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Old 10-03-2005, 01:17 PM   #1
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Car, Van, and Light Truck Disc Brakes and Fuel Economy

<p><strong>Publication:</strong>Lousiana State University</p><p><strong>Date:</strong>1993</p> <p><strong>GOAL: </strong><br>
To understand the relationship between a dragging disc brake and reduced fuel economy. </p>
<p><strong>OBJECTIVES: </strong><br>
The student will: 1. Properly identify a dragging disc brake. 2. Recognize that a dragging disc brake can reduce fuel economy. 3. Demonstrate how to correct disc brake malfunctions that result in a dragging disc brake. </p>
<p><strong>LESSON/INFORMATION: </strong><br>
Disc brake systems for cars and light trucks have been standard equipment for over a decade. By design, all disc brakes are self adjusting and therefore, they require no periodic readjusting to compensate for brake shoe wear. However, certain malfunctions may cause a disc brake to drag. This drag will reduce mpg and result in premature brake pad and rotor wear. Disc brakes are used primarily on front wheels but are also used on the rear of some vehicles. It is critical for both safety and maximum fuel economy that disc brakes operate properly! It is important to note that any brake system on a given vehicle operates in the proper front to rear sequence and that the left and right sides operate in unison to avoid undue premature lockup or lateral forces. </p>
<p><strong>Figure 1<br>
<img src="/files/gassavers/disc_brakes/auto-6.gif" width="361" height="401"></strong></p>
<p> Figure 1 illustrates a typical disc brake. Adjustment is automatic. What causes disc brakes to drag? Several malfunctions are possible. On the floating caliper, single piston design, the most likely culprits are a build-up of dirt, corrosion, or loss of the protective lubrication on the guide pins, sleeves or ways. These cause the caliper to not fully retract when the brake pedal is released. Figure 2a and 2b illustrate a typical single piston floating caliper disc brake at rest and when activated. </p>
<p><strong>Figure 2a.</strong></p>
<p><img src="/files/gassavers/disc_brakes/auto-7a.gif" alt="auto-7a" width="283" height="429"></p>
<p><strong>Figure 2b. </strong></p>
<p><strong><img src="/files/gassavers/disc_brakes/auto-7b.gif" width="569" height="474"> </strong></p>
<p> Cross sectional view of single-piston caliper. Note that the hydraulic pressure pushes the piston outward and pulls the cylinder inward. </p>
<p>The four piston, non-floating design is used only on larger pick-up trucks and vans (see Figure 3). Four piston types tend not to retract as a result of corrosion build up in the caliper hydraulic. Small incremental increases in corrosion inside the caliper bores prevent the pistons from fully retracting after each brake application. If one or more pistons fail to fully retract, the shoe will drag the rotor resulting in excessive drag and reduced mpg. </p>
<p> <strong>Figure 3. </strong> <br>
<img src="/files/gassavers/disc_brakes/auto-7c.gif" width="355" height="465"></p>
<p> 1. Caliper Assembly <br>
2. Outboard Piston Housing (Caliper Half or Caliper Casting) <br>
3. Inboard Piston Housing (Caliper Half or Caliper Casting) <br>
4. Internal Fluid Passages <br>
5. Piston Assembly <br>
6. Piston Seal <br>
7. Piston Boot (Dust Boot) <br>
8. Shoe and Lining Assembly <br>
9. Disc Brake Rotor </p>
<p>How are these problems corrected? If external corrosion, dirt, or lack of lubrication is the problem, disassemble in accordance with manufacturer's shop manual procedures. Clean parts with appropriate solvents. Inspect for wear, metal fatigue, and failure. Lubricate using the recommended lubricant and reassemble. If brake drag results from a piston or pistons not fully retracting, then caliper disassembly, inspection, and rebuilding will be necessary. Observe manufacturer's cautions in the shop manual. Note the recommended bleed down procedures for hydraulic systems in antilock braking systems and hydroboost type systems. </p>
<p> <strong>SUMMARY: </strong>Disc brake system failures are infrequent. Disc brakes can and do drag. Obvious indications of disc brake drag are: </p>
<p>1. Excessive wear of one brake pad (Note: it is normal for the inner pad of a single piston, floating type disc brake system to have a slightly higher wear rate than the outer pad.) </p>
<p>2. Vehicle pull. If all alignments are within specification, a car or truck can still pull to one side if a disc brake drags. </p>
<p>3. Tapered pad wear. This failure, most common on four piston, fixed caliper type, is caused by one or more pistons failing to fully retract. </p>
<p><strong>ACTIVITY: </strong><strong><br>
<p>1. Locate a vehicle with disc brakes. Note the type of system, i.e., 4-wheel disc, disc drive. Also note any assist systems, i.e., vacuum power, hydroboost, antilock single channel, three channel, etc. </p>
<p>2. Locate an appropriate manual to determine correct inspection procedure. </p>
<p>3. Block the wheels so the vehicle will not roll. Release the parking brake and place transmission in neutral. </p>
<p>4. Raise the axle and place a safety stand at an appropriate place to support the weight of the vehicle. </p>
<p>5. By hand, try to rotate the wheel and tire assembly. If it does not move freely, remove the tire or wheel assembly and proceed to check disc brake. Using the shop manual as a guide, identify the type of disc brake and check for: </p>
<li>A. Excessive pad wear, </li>
<li>B. Tapered pad wear, or </li>
<li>C. Build up of dirt, corrosion, or lack of lubricant. </li>
<p>If any of these are noted, repair according to manufacturer's manual. If the wheel and tire assembly did turn freely, the disc brake is retracting correctly. <strong>INFORMATION CHECK </strong></p>
<p> Indicate whether the statements below are true or false. If false, please explain why it is false. </p>
<p>_______ 1. Disc brakes need no periodic adjustment. <br>
_______ 2. One important disc brake difference is mounting type; they are either floating or fixed. <br>
_______ 3. As long as the wheel turns freely, no drag is evident and therefore no adjustment is needed. <br>
_______ 4. A weak return spring will cause a disc brake to drag. <br>
_______ 5. Which of these disc brake problems will cause a decrease in mpg? </p>
<li>A). Broken return spring </li>
<li>B). Corrosion on the caliper-to-mount way </li>
<li>C). Worn or improperly lubricated guide pins and sleeves </li>
<li>D). B and C </li>
<p> <strong>TEACHER'S NOTES </strong></p>
<p>An excessively tight disc brake causes vehicle pull, premature brake wear, and loss of mpg. A result is pad drag and reduced mpg. Even slight drag can reduce mpg. A non- releasing pad will not only wear out faster but will reduce the vehicle's mpg. It is important that students realize that even though disc brakes are virtually trouble free, over a period of time, lubricants get contaminated, are washed away, or dry out. The result is a brake pad that drags. </p>
<p><strong>ANSWERS TO INFORMATION CHECK </strong></p>
<p>1. True. These do need to be checked periodically for pad drag due to other failures. <br>
2. True. <br>
3. True and False. Depends on the age and conditions vehicle is operated under. Off road vehicles should be checked periodically. <br>
4. False. There are no return springs in disc brakes. <br>
5. D. There are no return springs. </p>
<p><strong>REFERENCES: </strong></p>
<p>Dales, David and Frank Thiessen. <strong>Automotive Principles and Service. </strong> Reston Automotive Series, Prentice Hall Co. Inc., Reston, VA. 1980. </p>
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