Simply put, it is how many axles a truck has and how many make it go. The most common example of an axle configuration that we hear every day for cars is 4x4 – that is four wheels – all of them driven by the engine. You can, of course get 4x4 trucks – but these are rare and are usually used by the military or the utilities – A Mercedes Unimog is the best example.
Just a Guide
This guide is intended as just that – we have tried to simplify the regulations to give the reader a flavour for the regulations. We recommend you look into the subject in more detail before making a purchase – the Freight Transport Association can provide some very detailed information that will be easier to understand once
you have read through our layman’s guide below.
Rigid Vs Articulated Trucks
Before talking about the axle configuration we need to clear up a distinction between a rigid truck (one that doesn’t tow a trailer) and an articulated one (artic) that does. This is important, as we still have to talk about the number of axles, but the weights that can be carried can change.
A rigid will always have a fixed number of wheels, but the artic (often called a tractor unit in the UK) can tow a trailer with one (not very common), two (more common), and three axles (the majority). There are trailers with more than three axles, but these are usually highly specialist heavy haulage trailers.
The maximum length for a rigid vehicle is 12 metres – any longer than this and you’ll need an artic or drawbar combination, (see below).
Rigid Axle combinations
The most common format of a truck is a 4x2 – this is basically the same as a standard car – four wheels, tow of which put the power down. If you look at a standard ‘4x2’ truck used for distribution, these will be in this format – and can legally weight up to 18 tonnes fully loaded – or Gross Vehicle Weight (GVW). The legal weight limit relies on the weight being spread over all of the axles of the truck – the more axles therefore the more weight it can carry. So if the weight of your goods and the vehicle is over 18 tonnes (8 tonnes on a front, steer axle and ten tonnes on the rear) then you’ll need to add an axle.
This means that you now have six wheels on the ground, but still only two of them are driven by the gearbox – or a 6x2. A 6x2 ‘rigid’ truck (one that isn’t towing a trailer) can therefore weigh 8 tonnes more than a 4x2 truck – or 26 tonnes GVW. There is a weight penalty for the extra axle, wheels and tyres, but a 6x2 can carry more weight than a 4x2.
As soon as we add an extra axle to a truck we are adding more weight, more rubber on the ground and moving more parts, increasing fuel consumption and wear. When a truck is empty or part-loaded then it doesn’t legally need all three axles, so trucks can be specified with a ‘lift axle’. This simply means that using hydraulics, the whole extra axle can be lifted off the ground and kept that way whilst driving. Lift axles can be places in between the two main – normal axles. These are called midlift axles. Put an axle behind the main drive axle and it becomes a ‘rear lift’. The application and weight distribution will determine whether a mid- or rear- lift axle is most appropriate.
If a truck doesn’t need an extra full axle, a smaller one can be installed on smaller wheels to stop the truck from breaking the laws, but also not carrying too much extra weight when not needed – these trucks won’t operate at the full gross vehicle weight of 26 tonnes – more likely at 23 tonnes.
For applications that will take the truck off good quality roads for a period – such as construction and quarry use – a truck will need better grip. For such applications we add an extra drive axle. Unlike a 4x4 where the front and rear axles do the driving, in a 6x4 it is the two sets of back axles that do the work, leaving the front ones to steer.
The most common UK truck for tipping and other arduous construction roles – hookloaders, for example. The truck has two sets of steering axles at the front of the vehicle and two sets of diving axles at the rear. Often referred to as ‘eight leggers’ or ‘eight wheelers’, it is these trucks that help build our roads and take away all our rubbish. An 8x4 rigid has a maximum permissible GVW of 32 tonnes in the UK.
Tractor Unit Axle configurations
A tractor unit can still only carry the same weight as a rigid truck – where the additional weight carrying capacity comes in is where it tows a trailer. Each additional axle on the trailer allows the vehicle to carry more weight. Instead of gross vehicle weight (GVW) of the truck we should refer to Gross Train Weight (GTW) which is the weight of the tractor unit and trailer and load. In reality, most people still refer to GVW for simplicity.
4x2 Tractor Units
The UK rules differ slightly to those on the continent which means that over recent years, the 4x2 format has become less popular than the 6x2, since three axle tractor units (6x2) are more tax efficient and are able to carry an extra four tonnes in the UK. International operators will have to opt for the 4x2, as there is no benefit to operating a 6x2 abroad.
The maximum permissible GTW for a 4x2 tractor unit in the UK and abroad is 40 tonnes, although many will run lower than this and pay a reduced road fund licence.
If an operator does not need to run at full weight, plastics, polystyrene, clothing etc. weighs relatively little and therefore a 4x2 tractor unit operating at 28 tonnes would do the trick. Local urban deliveries to supermarkets may use an ‘urban artic’ – a downsized 4x2 tractor unit and trailer to make deliveries into tight spots.
6x2 Tractor Units
The three-axle tractor unit segment has overtaken the 7.5 tonner to become the largest in the UK, (odd, since it is negligible in the rest of Europe). When combined with a three-axle trailer, these ‘artics’ are able to have a combined weight of 44 tonnes (GTW) in the UK. As with a 6x2 rigid, a 6x2 artic can have the additional axle added between the main axles – a Midlift, or behind the rear, drive axle – a rear lift. It is usually part of the standard fitment that the axles can be lifted to save fuel and rubber when running empty or part-loaded. For a 6x2 tractor unit to operate at 44 tonnes, the weight on each drive axle may not to exceed 10,500kg, and drive axles have to have ‘road friendly’ (air) suspension or not to exceed 8,500kg axle weight, the trailer to have road friendly suspension and the trailer has to be a triaxle. The truck has to be at least a Euro 2.
Favoured on the continent, a drawbar is allowed a greater overall length (18.5 metres against 16.5 metres of a standard tractor unit and trailer). Also a drawbar combination can drop a trailer and continue to make local deliveries in urban areas with the smaller ‘prime mover’ vehicle. A drawbar is a rigid truck with a body on that tows a trailer in a similar way that a car will tow a trailer, rather than backing underneath it as a tractor unit would. Again, the gross train Weight (GTV) depends on the number of axles the towing truck has and the number of axles on the trailer. Combinations with a total of six axles may weigh up to 44 tonnes, but five axle 40 tonnes drawbar combinations are more common in the UK.