4WD Glossary

[4ePajero]

Definitions of technical terms:

4WD Glossary


4x2: four wheels, two driven.
4x4: four wheels, all driven or drivable, 4x4, 6x6 etc.
6x6: six wheels, all driven or drivable [6x6].
ABS: Anti-Lock Braking system, but ABS 4wd.sofcom.com --> is actually an abbreviation for Anti Blockier System a Wabco Gmbh patent, Experience.
ACE: Active Cornering Enhancement, a BMW / Rover-ism ~ 1997, electronic system automatically manipulates suspension and/or brakes to assist cornering; also see ESP.
ADR: Australian Design Rules; it is 4wd.sofcom.com --> important to check ADR's if carrying out vehicle modifications.
All Wheel Drive: Taken literally AWD means all four wheels can be driven, but there is some ambiguity as to just what "driven" means. If a centre diff' is used between the front and rear axles then all wheels share equal torque (unless one slips and an LSD or diff' lock takes over). A cheaper arrangement is to drive one axle via a viscous coupling or similar. In this case one axle receives no torque until the other axle slips - is this all wheel drive? Debatable.
Anti-Roll Bar: torsion bar between left and right ends of an axle - it increases roll-stiffness which can improve on-road handling but can also restrict axle-articulation off-road.
Approach Angle: the angle at which a line drawn from the leading part of the vehicle's bodywork and touching the front tyre meets the ground. It gives an indication of the steepness of a step-up that a 4x4 can approach without its nose digging in. The bigger the angle, the closer it is to 90 degrees the better for offroad work. See also [web page] and departure angle.
ATV: All Terrain Vehicle, often very basic, possibly with skid-steering, balloon tyres, no suspension.
AWD: All Wheel Drive.
Back-lash: the free-play between gears - a certain amount of backlash is essential.
BAS: Brake Assist System (Mercedes Benz, 1990s), electronics detect sudden application of brakes and increase braking effect under the assumption that this is, or should be, a panic stop.
Boge Self-Levelling Unit: a self-contained unit which uses suspension movements to pressurize itself and so level the suspension under heavy load - as fitted to the Range Rover (Classic).
Capstan: type of winch.
Carawagon: (ex-) British firm that did camper conversions, esp' on Land Rovers and even Range Rovers.
Centre Differential: incorporated in the transfer case of a full-time 4WD vehicle to allow the front and rear axles to rotate at different speeds during cornering - is pioneered in the 1902 Spyker 4WD.
Champ: Austin Champ, British military four wheel drive see [web page].
CJ: Civilian Jeep.
Crew-Cab: body configuration, two rows of seats and generally four doors, also fitted with a tray (ute). Also known as double-cab.
Diff' Lock: a differential lock in an axle differential, ie. it makes the half-shafts rotate at the same speed.
CV Joint: Constant Velocity joint, used in drive-shafts where the input and output shaft must rotate smoothly at the same uniform speed of rotation.
CVT: Continuously Variable Transmission, e.g. [web page].
Dakar: city on the Atlantic coast of Senegal, finish of the Paris - Dakar, lately Granada - Dakar, offroad rally. (The start of the 2000 Dakar - Cairo rally.)
Dead Axle: uses a tube (or beam) to connect wheels but the tube does not contain a differential or half shafts. Drive is delivered to the wheels by separate half-shafts fitted with universal joints or cv joints.
Departure Angle: the angle at which a line drawn from a vehicle's rearmost bodywork and touching a rear tyre meets the ground. It gives an indication of the maximum steepness of a step-down that a vehicle can negotiate without dragging the rear bodywork. Long body overhangs reduce the departure angle. The closer the angle is to 90 degrees the better for offroad work. See also approach angle.
Differential: has one input and two outputs, e.g. allows left and right wheels on an axle to revolve at different speeds during cornering.
Differential Lock: dog-clutch, prevents differential action e.g. in slippery conditions. See [web page].
Direct Injection: where fuel is injected directly into the (main) cylinders of an engine, esp' in diesels or very rarely in petrol engines; more efficient than indirect injection where diesel is injected into a small chamber off the main cylinder.
DOHC: Double Overhead Cam Shafts (two per bank in a V-engine) enable valves to be further separated than with a single camshaft (SOHC), allowing valves to be larger and/or better placed, giving more power.
Dormobile: British firm carried out camper conversions including pop-top roofs, often on 4x2 vans, notably Bedfords, but also on four wheel drives.
DUKW: large [amphibious] military 6x6 vehicle dating from WWII.
