Extendable Trailers for Long Cargo: How Long Can You Go Before a Permit Is Required?
An extendable trailer (telescopic semi-trailer) is designed to carry long cargo such as steel beams, steel pipes, power poles or oversized structures. However, an extendable deck does not mean the vehicle can travel at unlimited length. When the total length of the vehicle combination including cargo exceeds the threshold for oversized cargo, a road circulation permit must be obtained before transport. This article explains how long a load an extendable trailer can carry and when a permit is required.
Table of contents:
II. When to use an extendable flatbed vs an extendable lowbed trailer?
I. How does an extendable trailer work?
An extendable trailer deck consists of nested frames (telescopic). For short cargo, the trailer retracts to work like a standard flatbed. For long cargo, the frame is pulled out and locked at the required length, so both ends of the load are supported instead of overhanging at the rear.
The maximum length depends on the model. For example, Faymonville's TeleMAX range can extend to about 4 times its retracted length, comes in 2 to 6 axle configurations and offers hydraulic steering as an option. When the deck is extended, the rear axle group sits further from the kingpin, so the combination's turning radius increases. That is why long extendable trailers usually come with steered axles. Extendable trailers commonly used in Vietnam are much shorter than these models.
II. When to use an extendable flatbed vs an extendable lowbed trailer?
Use an extendable flatbed when the cargo is long but the total height of vehicle and cargo stays under 4.2 m. Use an extendable lowbed when the cargo is both long and tall, because the low deck keeps the total height under 4.2 m. If the cargo only fits the length of a standard trailer deck, an extendable trailer is not needed.
Extendable flatbed trailer: a flat-deck semi-trailer with a standard deck height whose frame extends to increase load length. Suits long, slender, low-profile packages such as steel beams, steel pipes, power poles, spun concrete piles, steel roof trusses and rails.
Extendable lowbed trailer: a lowbed semi-trailer with a load deck close to the road surface and a gooseneck at the front connecting to the tractor; the low deck section extends. Suits cargo that is both long and tall, such as deep bridge girders, large steel structures, long tanks, machine frames and conveyor assemblies.
Criteria | Extendable flatbed | Extendable lowbed |
When to choose | Long cargo, total height of vehicle and cargo under 4.2 m | Long and tall cargo that would exceed 4.2 m on a standard deck |
Limits to control | Combination length, rear overhang, deflection of the package | Total height, ground clearance, combination length |
Main risks | Mid-span sag, longitudinal shifting when braking, wide tail swing in turns | Grounding, hitting overpass and power line clearances, difficulty on gate ramps |
Route survey focuses on | Turning radii, intersections, factory gates | Overpasses, power lines, speed bumps, railway crossings, slopes |
How to operate extendable flatbed and lowbed trailers
Choosing the right trailer solves only half the problem. The other half lies in how the deck is extended, how the cargo is blocked and lashed, and how the route is driven. Get one step wrong and the package can sag or shift, or the vehicle can be stopped mid-route for being out of gauge.
1. Before loading
Check the package dimensions, weight and center of gravity against the trailer manufacturer's load chart for the planned extension length. The permitted payload of an extendable trailer can change with extension length.
Take the support and lifting points from the cargo manufacturer's drawings or instructions; do not estimate them.
For an extendable lowbed: add the deck height, package height and dunnage to get the total height, then compare it with the 4.2 m limit.
2. Extending the deck and loading
Extend the deck while the trailer is empty, lock the frame pins and check again before lifting the cargo on.
Position the package's center of gravity so the load is distributed sensibly between the kingpin and the rear axle group, with no axle exceeding its permitted load.
For long, slender packages, add mid-span support with timber dunnage or load-spreading beams. On an extendable lowbed, do not place cargo on the gooseneck unless the manufacturer allows it.
3. Lashing
Lash against movement in both the longitudinal and transverse directions. Long cargo easily slides forward under hard braking, so tie-down lashing alone is not enough.
Calculate lashing forces according to EN 12195-1 or the CTU Code (IMO/ILO/UNECE). Use webbing or chains with a suitable working load limit and protect sharp edges with padding.
Mark the overhanging part of the cargo as required by regulations.
4. On the road
Extendable flatbed: watch the wide tail swing in turns. On narrow routes use a trailer with steered axles, and arrange escort personnel and vehicles if the road permit requires them.
Extendable lowbed: slow down over bumps, railway crossings and slopes. Stick to the surveyed route, because another route may have lower overpasses.
Re-check the lashings after the first stretch of the journey and at every stop.
