Mini Project report onSTUDY OF HYDRAULIC DISC BRAKESSubmitted in
partial fulfillment of the requirement for the award of degree
ofBACHELOR OF TECHNOLOGYINMechanical Engineering (Mechatronics)
ByBANDI NITHIN REDDY(10261A1408)G. CHAITHAN VINAY
KUMAR(10261A1412)J.V.N.J.R. KRISHNA TEJOMAYA(10261A1419)Under the
esteemed guidance ofMr. G. SREENIVASULU REDDYASST. PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING (MECHATRONICS)MAHATMA
GANDHI INSTITUTE OF TECHNOLOGY(Affiliated to Jawaharlal Nehru
Technological University, Hyderabad)Accredited by National Board of
Accreditation, New DelhiGandipet, Hyderabad-500075(A.P.)
www.mgit.ac.in2014
MAHATMA GANDHI INSTITUTE OF TECHNOLOGY(Affiliated to Jawaharlal
Nehru Technological University, Hyderabad)Accredited by National
Board of Accreditation, New DelhiGandipet, Hyderabad-500075(A.P.)
www.mgit.ac.in 2014
DEPARTMENT OF MECHANICAL ENGINEERING (MECHATRONICS)
CertificateThis is certify that the mini project report entitled
STUDY OF HYDRAULIC DISC BRAKES is being submitted byBANDI NITHIN
REDDY(10261A1408)G. CHAITHAN VINAY KUMAR(10261A1412)J.V.N.J.R.
KRISHNA TEJOMAYA(10261A1419)In partial fulfillment for the award of
the degree of bachelor of technology in mechanical engineering
(Mechatronics) is the bonafide work carried out by them under our
guidance. The results embodied in this project have not been
submitted to any other University or Institution for the award of
degree.
Internal Guide: Dr. K. SUDHAKAR REDDYG. SREENIVASULU REDDY Head
of DepartmentAssistant Professor Mechanical Engg(Mechatronics)
INTERNAL EXAMINER EXTERNAL EXAMINER
Acknowledgement
A comprehensive project report always requires goodwill,
encouragement, guidance and support of many people. We have great
pleasure in expressing our indebtness to G. Sreenivasulu Reddy
(M.Tech) Asst. Professor for his constant help and encouragement.
We acknowledge and express our deep sense of gratitude to him,
whose able guidance, incessant and encouragement and sound counsel
were instrumental in this work.We thank Dr. K. Sudhakar Reddy, Head
of Department of Mechanical Engg. (Mechatronics) and all other
staff members for guiding us through proper grooves.We show
gratitude to Dr. G. Chandra Mohan Reddy, our honourable Principal
for having provided all the facilities and support.It would be
impossible to refer in detail to many persons who have been
consulted in compilation of this work and elsewhere who have
contributed advice assistance, equipment and skilled services that
were invaluable to the progress of the work. We did excuse for not
naming them individually.
SUBMITTED BY
BANDI NITHIN REDDY(10261A1408)G. CHAITHAN VINAY
KUMAR(10261A1412)J.V.N.J.R. KRISHNA TEJOMAYA(10261A1419)
CONTENTSDESCRIPTION PAGE NO
I. ABSTRACT11. INTRODUCTION22. CHARACTERISTICS53. WORKING OF A
BRAKE84. BRAKE BASICS104.1 LEVERAGE104.2 HYDRAULIC11 4.3
FRICTION115. TYPES OF BRAKES145.1 DRUM BRAKES145.2 DISC BRAKE156.
TYPES OF DISC BRAKES166.1 FLOATING CALLIPER DISC BRAKE166.2 FIXED
CALLIPER DISC BRAKE176.3 SLIDING CALLIPER DISC BRAKE18 7.
COMPONENTS OF HYDRAULIC DISC BRAKES207.1 MASTER CYLINDER217.2
MASTER CYLINDER - OPEN OR CLOSED227.3 BRAKE LINES237.4 BRAKE
FLUID257.5 SLAVE CYLINDER/CALLIPER277.6 BRAKE PADS287.7 ROTORS318.
WORKING OF HYDRAULIC DISC BRAKE368.1 THE MASTER CYLINDER IN
ACTION378.2 COMPENSATING PORTS398.3 BYPASS PORTS408.4 SELF
ADJUSTMENT OF HYDRAULIC DISC BRAKE428.5 EMERGENCY BRAKES429. BRAKE
FADE449.1 BRAKE MODIFICATION TO REDUCE FADE449.2 DISC BRAKE
VENTS4510. ADVANTAGES4710.1 ADVANTAGES OF DISC BRAKES OVER DRUM
BRAKES4710.2 REASON FOR HIGH EFFICIENCY OF HYDRAULIC DISC
BRAKES5010.3 BETTER BRAKING BEHAVIOUR OF HYDRAULIC DISC
BRAKES5010.4 HIGHER SAFETY RESERVES5111. LIMITATIONS5212.
APPLICATIONS53II. CONCLUSION54III. REFERENCES55
LIST OF FIGURESPAGE NO1. Braking fundamentals82. Typical Braking
System93. Leverage114. Friction force versus weight125. Close look
at one of the blocks126. Types of Brakes147. Floating callipers
Disc Brake178. Fixed Caliper Disc Brake189. Brake Pad1910. Caliper
and Rotor1911. The layout of a typical brake system2112. The layers
that make up and average hydraulic brake line2413. Formula R1 brake
with braided brake lines2514. Exploded view of an Avid two-piece
calliper design2815. Organic Brake pads2916. Semi-metallic
pads3017. Sintered Brake pads3018. Ceramic Brake pads3119. Formula
Light weight, Avid G3 Clean Sweep, AshimaAiRotor3220. ISO standard
6-boltm Shimano CenterLock3321. Formula 2-Piece Stainless
Steel/Aluminium Rotor3422. Working of hydraulic disc brakes3623.
Brakes released 3724. Brakes applied3825. Functioning of
brakes3926. Compensating ports4027. Bypass ports4128. Master
Cylinder Return Operation4229. Emergency Brakes4330. Brake
Modification to Reduce Fade45
ABSTRACT
The current tendencies in automotive industry need intensive
investigation in problems of interaction of active safety systems
with brake system equipments. At the same time, the opportunities
to decrease the power take-off of single components, disc brake
systems. Disc brakes sometimes spelled as "disk" brakes, use a
flat, disc-shaped metal rotor that spins with the wheel. When the
brakes are applied, a calliper squeezes the brake pads against the
disc (just as you would stop a spinning disc by squeezing it
between your fingers), slowing the wheel.The disc brake used in the
automobile is divided into two parts: a rotating axis symmetrical
disc, and the stationary pads. The hydraulic disc brake is an
arrangement of braking mechanism which uses brake fluid, typically
containing ethylene glycol, to transfer pressure from the
controlling unit, which is usually near the operator of the
vehicle, to the actual brake mechanism, which is usually at or near
the wheel of the vehicle.The frictional heat, which is generated on
the interface of the disc and pads, can cause high temperature
during the braking process. Hence the automobiles generally use
disc brakes on the front wheels and drum brakes on the rear wheels.
The disc brakes have good stopping performance and are usually
safer and more efficient than drum brakes. The four wheel disc
brakes are more popular, swapping drums on all but the most basic
vehicles. Many two wheel automobiles design uses a drum brake for
the rear wheel. Brake technology began in the '60s as a serious
attempt to provide adequate braking for performance cars has ended
in an industry where brakes range from supremely adequate to
downright phenomenal. One of the first steps taken to improve
braking came in the early '70s when manufacturers, on a widespread
scale, switched from drum to disc brakes. Since the majority of a
vehicle's stopping power is contained in the front wheels, only the
front brakes were upgraded to disc during much of this period.
