Automotive Drivetrain; Running Gear

DRIVETRAIN

The drivetrain is made up of all components that transfer power from the engine to the driving wheels of the vehicle. The exact components used in a vehicle's drivetrain depend on whether the vehicle is equipped with rear-wheel drive, front-wheel drive, or four-wheel drive.

Today, most cars are front-wheel drive (FWD). Some larger luxury and performance cars are rear-wheel drive (RWD). Most pickup trucks, mini vans, and SUVs are also RWD vehicles. Power flow in an RWD vehicle passes through the clutch or torque converter, manual or automatic transmission, and the driveline (drive shaft assembly). Then it goes through the rear differential, the rear-driving axles, and onto the rear wheels.

Power flow through the drivetrain of FWD vehicles passes through the clutch or torque converter, then moves through a front differential, the driving axles, and onto the front wheels.

Four-wheel-drive (4WD) or all-wheel-drive (AWD) vehicles combine features of both rear- and front wheel-drive systems so that power can be delivered to all wheels either on a permanent or on-demand basis. Typically if a truck, pickup or SUV, has 4WD, the system is based on an RWD and a front drive axle is added. When a car has AWD or 4WD, the drivetrain is a modified FWD system. Modifications include a rear drive axle and an assembly that transfers some of the power to the rear axle.

|||| 0 The major components of a clutch assembly for a manual transmission. Flywheel; Clutch disc; Release bearing and hub; Clutch fork and linkage; Clutch housing; Clutch fork pivot; Pressure plate assembly; Crankshaft

Clutch

A clutch is used with manual transmissions/transaxles.

It mechanically connects the engine's flywheel to the transmission/transaxle input shaft. This is accomplished by a special friction plate that is splined to the input shaft of the transmission. When the clutch is engaged, the friction plate contacts the flywheel and transfers power to the input shaft.

When stopping, starting, and shifting from one gear to the next, the clutch is disengaged by pushing down on the clutch pedal. This moves the clutch plate away from the flywheel, stopping power flow to the transmission. The driver can then shift gears without damaging the transmission or transaxle. Releasing the clutch pedal reengages the clutch and allows power to flow from the engine to the transmission.

Manual Transmission

A manual or standard transmission is one in which the driver manually selects the gear of choice. Proper gear selection allows for good driveability and requires some driver education.

Whenever two or three gears have their teeth meshed together, a gearset is formed. The movement of one gear in the set will cause the others to move. If any of the gears in the set are a different size than the others, the gears will move at different speeds. The size ratio of a gearset is called the gear ratio of that gearset.

A manual transmission houses a number of individual gearsets, which produce different gear ratios. The driver selects the desired operating gear or gear ratio. A typical manual transmission has four or five forward gear ratios, neutral, and reverse.

Automatic Transmission

An automatic transmission does not need a clutch pedal and shifts through the forward gears without the control of the driver. Instead of a clutch, it uses a torque converter to transfer power from the engine's flywheel to the transmission input shaft. The torque converter allows for smooth transfer of power at all engine speeds.

Shifting in an automatic transmission is controlled by a hydraulic and/or electronic control system. In a hydraulic system, an intricate network of valves and other components use hydraulic pressure to control the operation of planetary gearsets. These gearsets provide the three or four forward speeds, neutral, park, and reverse gears normally found in automatic transmissions. Newer electronic shifting systems use electric solenoids to control shifting mechanisms.

Electronic shifting is precise and can be varied to suit certain operating conditions. All automatic transmission-equipped vehicles with OBD II have electronic shifting.

Driveline

Drivelines are used on RWD vehicles and 4WD vehicles. They connect the output shaft of the transmission to the gearing in the rear axle housing. They are also used to connect the output shaft to the front and rear drive axles on a 4WD vehicle.

A driveline consists of a hollow drive or propeller shaft that is connected to the transmission and drive axle differential by universal joints (U-joints). These U-joints allow the drive shaft to move with movement of the rear suspension, preventing damage to the shaft.

|||| 1 A common manual transaxle.

|||| 2 A cutaway of a six-speed automatic transmission shown with the torque converter in the housing.

|||| 3 The driveline connects the output from the transmission to the differential unit and drive axles.

Differential

On RWD vehicles, the drive shaft turns perpendicular to the forward motion of the vehicle. The differential gearing in the rear axle housing is designed to turn the direction of the power so that it can be used to drive the wheels of the vehicle. The power flows into the differential, where it changes direction, then flows to the rear axles and wheels.

The gearing in the differential also multiplies the torque it receives from the drive shaft by providing a final gear reduction. Also, it divides the torque between the left and right driving axles and wheels so that a differential wheel speed is possible. This means one wheel can turn faster than the other when going around turns.

Driving Axles

Driving axles are solid steel shafts that transfer the torque from the differential to the driving wheels.

A separate axle shaft is used for each driving wheel.

In an RWD vehicle, the driving axles are part of the differential and are enclosed in an axle housing that protects and supports these parts. Some rear drive axle units are mounted to an independent suspension and the drive axle assembly is similar to that of a FWD vehicle.

Each drive axle is connected to the side gears in the differential. The inner ends of the axles are splined to fit into the side gears. As the side gears are turned, the axles to which they are splined turn at the same speed.

The drive wheels are attached to the outer ends of the axles. The outer end of each axle has a flange mounted to it. A flange is a rim for attaching one part to another part. The flange, fitted with studs, at the end of an axle holds the wheel in place. Studs are threaded shafts, resembling bolts without heads. One end of the stud is screwed or pressed into the flange.

