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Muscle Car Brake Upgrade
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In addition to swapping big disc brakes for small disc brakes, another common brake system
upgrade involves converting a vehicle originally equipped with drum brakes to disc brakes. Up to
this point in the book, drum brakes have been ignored, as their poor thermal characteristics and
non-linear torque output make them undesirable in high-performance applications. However, for
this very reason, older vehicles originally equipped with drum brakes make ideal candidates for disc
brake conversions.
Chapter 13 exposes you to some of the unique steps involved in the process of converting a
vehicle with rear drum brakes to rear disc brakes. Because many of the brake system design
factors are similar to those already covered in Chapters 11 and 12, they are not recounted here.
Yet gain, balance, and thermal capacity still need to be primary considerations when upgrading
your vehicle, regardless of its age or intended use.
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The Vehicle
Compared to the race-prepared Porsche 911 examined in Chapter 12, a relatively stock 1972
Chevrolet Nova may seem a bit understated. Although both vehicles were built in the same year,
the Nova was much more likely to see time at the drag strip than at the racetrack. Its front disc and
rear drum brakes were common for vehicles of the era, and were relatively well-suited for boulevard
cruising and sprinting from stop sign to stop sign. However, improved braking performance shouldn’
t be limited to vehicles with road course aspirations, and the owner of this particular vehicle had
three distinct objectives in mind.
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When this Chevy Nova was built in
1972, drum brakes had been replaced
on most vehicles’ front axles by disc
brakes, but drums were still commonly
found out back. Primarily because of
their low cost, low weight, and superior
parking brake performance, drums can
still be found in modern rear brake
applications where thermal
requirements are low. (Baer)
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The Objective
The Nova’s original rear drum assembly measured a diminutive 9.5 inches in diameter and used a
7/8 -inch wheel cylinder, common hardware to be found hanging off the ends of a GM 10-bolt rear
end in 1972. While properly sized from a gain and balance perspective, the non-linear pedal feel of
the drum brake design made modulation difficult at best when braking near the vehicle’s limit of
adhesion. Thermally, though, the drum brake proved to be woefully inadequate, as no more than a
couple stops from moderate speeds were enough to produce rear brake pad fade and a dramatic
drop-off in brake system performance.
Therefore, in order to improve pedal feel and enhance the resistance to fade, the objective was to
replace the old-school drum brakes with a pair of modern disc brake assemblies. Of course, the
improved visual appearance of the disc brakes would score big points at the Saturday night cruise-
in as well.
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Solid axles, leaf springs, and drum
brakes—all standard fare for the times.
Like many cars of the era, this Nova’s
drums were cast with fins on the outer
diameter for enhanced convective
cooling, but were still thermally
inefficient even by contemporary
standards. (Baer)
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Picking The Right Parts
Rear Rotors
Since the owner desired to maintain the original 15-inch steel wheels, 12.0-inch diameter rear
rotors were the largest that could be installed. Measuring 0.8 inches in thickness, the vented friction
discs provided by Baer had substantially more thermal mass than the stock drum assemblies.
For primarily aesthetic reasons, the rotors were both slotted and cross-drilled. Although out of
place on a competition vehicle, this rotor treatment provided a unique look that was desired in this
particular application. It also helped that they matched the front rotor upgrade, which had been
performed earlier.
Although two-piece rotors were initially considered for their lower weight and enhanced visual
appeal, in the end, it was decided to use one-piece rotors to keep the budget reasonable. Curved
vanes were within financial reach though, and were expected to further aid convective cooling, even
during around-town use.
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In order to examine the inner workings
of a drum brake, it’s necessary to
remove the drum as shown above. The
design of the drum also dictates that
cooling air is not able to reach the
working elements of the drum brake
during operation, resulting in thermal
performance that is always inferior to
conventional disc brakes. (Baer)
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Drum Brakes 101
Although drum brakes and disc brakes may appear quite different at first glance, they both use
similar principles to convert hydraulic fluid pressure into torque, while at the same time converting
kinetic energy into heat. In both systems, pressure is converted into force, force is converted into
torque, and energy is converted at a friction interface.
Drum brake wheel cylinders perform the same function as disc brake calipers. Located inboard of a
rotating drum, two opposed pistons in each wheel cylinder are subjected to fluid pressure from the
hydraulic circuit. Based on the piston diameters, this pressure is converted into a pair of linear
forces acting against brake shoes, which function in the same capacity as the brake pads in a disc
brake system.
A variety of designs exist to transmit and amplify the wheel cylinder linear forces, but all serve to
expand the brake shoes out against the rotating drum. Based on the drum geometry, the sum of
these forces is then converted into torque. While there are many ways to arrange the internal
components to enhance gain, the end result is a pressure-to-torque relationship that is not typically
as linear as that provided by a disc brake assembly.
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Drum brake wheel cylinders convert
hydraulic fluid pressure into linear
force (yellow arrows) as a function of
their piston diameters. Through
various design techniques, this force is
amplified (red arrows) as it forces the
brake shoes out against the rotating
drum. It’s important to note that the
force distribution along the length of
the pad (as well as between the
leading and trailing pads) is never
uniform, resulting in uneven friction
material wear. (Delphi Corporation)
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Acting much like a disc brake rotor, the drum’s temperature rises as kinetic energy is converted into
heat at the friction interface. Unfortunately, the rubbing surface of the drum is located inside of the
brake assembly, resulting in inefficient convective cooling. For this reason, drums typically contain
fins on their outer diameter to enhance what little convective cooling is available externally.
There are many, many more design differences that differentiate drum brakes from disc brakes, but
in summary they are both engineered to accomplish the same task. For the enthusiast, though, the
poor thermal performance of drum brakes is reason enough to make them an inappropriate choice
for any high-performance application.
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Rear Calipers
Replacing the drum brake wheel cylinders were a pair of floating calipers with single 1.6-inch
diameter pistons, also provided by Baer. While not as visually exciting as fixed multi-piston calipers,
the tight packaging constraints of the steel wheels dictated the more compact floating design.
In contrast to the cast iron attachment brackets, the caliper bodies were fabricated from lightweight
aluminum. A pair of slider pins served to locate the two relative to one another while simultaneously
providing the required axial float.
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Bolting Them On
The very first step in any drum brake service procedure is to remove the cast iron drums from the
vehicle. Unfortunately, this can prove to be quite a challenge, since drums tend to firmly corrode
themselves in place over time. However, this is not altogether different from servicing a vehicle with
disc brakes, as you can usually break the drums free with some brute force and a mallet.
In this particular application, the corrosion was not significant, but because the brake shoes had
machined their way into the drum diameter, there was a ridge on the inside of the drum that
prevented its removal. For this reason, it was necessary to not only get out the mallet, but also to
externally adjust the drum internal components to completely retract the brake shoes.
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Converting a typical muscle car from
rear drum brakes to disc brakes
usually means taking apart the vehicle’
s rear end. While not very complex
from a mechanical perspective, it does
result in a significant mess if you’re not
using a large enough drain pan. Here,
the differential cover has already been
removed and one C-clip extracted.
(Baer)
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When removing the axle shafts from
the rear end, take your time to avoid
damaging the seals found at the ends
of the axle housing. The splines
machined in the ends of the axle shafts
can tear or rip the seals if you aren’t
careful. Of course, with the axles
removed, it might not be a bad idea to
replace the seals anyway. (Baer)
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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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Buy more and save on shipping!
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MRE
PO Box 47
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