Archive for the ‘Off-Road 4×4 Glossary’ Category

For the novice Jeep Off Roader’, here are some commonly used terms you may come across in your off roading adventures compliments of Wood Park Off-Road.

ABS. Anti-lock braking system; prevents wheels locking under maximum braking. Works on the principle of braking a wheel until it just begins to skid (this is the point where braking efficiency would drop off dramatically) and then releasing the brake pressure and re-applying the brakes. Wheel speed sensors identify the skid point and trigger a release in brake pressure. The cycle is repeated many times a second -with appropriate ‘cobblestone’ feed-back on the brake pedal to indicate you are in ABS mode. See also ‘Cadence braking’.

Approach angle. In side-view, the angle between the ground and a line, ahead of the vehicle, joining the periphery of the front wheel and (typically) the front bumper or other low component. It represents the size or steepness of a slope or obstacle that can be approached or climbed without striking bodywork.

Articulation. The ability of one axle to move – left wheel up, right wheel down or vice versa – relative to the chassis or its fellow axle. It is a measure of the ease with which wheels can stay in contact with the ground – and thus retain traction – on very ‘twisty’ off-road terrain.

Articulation angle, longitudinal. See ‘Longitudinal articulation angle’, wheels run within rigid casings without joints to allow vertical hinging as with independent suspension. In an off-road vehicle rigid axles have the advantage of maintaining maximum under-axle ground clearance at all times and always keeping the tire tread flat on the ground.

Bridle. A rope or cable attached to two points – typically the right and left chassis members – of a vehicle and converging to a point of attachment for a tow rope.

Cadence braking. A method of manual braking with the foot brake to simulate the action of ABS brakes – see above. Very effective in slippery conditions where brake locking has occurred or might otherwise occur, the driver applies the footbrake in a series of very rapid jabs at the pedal taking the wheels up to the point of brake locking and then releasing them before the inevitable fall-off in braking efficiency takes place. Effects improved braking in any extremely slippery conditions such as ice, snow, wet mud, or rain.

Capstan winch. A winch, generally mounted on or just behind the front bumper, usually run from an engagable extension to the engine crankshaft. The active component is usually a slowly revolving drum, about 15 cm in diameter, round which a rope may be wound to effect a winching operation. Has the advantage of being powered by the engine at idling speed and being a very low-stress unit that may be used all day without overheating or high electrical load.

Castor (or caster) angle. When the front wheels are moved right or left to steer the vehicle they each move about a steering axis. The aft inclination of this steering axis from the vertical (when viewed from the side) – about in the case of most Land Rovers – is the angle. Like casters on a tea trolley or office chair, this puts the ground contact point of the wheels the pivot axis and the result is a self-centering action tending to keep the front wheels pointing forward when in forward motion. Note that in deep sand with a ‘bow wave’ build-up of sand ahead of the wheels the effective ground contact point moves ahead of the steering axis and can give the effect of negative castor with runaway’ steering. The same thing happens when vehicle is traveling in reverse – the ground contact point being ‘ahead’ of the steering axis and again tending to make the front wheels ‘run away’ to full lock, failed climbing of steep off-road inclines.

Castor action. Tendency of front wheels to self-center when the steering wheel is released with the vehicle going forward. NB Opposite action takes place when in reverse – see Castor angle above. Castor action is a basic ingredient of steering feel.

Center differential.
A differential gear device installed at the point where the transfer box splits engine power between the front and rear axles via the front and rear propeller shafts. Working in the same way as the conventional rear axle differential on a two-wheel drive car, it allows differential rotation of front and rear shafts to accommodate the small rotational differences encountered in normal running, going round sharp corners etc. Such a device is essential in a vehicle having – for use on-road as well as off-road – full-time or permanent 4×4. Vehicles fitted with part-time or selectable 4×4 are not fitted with center differentials and thus cannot be used in four wheel drive on hard roads.

Continuous rolling contact. Description of a wheel in steady rolling contact with the ground without slip, wheel-spin or slide (as with locked brakes). Should be the aim at all times both on and off road.

Cross ply tire. Tire in which the sidewall reinforcement plies run diagonally from the bead towards the tread – each layer of textile at a different angle to its adjacent layer. Generally superseded by radial-ply tires whose thinner, more flexible sidewalls and braced tread yield better grip and lower rolling resistance. Because of thicker, multi-ply sidewalls, not so prone to sidewall damage as radials and can have low-cost applications when operating continuously on rock. However, reduced pressures in soft going can, due to the thick sidewalls, cause overheating and possibly de-lamination of the tire.