Since then, many manufacturers have adopted four-wheel disc brakes
on their high-end and performance models as well as their low-line
economy cars. Occasionally, however, as in the case of the 1999
Mazda Protgs, a manufacturer will revert from a previous four-wheel
disc setup to drum brakes for the rear of the car in order to cut
both production costs and purchase price. CHAPTER-1INTRODUCTIONA
brake is a mechanical device which inhibits motion. Most commonly
brakes use friction to convert kinetic energy into heat, though
other methods of energy conversion may be employed. For example
regenerative braking converts much of the energy to electrical
energy, which may be stored for later use. Other methods convert
kinetic energy into potential energy in such stored forms as
pressurized air or pressurized oil. Eddy current brakes use
magnetic fields to convert kinetic energy into electric current in
the brake disc, fin, or rail, which is converted into heat. Still
other braking methods even transform kinetic energy into different
forms, for example by transferring the energy to a rotating
flywheel.Brakes are generally applied to rotating axles or wheels,
but may also take other forms such as the surface of a moving fluid
(flaps deployed into water or air). Some vehicles use a combination
of braking mechanisms, such as drag racing cars with both wheel
brakes and a parachute, or airplanes with both wheel brakes and
drag flaps raised into the air during landing.Since kinetic energy
increases quadratically with velocity (), an object moving at 10m/s
has 100 times as much energy as one of the same mass moving at
1m/s, and consequently the theoretical braking distance, when
braking at the traction limit, is 100 times as long. In practice,
fast vehicles usually have significant air drag, and energy lost to
air drag rises quickly with speed.Almost all wheeled vehicles have
a brake of some sort. Even baggage carts and shopping carts may
have them for use on a moving ramp. Most fixed-wing aircraft are
fitted with wheel brakes on the undercarriage. Some aircraft also
feature air brakes designed to reduce their speed in flight.
Notable examples include gliders and some World War II-era
aircraft, primarily some fighter aircraft and many dive bombers of
the era. These allow the aircraft to maintain a safe speed in a
steep descent. The Saab B 17 dive bomber used the deployed
undercarriage as an air brake.Friction brakes on automobiles store
braking heat in the drum brake or disc brake while braking then
conduct it to the air gradually. When travelling downhill some
vehicles can use their engines to brake.When the brake pedal of a
modern vehicle with hydraulic brakes is pushed, ultimately a piston
pushes the brake pad against the brake disc which slows the wheel
down. On the brake drum it is similar as the cylinder pushes the
brake shoes against the drum which also slows the wheel down.
HISTORY OF DISC BRAKE Ever since the invention of the wheel, if
there has been "go" there has been a need for "whoa." As the level
of technology of human transportation has increased, the mechanical
devices used to slow down and stop vehicles has also become more
complex. In this report I will discuss the history of vehicular
braking technology and possible future developments. Before there
was a "horse-less carriage," wagons, and other animal drawn
vehicles relied on the animals power to both accelerate and
decelerate the vehicle. Eventually there was the development of
supplemental braking systems consisting of a hand lever to push a
wooden friction pad directly against the metal tread of the wheels.
In wet conditions these crude brakes would lose any effectiveness.
The early years of automotive development were an interesting time
for the designing engineers, "a period of innovation when there was
no established practice and virtually all ideas were new ones and
worth trying. Quite rapidly, however, the design of many components
stabilized in concept and so it was with brakes; the majority of
vehicles soon adopted drum brakes, each consisting of two shoes
which could be expanded inside a drum." In this chaotic era is the
first record of the disc brake. Dr. F.W. Lanchester patented a
design for a disc brake in 1902 in England. It was incorporated
into the Lanchester car produced between 1906 through 1914. These
early disc brakes were not as effective at stopping as the
contemporary drum brakes of that time and were soon forgotten.
Another important development occurred in the 1920s when drum
brakes were used at all four wheels instead of a single brake to
halt only the back axle and wheels such as on the Ford model T. The
disc brake was again utilized during World War II in the landing
gear of aircraft. The aircraft disc brake system was adapted for
use in automotive applications, first in racing in 1952, then in
production automobiles in 1956. United States auto manufacturers
did not start to incorporate disc brakes in lower priced
non-high-performance cars until the late 1960s.
CHAPTER-2CHARACTERISTICSBrakes are often described according to
several characteristics including:Peak force The peak force is the
maximum decelerating effect that can be obtained. The peak force is
often greater than the traction limit of the tires, in which case
the brake can cause a wheel skid.Continuous power dissipation
Brakes typically get hot in use, and fail when the temperature gets
too high. The greatest amount of power (energy per unit time) that
can be dissipated through the brake without failure is the
continuous power dissipation. Continuous power dissipation often
depends on e.g., the temperature and speed of ambient cooling
air.Fade As a brake heats, it may become less effective, called
brake fade. Some designs are inherently prone to fade, while other
designs are relatively immune. Further, use considerations, such as
cooling, often have a big effect on fade.Smoothness A brake that is
grabby, pulses, has chatter, or otherwise exerts varying brake
force may lead to skids. For example, railroad wheels have little
traction, and friction brakes without an anti-skid mechanism often
lead to skids, which increases maintenance costs and leads to a
"thump thump" feeling for riders inside.Power Brakes are often
described as "powerful" when a small human application force leads
to a braking force that is higher than typical for other brakes in
the same class. This notion of "powerful" does not relate to
continuous power dissipation, and may be confusing in that a brake
may be "powerful" and brake strongly with a gentle brake
application, yet have lower (worse) peak force than a less
"powerful" brake.Pedal feel Brake pedal feel encompasses subjective
perception of brake power output as a function of pedal travel.
Pedal travel is influenced by the fluid displacement of the brake
and other factors.Drag Brakes have varied amount of drag in the
off-brake condition depending on design of the system to
accommodate total system compliance and deformation that exists
under braking with ability to retract friction material from the
rubbing surface in the off-brake condition.Durability Friction
brakes have wear surfaces that must be renewed periodically. Wear
surfaces include the brake shoes or pads, and also the brake disc
or drum. There may be tradeoffs, for example a wear surface that
generates high peak force may also wear quickly.Weight Brakes are
often "added weight" in that they serve no other function. Further,
brakes are often mounted on wheels, and unsprung weight can
significantly hurt traction in some circumstances. "Weight" may
mean the brake itself, or may include additional support
structure.Noise Brakes usually create some minor noise when
applied, but often create squeal or grinding noises that are quite
loud.
2.1 Types of Braking Systems:-Brakes may be broadly described as
using friction, pumping, or electromagnetic. One brake may use
several principles: for example, a pump may pass fluid through an
orifice to create friction:Frictional brakes are most common and
can be divided broadly into "shoe" or "pad" brakes, using an
explicit wear surface, and hydrodynamic brakes, such as parachutes,
which use friction in a working fluid and do not explicitly wear.
Typically the term "friction brake" is used to mean pad/shoe brakes
and excludes hydrodynamic brakes, even though hydrodynamic brakes
use friction.Friction (pad/shoe) brakes are often rotating devices
with a stationary pad and a rotating wear surface. Common
configurations include shoes that contract to rub on the outside of
a rotating drum, such as a band brake; a rotating drum with shoes
that expand to rub the inside of a drum, commonly called a "drum
brake", although other drum configurations are possible; and pads
that pinch a rotating disc, commonly called a "disc brake". Other
brake configurations are used, but less often. For example, PCC
trolley brakes include a flat shoe which is clamped to the rail
with an electromagnet; the Murphy brake pinches a rotating drum,
and the Ausco Lambert disc brake uses a hollow disc (two parallel
discs with a structural bridge) with shoes that sit between the
disc surfaces and expand laterally.Pumping brakes are often used
where a pump is already part of the machinery. For example, an
internal-combustion piston motor can have the fuel supply stopped,
and then internal pumping losses of the engine create some braking.
