion – Inertia: Newton's First Law of Motion states that a body at rest will remain at rest unless it is acted upon by an outside force. In addition, a body in motion will remain in motion at a constant velocity (that is, at a constant speed in a straight line) unless it is acted upon by an outside force.
Pascal's Principle: Pascal's Principle states that when the pressure on any part of a confined fluid is changed, the pressure on every part of that fluid is changed by the same amount.
Putting it all together
Because we are discussing principles and basics, we won't factor in the geometric function of leverage increasing the pressure from the brake pedal to the master cylinder piston or the pressure increase caused by the power booster. Nor will we consider hold-off valves, proportioning valves or antilock systems.
If we start at the beginning, we have the potential energy contained in the fuel in the tank. Through the principles of chemistry (a whole different subject), that potential energy is transformed into heat energy. Through the principles and laws of physics, that heat energy is transformed into kinetic energy and the vehicle is rolling down the road.
According to Newton's First Law of Motion, the vehicle will continue to roll in a straight line, unless it is acted upon by an outside force. Air will cause some resistance, but most of the outside force comes from friction. That friction will come from many sources. No matter how smooth or how well lubricated, bearings and gears still have friction and will transform some of that kinetic energy into heat energy and in some cases sound energy.
Most of the energy transformation that occurs during braking is accomplished by friction caused by the brake friction material. Pascal's Principle works its magic and increases the pressure applied to the brake pedal into enough pressure applied to the brake friction material. This transforms the vast majority of the vehicle's kinetic energy into heat energy and in some cases sound energy.
Most people, however, don't care for that type of sound energy. If that heat is not dissipated to the atmosphere by conduction, convection and radiation, it can be transformed into light energy. But heaven help you if things go that far.
The Law of Conservation of Energy tells us that all of the potential energy contained in the fuel must be transformed into other types of energy. Almost all of that potential energy is transformed into heat energy, whether through the engine, drivetrain or brakes.
Is it any wonder that so much research and development time and money is spent finding ways to dissipate all that heat? Many brake rotors are vented, and open spoke wheels are becoming the norm. Some vehicles even have air scoops that direct fresh air onto the brakes.
Pascal's Principle is what allows brake pressure to be transferred to the brake calipers and/or wheel cylinders. The formula for the force at the brake slave cylinders (calipers/wheel cylinders) is:F
A
F
A
As you can see, pressure available to activate the brakes can be increased or decreased (as is the case in some wheel cylinders) very efficiently using Pascal's Principle. Of course, there is normally a lot more than 10 lb./in.
The reason I wanted to keep the math simple is evident if you use the brake system on a 1994 to 1995 Mercury Sable as an example. The master cylinder bore is 1 in., the caliper bore is ¼ in., and the wheel cylinder bore is 1 in. You use the formula PiR
The master cylinder piston area is:
3.14 x 0.5 in. x 0.5 in.
The caliper piston area is:
3.14 x ¼ in. x ¼ in.
You would need to divide 64 into 19 in order to convert the fraction to a decimal and then use PiR
It is also why disc brake calipers use square cut seal rings. The ring rolls very slightly when the brakes are applied, and the ring rolls back to retract the piston just enough to reduce the drag on the brake rotor when the brakes are released. For the same reason, drum brakes need to be adjusted to achieve a slight drag. If the brake friction material is too far from the rotor/drum surface, the brake pedal will have to push the master cylinder piston too far and a low pedal condition will exist.
I hope that by this point you have begun to realize how important it is to understand the science of physics when looking at brake systems. If it's been a while since you have studied physics and you are a little rusty, or if you have never had the opportunity to study the science, this should have been beneficial.
If you want to know more about physics and don't have an old textbook handy, visit one of your local bookstores for some self-study books on the subject.
Curt Marsh, AAM , is an ASE-certified Master Automotive Technician and L1, as well as an Indiana Certified Emission Repair Technician. He runs Marsh Garage, established in 1958.
