Steering Fundamentals

Transcription

1 CHAPTER 65 Fundamentals OBJECTIVES Upon completion of this chapter, you should be able to: List the parts of steering systems. Describe the principles of operation of steering systems. Compare linkage systems to rack and pinion. Describe how power steering systems operate. KEY TERMS ball socket flow control valve lock-to-lock parallelogram steering rack-and-pinion steering recirculating ball and nut steering gear spool valve steering ratio toe-out-on-turns turnbuckle STEERING SYSTEMS The steering system works with the suspension system. It allows the driver to steer the car while providing a comfortable amount of steering effort. system parts include the steering gear, the steering linkage, the steering wheel, and the steering column. There are two styles of steering. One has a gearbox and parallelogram linkage (Figure 65.1). The other is a long rack with linkage extending from its ends (Figure 65.2). Figure 65.2 Rack-and-pinion steering with linkage. Figure 65.1 A steering gearbox and parallelogram linkage. STEERING GEARS The two common automotive steering gear types are the conventional recirculating ball and nut steering gear and the rack-and-pinion steering. There are other types of steering gearbox designs, but these are the primary ones used in automobiles and light trucks. Ratio The steering ratio refers to the amount of steering wheel rotation. When the steering wheel is turned all the way in one direction, it stops against a lock. Turning the 1216

2 Fundamentals 1217 wheel all the way from one lock to the other is called lock-to-lock. A fast steering ratio is about three turns lock to lock or less. Slower ratios require the wheel to be turned about four times. The ratio of a steering gear varies, depending on whether or not the car has a power assist. Power steering cars usually have faster ratios. A 15:1 ratio means that when the steering wheel is turned 15 degrees the front wheels will turn 1 degree. The steering ratio is determined by the number of teeth on the driving gear compared to the number of teeth on the driven gear. With parallelogram steering, the length of the steering arms, pitman arms, and idler arms also play a part in determining steering ratio. Turning Radius The turning radius, or turning circle, refers to the amount of space it takes for a vehicle to be able to turn around. When a vehicle has a tight turning radius, it is easier to make a U-turn or maneuver into a parking space. RECIRCULATING BALL AND NUT STEERING GEAR In a recirculating ball and nut steering gear (Figure 65.3), a sector gear at the end of the pitman or sector shaft meshes with a ball nut driven by ball bearings that ride on the worm (steering) shaft to provide a smooth steering feel. The ball nut has curved channels that the ball bearings circulate within; the steering shaft also has bearing channels. The balls rotate and recirculate through tubes called ball returns. RACK-AND-PINION STEERING On a rack-and-pinion steering system (Figure 65.4), the end of the steering shaft has a pinion gear that meshes with the rack gear (Figure 65.5). These are used on Worm shaft Sector gear Ball nut Ball return tubes Pitman or sector shaft Figure 65.3 A recirculating ball and nut steering gear. many cars because they are lighter than standard steering gears. They are also easier to install during vehicle assembly at the factory. A typical rack and pinion often has a faster ratio than a recirculating ball steering gear. A spring-loaded damper preloads the rack gear. This prevents the rack from flexing, which would cause gear backlash (Figure 65.6). An adjustment screw or shims are used to adjust the amount of tension against the rack. The pinion shaft is usually supported by needle bearings. The rack is most often supported by plain bushings. wheel Reservoir Driver s airbag assembly pump Power-steering gearbox Figure 65.4 A power-assisted rack-and-pinion steering system. column

3 1218 CHAPTER 65 Rack gear support bushing Pinion (input) shaft Upper pinion bearing Pinion gear Inner tie-rod end Rack gear Lower pinion bearing Outer tie-rod Figure 65.5 Parts of a rack-and-pinion steering gear. Spring Shims Pinion Damper Hardened area Rack Figure 65.6 A damper holds tension against the rack. Rack-and-pinion steering systems are more easily damaged when the front wheels hit a curb or rock. They transmit more road shock, which can be felt through the steering wheel. The rack is mounted on rubber bushings to help cushion shocks (Figure 65.7). The rack-and-pinion assembly can be mounted in several locations. Some are on the sub-frame and others are on the firewall. STEERING LINKAGE The steering gear is connected to the wheels by the steering linkage. linkage parts vary, depending on the design used, but all designs include tie-rods, steering arms, and a steering knuckle. A comparison of the steering linkage parts of the two major steering systems is shown in Figure Rubber mounts Sub-frame Rack and pinion Figure 65.7 The rack on this short-and-long arm sub-frame is mounted on rubber bushings. A recirculating ball and nut steering gearbox can use a number of different linkage designs, depending on the suspension it is used with. Parallelogram

