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kap all phase & 6/11/03 11:37 am Page 35 Student Workbook LV40 Suspension Systems (3) LV40/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV40 SUSPENSION SYSTEMS (3) Contents Page Introduction Self-Levelling Suspension: The benefits 3 Principle of Air Suspension (Pneumatic): Progress check Ride Control Systems: Manual Control Automatic control Hydro-pneumatic suspension systems Progress check Hydro-pneumatic suspension damper action Progress check Hydro-pneumatic suspension accumulator Swash plate hydraulic pump Hydro-pneumatic height corrector Progress check Height corrector construction Oil flow in neutral (negligible) Oil flow neutral position to inlet position Oil flow inlet position to neutral position … Progress check Oil flow exhaust position to neutral position Page 20 21 Electronically Controlled HydroPneumatic Layout: Suspension in soft mode Suspension in firm mode MacPherson strut hydro-pneumatic (front) Hydro-pneumatic trailing arm (rear) Progress check 24 24 25 10 Active Hydro-Pneumatic Suspension 26 11 13 Routine Maintenance: Self levelling and ride control suspension systems (hydropneumatic) Test action of height correctors Check the condition and security of the height corrector linkages Other maintenance checks Common faults associated with selflevelling and ride control suspension systems (hydro-pneumatic) 7 14 14 15 16 17 17 18 22 23 23 27 27 28 28 28 28 19 -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Page Self energising suspension systems Self energising suspension system layout Underside view of unit and A-frame Ball joint and lower suspension unit mounting Self-levelling and ride control suspension systems (selfenergising suspension systems) Self-energising suspension unit faults Progress check Electronic Controlled Air Suspension: Electronic controlled air suspension (basic) Electronic controlled air suspension (detailed) Motor driven air compressor dryer Pneumatic suspension unit with damper Suspension control actuator details Suspension control actuator position Height control valves Height control sensors Vertical accelerator sensor Location of height control sensor Disc with vertical accelerator sensor 29 30 31 31 32 32 33 34 … Electronic control component functions Progress check Electronic circuit for air suspension Sensor and actuator relationship ECU functions Air suspension ECU Skid control ECU Engine ECU Electronic control of vehicle roll centre Progress check Electronic controlled air suspension Common faults associated with electronic controlled air suspension Page 42 43 44 45 46 46 46 46 46 48 49 51 35 35 36 37 38 38 39 39 40 41 41 -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Introduction On completion of this module you will be expected to effectively: • • • • describe the operating principles of self levelling suspension describe the operating principles of ride controlled systems explain the routine maintenance requirements for self levelling and ride controlled suspension systems describe the common faults associated with self levelling and ride controlled suspension systems Self-Levelling Suspension Self – levelling suspension systems have been used on cars for many years Some systems use only gas (pneumatic systems) and others are arrangements involving the use of both hydraulic fluid and gas (hydropneumatic) In addition self-energising systems use a large gas filled damper, together with a built in pump supporting normal springs and dampers Hydraulics – the study of pressures in liquids Pneumatics – the study of pressures in gases The benefits These suspensions systems are complex and relatively expensive, but they have the following advantages: • • • • constant ride height variable spring rate, which is dependent on load reduced body roll reduced pitching Electronic control has been added to these systems to make them very sophisticated and gives additional features over the standard layout These additional features are listed below: • • • • • anti-dive and anti-squat suspension characteristics can easily be incorporated improved damping features related to speed driver can alter spring stiffness and damper settings roll stiffness can be predicted by use of a steering wheel sensor ride height can be automatically lowered as speed increases giving better road holding and improved aerodynamics -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Principle of Air Suspension (Pneumatic) An air suspension system consists of three main components, namely • • • an air spring (with a concentric damper) formed by a rolling diaphragm enclosing the bottom of a chamber an engine driven air compressor/drier height control valves that are connected to the suspension and allow air in and out of the spring/damper unit Small-bore pipes and hoses connect all components The diagram below shows a typical air suspension layout as used on a luxury car Air suspension systems have been used on large goods vehicles for a number of years The variable rate spring characteristic proving particularly useful where the laden to un-laden ratio may be as high as 3:1 In the above diagram, you will also notice front suspension control actuators, which give the driver some control over the damper settings Air pressure level is in under the command of the control valve on the compressor -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue The above diagram shows the motor driven piston type air compressor, which has an air dryer attached to remove the water from the compressed air The air dryer is necessary particularly when the humidity is high When air is compressed, its volume is reduced dramatically, but since water is a liquid, it cannot be compressed Therefore, the water takes up a disproportionate amount of the space as the compressed air cools and needs to be removed to prevent any corrosion or damage to the system The above shows