Ultimate Off Road and 4x4 Glossary of Terms and Definitions

ABS: anti-lock braking system; prevents wheels from locking under maximum braking. It works on the principle of braking a wheel until it just begins to skid (this is where braking efficiency would drop off dramatically), releasing the brake pressure, and re-applying the brakes.

Administrative Closure: seasonal or emergency road/trail closures that can be in place for a year, with little or no public input.

Air down: lowering the tire pressure safely to improve off-pavement traction, always matching all four tires to the same pressure when four-wheeling. Increases all-around tire performance off-pavement, providing more grip area (tread) of the tire to surface area = better traction from a larger "footprint" of the tire.

All-wheel drive: unlike a 4WD system, AWD directs power to the wheels as needed. If the front wheels start to slip, the power automatically shifts to the rear wheels. Additionally, all-wheel drives do not have a low range in the transfer case. Some do not even have a transfer case. The AWD system kicks in automatically when it senses a loss of traction. Unlike its 4WD counterpart, it does not require any input from the driver. The system sends a variable amount of power to each wheel.

Approach angle: 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 front low component. It represents the size or steepness of a slope (incline angle) or obstacle that can be approached or climbed without front bumper or undercarriage damage.

Area of Critical Environmental Concern (ACEC): a term used by the Bureau of Land Management (BLM) in mostly desert areas where special attention is needed to protect and prevent damage to essential resources.

Articulation: simple measurement of the vertical movement of the tire (or axle and wheel). The ability of one axle to move - left tire up, right tire down, or vice versa - relative to the chassis. It measures the ease with which tires can stay in contact with the ground - and thus retain traction.

Axle Lock: dash button on Jeep vehicles that locks the axles when the vehicle is in 4LO, traveling less than 14 mph. The bottom of the button locks the rear axle, and pushing the button the second time also engages the front axle. Turn off and disengage the Axle Lock by pressing the top of the button.

Bead-lock wheel: a rim with the entire bead secured to the wheel with bolts, allowing maximum air-down options while retaining tire contact with the rim/wheel.

Breakover angle: the maximum clearance angle to suspension/body parts as a vehicle exits an obstacle or descends a hill – ground to low-hanging parts.

Bridle: in towing, a rope, strap, or cable attached to two points - typically the right and left chassis members - of a vehicle and converging to the point of attachment for a tow rope and also called a "V-Bridle" or "V-Chain Bridle" like a tow truck would use.

Build: a general reference to upgrading or modifying a 4WD vehicle, especially when upgrading from stock parts to aftermarket stronger modifications.

Cadence braking: a method of manual braking with the foot brake to simulate the action of ABS brakes. Very effective in slippery conditions where brake locking has occurred or might otherwise happen, 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 highly slippery conditions like ice, snow, wet mud, or rain.

Capstan winch: a vertical cylinder that rotates around an upright spindle, designed to lift heavy weights using cables, ropes, or chains.

Caster angle: an essential component of proper wheel alignment based on caster and camber angle, including toe-in and toe-out. When viewed from the side, the caster is the angle made by the axis of the kingpin (steering axis) with a vertical axis. A caster is created when the front wheels are moved right or left to steer the vehicle; they each move about a steering axis. The rear inclination of this steering axis from the vertical (when viewed from the side) is the angle. Like casters on an office chair, this makes 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 a negative caster with runaway steering. The same thing happens when a vehicle travels in reverse - the ground contact point being 'ahead' of the steering axis and again tends to make the front wheels 'run away' to full lock, failed climbing of steep off-road inclines.

Caster action: known as opposing caster angle, is the tendency of front wheels to self-center when the steering wheel is released with the vehicle going forward. The opposite action takes place when in reverse. Caster action is an essential ingredient of steering feel with opposing caster angle helping the vehicle drive straight and true.

CEQA: the California Environmental Quality Act: California's version of NEPA, which is used on state lands to identify projects and problems, get public input, and make decisions about how land will be used (or not).

Continuous rolling contact: is the description of a wheel in steady rolling contact with the ground without slip, wheel spin, or slide (as with locked brakes).