4 Fundamentals 1219 Idler arm and bracket Center link Pitman arm Tie-rods arm Figure Parts of a parallelogram steering system during a turn. Tie-rods Bearing Dust boot Figure 65.8 Both types of steering have tie-rods, steering arms, and a steering knuckle. steering is the most popular design used with the long and short arm suspension. PARALLELOGRAM STEERING LINKAGE A recirculating ball gear on a passenger car usually has parallelogram steering linkage. The name comes from the parallelogram shape made by the steering linkage during a turn (Figure 65.9). Parallelogram steering parts are shown in Figure Tie-rods on each side are connected by the center link. The pitman arm connects the steering box to the center link. An idler arm supports the center link on the passenger side (Figure 65.11). Figure Idler arm cutaways. Bearing Dust boot BALL SOCKETS Ball sockets connect the steering linkage parts. They allow parts to rotate during a turn and pivot as the steering deflects during a bump. TIE-RODS Tie-rod ends (Figure 65.12) are attached to pivot points at the front wheels. Not only do they transmit motion from the steering wheel to the front wheels, Tim Gilles Front of vehicle Seal Figure 65.9 The shape of a parallelogram is made by the steering linkage during a turn. Ball Seat Spring Figure A cutaway of a tie-rod end. Tim Gilles

5 1220 CHAPTER 65 Outer tierod end Adjusting sleeve Inner tierod end Center link Left-hand thread Right-hand thread Figure A threaded sleeve connects the inner and outer tie-rods. but they maintain the correct front wheel toe (the amount that the front tires are aimed inward or outward at the front). A threaded adjusting sleeve connects the inner and outer tie-rod ends (Figure 65.13). The sleeve has a right-hand thread on one end and a left-hand thread on the other end. When it is turned it acts like a turnbuckle. Turning it one way shortens the tie-rod assembly. Turning it the other way makes it longer. This allows the toe angle between the front tires to be adjustable. knuckle Spindle STEERING ARM The tie-rods attach to the front wheels at the steering arms (Figure 65.14). The steering arm is attached to the steering knuckle, which includes the spindle (Figure 65.15). During a turn, the inside wheel must turn sharper than the outside wheel because they follow different circular paths (Figure 65.16). The steering arms are angled inward, rather than being parallel to the frame. This angle, called the Ackerman angle, provides an important steering angle, called toe-out-on-turns (Figure 65.17). This angle is not adjustable. arm Figure arm and knuckle. Front of vehicle McQuay Norris Center link arm Tie-rods Angle outside wheel arms Figure The tie-rods attach to the front wheels at the steering arms. Angle inside wheel Common center Figure The inside wheel turns sharper than the outside wheel because they follow different circular paths. This is called toe-out-on-turns or turning radius.

