1 abs = braking systme

7/26/2019 1 ABS = BRAKING SYSTME

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Leverage and the Brake Pedal Design

Braking action on an automobile begins with the drivers foot on the brake pedal. Te driverapplies forceto the pedal (which we learn more about later), and the pedal transfers thatforce to the master cylinder pistons. Te brake pedal also multiplies the force of the driversfoot through leverage.

Te brake pedal is mounted on a lever with a pivot near the top of the lever. Te move-ment of the pedal causes a pushrod to move against a master cylinder. Te master cylinderis mounted inside the engine compartment on the rear bulkhead. Te master cylinder is ahydraulic pump that is operated by the driver through the brake pedal.

Most brake pedal installations are an example of what is called a second-class lever. In thescience of physics, a second-class lever has a pivot point (or fulcrum) at one end and forceapplied to the other end. A second-class lever transfers the output force in the same directionas the input force, and multiplies the input force, depending on where the output load is

placed. Te brake pedal installation shown in Figure 1-2 has a 10-inch lever, and the load (themaster cylinder pushrod) is 2 inches from the fulcrum (8 inches from the pedal). Te pedalratio, or the force multiplying factor, is the length of the lever divided by the distance of theload from the fulcrum. In this case, it is:

10

___

2 5:1

If the driver applies 50 pounds of force to the pedal, the lever increases the force to250 pounds at the master cylinder. When the driver applies 50 pounds of input force, thepedal may travel about 2.5 inches. When the lever applies 250 pounds of output force, the

Authors Note:A fulcrum is the point at which one lever pivots or sits to applyforce to another lever or device. A seesaw pivots on a fulcrum.

The bulkheadmay

be referred to as the

firewall.

Front disc brakes

Brakepedal

Parkingbrake lever

ABScomputer

Rear disc or drumbrakes

Hydraulic powerbrake booster

Master cylinderand ABS

hydraulic unit

ABS wiringharness

FIGURE 1-1 A typical automotive brake system comprises these major components and subsystems.

Copyright 2011 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.


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pushrod moves only 0.5 inch. Tus, as leverage in a second-class lever increases force, itreduces distance by the same factor:

2.5 inches

_________

5 0.5 inch

Service Brake History and Design

Modern automobile brakes evolved from the relatively crude brakes of horse-drawn vehicles.Te earliest motor vehicle brakes were pads or blocks applied by levers and linkage to theoutside of a solid tire on a wooden-spoked wheel. Te same principles of leverage that workin modern brake pedal installations increased the force of the brake pad applied to the solid

tire. Tese brakes worked well with speeds of 10 mph to 20 mph and little traffi c. Higherperformance (30 mph and beyond) and pneumatic tires meant that early wagon brakes wereshort-lived on automobiles.

By the end of the first decade of the twentieth century, automobiles were using eitherexternal-contracting band brakes or internal-expanding drum brakes. A few internal-expanding band brakes were tried on some early motor vehicles. External-contracting brakeshave a band lined with frictionmaterial wrapped around a drum located on the driveline oron the wheels. Te band is anchored at one end or at the center; levers and linkage tighten theband around the drum for braking force. Te service brakes on Fords famous Model werea single contracting band applied to a drum inside the transmission.

Band brakes, either internal or external, lose their effectiveness when higher braking force

is needed. When you study drum brakes, you will learn about the mechanical servo action ofbrake shoes. It is very diffi cult to develop servo action with an internal band brake, and higherbrake force is thus needed. Servo action on an external band brake tends to make the brakegrab at high brake forces and high drum speed. Other problems associated with band brakesincluded dirt and water damage and loss of friction with external bands and the tendency ofthese brakes to lock if the drum overheated and expanded too much. Internal band brakesalso suffered from band and drum overheating and reduced braking force.

As drum brakes evolved, internal-expanding shoe-and-drum brakes became the stan-dard. External-contracting band brakes were used as parking brakes until the late 1950s, buttheir days as service brakes were over by the late 1920s.

Leverageis the

use of a lever and

fulcrum to create

a mechanical

advantage, usually

to increase force

applied to an object.

A drum brakeis

a brake in which

friction is generated

by brake shoes

rubbing against the

inside surface of a

brake drum attached

to the wheel.

Frictionis the force

that resists motion

between the

surfaces of two

objects or forms of

matter.

10 inches

2 inches

50poundsforce

250poundsforce

Brake pedalpivot (fulcrum)

Brakepedal

Mastercylinder

Lever

Bulkhead

FIGURE 1-2 The brake pedal assembly uses leverage to increase force

applied to the master cylinder.

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Drum Brakes. By the mid-1920s, drum brakes with internal-expanding shoes were the gen-eral rule. Early drum brakes were operated mechanically by levers and linkage (Figure 1-3).Expensive luxury cars such as the 1921 Duesenberg Model A were among the first to havehydraulic drum brakes. Hydraulic brakes started to appear on lower-priced cars in themid-1920s with Chryslers Light Six, which became the Plymouth. Ford Motor Company,however, used mechanical brakes through the 1938 model year.

Te rigid brake shoes used with drum brakes could be made stronger than the flexiblebands of earlier brake designs. Tis eliminated breakage problems that occurred with greaterbraking forces that were required as automobiles got more powerful and faster. With hydrau-lic actuation, four-wheel drum brakes remained the standard braking system for most carsinto the middle and late 1960s. With the coming of Federal Motor Vehicle Safety Standards(FMVSS) 105 in 1967, brake systems had to pass specific performance tests that made frontdisc brakes the general rule in the 1970s. Even at the beginning of the twenty-first century,however, drum brakes are still used on the rear wheels of many cars and light trucks.

Disc Brakes. Modern automotive disc brakes were developed from aircraft brakes ofWorld War II. Known originally as spot brakes, disc brakes work by applying pressure totwo brake pads on opposite sides of a spinning rotor attached to the wheel hub (Figure 1-4).

FIGURE 1-3 Simple mechanical drum brake.

Authors Note:Tere were two major reasons for the increased use of hydraulic-applied brakes over the mechanically-applied ones: 1) Te four brakes never seem

to apply the same amount of braking force at the same time because 2) the brakelinkages required almost constant re-adjustment to make the brake work at all.

Authors Note:Te larger the vehicle, the more likely it is to have drum brakes onthe rear axles. Over-the-road trucks (semis) have drum on the trailer axles, usuallydrums on the tractors drive axles, and about a 50/50 split between drums and discson the steering axles.

Brake shoes

Drum

Brake pedal

pivot

Brake pedal Brake pedal rod

Cam

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