The crawl ratio: is found by multiplying the first-gear ratio, the low-range ratio, and the axle ratio. It is the ratio of torque at the wheel to torque at the engine's flywheel. The crawl ratio number reflects how often the final driveshaft will rotate per engine crankshaft rotation. Higher crawl ratio numbers (e.g., 200:1) transfer more force to the ground, giving better rock crawling capabilities at slower speeds.

Coordinated tow: when recovering a stuck vehicle, the process by which the engine power of both the tug and the stuck vehicle is coordinated - usually by a signal from an external spotter or by radio chatter between drivers - and the clutches/engines/gears of both vehicles are engaged at the same time to enhance the chance of a first-time recovery. Typically, on signal, both drivers "give it gas" simultaneously while hooked together.

Corner Travel Index™ (CTI): measures a vehicle's maximum axle/wheel articulation at the four corners of the vehicle, often referred to as "flex." The CTI is used in the off-roading industry to quantify the axle/wheel articulation to compare the performance potential of various vehicles and vehicle builds.

Corrugations: are deformations of an unsurfaced track taking the form of transverse, close-pitch undulations - i.e., at right angles to the direction of the track. They are sometimes referred to as 'washboards.'

Coupled brakes (trailer brakes): a brake system installed with certain large trailers whereby the trailer brakes are applied simultaneously, as are the towing vehicle's brakes. Vehicles must be specifically modified to operate this system with brake controllers, tilt sensors, and integrated wiring with appropriate towing lights.

Curb (Kerb) weight: is unladen weight, i.e., an empty vehicle plus a full tank of fuel with all other OEM fluids. Gross weight is when the rig is fully loaded with people, cargo, and fuel.

Departure angle: as seen from the side view, it is 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 banging body components.

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' (i.e., 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 occur on the fully extended wheels in diagonal suspension, as described above. However, a vehicle need not be in a totally diagonal suspension condition for diagonal wheel spin to occur; minor off-loading of diagonally opposed wheels or slippery ground under these wheels can provoke the condition. It can also happen when crossing ditches diagonally.

Differential: is a housed gear train that affects the rotation speed of gears/axle shafts. The "diff" takes power from the engine and delivers it to the wheels, usually the tire with less traction, thereby differentiating the power needs of the 4x4. Differentials allow the steering tires/wheels to turn at different speeds so the vehicle can take turns without undue stress/pressure on the tires. Typically, the inside tires move shorter distances than the outer tires/wheels. Differentials are sometimes called "pumpkins" due to their rounded shape.

Discontinuity of rolling contact: a generic term for wheel spin and wheel slide - as on locked brakes. See 'Continuous rolling contact' above.

Donuts: is a general term for bushings used to lift a vehicle body from the frame, providing more body-frame clearance and better ground clearance.

Droop: is a common term for total suspension travel, usually tested on a lift, allowing the tires to "droop" to full extension. Droop is typically measured from ride height down. Up travel or full compression, is measured from ride height up.

Drop Pitman Arm: aftermarket extension of the steering box to steering linkage that (usually) corrects steering geometry by reducing the drag-link angle.

Electronic Stability Control (ESC): also sometimes called Electronic Stability Program (ESP) or Dynamic Stability Control (DSC), is a computerized technology that improves a vehicle's stability by detecting and reducing loss of traction (skidding). When ESC detects loss of steering control, it automatically applies braking to wheels individually, such as the outer front wheel to counter oversteer or the inner rear wheel to counter understeer. Some ESC systems also reduce engine power until control is regained. ESC does not improve a vehicle's cornering performance; instead, it helps reduce the chance of the driver losing control of the rig.

Electronic traction control (ETC): is a feature in some modern-day vehicles that inhibits wheel spin by applying the brakes 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.

Endangered Species: a critter or plant is endangered when it is in danger of extinction. Conservation status categories of endangered species include threatened (and sometimes "rare.”) The Endangered Species Act (1973) (ESA) provides the process for determining the status of such species and what to do (or not do) about it. The ESA has been responsible for many trail and riding area closures.

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

Flotation: is a term used to describe the characteristics of a vehicle because of large aired-down tires, not to sink in soft ground, mud, or sand.

Four-corner awareness: is meant to describe the driver's responsibility to know the location of all four tires (corners) of the vehicle, especially while negotiating obstacles or when around pedestrians on the trail. Typically, the right rear tire (corner) is the least observed spot on a 4x4 in motion and often receives trail damage. Expert off-pavement drivers know exactly where all four tires are on an obstacle.