Some engines use a valve override called a Jake brake to greatly
increase pumping losses. Pumping brakes can dump energy as heat, or
can be regenerative brakes that recharge a pressure reservoir
called a hydraulic accumulator.Electromagnetic brakes are likewise
often used where an electric motor is already part of the
machinery. For example, many hybrid gasoline/electric vehicles use
the electric motor as a generator to charge electric batteries and
also as a regenerative brake. Some diesel/electric railroad
locomotives use the electric motors to generate electricity which
is then sent to a resistor bank and dumped as heat. Some vehicles,
such as some transit buses, do not already have an electric motor
but use a secondary "retarder" brake that is effectively a
generator with an internal short-circuit. Related types of such a
brake are eddy current brakes, and electro-mechanical brakes (which
actually are magnetically driven friction brakes, but nowadays are
often just called electromagnetic brakes as well).Brakes are one
the key parts of any vehicle, without which it is virtually not
possible to use the vehicle for travel. Clearly, a brake, which
serves to slow down the vehicle, should not be too weak. But
interestingly, when designing a brake system, it should also be
taken care that its not too efficient. A too strong a brake would
expose us continuously to the ill effects of a sudden brake
application in bus or car. If a vehicle is stopped abruptly or
strongly, the passenger may hit the front seat or whatever is
there. Hence, too efficient a brake system is not required!The
braking system is strongly relation to Newtons laws of motion.
Indeed, the above phenomenon is linked to Newtons second law of
motion, which states A body continues to be in its state of rest or
of motion unless external force acts on the same.On the other hand,
if a brake system is too weak, the stopping distance would increase
and hence may lead to accidents. Thus, a brake system should be
perfect enough to stop the vehicle at minimum safe distance,
without affecting the comfort of the passenger. In an endeavour to
achieve this there have been a lot of developments in the brake
system technology, right from Mechanical brakes to Air brakes in
automobiles. In this article we would like provide the relevant
information regarding the same
CHAPTER-3WORKING OF A BRAKEWe all know that pushing down on the
brake pedal slows a car to a stop. But we do not how does this
happen, how does our car transmit the force from our leg to its
wheels and how does it multiply the force so that it is enough to
stop something as big as a car.
Fig 1: Braking fundamentalsFriction and how it applies to
automobilesA brake system is designed to slow and halt the motion
of vehicle. To do this, various components within the brake system
must convert vehicles moving energy into heat. This is done by
using friction.Friction is the resistance to movement exerted by
two objects on each other. Two forms of friction play a part in
controlling a vehicle: Kinetic or moving, and static or stationary.
The amount of friction or resistance to movement depends upon the
type of material in contact, the smoothness of their rubbing
surfaces and the pressure holding them together.Thus, in a nutshell
a car brake works by applying a static surface to a moving surface
of a vehicle, thus causing friction and converting kinetic energy
into heat energy. The high-level mechanics are as follows.As the
brakes on a moving automobile are put into motion, rough-textures
brake pads or brake shoes are pressed against the rotating parts of
vehicle, be it disc or drum. The kinetic energy or momentum of the
vehicle is then converted into heat energy by kinetic friction of
the rubbing surfaces and the car or truck slows down.When vehicle
comes to stop, it is held in place by static friction. The friction
between surfaces of brakes as well as the friction between tires
and roads resists any movement. To overcome the static friction
that holds the car motionless, brakes are released. The heat energy
of combustion of in engine is converted into kinetic energy by
transmission and drive train, and the vehicle moves.
Fig 2: Typical Braking System
CHAPTER-4BRAKE BASICSWhen you depress your brake pedal, your car
transmits the force from your foot to its brakes through a fluid.
Since the actual brakes require a much greater force than you could
apply with your leg, your car must also multiply the force of your
foot. It does this in two ways: Mechanical advantage (Leverage)
Hydraulic force multiplicationThe brakes transmit the force to the
tires using friction, and the tires transmit that force to the road
using friction also. Before we begin our discussion on the
components of the brake system, let's cover these three principles:
Leverage Hydraulics Friction
4.1 LEVERAGEThe pedal is designed in such a way that it can
multiply the force from your leg several times before any force is
even transmitted to the brake fluid.
Fig 3: LeverageIn the figure above, a force F is being applied
to the left end of the lever. The left end of the lever is twice as
long (2X) as the right end (X). Therefore, on the right end of the
lever a force of 2F is available, but it acts through half of the
distance (Y) that the left end moves (2Y). Changing the relative
lengths of the left and right ends of the lever changes the
multipliers. 4.2 HYDRAULIC SYSTEMSThe basic idea behind any
hydraulic system is very simple: Force applied at one point is
transmitted to another point using an incompressible fluid, almost
always an oil of some sort. Most brake systems also multiply the
force in the process.4.3 FRICTIONFriction is a measure of how hard
it is to slide one object over another. Take a look at the figure
below. Both of the blocks are made from the same material, but one
is heavier. I think we all know which one will be harder for the
bulldozer to push.
Fig 4 : Friction force versus weight
To understand why this is, let's take a close look at one of the
blocks and the table: Fig 5: Close look at one of the blocksEven
though the blocks look smooth to the naked eye, they are actually
quite rough at the microscopic level. When you set the block down
on the table, the little peaks and valleys get squished together,
and some of them may actually weld together. The weight of the
heavier block causes it to squish together more, so it is even
harder to slide. Different materials have different microscopic
structures; for instance, it is harder to slide rubber against
rubber than it is to slide steel against steel. The type of
material determines the coefficient of friction, the ratio of the
force required to slide the block to the block's weight. If the
coefficient were 1.0 in our example, then it would take 100 pounds
of force to slide the 100-pound (45 kg) block, or 400 pounds (180
kg) of force to slide the 400-pound block. If the coefficient were
0.1, then it would take 10 pounds of force to slide to the
100-pound block or 40 pounds of force to slide the 400-pound block.
So the amount of force it takes to move a given block is
proportional to that block's weight. The more weight, the more
force required. This concept applies for devices like brakes and
clutches, where a pad is pressed against a spinning disc. The more
force that presses on the pad, the greater is the stopping
force.
A SIMPLE BRAKE SYSTEMThe distance from the pedal to the pivot is
four times the distance from the cylinder to the pivot, so the
force at the pedal will be increased by a factor of four before it
is transmitted to the cylinder. The diameter of the brake cylinder
is three times the diameter of the pedal cylinder. This further
multiplies the force by nine. All together, this system increases
the force of your foot by a factor of 36. If you put 10 pounds of
force on the pedal, 360 pounds (162 kg) will be generated at the
wheel squeezing the brake pads. There are a couple of problems with
this simple system. What if we have a leak? If it is a slow leak,
eventually there will not be enough fluid left to fill the brake
cylinder, and the brakes will not function. If it is a major leak,
then the first time you apply the brakes all of the fluid will
squirt out the leak and you will have complete brake failure.
CHAPTER-5TYPES OF BRAKESTYPES OF BRAKES1. DRUM BRAKES2. DISC
BRAKES (CALLIPER BRAKES)
Fig 6: Types of Brakes5.1 DRUM BRAKESThe drum brake has two
brake shoes and a piston. When you hit the brake pedal, the piston
pushes the brake shoes against the drum. This is where it gets a
little more complicated. as the brake shoes contact the drum, there
is a kind of wedging action, which has the effect of pressing the
shoes into the drum with more force. The extra braking force
provided by the wedging action allows drum brakes to use a smaller
piston than disc brakes. But, because of the wedging action, the
shoes must be pulled away from the drum when the brakes are
released. This is the reason for some of the springs. Other springs
help hold the brake shoes in place and return the adjuster arm
after it actuates.
