Truck Accident Reconstruction Glossary

We are often asked by retaining counsel to define certain industry terminologies utilized in our truck accident reports. After careful and patient defining of the artful language utilized, they are often referred to our website for further clarification.

We always remain available to educate those who retain us in the language of the trucking industry, whether we are serving as a truck accident expert witness or speaking at an industry conference.

The Truck Accident Reconstruction Glossary below contains many of the terms and phrases you will see in our reports and hear throughout discussions of accidents, testimony at trials, etc. Please feel free to bookmark this page for your future reference.

ABS: anti lock braking system

Acceleration: a change in velocity with respect to time. Values are either positive or negative with standard orientation.

Acceleration Scuff: friction marks that occur when the tires on the drive wheels of the vehicle are turning faster than they would normally turn, given the speed of the vehicle.

Air Compressor: The air compressor is the source of energy for the air brake system.

Air Dryer: The air dryer is an in-line filtration system whose primary function is to remove any liquid or water vapor from the compressor discharge air so that only clean and dry air is supplied to the air brake system.

Airlockers: air operated differential locks for traction purposes.

Air Coupling Device: The trailer air supply lines are connected to the truck-tractor by a strengthened flexible hosing. There are two lines that are color coded for easy identification. The blue hose is the service line and supplies air to the service brakes. The red line is the emergency line and supplies air to the parking and emergency brakes. The coupling device is commonly referred to as a “glad-hand”.

Air Reservoirs: The reservoir serves as a storage tank for the volume of compressed air needed to operate the air brake system.

Angular Momentum: a quantity associated with how an object moves around a reference point. It is often used to describe rotating objects. The angular momentum of an object is defined to be equal to its mass times its velocity about the point times its distance from that point.

Approach Angle: as a vehicle approaches a grade, it’s the angle of slope which it can climb up without any part of the front (bumper, shackle etc.) hitting the ground/rock

Articulation: the extent of movement between one tire and the other on the same axle

Asphaltic Concrete: a flexible roadway surface that meets the needs of most highways. It is constructed of an asphalt and rock mixture and is black in appearance.

Axis of Rotation: in a rotating object, the axis of rotation is the line about which the object is turning. The axis of rotation for a spinning record is its middle. The reference point for angular momentum is often chosen to be along the axis of rotation.

Brake Build-up (Brake Lag): occurs just after the brakes are applied and continues until they lock.

Brake Chamber: The brake chamber converts the energy of compressed air into the mechanical energy required for braking. There are three type brake chambers:

  1. Clamp
  2. Bolt
  3. Rotochamber

Brake Fade:  A reduction in braking effectiveness caused by excessive heat and wearing of components.

Brake Pad: The anti-friction material that lines plates and calipers used in disk brakes. Brake pads help to reduce heat and increase the life span of braking elements.

Brake Shoe: Anti-friction material that lines metal plates in brakes. Shoes help to reduce heat and increase the life span of braking elements.

Breakover Angle: when a vehicle crests a hill, it’s the angle at which the front tires still remain on the ground as the rear wheels continue to climb (see High Centered)

Center of Mass: An imaginary point within a rigid object where all the object’s weight is concentrated.

Chemical Energy: energy that is stored in your body and in molecules

Chord: a straight line that intersects a circle at two (2) points.

Closing Speed: The speed differential between two vehicles traveling on the same line.  The sum of the two speeds for opposing vehicles.  The following vehicle speed minus the lead vehicle speed for vehicles traveling in the same direction.

Coefficient of Kinetic Friction: ratio of the magnitude of the force of kinetic friction to the magnitude of the normal force.

Coefficient of Friction: The skid resistance of a roadway surface expressed in tenths and hundredths of one G.  The lower the number, the slicker the surface and the longer it will take a vehicle to stop at a given speed.

Collision Scrub: occurs during a traffic collision when the downward and rotating forces cause a smearing of tire material on the road surface.

Conservation: the idea that the total amount of certain quantities in nature always stays the same. Energy, linear momentum, and angular momentum are examples of quantities that obey the law of conservation.

