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Brake System Design
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If you skipped over the first two chapters, you missed the big news: brakes don’t stop your car. If
you didn’t skip them, well, now you have heard it again. In case you have not caught on by now, this
is a point that needs to be driven home.
Yet you know that when you press on the brake pedal the vehicle will, in most cases, slow down.
Usually, the more you push on the pedal, the more deceleration you feel. Heck, the vehicle might
even stop. Eventually, anyway.
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Although the tire ultimately stops the
car, it’s the combined contributions of
the brake system components that
multiply the driver’s leg force to stop
the car. For this reason, methodical
brake system design is required to
ensure that all performance
expectations are met. (StopTech)
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Chapter 1 taught you that between the effects of tire rolling resistance, driveline frictional losses,
and aerodynamic drag, the kinetic energy of the vehicle in motion can be absorbed without the
need for a separate vehicle brake system. However, there are often times when the rate of energy
conversion is not sufficient enough to produce an acceptable rate of deceleration (such as driving
around town, let alone on a race track). This is where the brake system steps in, and certain
modifications may prove useful to the casual driver, high-performance enthusiast, and pro racer
alike.
Now, before analyzing the benefits (and tradeoffs!) of adjustable proportioning valves, 6-piston
calipers, floating rotors, DOT 5.1 brake fluid, and stainless steel braided brake lines, it’s necessary
to take a high-level look at a typical brake system. Knowing the roles of the individual components
will better prepare you for the detailed discussions to come.
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Driver Applied Force
Brake systems are fitted to vehicles in order to increase their deceleration capability. They
accomplish this task by converting energy at a higher rate than the aforementioned passive
mechanisms. In fact, the rate of energy conversion is limited only by the tractive capability of the
tires and the thermal capacity of the brake system components.
None of this matters one bit, however, if the driver does not press on the pedal in the first place. If
you neglect the effects of tire rolling resistance, driveline frictional losses, and aerodynamic drag
for the rest of this chapter, it’s only the force exerted by the driver on the brake pedal that creates
slip (and hopefully force) at the contact patch. It’s not quite like Fred Flintstone, but all of the force
the brake system generates ultimately comes from the driver’s leg.
With that said, most people are not strong enough to decelerate a 3,000-lb vehicle at a reasonable
rate from even 20 mph using only their leg muscles. The brake system is therefore designed to
amplify the leg force generated by the driver (while of course still converting kinetic energy into
heat). This brings forward the concept of brake system gain.
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Gain
Gain is really nothing more than a fancy way of saying multiplication. The brake system gain relates
the amount of brake system force input to brake system force output. In equation form:
Brake system gain (unitless) = total brake force (lb) ÷ driver’s leg force (lb)
For example, if a leg force of 50 pounds on the brake pedal nets a total brake force of 2,000
pounds at the four contact patches, the brake system gain is 40. You could also say that the
system increased force at a 40-to-1 ratio, or that the gain was 40:1.
So where does the brake system gain come from? In brief, each of the brake system components is
designed to provide its own gain through some type of mechanical advantage. The overall brake
system gain is therefore equal to the individual brake system component gains all multiplied
together.
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The gain of a brake system
component is simply the
relationship between the force
coming in and the force going
out. In the case of a brake
booster, the ratio of these two
forces is called the boost gain.
(Randall Shafer)
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If a brake system is designed properly, even the very weakest driver should be capable of
generating enough leg force to decelerate the vehicle at the limit defined by the tire-to-road
interface. This dictates that every vehicle has a unique gain requirement. As a result, much of the
art of brake system design revolves simply around developing the appropriate amount of gain.
Force Distribution
In addition to amplifying the driver’s leg force, the brake system must also distribute all of this
amplified force to the four corners of the car, ultimately directing it to the four tire contact patches. It
may also need to modify the brake force distribution as a function of deceleration, speed, or vehicle
loading.
While brake force distribution is a critical responsibility of the brake system, you‘ll need to wait until
Chapter 4 to learn more. For the remainder of this chapter, the focus is on brake system gain.