5.2 DISC BRAKE The disc brake has a metal disc instead of a
drum. It has a flat shoe, or pad, located on each side of the disc.
To slow or stop the car, these two flat shoes are forced tightly
against the rotating disc, or rotor. Fluid pressure from the master
cylinder forces the pistons to move in. This action pushes the
friction pads of the shoes tightly against the disc. The friction
between the shoes and the disc slows and stops the disc.
CHAPTER-6TYPES OF DISC BRAKESThe Three Types of Disc Brakes
Are:-1. FLOATING CALIPER DISC BRAKES2. FIXED CALIPER DISC BRAKES3.
SLIDING CALIPER DISC CALIPER
6.1 FLOATING-CALIPER DISC BRAKES The calliper is the part that
holds the brake shoes on each side of the disc. In the
floating-calliper brake, two steel guide pins are threaded into the
steering-knuckle adapter. The calliper floats on four rubber
bushings which fit on the inner and outer ends of the two guide
pins. The bushings allow the calliper to swing in or out slightly
when the brakes are applied When the brakes are applied, the brake
fluid flows to the cylinder in the calliper and pushes the piston
out. The piston then forces the shoe against the disc. At the same
time, the pressure in the cylinder causes the calliper to pivot
inward. This movement brings the other shoe into tight contact with
the disc. As a result, the two shoes pinch the disc tightly to
produce the braking action
Fig 7: Floating calliper Disc Brake
6.2 FIXED-CALIPER DISC BRAKE This brake usually has four
pistons, two on each side of the disc. The reason for the name
fixed-calliper is that the calliper is bolted solidly to the
steering knuckle. When the brakes are applied, the calliper cannot
move. The four pistons are forced out of their calliper bores to
push the inner and outer brake shoes in against the disc. Some
brakes of this type have used only two pistons, one on each side of
the disc
Fig 8: Fixed Caliper Disc Brake6.3 SLIDING-CALIPER DISC BRAKEThe
sliding-calliper disc brake is similar to the floating-calliper
disc brake. The difference is that sliding-calliper is suspended
from rubber bushings on bolts. This permits the calliper to slide
on the bolts when the brakes are applied. Proper function of the
brake depends on (1) the rotor must be straight and smooth, (2) the
calliper mechanism must be properly aligned with the rotor, (3) the
pads must be positioned correctly, (4) there must be enough "pad"
left, and (5) the lever mechanism must push the pads tightly
against the rotor, with "lever" to spare.Most modern cars have disc
brakes on the front wheels, and some have disc brakes on all four
wheels. This is the part of the brake system that does the actual
work of stopping the carThe most common type of disc brake on
modern cars is the single-piston floating calliper.
MAIN PARTS:The main components of a disc brake are: The brake
pads The calliper, which contains a piston The rotor, which is
mounted to the hub BRAKE PAD
Fig 9: Brake PadCALIPER AND ROTOR
Fig 10: Caliper and RotorCHAPTER-7COMPONENTS OF HYDRAULIC DISC
BRAKENow that we understand hydraulics let's take a look at the
different parts which make up the hydraulic brake. The entire
braking system can be broken down into the following main parts: 1.
Master cylinder (Lever)2. Lines3. Fluid4. Slave cylinder
(Calliper)5. Pads6. RotorNext we will explain these components in
more detail.
Fig 11: The layout of a typical brake system
7.1 MASTER CYLINDER
Converts mechanical force from the brake pedal, power booster
and push rod into hydraulic pressure
Contain pistons, piston seals, return springs and internal brake
fluid ports.
Also has a fluid reservoir that may either be an integral part
of the unit or remotely mounted. The reservoir itself will have a
removable cap with a rubber diaphragm seal that must be in good
condition to seal properly.
Most reservoirs also have a low brake fluid level switch to
alert the driver of a low fluid condition.Master Cylinder/LeverThe
master cylinder, mounted to the handlebar, houses the brake lever
and together they produce the input force needed to push hydraulic
brake fluid to the slave cylinder (or calliper) and cause the brake
pads to clamp the rotor. The lever stroke can be divided into 3
categories: 1. Dead-stroke - This is the initial part of the lever
stroke when the primary seal pushes fluid toward the reservoir
before it goes on to push fluid on to the calliper via the brake
lines.
2. Pad Gap Stroke - This is the part between the calliper
beginning to push the pistons out of their housings and the pads
contacting the disc (as the dead space between the pads and rotor
is taken up).3. Contact & Modulation - The pads are now
clamping the rotor and by stroking the lever further, additional
brake power will be generated. Modulation is rider controlled and
not necessarily a characteristic of the braking system; however
some brakes may allow the rider to better modulate or control the
braking forces than others.
7.2 MASTER CYLINDER - OPEN OR CLOSEDMaster cylinder systems can
be categorized into two groups - open and closed.An open system
includes a reservoir and bladder which allow for fluid to be added
or removed from the braking system automatically during use.
Reservoirs are the overflow for fluid which has expanded due to
heat produced by braking. The bladder has the ability to expand and
contract therefore as the fluid expands the bladder will compensate
without any adverse effects on the 'feel' of the brake. Reservoirs
also provide the additional fluid needed as the pads begin to wear
resulting in the need for the pistons to protrude further to
compensate for the reduced pad material.A closed system also
utilizes a reservoir of brake fluid however the lack of an internal
bladder to compensate for the expansion in brake fluid and also to
compensate for pad wear means that any adjustments to the levels of
brake fluid within the working system need to be made manually.
7.3 BRAKE LINESHydraulic brake lines or hoses play the important
role of connecting the two main working parts of the brake, i.e.
the master cylinder and slave cylinder. We've already mentioned
that hydraulic systems can be very versatile in that their lines or
hoses can be routed almost anywhere so let's take a closer look.
Hose ConstructionHydraulic hoses are multi-layered in their
construction and usually consist of 3 layers:
1. Inner Tube - This layer of tubing is designed to hold the
fluid. Teflon is usually the material of choice here as it does not
react or corrode with brake fluid.2. Aramid (Kevlar) Layer -
provides the strength and structure of the hose. This woven layer
is flexible and handles the high pressures of the hydraulic system
efficiently in that it should not expand. Kevlar is also very
light, which is a desirable attribute for any cycle component, and
also it can be cut easily and re-assembled using standard hose
fittings.3. Outer Casing - Serves as a protection layer for both
the Kevlar layer and the bike frame to reduce abrasions.
Fig 12: The layers that make up an average hydraulic brake
lineSteel Braided Brake LinesSteel braided hoses can provide some
advantages over standard hydraulic hoses. Steel braided hoses are
also usually a 3-layer construction, the inner most layer contains
the brake fluid and there is an outer most layer which provides
protection against abrasions. The key difference is in the middle
layer which is made up of a stainless steel braid.This stainless
steel layer is designed to be more resistant against expansion than
that of standard lines. This can be an advantage because when the
brake lever is applied we want all of the force we put in to be
transferred to the calliper to cause braking. Any expansion in the
hydraulic line due to the pressures within will mean that some of
that pressure will not be transferred to the calliper. This will be
wasted effort and will require additional lever input by the rider
to compensate.Steel braided lines may also be more appealing
aesthetically. Many riders believe that they look better than the
standard, boring black hoses that are supplied with the vast
majority of brakes on the market.
