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Buy with confidence!
If for any reason
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All international
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Brake Pedals, Boosters and Master Cylinders
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The apply system (sometimes called the actuation system) exists in order to amplify the driver’s leg
force and then convert it into hydraulic fluid pressure. In nearly all cases, the apply system
functionality is not assigned to a single device. Instead, most apply systems accomplish this task by
dividing the total responsibility among several discrete components.
While there are many different types of apply systems, most conventional vehicles use a
combination of brake pedals, brake boosters, and master cylinders to get the job done. Variations
on this recipe abound, but the fundamental objective remains the same: increase the applied force
and turn it into pressure.
There is, however, one addendum. As you just learned in Chapter 4, brake pressure distribution
between the front brakes and rear brakes can be used to optimize a vehicle’s brake balance.
Consequently, the apply system may also contain additional devices which limit, regulate, or
proportion the rear brake line pressure.
The pages that follow walk through these various components in detail, starting with the driver’s
primary interface to the brake system: the brake pedal.
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Most brake pedal pads are
constructed from steel with textured
molded rubber covers. The pedal
shown here is actually made from cast
aluminum for weight savings.
Regardless of the aesthetic impact, the
material of the pad itself doesn’t impact
the brake pedal output. (Randall
Shafer)
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Brake Pedal Components
The brake pedal’s primary role is to mechanically increase the force exerted by the driver’s leg on
the brake pedal pad. Remember, it’s essentially the amplified force from the driver’s leg that
creates slip at the tire contact patches.
The brake pedal accomplishes this task as a direct result of its lever-based geometry. There are
many different designs available, but all rely on the following series of individual components to get
the job done.
Brake Pedal Pad
The brake pedal pad is simply where the driver provides mechanical force input to the brake
system. Most OEM systems use a ribbed rubber pad molded over a steel or composite core, but
exotic-looking brake pedal pads made from cast aluminum and/or with weight-saving holes are now
becoming more commonplace. Regardless of its construction, the pad should be large enough that
the driver will not miss the pedal in a panic, small enough that it won’t get hit by accident, and
textured enough to prevent the foot from slipping off in the heat of battle.
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In most modern production vehicles,
the brake pedal fulcrum is located at
one end of the brake pedal arm.
However, in many hot rod applications,
the fulcrum (green arrow) is found
between the brake pedal pad (yellow
arrow, inside vehicle) and output rod
(red arrow, under vehicle). In either
case, the pedal ratio is calculated
using the same equation. (Randall
Shafer)
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Fulcrum
The fulcrum is found on the opposite end of the brake pedal assembly from the brake pedal pad.
This is the pivot point for the moving components of the brake pedal assembly. Usually it’s nothing
more than a steel pin riding in a self-lubricating bronze bushing.
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If your vehicle is equipped with a
vacuum booster, the brake pedal
output rod is most likely attached to
the brake booster with a clevis joint.
Shown here sticking out of the back
side of the booster, the eyelet fits
around a pin on the brake pedal arm,
mechanically coupling the two
components together. (Delphi
Corporation)
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Output Rod
The output rod transmits the brake pedal output force from the brake pedal assembly to the next
device in the brake system (typically a brake booster of some sort). Since it must swing through an
arc as it travels, it’s usually located with a clevis bracket or, in racing applications, a spherical
bearing, to allow for an angular misalignment between the two adjacent components.
Brake Pedal Arm
The brake pedal arm is the largest component in the brake pedal assembly. Its function is to locate
the brake pedal pad at one end, the fulcrum at the opposite end, and the output rod somewhere in-
between. In some racing applications the fulcrum is placed between the pad and the output rod, but
the concept is still the same.
Due to the large leg forces a driver may be capable of applying, the brake pedal arm must be
structurally sound to prevent deformation or collapse during severe use. For this reason, it’s
usually designed to resemble an I-beam in cross-section with webbing often added for additional
strength.
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Ideally, the brake pedal arm should
not bend or deflect during use. To
make the arm as stiff as possible, it
can be shaped like an I-beam or can
be boxed along its entire length. This
pedal simply uses extremely thick
steel for its construction—crude, but
effective. (Randall Shafer)
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Brake Switch
The final component of the brake pedal assembly is the brake switch. This is the device used to
illuminate the tail lamps of the vehicle when the brakes are applied. In most applications, it consists
of a mechanical plunger on/off switch actuated by the brake pedal arm, but in some custom
applications it’s common to have a pressure-actuated switch mounted in one of the master cylinder
hydraulic lines.
