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Glossary Of 4wd Terms

ABS - or anti-lock braking system prevents wheels from locking by sensing wheel slip and releasing brake pressure appropriately, maintaining steering control under maximum braking.

Approach angle - the angle a 4WD can take a steep climb or descent without scraping the front on the ground.

Bash plate - fitted underneath vital components like the engine, transmission and fuel tank.

Usually made of thick steel and designed to take hard knocks off-road.

Bullbar - fits to the front of the vehicle and replaces the bumper bar.

Usually one main bar with tubular hoops rising above to protect the headlights, radiator and front panel structure from damage in particular wildlife strikes in the country.

Made from steel, aluminium and lately, polycarbonate material.

Departure angle - the angle the rear of a 4WD can take a climb or descent without scraping on the ground.

Differential - drives both half-shafts at the same time but allows them to be driven at different speeds, as when turning.

A CENTRE DIFFERENTIAL works under the same principle only it drives front and rear driveshafts (which connect to front and rear differentials) and allows each driveshaft to be driven at different speeds.

Driveshaft - or propshaft connects the transmission to the differential.

Electromagnetic clutch - relies on sensors to detect wheelspin on (usually) the rear axle and then electrically engages a clutch to apportion drive to the front axle, providing 4WD.

Free-wheeling hubs - on a part-time 4WD allow manual disconnection of the front axle driveshafts at the wheels, saving on wear.

Auto free-wheeling hubs are the same except they automatically disengage when the vehicle is reversed a few metres.

Full-time - or constant 4WD sends power to all four wheels all the time, and unlike part-time 4WDs can be safely driven on hard road surfaces.

Some have a selectable mechanical centre differential lock so they then deliver torque in a fixed 50-50 split front/rear (like part-time 4WD) for better off-road grip.

Examples: All-Wheel Drive (Subaru); Full Time 4WD (Toyota).

Ground clearance - the measurement taken between the lowest central point of the car and the ground.

Halfshaft - the shaft connecting the differential to the wheel.

HDC - Hill Descent Control, Land Rover's system to do away with low-range gearing for steep off-road descents.

Using ABS, it applies brake pressure to all four wheels to restrict speed downhill.

High range - the normal set of gears used for driving at normal road speeds.

Low-range reduction - an extra set of gears for slow-speed driving (usually only the lowest two ratios are required).

Important for steep slippery off-road trails.

LSD - Limited-Slip Differential, just like in some two-wheel drives.

A mechanical version of traction control, it limits wheelspin on an axle.

Examples: Trac-Lok and Vari-Lok (Jeep).

LWB - long wheelbase, a term often used for a 4WD wagon which is also offered as a short wheelbase (SWB).

Nudge bar - single tubular hoop, usually made from aluminium or steel, fitted to the front of the vehicle, suitable for minor parking bingles.

Inadequate for deflecting a big red 'roo.

On-demand 4WD - sophisticated part-time system that runs in two-wheel drive until 4WD is required.

The system engages 4WD automatically.

It has a 'switch' which senses when to 'close' and engage 4WD.

This 'switch' is most often a viscous coupling or electromagnetic clutch.

Examples: Control Trac (Ford); Real Time (Honda); Syncro (Volkswagen); Torque On Demand (Holden); Quadra-Trac, Quadra-Drive (Jeep).

PART-TIME 4WD - allows use of 4WD when you're driving on slippery roads or off-road.

Otherwise only two-wheel drive can be used or else transfer case damage occurs due to wind-up (see below).

Examples: Command-Trac (Jeep); Easy Select (Mitsubishi).

Ramp-over angle - angle a 4WD can drive over a mound without scraping underneath.

Selectable full-time 4WD - a constant 4WD with a front prop shaft which, if desired, can be disconnected to run in two-wheel drive (saving on wear and rolling resistance).

Examples: Super Select (Mitsubishi); Selec-Trac (Jeep).

Side steps - a bar or board made of steel or alloy which runs along the length of the sill to allow easier entry to the cabin, and in the case of sturdier examples, protect the sills while off-road.

Snatch strap - like a giant elastic band used to 'snatch' a car out of a bog.

Traction control - uses ABS to apply braking force to a slipping wheel, allowing drive to be directed to the wheels with most grip.

Examples: ETC (Land Rover and Range Rover); 4-ETS (Mercedes-Benz).

Transfer case - part of the transmission which channels drive to front and rear axles and may contain the gearset for low range.

Viscous coupling - is an enclosed tube connected between transfer case and driveshaft which has viscous fluid within.

This fluid heats up when a pair of wheels start to spin and 'locks' the driveshaft, giving 4WD.

Wheel articulation - lateral movement of a live axle; for independent suspension this is described as wheel travel.

Wind up - eventually happens when a part-time 4WD (or full-time 4WD with centre differential locked) is driven on paved roads, potentially causing much damage to the transfer case.

Winch - usually fitted to the front of the vehicle, it is a steel cable tied to a drum that is driven by a powerful electric motor to retrieve a bogged vehicle.

The hooked end of the cable is usually attached to a fixed point and the cable reeled in, pulling the vehicle out of its predicament.

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