4x4 Glossary

4 x 4 – This general term is used to describe a vehicle with four-wheel drive. The first figure is the number of wheels, and the second is the number of powered wheels.

4x2 – This term refers to a two-wheel drive vehicle with four wheels.

Angle of Approach – When viewed from the side, this is the angle between the ground and a line running from the tyre to the front bumper or other low-hanging component. This angle gives an indication of how steep a ramp a vehicle can negotiate without damage. It gives a good indication of a vehicle’s ability to climb over rocks and logs without hitting and potentially damaging the front part of the vehicle’s body or chassis.

Angle of Departure – In side view, this is the angle between the ground and a line running from the rear tyre to the rear bumper or other low-hanging bodywork. Similar to the approach angle, the departure angle indicates a vehicle’s ability to drive off a ramp or obstacle without damaging the rear.

Anti-Lock Braking System (ABS) – This braking system senses any significant difference in wheel speed, from one wheel to another, when a vehicle is braking hard. When any of the wheels begin to lock up (completely stop rotating), ABS automatically reduces the braking forces to that wheel or wheels in order to keep all the wheels rolling – to prevent break-induced skidding. ABS can control all four wheels (cars and most 4x4s have this system) or any two. The system can group wheels together in "channels" of operation (i.e. a three-channel ABS system on a four-wheeled vehicle) or have one channel for each wheel (four-channel ABS).

Anti-Roll Bar – This suspension element is a long torsion bar (essentially a rod that can twist) mounted across the vehicle from one wheel to the wheel on the opposite side. By transferring movement and force from one side of the vehicle to the other, an anti-roll bar car restrict body lean (or roll) during cornering. Vehicles may have anti-roll bars in the front, rear or both.

Arms, Links, Rods, Wishbones – These terms describe a variety of suspension components that "locate" the wheels or axles and restrict their movements for optimum handling and traction. The complex relationship among suspension components as they move is called the suspension geometry.

Axle Articulation – This term indicates the ability of one axle to move relative to the other – left wheel up, right wheel down. It is the measure of the ease with which wheels stay in contact with the ground (and retain traction) on a very bumpy uneven track.

Ball Joint – A ball joint is a flexible joint consisting of a ball in a socket. It is most often used in suspensions because it accommodates a wide range of motion in two directions.

Beam Axle – A beam axle is a rigid axle supporting non-driven wheels.

Brake Fade – As brakes heat up with hard or repeated use, their effectiveness usually diminishes.

Brake Lockup – In braking, lockup describes the point at which a tyre starts to skid in an emergency stopping situation. A tyre’s maximum braking force is developed when it is on the verge of lock-up, so a car’s shortest stopping distances are produced when its front and rear tyres are held just short of lockup. Anti-lock brakes (ABS) prevent wheel lockup, giving the advantage of simultaneous steering and braking ability (a locked wheel cannot be steered).

Bush – A suspension bushing is a type of bearing for suspension components. Typically made of rubber or a synthetic material, a bush can be "tuned" to affect ride quality and handling, and minimise the transmission of vibrations to the passenger cabin.

Cadence Braking – A method of manual braking with the foot brake to stimulate the action of ABS brakes. (See Anti-Lock Braking System above).

Camber – When viewed from the front of the vehicle, camber is the angle of the tyre/wheel compared with true vertical. When the tops of the front wheels are tilted inward, a vehicle’s camber is negative; when tilted outward, camber is positive. Camber influences lateral control.

Castor – This angle describes the inclination of the front wheels, when viewed from the side, as the wheels "travel" from the suspension’s lowest to its highest points – in effect, the angle between a vertical line and the vehicle’s steering axis. Just as an office chair of shopping trolley uses castors to roll forward more easily, a high degree of castor in a vehicle makes the wheels "self-centre" more readily.

Chassis – This term can describe a vehicle’s structural elements or the collection of mechanical components attached to its frame. In vehicles with monocoque or "unibody" construction, the chassis comprises everything but the bolted-on body panels of the car. In vehicles with a separate frame, chassis usually refers to the frame.