Four-wheel drive (4x4): vehicle transmission system in which engine power is applied to all four wheels, sometimes simultaneously or part-time. The term 4x4 (four by four) has the specific connotation that it is a four (wheeled vehicle driven) by four (wheels); however, the term is a misnomer as unless the axles have posi-traction/limited slip, or lockers, a 4x4 is actually two-wheel drive. One front and one rear tire receive torque (power) when in 4x4 unless other serious mods are in place.

Full-float: "floating" axles remove the vehicle weight off the axles and "floats" the rear end housing. In this case, if the axle breaks, the 4x4 can still roll freely. Semi-float means the weight is on the axle shaft itself. The full floating axle has the wheel hub and rod attached together. The axle housing has the wheel mounted onto it rather than onto the axle rod. This means the rod doesn't get the bending effect like with the semi-floating axle. The rod will get the torque effect only, which gets transmitted to the wheels from the differential.

Full size: vehicles larger than "short-wheelbase" such as Jeep Gladiators, Grand Cherokees, and Jeep Wagoneers.

Ground clearance: commonly a measurement of ride height. It is the space between the ground and a given mechanical vehicle part. Usually, when quoted for a vehicle, taken as the least for any component on the vehicle – ground clearance is the space under the differential casing. But note there is a difference between under-axle and underbelly clearance.

Heel and toe wear (cupping): jargon for the uneven front to rear wear on individual blocks of a bold off-road tire tread when used on roads

High-centered: is jargon for a vehicle stuck on an obstacle near the mid-ship (center), usually on the frame or belly skid.

High range: is the transmission status when the two-speed transfer case is in the high range position - for normal, on-road, day-to-day use.

Hi-lift jack: is a versatile, lever-operated, long, narrow, mechanical bumper jack capable of lifting a vehicle high off the ground.

Hill-descent: is a driver-assistance feature that holds the 4x4 at a specific speed without any input from the driver while traversing down descents in rough ground. It is like a super-slow cruise control.

Hi-lo transfer case shifter: is sometimes used to describe the transfer case lever. One lever selects 2WD, neutral, or 4WD, while the other lever selects high or low range, depending on the vehicle.

Jerk strap: a tow strap, usually nylon or synthetic rope with a built-in stretch, used to "jerk" or yank out a stuck vehicle. The yanking/towing vehicle is in motion before taking up the slack in the tow/yank rope, providing a sudden forward motion to the stuck vehicle – a yank.

KERR: is a European term for kinetic Energy Recovery Rope. A descriptive term coined to describe specially constructed nylon type ropes capable of stretching during a "snatch tow" recovery, where the towing vehicle is in motion before taking up the slack in the tow rope – more commonly called a yank or jerk recovery tow.

GVW: gross vehicle weight - the maximum permitted laden weight of a vehicle, including payload, fuel, and driver. GVWR is the vehicle's gross weight rating, the vehicle's maximum weight rating as defined by the manufacturer. GVWR includes the vehicle's chassis, body, engine, engine fluids, fuel, accessories, driver, passengers, and cargo, but excluding that of any trailers.

Kinetic energy: is the energy of motion, proportional to the vehicle's total weight and the square of its speed. Thus, if a vehicle's weight doubles, its KE also increases twice, but if its speed doubles, its KE increases by two squared, i.e., four times. See 'snatch-towing.'

Laden weight: is the weight of a vehicle carrying some or full payload. See also GVW concerning loading to maximum permitted weight. Also see "Curb Weight."

Landuse or Land Use: jargon for efforts to save trails, sustain motorized recreation, and keep it alive and well, usually led by knowledgeable, experienced advocates versed in government bureaucracy and environmental policy and sometimes blended in with access, both meaning the designation of how the public uses our lands (and waterways).

Lifted: is jargon for a 4x4 that is raised above stock height with lift blocks, pucks, donuts, or suspension upgrades.

Limited slip: a traction device in the differential that limits the slip of tires/gears/axles but does not lock up the axles together (like actual lockers).

Line: the specific route taken over or through an obstacle. It is common to hear a driver struggling with an obstacle say, "I took the wrong line."

