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.2 The mathematical formula is: P = F/A
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.