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Brake System Overview
It’s now time to analyze the mechanical attributes, the functional responsibilities, and gain
characteristics of each individual brake system component. Note that this is required reading before
jumping to Chapters 5 through 10 which go into much deeper detail!
The Brake Pedal
Most people are already familiar with the brake pedal pad—it’s where you press to make the your
vehicle stop! But while most of you are aware of the part of the pedal that makes contact with your
foot, two equally important components of the pedal assembly, the output rod and fulcrum, are
generally out of sight. Together, these three separate parts define the brake pedal assembly.
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The brake pedal is really just a
big lever under the dash. As
driver applies leg force to the
brake pedal, the output force is
amplified based on the pedal’s
geometry. (Randall Shafer)
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The primary function of the brake pedal assembly is to harness and multiply the force exerted by
the driver’s leg. The amount of amplification, or gain, is a function of the brake pedal leverage.
You probably learned the concept of leverage on a teeter-totter—the farther you sit from the middle
(the pivot point, or fulcrum), the more weight you can lift on the other end. In the case of the brake
pedal assembly, the fulcrum is at the top of the brake pedal arm, the brake pedal pad is on the
opposite end, and the output rod is somewhere in between. Based on the distance between these
features, the pedal ratio can be defined as:
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Pedal ratio (unitless) = distance, pad to fulcrum (in) ÷ distance, output rod to fulcrum (in)
Because the distance from the pad to the fulcrum is longer than the distance from the output rod to
the fulcrum, the pedal ratio is a value greater than one. For example, if the distance from the pad to
fulcrum was 12.00 inches and the distance from the output rod to the fulcrum was 3.75 inches, the
pedal ratio would be 3.2:1.
In order to calculate the brake pedal output force, one simply needs to multiply the driver leg force
and the pedal ratio as follows:
Brake pedal output force (lb) = driver leg force (lb) x pedal ratio (unitless)
To put some real-world numbers into the equation, if a driver leg force of 41 pounds were multiplied
by the previously defined 3.2:1 pedal ratio, the output rod force would equal approximately 131
pounds. At first glance this would appear to be a good thing, and in one regard, it is.
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Because the driver can apply a
significant amount of leg force to the
brake pedal arm, it must be made
strong enough to avoid excessive
bending or deflection. The photo
above compares an I-beam-shaped
brake pedal arm on the right to a
thinner clutch pedal arm on the left.
(Randall Shafer)
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Unfortunately, gain isn’t a free lunch. More gain brings more force multiplication, but at the cost of
increased pedal stroke, or travel, which is generally viewed as undesirable. For example, doubling
the pedal ratio to 6.4:1 would double the output (approximately 262 pounds of output rod force),
but would require the pedal to travel twice as long of a distance to achieve this result.
The primary design compromise for the brake pedal therefore becomes juggling pedal ratio and
pedal travel. You can’t have your cake and eat it too.
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The Brake Booster
The brake pedal output rod force is fed through the firewall and into the back of the brake booster.
Brake boosters, or simply boosters, come in many colors, shapes, and sizes, yet they are all
designed to do the same thing—they amplify brake pedal output rod force. The booster’s inner
workings can be quite complex, but fundamentally they rely on a pressure differential working
across an internal diaphragm or piston to create a booster output force that is proportional to the
brake pedal output rod force. In equation form:
Booster output force (lb) = brake pedal output force (lb) x boost gain (unitless)
Continuing the example, given a boost gain of 5.9:1 (typical for a conventional passenger vehicle
brake booster), you can calculate that a 131-lb brake pedal output rod force would be translated
into 774 pounds of booster output force.
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Vehicles that advertise “power brakes”
(like this 1969 Chevelle SS) use a
brake booster to increase the brake
pedal output force. Because the
booster uses engine intake manifold
vacuum to perform this task, making
changes to a vehicle’s camshaft timing,
runner design, air cleaner geometry,
and a host of other horsepower
modifiers can degrade the booster’s
performance. (Randall Shafer)
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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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