The Suspension System
The Suspension System has become more advanced through the years. These advances have been made to provide better and safer handling and a better ride. Today, front and rear suspensions have many parts and can be quite complex. As a vehicle moves, the suspension and tires must react to the current driving conditions.
Specifically, the suspension system:
Supports the weight of the vehicleKeeps the tires in contact with the roadControls the direction of the vehicle’s travelAttempts to maintain the correct vehicle ride heightMaintains proper wheel alignmentReduces the effect of shock forces as the vehicle travels on an irregular surfaceFrames 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.
Th e 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.
Th e unitized body provides a steel box around the passenger compartment to provide passenger protection in a collision. 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. Th e 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 .
Vehicle weight plays a signifi cant role in fuel consumption. One automotive design e ngineer states that “Fuel economy improvements are almost linear with weight reduction. A 30 percent reduction in vehicle weight provides approximately a 30 percent improvement in fuel economy.” If a Toyota Prius weighs 3,300 lb (1,497 kg) and provides 50 miles per gallon (mpg), the same Prius would provide 55 mpg if it weighed 3,000 lb (1,360 kg).
Carbon dioxide (CO2) emissions are a major concern for automotive manufacturers, because CO2 is a greenhouse gas that contributes to global warming. Vehicle manufacturers are facing increasingly stringent CO2 emission standards.
CO2 emissions are proportional to fuel consumption.
Reduced fuel consumption results in lower CO2 emissions.
Th erefore, reducing vehicle weight results in less fuel consumption and lower CO2 emissions.
Reduced weight also contributes to improved vehicle performance.
Th e unitized body provides a steel box around the passenger compartment to provide passenger protection in a collision. 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. Th e 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 (Figure 1-3). 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 .
Vehicle weight plays a signifi cant role in fuel consumption. One automotive design e ngineer states that “Fuel economy improvements are almost linear with weight reduction.
A 30 percent reduction in vehicle weight provides approximately a 30 percent improvement in fuel economy.” If a Toyota Prius weighs 3,300 lb (1,497 kg) and provides 50 miles per gallon (mpg), the same Prius would provide 55 mpg if it weighed 3,000 lb (1,360 kg). Carbon dioxide (CO2) emissions are a major concern for automotive manufacturers, because CO2 is a greenhouse gas that contributes to global warming.
Vehicle manufacturers are facing increasingly stringent CO2 emission standards.
CO2 emissions are proportional to fuel consumption.
Reduced fuel consumption results in lower CO2 emissions. Th erefore, reducing vehicle weight results in less fuel consumption and lower CO2 emissions. Reduced weight also contributes to improved vehicle performance.
Front Suspension Systems
The front and rear suspension systems are extremely important to provide proper wheel position, steering control, ride quality, and tire life. Th e impact of the tires striking road irregularities must be absorbed by the suspension systems.
Th e 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 .
Th is type of front suspension system has a long lower control arm and a shorter upper control arm. Th e 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 .
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.
Th is 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 aff ects the opposite rear wheel. Some semiindependent 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 aff ecting 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 diff erent confi gurations.
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 .
Shock Absorbers and Struts
Each corner of the vehicle has a shock absorber or strut connected from the suspension s ystem to the chassis.
Shock absorbers control spring action and wheel oscillations to p rovide 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 fi lled 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 . Th e 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 fl ow from below the shock absorber piston and valve to the area above the valve. Upward wheel movement is called jounce travel.
The strut valves are designed to provide precise oil fl ow 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 fl ow from above the shock absorber piston and valve to the area below the valve .
Downward wheel movement is called rebound travel. The strut valves provide precise control of the oil fl ow, 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 (Figure 1-16).
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 fi rm ride for hard cornering, braking, or fast acceleration.
Computer-controlled s uspension systems reduce body sway during hard cornering, and thus contribute to improved ride quality and vehicle safety. Some computer-controlled suspension systems are driveradjustable 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 fl uid 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 fl ows easily through the shock absorber valves to provide a softer ride.
If the suspension computer supplies current fl ow 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.
Th e computer can provide a large variation in current fl ow 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.
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.
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