chap79 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 79, the reader should
be able to:

•
•
•

Prepare for ASE Brakes (A5) certification test
content area “E” (Miscellaneous Systems
Diagnosis and Repair).

Discuss the construction of brake drums and
rotors.
Explain the formation of hard spots in drums
and rotors.

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OBJECTIVES:
After studying Chapter 79, the reader should
be able to:

•

•
•

Describe how to measure and inspect drums
and rotors before machining.
Discuss how surface finish is measured and
its importance to satisfactory brake service.
Demonstrate how to machine a brake drum
and rotor correctly.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:

bellmouth • chill spots • composite rotors • convection
discs (disks) • eccentric distortion • elastic limit
hard spots • heat checking
lateral runout (LRO) • microinches (μin.) • out-of-round
parallelism • ra • root mean square (RMS)
scoring • scratch cut • self-aligning spacer (SAS)
thickness variation (TV)
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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BRAKE DRUMS
Brake drums are constructed of 
cast iron with mild steel centers
and drilled for the lug studs.  

Cast iron contains about 3% 
carbon, which makes the drum 
hard, yet brittle. 
Figure 79–1 Types of brake drums.
Regardless of the design, all types
use cast iron as a friction surface.

Any pounding to remove 
drums should be done on the 
center mild steel portion, which 
can take this force without 
damage. 
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Even aluminum brake drums use cast iron for the friction surface 
area. Besides saving weight, aluminum brake drums transfer heat 
to the surrounding air faster than cast iron or steel. 
Brake drums and rotors are the major energy­absorbing parts of the 
braking system. Friction between the friction material and the 
drum or rotor creates heat. This heat is absorbed by the drum or 
rotor and travels from the friction surface to the remainder of the 
drum or rotor by heat convection.
As energy continues to be absorbed, the drum or rotor increases in 
temperature. Airflow across the drum or rotor helps to dissipate the 
heat and keep the temperature rise under control.
See Figure 79–2 through 79–4.
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Figure 79–2 The airflow through cooling vents helps brakes from overheating.

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Figure 79–3 This air scoop is part of the
water/dirt shield attached next to the rotor.

Figure 79–4 Most race cars are equipped
with scoops to direct air past the brakes to
dissipate the heat quickly. The cooler the air,
the more efficient and effective the brakes.

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BRAKE DRUM AND ROTOR DAMAGE
Besides wear, drums and rotors often experience damage to their 
friction surfaces. Because drum and rotor damage is caused by 
extremes of operation, it is most commonly found on front brakes, 
which experience more severe use than rear brakes.

Scoring  An extreme form of drum and rotor wear, scoring is 
scratches, deep grooves, and a generally rough finish on the
friction surface. 
Drum brakes are more likely to become scored because their 
closed construction holds dirt, sand, and abrasive dust in the 
friction assembly. Severe drum scoring often results when metal 
parts of the friction assembly fatigue, break loose, and are trapped 
between the linings and drum.  
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Figure 79–5 Scored drums and rotors often result in metal-to-metal contact.

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Cracking  Cracks in a brake drum or rotor are caused by the stress 
of severe braking or an impact during an accident.
Generally, drums and rotors that have been previously machined 
are more susceptible to cracking than new parts.
Cracks can appear anywhere on a drum or rotor, although on 
drums they are most often found near the bolt circle on the web, or 
at the open edge of the friction surface.
Rotors generally crack first at the edge of their friction surfaces.
See Figure 79–6.

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The Tap Test
Using a steel hammer, lightly tap a brake drum or rotor. It should ring if it is
not cracked. If a dull thud is heard, the drum or rotor is likely cracked and
should be replaced.

Figure 79–6
Cracked drums or rotors must be replaced.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Heat Checking  A lesser form of drum and rotor cracking is called 
heat checking: many small, interlaced cracks on the friction 
surface. 
Figure 79–7 A heat-checked
surface of a disc brake rotor.

These cracks typically 
penetrate only a few 
thousandths of an inch 
into the metal and 
seldom pass through 
the structure of the 
drum or rotor. 

Light heat checking can often be machined away. In more severe 
cases the drum or rotor must be replaced.
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Hard or Chill Spots  If brake temperatures become too great, 
localized impurities in the metal can be burned away, altering the 
structure of the metal and causing hard spots, also called chill 
spots, to appear. Hard spots are roughly circular, bluish/gold 
glassy appearing areas on the friction surface. 

Figure 79–8 These dark hard spots are
created by heat that actually changes the

metallurgy of the cast-iron drum. Most
experts recommend replacement of any
brake drum that has these hard spots.