Coil Spring – Used in a suspension system, a coil spring consists of a bar of resilient metal (usually a steel alloy) wound into a spiral form. It can be compressed repeatedly or extended without permanent deformation. Their function is to support the weight of the vehicle.

Constant-Velocity Joint – A "CV" joint is a type of universal joint, usually used between the front half-shafts and front spindles/wheels in a front-wheel-drive or four-wheel-drive vehicle. CV joints are engineered to transmit power more smoothly (compared with universal joints) as they move through two planes. They are also less prone to driveline vibrations.

Crossmember – This component of a chassis (or subframe), placed transversely, connects to and strengthens longitudinal chassis members.

Differential – A differential transmits power from the transmission or transfer gearbox to the wheels, and allows the wheels to turn at different speeds (so the vehicle can turn corners without drivetrain wind-up – a situation in which the wheels on the outside of a curve turn faster and travel a greater distance than the inside wheels). While most differentials are mechanical gear-driven devices, sometimes a multi-plate clutch pack (MPT) can act in place of a differential.

Differential Lock – The main disadvantage of an "open" differential is that it will send power to the wheel (or axle) with the least traction. A differential lock literally locks out the differential action and forces power to be split equally between each wheel (or each axle) for maximum traction. In a rear-wheel-drive vehicle, for instance, locking the rear differential locks the left and right rear wheels together so that both receive equal power. In a full-time or permanent 4x4 vehicle, locking the centre differential locks the front and rear driveshafts together sending equal power to the front and rear axles. While locking a differential helps increase traction, it makes steering more difficult on high-traction surfaces because it tends to force the vehicle to travel in a straight line.

Disc Brakes – This type of braking system consists of a disc or rotor that rotates at the same speed as the wheel to which it is attached, straddled by a calliper. The calliper contains brake pads, operated by small pistons, that squeeze against the surface of the disc to slow it down or stop it. Disc brakes operate more efficiently at high temperatures and wet conditions than drum brakes.

DOHC – Double overhead camshaft engines use two camshafts located in each cylinder head. One camshaft operates the intake valves, the other actuates the exhaust valves.

Driveshaft – The driveshaft transmits power from the transmission or transfer case to the differential. A four-wheel-drive vehicle has at least two driveshafts – one running to the front differential and one to the rear. Also known as propeller shafts, or prop shafts.

Drivetrain – This term, also called a powertrain, describes all of a vehicle’s components that produce power and transmit power to the wheels – the engine, transmission, transfer case, driveshafts, differentials, axle shafts and wheel hubs.

Drum Brakes – A drum brake uses a drum-shaped (cylindrical) housing, usually made of cast iron, that is attached to the wheel and rotates with it. Inside the drum are curved brake shoes that are forced into contract with the inner drum to provide braking. Drum brakes are simple and generally effective, but less efficient than disc brakes under heavy use or when wet.

Engine Braking – This term describes the driving technique of slowing a vehicle by taking your foot off the throttle, particularly in a lower gear (such as first gear-Low Range). Engine braking uses the compression of the engine and the low gearing of the transmission/transfer gearbox to slow the vehicle.

Full-Time 4x4 – This drive system offers both a 4x2 and 4x4 mode. 4x4 can be engaged on dry roads for normal on-road driving because this system uses some type of centre differential. A typical full-time 4x4 system offers 4x2 and 4x4 "Auto" and 4x4 Low. Virtually all full-time 4x4 systems also have a two-speed transfer gearbox.

Gear Ratio – This is a numerical ratio of a series of gears in relation to each other based on the number of turns of the input shaft, compared with turns of the output shaft. Gear ratios are determined by the number of teeth on each gear. For instance, a gear with 36 teeth meshed with a gear 12 teeth gives a 36/12 or 3/1 ratio. This is usually expressed as 3.00:1,

Ground Clearance – With the vehicle stationary, ground clearance is the measurement from the lowest-hanging point under the vehicle (usually a differential or the exhaust system) to the ground. A high ground clearance allows a vehicle to more easily drive off-road or through heavy snow without damaging under-body components.

GVW – Gross Vehicle Weight is the combination of curb weight plus payload (including driver and fuel). It is the maximum rated (by the manufacturer) amount the vehicle can weight and safely operate.

