chap87 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 87, the reader should be able to: • • • Prepare for ASE Suspension and Steering (A4) certification test content area “B” (Suspension System Diagnosis and Repair) Describe the various types and styles of rear suspension Explain the difference among the different types of rear axles Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 87, the reader should be able to: • • List the steps necessary to replace rear shock absorbers Explain how to replace rear leaf and coil springs Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: axle windup • chapman strut • hotchkiss drive IRS • live axle • panhard rod semi-independent suspension • semi-trailing arm • solid axle torque arm • track rod • trailing arm watts linkage Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 All suspensions have two basic jobs: keeping the tires on the ground and providing a smooth ride Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SOLID REAR AXLES A solid axle can be used on  either a rear­wheel­drive or  front­wheel­drive vehicle.  Figure 87–1 Solid axles are used on rearwheel-drive vehicles as well as front-wheel drive vehicles On a rear­wheel­drive vehicle,  a solid rear axle consists of the  differential gears and axle  shafts inside a solid housing.  On a front­wheel­drive, a  solid axle is a U­shaped or  tubular beam that may contain  a torsion bar, rod, or tube Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Solid axles have some handling characteristics that are inferior to  those of an independent suspension. Disadvantages of a solid axle  include the following: Increased proportion of unsprung weight Side­to­side road shock transference Poorer tire adhesion Increasing the proportion of unsprung weight decreases ride  quality. Transferring road shock from side­to­side causes wheel  tramp and shimmy, and poor tire adhesion accelerates tire wear See Figure 87–2 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 87–2 A solid axle supports the springs, so the axle and suspension components are unsprung weight When one wheel rides over a bump, the shock transfers through the solid axle to the opposite side, leading to unstable handling Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The sprung weight of the frame and body must be heavy to oppose  the unsprung weight of a solid axle, especially on a driven axle Unsprung weight, wheel tramp, and shimmy all reduce tire  adhesion. The effects of these problems are more noticeable in  rear­wheel­drive vehicles due to the weight of the rear axle and  differential assembly On a typical front­wheel­drive vehicle, the simple axle beam is not  heavy enough to decrease ride quality or cause extreme tire wear.  The rear tires of a front­wheel­drive model always wear more  slowly than the front tires because the rear axle is so much lighter  than the power train If engine torque is applied to the rear axle to drive the vehicle, the  axle is referred to as a live axle Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 What is Axle Windup? Axle windup is a product of the law of physics, which states that every action produces an equal and opposite reaction As the axle shafts rotate in one direction to drive the wheels, the axle housing attempts to rotate in the opposite direction The force of this reaction tends to lift the front end of the vehicle during acceleration Axle windup is a particular problem with a solid, driven rear axle because the axle housing concentrates reacting force Under extreme acceleration, the reacting force can actually tilt the drive shaft upward and lift the front wheels off the ground Leaf springs, control arms, pinion snubbers, and torque arms all are means of controlling axle windup Figure 87–3 When the axle housing reacts against the force of axle shaft rotation, the front of the differential tilts upward, creating axle windup Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 REAR SHOCK REPLACEMENT Before removing the rear shock absorbers, the rear axle must be  supported to prevent stretching of the rear brake flexible hose Shocks are attached to the frame or body of the vehicle at the top  and to a bracket on the rear axle housing at the bottom Often, the top of the rear shock absorber is fastened inside the  vehicle.  NOTE: Shocks and/or struts should always be replaced as a pair Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Air Shock Installation  Air­adjustable shock absorbers are a  popular replacement for conventional rear shock absorbers. Air  shocks can be used to level the vehicle while towing a trailer or  when heavily loaded. When the load is removed, air can be  released from the shocks to return the vehicle height to normal Most replacement air shocks are directional and labeled left and  right. This ensures the plastic air hose line exits the shock toward  the center or rear of the vehicle and is kept away from the wheels The plastic air shock line attaches to the shock absorber with an O­ ring or brass ferrule and nut. An air leak can result if this O­ring or  ferrule is not installed according to the manufacturer’s  recommendations. Route the plastic air line along the body,  keeping it away from the exhaust and any other body parts where  the line could be damaged.