Co-ordinated tow. When recovering a stuck vehicle, the process by which the engine power of both the tug and the stuck vehicle are co-ordinated – usually by a signal from an external marshaller – and the clutches of both vehicles are engaged at the same time to enhance the chance of a first-time recovery.

Corrugations. Deformation of an unsurfaced track taking the form of transverse, close-pitch undulations – ie at right angles to the direction of the track. Sometimes referred to as ‘washboard’.

Coupled brakes. Brake system installed with certain large trailers whereby the trailer brakes are applied at the same time as are the brakes of the towing vehicle. Vehicles must be specifically modified to operate this system – with appropriate trailers.

Departure angle. In side view, the angle between the ground and a line, aft of the vehicle, joining the periphery of the rear wheel and (typically) the rear chassis member or other low component. It represents the size or steepness of a slope or obstacle that can be approached or climbed in reverse without striking bodywork.

Diagonal suspension. A manifestation occurring off-road when a vehicle is, for example, diagonally crossing a small but well-defined ridge. When the ridge is so severe that, say, the right front wheel and the rear left wheels are on full ‘bump’ (ie fully up in the wheel arches) and the other wheels are hanging down to the full extent of wheel travel, the vehicle may be described as being diagonally suspended or on diagonal suspension. Some also refer to this state as being ‘cross-axled’.

Diagonal wheel-spin. The wheel-spin that can take place on the fully extended wheels in a condition of diagonal suspension as described above. However, a vehicle need not be in a totally diagonal suspension condition for diagonal wheel-spin to take place; minor off-loading of diagonally opposed wheels or the presence of slippery ground under these wheels can provoke the condition. Can also occur crossing ditches diagonally.

Diff-lock. See first ‘Center differential’ above. Locking of the center differential, activated by moving the transfer gearbox lever to the left and confirmed by illumination of the ‘DIFF-LOCK’ indicator light, puts the differential function on hold. Where traction conditions or grip are different front and rear there would be a tendency for the center differential to permit the front wheels, say, to spin ineffectively while they are on wet clay and cause the rear wheels, on grippier ground, to stop rotating. The diff-lock locks the center differential, thus locking front and rear prop shafts together, ensuring they revolve at the same speed and enhancing traction. Diff-lock is usually engaged for difficult off-road conditions but should never remain engaged on hard grippy roads.

Differential casing. Not to be confused with the center differential, each axle, of course, has a normal cross-axle differential at the point where the propeller shaft from the transfer gearbox meets the axle. The size of the crown wheel and pinion plus differential demands a bulge in the axle casing – referred to as the diff casing. It has special significance in off-road vehicles because it is the lowest point of the axle and thus the point of least ground clearance –

Discontinuity of rolling contact. Generic term for wheel-spin and wheel slide – as on locked brakes. See ‘Continuous rolling contact’ above.

EAS – electronic air suspension. Introduced in the 1993 model year on certain Range Rover models further to enhance standards of road noise insulation, ride and handling, the system substitutes air bags and a live-line pneumatic system, (ie an electrically driven compressor, air pressure reservoir and associated controls) for the steel coil springs used on the rest of the Land Rover model range. Logic- controlled by an electronic control unit, height sensors and driver controls, the system maintains front and rear self-leveling in the five height modes listed below. These notes show the versatility of the system and the purpose for which it was designed. However, for the casual driver, new to the vehicle, no prior knowledge or expertise is required; FAS will cycle automatically through appropriate modes according to prior programming. The driver need not even know EAS is fitted. On engine start-up EAS assumes the last selected ride height.

Electronic traction control – ETC. ETC is a standard/optional feature, available only on ABS-equipped Range Rovers. It inhibits wheel-spin by applying brake to a spinning rear wheel and thus enhances traction on ice, snow or in severe off-road conditions. It utilizes ABS sensors for wheel speed determination and brakes the spinning wheel to, through the axle differential, apply torque to the stationary wheel. Like ABS. it is especially effective in maintaining control when one side of the vehicle is on a more slippery surface than the other – a so-called ‘split-p surface. A dashboard light illuminates when the system is operating. The function is inhibited above 50 kph, a speed above which unintentional wheel spin is unlikely to occur.

Engine braking. Vehicle retardation derived from engaging a low gear and taking your foot off the throttle.

