How Many Axles Do You Need? Lowbed Selection by Payload
Автор STU Trailers Engineering Team30 июля 2026 г.9 мин чтения
Axle count is decided by the road, not by the trailer. Structural capacity tells you what a chassis can carry; national axle load limits tell you what you can legally put on it. Here is how to work out which number you actually need.
Axle count is the first number most buyers ask about and the one most often specified wrongly. The reason is that two separate limits govern it and they rarely give the same answer. Structural capacity is what the chassis can physically carry. Legal capacity is what national axle load rules allow you to put on the road. A trailer can be structurally capable of 60 tonnes and still be illegal on your route, and adding axles is usually the cheapest way to resolve that gap.
The short answer
As a working guide: two axles suit loads up to roughly 30 tonnes, three axles cover 30 to 45 tonnes, four axles handle 45 to 70 tonnes, and five to eight axles are specified above that for cranes, transformers, bridge sections and heavy plant. These are structural bands. Before ordering, check them against the axle load limits of every country the unit will run in, because the legal limit binds first more often than the structural one does. If the difference between a lowbed and a low loader is still unclear, the two terms describe the same trailer family in most European markets, and the specification numbers matter far more than the name.
Structural capacity versus legal capacity
Structural capacity is an engineering figure. It depends on chassis design, steel grade, beam geometry and the rated capacity of the axles fitted. It answers the question of what the trailer can carry without deforming or failing.
Legal capacity is a regulatory figure. It depends on how much weight each axle and each axle group may impose on the road surface, which varies by country and sometimes by road class within a country. The baseline in the EU framework is 10 tonnes on a single non-driving axle, with tandem and tridem groups governed by their own tables and by the spacing between axles.
The practical consequence is that spreading a load across more axles reduces the weight each one carries, which is frequently what makes a transport legal rather than abnormal. An operator moving 50 tonnes on four axles may travel as a normal load where the same 50 tonnes on three axles would require a permit for every journey. Over a trailer's working life the cost of that extra axle is usually less than the permits it avoids.
Working through the bands
Two axles
Suitable for machines up to roughly 30 tonnes: mini and midi excavators, compact loaders, telehandlers, rollers and general plant movement for contractors and rental operations. The advantage is low tare weight and a shorter, more manoeuvrable unit that reaches sites a longer combination cannot. The limitation is that it runs out of legal headroom quickly as machine weight rises.
Three axles
The most common configuration in machinery transport, covering roughly 30 to 45 tonnes. This band includes the large majority of tracked excavators, wheel loaders, dozers and crushing equipment, which is why most dealers, contractors and quarry operators settle here. Three axles balance payload against tare weight and length, and a well-specified three-axle unit with hydraulic ramps and detachable platform sections covers a surprisingly wide machine range on its own.
Four axles
For 45 to 70 tonnes, and for operators who sit near the top of the three-axle band often enough that permits become routine. The fourth axle buys legal headroom rather than structural capacity in most cases. Operators frequently discover this only after a year of permit applications, having specified for the machine they owned rather than the one they were about to buy.
Five to eight axles
Above 70 tonnes the specification becomes project-driven rather than catalogue-driven. Six and eight-axle extendable configurations carry transports above 100 tonnes: mobile and crawler cranes, transformers, bridge girders, wind tower sections, long steel beams and pipeline segments. At this level axle count works together with hydraulic steering, pendular axles and extension length as a single design problem rather than as separate options.
Axle capacity is a second number
Axle count alone does not describe capacity, because axles themselves are rated differently. STU specifies axles in 9, 11, 12 and 14-tonne capacities from SAF, BPW, GIGANT, VALX, TRAX and MPD. Three axles rated at 14 tonnes carry substantially more than three rated at 9 tonnes, so a specification that states only the count is incomplete.
Higher-rated axles add weight and cost, so the correct choice is the rating that matches the load rather than the highest available. This is also where tyre configuration enters: single or twin tyres change both the load rating and the deck height, and deck height is often the constraint that decides whether a tall machine travels legally at all.
Tare weight is payload you already spent
Every kilogram of trailer is a kilogram of cargo you cannot carry within the same gross combination limit. This is why steel grade appears on a lowbed specification sheet. Chassis built from S690QL or Strenx 700MC high-strength steel achieve the same structural capacity at lower mass than lower-grade construction, and that difference converts directly into payload on every journey for the life of the asset.
The relationship works against you as options accumulate. Extension mechanisms, detachable goosenecks, pendular axles and hydraulic suspension all add weight in exchange for capability. Each is worth specifying when the work requires it and worth omitting when it does not, because the alternative is carrying the hardware and losing the payload on every load.