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Rear Coil Springs  Replacement  Coil springs in the rear are easily replaced on both  front­wheel­drive and rear­wheel­drive vehicles. The procedure: Raise the vehicle safely on a hoist Remove both rear wheels Support the rear axle assembly with tall safety stands Remove the lower shock absorber mounting bolts/nuts and  disconnect the shock absorber from the rear axle assembly Slowly lower the rear axle assembly by either lowering the  height of the adjustable safety stands or raising the height  of the vehicle on the hoist Lower the rear axle just enough to remove the coil springs See Figure 87–20 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 87–20 The shock absorber needs to be disconnected before removing the coil spring Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CAUTION: The shock absorber is usually the only component that  limits the downward movement of the rear axle to allow removal of the  rear coil springs. Some vehicles may be equipped with rear suspension  height sensors for the adjustable suspension system or an adjustable rear  proportioning valve for the rear brakes. Some vehicles also require that the  rear stabilizer bar or track rod be disconnected or removed before lowering  the rear axle assembly. Always consult service information for the  procedure and torque specifications for the vehicle being serviced Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Rear Leaf Springs  Replacement  Rear leaf springs often need replacement due to one  of the following common causes: Individual leaves of a leaf spring often crack, then break   When a leaf spring breaks, the load­carrying capacity of the  vehicle decreases and it often sags on the side with the broken  spring. Metal fatigue, corrosion, and overloading are three of the  most common causes of leaf spring breakage See Figure 87–21 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 87–21 The center bolt is used to hold the leaves of the leaf spring together However, the role for the center bolt also weakens the leaf spring The crack shown is what a technician discovered when the leaf spring was removed during the diagnosis of a sagging rear suspension Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 NOTE: When one rear spring on one side sags, the opposite front end of  the vehicle tends to rise. For example, if the right rear spring breaks or  sags down, the left front of the vehicle tends to rise higher than the right  front. This unequal vehicle height can make the vehicle difficult to handle,  especially around corners or curves If the center bolt breaks, the individual leaves can move and  the rear axle is no longer held in the correct location. When one  side of the rear axle is behind the other side, the vehicle will  dog track Dog tracking refers to the sideways angle of the vehicle while  traveling straight. It is commonly caused by the rear axle  steering the vehicle toward one side, while the driver controls  the direction of the vehicle with the front wheels Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 To replace leaf springs in the rear of a rear­wheel­drive vehicle,  follow these steps: Raise the vehicle safely on a hoist Support the rear axle with safety stands Remove the rear shackle bolts and forward mounting bolt  or mounting bracket Remove the U­bolts Being careful of any nearby brake line, remove the spring Install the new spring, being careful to position the center  bolt correctly into the hole on the axle pedestal Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 TROUBLESHOOTING REAR ELECTRONIC LEVELING SYSTEMS Check for normal operation. Some leveling systems require the  ignition key be on (run), while other systems operate all the time Begin troubleshooting by placing approximately 300 lb (135 kg)  on the rear of the vehicle. If the compressor does not operate,  check to see if the sensor is connected to a rear suspension  member and that the electrical connections are not corroded Also check the condition of the compressor ground wire. It must  be tight and free of rust and corrosion where it attaches to the  vehicle body. If the compressor still does not run, check to see if  12 volts are available at the power lead to the compressor.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 If necessary, use a fused jumper wire directly from the positive (+)  of the battery to the power lead of the compressor. If the  compressor does not operate, it must be replaced If the ride height compressor runs excessively, check the air  compressor, the air lines, and the air shocks (or struts) with soapy  water for air leaks Most air shocks or air struts are not repairable and must be  replaced. Most electronic leveling systems provide some  adjustments of the rear ride height by adjusting the linkage  between the height sensor and the rear suspension See Figure 87–22 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 87–22 Most electronic level-control sensors can be adjusted, such as this GM unit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Solid rear axles are commonly used on rear­wheel­drive and  front­wheel­drive vehicles A Hotchkiss rear suspension uses the leaf springs to absorb  axle windup Trailing arms run parallel to the centerline of the vehicle and  are used to locate a solid rear axle A track rod (Panhard rod) or Watts linkage is used to keep the  rear axle centered under the vehicle Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Independent rear suspension (IRS) usually uses coil springs  but can use a transversely mounted leaf spring The rear suspension should be supported whenever replacing  the rear shock absorbers Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 ... means of controlling axle windup Figure 87–3 When the axle housing reacts against the force of axle shaft rotation, the front of the differential tilts upward, creating axle windup Automotive Technology: ... The design is based on a crossbeam  that is similar to the beam axle? ?of? ?a  solid, nondriven rear suspension.  The crossbeam is placed ahead,  rather than? ?at? ?the centerline,? ?of the wheels.  Continued Automotive Technology: ... supported to prevent stretching? ?of? ?the rear brake flexible hose Shocks are attached to the frame or body? ?of? ?the vehicle? ?at? ?the top  and to a bracket on the rear axle housing? ?at? ?the bottom Often, the top? ?of? ?the rear shock absorber is fastened inside the