the construction of a front and rear rolling bellows air spring or pneumatic cylinder unit Note: The concentric damper position, which also forms the bottom suspension unit mounting, and the sub chamber mounted above the main chamber The sub chamber can be connected and disconnected from the main air chamber to decrease and increase the spring rate respectively, i.e make the spring softer or return it to normal Open and closing the sub chamber is carried out by the suspension control actuators, which also control the damper settings -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Progress check Answer the following questions: Name the three main types of self-levelling suspension systems How can these systems be made more effective? -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Ride Control Systems These come under two headings: Manual control This is where the driver has some control over the ride height and damper settings The first application of this system was the use of manually adjustable dampers, which involved the driver reaching under the wheel arc and twisting the damper body to increase or decrease the stiffness This was later modified to cable operation to enable the driver to adjust damper settings from his or her seat whilst on the move Electric control was later added to make operation easier As mentioned earlier, the use of air suspension allows the spring rate to be altered If metal springs are used it is not possible to alter the spring rate or stiffness since this is determined by the physical size and length of the spring Automatic control Usually controlled electronically by an ECU receiving signals from various sensors around the vehicle In this case the ride height and damper settings (and spring rate with air suspension) are controlled automatically by an ECU, which has set suspension parameters programmed to control the suspension system to give the best ride and road holding characteristics In this system the driver may have some control over the damper settings, i.e hard or soft ride, but the ECU will override these if vehicle handling or safety is compromised -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Rebound Adjustment Range Velocity = M/S Compression Forces Forces (N) The above graph is showing the force – suspension movement velocity of an adjustable damper Note: The compression or bump forces are less than the rebound forces This allows the spring to rise quickly, as the road wheel follows the road surface, but for the travel to be resisted on the way down on rebound Hydro-pneumatic suspension systems The principle of this suspension is very simple Spherical gas springs are used and suspension movement is transferred to the spring hydraulically by means of a piston running inside a cylinder The piston rod is attached to the upper suspension arm in a double wishbone suspension layout MacPherson struts are also used, as shown later Each sphere contains a diaphragm behind which a quantity of nitrogen is trapped under a pressure of approximately 50 bar A height corrector is attached to the anti roll bar and when a load is placed in the car, the car body sinks This movement is registered by the height corrector, which opens a valve to admit hydraulic fluid under pressure to "lengthen" the hydraulic strut and thereby re-establish the correct ride height -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Hydro-pneumatic suspension Damper valve Nitrogen gas Mineral based hydraulic fluid Height corrector This diagram illustrates the layout of a double wishbone IFS hydro-pneumatic suspension system Note: The position of the damper valve between the cylinder and the oil space below the sphere and the height corrector, which is linked to the anti-roll bar Later diagrams show that only one height corrector is used at the front and only one at the rear -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Pneumatic suspension unit with damper Location for suspension control actuators Variable damping force rotary valve An air suspension unit is illustrated above It shows the location for the suspension control actuators, which alter the damper settings by turning the rotary valve They also control the spring rate as mentioned previously Actuators alter the damping force by turning the damper control rod It is operated by a stepper motor which has nine steps each of 15 degrees - 37 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Suspension control actuator details Suspension control actuator position - 38 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Height control valves The height control valves are shown above These supply and discharge the air, to and from respectively, the air suspension units They have two solenoid valves Height control sensors The above diagram shows details of the electrical height control sensors which work on the principle of a variable resistor The ride height is thus proportional to the voltage and this signal is fed to the suspension ECU - 39 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Vertical accelerator sensor The above diagram shows the vertical acceleration sensor The air suspension ECU calculates the sprung mass velocity of the wheels according to the signals received from the acceleration sensors Also, by way of the height control sensors, the ECU calculates the relative velocity between the sprung and un-sprung mass and sets the damping force for optimum suspension performance and road holding - 40 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Location of height control sensor The location of the height control sensors is shown above Note: This is one of the few mechanical parts on this type of air suspension Disc with vertical accelerator sensor Another view of the acceleration sensor is shown above The inertia of the Piezoelectric disc generates an electric signal in response to the vertical acceleration of the wheel