Automotive Suspension & Steering Systems

 

The suspension system must provide proper steering control and ride quality. Performing these functions is extremely important to maintain vehicle safety and customer satisfaction. For example, if the suspension system allows excessive vertical wheel oscillations, the driver may lose control of the steering when driving on an irregular road surface. This loss of steering control can result in a vehicle collision and personal injury. Excessive vertical wheel oscillations transfer undesirable vibrations from the wheel(s) to the passenger compartment, which causes customer dissatisfaction with the ride quality. The suspension system and frame must also position the wheels and tires properly to provide normal tire life and proper steering control. If the suspension system does not position each wheel and tire properly, wheel alignment angles are incorrect and usually cause excessive tire tread wear. Improper wheel and tire position can also cause the steering to pull to one side. When the suspension system positions the wheels and tires properly, the steering should remain in the straight-ahead position if the car is driven straight ahead on a reasonably straight, smooth road surface. However, if the wheels and tires are not properly positioned, the steering can be erratic, and excessive steering effort is required to maintain the steering in the straight-ahead position. The steering system is also extremely important to maintain vehicle safety and reduce driver fatigue. For example, if a steering system component is suddenly disconnected, the driver may experience a complete loss of steering control, resulting in a vehicle collision and personal injury. Loose steering system components can cause erratic steering, which causes the driver to continually turn the steering wheel in either direction to try and keep the vehicle moving straight ahead. This condition results in premature driver fatigue.

 

 

Frames and Unitized Bodies

 

Some vehicles, such as rear-wheel-drive cars, sport utility vehicles (SUVs), and trucks, have a frame that is separate from the body. Other vehicles have a unitized body that combines the frame and body in one unit, eliminating the external frame. In a unitized body, the body design rather than a heavy steel frame provides strength and rigidity. All parts of a unitized body are load-carrying members, and these body parts are welded together to form a strong assembly.

 

 

 

The frame or unitized body serves the following purposes:

 

1. Allows the vehicle to support its total weight, including the weight of the vehicle and

cargo.

2. Allows the vehicle to absorb stress when driving on rough road surfaces.

3. Enables the vehicle to absorb torque from the engine and drive train.

4. Provides attachment points for suspension and other components. The unitized body provides a steel box around the passenger compartment to provide passenger protection in a collision. Modern automotive bodies use a wide variety of materials, such as hydroformed aluminum rails, magnesium roof frames, and carbon fiber-balsa wood composite floor panels. In most unitized bodies, special steel panels are inserted in the doors to protect the vehicle occupants in a side collision. Some unitized body components are manufactured from high-strength or ultra-high-strength steels. The unitized body design is typically used in small- and mid-sized front-wheel-drive cars. A steel cradle is mounted under the front of the unitized body to support the engine and transaxle. Rubber and steel mounts support the engine and transaxle on the cradle. Large rubber bushings are mounted between the cradle and the unitized body to help prevent engine vibration from reaching the passenger compartment. Some unitized bodies have a partial frame mounted under the rear of the vehicle to provide additional strength and facilitate the attachment of rear suspension components.

 

 

Front Suspension Systems

 

The front and rear suspension systems are extremely important to provide proper wheel position, steering control, ride quality, and tire life. The impact of the tires striking road irregularities must be absorbed by the suspension systems. The suspension systems must supply proper ride quality to maintain customer satisfaction and reduce driver fatigue, as well as provide proper wheel and tire position to maintain directional stability when driving. Proper wheel position also ensures normal tire tread life. Typical components in a short-and-long arm (SLA) front suspension system are illustrated in Figure. This type of front suspension system has a long, lower control arm and a shorter upper control arm. The main front suspension components serve the following purposes:

 

1. Upper and lower control arms—control lateral (side-to-side) wheel movement.

2. Upper and lower control arm bushings—allow upward and downward control arm movement and absorb wheel impacts and vibrations.

