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ng is made of a length of round spring-steel rod wound into a coil (Fig 49-3). Figure 49-1 shows front and rear suspension systems using coil springs. Some coil springs are made from a tapered rod (Fig. 49-3). This gives the springs a variable spring rate (Ø49-5). As the spring is compressed, its resistance to further compression increases.
LEAF SPRING Two types of leaf springs are single-leaf and multileaf springs (Fig. 49-4). These have several flexible steel plates of graduated length, stacked and held together by clips. In operation, the spring bends to absorb road shocks. The plates bend and slide on each other to permit this action. Single-leaf springs are described in Ø49-13
TORTION BAR The torsion bar is a straight rod of spring steel, rigidly fastened at one end to the vehicle frame or body. The other end attaches to an upper or lower control arm (Fig. 49-5). As the control arm swings up and down in response to wheel movement, the torsion bar twists to provide spring action.
AIR SPRING The air spring (Fig 49-6) is a rubber cylinder or air bag filled with compressed air. A plastic piston on the lower control arm moves up and down with the lower control arm. This causes the compressed air to provide spring action. If the load in the vehicle changes, a valve at the top of the air bag opens to add or release air. An air compressor connected to the valve keeps the air springs inflated.
49-4 Sprung and unsprung weight
	The total weight of the vehicle includes the sprung weight and the unsprung weight. The sprung weight is the weight supported by springs. The unsprung weight is the part not supported by springs. This includes the weight of drive axles, axle shafts, wheels, and tires.
	The unsprung weight is kept as low as possible. The roughness of the ride increases as unsprung weight increases. To take an extreme example, suppose the unsprung weight equals the sprung weight. As the unsprung weight moves up and down, due to the wheels meeting road bumps and holes, the sprung weight would move up and down the same amount. For this reason, the unsprung weight should be only a small part of the total weight of the vehicle.
49-5 Spring rate
	The softness or hardness of a spring is its spring rate. This is the load required to move a spring a specified distance. The rate of a spring that compresses uniformly (a linear-rate spring) is the weight required to compress is 1 inch [25.4 mm]. If 600 pounds [272 kg] compresses the spring 3 inches [76 mm], then 1200 pounds [544 kg] will compress it 6 inches [152 mm].
	Variable-rate springs do not move or deflect at a constant or linear rate. The coil spring in Fig 49-3 is one type of variable-rate spring. Winding the coils from a tapered rod provides the variable rate. The spring rate varies from an initial 72.2 pounds per inch [1.29 kg/mm] to 163.5 pounds per inch [2.92 kg/mm]. Other variable-rate coil springs have the coils closer together at the top than at the bottom, or are wound in a cone or barrel shape.
50-1 Purpose of the steering system
	The steering system (Figs. 49-18 and 49-22) allows the driver to control the direction of vehicle travel. This is made possible by linkage that connects the steering wheel to the steerable wheels and tires. The steering system may be either manual or power. When the only energy source for the steering system is the force the driver applies to the steering wheel, the vehilcle has manual steering. Power steering uses a hydraulic pump or electric motor to assist the driver’s effort. Most vehicles have power steering to make parking easier.
	The basic operation is the same for both manual and power steering. As the driver turns the steering wheel, the movement is carried to the steering gear (Fig. 50-1). It changes the rotary motion of the steering wheel into straightline or linear motion. The linear motion acts through steering linkage or tie rods attached to the steering-knuckle arms (Ø49-19) or steering arms. The steering knuckles then pivot inward or outward on ball joints (Ø49-20). This moves the wheels and tires to the left or right for steering.
52-1 Automotive brakes
 Figure 52-1 shows the brake system in an automobile. It has two types of brakes:
The service brakes, operated by a food pedal, which slow or stop the vehicle.
The parking brakes, operated by a food pedal or hand lever, which hold the vehicle stationary when applied.
Most automotive services brakes are hydraulic brakes. They operate hydraulically by pressure applied through a liquid. The service or foundation brakes on many medium and heavy-duty trucks and buses are oprated by air pressure (pneumatic). These are air brakes. Many boat and camping trailers have electric brakes. All these braking system depend on friction( 52-2) between moving parts and stationary parts for their stopping force.