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Hard spots create a number of problems including the following:
They are harder than surrounding areas of the friction surface, 
and do not wear at the same rate. Once the spots begin to 
stand out from the rest of the friction surface, they cause rapid 
brake lining wear.
The friction coefficient of hard spots is less than that of 

surrounding areas so braking power is reduced or becomes 
uneven. This can cause the brakes to chatter, or result in a 
hard or pulsating brake pedal.
A drum or rotor is more likely to crack in the area of hard 
spots than elsewhere.
Most vehicle manufacturers recommend that the drum or rotor 
should be replaced if hard spots are found.
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BRAKE DRUM DISTORTION
To ensure smooth brake application without pedal pulsation or 
other problems, brake drum friction surfaces must remain in a 
fixed position in relation to the shoes. 

In some cases, a position variation of less than a thousandth of
an inch will create braking problems. Distortion puts the drum 
friction surfaces out of proper alignment with the shoes.
Storing Drums and Rotors
A common cause of distortion in new brake drums and rotors is improper
storage. Drums and rotors should always be stored lying flat; they should
never be stood on edge. Distortion of new drums and rotors is common, so
they should be routinely checked before installation.
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Drum Distortion  The friction surface of a brake drum in perfect 
condition is parallel to the axis of the axle, and rotates in a precise 

circle centered on the axle or hub.
All brake drums suffer from distortion during brake operation, but 
usually return to their original shape once the brakes are released.
When the friction surface does not return to its proper shape, is no 
longer parallel to the axle, or does not rotate in a precise circle 
around the axle, the drum is distorted.

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Bellmouth Drums  When an open edge of a brake drum friction 
surface has a larger diameter than the closed edge, the drum is 
suffering from bellmouth distortion. 

Bellmouth distortion is caused by poor drum rigidity combined with 
high heat and brake application force. It occurs when a drum suffers 
mechanical fade and its open edge, unsupported by the drum web, 
expands more than its closed edge.
When the brakes are applied harder to compensate for the fade, the 
shoes distort the open edge of the drum so far outward that the 
elastic limit of its metal is exceeded.
Once this happens, the drum will not return to its original shape 
after it cools. Repeated occurrences of this process eventually cause 
the drum to take on a bellmouth shape.
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Bellmouth distortion is especially common on wide drums and 
commonly occurs when a drum has had too much metal machined 
from it. If new shoes are installed in a bellmouthed drum, brake 
fade and unusual lining wear will result. 

Figure 79–9 Bellmouth
brake drum distortion.

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Out­of­Round Drums  Uneven heat distribution can sometimes 
cause out­of­round distortion in which the drum radius varies 

when measured at different points around its circumference.
Out­of­round distortion can take place when a vehicle drives 
through a puddle after a series of hard stops and water splashed on 
the brakes causes rapid and uneven cooling of the drums. It can 
also result if the parking brake is firmly applied after a series of 
hard stops before the drums have had a chance to cool. 
The most common symptom of an out­of­round drum is a pulsating 
brake pedal when the brakes are applied at all speeds. Out­of­round 
drums can also cause a vibration or brake chatter at speeds above 
approximately 40 mph (60 km/h). 
See Figure 79­10.
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The shoes extend out against the heat­expanded drum and prevent it 
from contracting to its original circular shape as it cools, and force the 
drum into an out­of­round shape.

In extreme cases, an out­
of­round drum can result 
in erratic braking action, 
and possibly cause the 
brakes to grab with every 
revolution of the wheel.

Figure 79–10 Out-of-round brake
drum distortion.

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Eccentric Drums Exists when the 
center of the circle described by 
the brake drum friction surface is 
other than the center of the axle or 
the bolt circle of the drum web.

If driven for an extended time 
with an eccentric drum, the 
linings may slowly wear the 
friction surface “round” again. 

Figure 79–11
Eccentric brake drum distortion.

As tension on lug nuts or bolts is 
released, the drum will relax back 
to an out­of­round condition not 
apparent before the wheel was 
removed.
Continued

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The Parking Brake Trick
Whenever attempting to diagnose a brake pedal pulsation, drive to a
deserted area or parking lot and try stopping the vehicle using the parking
brake. If a vibration occurs, the problem is due to a fault with the rear
brakes. If a vibration does not occur except when using the service brakes,
the problem is most likely due to a fault with the front brakes.

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REMOVING DRUMS
Metal clips called tinnerman nuts are installed at the factory to 
keep the brake drums from falling off during vehicle assembly. 
These clips can be removed and do not need to be reinstalled.
The first inspection step after removing a brake drum is to check 
it for warpage using a straightedge. A warped drum is often a 
source of vibration. A brake drum that is out­of­round can cause a 
brake pedal pulsation during braking.  See Figure 79­12.
NOTE: To help diagnose if the front brakes or rear brakes are the cause 
of the vibration, try slowing the vehicle using the parking brake. If 
vibration occurs, the problem is due to the rear brakes.

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Figure 79–12 A straightedge can be used to check for brake drum warpage.

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