3. Coil springs—allow proper suspension ride height and control suspension travel during driving maneuvers.

4. Ball joints—allow the knuckle and wheels to turn to the right or left.

5. Steering knuckles—provide mounting surfaces for the wheel bearings and hubs.

6. Shock absorbers—control spring action when driving on irregular road surfaces.

7. Strut rod—controls fore-and-aft wheel movement.

8. Stabilizer bar—reduces body sway when a front wheel strikes a road irregularity.

 

 

A MacPherson strut front suspension system has no upper control arm and ball joint; instead, a strut is connected from the top of the knuckle to an upper strut mount bolted to the reinforced strut tower in the unitized body (Figure 1-6). The strut supports the top of the knuckle and also performs the same function as the shock absorber in a SLA suspension system. The coil spring is mounted between a lower support on the strut and the upper strut mount. Insulators are mounted between the ends of the coil spring and the mounting locations. A bearing in the upper strut mount allows the strut and coil spring to rotate with the spindle when the front wheels are turned.

  

 

Rear Suspension Systems

 

A typical live-axle rear suspension system has a one-piece rear axle housing. Trailing arms are connected from the rear axle housing to the chassis through rubber bushings. The coil springs are mounted between the trailing arms and the chassis. Because the rear axle housing is a one-piece assembly, vertical movement of one rear wheel causes the opposite rear wheel to be tipped outward at the top. This action increases tire tread wear and reduces ride quality and traction between the tire tread and road surface. Many front-wheel drive cars have a semi-independent rear suspension system with an inverted steel U-section connected between the rear spindles. The inverted U-section

 

 

usually contains a tubular stabilizer bar. When one rear wheel strikes a road irregularity, the inverted U-section and stabilizer bar twist, allowing some independent rear wheel movement before the wheel movement affects the opposite rear wheel. Some semi-independent rear suspension systems have a track bar and brace connected from the inverted U-section to the chassis to reduce lateral rear axle movement. Many vehicles have an independent rear suspension system, wherein each rear wheel can move independently without affecting the position of the opposite rear wheel. This type of suspension system reduces rear tire wear and provides improved steering control. Independent rear suspension systems have a number of different configurations. A MacPherson strut independent rear suspension system has a strut and coil spring assembly connected from the top of the spindle through a upper strut mount to the chassis. No provision for strut rotation is required, because the rear wheels are not steered. Some independent rear suspension systems have a multilink design, wherein an adjustment link connected from the rear spindle to the chassis allows rear wheel position adjustment.

 

 

 

Tires, Wheels, and Hubs

 

Tire Purpose

Tires are extremely important because they play a large part in providing vehicle safety and ride quality! Tires are the only point of contact between the vehicle and the road surface. Vehicle tires provide these functions:

1. Tires must support the vehicle weight safely and firmly.

2. Tires must provide a comfortable ride.

3. Tires must supply adequate traction on various road surfaces to drive and steer the vehicle.

4. Tires must contribute to proper steering control and directional stability of the vehicle.

5. Tires must absorb high stresses when cornering, accelerating, and braking.

6. Tire treads must be designed to propel water off the tread and away from the tire when driving on wet highways. This action prevents water from lifting the tires off the road surface, which decreases tire traction.

 

 

Wheel Rim Purpose

 

Wheel rims can be manufactured from steel, cast aluminum, forged aluminum, pressure-cast chrome-plated aluminum, or magnesium alloy. Wheel rims must retain the tires safely under all operating conditions without distortion. Tire and wheels must form air-tight containers at all temperatures so air does not leak from the assembly. Wheel rims must position the tires at the proper distance inward or outward from the vertical mounting surface of the wheel. 

The distance between the vertical wheel rim mounting surface and the centerline of the wheel rim is called wheel offset. If the wheel centerline is located outboard from the vertical wheel mounting surface, the wheel has positive offset. Conversely, if the wheel centerline is located inboard from the vertical wheel mounting surface, the wheel has negative offset. Wheel rims typically have four to six mounting openings that fit over studs in the wheel hub. When a wheel rim is installed on the hub studs, tapered nuts are then tightened to the specified torque to retain the wheel and tire assembly on the hub. On many wheel rims, the openings in the wheel rim are tapered to match the tapers on the retaining nuts. These tapered openings and matching tapered nuts center the wheel rim on the hub.

 

 

Wheel Hubs

Wheel hubs must provide a secure mounting surface for the wheel rim and tire assembly. Wheel hubs also contain the wheel bearings that provide smooth wheel rotation with reduced friction. Wheel bearings must have a minimum amount of end play to greatly reduce wheel lateral movement. The wheel hub and bearing assemblies must carry the load supplied by the vehicle weight, and these assemblies must also guide the wheel and tire assembly (Figure 1-13). The vehicle weight is supplied to the wheel hub and bearing assembly in a vertical direction. This type of bearing load is called a radial bearing load. When the vehicle turns a corner, the wheel hubs and bearings must carry thrust bearing loads supplied in a horizontal direction and angular bearing loads supplied in a direction between the horizontal and the vertical.