ECU: Electronic Control Unit - heart of the engine management system of a modern car.
Efi: Electronic Fuel Injection.
ESP: Electronic Stability Program, a Mercedes-Benz-ism from the late 1990s, e.g. on the Mclass - electronics manipulate the brakes and engine torque to prevent or reduce skids; also see ACE.
F-Head: see IOEV.
FC: Forward-Control or cab-over design, with the cab over the engine and front wheels - makes for a larger load space. [Example.]
Four Wheel Drive: Taken literally 4WD means four wheels, all of which are (or can) be driven. See full time and 4wd.sofcom.com --> part time 4WD.
Full Time 4WD: all four (or more) wheels are driven at all times. Strictly, a centre diff' should be employed to share equal torque to front and rear axles, but see `all wheel drive'.
Fully Floating Axle: this is nothing to do with amphibious vehicles, rather it refers to a fully floating half-shaft in a live axle. In a F.F. axle, the wheel hub is fully supported by twin taper roller bearings. The half shaft therefore only transmits torque from the differential to the hub. If the half shaft breaks, the wheel won't fall off!
FWD: Four Wheel Drive Auto Co (1911-).
Gipsy: Austin Gipsy (UK).
GPA: General purpose amphibious, the amphibious Jeep or Seep; built by Ford.
GPW: WWII Jeeps as built by Ford.
Green Laning: following (British) public rights of way, or green lanes, in a four wheel drive vehicle.
Haflinger: small 4WD (was) made by Steyr Daimler Puch.
HDC: Hill Descent Control, electronic system applies brakes automatically to crawl down a steep hill, as on the Freelander, ~ 1997.
HMMWV: High Mobility Multi-purpose Wheeled Vehicle, aka [Hummer] or HumVee.
Hookes Joint: universal joint.
HT: Hard-Top.
Hub Reduction Gears: reduction gears in the hub carrier, often used in conjunction with a portal axle.
Hummer or Humvee: see HMMWV.
Hydraulic Lifter: self-adjusting tappet, uses oil-pressure and one-way valve to control the backlash between cam-shaft and engine valves (as opposed to solid lifter).
Hydraulic Stall: if water is drawn into the engine cylinders, e.g. during wading, it will stop the engine immediately, often bending the con-rods, because water is incompressible.
Indirect Injection: where fuel is injected into a small combustion chamber that leads off the main cylinder in an engine - especially in (some) diesel engines.
IFS: Independent Front Suspension (eg double wishbones)
IRS: Independent Rear Suspension (eg double wishbones)
Intercooler: a small radiator used to lower the temperature of air compressed by a turbo-charger (or super-charger). This increases its density, so more air can enter the cylinders - more power.
IOEV: Inlet Over Exhaust Valve - the inlet valve is in the cylinder head but the exhaust valve is in the block, e.g. Willys CJ3B Jeep F-head, or Land-Rover Series one engine.
Jeep: The term was first applied to a vehicle in the form of the WWII Jeep MB.
Kinetic Recovery: the use of momentum and a snatch strap to extricate a bogged vehicle. See [web page].
LandCruiser: 4x4 by Toyota.
Live Axle: uses a rigid axle tube. Half-shafts pass down centre of tube (cf dead axle). This system is strong and simple, and ground clearance does not reduce under braking, but it has more unsprung weight than independent suspension. See [web page].
Locker: replaces the planet and side-gears in a diff', torque goes to the non-slipping wheel, may give "twitchy" handling. See [web page].
LPG: Liquified Petroleum Gas, butane, propane and/or propylene compressed. Popular dual-fuel conversion for large petrol cars and four wheel drives because LPG is very cheap in Australia, 20c to 25c/L v. 70c to 75c/L (1997). Power is slightly reduced. Those engines susceptible to burning valves should perhaps not be converted.
LSD: Limited Slip Differential See [web page].
LWB: Long Wheel Base, usually counting anything over 100" (2540mm).
MA: Willys Overland MA, which developed into the MB model, won the US army competition for what came to be known as the Jeep, as described in the Rifkind Report.
Military Vehicles: web page.
MPFI: see MPI.
MPI: Multi-Point Injection, Efi system where fuel is injected into the intake of each cylinder rather than at one point (throttle body injection) - allows more precise fuel metering.
Mule: minimalist four wheel drive - M274 Mechanical Mule
Mutt: Military Utility Tactical Truck M151 (they must do it deliberately). See [web page].
OHC: Over Head Camshaft, cam shaft is in the head, above the valves.