This is the signal sent to the ECU The steering angle sensor is shown above and measures the degree and rate of steering wheel movement This signal is sent to the suspension ECU - 41 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Electronic control component functions - 42 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Progress check Answer the following question: What components are used to carry out the following functions? a) Alter the level of damping and spring rate: b) Measure the ride height of the vehicle: c) Control the air flow in and out of the pneumatic cylinders: - 43 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Electronic circuit for air suspension - 44 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Sensor and actuator relationship The above shows the relationship between the sensors and the actuators - 45 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue ECU functions Air suspension ECU This carries the following functions: • Controls the damping force and vehicle ride height in accordance with operating/driving conditions • It receives signals from height control sensors, vertical acceleration sensors, steering angle sensor, ride height control switch and damper setting switch It sends signals to suspension control actuators, height control solenoids, exhaust valve solenoid, height control relay and air compressor • Controls and operates the suspension malfunction warning • Operates a diagnostic check in the event of a suspension malfunction Skid control ECU Transfers signals from stop light switch and speed sensor to suspension ECU Engine ECU Transfers signals from throttle position, gear position and sensors and IC regulator to suspension ECU Electronic control of vehicle roll centre When a vehicle with conventional springs corners, a degree of roll takes place dependent on the stiffness or rate, of the springs and the strength of the antiroll bar The point at which roll takes place is called the roll centre and its position depends on the design of the suspension For example, the roll centre of a vehicle with leaf springs is relatively high and by carefully designing an IFS or IRS the roll centre can be lowered The centrifugal force generated when cornering, acts through the centre of gravity and its position relative to the roll axis, determines the degree of roll and the cornering characteristics - 46 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Centre of Gravity Roll Imaginary centre control on inside of turn Outer spring lowered to drop roll centre With electronic control of air suspension the air pressure in the pneumatic cylinders can be adjusted to lower the roll centre and determine its ideal position for the cornering conditions This is shown in the diagram above - 47 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Progress check Answer the following questions: List three sensors, which feed signals to the suspension ECU: What two ECU’s feed signals to the suspension ECU? - 48 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Electronic controlled air suspension The maintenance of the mechanical components is much the same as a conventional system and has been covered in earlier module This area of maintenance should be checked before carrying out any work on the air suspension system Note: Before jacking a vehicle with air suspension, switch off the engine or isolate the control system If this is not done then the height control sensors will try to alter the ride height The following checks should be carried out • The security and condition of height control sensors should be checked The diagram below shows the position of the links between the sensors and suspension arms These may need to be adjusted in accordance with the manufacturers’ settings • Check condition of compressor air filter and dryer Filters may need changing together with any desiccant cartridge in the air dryer • Check condition and security of all mechanical suspension components • Check for air leaks using a solution of soap detergent and water This check should also be carried out with someone operating the height control switch Adjustment - 49 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue The following two diagrams show the method of checking the response time of the suspension, to the driver altering the vehicle ride height by operating the height control switch This diagram highlights response time check (suspension rising) This diagram highlights response time check (suspension falling) - 50 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Common faults associated with electronic controlled air suspension Apart from mechanical faults with the height control sensors, compressor air filter and dryer and air leaks from hoses there are few common faults The air suspension ECU has a fault diagnosis facility and this is indicated to the driver by way of a warning light If a malfunction occurs in any of the sensors or actuators, the ECU prohibits the vehicle height control and/or the damping force control This enables the vehicle to be driven home with the suspension acting as a simple air suspension system Any fault can be identified using codes Some manufacturers use hand held testers, which enable the suspension to be put through a simulation test to check all sensors and actuators - 51 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue ... Rights Reserved LV40: Suspension Systems (3) Issue Electronic circuit for air suspension - 44 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Sensor... Reserved LV40: Suspension Systems (3) Issue Suspension control actuator details Suspension control actuator position - 38 Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension. .. self-levelling suspension systems How can these systems be made more effective? -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV40: Suspension Systems (3) Issue Ride Control Systems