Half-Shaft – A half-shaft is an articulating rotating shaft used in independent suspension systems to transmit power from a differential to a wheel. The term is also used to describe an axle shaft.

Independent Suspension – This describes a suspension system wherein each wheel on an axle is sprung separately and can react independently to bumps. A non-independent suspension uses a solid, beam or rigid axle to "connect" the suspension of two wheels. Both systems have advantages and disadvantages.

Ladder Frame Chassis – A ladder frame is shaped like a large ladder. Two long "rails" run along the sides, with cross-members connecting the two. This type of frame is used in some 4x4 vehicles. However, differences exist in the shape, structure and thickness of the various frame elements.

Leaf Spring – A long, flat, flexible piece of steel (or a composite material) curved into an arc that bends when forces act upon it. Leaf springs, most often used in pick-up trucks usually mount a solid axle to a vehicle.

Limited-Slip Differential – This differential has a mechanism to limit the speed differences between its two outputs. A limited slip differential ensures that some torque is always distributed to both wheels, even when one has very little traction.

Live Axle – A live axle is a solid axle that transmits power to a pair of wheels. It is composed of a rigid axle with a differential and axle shafts to power two wheels. It is called "live" because it has engine power flowing through it. A solid axle that does not transmit power is called a beam axle.

Low Lock – All full-time 4x4 systems and most competitive permanent 4x4 systems lock the centre differential when you engage Low Range. (Part-time systems have no centre differential, and so are in effect locked when in 4x4 High or 4x4 Low). The assumption is that when you are 4x4 Low, you want the extra traction of a locked centre differential. However, locking the centre differential limits Low Range to off-road use.

Monocoque Construction – This is a type of body construction that doesn’t require a separate frame to provide structural strength or support for the car’s mechanical components. A monocoque design uses many strong but light structural elements as an integral part of its construction.

Multi-Purpose Vehicle (MPV) – This is a high vehicle, designed primarily for transporting more than five people in comfort, on-road, and offering some seat flexibility for load carrying. Examples include the Toyota Previa.

Overdrive – Any gearset in which the output shaft turns faster than the input shaft is considered overdrive (the gear ratio is less than 1:1). Overdrive gears are used in most modern transmissions – usually for top gear only – because they reduce engine rmp and improve fuel economy. (A typical overdrive ratio is 0.78:1).

Overhead Cam – Overhead camshaft describes a type of valvetrain arrangement in which the engine’s camshaft is in its cylinder head – above (or over) the valves. When the camshaft is placed close to the valves, the valvetrain components can be stiffer and lighter, allow the valves to open and close more rapidly and the engine to run at higher rpm. In a single-overhead-cam (SOHC) layout, one camshaft actuates all of the valves in a cylinder head. In a double-overhead-camshaft (DOHC) layout, one camshaft actuates the intake valves and one camshaft operates the exhaust valves.

Part-time 4x4 – The most basic type of four-wheel-drive system, part-time 4x4 operates in 4x2 mode (usually rear-wheel drive) all of the time on dry road, 4x4 mode can be engaged only off-road or when the tarmac is very slippery because this system forces the front and rear wheels to travel at the same speed. Using 4x4 on dry road can damage the drivetrain and cause extreme tyre wear. Virtually, all part-time 4x4 systems also employ a two-speed transfer box.

Permanent 4x4 – As the name suggests, permanent 4x4 sends power to all four wheels continuously, on-road or off-road. There is no 4x2 capability, and the driver does not need to determine when to engage 4x4 mode. Drive selections typically include 4x4 High and 4x4 Low. Most permanent 4x4 systems also have a locking centre differential and a two-speed transfer case.

Powertrain – This term describes an engine and transmission combination. See drivetrain.

Pushrod – In the valvetrain of an overhead valve (OHV) engine, pushrods are used to transfer the reciprocating motion from the camshaft to the rocker arms and then to the valves. Pushrod engines are generally shorter than overhead cam engines and often operate at lower engine speeds.

Rack and Pinion Steering – This system ensures on-road steering precision, as it is the most direct system available – giving optimum feedback and a fast, predictable response.