Emergency flotation (pressure)
. Very low tire pressure (about 60% of normal road pressures), always associated with a low maximum permitted speed (20 kph or 12 mph) used for traversing or recovery from very soft ground. Such low pressures cause extreme tire sidewall flexing – hence the speed limitation.

Emergency soft. Another name for emergency flotation tire pressure – see above.

Fatigue life. Number of specified load reversals at which a metal component will fail. In the context of this book see fatigue life of nylon snatch-towing ropes -‘Recovery – snatch-towing’.

Flotation. Characteristic of a vehicle, by reason of large softly inflated tires, not to sink on soft going such as mud or sand.

Four-wheel drive (4×4). Vehicle transmission system in which engine power is applied to all four wheels. The term 4×4 (four by four) has the specific connotation that it is a four (wheeled vehicle driven) by four (wheels).

Full-time 4×4. A transmission system on a four-wheeled vehicle in which all four wheels are driven by the engine all the time. (As opposed to a vehicle that is normally in two-wheel drive with four-wheel drive selected by a separate lever when required.)

Geometric limitations. A term coined for this book to describe the limitations and extent of approach and departure angles, ramp angle, steering lock, articulation and – an even newer term – longitudinal articulation angle.

Ground clearance. Space between the ground and a given mechanical part of the vehicle. Usually, when quoted for a vehicle, taken as the least for any component on the vehicle – the space under the differential casing. But note difference between under-axle and underbelly clearance.

Ground stress. Term coined for this book to indicate how much strength is being asked of a particular piece of ground in terms of flotation or lateral shear to accommodate traction, braking or acceleration.

Heel and toe wear. Jargon for the uneven front to rear wear on individual blocks of a bold off-road tire tread when used on roads.

High box. Status of the transmission when the two-speed transfer gearbox lever is in the high ratio position – for normal, on-road, day-to-day use.

High ratio. Term to describe the transmission when the transfer gearbox lever is in the high position – high box above.

Hi-lift jack. Versatile lever-operated mechanical bumper jack capable of a lift of a metre or more.

Hi-lo lever. Term sometimes used to describe the transfer gearbox lever. High Load suspension. An option on the Defender 90 enabling payload to be raised by about 150 kg.

Kerb weight. Unladen weight, ie empty vehicle plus full fuel plus 75 kg driver.

KERR. Kinetic Energy Recovery Rope. Descriptive term coined to describe specially specified nylon ropes capable of stretching during snatch tow.

GVW. Gross vehicle weight – the maximum permitted laden weight of a vehicle including payload, fuel and driver.

Kinetic energy. Energy of motion, proportional to the total weight of the vehicle and the square of its speed. Thus if a vehicle’s weight doubles its KE also goes up two times; but if its speed doubles its KE increases by two squared, ie four times. See ‘Recovery – snatch-towing’

Laden. Vehicle carrying some or full payload. See also GVW above concerning loading to maximum permitted weight.

Leveled suspension. A means of eliminating the ‘squat’ of the rear suspension under load by a hydraulic self-leveling unit between the chassis and the center of the rear axle.

Longitudinal articulation coefficient (CLA) single number that conveys the off-road, ‘twisty ground’ potential of an off-road vehicle. A given max wheel movement enhances this capability more on a short wheelbase than on a long wheelbase vehicle. (CLA) is a non-dimensional parameter, provisionally coined, (higher values mean better articulation) that takes account of both wheel movement and wheelbase; it relates to the angle between the ground and a line joining the front and rear hubs (or tyre periphery) when one wheel is on full bump and the other fully.

Low box. Status of the transmission when the transfer gearbox lever is in the low position – for difficult off-road conditions demanding greater traction or low speed control.

Low ratio. Term to describe the transmission when transfer gearbox lever is in the low position.

Marshaling. (‘Marshaller’ derived from ground-crew who marshal aircraft on airport aprons.) In the context of off-road operations, taken to mean the detailed direction of a vehicle by a marshaller outside the vehicle who is able to see all four wheels and also the difficult ground being traversed. Marshaling should be undertaken when there is the danger of damaging tire sidewalls or the underside of the vehicle on rocks or other obstacles.

M+S tires. Mud and snow tires. A generic term for 4×4 tires with a road-oriented, not especially bold, tread pattern suitable for mild snow and mud conditions.

Mud tires. Bold, open-tread tires optimized for mud with disadvantages on hard roads.

Multi-purpose tire. Combination / compromise between on-road and mud tires.