Conserve: stay the same or constant. To say that something is conserved means is to say that the amount of it stays the same and obeys conservation.

Coordinate Method: technique of scene documentation where items of evidence are located by measuring the distance from two (2) reference lines. Similar in concept to locating points in the Cartesian plane.

Counter Torque: A rotational force that opposes the directional force of torque. Plugging and dynamic braking apply counter torque to a rotor.

Delineators: those devices that direct, channel and separate vehicular traffic.

Delta-V: The velocity vector difference between a per and post impact.

Departure Angle: the angle between your rear tires and the first rear protrusion that would scrape the ground as you descend off of a hill

Differential: also referred to as “pumpkin” it’s the device that transmits power to two points allowing one to go faster than the other

Divided Attention: the ability or lack thereof to do more than one thing at the same time.

Drag Link: Connects the pitman arm to the steering knuckle arm or spindle arm on the near side (typically left side) of the vehicle.

Effective Drag Factor: the ratio of the velocity squared to the quantity of the skidding distance multiplied by twice the gravitational constant.

Energy: a quantity associated with a system. Energy is often related to the motion of an object or the potential for motion. Kinetic energy, potential energy, chemical energy, and heat energy are examples of forms of energy.

Fifth Wheel: The fifth wheel is the coupling device between the truck-tractor and trailer; however, the fifth wheel is mounted to the truck-tractor.

Final Rest Position: controlled or uncontrolled post-collision resting position of vehicles, pedestrians, debris, etc.

Floating Axle: in the “full” case, the drive shaft is independent (floating) of the hub bearings, in the “semi” state the shaft supports the hub bearings

Fog Line: The solid white line on the right edge of a roadway.

Force: an influence upon an object that causes motion or a change in motion

Four Wheel Drive: all four wheels are driven (also referred to as 4wd or 4×4 4by4)

Four Wheeling Hub: hubs are devices in the front wheels which makes you able to disengage the front drive train, to free wheel, and when needed, lock the hubs in for four wheeling situations

Friction: the resistance of an object to movement across a surface. A way that a form of energy, such as kinetic energy, is changed into another form, usually heat energy. Friction causes you to slow down when you are swinging. It also causes a spinning top to eventually fall over.

Friction Brake: A brake that engages when two surfaces press together and transfer energy through friction. Friction brakes are the most common type available.

G-Force: A term used when accelerations are expressed as a multiple of the acceleration of gravity.  The acceleration of gravity equaling 1G or 32.2 feet per second squared.

Gaze Nystagmus: Involuntary oscillation of the eyes as they gaze at an object moving through the field of vision.

Gravitational Acceleration (g): the constant describing the acceleration of any object falling toward the earth. Near the earth, gravitational acceleration is approximately equal to 9.8 meters per second per second (m/s2).

Gravitational Potential Energy: a type of potential energy. Gravitational potential energy is associated in the interaction of an object with the earth. It is defined to be equal to the mass of the object times the gravitational acceleration times the distance of the object from the ground.

Ground Clearance: this is the amount of space between the lower most hanging part of your vehicle’s undercarriage and the flat ground.

Gouge Mark: A pavement scar made by a crash-involved vehicle.

Heat Energy: a type of energy associated with temperature. Actual amounts of heat energy are difficult to measure.

High Centered: when obstacles or terrain are broached by the front tires and the vehicle is suspended on it’s undercarriage so that three or more tires lack traction

Impact Aspersion: occurs when a fluid container is ruptured under extreme pressures of impact and then aspersed onto the roadway.

Impending Skidmarks: also known as shadow skidmarks are small fragments of rubber, ground or scraped from the tire, at the tire and roadway interface, as the rotating tire slows to a locked position.

Inertia: The tendency of an object to stay in its state of rest or motion until acted on by an external force. Torque and braking must overcome inertia to accelerate or decelerate a motor.

Jack: a device used to suspend a portion of your vehicle off of the weight of the tire in order to make repairs, also can be used to help get a vehicle ‘unstuck’ (popular brand name is “HiLift”).

