In order to offer the operator a significant amount of control over the vehicle, certain systems must be installed. The drive train connects the engine to the wheels while also providing various speeds for the driver. The brake system gives the operator a way to slow or stop the vehicle and the steering system lets the driver select or change the direction and course of the vehicle.
The Drive Train
- A Clutch or Torque Converter
- A Transmission and Differential
- A Series of Drive Shafts
A clutch is used with a manual transmission while a torque converter is used with an automatic transmission. The clutch is operated through the use of a foot pedal and is controlled by the driver. When the clutch is engaged the engine is connected to the transmission, when the clutch is disengaged the engine can continue to run even though the vehicle maybe at a stand still.
A torque converter engages and disengages itself depending on the speed of the engine. A torque converter is a fluid coupling and is divided into two (2) halves. Fluid pressure in one half of the torque converter generated by the speed of the engine forces the other half of the torque converter to begin turning. The vehicle speed increases until the two (2) halves are turning at the same speed. When the engine is slowly running (idling) at about 1,000 R.P.M. (revolutions per minute) the vehicle can be held at a stand still however as the engine speed is increased the vehicle begins to move.
A transmission or gear box is used to provide the driver with different speeds through the use of a series of gears. An automatic transmission changes gears on its own while the gears of a manual transmission must be changed by the driver. Like the gears on a bicycle the driver selects the appropriate gear for the road. When the bicycle is stationary the large sprocket on the back wheel provides the rider with enough torque to get the bicycle moving without too much effort on the rider’s part however the bicycle does not move very fast. Once the bicycle is in motion, the rider selects the next smaller sprocket. This allows the rider to gain more speed without any extra effort. As the bicycle moves fast the rider continues to select the next smaller sprocket until the chain is on the smallest sprocket (top speed). As the bicycle is slowing down the rider must reverse the process and start moving towards the larger sprocket until the chain is on the largest sprocket and the bicycle is stopped once again.
A differential allows the left and right drive wheels to turn at different speeds when turning. This is necessary because the outer wheel must travel a greater distance than the inner wheel when turning. As the inner wheel slows down the outer wheel speeds up by the same amount. If the inner wheel is stopped then the outer wheel will be traveling at twice the speed indicated on the speedometer.
The drive shafts connect the transmission and/or differential(s) to the drive wheels depending on if the vehicle is front wheel drive, rear wheel drive or 4-wheel drive. For front wheel drive the transmission and differential are combined in a single unit called a transaxle and therefore there are only two (2) drive shafts needed, one for the left wheel and one for the right. With rear wheel drive there are three (3) drive shafts necessary, one main drive shaft connects the transmission to the differential at the rear of the vehicle and the other two (2) shafts connect to the left and right drive wheels. 4-wheel drive usually requires a transfer case, two (2) differentials and six (6) drive shafts. The transfer case is mounted on to the transmission with two (2) of the drive shafts connected to it, one for the front wheels and one for the rear. These drive shafts then connect to a differential which then feeds the corresponding drive wheels.
The Hydraulic Brake System
The hydraulic brake system uses the principle of hydraulics to transmit the motion and amplify the pressure exerted by the driver on the brake pedal to the individual braking components at each of the wheels. The main components of the system are:
- The master cylinder and reservoir
- The front brake callipers, brake pads and rotors
- The rear wheel cylinders, brake shoe assembly and drums
The master cylinder is located directly in front of the driver in the engine compartment and is operated through the use of the brake pedal. The pedal is connected to a plunger inside the master cylinder where pressure is applied to the fluid and transmitted to the wheel units by brake lines and hoses. The front brakes handle approximately 70% of the braking while the rear brakes handle only 30% therefore the front have disc brakes while the rear use drum brakes. Disc brakes are capable of producing a much greater braking force than drum. Disc brakes consist of a brake caliper, pads and rotor. The brake caliper and piston clamps the brake rotor much the same way the brakes on a bicycle grab the rim of the wheel. The brake pads allow for wear due to the friction needed to stop the vehicle. The drum brakes consist of a wheel cylinder, brake shoes and a drum. The wheel cylinder is located at the top of the assembly and has two (2) pistons which force the brake shoes outward. The shoes contact the brake drum generating the friction necessary to stop the vehicle.
The Steering System
The rack and pinion steering system is one of the most popular systems available. Each front wheel turns on its own axis and is connected to the steering unit by a steering arm and tie rod. The steering arms are bent inward to allow the inner wheel to turn at a greater angle than the outer wheel when turning. The movement of these parts is controlled by the steering wheel and shaft which has a pinion gear at the end. The pinion gear is engaged with a steering rack which slides either left or right depending on which way the pinion gear is rotated.
Key Terms and Definitions
- Brake Caliper: A hydraulic unit which forms the cylinder and contains the piston. It provides braking by producing a clamping action on the brake rotor.
- Brake Pad: A replaceable friction surface used with disc brakes.
- Brake Rotor: A metal disc, mounted on the wheel, against which brake pads are forced in order to stop the vehicle.
- Brake Shoe: Curved replaceable friction surfaces used with drum brakes.
- Brake Drum: Cast iron housing mounted on the wheel. The brake shoes are forced out against the inner surface exerting a braking effect upon wheel.
- Clutch: A device that allows the driver to engage and disengage the engine from the transmission.
- Differential: A unit that allows the wheels to turn at different speeds when turning.
- Drive Shaft: A steel shaft or tube that connects the transmission to the differential or drive wheels.
- Drive Train: All the collective parts that transmit power/motion to the wheels.
- Fluid Coupling: A unit that transfers power/motion through the use of fluid.
- Hydraulics: The use of fluids to create motion.
- Master Cylinder: A unit that generates pressure in a hydraulic brake system.
- Reservoir: A tank or bottle used to hold excess fluid.
- Torque Converter: A fluid coupling used in an automatic transmission.
- Transaxle: A combination of transmission and differential in one case used in front wheel drive vehicles.
- Transmission: A device that uses gearing to alter torque and speed output by changing the ratio between the engine R.P.M. and the drive wheels R.P.M.
- Transfer Case: A gear box driven by the transmission that transmits motion to both front and rear wheels; Used on 4-wheel drive vehicles
- Wheel Cylinder: A hydraulic unit used in drum brakes to actuate the brake shoes
Safety is an on going concern in the shop and should never be taken lightly. When inspecting and/or servicing a vehicle or any of its components always adhere to both the general shop safety rules as well as any additional safety rules specific to that area.
- Transmissions are extremely heavy and should never be carried alone. When removing or installing a transmission be sure to have someone help you.
- Transmissions have sharp edges and should be handled with care. Never run your hands freely across any surfaces without some form of protection (work gloves or a rag).
- Brakes are capable of producing a tremendous amount of heat. When inspecting or servicing the brakes, use caution, as the wheel and brake components may not have had sufficient time to cool.
- Never use compressed air to blow brake dust off a wheel brake assembly. Use a special brake vacuum to remove unwanted dust. Older brake linings were made of asbestos, which is carcinogenic.
- Only use approved brake fluid in the brake system (DOT 3 or 4). Any other type of fluid may cause a sudden brake failure by destroying the rubber seals in the brake system.
- Avoid using hammers while removing and/or installing brake components. Many parts are cast and could develop stress cracks resulting in failure during operation.