Pendular axles and suspension
Standard air suspension is appropriate for road transport and prepared sites. Pendular axles allow far greater individual wheel travel, which keeps load distribution stable across uneven ground and protects the chassis from torsional stress that road suspension is not designed to absorb.
The application is specific: haul roads, quarry benches, unmade site access and heavy transports where the ground cannot be relied on. For an operator running motorway distribution with occasional site delivery, pendular axles are cost and weight without return. For a mining or heavy haulage operator, they are what keeps the trailer serviceable.
Steering, and when axle count forces the decision
As axle count and deck length grow, the rear of the combination tracks progressively wider through turns. At two and three axles this is manageable. From four axles upwards, and particularly on extendable units, steering stops being an upgrade and becomes a requirement for site access.
Self-steering axles reduce tyre scrub and turning radius at modest cost. Mechanical steering follows the tractor's path more tightly through geometry alone. Hydraulic steering, operated from a wireless remote or a control panel, gives direct control over rear axle position and is what allows long combinations to clear roundabouts, gate approaches and urban corners that would otherwise force a route change.
Securing capacity scales with the load
A heavier machine needs more restraint, and restraint has to come from lashing points engineered into the chassis rather than improvised by the driver. STU lowbeds are supplied with eight 16-tonne lashing rings per side as standard, which gives enough positions and enough rated capacity to secure machines of very different geometries without compromise.
Under EN 12195 the required restraint is calculated from the load's mass and the friction between machine and deck, in each direction of travel. More points at the right positions mean chains pull at effective angles. Too few points, or points in the wrong places, force compromises that show up as cargo damage and enforcement problems.
Specify for the fleet you will have
The most expensive specification error in this category is sizing for today's heaviest machine. Trailers last a decade or more; machine fleets turn over faster and generally get heavier. A three-axle unit bought for a 40-tonne excavator becomes a permit generator the moment a 50-tonne machine arrives.
The practical test is straightforward. List every machine you expect to move within the next five years, take the heaviest, add its transport height to the deck height you are considering, and check both weight and height against the limits on your routes. Specify to that result rather than to current inventory.
What catches operators out
Three things, repeatedly. Specifying axle count from structural tables and discovering the legal limit binds first. Stating a count without stating axle rating, then receiving a compliant trailer with less capacity than expected. And adding every available option, then losing more payload to tare weight than the options return in capability.
Frequently asked questions
How many axles do I need for a 40-tonne excavator?
Three axles will normally carry a 40-tonne machine structurally, but the deciding factor is the axle load limit on your route. Where national limits are tight or the route crosses several countries, four axles or a self-steering configuration may be required to keep the movement legal.
How many axles for 60 tonnes?
Four axles is the usual answer, with axle rating and route limits determining whether that is sufficient or whether a fifth is needed. Above 70 tonnes the specification moves into project territory and is designed around the specific load rather than selected from a range.
Does adding an axle always increase payload?
It increases legal payload by spreading weight across more contact points, which is usually the binding constraint. It also adds tare weight, so the net gain is smaller than the axle's rating suggests. On routes where axle load limits bind, the trade is almost always worth making.
What is the difference between 9-tonne and 14-tonne axles?
The rated load each axle may carry. Higher-rated axles allow more payload on the same axle count, at the cost of additional weight and price. The correct rating is the one matched to the intended load rather than the highest available.
Can I add axles to an existing trailer later?
Not practically. Axle count is designed into the chassis geometry, braking system and load distribution from the start. Retrofitting is rarely economical compared with specifying correctly at the order stage, which is why the five-year fleet question matters before ordering rather than after.
How STU specifies axle configurations
STU Trailers builds lowbed and low loader trailers from 2 to 8 axles, in fixed and extendable configurations, with 6 and 8-axle extendable units built for transports above 100 tonnes. Axles are available from SAF, BPW, GIGANT, VALX, TRAX and MPD in 9, 11, 12 and 14-tonne capacities, with single or twin tyres and pendular options where ground conditions require them. Braking is WABCO or Knorr-Bremse EBS.
Chassis are laser-cut and welded from S690QL or Strenx 700MC high-strength steel, with detachable or hydraulic goosenecks, hydraulic suspension and eight 16-tonne lashing rings per side specified according to the work. Because every chassis is engineered to order rather than pulled from stock, axle count, axle rating, deck height, extension length and steering are matched to the customer's machine inventory and the route limits it will operate under. Every unit is load-tested before it leaves the factory.