OHV: Over Head Valves where the inlet and outlet valves are in the cylinder head (in contrast to side-valves). Valves are operated by push-rods from a cam-shaft at the side of the engine.
Open Differential: a "normal" differential without diff' lock or LSD features - axle looses traction if one wheel slips.
Overdrive: a top gear that is higher than 1:1. Originally overdrives were only provided as optional units bolted onto the original gearbox, and often employing epicyclic gears. The term is now also applied to the top (usually 5th) gear ratio in the main gearbox when it is higher than 1:1.
Oversteer: tendency of a car to turn more sharply than the driver intended, probably leading to a rear wheel skid and eventually a spin, especially rear wheel drive vehicles when too much acceleration is applied.
Panhard Rod: provides sideways location of a live axle.
Part-Time 4WD: a vehicle that is normally in two-wheel drive but one that can engage drive to the other two wheels, usually through a dog-clutch.
Portal Axle: system where the half-shaft enters the hub-carrier higher than the stub axle, thus raising the diff' and half-shafts for extra ground clearance. Drive is carried to the hub by gears. See Hummer, Pinzgauer or Unimog.
PTO: Power Take Off, unit bolts onto transfer case (usually), incorporates a dog clutch and is used to drive a winch or other special equipment.
Quattro: A series of four wheel drive road / rally cars by Audi.
Ramp-Over Angle: The minimum internal angle of an inverted-V ramp that a vehicle can drive over 4wd.sofcom.com --> without dragging its belly, ie. a measure of the sharpest bank it can drive over. A typical figure is about 150 degrees; the smaller the better offroad. NB. sometimes the ramp's `external angle', 180-ROA, is given, typically a figure of about 30 degrees; subtract from 180 to convert. See [web page].
Register: a club that specialises in the history and the restoration of vehicles, usually to original condition.
RST-V: US Marines' plans for what is in effect the Jeep for 2000+ - Reconnaissance, Surveillance and Targeting Vehicle.
SAS: Solid Axle Suspension / Solid Axle Swop
SFA: Solid Front Axle
SRA: Solid Rear Axle
Scout: four wheel drive made by International Harvester.
Seep: the [amphibious] Jeep, all based on the Ford built Jeeps.
Series: the [SI-SIII] leaf-sprung Land-Rovers, 1948-1983.
Side Valve: where the inlet and outlet valves are in the cylinder block rather than the cylinder head (OHV), e.g. as in the Willys Overland Jeep MB.
Snatch Strap: elastic strap used in kinetic recovery. See [web page].
SOHC: Single Over-Head Cam, operates engine valves. See DOHC, OHC.
Solid Lifter: Non self-adjusting (i.e. solid) tappet, see Hydraulic lifter.
Speeding Ticket: Voluntary form of taxation.
ST: Soft-Top.
Super-Charger: A turbine driven directly by the engine to compress the engine intake air.
Sway bar: see anti-roll bar.
SWB: Short Wheel Base, usually counting anything under 100" (2540mm).
Tirfor: Brand of hand-winch which pulls the rope through it.
Torsen: Type of limited slip differential, incorporates pairs of worm gears driving the side-gears.
Torsion Bar: Type of spring, rod or bar twisted by suspension movement, esp' independent front suspension.
Transfer-Case: Secondary gearbox used to transfer drive sideways so that it may then be carried forward, around the engine, to the front diff' and axle. Usually also provides high and low gear ratios.
Transmission Wind-Up: Stress in the transmission system caused by wheels travelling different distances (e.g. cornering) but unable to be relieved because diff' locks are engaged or because part-time 4WD is engaged.
Trial: A four wheel drive competition on a closed course that emphasises precise vehicle control rather than outright speed.
Turbo-Charger: Exhaust gases drive a turbine which drives a compressor which raises the engine air intake pressure or charges the engine with more air - more power, [web page].
Tyre Dimensions are explained [here].
Understeer: Tendency of a car to turn less sharply than the driver intended, especially a front wheel drive vehicle under hard acceleration.
Unimog: All wheel drive trucks made by Mercedes Benz See [web page].
Universal Joint: See [web page].
Viscous Coupling: An arrangement of discs in a viscous fluid that allows two shafts to rotate at the same or only slightly different speeds. It can be incorporated in a differential to allow a slow, but not a high speed, differential action, ie. preventing wheel-spin without transmission wind-up. See [example]
Willys Overland: Makers of the WWII [Jeep].
Winch: Various kinds, see [examples].
Wishbone: A-frame component of independent suspension systems.