Tachometer
An instrument that indicates the revolutions per minute (RPM) of the engine.
Tag Axle
An auxiliary axle installed in conjunction with the rear axle(s) of a truck chassis. A tag axle is installed behind the rearmost axle, thus extending the length of the wheelbase. See Tandem Axle.
Tail Plate
Rearmost part of the towing or recovery vehicle body.
Tandem Axle
Two axles mounted as a group (three axles placed together are often referred to as a Tri-axle Tandem). There are three tandem-axle drive types: (1) Dual-drive Tandem, both axles have drive mechanisms and are connected to the engine power unit; (2) Pusher Tandem, only the rearmost axle is a driving type and forward unit is free rolling (load-carrying only), commonly called Dead Axle; (3) Trailing Axle Tandem (Tag Axle), forward unit of tandem is a driving type while rear unit is freely rolling. Two trailer axles are also called tandem axles.
Tare Weight
The total weight of an empty vehicle in a condition ready to receive payload. Same as Curb Weight.
Telematics
The integrated use of telecommunications and informatics, also known as Information and Communications Technology (ICT). More specifically, telematics is the science of sending, receiving and storing information via telecommunication devices, including cellular communications and satellite systems.
- Active Telematics —Systems that are typically capable of interfacing with a vehicle on a real-time basis.
- Passive Telematics —Systems that may send or receive information but cannot actively interface with a vehicle.
Thermal Efficiency
The efficiency of an engine in converting heat energy from combustion of fuel into mechanical work.
T-Hook
Attachment device used for towing.
Tiedown Assemblies
Device(s) used to restrain cargo. Also called Cargo Control or Restraint Equipment.
Tilt Cab
Vehicle designed with engine beneath cab and having provision for tilting cab forward on a pivot near front bumper to provide easy access to engine.
Tilt Cylinder
Cylinders used to change the attitude of a structure or body.
Tire Clearance
Space between tires and nearest part of the body or under-construction.
Tire Loaded Radius
The distance from the center of the wheel to the road with tire loaded to rated capacity. Static radius applies when vehicle is at rest; rolling radius for a vehicle when in motion. The latter dimension is usually slightly greater than the static radius and is the figure used in determining the tire revolutions per mile.
Torque
The rotating or twisting force developed by the truck engine. This is one of the two factors in figuring horsepower and is always expressed in pound-feet. At a given RPM, the higher the torque, the greater the horsepower. The higher the torque rating of a truck engine, the greater its ability to climb hills and increase speeds. A high-torque truck engine eliminates a lot of gear shifting for the driver.
Torque Converter
Used in truck and car automatic transmissions. Torque is multiplied by the action of various turbine-like elements on a fluid.
Torque Multiplication
The truck transmission as well as rear-axle gears multiplies the engine’s torque. This is done by reducing engine speed through gears, thus increasing torque by reducing revolutions per minute.
Tow Bar
A device for positioning a towed vehicle behind a recovery vehicle.
Tow Chains
Chain assemblies used as a primary coupling between towing and towed vehicles (not the same as Safety Chain).
Tow Sling
A device used for lifting and towing vehicles with the load supported on rubber belts and chains.
Tow Vehicle
Vehicle used to lift/tow other vehicles.
Tractive Effort
The force available at the road surface contacting the driving wheels of the truck. Determined by engine torque, transmission ratio, axle ratio, tire size and frictional losses in the driveline. Also called Rim Pull.
Tractive Factor or Performance Factor
This is tractive effort per thousand pounds of gross vehicle weight. A means of measuring the performance potential of a truck or tractor.
Tractor
A tractor, or truck tractor, is a truck designed primarily for drawing other motor vehicles and not so constructed as to carry a load other than a part of the weight of the vehicle and the load so drawn.
Tractor Breakaway Valve
Coupled between the tractor and trailer emergency brake system, the tractor breakaway valve provides an air supply to the trailer emergency system for normal operating conditions. In case of trailer brake system failure, the breakaway valve automatically seals off the flow of air pressure from the tractor to the trailer preventing the loss of air pressure from the tractor braking systems and activates the trailer emergency brake. In conjunction with a breakaway valve, a dash-mounted manual control valve is located in the cab. This manual control is used to charge the trailer brake system reservoir for normal operation. In the event of loss of air pressure in the normal braking system, this manual control can be used to seal off the tractor brake system.
Trailer
A motor vehicle with or without motive power, designed for carrying persons or property and for being drawn by another motor vehicle.
Trailer Converter Dolly
A trailer chassis equipped with one or more axles, a lower half of a fifth wheel and a drawbar.
Transmission
A gear reduction device that contains an assembly of gears and associated parts that transmits power from the engine to the driving axle(s). A transmission contains a number of gears that when a connection is made between a specific set provides a choice of ratio. Connection is made by sliding the teeth of one gear into mesh with another, or by engaging a tooth-type clutch that has one part fastened to a gear already meshed to another and the other part splined to a shaft. Synchromesh transmissions use gear-speed synchronizers to ease engagement.
Tread
(1) The distance between the centers of tires on the same axle at the points where they contact the road surface. Duals are measured from the center of dual wheels; (2) That portion of a tire that comes into contact with the road.
Tread Rib
A tread section running circumferentially around a tire.
Tread Separation
Pulling away of the tread from the tire carcass.
Truck
A motor vehicle with motive power, except a trailer, designed primarily for the transportation of property or special purpose equipment.
Truck Hitch
A device for positioning and supporting one end of a towed vehicle behind a recovery vehicle.
Truck Tractor
A truck designed primarily for pulling a semi-trailer and not so constructed as to carry a load other than a part of the weight of the semi-trailer. See Tractor.
Trunnion
The axis, pivot point or center point between axles. A Trunnion Bar is used in single-point tandem suspensions.
Turning Radius
The shortest distance in feet required for a given truck to negotiate a U-turn or make a 180-degree turn. The smaller the turning radius of the truck, the greater its maneuverability and consequent ability to handle well in heavy traffic or congested areas.
Twin Screw
Slang term for Tandem Drive.
Two-speed Axle
A rear axle arrangement whereby the driver can select one of two ratios. A truck with a two-speed axle and a five-speed transmission would have 10 forward speeds.