Jackknife: A dynamic that occurs in a combination vehicle when the vehicle towing a trailer brakes more heavily than the trailer.  This braking imbalance will cause the rear of the towing vehicle to move right or left depending on the external forces applied to it.  Jackknifing can also be caused by an extreme Oversteer condition in the towing vehicle.

Kilogram (kg): a unit or measurement for mass. A one-kilogram mass on a scale reads about 2.2 pounds.

Kinematics: the branch of mechanics that uses mathematics to describe motion, without reference to the forces or masses involved.

Kinetic Energy: a type of energy describing an object’s motion. Kinetic energy is defined to be equal to 1/2 times the object’s mass times the square of the velocity of the object.

Kinetic Friction: describes the frictional forces present between surfaces in relative motion.

King Pin: The king pin is a cylindrical shaped implement that protrudes from the bottom front of the trailer that connects to the fifth wheel. This allows for a rotational affect causing the pivoting articulation of the trailer.

Linear Momentum: a quantity associated with the motion of an object along a straight path. The linear momentum of an object is defined to be equal to its mass times its velocity.

Live Axle: an axle supporting the weight of the vehicle containing the power to the wheels

Locker: or differential locker, is a device that forces the diff to allocate equal power to both points (both tires) thus keeping power to a slipping tire

Mechanical Lag Time: The time that expires from first brake application to activation of the foundation brakes.

Meter (m): a unit for measurement for length. One meter is about equal to 3.3 feet or 1.1 yards.

Middle Ordinate: the distance of the center point of the chord, to the outer edge of the inertial scuffmark. Used in determining the radius of the circle that most closely approximates the shape of the arc.

Momentum: the mass of an object multiplied by its velocity. It is a vectored quantity with both magnitude and direction.

Normal Force: the force that a body exerts on another body at right angles to their mutual interface.

Off Tracking: A dynamic that occurs in a turning combination vehicle whereby the trailer tires track a different radius than the towing vehicle tires.  The trailer tires usually track a smaller radius except at high speeds.

Pedestrian Manifestations: those items of evidence deposited at the collision scene by the pedestrian during or following impact.

Perception Reaction Time: The time needed by a driver for seeing a hazard, recognizing that hazard, deciding on an evasive action, and initiating that action.

Pitman Arm: This component transfers steering motion from the gearbox to the steering linkage. It is attached to the shaft of the steering box and connects to one end of the drag link.

Point of No Escape (by braking): consists of te perception/reaction distance plus te slide-to-stop distance for the specified velocity.

Portland Cement Concrete: a rigid roadway surface designed for roadways that are continuously exposed to large trucks, large volumes of traffic and sections of roadway where the grade is 12 percent or greater. The color is normally white or grey.

Potential Energy: a type of energy having to do with how two or more objects are interacting. Potential energy is stored in the interaction of an object with another object. Gravitational potential energy is a type of potential energy.

Pushrod and Yoke: The pushrod is a threaded rod that is on one end attached to the diaphragm plate located inside the brake chamber. The pushrod travels out of the brake chamber in a linear direction. At the end of the pushrod is a yoke that is attached to a slack adjuster with a clevis pin.

Quantity: a characteristic of a system that can be well described and possibly assigned values. Energy, linear momentum, and angular momentum are examples of quantities associated with all systems.

Quick Release Valve: The function of the quick release valve is to rapidly exhaust air from the brake chambers once the foot treadle is released.

Radius: when used to describe angular momentum, it is the distance of the object from the reference point

Raised Pavement Markers (R.P.M.): Non-reflectorized ceramic pavement markers, also known as “Bott’s Dotts”, are most often used to delineate travel lane lines. Reflectorized RPM’s come in many different colors and are used to delineate travel lane lines and outside boundaries.

Reference Point: in angular momentum, the point in space around which the motion of an object is described. The angular momentum of an object is defined in relation to some point, the reference point. The velocity and radius of an object are measured from the reference point. The reference point is often chosen to be along the axis of rotation of a rotating object.

Regrooved Tire: A regrooved tire is one that has new grooves cut into it.

Recapped Tire: A recapped tire is made by bonding new tread rubber to a used tire.

