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Brake Calipers
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While calipers need to convert
hydraulic fluid pressure into clamp
force, they also must look good doing
it. A little advertising never hurts either.
Can you guess the manufacturer of
this caliper? (Randall Shafer/Baer)
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In recent years, brake calipers have transformed into a prominent automotive accessory for the
image-conscious consumer. Yellow, red, silver, black, and even bright gold examples can be found
on the front and/or rear axles of many performance vehicles. Caliper bodies have even been
converted into miniature billboards for the caliper manufacturers themselves.
While these new caliper trends are pleasing to the eye, the basic role of the caliper has remained
unchanged since its inception. The caliper must simply convert the hydraulic fluid pressure
generated in the master cylinder into a linear mechanical clamping force against the brake pads. At
the same time, the caliper will usually locate the brake pads and supports the torque generated by
the brake rotor, but these are secondary functions.
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Hydraulic Gain
As you have already learned, the caliper clamp force can be calculated based upon the brake fluid
pressure generated by the master cylinder and the inboard caliper piston area as follows:
Caliper clamp force (lb) = master cylinder pressure (psi) x effective caliper piston area (in2)
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Note that although it wasn’t mentioned explicitly back in Chapter 3, the effective caliper piston area
is equal to the inboard caliper piston area multiplied by two.
Based on this relationship, it’s common to select calipers and master cylinders in such a way as to
amplify the force being applied to the master cylinder piston. Calculated much like the brake pedal
ratio, the hydraulic gain of the system is equal to the effective caliper piston area divided by the
master cylinder piston area. A more detailed description of hydraulic gain can be found in the
sidebar, but in summary, the hydraulic gain can be increased by reducing the master cylinder area
or by increasing the effective caliper piston area.
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Caliper sizing and selection is
generally a function of the required
hydraulic gain, with larger calipers
providing more output than smaller
calipers. For this reason, calipers like
this eight-piston monster designed for
the Hummer H2 would be complete
overkill on a smaller, lighter vehicle.
(Randall Shafer/StopTech)
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Caliper Components
It has already been stated that the caliper functionally resembles a common master cylinder, but in
order to fit around the spinning rotor it must be shaped like a C-clamp. This isn’t necessarily a good
thing, as the clamping force generated at the open end of the caliper will always attempt to spread
the caliper body apart. This distorts the caliper body, which exaggerates the P-V relationship. In
short, more caliper deflection results in more brake pedal travel.
Two significant mechanical attributes of a caliper are its stiffness and its strength. Not to be used
interchangeably, stiffness indicates how much deflection a caliper exhibits for a given amount of
clamp force, while strength is a measure of the absolute force that can be sustained before failure
of the caliper. Consequently, high stiffness is desired for good pedal feel while high strength is
required for mechanical integrity.
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Body
The caliper body is typically made of cast iron in production vehicles. It locates the piston and
supports the clamp force exerted on the brake pads. While cast iron is acceptable from stiffness
and strength perspectives (especially at elevated temperatures), its weight makes it undesirable in
performance applications. Consequently, aluminum alloys are employed when circumstances
dictate the lowest weight possible (in fact, aluminum caliper bodies are nearly universal on modern
high-performance vehicles), but their reduced stiffness can lead to excessive caliper deflection
without appropriate design countermeasures.
The body consists of three main parts: an inboard body section (which almost always contains at
least one piston bore), an outboard body section (which may or may not contain additional piston
bores), and a bridge, which connects the two. In some applications, calipers can be fabricated from
three separate components, but most often times are combined in a number of creative ways.
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Caliper strength and stiffness are two
key characteristics for optimum
performance. Recent advances in
computer modeling and simulation
have allowed for significant
improvements in both of these areas
without the penalty of increased
weight. (StopTech)
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Hydraulic Gain Example
If a vehicle was designed with a 0.75-inch-diameter master cylinder piston and a 2.00-inch-diameter
caliper piston, one could calculate their effective areas as follows:
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Diameter (in) Inboard Area (in2) Effective
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Master cylinder 0.75 n/a 0.44
Caliper 2.00 3.14 6.28
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Recalling from the body text, the hydraulic gain of the brake system is simply the effective area of
the caliper divided by the effective area of the master cylinder, or in equation form:
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Hydraulic gain (unitless) = effective caliper piston area (in2) ÷ master cylinder piston area (in2)
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Consequently, the hydraulic gain of this system would be 14.2:1 (6.28 square inches divided by
0.44 square inches). In other words, for every pound of force applied to the master cylinder piston,
14.2 pounds of clamp force is generated by the caliper.
The hydraulic relationship also dictates that the linear travel experienced by the master cylinder
piston will be 14.2 times greater than the linear travel experienced by the caliper piston relative to
the caliper body. For example, if the caliper piston required 0.010 inches of travel to overcome
compliance, the master cylinder piston would need to travel approximately 0.142 inches (14.2 times
as far) to accommodate the P-V need.
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The basic caliper structure consists of
body sections and bridge sections. In
some applications, these may be
discrete components assembled
together. In the case of this early
Porsche 911 Turbo caliper, two piston
housings are bolted to two bridge
sections to finalize the caliper
assembly. (Randall Shafer)
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Bridge Reinforcement
In order to facilitate caliper inspection and brake pad replacement without caliper removal, it’s
common to have large openings in the caliper bridge. Unfortunately, these openings can greatly
reduce the stiffness of the caliper, resulting in poor brake pedal feel. The term open caliper is often
used to describe this type of arrangement.