 

 

Shock Absorbers and Struts

 

Each corner of the vehicle has a shock absorber or strut connected from the suspension system to the chassis. Shock absorbers control spring action and wheel oscillations to provide a comfortable ride. Controlling spring action and wheel oscillations also improve vehicle safety because the struts help to keep each tire tread in contact with the road surface. If the struts are worn out, excessive wheel oscillations when driving on irregular road surfaces can cause the driver to lose control of the vehicle. Struts also reduce body sway and lean while turning a corner. Struts reduce the tendency of the tire tread to lift off the road surface. This action improves tire tread life, traction, steering control, and directional stability.

 

Struts contain a sealed lower chamber filled with a special oil. Many shock absorbers have a nitrogen gas charge on top of the oil. This gas charge helps to prevent the shock absorber oil from foaming. A circular steel mount containing a rubber bushing is attached to the bottom end of the lower chamber, and this lower mounting is bolted to the suspension system. The upper strut housing is connected to a piston rod that extends into the lower chamber. A piston valve assembly is attached to the lower end of the piston rod. The upper strut mount is similar to the lower mounting, and the upper mount is bolted to the chassis.

 

 

When a wheel strikes a road irregularity, the wheel and suspension move upward, and the spring in the suspension system is compressed. This action forces the lower shock absorber chamber to move upward, and the oil must flow from below the shock absorber valve to the area above the valve. Upward wheel movement is called jounce travel. The strut valves are designed to provide precise oil flow control, and thus control the speed of upward wheel movement. When a spring is compressed, it stores energy and then immediately expands with an equal amount of energy. When the spring expands, the tire and wheel assembly is forced downward. Under this condition, the lower strut chamber is forced downward, and oil must flow from above the shock absorber valve to the area below the valve (Figure 1-15). Downward wheel movement is called rebound travel. The strut valves provide precise control of the oil flow, and this action controls spring action and wheel oscillations. Shock absorbers and valves are usually designed to provide more control during the rebound travel compared to the jounce travel.

 

Internal strut design is similar to shock absorber design, but struts also support the top of the steering knuckle. In most suspension systems, the lower end of the strut is attached to the top of the steering knuckle, and a special mount is connected between the upper end of the strut and the chassis. On front suspension systems, the upper strut mount must allow strut and spring rotation when the front wheels are turned to the right or the left. The upper strut mount isolates wheel and suspension vibrations from the chassis.

 

 

 

Computer-Controlled Suspension Systems and Shock Absorbers

 

Many vehicles are equipped with computer-controlled suspension systems that provide a soft, comfortable ride for normal highway driving, and then automatically and very quickly switch to a firm ride for hard cornering, braking, or fast acceleration. Computer-controlled suspension systems reduce body sway during hard cornering, and thus contribute to improved ride quality and vehicle safety. Some computer-controlled suspension systems are driver-adjustable with up to four suspension modes to allow the driver to tailor the ride quality to the driving style. Some computer-controlled suspensions systems have electronically actuated solenoids in each shock absorber or strut. These solenoids rotate the shock absorber or strut valves to adjust the valve openings and shock absorber control. Other shock absorbers or struts contain a magneto-rheological fluid which is a synthetic oil containing suspended iron particles. A computer- controlled electric winding is designed into the shock absorber housing. When there is no current flow through the winding, the iron particles are randomly dispersed in the oil. Under this condition,

the oil consistency is thinner and the oil flows easily through the shock absorber valves to provide a softer ride. If the suspension computer supplies current flow to the shock absorber windings, the iron particles are aligned so the oil has a jelly-like consistency. This action instantly provides a much firmer ride. The computer can provide a large variation in current flow through the shock absorber windings and a wide range of ride control. Input sensors at each corner of the vehicle inform the suspension computer the velocity of the wheel jounce and rebound, and the computer uses these input signals to operate the shock absorber windings or actuators.

