brkdrnroto

7/29/2019 brkdrnroto

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Chapter 11

Brake DrumsAnd

Rotors


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Brake Drum Construction

Drum Web - closed edge of the drum that containsthe holes that allow the drum to be mounted.

To improve Cooling Axial Fins Perpendicular tothe axle -common on large truck brakes

Radial cooling fins - raised ridges cast parallel tothe axle.

Some drums have a damping spring to quietvibrations that would be amplified as a brakesqueal.


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Solid Drums

Made entirely from gray cast iron.

They are heavy and can absorb a great deal of heat

before they suffer mechanical fade. Low cost, excellent wear characteristics.

Cast iron is soft but the heat from braking hardensthe metal to create a wear resistant skin for the

friction surface. It also has a high coefficient of friction compared

to other metals.


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Solid Drums

It also has free graphite particles that act as

a lubricant between the drum and the

linings. These particles reduce drum wear.

Problems - brittle and will crack if over

stressed. Hard to see cracks. In late model

cars the heavy drums affect fuel economy.


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Composite Drums

Made of two metals joined together.

All composite drums have cast iron friction

surfaces. Steel and iron composites - sheet steel web with

cast iron drum

Stamped Steel outer drum with a cast iron

liner - also called a centrifugal cast brake drumbecause of the way the molten iron is poured intothe stamped steel and spun.

Creates good heat transfer


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Composite Drums

Benefits - less expensive, less weight

Drawbacks - less able to absorb heat.


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Composite Drums

Aluminum and Iron composite drums - castaluminum outer drum with a cast iron

lining. Called a Bimetallic drum. The aluminum is formed around the cast

iron lining.

Benefits - less weight, conducts heat fourtimes faster than iron- cools better

Drawbacks - costly.


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Brake Rotor Construction

All production automobile rotors are cast as

one piece from gray cast iron because of the

low cost, good wear and friction and ease ofmachining.

On some rotors the hub assembly

containing the wheel bearings is cast intothe center of the rotor.


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Solid & Vented Rotors

Solid brake rotors have there friction surfaces on

opposite sides of a solid piece of metal. They do

not cool as well as vented rotors and are usuallyfound on lighter applications.

Vented rotors have radial cooling passages

between its friction surfaces.

Vented rotors can be balanced at the factory and these

weights should not be removed from the cooling fins.


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Solid & Vented Rotors

Uni directional vented rotors are designed

as specific left and right hand models. The

fins should be facing rearward at the top ofthe rotor.

Drilled rotors do not significantly increase

brake cooling


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Drum and Rotor Mounting

Fixed - cast as one piece with the hubassembly containing the wheel bearings.

Mainly used on non- driven axles. Floating - all floating drums and rotors are

held in place by the wheel and lug nuts

They are less expensive and easy to service,they have less heat transfer to the bearings.

May be held on with speed nuts (tinnermannuts)


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Drum Wear

Drum taper wear - the closed edge of the drum

fiction surface wears more than the open edge.

Caused by the excessive application force andheat of repeated heavy braking.

These factors cause the open end to expand leaving the

inner closed end to work against the friction area.

Brake dust dirt and grit also contribute to taper wear.

Causes low brake pedal - needs to travel more.


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Drum Wear

Drum Barrel Wear - the center of the

friction surface is warn more than the ends.

Caused by the web of the shoe transmittingmore force to the center of the drum than the

outer edges.

It does not present any operating hazards andhas no obvious symptoms to the driver.


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Worn Drums

Scored

Bell Mouthed

Concaved or

Barrel Shape


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Rotor Wear

Taper Variation. - A difference of thickness

from the outer edges to the inner edges of

the friction surface. most rotors wear more on the outer edge

than the inner edge.

Inner edge wear is called a dished rotor.


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Brake Drum and Rotor Damage

Scoring - extreme wear on a drum or rotor friction

surface.

Scoring is often caused by metal to metal contact orforeign materials between the linings and friction

surface.

Cracking - caused by severe braking or impact.

drums around the bolt hole or the outer edge rotors usually on the outer edge

If it's cracked replace it.


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Check for a scored rotor


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Drum & Rotor Distortion

Drum

Bellmouth fig. 11-22

Out of round fig. 11-23

Eccentric fig. 11-24

Rotor

Lateral run out fig. 11-26

Lack of parallelism fig. 11-27


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(continues)

Measuring Rotors

Minimum thickness often cast or stamped on the rotor

allow 0.015 to 0.030 for wear

Lateral run out usual specifications range from 0.002 to 0.005

tighten the wheel bearing if hub-type rotor

install lug nuts if hubless-type rotor

use dial indicator to measure run out as you rotatethe rotor


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Special Considerations

Drums on the same axle should bemachined within .010 - .020 of each

other. Rotors with fixed calipers should have the

same amount of metal removed on eachside.

Semi metallic pads need a smoother nondirectional finish for proper operation.

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