Fig 13:2011 Formula R1 brake with braided brake lines7.4 BRAKE
FLUIDHydraulic braking systems typically use one of two types of
brake fluid - DOT fluid or mineral oil. An important thing to note
before we get into the properties of each is that the two fluids
should never be mixed. They are made up of very different chemicals
and the seals within the braking system are suited to either fluid
or not both; therefore mixing or replacing one fluid with the other
is likely to corrode the internals of your brake.On the other hand,
mixing fluid from the same family is allowed but not generally
advised. For example you may mix DOT 4 fluid with DOT 5.1 without
harming your braking system. DOT Brake FluidDOT brake fluid is
approved and controlled by the Department of Transportation. It has
to meet certain performance criteria to be used within braking
systems and is classified by its performance properties - mainly
its boiling points.DOT 3, 4 and 5.1 brake fluids are glycol-ether
based and are made up of various solvents and chemicals.
Glycol-ether brake fluids are hygroscopic, which means they absorb
water from the environment even at normal atmospheric pressure
levels. The typical absorption rate is quoted to be around 3% per
year. This water content within the brake fluid will affect the
performance by reducing its boiling points that is why it is
recommended to change brake fluid every 1-2 years at most.
The table below shows DOT brakes fluid in its various
derivatives with its corresponding boiling temperatures. Wet
boiling point refers to fluid with water content after 1 years'
service.
DOT FluidDry Boiling PointWet Boiling Point
DOT 3205 C (401 F)140 C (284 F)
DOT 4230 C (446 F)155 C (311 F)
DOT 5260 C (500 F)180 C (356 F)
DOT 5.1
270 C (518 F)
190 C (374 F)
DOT brake fluid is commonly used in Avid, Formula, Hayes and
Hope brakes. DOT 5 Brake FluidDOT 5 brake fluids (not to be
mistaken for DOT 5.1) are very different from other DOT fluids as
it is silicone based and not glycol-ether based. This silicone
based brake fluid is hydrophobic (non water absorbing) and must
never be mixed with any other DOT brake fluid.
DOT 5 can maintain an acceptable boiling point throughout its
service life although the way in which it repels water can cause
any water content to pool and freeze/boil in the system over time -
the main reason that hygroscopic fluids are more commonly used.
Mineral OilMineral oil is less controlled as a brake fluid, unlike
DOT fluid which is required to meet a specific criteria, therefore
less is known regarding its performance and boiling points from
brand to brand.Manufacturers such as Shimano and Magura design
their brakes around their own brand of mineral oil and should never
be introduced to DOT brake fluid as this will likely have an
adverse effect on the brake's seals.An advantage of mineral oil is
that, unlike most DOT fluids, it does not absorb water. This means
that the brake will not need to be serviced as often, but any water
content within the braking system could pool and freeze/boil
adversely affecting the performance of the brake.
Mineral oil is also non-corrosive meaning handling of the fluid
and spillages are less of a concern. 7.5 SLAVE CYLINDER/CALLIPERThe
brake callipers reside at each wheel and respond to the lever input
generated by the user. This lever input is converted to clamping
force as the pistons move the brake pads to contact the rotor.
Callipers can be fixed by a rigid mount to the frame or floating.
Fixed callipers are combined with a fixed rotor which offers the
only way of achieving zero free running drag one drawback of this
design is that it is much less tolerant of rotor imperfections.
Floating callipers slide axially and self-centre with each braking
application. ConstructionCalliper construction can fall into two
categories - mono-block and two piece. The difference here is the
'bridge' design, the bridge is the part of the calliper above the
pistons which connects the two halves together and provides the
strength to endure the clamping forces generated by the
pistons.
1. Mono-block - A mono-block calliper is actually a one piece
design formed from one piece of material. This can offer a unique
design and usually a lighter calliper as there is no need for steel
bolts joining both halves as in a two piece design. Also the lack
of a transfer port seal means there is one less opportunity for
fluid leaks at the half way seam. Servicing a mono-block calliper
can be tricky however and manufacturing and assembly are usually
more difficult.2. Two piece - These two piece callipers are
constructed as two separate halves and are then held together with
steel bolts which can provide additional strength over a mono-block
design. Servicing, manufacturing and assembly are simplified. Steel
bolts and additional seals are a means of additional weight and can
be problematic during servicing.
Fig 14: Exploded view of an Avid two-piece calliper
designPistonsThe pistons are the cylindrical components housed
within the calliper body. Upon lever input they protrude to push
the brake pads which contact the rotor. The number of pistons
within a calliper or brake can differ. Many hydraulic mountain bike
brakes have 2 piston callipers, some may have 4 pistons. Whereas
some automobile brake callipers have 6 or even 8 pistons. It is an
important note that brake power is not determined by piston
quantity. A more reliable indicator would be total piston contact
area, e.g. 4 smaller pistons can be just as powerful as 2 larger
pistons. Pistons can be either opposed or single sided. Opposed
pistons both protrude with lever input to push the brake pads equal
amounts to meet the rotor at both sides. Whereas single sided
calliper pistons stroke on one side and float the rotor to the
opposite pad. 7.6 BRAKE PADSChoosing the right brake pads can mean
the difference between a great and a poor performing brake. With
the sheer diversity of brake pad materials out there it is quite
easy to get it wrong when the time comes to replace the pads.Let's
jump right in and take a look at the different pad materials
available and their properties. OrganicOrganic brake pads contain
no metal content. They are made up of a variation of materials
which used to include asbestos until its use was banned. These days
you will commonly find materials such as rubber, Kevlar and even
glass. These various materials are then bonded with a
high-heat-withstanding resin. An advantage of organic pads is that
they're made up of materials that don't pollute as they wear. They
are also softer than other brake pads and as a result quieter. Also
they inflict much less wear upon the brakes' rotor. However organic
pads wear down faster and they perform especially poorly in wet
gritty conditions (UK readers take note).Organic pads then are
probably more suited to less aggressive riding in mostly dry
conditions. Fig 15: Organic Brake padsSemi-metallicThe metallic
content of semi-metallic pads can vary from anything between 30%
and 65%. The introduction of metal content into the friction
material changes things slightly. It can improve the lifespan of
the pad quite significantly as metal wears slower than organic
materials. Also heat dissipation is improved as it is transferred
between the pad material and the backing plate. Some disadvantages
can include increased noise during use and the harder compound
means increased wear on the rotor.
Fig 16: Semi-metallic padsSinteredSintered brake pads are made
up of hardened metallic ingredients which are bound together with
pressure and high temperature. The advantages of this compound are
better heat dissipation, a longer lasting pad, better resistance to
fading and superior performance in wet conditions. The trade-offs
are more noise, longer bed-in time and a poor initial bite until
the friction material has chance to warm.
Fig 17: Sintered Brake padsCeramicCeramic brake pads are now
seen more and more as an alternative/upgrade mountain bike brake
pad. Traditionally ceramic brake pads would only be seen on high
performance racing cars with brakes which need to perform under
intense heat. Heat like that is not usually a problem for the
average mountain bike brake and therefore for most people ceramic
pads would be overkills however they might have other desirable
properties. The advantages of a ceramic material then is one which
can cope with extreme heat and keep performing strongly; this is in
part down to its great dissipating abilities. They also last longer
than other pads and noise is less of an issue. They're also easier
on brake rotors and produce a lot less dust that other brake pad
compounds.