Relay Valve: The relay valve is found on or near both the drive axle brakes and semitrailer axle brakes and ensures simultaneous application of the front and rear brakes on a combination unit when an air brake application is made with the foot treadle.

Rim Gouge: A scar on the pavement made by a wheel rim.  Usually related to a missing or under-inflated tire.

Safe Stopping Distance: same value as the point of no escape.

Scientific Method: 1.) make observations of available facts; 2.) develop a hypothesis; and 3.) test the hypothesis

Scrape: An area of pavement covered by many scratches or striations made by sliding metal.

Scratch: A light scar made on the pavement, made by a sliding metal part.

Scuffmarks: friction marks left on the roadway by tires that are free to rotate.

Shackle: for recovery, it’s the “U” shaped device with a screwable pin across the open top that results in a “D” shape so that the two ends of a strap can be securely attached from which the hook of winch cable can be attached as an anchor point

Shackle (suspension): a coupling device usually found on leaf springs, used to attach the frame and the suspension component

Short/Long Wheel Base: the manufacturers wheel base dimension measured from the center point of a front wheel to the center point of the same side rear wheel (wheelbase from front axle to rear axle) ‘short’ refers to “Wrangler” type lengths, long refers to “Grand Cherokee” type measurements

Skidmarks: dark deposits of tire and roadway material normally deposited on a roadway surface by a tire that is not free to rotate.

Slack Adjuster: The slack adjuster is the link between the pushrod and the S-cam brake shaft. It develops a movement around the longitudinal axis of the S-cam shaft.

Snatch Strap: another name for the recover strap, in this case referring to a maneuver that carefully makes a concerted ‘tug’ between one vehicle and the stuck vehicle, to ‘snatch’ or ‘yank’ the vehicle forward or backward, over an obstacle or out of gripping terrain

Speed: magnitude void of direction. How fast an object is moving

Sporadic Trickle: the fluid trail that usually starts after impact aspersion and continues to the vehicles final rest position or until the fluid container is empty.

Static Friction: describes the frictional forces present that are between surfaces at rest with each other.

Stop Lamp Switch: The stop lamp switch operates the vehicle’s stop lamps by completing an electrical circuit each time a brake application is made.

Stopping Distance: The total distance needed to perceive, react, and brake to a stop.

Strap: a specially designed band of material that can be connected between vehicles to pull one vehicle forward, there are different widths and strengths for different weight applications

Suspension Travel: how far between the highest and lowest distance a wheel can ‘travel’ from full compression to full hang or droop

System: a group of objects.

Tie Rod End: The small connecting part between each end of the tie rod bar and the corresponding steering arm.

Tractor Protection Valve: The tractor protection valve protects the tractor air supply under trailer breakaway conditions or when a sudden air loss evacuates the system.

Torque: simply defined as the twisting force from the power of the engine

Transfer Case: the secondary gear box splitting the power to the front and rear of the vehicle

Transmission: the unit transmitting power from the engine to the wheels

Tree Strap: a small band of strap that is used around the base of a strong tree, a shackle point attached at the two ends, and then a winch cable can be connected for self recovery (this protects the tree – never put metal cable around a tree!)

Triangulation: technique for scene documentation where items of evidence are located by measuring the distance between two (2) reference points.

Velocity: a change in position with respect to time. Has properties of both magnitude and direction. The speed of an object in a certain direction. Velocity is usually measured in meters per second (m/s)

Wheel Separation: Wheel separation is when one or more wheels become separated from the truck-tractor, or the trailer. Typical wheel separation is caused by wheel seal failure, causing bearing failure. A different type of wheel separation is as a result of stud/lug failure.

Yaw: a sidewise movement of a vehicle in turning; movement of a vehicle in another direction than that in which it is headed; sidewise motion produced when centrifugal force exceeds traction force. Often the result of overreaction or exceeding the critical speed. Sometimes revealed by tiremarks on the roadway.

Yawmark: a scuffmark made while a vehicle is yawing; the mark made on the road by a rotating tire which is slipping in a direction parallel to the axle of the wheel.