An extendable (telescopic) trailer is a semi-trailer whose platform length can be increased thanks to chassis beams that slide inside one another. When empty it travels at standard trailer length; when extended it carries long loads such as beams, pipes, tower segments and precast elements. If the overall length together with the tractor exceeds 16.50 metres, a special permit from the General Directorate of Highways (KGM) is required.
How is a telescopic trailer built?
The main chassis of the trailer consists of two-part box beams: the outer beam is fixed at the front section, and the inner beam slides out of it towards the rear. The beams are drilled at set intervals; once the trailer has been extended to the required length, pins are inserted to lock it. Single-stage models have one inner beam, while double- or triple-stage models have two or three. The amount of extension varies by model; in many designs the platform can be extended to roughly twice its closed length.
The telescopic design can be applied to different platform types. Flatbed models are used for beams and pipes, while low-bed models are used for loads that are both long and tall. The differences between designs with a lowered deck are explained in the semi-lowbed trailer and lowbed trailer entries.
What loads is it used for?
- Beams: Steel or prestressed concrete beams for bridges and viaducts, which can reach 25–40 metres.
- Pipes and pipe bundles: Steel pipes longer than 12 metres in pipeline projects.
- Structural steel elements: Factory roof trusses, columns, crane girders.
- Wind farm components: Tower segments and some blade types. Separate blade carriers are used for very long blades; the methods are covered in the wind turbine blade transport entry.
- Precast elements: Long concrete panels and piles.
For short but heavy loads such as construction machinery, a telescopic trailer is not the first choice; trailer selection for these loads is compared in the which trailer for construction machinery transport entry.
How does the length permit work?
Article 128 of the Highway Traffic Regulation (Karayolları Trafik Yönetmeliği) sets the maximum length of a tractor and semi-trailer combination at 16.50 metres. When the platform is extended, this limit is often exceeded; if the load is indivisible, a Special Load Transport Permit (Özel Yük Taşıma İzin Belgesi) is obtained from the General Directorate of Highways (Karayolları Genel Müdürlüğü). The permit specifies the route, travel hours and, where necessary, an escort requirement; the application steps are explained in the KGM special permit entry. Any part of the load overhanging the rear of the trailer is also counted in the overall length and must be marked.
Why is the turning radius a problem?
As the trailer gets longer, the distance between the king pin and the rear axles increases. On a bend, the rear axles do not follow the tractor's track but cut inwards. This off-tracking means mounting kerbs, central reservations or traffic signs at junctions, or crossing into the opposite lane. The rear end of a long load, meanwhile, swings out towards the outside of the bend. That is why junctions, passages through villages and motorway slip roads are measured in advance for long transports.
What do rear-steered trailers offer?
On many telescopic trailers the rear axles are steered. In a forced-steering system, the axles turn mechanically or hydraulically according to the angle between the tractor and the trailer. In addition, there is remote-controlled manual steering: at a tight junction, an operator walks alongside and steers the rear of the trailer. This allows the trailer to turn in a much smaller space than its length would otherwise require.
| Feature | Standard flatbed trailer | Telescopic trailer |
|---|---|---|
| Platform length | Fixed, approx. 13.6 m | Adjustable, extended to suit the load |
| Steered axles | Usually none | Forced and manual steering on many models |
| Permit requirement | Not required within legal dimensions | Often required when extended |
| Empty return | Normal vehicle | Closed and returned to legal dimensions |
Common mistakes
- Not checking after extension that the locking pins are seated on both sides.
- Positioning the load so that its centre of gravity sits unevenly between the axle group and the king pin.
- Supporting a flexible load only at both ends and letting it sag in the middle.
- Carrying out the junction survey on a map only.
Permits, escorts and vehicle planning for long loads are handled together under out-of-gauge and heavy cargo transport.