The wheel fits over the studs, and a nut, called the lug nut, is tightened over the open end of the stud. This holds the wheel in place.

The differential carrier supports the inner end of each axle. A bearing inside the axle housing supports the outer end of the axle shaft. This bearing, called the axle bearing, allows the axle to rotate smoothly inside the axle housing.

Transaxle

A transaxle is used on FWD vehicles. It’s made up of a transmission and differential housed in a single unit. The gearsets in the transaxle provide the required gear ratios and direct the power flow into the differential. The differential gearing provides the final gear reduction and splits the power flow between the left and right drive axles.

The drive axles extend from the sides of the trans axle. The outer ends of the axles are fitted to the hubs of the drive wheels. Constant velocity (CV) joints mounted on each end of the drive axles allow for changes in length and angle without affecting the power flow to the wheels.

Four-Wheel-Drive System

4WD or AWD vehicles combine the features of RWD transmissions and FWD transaxles. Additional transfer case gearing splits the power flow between a differential driving the front wheels and a rear differential that drives the rear wheels. This transfer case can be a housing bolted directly to the transmission/transaxle, or it can be a separate housing mounted somewhere in the driveline. Most RWD-based 4WD vehicles have a drive shaft connecting the output of the transmission to the rear axle and another connecting the out put of the transfer case to the front drive axle. Typically, AWD cars have a center differential, which splits the torque between the front and rear drive axles.

|||| 4 A cutaway of an automatic transaxle.

RUNNING GEAR

The running gear of a vehicle includes those parts that are used to control the vehicle, which includes the wheels and tires and the suspension, steering, and brake systems.

Suspension System

The suspension system includes such components as the springs, shock absorbers, MacPherson struts, torsion bars, axles, and connecting linkages. These components are designed to support the body and frame, the engine, and the drivelines. Without these systems, the comfort and ease of driving the vehicle would be reduced.

Springs or torsion bars are used to support the axles of the vehicle. The two types of springs commonly used are the coil spring and the leaf spring.

Torsion bars are also used and are long spring steel rods. One end of the rod is connected to the frame, whereas the other end is connected to the movable parts of the axles. As the axles move up and down, the rod twists and acts as a spring.

Shock absorbers dampen the upward and downward movement of the springs. This is necessary to limit the car's reaction to a bump in the road.

Steering System

The steering system allows the driver to control the direction of the vehicle. It includes the steering wheel, steering gear, steering shaft, and steering linkage.

Two basic types of steering systems are used today: the rack-and-pinion and recirculating ball systems. The rack-and-pinion system is commonly used in passenger cars. The recirculating ball system is normally used only on pickup trucks, SUVs, and full-size luxury cars.

Steering gears provide a gear reduction to make changing the direction of the wheels easier. On most vehicles, the steering gear is also power assisted to ease the effort of turning the wheels. In a power assisted system, a pump provides hydraulic fluid under pressure to the steering gear. Pressurized fluid is directed to one side or the other of the steering gear to make it easier to turn the wheels.

Some vehicles are equipped with speed-sensitive power steering systems. These change the amount of power assist according to vehicle speed. Power assist is the greatest when the vehicle is moving slowly and decreases as speed increases.

|||| A strut assembly of a typical suspension system, by Ford Motor Company.

|||| A typical hydraulic brake system with disc brakes at the front and rear wheels. Front brake caliper Front brake hoses Rear brake hose Combination valve Master cylinder Rear wheel cylinder Brake lines

|||| (A) A parallelogram-type steering sys tem. (B) A rack and pinion steering system. Federal-Mogul Corporation

|||| A disc brake unit with a wheel speed sensor for ABS. Chrysler

|||| An alloy wheel with high-performance tires.

|||| The electric motor in this hybrid arrangement fits between the engine and the transmission.

Brakes

Obviously, the brake system is used to slow down and stop a vehicle. Brakes, located at each wheel, use friction to slow and stop a vehicle.

The brakes are activated when the driver presses down on the brake pedal. The brake pedal is connected to a plunger in a master cylinder, which is filled with hydraulic fluid. As pressure is put on the brake pedal, a force is applied to the hydraulic fluid in the master cylinder. This force is increased by the master cylinder and transferred through brake hoses and lines to the four brake assemblies.

Two types of brakes are used-disc brakes and drum brakes. Many vehicles use a combination of the two types: disc brakes at the front wheels and drum brakes at the rear wheels; others have disc brakes at all wheels.

Most vehicles have power-assisted brakes. Many vehicles use a vacuum brake booster to increase the pressure applied to the plunger in the master cylinder. Others use hydraulic pressure from the power steering pump to increase the pressure on the brake fluid. Both of these systems lessen the amount of pressure that must be applied to the brake pedal and increase the responsiveness of the brake system.

Nearly all late-model vehicles have an antilock brake system (ABS). The purpose of ABS is to prevent skidding during hard braking to give the driver control of the vehicle during hard stops.

Wheels and Tires

The only contact a vehicle has with the road is through its tires and wheels. Tires are filled with air and made of forms of rubber and other materials to give them strength. Wheels are made of metal and are bolted to the axles or spindles. Wheels hold the tires in place. Wheels and tires come in many different sizes. Their sizes must be matched to one another and to the vehicle.

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