LV30 suspension systems(2)

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kap all phase & 6/11/03 11:37 am Page 33 Student Workbook LV30 Suspension Systems (2) LV30/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV30 SUSPENSION SYSTEMS (2) Contents Page Forces Acting on Suspension: Torque reaction in a live axle Brake reaction on a live axle Forces on leaf spring during cornering Bump steering effect of leaf sprung live axle Limitations of leaf spring suspension Progress check Typical forces in an independent suspension Exercise Exercise Use of an offset spring to reduce bending forces in the stub axle Progress check Suspension system terminology Progress check 13 14 15 18 Compliance 19 Advantages and Disadvantages of Different Suspension Systems: Non-independent rigid axle suspension Disadvantages of non-independent suspension systems Page Independent front and rear suspension Progress check Hydro-pneumatic and air suspension Progress check Routine suspension maintenance checks IFS layout – front wheel IFS layout – rear wheel Rear suspension with leaf spring Exercise Check for component wear Progress check 6 11 12 Identification of Common Faults Associated with Suspension Systems: Common suspension system faults and common causes Damper bounce test Progress check 22 23 24 25 26 27 27 28 29 30 32 33 33 34 36 20 20 21 -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Forces Acting on Suspensions Newton’s third law of motion tells us that forces always act in “pairs” of equal and opposite forces It states that “To every action there is an equal and opposite reaction.” This can be seen in the picture which shows a car towing another car A B A is the action or force applied by the tow car and B is the opposing force or reaction in the towrope caused by the mass or weight of the second car The statement, “To every action there is an equal and opposite reaction”, should be considered when looking at the forces in a suspension system The suspension has to resist the following forces: • forces produced by driving torque from the transmission • forces produced when brakes are applied • forces produced during cornering • plus the normal forces produced in the suspension as the vehicle negotiates bumps on the road surface -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Torque reaction in a live axle The illustration shows what happens to the axle and leaf springs when drive is passed to the wheels Action of “axle tramping” As the torque is applied to the wheels they are reluctant to turn due to the inertia of the car Therefore there is a tendency for the axle to twist in the opposite direction and “wind up” the springs as shown The torque is applied to move the wheels in a forward direction but the axle tends to twist in a reverse direction, i.e action and reaction This causes the leaf springs to bend in the direction shown, as they resist the torque reaction If the springs were not there to resist this force, then instead of the wheels rotating forwards, the axle would rotate backwards, as this is the line of least resistance Under severe conditions the springs will “wind up” until the wheels spin This releases the torque reaction in the springs as the force due to the driving torque is lost Remember that if there is no action or load, then there will be no reaction The axle is returned to its normal position by the springs, wheel adhesion is reapplied and the action is repeated This cyclic action of “wind up” and release is repeated causing “axle tramp” -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Brake reaction on a live axle The red line shows the curve of the spring under braking When the brakes are applied the wheels and axle tend to rotate as one in the same direction This causes the springs to be bent in the direction shown i.e in the opposite direction to the way they bend when resisting torque reaction Forces on leaf spring during cornering When a vehicle corners, the centrifugal force and opposing cornering force cause the springs to bend laterally or sideways slightly It is the cornering force on the tyre that causes the vehicle to turn a corner A Panhard rod or lateral control rod is sometimes used to eliminate this bending of the spring -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue A Panhard rod is shown and in this example it is used on a live axle that is suspended on coil springs However it can be used to good effect on a leaf spring suspended live axle Springs bending laterally Centrifugal force Cornering force opposing centrifugal force The effects of cornering forces on a leaf spring suspended live axle are shown in this diagram The lateral, or sideways, bending that takes place also moves the axle in this direction slightly This will have an adverse affect on the steering and road holding of the vehicle We shall see later some other disadvantages of the simple leaf sprung rigid axle Bump steering effect of leaf sprung live axle b a Distance a is greater than b causing rear axle to steer The action of a leaf sprung live axle passing over road surface bumps can cause a steering effect to take place on the axle -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Vehicle roll during cornering can cause a similar steering effect since the inside spring is effectively in the rebound position and the spring on the outside of the curve is under bump conditions These effects are called bump steer and roll steer respectively Bump steer and roll steer can occur with independent suspension systems However, it more noticeable and the effects are greater with the rigid leaf sprung axle, which is a very basic design that has its roots in the horse drawn cart The reason for this bump steer (and roll steer) is shown here As the wheel moves upwards the leaf spring is flattened and the leading part of the spring, from the fixed shackle, becomes longer See (a) in the diagram The centre line of the axle is thus moved away from its normal position of 90 degrees to the front/rear or longitudinal axis of the vehicle This causes a steering effect to take place in much the same way as the steering on a horse drawn cart in pre-Ackermann days In addition, during roll conditions the spring on the other side moves down which causes the front part of the spring to shorten, see (b) in the diagram Limitations of leaf spring suspension As covered in Phase Suspension Systems LV16 the fact that the leaf spring has to carry out two tasks means that it has limitations as a suspension system -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Progress check Answer the following questions: What are the two functions that a leaf spring has to perform? Why does it not carry out these functions ideally? -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Typical forces in an independent suspension The next few diagrams show typical forces produced in an independent suspension system during driving, braking and cornering A MacPherson strut IFS (right hand) has been chosen for simplicity but the principles can be applied to any type IFS or IRS Bending force on strut Centrifugal force Wishbone in tension Opposing cornering force The first diagram illustrates the forces in the suspension as the vehicle negotiates a right turn Note: Direction of the centrifugal force due to cornering and the opposing cornering force applied to the wheel at the road surface Look at the direction of the forces produced in the suspension components -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Independent front and rear suspension This type of suspension addresses most of the disadvantages of the rigid axle suspension e.g: • Increased spring deflection and therefore ride comfort - due to the suspension design • Accurate control of steering geometry and therefore reduced bump steer and roll steer This is due to the suspension design which maintains the wheels in the correct position with little or no change in geometry as the wheels follow the road surface • Improved roll stiffness as the springs tend to move further apart, plus the extensive use of anti-roll bars • Reduced un-sprung weight giving improved wheel to road contact and therefore road holding This is due to the suspension design involving lighter moving parts and greater wheel to road contact • Greater flexibility regarding engine positioning in frame Un-sprung weight consists of; the weight of the wheels, half the weight of the springs, dampers and drive shafts and less than half the weight of the suspension arms and anti-roll bars This diagram illustrates which parts of the independent suspension system are un-sprung weight -22Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Progress check Answer the following: Give four advantages of independent suspension systems: Give two advantages of rigid axles: -23Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Hydro-pneumatic and air suspension These suspensions systems are complex and relatively expensive They have the following advantages: • constant ride height • variable spring rate, which is dependent on load • reduced body roll • reduced pitching • ride height can be automatically lowered with electronic control of air suspension giving improved road holding A layout of a car air suspension system is shown Note: The suspension design is almost the same as a normal IFS and IRS system except for the air springs and control actuators (levelling valves) -24Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Progress check Answer the following: Give three advantages of air suspension on a car: -25Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Routine suspension maintenance checks These checks are relatively simple and are as follows: • check security of suspension fittings and components • check operation of dampers • check ride heights • check for wear and leaks This shows a typical vehicle under body and indicates the components that need routine checking for tightness The following two diagrams show tightening checks that have to be carried out on typical IFS and IRS systems respectively -26Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue IFS layout - front wheel IFS layout - rear wheel -27Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Rear suspension with leaf spring The areas that need a tightness check on a leaf spring rear suspension The layout shows a “dead” or non-driven axle, but the same areas would need checking on a live or driven axle Check for damper leaks in this area The photograph shows where to check for damper leaks Damper operation can be checked by a simple “bounce test”, as shown in the next section Uneven tyre wear and poor road holding often indicate weak damper operation -28Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Exercise Checking the suspension ride height of a vehicle This task should be carried out on flat non-sloping workshop floor Checking ride heights Checking ride heights Ride heights should be checked in accordance with manufacturers instructions E.g tyre pressure settings, no person onboard and specified amount of fuel in tank The measurement is best taken with a solid rule or bar calibrated to the manufacturers’ dimensions Use of a tape rule is not recommended -29Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Check for component wear Checking lower suspension ball joint wear Checking lower suspension ball joint wear MacPherson strut suspension layout Checking for wear in the wishbone suspension Checking for wear in wishbone suspension -30Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Checking for wear for in top wishbone Checking wear in top wishbone -31Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Progress check Answer the following: List four routine suspension maintenance checks that need to be carried out: -32Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Identification of Common Faults Associated with Suspension Systems Common suspension system faults and common causes Incorrect ride height (unequal or low) and common causes: • broken spring or fault with spring seating e.g corrosion at MacPherson strut top mounting • damaged or bent suspension arms as a result of kerbing etc Cracked, perished or worn mountings or bushes: • usually due to high mileage or long vehicle life Suspension noise: • excessive free play or wear in components or mountings • loose suspension components or fixings • worn damper • lack of lubricant Excessive travel or movement in suspension components: • generally due to worn, loose or broken suspension components • worn dampers Fluid leakage from dampers and hydraulic components: • worn dampers due to high mileage or arduous service conditions • loose hydraulic connections or cracked pipes or hoses Worn dampers: • Carry out bounce test and observe results -33- Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue bounce DamperDamper bounce test test Vehicle should be pushed down on suspension at each corner and the degree of bounce observed The first diagram shows the vertical motion of the vehicle on the suspension during a bounce test This shows a serviceable damper Notice how the there are only or strokes before the damper absorbs the oscillation of the spring The second diagram illustrates the same action but with a worn damper In this case there are eight strokes before the spring force is damped out Only experience will enable a technician to decide how many strokes a spring can move before the damper is considered to be unserviceable The test should be backed up with a road test and if necessary the damper removed and its action compared with a new unit Start Finish Vertical motion of vehicle body on suspension Serviceable damper action is shown Note: There are only three strokes before the vertical motion is damped out -34Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Finish Start Un-serviceable damper action is shown Note: There are eight strokes before the vertical motion is damped out Abnormal tyre wear can be attributed to a number of suspension faults For example, worn, damaged or loose suspension components affecting wheel alignment settings will increase the rate of tyre wear, and reduce passenger comfort -35Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Progress check Answer the following: List six possible suspension faults: -36Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue ... All Rights Reserved LV30: Suspension Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV30: Suspension Systems (2) Issue Forces Acting on Suspensions Newton’s... Reserved LV30: Suspension Systems (2) Issue Suspension system terminology Upward movement of the suspension is called bump Downward movement is called rebound Vehicle movement on the suspension. .. All Rights Reserved LV30: Suspension Systems (2) Issue Independent front and rear suspension This type of suspension addresses most of the disadvantages of the rigid axle suspension e.g: • Increased