 

the shock absorber fluid in relation to the wheel jounce velocity before the wheel moves downward in the rebound stroke and strikes the road surface. Some computer-controlled suspension systems have air springs in place of coil springs. Front and rear height sensors inform the suspension computer regarding the suspension height, and the computer operates an air compressor and air spring control valves to control the amount of air in the air springs, and thus control suspension height. Some air suspension systems also have computer-controlled shock absorbers or struts.

 

Steering Systems

 

Steering systems are essential to provide vehicle safety, steering quality, and steering control! Steering system problems can cause the steering to pull to one side when driving straight ahead, excessive steering effort, wheel shimmy, or excessive steering wheel free-play. These problems all reduce vehicle safety and increase driver fatigue. Therefore, steering systems must be properly maintained.

 

 

Steering Columns and Steering Linkage Mechanisms

 

The steering column connects the steering wheel to the steering gear. The steering wheel is connected to the steering shaft, and this shaft extends through the center of the steering column. The lower end of the steering shaft is connected through a universal joint or flexible coupling to the shaft from the steering gear. The steering shaft is supported on bearings in the steering column. Some steering columns are designed to collapse or move away from the driver, if the driver is thrown against the steering wheel in a collision. Some steering columns are designed so the driver can tilt the steering wheel downward or upward to provide increased driver comfort and facilitate entering and exiting the driver’s seat. Some steering columns also provide a telescoping action so the steering wheel can be moved closer to, or farther away from, the driver. Other steering columns do not have any tilt or telescoping action.

 A mounting bracket retains the steering column to the instrument panel. On most vehicles, the ignition lock cylinder and ignition switch are mounted in the steering column. Removing the key from the ignition switch locks the steering column and the gear shift on many vehicles. The steering column usually contains a combination signal light, wipe/wash, dimmer, and cruise control switch. This switch may be called a smart switch. The switch for the hazard warning lights is also mounted in the steering column. An air bag inflator module is mounted in the top of the steering wheel, and a clock spring electrical connector under the steering wheel maintains electrical contact between the inflator module and the air bag electrical system. Steering linkages connect the steering gear to the steering arms on the front wheels. In a parallelogram steering linkage, a pitman arm is connected from the steering gear to a center link. A pivoted idler arm bolted to the chassis supports the other end of the center link. Tie rods are connected from the center link to the steering arms attached to the front wheels. Pivoted ball studs are mounted in the inner ends of the tie rods, and outer tie rod ends are threaded.

 

 

Recirculating Ball Steering Gears

 

Some vehicles are equipped with a recirculating ball steering gear, wherein the steering shaft is attached to a worm gear in the steering gear. A ball nut with internal grooves is mounted over the worm gear. Ball bearings are mounted between the worm gear and ball nut grooves to reduce friction and provide reduced steering effort. Outer grooves on the ball nut are meshed with matching teeth on the sector shaft. The lower end of the sector shaft is splined to the pitman arm. When the steering wheel is turned, the ball nut moves upward or downward on the worm gear, which rotates the sector shaft to provide the desired steering action. Recirculating ball steering gears can be manual-type with no hydraulic assist, or power-type with hydraulic assist from the power steering pump.

 

 

Rack and Pinion Steering Gears

 

Rack and pinion steering gears and linkages are more compact than recirculating ball steering gears and parallelogram steering linkages. Therefore, rack and pinion steering gears are usually installed on smaller, front-wheel drive vehicles. Rack and pinion steering gears transfer more road shock from the front wheels to the steering gear and steering wheel, because the tie rods are connected directly to the rack in the steering gear. In a rack and pinion steering gear, a toothed rack is mounted on bushings in the rack housing. The rack teeth are meshed with teeth on a pinion gear mounted near one end of the gear. The pinion gear is mounted on bearings in the gear housing. The steering shaft from the steering gear is attached to the upper end of the pinion gear. When the steering wheel is turned, the rotation of the pinion gear moves the rack inward or outward to provide the desired steering action. Rack and pinion steering gears can be manual-type or power assisted by fluid pressure from the power steering pump. Power rack and pinion steering gears have a piston near the center of the rack, and fluid pressure is supplied from the power steering pump to sealed chambers on either side of the rack piston to provide steering assistance.

