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'.
Air suspension. - see EAS.
Anti-lock brakes. See ABS
above.
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 tyre 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 travelling 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-centre 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.
Centre 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 4x4. Vehicles fitted with part-time or selectable 4x4 are not
fitted with centre 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-axled. See 'Diagonal
suspension'.
Cross ply tyre. Tyre 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 tyres 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
tyre.
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 'Centre
differential' above. Locking of the centre 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 centre 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 centre 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 centre
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-levelling 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 utilises 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 tyre
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 tyre
sidewall flexing - hence the speed limitation.
Emergency
soft. Another name for emergency flotation tyre pressure - see
above.
ETC. See 'Electronic
traction control' 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 tyres, not to sink on soft going such as
mud or sand.
Four-wheel drive (4x4). Vehicle transmission
system in which engine power is applied to all four wheels. The term 4x4
(four by four) has the specific connotation that it is a four (wheeled
vehicle driven) by four (wheels).
Full-time 4x4. 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 tyre 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.
Handbrake. See 'Transmission
brake'.
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.
Levelled
suspension. A means of eliminating the 'squat' of the rear suspension
under load by a hydraulic self-levelling unit between the chassis and the
centre of the rear axle. Standard on coil sprung Range Rovers, Defender
110 Station Wagons and County; special order on any other Defender
110.
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.
Marshalling. ('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. Marshalling should be
undertaken when there is the danger of damaging tyre sidewalls or the
underside of the vehicle on rocks or other obstacles.
M+S
tyres. Mud and snow tyres. A generic term for 4x4 tyres with a
road-oriented, not especially bold, tread pattern suitable for mild snow
and mud conditions.
Mud tyres. Bold, open-tread tyres
optimised for mud with disadvantages on hard roads.
Multi-purpose tyre. Combination / compromise between
on-road and mud tyres.
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 in this book 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.
Part-time 4x4. See 'Selectable
four-wheel drive'.
Permanent four-wheel drive. See 'Full-time
4x4' above.
Radial ply
tyre. A type of tyre construction in which sidewall structural plies
run radially out towards the tread instead of criss-cross diagonally. With
their thinner, more flexible sidewalls, radial tyres have lower rolling
resistance than cross-ply tyres (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 tyres 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; a Defender 90 will be better than a
Defender 130 in this respect.
Ramp breakover angle. The
fuller title of 'Ramp
angle' above.
Range change. Term sometimes used for the
transfer gearbox lever.
Reduced inflation. Lowering tyre
pressures to increase flotation in soft ground conditions such as mud or
soft sand.
Rolling contact - see 'Continuous
rolling contact'.
Sand
ladders. A pair of aluminium 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 tyres. Term often used
to mean desert tyre - implying an ability to cope with desert rock and
stones as well as sand. These tyres are characterised by tread blocks of a
gentle, shouldered profile with no bold, right-angled edges such as a mud
tyre would have. Radial construction is far more suited to the low
inflation pressures sometimes used in sand. Despite their appearance,
'balloon' tyres 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 characterised 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 centre differential
Self
centring. 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 utilised to enhance safety when driving in deep wheel ruts on slippery
ground.
Self-levelling suspension. See 'Levelled
suspension'
Sidewall. The external 'walls' of a tyre
between the tread and the bead or wheel rim. This area is particularly
vulnerable on radial ply tyres to damage in off-road operations from
oblique rubbing contact with side-swiping sharp rocks. Driver awareness
essential.
Sidewall awareness. Awareness by sensitive
drivers of the susceptibility to damage of the tyre sidewall. An attribute
worth developing.
Sidewall deflection. Outward movement of
the tyre 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 tyres 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 tyre
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 centre differential
lock.
Tractive 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.
Trailer nose-load. See 'Nose
load'
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 fulfils 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 'Centre
differential',. A 4x4 with no centre differential or one driven with
the centre diff locked (ie in both cases the front and rear propeller
shafts locked together) is unable to accommodate the small differences in
distance normally travelled 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 travelled.
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 centre
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.
