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Brake Balance
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One of the most critical, yet least understood, brake system attributes is brake system balance.
This single design parameter can make or break (no pun intended) a vehicle’s stopping distance
performance. Even with the very best brake system components installed on your vehicle, improper
brake system balance can prevent the tires from operating at their maximum decelerations
simultaneously, resulting in vehicle deceleration performance that is far from optimized.
Improper brake system balance can also create undesirable vehicle dynamic responses. From a
premature loss of vehicle steering during braking to dynamic instability while braking in a turn, the
ramifications of improper balance can extend far beyond a few additional feet of stopping distance.
Unfortunately for the automotive enthusiast, screwing up a vehicle’s brake balance is pretty darn
easy to do. Later in this chapter you’ll be presented with a table of those factors that can influence
brake balance, but let it suffice to say that just about anything and everything brake related,
suspension related, and tire related can have an effect (both positive and negative) on brake
balance.
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A vehicle with a balanced brake system creates brake forces at all four tires simultaneously that are
equal to the maximum forces that each tire can sustain independently. You could also say that a
balanced brake system is one that brings all four tires to their independent maximum coefficients of
friction at the same time. In either case, defining perfect brake balance is quite a bit easier than
designing a system that can pull it off.
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Even with the very best brake system
components, improper brake balance
can wreak havoc on vehicle braking
dynamics. Stopping distance certainly
can suffer as well. (Wayne
Flynn/pdxsports.com)
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Tires generate brake forces through
adhesive, deformation, and
mechanical wearing modes of
operation. Based on the surface,
condition, and level of slip, a tire may
be operating in one, two, or all three
modes simultaneously. Tire smoke
usually indicates too much mechanical
wearing! (The Tire Rack)
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In Chapter 2 you learned that the maximum brake force a particular tire can generate is equal to
the coefficient of friction of the tire-road interface (mu in the equation below) multiplied by the
amount of weight being supported by that corner of the car:
Brake force at one tire (lb) = corner weight (lb) x mu (unitless)
To use real numbers, a single tire supporting 500 pounds of the total vehicle weight with a peak
coefficient of friction of 0.9 (a typical value for an all-season tire on a dry asphalt road) could
generate, in theory, a maximum of 450 pounds of braking force. Recall that this also would result in
a maximum deceleration contribution of 0.9g at that one wheel.
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In this example, a single tire is
supporting 750 pounds of vehicle
weight (red arrow) with a peak
coefficient of friction, or mu, of 1.1
(blue star). Therefore, this tire could
generate, in theory, a maximum of 825
pounds of braking force (yellow arrow).
The brake force would oppose the
direction of travel (green arrow).
(Randall Shafer)
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Now if you were to place an additional 200 pounds on the same tire (700 pounds total), the
maximum brake force rises to 630 pounds (this assumes that the peak coefficient of friction remains
at 0.9). From this calculation you can see that an increase in maximum brake force does not result
in higher deceleration (still 0.9g in this case). Why? Because the tire has more weight on it, and
that additional weight requires its own additional force to decelerate.
Based on this relationship, you can also predict that reducing the weight on the tire reduces the
maximum brake force sustainable by that corner. In the example above, if the weight were reduced
to 300 pounds, a maximum of only 270 pounds of brake force would be available at that corner
(again, assuming the same coefficient of friction).
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In order to determine a vehicle’s static
weight distribution, weigh the front and
rear axles of the car. The percent front
weight is the front axle weight divided
by the total weight. The percent rear
weight is the rear axle weight divided
by the total weight. A nice set of scales
will do the math for you, though.
(Randall Shafer)
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From all of these equations, ideal brake balance can be boiled down to one simple relationship. For
perfect brake balance under all conditions:
Front brake force (lb) / front vehicle weight (lb) = rear brake force (lb) / rear vehicle weight (lb)
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Therefore, one could surmise that in order to design a perfectly balanced brake system you would
just:
1. Weigh the front axle and rear axle of the car.
2. Design the front axle and rear axle brake components to deliver brake forces in the same ratio
as the front-to-rear weight distribution.