5. After unloading
Retract and lock the deck before running empty, so the combination is shorter and easier to turn and enter yards.
Cargo that is both long and heavy enough to require spreading the load over many axles should move to OOG trailers selected by payload and number of axles, including hydraulic modular trailers.
III. How far can an extendable trailer extend before a road permit is required?
The key threshold is 20 m overall length of the whole vehicle combination, including the tractor, trailer and cargo. The current rules are in Circular 12/2025/TT-BXD, as amended by Circular 19/2026/TT-BXD (consolidated text 26/VBHN-BXD dated 02/06/2026).
Case | Rule | Consequence |
Ordinary cargo loaded on a vehicle | Cargo length ≤ 1.1 times the vehicle's overall length, ≤ 20.0 m, and not overhanging the front/rear of the body by more than 10% of the vehicle's overall length (Article 17.2) | No permit needed if all 3 conditions are met and there is no other overweight or oversize condition |
Vehicle combination including cargo longer than 20.0 m | Classified as oversized cargo (Article 10.1.a) | A road circulation permit is required |
Applying for a permit | Submitted to the Department of Construction where the company is registered, or at the cargo's origin or destination (Article 24) | Processed within 2 working days of receiving a complete application (Article 23.3.b) |
Permit validity | Up to 90 days; up to 30 days if the route is not fully upgraded or escorts/support vehicles are required (Article 22.4) | Multi-trip shipments must plan for re-applications |
Illustrative example: a 15 m steel beam on an extendable trailer. Add the tractor cab, the gap between the cab and the end of the beam, and the rear section of the trailer, and the overall length of the combination easily exceeds 20 m even though the beam alone is under 20 m. That is why the whole combination must be measured after loading, rather than using the package length. This measurement is also the figure declared in the permit application.
Length is not the only criterion. If width, height or axle load is exceeded at the same time, additional road survey thresholds apply; see our article on oversized and heavy cargo transport.
IV. What are the risks of hauling long cargo on an extendable trailer?
Risks related to blocking, lashing and driving the route, such as wide tail swing in turns, mid-span sag and longitudinal shifting under braking, are covered in the operating section of part II. There are also two risks that are easily overlooked because they sit in the permit stage:
The tractor lacks towing capacity. A permit is not issued if the trailer exceeds the tractor's designed towing mass (Article 22.3).
Conditions attached to the permit. A road circulation permit may require escort personnel, support vehicles and warnings for other road users (Article 22.4). As a result, travel times usually cannot be chosen freely.
Related articles:
- Flatbed vs lowbed trailers: how do they differ and when should you use each?
- Cranes, forklifts and lifting equipment for project cargo on siteV. What information is needed to plan a long-cargo move?
Send the 5 groups of information below and H-Cargo can recommend the trailer type, the extension length needed and whether a permit is required:
Length, width and height of the package including packaging; weight; support and lifting point positions.
Pickup and delivery points, with photos of the gates and internal roads.
Whether the cargo can be supported at both ends only, or needs additional support in the middle.
Number of packages and trips.
Required delivery date at site.
The full process from survey to handover is covered in our article on the 6-step project cargo transport process.
Have long cargo to move from port to site? Send the dimensions and delivery point, and H-Cargo will advise on the trailer, route and permits as part of our project cargo transport services.
VI. Frequently asked questions
What is the maximum length of an extendable trailer?
It depends on the model. How far the deck extends is a manufacturer specification. How long the combination may travel on the road is set by the road circulation permit once the combination exceeds 20 m.
Does an 18 m load on an extendable trailer need a permit?
Almost certainly yes. An 18 m load sits behind the tractor cab, so the overall length of the combination will exceed 20 m. Beyond that threshold it is oversized cargo and a road circulation permit is required.
Who applies for the road circulation permit?
The transport company or vehicle owner submits the application to the Department of Construction. The cargo owner provides the package specifications.
VII. Conclusion
An extendable trailer is an effective way to move long cargo, but whether a road circulation permit is needed depends on the actual dimensions of the vehicle combination after loading. To avoid extra costs and delays, survey the route and the pickup and delivery points and choose the right trailer type from the start. If you need advice on moving long or oversized cargo, contact H-Cargo for fast, accurate support.
Author: Navy Ngo - Project team
Contact us for a free consultation:
H-Cargo International Logistics
113-115 Ung Van Khiem St., Ward Thanh My Tay, Ho Chi Minh City
Tel: +84 888 909 186
Email: project@hcargovn.com
Web: www.hcargovn.com
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