6 Fundamentals 1221 Inside of turn 20 Outside of turn 18 Front of vehicle Linkage arms Figure The steering arms are bent at an angle to allow the fronts of the tires to toe out during a turn. Figure Inner tie-rods attach at the center of this rack. RACK-AND-PINION STEERING LINKAGE Rack and pinion does not have the complicated steering linkage used with a recirculating ball and nut steering box. In most systems, two tie-rods come out of the steering rack (see Figure 65.7). They have conventional tie-rod end ball sockets, but only on the outer ends. These attach to the steering knuckles. The inner tie-rod ends are ball sockets (Figure 65.18). They are enclosed within rubber bellows or boots on the rack. In another type of rack-and-pinion system, the inner tie-rods attach to the center of the rack gear (Figure 65.19). Damper Some steering linkages use a steering damper, a horizontally mounted shock absorber, to minimize the effect of road shocks to the steering wheel (Figure 65.20). STEERING COLUMN The steering wheel is splined to a steering shaft located in the center of the steering column. A locknut retains the steering wheel to the shaft. The shaft is supported by bearings at the top and bottom of the steering column. The ignition switch and lock (between the shift selector and the steering wheel) are built into the steering Socket seat Inner tierod end Ball Tie-rod Figure Two types of inner tie-rod sockets. Outer tie-rod threads here Tim Gilles damper Figure A steering damper absorbs road shocks to the steering linkage. column. The turn signal switch, transmission shift selector, and horn control are also included in the steering column. Most newer vehicles use the European style of controls, which includes a headlight and dimmer switch, windshield wiper and washer controls, and cruise control. Tilt and Collapsible Columns Tilt steering wheels and collapsible columns are used on many vehicles. Tilt columns provide the driver with a means of adjusting the angle of the steering wheel. A tilt column includes a short section of shaft beneath the steering wheel, connected to the steering column using gears or a universal joint. A typical tilt column uses a spring-loaded ratcheting mechanism to hold the steering wheel in position (Figure 65.21). A tilt release lever compresses the spring to release tension on the ratchet when steering wheel position adjustment is desired. Some columns are telescopic, allowing the height of the steering wheel in relation to the instrument panel to be adjusted as well. Air bags have been installed on the steering wheel in most vehicles since the early 1990s, and, by law, steering columns and shafts are required to be collapsible in case of an accident (Figure 65.22). There are different designs of collapsible steering columns and shafts. One type uses a two-piece outer section retained with plastic shear pins (Figure 65.23). It is combined with a two-piece inner steering shaft, also retained with plastic shear pins.

7 1222 CHAPTER 65 Plastic shear pins Outer section Figure A tilt steering column has a mechanism that releases tension on a ratchet to allow the steering wheel angle to be changed. Inner section column Shear pins wheel mount Inner shaft shaft Outer shaft Figure A collapsible steering column and shaft. Tilt mechanism gearbox is mounted to the frame, there is movement or flex between the two. There is a flexible coupling or a universal joint between the steering shaft and the splined input shaft of the steering gear (Figure 65.24). Collapsible steering column Figure This tilting steering column can also collapse during an impact. The insets show how the collapsing mechanism operates. Service to the steering column and air bags, called supplemental inflatable restraints (SIR), is covered in Chapter 66. The steering shaft connects the steering wheel to the steering gear. Because the steering column is mounted to the body of the car and the steering Flexible coupling (Rag joint) Universal joint Figure A flexible coupling or a universal joint provides the attachment between the steering shaft and the splined input shaft of the steering gear.

8 Fundamentals 1223 It allows a small amount of misalignment between the steering column and the steering gear. The flex coupling also prevents the transfer of road shocks to the steering wheel. When there is a greater angle, a universal joint is used. When the transmission shift selector is located in the steering column, a slotted lock plate attached to the upper steering shaft is engaged with a lever to lock the steering wheel when the shift lever is in park and the ignition switch is off. POWER STEERING systems on most vehicles in the United States and Canada are power assisted, although some manual units are still made for some smaller cars. Most power steering is hydraulic, with pressure supplied from the crankshaft by a belt-driven pump (Figure 65.25). There are also some newer vehicles that use electric motor-assisted steering. These are found on hybrid vehicles and vehicles with intelligent parking assist systems. Roller type Vane type Roller Vane POWER STEERING PUMP A pump that is driven by a belt from the crankshaft supplies hydraulic power to assist steering effort. Three main types of power steering pumps have been used on cars (Figure 65.26). They are the roller, vane, and slipper types. The vane type is the most popular. All of the types of pumps work in the same way. The pump shaft is turned, drawing oil into the pump. The pump pushes the oil into a smaller area where it is trapped and pressurized for delivery to the steering gear. Slipper type Slipper Figure Types of power steering pumps. Power Pump Operation The power steering pump works under difficult conditions. assist is needed most when the car is stopped or nearly stopped. Therefore, a steering pump Belt pulley Pump Return line Pressure line gear Figure Parts of a hydraulic power steering system. must deliver sufficient flow to be able to provide steering assist at low engine rpm and idle. A pump develops more unneeded flow at higher engine rpm and vehicle speeds. Excess fluid flow at cruising rpm must be diverted back to the inlet side of the pump so it can be returned to the reservoir without creating pressure. Most pumps have a two-stage pressure relief valve. A flow control valve monitors the turning effort on the steering wheel to provide the correct amount of assist (Figure 65.27). Its two functions are to control flow and limit maximum pressure. The flow control valve is almost always working. The pressure relief part of the valve hardly ever opens. It opens when the steering wheel is held all the way to one side, against a steering lock. This causes noise as pressure is bled off to the fluid intake side of the pump. When the pump turns at low speed, fluid action is as shown in Figure At higher speeds, fluid flow will become excessive. The by-pass port opens, allowing the excess fluid to return to the pump inlet.