3. Pat yourself on the back for achieving perfect brake balance.
For example, a vehicle with equal (or 50/50) front-to-rear weight distribution would appear to
require front and rear brakes that generate the same amount of brake force simultaneously. In
other words, the front brakes would provide 50 percent of the total required brake force and the
rear brakes would provide the other 50 percent of the total required brake force.
Looking at a different scenario, it would appear that a vehicle with 60/40 front-to-rear weight
distribution would require front brakes that provide 60 percent of the total brake force while the rear
brakes would contribute the remaining 40 percent of the total required brake force. Why? Because
of the extra weight being supported by the nose of the car.
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In mini-summary, a perfectly balanced brake system generates brake forces at the front and rear
axles in exact proportion to the front and rear axle weights. Like most things in life though,
calculating brake balance is not as simple as it may appear and designing a braking system to
these static conditions would neglect the most important factor in the brake balance equation—the
effect of weight transfer during braking.
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Static Weight Distribution
Let’s assume you have a 2,500-pound vehicle with an unknown static weight distribution. If you are
only concerned with the vehicle at rest, it’s easy to determine the weight on each wheel. You just
need to find some corner weight scales and weigh it.
The sum of the front individual corner weights (left front + right front) is equal to the front axle
weight, and the sum of the rear corner weights (left rear + right rear) is equal to the rear axle
weight. For example, if the sum of the front individual corner weights is 1,250 pounds and if the sum
of the rear individual corner weights is also equal to 1,250 pounds, then you could say that the
vehicle has 50/50 weight distribution. That is, half of the vehicle’s weight (50 percent) is being
supported by the front axle, and the other half of the vehicle’s weight (the remaining 50 percent) is
being supported by the rear axle.
The total weight of the vehicle is equal to the sum of the two axle weights (our original 2,500
pounds), and this weight can be thought of as acting through (or existing at) the vehicle’s center of
gravity, or CG. Simple, right?
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Either at rest or while coasting, every
vehicle has a static weight distribution.
Due to its rear-engine layout, this
Porsche 911 has approximately 38
percent of its weight on the front axle
and 62 percent of its weight on the
rear axle prior to braking. (Daniel
Mainzer)
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Real-Life Brake Balance Success Story
How big of an impact can brake balance have on vehicle performance? It varies by application, but
even with the very best brake system components, super sticky tires, and impeccable installation,
skewing your brake balance can lengthen stopping distances dramatically. So much for those fancy
red calipers…
To illustrate this point, here is a real-life brake balance success story reproduced with permission
from Grassroots Motorsports during their Porsche 914-4 restoration.
“Our initial stopping distance measurements were not quite world-class. Even though we had
installed Yokohama AVS Intermediate 195/60ZR15s at all four corners, we were recording stopping
distances of 150 to 160 feet from 60 mph. There was obviously room for improvement.
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“We then began to slowly adjust the proportioning valve until we were just barely on the verge of
rear lock-up. We dialed back a tiny bit for a safety factor and again ran our stopping distance tests.
Note that if you are doing this at home, you should be prepared just in case you go a bit too far and
need to deal with the back end of the car getting all out of shape. A large parking lot or airstrip (as
opposed to a crowded four-lane highway) is really the best place for this sort of thing.
“As stated earlier, the adjustable proportioning valve is a must-have item for anyone performing a
914-4 caliper swap. Our new stopping distance from 60 mph was now a scant 121 feet—on par with
many of today’s premier sports cars. Apparently the brake bias was significantly holding us back
from optimizing our new components.”
In this particular application, the stopping distance from 60 mph was reduced by approximately 34
feet—a whopping 22 percent! If you consider that out-braking your opponent by just two feet every
lap for a twenty lap sprint race can result in a three to four car-length advantage at the checkered
flag, a 22-percent decrease in stopping distance in every braking zone is sure to get everyone’s
attention.
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Paying attention to brake balance can
pay huge dividends at the track. The
60 mph stopping distance of the
Porsche 914-4 shown here went from
160 feet to 121 feet simply by setting
the brake proportioning valve to an
optimum position. (David S.
Wallens/Classic Motorsports)
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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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MRE
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