Locked up: refers to positive locking devices in both front and rear differentials like "lockers." It may be used to refer to one axle, as in someone asking, "Is your rear-end locked up?"

Lockers: are a power-differentiating device that allows engine power to be delivered to both wheels on one axle simultaneously, providing maximum traction by engaging the spider gears together like a farm tractor, eliminating unequal rolling traction.

Low gears: the transmission status when the transfer case is in the low position - for challenging off-road conditions demanding greater traction or low-speed control.

Low ratio: describes the transmission when the transfer case is in low.

M+S Tires: mud and snow tires. A generic term for 4x4 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. Some mud tires with large, bold blocks wear out faster and are nosier on pavement.

Multi-purpose tire: usually called an all-terrain tire, is a combination/compromise between on-road and mud tires.

NEPA: the National Environmental Policy Act (NEPA) defines the level of documentation needed for projects on public (federal) land and how land will be used. NEPA provides the public with the ability to input comments.

Off camber: a section of sideways trail that causes a 4x4 to lean one side or the other, thereby increasing the likelihood of a rollover.

Off-road: is slang and a bit of a misnomer for driving off-pavement or off-highway yet still on designated, legal trails/roads.

Off Road Plus (+): Jeep function, when activated, is designed to improve the user experience when using specific off-road driving modes. It does not function in 2WD High mode. When shifting into 4-Hi under Off-Road Plus mode, you'll see more responsive acceleration and a change in how the vehicle performs in softer terrain. The mode dials in a more tuned traction control, allowing additional wheel slip in sandy conditions to ensure the vehicle doesn't bog down and lose momentum. Additionally, this mode optimizes transmission behavior, holding gearing longer and keeping the revs high for continued power. This is crucial when attempting to traverse a muddy patch on the trail or looking to crest a particularly tall series of sand dunes. Using Off-Road Plus in 4-High also modifies the Jeep's Electronic Stability Control, making it less obtrusive and generally keeping it from impeding movement so quickly — helping the vehicle overcome traction issues in sand, mud, or snow.

On-foot recon: inspecting a difficult off-road obstacle on foot before committing your vehicle to it. It is sometimes referred to as pre-walking or pre-scouting.

Open differential: is a stock or unmodified vehicle with no lockers or limited slip mechanism in the pumpkin/differential. The open diff will distribute power to the one tire with the least resistance (least traction, most spin). An open differential is a device that splits the power from the transmission to the driven wheels and allows them to rotate at different speeds.

Over-torque: is the concept of applying too much torque (or power) to the wheels so that they break their grip with the ground and spin – sometimes referred to as "peeling out."

Pintle (NATO towing hook): large, robust, four-bolt attachment towing pintle with top Closure and, usually, 360 rotational capabilities about the longitudinal axis originally specified for NATO military vehicles. Suitable for off-road towing, albeit because a trailer towing eye will not be a close fit over the hook, it generates quite a bit of noise with back-and-forth banging.

Power Take Off Winch (PTO): a winch generally mounted on or just behind the front bumper usually runs from an engageable extension to the engine crankshaft. The active component is usually a slowly revolving round drum on which a rope or cable may be wound to effect a winching operation. It 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 demanding a high electrical load.

Pucks: is a general term for bushings used to lift a vehicle body from the frame, providing more body-frame clearance and better ground clearance

Pumpkin: is slang for the front or rear differential, sometimes shaped like a pumpkin. It can also refer to just the removable center section of a differential housing.

Ramp break over angle: is a measure of an automobile's ability to be driven over the crest formed by two converging surfaces without scraping its underside. The ramp breakover angle varies inversely to the wheelbase – the longer the wheelbase, the less ramp breakover angle.

Ramp Travel Index (RTI): is a formula to measure a vehicle's ability to flex its suspension, also known as axle articulation. The RTI rating is used mainly in the off-roading industry to test and describe chassis limits of modified vehicles. The ramps vary between 15 and 30 degrees of angle for the vehicle to ride up. The driver drives one tire up the ramp until the back rear (opposite) tire wants to lift off the ground.

Reduced inflation: lowering tire pressures to increase flotation in soft ground conditions such as mud or soft sand. Better known as "airing down."