 

 

Power Steering Pumps

A belt surrounding the crankshaft pulley and the power steering pump pulley drives the power steering pump. The power steering pump drive belt can be a V-type or a ribbed V-type. The ribbed V-belt contains a number of small, ribbed grooves on the underside and a flat upper side. The ribbed V-belt surrounds the pulleys on all the belt-driven components, allowing these components to be on the same vertical plane. This arrangement saves a considerable amount of under hood space. The smooth side of the ribbed V-belt can be used to drive some components. Some power steering pumps have an integral fluid reservoir; other power steering systems have a remote fluid reservoir. The pump supplies fluid through a high-pressure hose to the steering gear, and the fluid is returned from the steering gear through a low-pressure hose to the pump. In some power steering pumps, the pump shaft is connected from the pulley to the rotor, and a number of vanes are mounted in rotor slots. The rotor and vane assembly rotates in the center of an elliptical cam ring. As the vanes rotate inside the cam ring, the spaces between the vanes change in volume. When the spaces between the vanes become smaller, pressure is applied to the fluid because the fluid cannot be compressed. This pressurized fluid is forced through the pump outlet fitting and high-pressure hose to the steering gear. A flow control valve with an integral pressure relief valve controls pump pressure.

 

 

Electronic Power Steering Gears

 

Some vehicles have an electronic power steering gear in place of the hydraulic power steering gear and pump. An electronic power steering gear contains a computer-driven reversible electric motor. The electronic power steering gear is similar to a rack and pinion steering gear, but contains an electric motor in place of hydraulic controls. The armature in the electric motor is connected through a set of gears to a worm shaft, and a ball nut is mounted over the worm shaft. The armature is hollow, and the rack extends through the armature. The ball nut and worm shaft are similar to the ones in a recirculating ball steering gear. The ball nut is coupled to the rack, and fields in the motor surround the armature. When the computer supplies current to the armature, the armature rotates and turns the worm shaft. Worm shaft rotation moves the ball nut and helps to move the rack to the right or left to provide steering assistance. The power steering computer supplies current to the armature in the electric motor to help rotate the armature in the proper direction. Voltage signals are sent from a vehicle speed sensor (VSS) and steering wheel rotational sensor mounted on the steering gear pinion. The computer uses this information to provide the proper amount and direction of current to the steering gear armature.

 

 

Four-Wheel Steering

 

Some vehicles are equipped with four-wheel steering, wherein a separate computer controls a rear, electronic power steering gear. The electric motor on the rear steering gear drives the steering gear pinion. Input signals are sent to the rear steering computer from the front steering position sensor and the VSS. At low speeds, the computer and rear steering gear steer the rear wheels up to 12° in the opposite direction to the front wheels. The term negative-phase steering is applied to the mode when the rear wheels are steered in the opposite direction to the front wheels. This rear wheel steering action allows the vehicle to have a shorter turning circle and facilitates parking in small spaces. At higher vehicle speeds, the rear wheels are steered 1° in the same direction as the front wheels. The term positive-phase steering is applied to the mode when the rear wheels are steered in the same direction as the front wheels. This action reduces vehicle sideslip when changing lanes at higher speeds.

 

 

Wheel Alignment

Proper wheel alignment is extremely important to provide steering control, ride quality, and normal tire tread life. Improper wheel alignment may cause steering wander, steering pull to the right or left, or improper steering wheel return after turning a corner. Incorrect wheel alignment may contribute to harsh ride quality. Wheel alignment angles that are not within specifications may cause rapid tire tread wear

 

1. Rear Wheel Tracking
2. Rear Wheel Toe
3. Rear Wheel Camber
4. Front Wheel Camber
5. Front Wheel Caster

 

Steering Axis Inclination

Steering axis inclination (SAI) is the angle of a line through the centerline of the upper strut mount and the lower ball joint in relation to the true vertical centerline of the tire viewed from the front of the vehicle. The included angle is the sum of the SAI and camber angle. If the camber angle is negative, this angle must be subtracted from the SAI to calculate the included angle. When the SAI angle is tilted toward the center of the vehicle and the wheels are straight ahead, the height of the front spindles are raised closer to the chassis. This action allows gravity to lower the height of the vehicle. When the front wheels are turned, each knuckle moves through an arc that tends to force the tire into the ground. Because this reaction cannot occur, the chassis lifts when the front wheels are turned. When the steering wheel is released after a turn, the vehicle weight has a tendency to settle to its lowest point. Therefore, SAI helps return the wheels to the straight-ahead position after a turn, and also tends to maintain the front wheels in the straight-ahead position. However, SAI does increase steering effort, because the chassis has to lift slightly when cornering.