Ramp Breakover Angle – This is a measure of a vehicle’s ability to drive over a sharp ridge or ramp without touching its underside. The included angle measures the angle inside the ramp; the excluded angle measures the combined angles outside the ramp to the horizontal. A short wheelbase vehicle with large tyres will have the best ramp breakover angle.

Recirculating-Ball Steering – This steering system uses steel balls inside a larger screw-and-nut mechanism to turn the wheels. The ball bearings in the recirculating track reduce friction. This type of steering is considered durable and easy to operate.

Recovery Strap – A recovery strap is a strap made of elastic nylon with loops on both ends. Unlike a simple rope or chain, a recovery strap uses kinetic energy (like a rubber band) to help free a stuck vehicle using another vehicle.

Rigid Axle – See beam axle.

Shock Absorber – A device used primarily to dampen suspension oscillations, shock absorbers respond to motion.

Single Overhead Cam – An SOHC engine uses one camshaft in each cylinder head to operate both the exhaust valves and the intake valves and the intake valves.

Strut – A strut is a suspension element in which a reinforced shock absorber is used as one of the wheel’s locating members, typically by solidly bolting the wheel hub to the bottom end of the strut.

Sub-frame – A subframe is a small, separate frame usually attached to a unitized body vehicle. A front subframe might be used to "cradle" the engine and transmission, while a rear subframe would attach the rear suspension to the unibody structure.

Sump Guard – This term refers to a protection cover or plate under a vehicle that covers vulnerable components, such as the transmission/transfer gearbox, engine oil pan or fuel tank.

Suspension Travel – This term refers to the amount of vertical wheel movement allowed by the suspension.

Toe-In-Toe-Out – This alignment measurement is the amount by which the front wheels (when viewed from above) are not parallel to each other. Toe can affect handling and steering feel.

Torque – Torque is the rotational equivalent of force, measured in Newton metres (Nm). In an engine, torque is the twisting force measured at the crankshaft.

Tension Bar – This is a type of spring made of a long solid or tubular rod with one end fixed to the chassis and the other twisted by a lever connected to the suspension.

Traction – This is the concept of achieving (and maintaining) grip between the wheels and the ground/road surface without slip, skid or sinkage.

Transfer Gearbox – A transfer box (or transfer case) is a system of gears or an auxiliary transmission used in 4x4 vehicles that transfers power from the transmission to the front and rear driveshafts. Transfer gear-boxes typically have two gear ranges, High and Low. High Range is used for typical on-highway or light off-road use. Low Range is used for serious off-road conditions. Putting a transfer gearbox in "neutral" disconnects the transmission from the wheels. Unless the brakes are applied (foot brake or handbrake), a vehicle can roll when the transfer gearbox is in "neutral" even if the transmission is in "park" (automatic) or a forward/reverse gear (manual).

Transmission – A transmission is a gearbox (either manual or automatic) with a number of different ratios to match the engine’s rmp and torque to various driving situations.

Universal Joint (UJ) – A UJ "connects" two shafts that aren’t in a straight line. Depending on its design, a universal joint can accommodate a large variation between the angle of the input shaft and that of the output shaft.

Viscous Coupling Unit (VCU) – This complex device relies on the characteristics of a silicon fluid inside it. It can act as a differential, as a means to restrict wheel spin or both. Inside are two sets of slotted metal plates – one connected to a front shaft, the other to a rear shaft. When there is a significant speed difference between the shafts, the silicon fluid heats up, expands and becomes thinner, limiting the speed difference and thereby slippage right-to-left or front-to-rear. In effect, it "locks" the shafts together (although not mechanically, as in a locking differential). When there is little or no speed difference between the shafts, the viscous coupling does not work.

Winch – A winch is an externally mounted device consisting of a cable spooled onto a drum. It is used to pull heavy or bulky objects or to retrieve a vehicle that is stuck. The drum can be driven by the engine, by hydraulic power or electrically.

Worm-and-Roller Steering – This steering mechanism uses an hourglass-shaped worm and a pinion roller in external contact. This type of steering gear is known for its durability.