9 1224 CHAPTER 65 Bypass port Metering pin Variable orifice Pressure relief ball Intake Discharge Orifice Figure A control valve regulates flow in the power steering pump. Power Consumption Power steering pumps require a considerable amount of horsepower to operate. Sometimes on small vehicles, when steering effort is high, the air-conditioning compressor will be shut off to compensate for the draw of the power steering system. Many vehicles have computer-controlled charging systems that also shut off the direct current (DC) generator at idle when the power steering system is under load. Some late-model rack-and-pinion steering units are powered by an electric motor. These are mostly found on hybrid-electric vehicles that require electrical power assist when the engine is shut off during idle-stop and off-idle operation. Ford Motor Company Recirculating Ball Power Integral recirculating ball gearboxes have a ball nut and sector gear just like a manual gearbox. The ball nut is housed within a power piston. Pressurized oil enters a chamber on either side of the power piston to provide steering assist (Figure 65.29). To sense and control power steering assist, gearboxes use either a pivot lever or a torsion bar acting on a spool valve. On the pivot lever type, turning effort causes a spool valve to be moved by a pivot lever (Figure 65.30). When the spool valve moves, fluid is directed to one side or the other of the power piston to provide the assist. On the torsion bar steering gear, also called a rotary valve steering gear, a small, sensitive torsion bar twists Left turn TYPES OF POWER STEERING Power steering systems are either rack-and-pinion or recirculating ball and nut with a hydraulic control system added. Most are integral power steering systems (see Figure 65.25). Integral means part of. An integral power steering has the power steering components located within the steering gear. Integral units can be either recirculating ball or rack and pinion (Figure 65.28). Right turn Figure Pressurized oil enters a chamber on either side of the power piston to provide steering assist. Spool valve Spool valve Integral power steering gear Figure Types of integral power steering. Power rack

10 Fundamentals 1225 Sector shaft adjusting screw Pivot lever Worm shaft Power piston Sector shaft Figure A pivot lever-type power steering gear. VINTAGE POWER STEERING A linkage-type gearbox, found on older cars and light trucks, uses a standard gearbox with a piston attached to the steering linkage. If you had a 1965 Mustang with power steering, it used this system. Vintage Figure 65.1 shows fluid assist during a turn. Check valve (in control valve housing) Pump Reservoir Power cylinder in response to steering effort. This turns a rotary spool valve to direct pressure to the desired side of the power piston (Figure 65.31). Rack-and-Pinion Power In a rack-and-pinion system, fluid is directed to a chamber on either side of the rack, in a manner similar to the linkage-type power steering used on older cars. Figure shows how fluid is directed in a power rack-and-pinion steering gear during left and right turns. To sense and control the amount of assist, some racks use a torsion bar attached to the input shaft. Other units use a spool valve that moves in response to up-and-down movement of the pinion gear as it tries to drive the rack. Control valve Reaction valve Pressure Return Reaction pressure Vintage Figure 65.1 Fluid flow in a linkage power steering system during a left turn. Ford Motor Company ELECTRONICALLY CONTROLLED VARIABLE EFFORT POWER STEERING Power steering allows a vehicle to be easily steered at low speeds. Once the vehicle achieves a reasonable speed, however, a fixed level of power assist is no longer needed and can interfere with a driver s feel for the road. On many late-model vehicles, the speed of the vehicle determines how much power assist is given.

11 1226 CHAPTER 65 Pin Torsion bar shaft Stub shaft Valve spool Sector shaft Power piston Figure A torsion bar-type power steering gear. Worm shaft Left turn Left turn tube Tie-rod Bellows Rack Piston Housing tube (power cylinder) Right turn Right turn tube Piston Figure Fluid power assist in a rack-and-pinion unit. Rack Hunter Engineering Company

