Glossary › D


The coefficient describing the load which can be carried by a trailer coupling. This coefficient is calculated from the weight of the tractor unit and trailer.


Digital Audio Broadcasting. Digital radio system that alongside an audio signal can also transmits text and graphic data.

Dampness seal

Required on the outer skin of a refrigerated vehicle to ensure optimum retention of cold temperature.


Leading German manufacturer of hydraulic tipping cylinders and tail-lifts.


German Patent Office


The vertical drawbar load when travelling forward or reverse (tandem trailers).

Dealers buying value

Based on the vehicle value less the estimated costs for resale of a used body, trailer or semi-trailer. Included are the costs of demounting, repair, transport, testing, painting and purchasing.


Material deflection on long semi-trailers which when unloaded return to the rest position.


German institute for vehicle testing, founded in 1925.

Depot vehicle

Generally these are vehicles which, due to their build style and load capacity, are used to transport between a manufacturer and a central depot.


Depreciation of capital costs.

Detachable gooseneck

Heavy low loaders may have a gooseneck that can be mechanically or hydraulically detached from the load deck so that the deck may be driven on/off from the front.

Deutsche Bundesbahn Guidelines

General guidelines from German Railways covering rail vehicles (wagons, containers, etc).

Diagonal short coupling system

System manufactured by the German company Orthaus using a scissor principle to stabilise connection between tractor unit and trailer.

DIN Standards

German Industrial Standards which cover essential components for trailers.

DIN towing eyes

Towing eyes of 40 mm and 50 mm diameter for trailers.

Disc brakes

Used on trailers over 18 tonnes gross weight with 19.5″ or bigger wheels and now provided as standard by many manufacturers.

Discharge aid

Found in the discharge cones of silo vehicles.

Discharge equipment

All pneumatic or mechanical discharge systems for trailers.

Discharge fittings

Measuring equipment for the emptying of liquid transporters.

Discharge hatch

Discharge hatch in the bottom of trough bodywork.

Discharge port

For example the reference width of the discharge aperture of a silo vehicle.


Company cooperation between manufacturers Daimler Chrysler and Hendrickson for the manufacture of complete trailer axle systems.

Dividing walls

Dividing walls in animal transporters. Usually made with through holes to reduce weight and allow airflow.

Document holder

Equipment to hold vehicle documents especially used for unaccompanied rail or sea transport.

Dolly axle

Single axles with towing equipment, which are used to help transport or couple a semi-trailer. Can also be a used as an additional axle to distribute or increase load.


Filling aperture on a tank vehicle.

Dome collar

Collar around a tank dome aperture.

Dome cover

The cover on filling apertures of tank vehicles.

Domex 690

This is a special fine grain steel that is commonly used in vehicle manufacture.

Door locking gear

Rod operated locking equipment used on rear barn doors. Also known as power brace locks.

Door retainer

Various types of equipment are used to hold rear doors open e.g. hooks and eyes, magnetic etc.

Double axle bogie

Two connected and commonly suspended trailer axles.

Double barn doors

Can be full or part height double leaf doors.

Double deck loading

Loading of trailers on decks at two levels. Upper deck is height adjustable.

Double floor

Required for heavy point loads.

Double lock

ISO standard locking system for swap bodies and containers.

Double measuring equipment

Measuring equipment on tank vehicles, which allows for example the simultaneous discharge of petrol and diesel.

Double turntable

Only required for short coupling systems that require a second turntable.


This name is used by a leading manufacturer of high volume dry freight semi-trailers with double floor system. Double floor systems are also manufactured by others. The idea of these double decks is to maximise the use of the available space inside a vehicle. The second deck may be manually, mechanically or hydraulically positioned.

Double-T profile

Most commonly used profile for main rails of trailers and semi-trailers.

Draul System

Swedish load pushing system using hydraulic cylinders for timber transport.

Draw bar fork width

The distance between the fixing blocks of a drawbar fork. This is often also the distance between the axle suspension springs.


Used to tow car trailers or heavy tandem trailers.

Drawbar adjustment, hydraulic

Hydraulic-manual height adjustment of a tandem trailer drawbar.

Drawbar adjustment, mechanical

Mechanical height adjustment of a tandem trailer drawbar.

Drawbar fork

Colloquial term for V-shaped drawbar.

Drawbar load

The vertical load applied to the tractor unit via the trailer coupling. This load is part of the total load of a tandem trailer.

Drawbar support

Prevents a drawbar falling to the ground. May also be connected to the braking system whereby the brakes are applied if the drawbar is allowed to fall – no longer manufactured.


Low loaders may be driven onto from the rear.

Drive-on tracks

Car transporter trailers have retractable tracks that can be driven over from trailer to truck.

Drop-side locks, flush

Drop-side locks that are flush fitting to allow maximum internal load area.


Aluminium or steel hinged side panels.

Dry freight body

General term for box bodies for food transport. Lightly insulated, at least with roof insulation.


Leading manufacturer of tyres.


Quick coupling system for air connections.

Dynamic balance

An axle component that carries forward and reverse loads.

Emergency brake

Known as the third brake. Electro-mechanically applied by the tractor unit to brake trailers on steep inclines or under normal conditions to assist the air braking system. A loaded vehicle on an incline of 7% over 6km will be braked so that a speed of 30 km/h will not be exceeded.

Triple bridge vehicle

Road train combination with a total weight of 17.5 t comprising a double axle tractor unit up to 7.5 t and a single axle trailer up to 10 t axle load. In Germany these vehicles could previously be driven with a third class HGV licence taking advantage of a loophole in the German regulations but this is no longer the case.

Turning point

This is the coupling point of a trailer on the tractor unit.

Turning radius

The distance from the king pin to the neck of a semi-trailer.

Turntable-rod steering

Steering of the rear trailer axle is achieved from the king-pin via rods.