52-15 Types of disc brakes
The disc brake (fig 52-17) has a metal disc or rotor instead of a drum. It uses a pair of flat, lined shoes or pads that are forced against the rotating disc to produce braking. The pads are held in a caliper (figs 52-17 and 52-18) that straddles the disc. The caliper has one or more pistons, with a seal and dust boot for each. During braking, hydraulic pressure behind each piston in fig. 52-17 pushes it outward. This forces the pad into contact with the disc. The resulting frictional contact slows and stops the disc and wheel.
There are three types of disc brakes. Figure 52-17 shows a fixed-caliper disc brake. The other two are the floating-caliper and sliding-caliper. Each differs in how the caliper mounts and operates.
 Note: All three types of disc brakes work in the same general way. However, vehicle manufacturers have used many variations of each. Typical examples are described below. Refer to the vehicle service manual for information about the brakes on a specific vehicle.
fixed-caliper disc brake: A fixed caliper (figs 52-17 and 52-19A) has pistons on both sides of the disc. Some use two pistons, one on each side. Others use four pistons with two on each side. The caliper is rigidly attached to a steering knuckle or other stationary vehicle part. Only the pistons and pads move when the brakes are applied.
floating-caliper disc brake: A typical floating caliper (fig 52-19B and 52-20) has only one piston, located on the inboard side of the disc. The caliper moves or “floats” on rubber bushings on one or two steel guide pins. The bushings allow the caliper to move slightly when the brakes are applied. Some floating calipers have two pistons on the inboard side of the disc.
Applying the brakes causes brakes causes brake fluid to flow into the caliper (fig 52-21). This pushes the piston outward so the inboard shoe is forced against the disc. At the same time, the pressure pushes against the caliper with an equal and opposite force. This reaction causes the caliper to move slightly on the bushings, bringing the outboard shoe into contact with the disc. The two pads clamp the disc to produce the braking action.
sliding-caliper disc brake: Figure 51-12 shows a sliding-caliper disc brake. It is similar to the floating-caliper brake. Both calipers move slightly when the brakes are applied. However, the sliding caliper slides on machined surfaces on the steering-knuckle adapter or anchor plate. No guide pins are used.
53-1 Purpose of antilock braking
Tires skid when they slow or decelerate faster than the vehicle. One way to help prevent skidding is to keep the brakes from locking. This is the purpose of the antilock-braking system (ABS). During normal braking, the antilock-braking system (fig.53-1) has no affect on the service brakes. However during hard or severe braking, the antilock-braking system prevent wheel lockup.	
The system allows the brakes to apply until the tires are almost starting top skid. Then the antilock-braking system can vary or modulate the hydraulic pressure to the brake at each wheel. This “pumping the brakes” keeps the rate of wheel deceleration below the speed at which the wheels can lock.
53-2 Operation of the antilock-braking system
Figure 53-1 shows a vehicle equipped with a vacuum brake booster (52-32) and four-wheel antilock brakes.The brake lines from the master cylinder connect to a hydraulic unit or actuator. Lines from the actuator connect to the wheel brakes. The actuator is controlled by the ABS control module.
Wheel-speed sensors (fig.53-1 and 53-2) at each wheel continuosly send wheel-speed information to the ABS control module. There is ABS action until the stoplight switch signals the control module that the brake pedal has been depressed. When the control module senses a rapid drop in wheel speed, it signals the actuator to adjust or modulate the brake pressure to that wheel.This prevents wheel lockup.
53-9 Purpose of traction control
Any time a tire is given more torque than it can transfer to the road, the tire loses traction and spins. This usually occurs during acceleration. To prevent unwanted wheelspin, some vehicles with ABS also have a traction-control system (TCS). When a wheel is about to spin. The traction-control system (Fig.53-10) applies the brake at that wheel. This slows the wheel until the chance of wheel spin has passed.
53-10 Operation of traction-control system
The antilock-braking system and traction-control system share many parts. The wheel-speed sensors report wheel speed to the ABS/TCS control module (Fig. 53-10). When a wheel slows so quickly that it is about to skid, the ABS holds or releases the brake pressure at that wheel. If wheel speed increases so quickly that the wheel is about to spin, the TCS applies the brake at that wheel. This slows the wheel and prevent wheel spin.
The TCS can also reduce engine speed and torque if braking alone does not prevent wheelspin. When this is necessary, the ABS/TCS control module signals the engine control module. It then retards the spark and reduces the amount of fuel delivered by the fuel injectors.