12 Fundamentals 1227 One type controls fluid output from the pump. Another type controls the amount of fluid pressure available in the power steering gear. Pump-Controlled Assist With pump-controlled steering assist, an electronically managed actuator solenoid changes fluid flow in the power steering pump control valve. This is done with a pulse width modulated signal, like in fuel injection systems. Maximum power assist occurs at 1,500 rpm (fast idle) while the vehicle is at rest. The computer varies the solenoid on-time, which allows pump pressure to be higher. As the speed of the vehicle increases, the amount of pump flow is decreased. This increases the steering effort and gives the driver a better feel for the road. -Controlled Assist With a steering gear controlled steering assist (Figure 65.33) the amount of boost available at the power steering gear is sensed by a module that responds by changing fluid flow in the pump control valve to provide the correct amount of assist. General Motors electronic variable assist steering system, Magnasteer, has a rotary actuator attached to the input shaft of the hydraulically powered rackand-pinion steering gear (Figure 65.34). A control module varies a supply of electrical current to the actuator, which uses electromagnetic force on the steering gear input shaft to increase or decrease steering effort. Turning the steering wheel rotates the input shaft/ spool valve with its 16 permanent magnet segments. The rotary actuator solenoid that surrounds the input shaft has 16 matching segments, electromagnetically powered by a coil. At high speeds, the control module causes the electromagnetic segments to attract the permanent magnet segments, increasing steering effort to improve road feel. At low speeds, the module reverses the polarity on the actuator s magnetic segments, reducing steering effort. Rack-and-pinion steering gear Magnasteer rotary actuator Figure The Magnasteer system uses a magnetic actuator. More advanced variable-effort steering systems can adjust steering effort in response to lateral forces. A computer senses G-forces from a lateral accelerometer and compares the signal with vehicle speed and a signal from the digital steering angle sensor that tells how fast the steering wheel is being turned (Figure 65.35). The steering effort is adjusted to fit the condition. Four-Wheel Some automotive and truck manufacturers have produced four-wheel steering systems as an extra-cost item Photo cell Sensor ring Solenoid control valve Speed proportional rack-and-pinion power steering gear Solenid control module Figure A speed proportional power steering unit. Figure A digital steering angle sensor tells the computer how fast and how far the steering wheel is turned.

13 1228 CHAPTER 65 Center of turning radius Electrohydraulic steering gear Conventional Four wheel Figure The action of conventional front-wheel steering and four-wheel steering when parallel parking. in a few select vehicles. All four wheels steer, improving handling and helping the vehicle make tighter turns. The turning radius of a truck with four-wheel steering is similar to that of a compact car. At low to medium speeds the rear wheels steer in a direction opposite to the front wheels. At high speeds, they turn in the same direction as the front wheels to improve maneuverability during lane changes. Front wheels do most of the steering and rear-wheel turning is generally limited to 5 to 6 degrees during an opposite-direction turn. During a same-direction turn, rear-wheel steering is limited to about 1 to 1.5 degrees. If the rear-wheel turning radius were not limited, the wheels would bump into the curb as the vehicle attempted to maneuver out of a parallel parking spot (Figure 65.36). There are different types of four-wheel steering, including mechanical, hydraulic, and electric/hydraulic. ELECTRONICALLY CONTROLLED STEERING SYSTEMS Electronically controlled steering gears can be steerby-wire with no mechanical connection, or they can be mechanically connected using an electrohydraulic steering gear (Figure 65.37) or electric motor. Steerby-wire systems are used in some tractors and forklifts but due to safety considerations are only found in automotive concept cars (Figure 65.38). Some active front-steering (AFS) systems are capable of electronically changing the steering gear ratio so the electronic control unit (ECU) steers the front wheels at a different rate than the steering wheel is turned by the driver. The assist requirements of highway and in-town steering are different, Figure An electrohydraulic steering gear provides active front steering control mechanically, without driveby-wire. Control unit control signal SW sensor signal SW control signal sensor signal Motor drive Tierod Tierod Sensor motor gear Motor drive Sensor wheel motor wheel Coupler Figure An electronically controlled drive-by-wire active front steering system. and the AFS system changes between low- and highspeed steering assist seamlessly. At lower vehicle speeds, steering assist is increased. At higher speeds, when power assist is not necessary, steering assist decreases. Electric Gears Electric motor powered steering systems use a rackand-pinion steering gear. Some motors operate directly on the steering gear at the pinion shaft (Figure 65.39). Others act on the top of the steering shaft, with

14 Fundamentals 1229 gear Electric motor assist Figure An electric steering assist located on the steering gear. ECU shaft Electric motor assist gear Figure An electric steering assist located at the top of the steering shaft in the passenger compartment. the motor located in the passenger compartment (Figure 65.40). An electric motor can operate when the engine is not running, so an electric power steering system is the design used on hybrid and fuel cell vehicles. Electric steering improves fuel economy because it only operates when needed, but electric steering requires at least a 42-volt electrical system, so it has cost and safety considerations. The ECU controls the amount of steering assist by regulating the amount of current applied to the steering assist motor. When the driver turns the steering wheel, the steering gear input shaft twists a torsion bar until the twisting effort equals the opposing force.