Sand tracks: are the generic name, also known as Traction Mats, for any item made to lay beneath the tires in soft sand to provide grip and floatation. It may be PSP (pierced steel planking), aluminum, rubberized, or artificial plastic-type material.

Sand tires: is a term often used to mean a desert tire, implying an ability to cope with desert rock, stones, and sand. In the duning community, a sand tire is often designed to eliminate the likelihood of sinking by having a wider tire, often with bold tread patterns for traction in soft sand.

Salt flat: is a 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.

Scrub radius: the distance between the point where the steering axis (or kingpin) would intersect with the ground and the center of the tire contact patch. It can be positive, negative, or zero, depending on the alignment, suspension, and wheel offset. It influences the vehicle's steering response, stability, and tire wear.

Selec-Speed Control (SCC): is standard on modern Jeeps with automatic transmissions and off-road packages, which are different from hill descent. Selec-Speed Control is like cruise control for off-pavement driving. SCC maintains a constant speed by controlling engine torque and the brakes, keeping the vehicle's momentum stable and allowing the driver to focus solely on steering.

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

SEMA: Specialty Equipment Manufacturing Association that holds an annual SEMA Show in Las Vegas that most of the off-road world attends as vendors or jobbers.

Sidewall: is a term that describes 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 from oblique rubbing contact with side-swiping sharp rocks in off-road operations. Driver awareness is essential.

Sidewall deflection: the tire sidewall's outward movement in the ground contact patch region 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 severe damage to the tire.

Shock loading: means the arrest of mechanical motion in an excessively abrupt way or the application of sharp load reversals in such a way as to risk structural failure. For example, applying the handbrake while the vehicle is in motion can cause unacceptable shock loading of the rear axle half-shafts. Engaging a diff-lock while one or more wheels are spinning could also result in a severe and damaging shock load to the transmission.

Spotting: the detailed direction given to the driver of a vehicle by someone outside the vehicle who can see all four wheels and the obstacle being traversed. Use spotters when there is a danger of damaging tire sidewalls or the vehicle's underside on rocks or other obstacles. Only one spotter must communicate with the driver – and that spotter should be a trusted and experienced four-wheeler. Spotter and driver agree beforehand, face to face, on signals to be used while spotting. Terminology is also essential – rather than left or right, most spotters/drivers use driver/passenger for the lingo, eliminating confusion on who's left or right. Important spotting rule- if the driver cannot for any reason see the spotter, the driver does nothing – no steering, no moving, no wheel changes, no nothing. The driver waits until seeing the spotter before making any movements.

Spotter: the one who gives detailed directions to the driver inside a vehicle while being utside the vehicle and can see all four wheels and the obstacle being traversed.

Steel-belted tire: the technical name for the belts or crown plies that provide the rigid base for the tread.

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 guide for the Marlow Ropes Recovaline is 40% stretch; this limit should NEVER be approached.

Suspension: is a generic term to describe the undercarriage weight-handling, body height, load-leveling, and powertrain components of a 4x4. Typical suspension types include solid axle, four-link, and independent front suspension.

Sustainable: in landuse, a term meaning "able to be maintained at a certain rate or level," such as trail use. It is also defined as conserving an ecological balance by avoiding depletion of resources. In the 4x4 world, it means managing all factors that impact the trail and its surroundings so that trail use can continue into the future.

Sway bar (disconnect): a feature (button on the dash) (Jeep JK, JL, and JT models) is an electronic disconnect for the sway bar, allowing for a better ride with more vehicle articulation over uneven terrain or obstacles. Sometimes referred to as stabilizer bars, typically on the front and rear of a vehicle, they can be solid or tubular and balance the steering while turning. The sway bar is usually engaged while driving on pavement, then disengaged (disconnected) when off-road.

T-case (transfer case): is a device usually attached to the transmission that serves as a gearbox that splits the engine power to the front and rear axles, transferring power from the engine to the wheels. The t-case also provides lower gear ratios for rock crawling and slow rolling.

Tail Gunner: is the title (and often radio call sign) for the last vehicle in a convoy whose job entails ensuring no one is left behind while communicating the convoy's progress to the Trail Boss/Leader.