Fig 18: Ceramic Brake pads
7.7 ROTORSRotor size has a direct effect on braking power. The
larger the brake rotor the more power will be produced for any
given input. This can be a concern with larger rotors as they tend
to have more of a 'grabby' feel making the brake more difficult to
modulate. Mountain bike rotors tend to range in size from 160mm to
203mm, with smaller rotors geared toward XC type riding and larger
rotors designed for downhill riding. Rotor DesignImportant
specifications of rotor design include hardness, thickness and rub
area.The material used to manufacture rotors must be hard and
durable due to the aggressive forces inflicted upon them from the
pad friction material. This has a direct impact on rotor wear.
Rotors must also have no thickness variations. Differences in
thickness around the circumference of the rotor can have undesired
effects on the braking system including pulsing as thicker and
thinner sections pass between the pads. Rotors also need to run
true. Any lateral wobble in the rotor during use can cause the
brake to contact the pads intermittently during riding.
Fig 19: Left to right: Formula Lightweight, Avid G3 Clean Sweep,
AshimaAiRotorA rotor's rub area can take the form of many different
designs. The three rotors above show this in detail. Rub area
design can affect the weight and strength of the rotor. It also has
a direct effect on pad lifetime. TYPES OF ROTORSThe two types of
rotor on the market today are ISO standard 6-bolt rotors and
CenterLock rotors. Both have their pros and cons.6 Bolt - Readily
available and interchangeable between many brake models, this is
the most common rotor fixing system in use today and was adopted by
all manufacturers in the late 1990's. With no shortage of hub
options, cross-compatibility with other products is rarely a
problem. However installation of six fixing bolts can be cumbersome
and there is always the risk of stripping a thread on fixing bolts
and hub mounting points.CenterLock - The Shimano CenterLock system
eliminates the risk of stripping threads as there are no bolts to
worry about, just one centre locking ring. Installation and removal
is also simplified, although you will need a CenterLock tool. Lack
of mass-market adoption means that hub choices are limited and
brake choice may also be limited due to odd sized rotors.
CenterLock rotors are also generally slightly heavier and can come
at a price premium.
Fig 20: Left to right: ISO standard 6-bolt, Shimano
CenterLock2-Piece Rotors2-Piece rotors are supplied as standard
with some higher priced brake sets and can also be bought
separately as an upgrade.In contrast to standard stainless steel
rotors, 2-piece rotors combine a stainless steel rub area with an
aluminium carrier (or spider). The advantage of the alloy carrier
is a cooler running disc as aluminium has superior heat dissipation
qualities to that of stainless steel. This will also help to keep
your pads, calliper and fluid cooler. Aluminium is also lighter
than stainless steel so a reduction in weight can be expected.
Fig 21: Formula 2-Piece Stainless Steel / Aluminium Rotor
Reason why Brakes FailHydraulic brakes can fail or temporarily
stop working for numerous reasons such as a simple (but potentially
catastrophic) fluid leak or eventual brake fade after prolonged
use. Knowing the causes of brake failure can be valuable knowledge
in curing the problem and preventing future episodes.As we know
there are a couple of important principles behind hydraulic brakes.
Hydraulics relies on pressure within the system and brakes rely on
friction. Absence of either will result in failure of the system.
For example, a loss of brake fluid will decrease the pressure
within the system as the lever has nothing to transfer the input
forces to. On the other hand if brake fluid contacts the brake pads
or rotor, a loss of friction will occur due to the lubricating
nature of brake fluid.The above examples should be obvious to most
but what about the less obvious causes of brake failure? Earlier we
mentioned brake fade, a term which I bet many of you have heard,
however did you know that there are multiple types of brake fade?
Below is an overview of the three different types. Pad FadeAll
friction material (the stuff your pads are made of) has a
coefficient of friction curve over temperature. Friction materials
have an optimal working temperature where the coefficient of
friction is at its highest. Further hard use of the brake will send
the friction material over the optimal working temperature causing
the coefficient of friction curve to decline.This high temperature
can cause certain elements within the friction material to melt or
smear causing a lubrication effect; this is the classic glazed pad.
Usually the binding resin starts to fail first, and then even the
metallic particles of the friction material can melt. At very high
temperatures the friction material can start to vaporize causing
the pad to slide on a layer of vaporized material which acts as a
lubricant.The characteristics of pad fade are a firm, non-spongy
lever feel in a brake that won't stop, even if you are squeezing as
hard as you can. Usually the onset is slow giving you time to
compensate but some friction materials have a sudden drop off of
friction under high temperatures resulting in sudden fade.
Green FadeGreen fade is perhaps the most dangerous type of fade
which manifests itself on brand new brake pads. Brake pads are made
of different types of heat resistant materials bound together with
a resin binder. On a new brake pad these resins will cure when used
hard on their first few heat cycles and the new pad can hydroplane
on this layer of excreted gas.
Green fade is considered the most dangerous as it can catch
users unaware given its quick onset. Many people would consider new
brake pads to be perfect and may be used hard from the word
'go'.Correct bedding-in of the brake pads can prevent green fade.
This process removes the top layer of the friction material and
keys the new pad and rotor together under controlled conditions.
Fluid FadeFluid fade is caused by heat induced boiling of the brake
fluid in the callipers and brake lines. When used under extreme
conditions heat from the pads can transfer to the calliper and
brake fluid causing it to boil, producing bubbles in the braking
system. Since bubbles are compressible this results in a spongy
lever feel and prevents the lever input from being sent to the
calliper.The major cause of fluid fade is absorbed water from the
air under normal atmospheric conditions which reduces the boiling
temperature of the brake fluid. DOT brake fluid has an affinity for
absorbing water from the air around it, especially in hot humid
conditions. This is the main reason why we replace brake fluid on
an annual basis.Fortunately fluid fade has a gradual onset giving
the user time to compensate for potential loss of
braking.CHAPTER-8WORKING OF HYDRAULIC DISC BRAKES
Fig 22: Working of hydraulic disc brakes The master cylinder is
where the brake fluid starts. The pedal is attached to the master
cylinder plunger. When the pedal is depressed it pushed the plunger
which pushes the brake fluid down the brake lines. The brake lines
are connected to the slave cylinders. When the brake fluid reaches
the slave cylinders it presses out a piston to which is attached a
brake pad. The brake pad then clamps against the rotor. All air
must be bled from the system. (Air is compressible and if you have
any in the system you will have a soft pedal.) As oil is virtually
uncompressible it works as a solid link from pedal to brake.
8.1 THE MASTER CYLINDER IN ACTIONAs you can see in figure there
are two pistons (primary and secondary) and two springs inside the
master cylinder.
When the brake pedal is pressed, a push rod moves the primary
piston forward which begins to build pressure in the primary
chamber and lines. As the brake pedal is depressed further, the
pressure continues to increase.
Fluid pressure between the primary and secondary piston then
forces the secondary piston forward and pressurizes the fluid in
the secondary circuit.
If the brakes are operating properly, the pressure will be the
same in both circuits.
Fig 23: Brakes released
Fig 24: Brakes applied
If there is a leak in one of the brake circuits, that circuit
will not be able to maintain pressure. Figure shows what happens
when one of the circuits develops a leak. In this example, the leak
is in the primary circuit and the pressure between the primary and
secondary pistons is lost. This pressure loss causes the primary
piston to mechanically contact the secondary piston and the master
cylinder now behaves as if it has only one piston. The secondary
circuit will continue to function correctly, however the driver
will have to press the pedal further to activate it. In addition,
since only two wheels now have pressure, the braking power will be
reduced.
Fig 25: Functioning of the brakes
8.2 COMPENSATING PORTS
Small holes those are located between the master cylinder
reservoir and the front side, or pressure side, of the master
cylinder pistons.
When the master cylinder pistons are in the at-rest position (no
braking-figure 9), the piston seals uncover the compensating ports
and open the passages between the reservoir and the wheel brake
channel.
