Radiator Glossary of Terms
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R-12
The automotive term for dichlorodifluoromethane, also known as "Freon," a type of manmade CFC refrigerant used in all 1992 and earlier automotive A/C systems. R12 is being phased out because of its harmful effects on the ozone layer when it leaks or is ve
R-134A
The automotive term for tetrafluoroethane, also known as "SUVA," a manmade refrigerant that contains no chlorine and is considered to be "ozone safe." Used in most 1995 and newer automotive A/C systems.
RABS
Rear Antilock Brake System. Ford's name for rear wheel ABS. See RWAL and Rear Wheel ABS.
Race
The surface on the inner or outer ring of a bearing on which the balls, needles or rollers move.
Rack & Pinion Steering
A type of lightweight steering gear that uses a worm-like gear (the pinion) to drive a horizontal bar (the rack). The primary advantage of rack & pinion steering is that it is lightweight and uses fewer parts than a reciprocating ball steering gear.
Radial
The tire construction utilizing plies that run radially from bead to bead under the tread. This construction requires a belt to stabilize the tread and define the tire diameter.
Radial Runout
Variation (out-of-round) in the radius or circumference of a wheel or tire. It is measured by placing a dial indicator on the inside edge of the rim or tire tread. Too much radial runout can cause up-and-down vibrations similar to those caused by a static
Radial Tire
A type of tire that's constructed with the reinforcing belts sideways under the tread rather than lengthwise. This makes the tire more flexible which reduces rolling resistance to improve fuel economy (See Tire Ratings). A radial tire can be identified by
Radiator
The part of the cooling system that gets rid of the engine heat. Coolant from the engine flows past the thermostat and into the radiator where it is cooled by air passing through the fins. Internal corrosion and hairline cracks caused by vibration are the
Rear Axle Steer
A steering pull or lead to one side caused by misalignment of the rear wheels or axle. Misalignment creates a thrust angle that causes the vehicle to lead to one side resulting in an off-center steering wheel and accelerated toe wear in the front tires.
Rear Toe
The toe setting of the rear wheels. Rear toe is not adjustable on rear-wheel drive cars with solid axle housings but is adjustable on many front-wheel drive cars and minivans. If rear toe is unequal, it can produce a diagonal wear pattern (heel and toe we
Rear Wheel ABS
A type of ABS system that only involves the rear wheels. Commonly used on pickup trucks and vans, rear-wheel ABS provides skid control with varying vehicle loads. This type of ABS system uses only a single speed sensor in the transmission or differential
Rear-Wheel Drive (RWD)
A method of driving a vehicle whereby engine power is applied to the rear wheels. Power from the engine flows through the transmission, down the drive shaft, through the differential to the rear axles and wheels.
Rebound
This term has nothing to do with basketball. What it refers to is the suspension springing back after it's been momentarily compressed (See Jounce).
Rebuilt Parts
Rebuilt parts are those that have been salvaged and reconditioned to good-as-new condition. Rebuilt parts include alternators, starters, water pumps, clutches, brake calipers, brake shoes, master brake cylinders and fuel pumps. Savings compared to equival
Receiver-Dryer
A container for storing liquid refrigerant from the condenser. This component also contains a bag of desiccant that absorbs small amounts of moisture from the refrigerant.
Recirculating Ball Steering
A type of steering gear normally used with a parallelogram steering linkage. So named because of the ball bearings that are recirculated in the gear box between the worm and sector gears to reduce friction.
Recovery & Recycling
A mandatory requirement for all facilities that perform A/C service work. Venting refrigerant into the atmosphere is no longer permitted. All refrigerant (R12 or R134a) must be recovered from the vehicle's A/C system prior to opening the system for repair
Reference Voltage
In computerized engine management systems, a five volt signal sent out from the computer to a variable resistance sensor such as a TPS. The computer then reads the voltage value of the return signal. Called "V-ref."
Refrigerant
The working agent in an A/C system that absorbs, carries and releases heat. The two primary automotive refrigerants are R12 and R134a, but many other substances have similar properties (primarily a low boiling temperature) that allow them to be used as "a
Refrigeration
The removal of heat by mechanical means.
Refrigeration Cycle
The complete course of refrigerant back to its starting point. During the refrigeration cycle, refrigerant circulates through the system changing temperature, pressure and physical state (liquid & vapor). This allows heat to be absorbed from air entering
Regulator
A device that maintains the amperage and/or voltage levels of a circuit at predetermined values.
Relay
A switch that automatically opens and/or closes a circuit.
Reservoir
Chamber connected to the master cylinder (usually by hoses) and used for storing hydraulic fluid.
Resistance
The opposition to the flow of current through a circuit or electrical device, and is measured in ohms. Resistance is equal to the voltage divided by the amperage.
Resistor
A device, usually made of wire, which offers a preset amount of resistance in an electrical circuit.
Retrofit
To replace an older component, system or refrigerant with a newer one. With respect to A/C systems, retrofit refers to replacing R12 with R134a. Changing refrigerants requires changing compressor lubricants and service fitting, and may also require other
Revolutions Per Mile
The measured number of revolutions made by a tire traveling one mile.
Rheology
The study of fluid-flow properties.
Ride Height
The distance between a specified point on the chassis, suspension or body and the ground. Measuring ride height is an indirect method of determining spring height, which is important because it affects camber, caster and toe. Low ride height indicates wea
Rim
The portion of a wheel incorporating the well, seats, and flange onto which a tire is mounted.
Rim Diameter
The diameter of the rim bead seats that support a tire, normally indicated in whole numbers in inches for passenger cars.
Rim Width
The distance between rim flanges.
Ring Gear
The name given to a ring-shaped gear attached to a differential case, or affixed to a flywheel or as pad of a planetary gear set.
Road Crown
The slope of a road surface to the outside for proper drainage. Excessive road crown can cause a vehicle to lead to the right. Reducing caster on the left front wheel is sometimes used to compensate for road crown.
Roller Bearing
A bearing made up of hardened inner and outer races between which hardened steel rollers move.
Rotation
The systematic movement of tires from one vehicle position to another to maximize tread life and minimize irregular wear.
Rotor
1. The disc-shaped part of a disc brake assembly, upon which the brake pads bear; also called, brake disc. 2. The device mounted atop the distributor shaft, which passes current to the distributor cap tower contacts.
RPM
Abbreviation for Revolutions Per Minute. Engine speed is often expressed as so many rpm.
Runout
The measure of the out of roundness of the tire causing a vibration that cannot be balanced.
Run-Out
Rotors which are warped or out-of-true have excess "run-out", meaning the surface varies or wobbles as it rotates around a fixed point.
Rust And Corrosion Inhibitors
The additives that protect lubricated surfaces from rusting and corrosion.
Rustproofing
The process of applying rust-inhibiting chemicals, waxes or sealers to the underside and inside of the vehicle's body as well as any other rust-prone areas. Not to be confused with undercoating which treats only the underside of the vehicle. Commercial ru
RWAL
Rear Wheel Anti-Lock brakes. A term used by General Motors and Chrysler for rear-wheel antilock braking.
Rectifier
A part of an alternator that used diodes to convert alternating current into direct current. It usually consists of three pairs of diodes.
Reduction Catalyst
The section of a three-way catalytic converter that breaks NOx down into harmless nitrogen and oxygen through a reduction reaction.

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