NATO towing hook. Large, robust, four-bolt attachment towing pintle with top-closure and, usually, 3600 rotational capability about the longitudinal axis originally specified for NATO 7.5 tonne military vehicles. Suitable for off-road towing albeit, due to the fact that a trailer towing eye will not be a close fit over the hook, it generates quite a bit of ‘goods train’ fore and aft banging.

Nose load. Trailers should be nose heavy; the nose-load is the amount of nose-heaviness (sometimes called trailer preponderance’) measured at the tow-hitch and must be considered part of the towing vehicle’s payload.

On-foot recce. Inspecting a difficult off-road obstacle on foot before committing your vehicle to it.

Overrun brakes. Trailer brakes activated by the tendency of the trailer to overtake -or overrun – the towing vehicle when the vehicle brakes or slows down.

Over-torque. Used to convey the concept of applying too much torque (or power) to the wheels so that they break their grip with the ground and spin. Radial ply tire. A type of tire construction in which sidewall structural plies run radially out towards the tread instead of criss-cross diagonally. With their thinner, more flexible sidewalls, radial tires have lower rolling resistance than cross-ply tires (yielding better fuel consumption) as well as giving longer tread life. They can accommodate the use of low inflation pressures without overheating, due to their flexible sidewalls, but are sometimes more prone to sidewall damage when operating in rocky or stony conditions. Because radial tires invariably also have a braced tread area of great dimensional stability, they ‘track-lay’ the tread (like a bulldozer), do not suffer from ‘tread shuffle’ and so achieve more traction in limiting off-road conditions.

Ramp angle. A measure of vehicle under-belly clearance or the ability to drive over a sharp ridge or ramp without touching the underside of the vehicle on the obstacle. The ramp angle is the angle measured from the lowest part of the chassis at mid-wheelbase down to the periphery of front and rear wheels. Obviously a short wheelbase vehicle with large wheels will have the smallest ramp angle and best under-belly clearance.

Ramp breakover angle. The fuller title of ‘Ramp angle’ above.

Range change. Term sometimes used for the transfer gearbox lever.

Reduced inflation. Lowering tire pressures to increase flotation in soft ground conditions such as mud or soft sand.

Sand ladders. A pair of aluminum ladders, about 170 cm long, specially made with rungs closer than normal, to lay beneath the vehicle wheels in soft sand to give grip and flotation.

Sand tracks. Generic name sometimes given to any item fulfilling the role of a sand ladder. May be PSP (pierced steel planking).

Sand channels. Term often interchangeable with sand tracks, channels were used for this purpose. Can include articulated sand planks.

Sand tires. Term often used to mean desert tire – implying an ability to cope with desert rock and stones as well as sand. These tires are characterized by tread blocks of a gentle, shouldered profile with no bold, right-angled edges such as a mud tire would have. Radial construction is far more suited to the low inflation pressures sometimes used in sand. Despite their appearance, ‘balloon’ tires with circumferential groove treads are considerably less effective in sand than a radial such as the Michelin XS.

Salt flat. Salt marsh of very unreliable consistency and bearing strength found in desert regions and characterized by a top crust of varying thickness and strength with soft salt mud of great depth beneath it.

Selectable four-wheel drive. A four-wheeled vehicle which proceeds normally in two-wheel drive but on which, by means of a lever control, four-wheel drive may be selected. It is important to remember that such vehicles in four-wheel drive do not have the benefit of a center differential.

Self centering. The characteristic of front (steered) wheels to resume the straight-ahead position due to castor angle (See ‘Castor angle’) when the steering wheel is released. This characteristic can be utilized to enhance safety when driving in deep wheel ruts on slippery ground.

Sidewall. The external ‘walls’ of a tire between the tread and the bead or wheel rim. This area is particularly vulnerable on radial ply tires to damage in off-road operations from oblique rubbing contact with side-swiping sharp rocks. Driver awareness is essential.

Sidewall awareness. Awareness by sensitive drivers of the susceptibility to damage of the tire sidewall. An attribute worth developing.

Sidewall deflection. Outward movement of the tire sidewall in the region of the ground contact patch due to low inflation pressures or hitting a sharp bump with excess speed. It is important not to run tires at less than recommended inflation pressures for given maximum speeds and loads since by doing so you will exceed the manufacturer’s specified limits for sidewall deflection and thus cause overheating and serious damage to the tire.

Shock loading. In the context of this book, taken to mean the arrest of mechanical motion in an excessively abrupt way or the application of sharp load reversals in a such a way as to risk structural failure. For example, the application of the handbrake whilst the vehicle is in motion can cause unacceptable shock loading of the rear axle half-shafts. Engaging diff-lock whilst one or more wheel is spinning could also result in severe and damaging shock load to the transmission.