Consequently, select manufacturers implement an auxiliary bridge reinforcement to regain the
stiffness lost by open caliper design. This component is not required with closed calipers where pad
replacement is performed by removing the entire caliper body from the vehicle.
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In a closed caliper design, a structural
reinforcement connects the inboard
and outboard caliper body sections.
Based on the manufacturer, it may be
a removable piece bolted in place as
shown above, or it may be integral to
the body assembly. (Randall
Shafer/StopTech)
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Bracket
Based on the individual design, the caliper body may require a bracket to attach it to the
suspension upright, or knuckle. Made from either aluminum or cast iron, the bracket may also serve
to locate the brake pads in some applications. In the case of the floating caliper, the bracket also
contains parallel channels for the caliper slider pins (more to come in just a few paragraphs) and is
most likely designed to support the brake pad friction force as well.
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Most calipers require a bracket to
attach them to the vehicle suspension.
Simple brackets, like the one shown
above in yellow, locate the caliper
relative to the rotor without performing
auxiliary functions. (StopTech)
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In many applications, the caliper
mounting bracket must do much more
than simply position the caliper relative
to the rotor. For example, the
C-shaped channels machined into
either end of the bracket shown above
serve to locate the brake pads as well
as to provide axial (sliding) movement
of the entire caliper assembly. (Randall
Shafer/Baer)
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Abutment Plates
In aluminum caliper applications that need to support brake pad friction force, a pair of brake pad
abutment plates are used to provide a durable surface at the sliding interface. These plates are
usually fabricated from a thin piece of hard steel and allow for free motion of the steel brake pad
backing plates relative to the aluminum caliper body without deformation of the softer body material.
Cast iron caliper bodies, due to their unique mechanical properties, typically do not require
abutment plates. Note also that the abutment plates may be located in the caliper bracket in some
applications.
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Since most high-performance calipers
are fabricated from aluminum,
abutment plates should be added to
the caliper body to prevent wear at the
brake pad interface. Calipers made
from cast iron, however, don’t require
this additional design feature. (Randall
Shafer/Baer)
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Piston(s)
The caliper piston (or pistons) transmits hydraulic fluid pressure to the brake pad backing plates.
Based on the application, a caliper may have between one and eight pistons, but all function in the
same manner. Larger-diameter pistons generate higher forces and smaller diameter pistons
generate lower forces.
In most production applications, pistons are fabricated from steel or aluminum. Titanium is used in
select racing applications for its low thermal conductivity and decreased weight, but the cost of this
material makes it prohibitive on typical production vehicles. Phenolic materials are found in some
OEM applications, but their poor resistance to high temperatures makes them generally
inappropriate for high-performance use.
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Thermal Insulators
In addition to titanium pistons, thermal insulators may be sleeved inside of steel or aluminum
pistons to reduce heat transfer from the brake pad backing plate to the piston (and ultimately to the
brake fluid). Titanium and ceramic button inserts can be quite effective at keeping heat out of the
hydraulic system, and they’re less expensive to manufacture than billet titanium pistons.
Ceramic materials are also applied to the interior surfaces of the caliper body in some racing
applications. These exotic coatings are designed to reduce heat transfer from the rotor to the
caliper body, and ultimately the brake fluid.
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Previous | Next
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This has been a sample page from
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High-Performance Brake Systems
Design, Selection, and Installation
by James Walker, Jr.
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High-Performance Brake Systems: Design, Selection, and
Installation gives you the knowledge to upgrade your brakes the
right way the first time. Author James Walker, Jr. doesn’t just tell
you what to do—he uses over 330 photos and plain English to
help you understand how and why your brake system works, what
each of the components does, and how to intelligently upgrade
your brakes for better performance. There are chapters showing
you how to choose and install the most effective rotors, calipers,
pads, and tires for your sports car, muscle car, race car, and
street rod. You will even find special sidebars detailing how each
upgrade will affect your ABS.
Brakes might be one of the most important, yet least understood,
vehicle systems. Brakes are relied upon day in and day out
without giving a second thought to their condition, let alone their
purpose, function, or design. Brake systems can be intimidating,
and they aren’t usually the first thing the average horsepower
junkie chooses to upgrade. But there’s no reason to wait until you
have a problem to learn how your brakes work. Whether you are
a casual enthusiast, a weekend warrior, or a professional racer,
this book will tell you everything you need to know about brakes.
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Click below to view a sample
page from each chapter
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Chap. 1 - Energy Conversion
Chap. 2 - Tires Stop the Car
Chap. 3 - System Design
Chap. 4 - Brake Balance
Chap. 5 - Pedal & Master Cyl
Chap. 6 - Brake Fluid
Chap. 7 - Lines and Hoses
Chap. 8 - Brake Calipers
Chap. 9 - Brake Pads
Chap. 10 - Brake Rotors
Chap. 11 - Sports Car Brakes
Chap. 12 - Race Car Brakes
Chap. 13 - Muscle Car Brakes
Chap. 14 - Street Rod Brakes
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8-1/2 x 11"
Softbound
144 pages
330+ color photos
Item: SA126
Price: $21.95
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Click here to buy now!
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This is a great book that any performance enthusiast will love!
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Other items you might be interested in
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Sport Compact Turbos & Blowers
While most owners of sport compacts can afford the simple bolt-ons
available, some owners want to take their modifications a step further.
There is intense competition to be the fastest, and quite often the only
way to win is to go to the next level – by installing a supercharger /
blower or turbocharger on your engine. This book is an enthusiast’s
guide to understanding and using turbochargers and superchargers
on sport compact cars.
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Price:
$18.95
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