Tires, cross-ply tire: a tire in which the sidewall reinforcement plies run diagonally (45-degree angle) from the bead towards the tread - each layer of cords at a different angle to its adjacent layer, crisscrossing the cords. Generally superseded by radial-ply tires whose thinner, more flexible sidewalls and braced tread yields better grip and lower rolling resistance. In radial ply tires, the cords are arranged at a 90-degree angle to the wheel's center line and overlap.

Tires, radial ply tire: a type of tire construction in which sidewall structural plies run radially out towards the tread instead of crisscrossing diagonally. With their thinner, more flexible sidewalls, radial tires have lower rolling resistance than cross-ply tires (yielding better fuel consumption) and give longer tread life.

Tire bead: provides the clamping mechanism to ensure an airtight fit of the tire properly seated to the rim/wheel.

Tire sidewall: what protects the side of the tire from impacts with curbs, obstacles, and the road. Tire details and manufacturing specs, such as tire width and speed rating, are listed here.

Tire tread: the outer visual of a tire; the basis for traction and cornering grip, designed to resist wear, abrasion, and heat.

Tire type: jargon used to identify the use of tires like passenger, all-terrain, mud-snow, and truck.

Tongue weight: is the amount of nose/tongue-heaviness (sometimes called trailer preponderance) measured at the tow hitch and must be considered part of the towing vehicle's payload.

Tow hooks: metal hooks (like D-rings) on the front/rear of a vehicle, usually attached/anchored to the frame.

Tranny: slang for transmission.

Traction: the concept of achieving grip between the wheels and the ground without slip, skid, or sinkage.

Traction Control: see 'Electronic Traction Control, ETC'.

Traction mats: are the generic name, also known as Sand Tracks, for any item made to lay beneath the tires in soft sand to provide grip and floatation. It may be PSP (pierced steel planking), aluminum, rubberized, or artificial plastic-type material.

Trail Boss or Trail Leader: the person in the lead of a convoy, directing and leading the progress of a trail ride.

Trailer Queen: is a vehicle that spends more time on a trailer than on a trail. Usually, a show rig or SEMA build.

Transfer case (T-case): a gearbox that transfers power from the engine to the wheels on a four-wheel drive vehicle.

Travel Management Rule (TMR): part of the route designation process where all trails and roads must be marked on a map, intending to provide a sustainable system of roads while limiting or excluding cross-country travel (off trail).

Tread Lightly: is an organization and a set of principles of responsible, sustainable motorized recreation, including respecting the land, minimizing impacts, packing out trash, obeying the laws and rules, and being a responsible four-wheeler.

Tree Hugger: is a slang and somewhat derogatory nickname for extreme environmentalists.

Tree Saver: a wide strap of webbed/synthetic and sewn material used to wrap around a tree anchor point to save the tree from damage during recovery operations.

Unladen: Vehicle carrying fuel, driver but no payload or other load - see 'Kerb' above.

Viscous coupling unit (VCU): is often found in all-wheel-drive vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set. Viscous couplings are used as a center differential in some four-wheel drive vehicles. The viscous coupling is based on the principles of a hermetic container. The viscous coupling is a cylinder with a shaft or axle protruding from either end. Inside, plates are attached to each shaft to spin alongside each other. One axle and set of plates turn in time with the front wheels, and the other axle and set of plates turn in time with the back wheels.

Weep holes: oil drain holes in the bottom of the clutch housing (and the camshaft drive-belt housing on some engines) to preclude the possibility of the clutch or cam belts becoming contaminated in the event of oil leaks from the adjacent bearings. Weep holes allow water to escape and prevent damage. Wadding plugs is another name for these holes/plugs.

Washboard road: also called corrugation of roads, is a series of ripples spaced close together, which occurs with the passage of tires rolling over unpaved roads at speeds sufficient to cause bouncing, giving the appearance of a laundry washboard.

Wheel Travel: the distance measured between up and down as the suspension works.

Wilderness: based on the 1964 Wilderness Act by Congress, a land designation that allows no motorized or mechanized travel of any type, including bicycles.

Yank strap: a tow strap, usually nylon or synthetic rope with a built-in stretch, used to "jerk" or yank out a stuck vehicle. The yanking/towing vehicle is in motion before taking up the slack in the tow/yank rope, providing a sudden forward motion to the stuck vehicle – a yank.


This glossary was compiled with the help of Off Road Hall of Famer and Author Del Albright