Small gear lever. Don’t be embarrassed if you can’t remember the name for the transfer gear lever..!

Snatch tow. A method of recovering a stuck vehicle in which the towing vehicle is in motion before taking up the slack in the tow rope. Use only using special-purpose stretch ropes and specified procedures for this.

Steering lock. The extent to which the steering wheel may be moved to the right or left. Thus ‘full lock’ implies movement of the steering wheel as far as it will go right or left.

Stretch limit (KERR ropes). The extent to which a kinetic energy recovery rope will stretch before it is in danger of breaking. A gnide for the Marlow Ropes Recovaline is 40% stretch; this limit should NEVER be approached.

Traction. In the context of this book the concept of achieving grip between the wheels and the ground without slip, skid or sinkage.

Traction Control. See ‘Electronic Traction Control, ETC’.

Traction controls. Here taken to mean the lever controlling the transfer gearbox and center differential lock.

Traction effort. The amount of ‘pull’ exerted by a vehicle as a result of traction.

Trailer preponderance. Sometimes used to denote down-load on the vehicle towing hitch.

Transfer box. Originally the name implied the transfer of power from the main gearbox to the front axle as well as the rear axle on a four-wheel drive vehicle. In all Land Rover products a two-speed transfer box is fitted so it has the additional role of permitting power from the gearbox to go to the axles at normal 1:1 gearing (high ratio) or geared down by nearly 2:1 (low ratio).

Transfer Gear Lever. The small gear lever in the cab next to the main gear lever. It controls whether the transmission is in ‘high ratio’ or ‘low ratio’ in the transfer box. The same lever also controls the engagement of the diff lock – see above -except in the Range Rover where a viscous coupling fulfills this requirement automatically.

Transmission brake. The handbrake on all Land Rovers operates by gripping the rear propeller shaft at the point where it leaves the transfer gearbox and is thus called a transmission brake. It should be used as a parking brake only and should never be operated whilst the vehicle is in motion except in emergency.

Transmission wind-up. Read first ‘Center differential’,. A 4×4 with no center differential or one driven with the center diff locked (ie in both cases the front and rear propeller shafts locked together) is unable to accommodate the small differences in distance normally traveled by the front wheels compared to the rear wheels. The diff-lock ensures both propeller shafts rotate exactly the same amount despite the small differences in distance actually traveled. This results in some wheel slip and skid which, on loose ground, can take place without any harm. On hard roads, however, the superior wheel grip makes it difficult for the wheels to slip much and in the process of trying to do so considerable torsional stress builds up in the transmission. This is known as transmission wind-up and can sometimes exert so much stress that the diff-lock gears will not disengage when so selected. You will also sense very heavy steering. If this occurs due to your forgetting to de-select diff-lock on hard ground (or at any other time) and the diff lock will not disengage, the solution is to reverse the vehicle some distance until the diff-lock warning light extinguishes.

Unladen. Vehicle carrying fuel, driver but no payload or other load – see ‘Kerb’ above.

Viscous coupling unit (VCU). A unit fitted as standard to all Range Rovers across the center differential (not instead of it) automatically to effect locking of the differential when a significant speed difference between front and rear propeller shafts is sensed. Conceptually it comprises a cylinder attached to the rear prop shaft into which an extension of the front prop shaft is introduced. Discs are attached alternately to the front prop shaft and the inside of the cylinder so that they interleave very closely within the cylinder. The cylinder is sealed at both ends and is filled with a special silicone fluid which has the characteristic of markedly increasing its viscosity when stirred. Thus when one prop shaft rotates relative to the other one – the situation of front (or rear) axle wheel-spin – the fluid increases its viscosity enough to lock the shafts together. When relative rotation ceases the viscosity changes back to its original value and the shafts are unlocked. The viscous coupling unit (VCU) has the advantage of being automatic on both engagement and disengagement and its action is gradual and without shock-loading to the transmission.

Wading plugs. Oil drain holes are provided in the bottom of the clutch housing (and the camshaft drive-belt housing on Tdi and 2.5D engines) to preclude the possibility of the clutch or cam belts becoming contaminated in the event of oil leaks from the adjacent bearings. Wading plugs should be fitted to block these holes when driving through water over 30 cm deep and subsequently removed.

Article Source:
Wood Park Off-Road. Off-Road 4×4 Glossary. 2009. http://www.woodpark-offroad.com

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