There are many mechanical switches in use that allow you to adjust the plunger mechanism. In
general, they should never be modified once they leave the factory. However, if you are changing
other aspects of the apply system, it may be necessary to adjust the threaded mechanism to only
illuminate the brake lamps when the driver applies the brakes.
After adjusting the brake switch, it’s a good idea to put the vehicle on jack stands and turn each of
the road wheels by hand, since an over-adjusted brake switch can result in continuous brake drag.
If the wheels are more difficult to rotate than before the adjustment, chances are the switch needs
to be returned to its original position.
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Although many brake switches employ
a threaded mount for simple
adjustment, this doesn’t mean that you
should tamper with its setting. The
adjustment feature is typically used for
manufacturing flexibility at the vehicle
assembly plant and not for tweaking
later. (Randall Shafer)
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OEM Brake Pedals
The brake pedal arm geometry (the location of the output rod relative to the brake pedal pad and
the fulcrum) defines the pedal ratio. Note that this relationship holds true regardless of the location
of the output rod relative to the fulcrum:
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The brake pedal ratio is calculated by
dividing the linear distance from the
brake pedal pad to the fulcrum by the
length from the output rod to the
fulcrum. The larger the difference
between the two distances, the more
gain the brake pedal provides.
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Pedal ratio (unitless) = distance, pad to fulcrum (in) / distance, output rod to fulcrum (in)
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Typical brake pedal ratios (on conventional vehicles with vacuum boosters) are in the range of
3.5:1 to 4.5:1. Back in the days before booster assist became common, pedal ratios were more
often in the range of 6.0:1 to 9.0:1. While these older designs certainly provided more mechanical
advantage than most modern brake pedals, the corresponding increase in pedal travel made them
undesirable.
As you learned back in Chapter 3, the primary design exercise becomes trading pedal ratio for
pedal travel. In most street applications though, this is an academic point at best because modifying
the brake pedal arm is a complex task best left to an experienced fabricator. This is not an area for
novice experimentation.
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Racing Brake Pedals
Despite the warning above, there may be times when modification of the brake pedal becomes
necessary to offset a change elsewhere in the brake system. The most common need arises when
a vacuum booster is removed from the brake system in a racing application. In this situation, the
reduction in gain must be offset through a variety of different means starting with the brake pedal
ratio.
Given the space and packaging limitations of a typical passenger vehicle, the maximum brake pedal
ratio attainable is approximately 5.5:1 to 6.5:1. A brake pedal with a higher ratio usually won’t fit
under the dash conveniently, at least if you are trying to modify the stock brake pedal arm to
achieve this level of gain (which you probably shouldn’t be doing anyway).
Consequently, the best way to achieve a higher brake pedal ratio is to replace the OEM brake
pedal with an aftermarket unit. Several makes and models exist, and should be selected based on
your mounting needs (floor mount, firewall mount, or overhung), packaging constraints, and, of
course, desired pedal ratio.
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If you need more gain than the OEM
brake pedal is capable of providing,
invest in a racing brake pedal
arrangement. Most, like the assembly
shown here, aren’t designed to be
used with vacuum boosters, so the
master cylinders are mounted directly
to the bracket. (Randall Shafer)
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Brake Boosters
Although the details weren’t covered in Chapter 3, you’ve already learned that the brake booster is
designed to further amplify the brake pedal output rod force. The ratio of input force to output force
can be expressed as a linear gain as follows:
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Booster output force (lb) = brake pedal output force (lb) x boost gain (unitless)
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While there are a number of different designs in use, an overwhelming majority of vehicles use a
vacuum-assisted booster. The rest either use a hydraulically assisted device to perform the booster
function, or they simply go without.
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From the outside, vacuum boosters
look deceivingly simple. However, their
complex inner workings are finely
tuned to provide additional gain for the
force delivered by the brake pedal.
Resist the temptation to open one up,
for there are no user-serviceable parts
inside! (Randall Shafer/Delphi
Corporation)
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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:
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MRE
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