15 1230 CHAPTER 65 A torque sensor monitors the amount the torsion bar twists. It compares magnetic inductance between an input and an output coil, called resolvers (Figure 65.41). The turning effort of the driver on the steering wheel is reflected by the output signal from the torque sensor to the ECU. Inputs to the computer decisionmaking process include other vehicle conditions, like vehicle speed, steering angle, and the skid control system s yaw and deceleration rate. Electric steering gears are also used with intelligent parking assist systems that help a driver when parallel parking. Planetary Gear Active There are active steering systems, like the one used by BMW, that use a planetary gearset between the steering wheel and the steering gear. The input is the sun gear and the output is the planetary carrier. The ring rear is held, but its speed is regulated by a computercontrolled electric motor. The computer controls the speed the electric motor turns the ring gear, providing a variable steering gear ratio. Input speed sensor Output speed sensor gear pinion Torsion bar Torque sensor input to ECU gear rack Figure When the torsion bar twists, sensors at each end monitor the turning effort of the driver. REVIEW QUESTIONS 1. One of the two most common types of automotive steering gears is the recirculating ball and nut. What is the name of the other one? 2. What is the term that compares the number of teeth on the driving gear to the number of teeth on the driven gear? 3. When the steering wheel is turned all the way in one direction, it stops against a. 4. What is the name of the steering part that is angled so that the front wheels toe out during a turn? 5. What is another type of steering shaft coupling besides a flex coupling? 6. What are the names of the three main types of steering pumps that have been used on automobiles? 7. Which is the most common type of power steering pump design? 8. What is the name of the power steering design that has the power steering components located within the steering gear? 9. What is the name of the vintage power steering design that uses a standard gearbox with a hydraulic-assist piston attached to the steering linkage? 10. What type of power steering is used on hybrid vehicles? ASE-STYLE REVIEW QUESTIONS 1. Which of the following is/are true about turning radius? a. The outer wheel turns sharper than the inner wheel. b. Turning radius is an adjustable angle. c. Both A and B d. Neither A nor B 2. Front tires are supposed to toe during a turn. a. Outward b. Inward c. Both A and B d. Neither A nor B

16 Fundamentals Technician A says that steering assist is needed most when the car is stopped or nearly stopped. Technician B says that the power steering must be able to provide more steering assist at low engine rpm and idle. Who is right? a. Technician A b. Both A and B c. Technician B d. Neither A nor B 4. Two technicians are comparing rack-and-pinion steering to recirculating ball steering. Technician A says that recirculating ball steering is more easily damaged when a wheel hits a rock. Technician B says that recirculating ball steering gears are mounted on rubber bushings to help cushion shocks. Who is right? a. Technician A b. Both A and B c. Technician B d. Neither A nor B 5. All of the following are true about steering columns except: a. Some steering columns can tilt, allowing the angle of the steering wheel to be changed. b. Some steering columns can telescope, allowing the length of the steering column to be changed. c. columns can collapse during an accident. d. The steering shaft is held in position in the steering column by a universal joint at the top and a rag joint at the bottom. 6. Which of the following is/are true about electric motor assist power steering? a. They are rack-and-pinion steering gears. b. The motor can be located in the passenger compartment. c. Both A and B d. Neither A nor B 7. The steering design used with short-and-long arm suspensions is the: a. Rack and pinion b. Recirculating ball and nut c. Both A and B d. Neither A nor B 8. A fast steering ratio is about turns lock to lock. a. Three b. Five c. Four d. Ten 9. In the figure shown below, the part indicated by the arrow is a(n): a. Idler arm b. Center link c. Tie-rod d. Pitman arm 10. An electronically controlled power steering system: a. Increases assist at lower vehicle speeds b. Decreases assist as vehicle speed increases c. Both A and B d. Neither A nor B