Question 1

What is a Sliding Fifth Wheel?

The Truck Team · Accepted Answer

The fifth wheel is the attachment on the rear of the tractor unit that couples the tractor unit (prime mover) to the semi trailer. The shape of the fifth wheel means that the trailer king pin is guided to the centre of the fifth wheel where it locks into place. The flat surface of the fifth wheel is greased to facilitate the swivelling required on the articulated unit and trailer. The 'sliding' part is not while the system is in use, but before attaching, the fifth wheel can be moved further forwards or to the rear depending on the length of the neck of the trailer - the longer the neck, the further back the fifth wheel needs to be to protect the rear of the cab and for optimum weight distribution across axles. It is the fifth wheel that makes it a tractor unit as opposed to a rigid. Without the fifth wheel and the trailer that uses it to couple together, the tractor unit is of almost no use! You will always find a fantastic choice of used tractor units for sale with sliding fifth wheels. Be aware that the sellers often describe them simply as 'sliders' [caption id="attachment_4551794" align="aligncenter" width="1280"]

Sliding fifth Wheel on DAF Tractor Unit - Note the rails that the fifth wheel can slide along[/caption]

Question 2

What is Midlift Vs Rear Lift?

The Truck Team · Accepted Answer

These terms only really apply to 6x2 tractor units. These trucks have three axles, but only one of them drives the truck. Let's start off with a more simple example - a 4x2 unit - like most cars they steer at the front and drive from the rear. The drive axles will have two sets of wheels and tyres on as opposed to one set at the front. Now if you add an extra axle to make the tractor unit a 6x2 and able to carry a train weight of 44 tonnes, you could put the extra axle between the two existing axles (mid) or behind the drive axle (rear). Now for the 'lift' part. Unfortunately trucks have to run partially loaded or even empty at times. When this happens, the 6x2 tractor unit no longer requires the third axle to operate legally, (only needed between 4o and 44 tonne GTW). Using the extra axle adds to tyre wear as well as worsens the fuel economy, so it's best to take it out of play. A truck with a 'Midlift' or 'Rearlift' (also known as tag axle) can lift the third axle out of the way. A further enhancement for the midlift axles is to be able to assist with the steering of the tractor unit when deployed. This is called a 'Mid-Lift & Steer' axle.

Infographic - Tractro Unit Axle Configuration

Question 3

WHAT POWER TRACTOR UNIT SHOULD I BUY?

The Truck Team · Accepted Answer

This totally depends on what weight you will be operating at, the distances, payload requirements and typical terrain. There is no point in paying over the odds on a higher horsepower tractor unit just for the sake if it. That said, if you don’t really need a 460hp model, but that is what is available in large quantities in the market, then why not opt for the higher power? There is no reason why not to if the 460hp makes sound financial sense - but make sure that the higher output is from the same sized engine. If you are changing from an eleven litre to a fourteen litre for the higher power it may have a negative impact on fuel economy, but it will definitely have an impact on payload – vital for those short haul bulk jobs, such as fuel and bulk tipping work. In general, the more important the payload, the smaller swept volume engine should pay operating dividends. As far as the terrain is concerned when specifying the best power output, for most UK routes, 420 to 460hp trucks are more than enough when operating at 44 tonnes gross train weight. For trucks that do have to go abroad or spend a lot of time on the UK’s hilliest routes, then an upgrade to a 500hp may seem suitable. As far as the top power 600hp plus trucks in the UK – then it should be for heavy haulage operation or huge egos (a.k.a. flagship trucks).

Question 4

WHAT SIZE TRACTOR UNIT CAB DO I NEED?

The Truck Team · Accepted Answer

The answer isn’t ‘how tall are you?’ Everyone would like to have the most room possible in their working environment but there is a trade-off between the size of the cab and its weight/cost. For example, there is no point having a high roof sleeper cab with a flat floor on local tipping operations or for delivering fuel to petrol stations. In these payload intensive operations every kilo counts and 100 kilos for a taller cab would reap little benefit. Bear in mind that this is important not just for the differing size cabs within a manufacturer’s specific model range, but it may be the case that a totally different model may suit better. Look at DAF Trucks as an example. Sure, you can have a Super Space Cab XF (or XG+ nowadays) but you have to carry all of that cab around. Opt for the smaller CF (Soon to be XD) cab – especially the day cab – and you could be saving hundreds of kilos and drive a truck that is narrower and easier to drive locally. On the other hand, if you mean to attract drivers and they often have nights out in the cab, then the largest, most luxurious cabs make most sense. Think Topline, MegaSpace, Super Space Globetrotter XL and you won’t go far wrong.

Question 5

WHAT'S THE MOST POPULAR MAKE & MODEL OF TRACTOR UNITS?

The Truck Team · Accepted Answer

Whilst nearly every truck manufacturer has a tractor unit range, it is historically the Scania R-Series that has topped the shopping list for UK tractor unit operators – especially the Scania V8. More recently, Scania has introduced the S-Series which will no doubt take over at the top of the list. The Volvo FH – especially the Globetrotter/Globetrotter XL is a desirable truck which has started to see some competition with the improved Mercedes Actros in recent years since the launch of the Euro 6 versions. Sitting centre stage are the DAF XF trucks – excellent value but are some put off by the ageing cab design, despite DAF’s efforts at modernising the 1990’s cab? Joining DAF is MAN with their TGX and TGS range of tractor units with the less favourable brands in the UK of Iveco's Stralis and Renault Truck's Range T bringing up the rear. These days this is less about the actual trucks themselves – nobody makes a bad truck these days – it is more about the reputation of the brand earned over the years. The transport industry has a very long memory!

Question 6

WHICH AXLE CONFIGURATION DO I NEED FOR A TRACTOR UNIT?

The Truck Team · Accepted Answer

In general, UK operators tend to opt for a 6x2 truck. Looking at sales statistics, there are more than seven ties the number of 6x2 tractor units sold in the UK than 4x2 units. This is because the gross train weight when used with a tri-axle trailer is 44 tonnes for a 6x2 and just 40 tonnes for a 4x2. Nearly all of these trucks have a mid-lift or rear lift axle which means that it is lifted out of use when not fully loaded over the 40 tonnes. So, if your operation could run at maximum weights, then it has to be a 6x2. However, if you are a rare UK operator that spends most of the time on the continent, then a 4x2 will be more sensible as only the UK has the extra four tonne allowance meaning the extra axle on a 6x2 would be a waste of space as you could only operate at 40 tonnes regardless. Other 4x2 tractor unit operators will be high volume, low weight operations (think feather pillows and polystyrene). There are some niche axle configurations for tractor units, but these are highly specialist. A 6x4 double drive tractor unit may be of benefit to you if you are operating locally and need additional traction on building sites – but you will lose payload and add cost to the truck, so make sure that it is really required. These 6x4 trucks are usually used with double reduction hubs on heavy haulage work. It is in this realm that you will find 8x4 tractor units as well. Sometimes the extra axle may be because the tractor unit is fitted with a particularly heavy crane as well. So, to summarise – if you will never have the need to run over 40 tonnes or, if you are mainly running abroad, then choose a 4x2, otherwise a 6x2 should be your choice of tractor unit.

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