Allow for the normal expansion and contraction of brake fluid
due to changes in temperature.
Fig 26: Compensating Ports
Assist in fluid return after brake release (See Bypass Port
section below).
Note: When the brakes are released, the piston seals on both the
primary and secondary pistons are located between the compensating
port and the bypass port. During braking, the piston seals close
the compensating port passages to the reservoir which prevents high
pressure fluid from entering the reservoir.
8.3 BYPASS PORTS
The bypass ports, like the compensating ports, are passages that
are open between the reservoir and the master cylinder chambers
(fig. 10). However, the bypass ports are open to the low pressure
or back side of the pistons.
Allow the master cylinder pistons to return to the at-rest
position rapidly.
Fig 27: Bypass Ports
During brake release, the following occurs: Strong springs in
the master cylinder force the pistons back to the at-rest position
faster than the brake fluid can return through the hydraulic
channels. The pistons must return rapidly so they can be ready for
another forward stroke, if necessary. This rapid piston return
movement could create a vacuum in the master cylinder high pressure
chambers, which would delay brake release. The bypass ports allow
brake fluid from the reservoir to fill the low-pressure piston
chambers. Brake fluid from the low pressure chambers then passes
through holes in the pistons and bypasses the piston lip seals. The
pistons can then return without any dragging.Since this return
action causes additional fluid to be moved to the front of the
piston, it results in an excess amount of fluid being present
there, as even more fluid returns from the callipers and wheel
cylinders. This excess fluid is easily returned to the reservoir
through the now-open compensating ports.Note: Piston dragging can
also occur if the seals are installed backward.
Fig 28: Master Cylinder Return Operation: applied (left);
releasing (right)
8.4 SELF ADJUSTMENT OF DISC BRAKES:Disc brakes are self
adjusting. Each piston has a seal on it to prevent fluid leakage.
When the brakes are applied, the piston moves toward the disc. This
distorts the piston seal. When the brakes are released, the seal
relaxes and returns to its original position. This pulls the piston
away from the disc. As the brakes linings wear, the piston over
travels and takes a new position in relation to the seal. This
action provides self-adjustment of disc brakes.8.5 EMERGENCY
BRAKES:In cars with disc brakes on all four wheels, an emergency
brake has to be actuated by a separate mechanism than the primary
brakes in case of a total primary brake failure. Most cars use a
cable to actuate the emergency brake.
Fig 29: Emergency BrakesSome cars with four-wheel disc brakes
have a separate drum brake integrated into the hub of the rear
wheels. This drum brake is only for the emergency brake system, and
it is actuated only by the cable; it has no hydraulics.
CHAPTER-9BRAKE FADEVehicle braking system fade, or brake fade,
is the reduction in stopping power that can occur after repeated or
sustained application of the brakes, especially in high load or
high speed conditions. Brake fade can be a factor in any vehicle
that utilizes a friction braking system including automobiles,
trucks, motorcycles, airplanes, and even bicycles.Brake fade is
caused by a build-up of heat in the braking surfaces and the
subsequent changes and reactions in the brake system components and
can be experienced with both drum brakes and disc brakes. Loss of
stopping power, or fade, can be caused by friction fade, mechanical
fade, or fluid fade. Brake fade can be significantly reduced by
appropriate equipment and materials design and selection, as well
as good cooling.Brake fade occurs most often during high
performance driving or when going down a long, steep hill. Owing to
their configuration fade is more prevalent in drum brakes. Disc
brakes are much more resistant to brake fade and have come to be a
standard feature in front brakes for most vehicles.
9.1 BRAKE MODIFICATION TO REDUCE FADEHigh performance brake
components provide enhanced stopping power by improving friction
while reducing brake fade. Improved friction is provided by lining
materials that have a higher coefficient of friction than standard
brake pads, while brake fade is reduced through the use of more
expensive binding resins with a higher melting point, along with
slotted, drilled, or dimpled discs/rotors that reduce the gaseous
boundary layer, in addition to providing enhanced heat dissipation.
Heat build-up in brakes can be further addressed by body
modifications that direct cold air to the brakes.The "gaseous
boundary layer" is a hot rod mechanics explanation for failing
self-servo effect of drum brakes because it felt like a brick under
the brake pedal when it occurred. To counter this effect, brake
shoes were drilled and slotted to vent gas. In spite of that, drum
brakes were abandoned for their self-servo effect. Discs do not
have that because application force is applied at right angles to
the resulting braking force. There is no interaction.Drum brake
fade can be reduced and overall performance enhanced somewhat by an
old "hot rudder" technique of drum drilling. A carefully chosen
pattern of holes is drilled through the drum working section; drum
rotation centrifugally pumps a small amount air through the shoe to
drum gap, removing heat; fade caused by water-wet brakes is reduced
since the water is centrifugally driven out; and some
brake-material dust exits the holes. Brake drum drilling requires
careful detailed knowledge of brake drum physics and is an advanced
technique probably best left to professionals. There are
performance-brake shops that will make the necessary modifications
safely.
9.2 DISC BRAKE VENTS
Fig 30: Brake Modification to Reduce FadeA moving car has a
certain amount of kinetic energy, and the brakes have to remove
this energy from the car in order to stop it. How do the brakes do
this? Each time you stop your car, your brakes convert the kinetic
energy to heat generated by the friction between the pads and the
disc. Most car disc brakes are vented. Brake fade caused by
overheating brake fluid (often called Pedal Fade) can also be
reduced through the use of thermal barriers that are placed between
the brake pad and the brake calliper piston. These reduce the
transfer of heat from the pad to the calliper and in turn hydraulic
brake fluid. Some high-performance racing callipers already include
such brake heat shields made from titanium or ceramic materials.
However, it is also possible to purchase aftermarket titanium brake
heat shields that will fit your existing brake system to provide
protection from brake heat. These inserts are precision cut to
cover as much of the pad as possible. These Titanium Brake shims
are an easy to install, low cost solution that are popular with
racers and track day enthusiasts.Another technique employed to
prevent brake fade is the incorporation of fade stop brake coolers.
Like titanium heat shields the brake coolers are designed to slide
between the brake pad backing plate and the calliper piston. They
are constructed from a high thermal conductivity, high yield
strength metal composite which conducts the heat from the interface
to a heat sink which is external to the calliper and in the
airflow. They have been shown to decrease calliper piston
temperatures by over twenty percent and to also significantly
decrease the time needed to cool down. Unlike titanium heat
shields, however, the brake coolers actually transfer the heat to
the surrounding environment and thus keep the pads cooler.
CHAPTER-10ADVANTAGES10.1 ADVANTAGES OF DISC BRAKES OVER DRUM
BRAKESAs with almost any artifact of technology, drum brakes and
disc brakes both have advantages and disadvantages. Drum brakes
still have the edge in cheaper cost and lower complexity. This is
why most cars built today use disc brakes in front but drum brakes
in the back wheels, four wheel discs being an extra cost option or
shouted as a high performance feature. Since the weight shift of a
decelerating car puts most of the load on the front wheels, the
usage of disc brakes on only the front wheels is accepted
manufacturing practice. Drum brakes had another advantage compared
to early disc brake systems. The geometry of the brake shoes inside
the drums can be designed for a mechanical self-boosting action.
The rotation of the brake drum will push a leading shoe brake pad
into pressing harder against the drum. Early disc brake systems
required an outside mechanical brake booster such as a vacuum
assist or hydraulic pump to generate the pressure for primitive
friction materials to apply the necessary braking force. All
friction braking technology uses the process of converting the
kinetic energy of a vehicles forward motion into thermal energy:
heat. The enemy of all braking systems is excessive heat. Drums are
inferior to discs in dissipating excessive heat:"The common
automotive drum brake consists essentially of two shoes which may
be expanded against the inner cylindrical surface of a drum.The
greater part of heat generated when a brake is applied has to pass
through the drum to its outer surface in order to be dissipated to
atmosphere, and at the same time (the drum is) subject to quite
severe stresses due to the distortion induced by the opposed shoes
acting inside the open ended drum.The conventional disc brake, on
the other hand, consists essentially of a flat disc on either side
of which are friction pads; equal and opposite forces may be
applied to these pads to press their working surfaces into contact
with the braking path of the discs. The heat produced by the
conversion of energy is dissipated directly from the surfaces at
which it is generated and the deflection of the braking path of the
disc is very small so that the stressing of the material is not as
severe as with the drum." The result of overheated brakes is brake
fade...the same amount of force at the pedal no longer provides the
same amount of stopping power. The high heat decreases the relative
coefficient of friction between the friction material and the drum
or disc. Drum brakes also suffer another setback when overheating:
The inside radii of the drum expand, the brake shoe outside radii
no longer matches, and the actual contact surface is decreased.
Another advantage of disc brakes over drum brakes is that of
weight. There are two different areas where minimizing weight is
important. The first is unsprung weight. This is the total amount
of weight of all the moving components of a car between the road
and the suspension mounting points on the cars frame. Auto designs
have gone to such lengths to reduce unsprung weight that some, such
as the E-type Jaguar, moved the rear brakes inboard, next to the
differential, connected to the drive shafts instead of on the rear
wheel hubs. The second "weighty" factor is more of an issue on
motorcycles: gyroscopic weight. The heavier the wheel unit, the
more is gyroscopic resistance to changing direction. Thus the bikes
steering would be higher effort with heavier drum brakes than with
lighter discs. Modern race car disc brakes have hollow internal
vents, cross drilling and other weight saving and cooling features.
Most early brake drums and discs were made out of cast iron.
Current OEM motorcycle disc brakes are usually stainless steel for
corrosion resistance, but after-market racing component brake discs
are still made from cast iron for the improved friction qualities.
Other exotic materials have been used in racing applications.
Carbon fibre composite discs gripped by carbon fibre pads were
common in formula one motorcycles and cars in the early 1990s, but
were outlawed by the respective racing sanctioning organizations
due to sometimes spectacular failure. The carbon/carbon brakes also
only worked properly at the very high temperatures of racing
conditions and would not get hot enough to work in street
applications. A recent Ducati concept show bike uses brake discs of
selenium, developed by the Russian aerospace industry, which claim
to have the friction coefficient of cast iron with the light weight
of carbon fibre. Another area of development of the disc brake is
the architecture of the brake calliper. Early designs had a rigidly
mounted calliper gripping with opposed hydraulic pistons pushing
the brake pads against a disc mounted securely to the wheel hub.
Later developments included a single piston calliper floating on
slider pins. This system had improved, more even pad wear. Most
modern automobiles and my 1982 Kawasaki motorcycle use this type
calliper. Current design paradigm for motorcycle brakes have up to
six pistons, opposed to grip both sides of a thin, large radius
disc that is "floating" on pins to provide a small amount of
lateral movement; two discs per front wheel. Improvements in
control have been made available with the application of Anti-Lock
Brake technology. Wheel sensors convey rotation speed of each wheel
to a computer that senses when any of them are locked up or in a
skid, and modulates individual wheel brake hydraulic pressure to
avoid wheel skidding and loss of vehicular control. The use of
exotic materials for additional weight savings would be likely for
the future of motor vehicle braking. Discs mounted to the wheels
rim gripped by an internally located calliper are not necessarily a
new design (Porsche, 1963) but could be a futuristic looking option
for motorcycle wheels. Electric vehicles of the future will likely
utilize regenerative braking, the electric motors become generators
to convert kinetic energy back to electricity to recharge the
batteries. As production vehicles become increasingly quicker, the
need for "whoa" will always accompany the "go".
10.2 REASON FOR HIGH EFFICIENCY OF DISC BRAKES Flat brake disc
(axial brake) under high pressure versus round brake drum (radial
brake) during braking Full friction surface of the brake pad on the
plane brake disc. No loss of brake power due to overheating or
partial contact from brake drum parts expansion. Disc brakes can
withstand higher loads and its efficiency is maintained
considerably longer even under the highest stresses Higher residual
brake force after repeating braking Brake discs can withstand
extremely high temperatures Full contact of brake pads achieves
maximum effect. No verification of brake pads. Dangerous fading or
slipping is almost completely eliminated.
10.3 BETTER BRAKING BEHAVIOUR OF HYDRAULIC DISC BRAKES Driver
friendly braking behaviour. Sensitive braking in all situations and
better Sensitive brake application and better brake feeling Uniform
braking from small fluctuations in brake forces Retardation values
retained even under heavy stresses Minimal "pulling to one side"
due to uneven brake forces Disc brake axial arrangement permits a
simple and compact design Linear characteristics lead to an even
progression of brake force Basic design principle makes for higher
efficiency Low hysteresis is particularly suitable to ABS control
cycles
10.4 HIGHER SAFETY RESERVES Minimal braking effect from high
temperatures and extreme driving requirements Minimal heat fading
No brake disc distortion from extreme heat due to internal
ventilation with directional stability and large power reserve
under high stress The decisive safety aspects of the disc brake
design are shorter braking distances High power and safety reserves
for emergencies Constant braking power under high stresses
Shortened braking distance under emergency braking with
considerably improved directional stability
CHAPTER-11
LIMITATIONS
The limitations of hydraulic disc brakes: Braking systems fails
if there is leakage in the brake lines. The brake shoes are liable
to get ruined if the brake fluid leaks out. Presence of air inside
the tubing ruins the whole system. Pad wear is more. Hand brakes
are not effective if disc brakes are used in rear wheels also.
(Hand brakes are better with mechanical brakes).
CHAPTER-12APPLICATIONS
The applications of Hydraulic Disc Brakes are:
Hydraulic Disc brakes are used primarily in motor vehicles,
tanks, but also in machinery and equipment, and aircraft, bicycles,
carriages and railway. The disc brakes have been widely used in
cars and trucks, especially in the premium sedan. The disc brakes
on the new mine hoist brake. The disc brake inertia is small, fast
action, high sensitivity, and adjustable braking torque. The
multi-rope friction hoist all use disc brakes.
II.
CONCLUSIONMany trucks and buses are equipped with hydraulic
actuated disc brakes. The high contact forces are transmitted
mechanically via needle mounted actuating device.In view of the
fact that the air can circulate freely between the disc and the
brake shoe, disc brakes are cooled much better, especially since it
is possible to do so ventilated discs extra holes. The gases
resulting from friction, dust, dirt, do not stay on the working
surfaces. These brakes are not sticky.The disc brakes have been
widely used in cars and trucks, especially in the premium sedan.
The disc brakes on the new mine hoist brake. The disc brake inertia
is small, fast action, high sensitivity and adjustable braking
torque.
1
III. REFERENCES TechCenter By Karl Brauer, Editor in Chief,
Edmunds.com
http://cars.about.com/od/thingsyouneedtoknow/ig/Disc-brakes
http://en.wikipedia.org/wiki/Disc_brake
http://www.kobelt.com/pdf/brochure_brake.pdf
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake.htm
http://www.sae.org/search?searchfield=brake%20system
http://www.hinduonnet.com/thehindu/2000/05/25/ceramic brake
disc
Automotive Engineering International Online Global Viewpoints,
Nov_ 1999