Copyright 1999 FleetWatch magazine and FleetWatch On-Line.

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Past Issues
July 1999

The Choice - Interlink or Trailor

Which type of combination is preferred - the interlink or the 6 x 4 freight carrier drawing a four-axle trailor. Obviously there will always be "horses for courses" and the job-to-be-done will dictate the better combination. However, there are a large number of applications in which either the interlink or the truck / trailor can be used. So which is the preferred one in terms of total economy taking into account factors such as payload, purchase price, operating cost, earning ability, flexibility, utilisation and others.

Before dealing with a comparison analysis, it is necessary to consider a number of related factors which influence the position.

The Interlink vehicle combination is not permitted in Europe, Britain and North America - except in very restricted applications in the USA. In fact, no trailor of any type is followed behind an articulated vehicle. However, all the countries mentioned do allow the truck / trailer combination. Why is this?

One reason is the limitation of the permitted maximum combination in most countries. For example, Britain was due to allow up to 40 tons for a five-axle artic and 41 tons for a six-axle artic on January 1, 1999. In South Africa, the permitted maximum is 56 tons.

Another reason why many overseas countries do not use Interlinks is the limitation on the overall length of a combination of vehicles. Not all the countries are the same but the maximum is 18 m which does not justify an Interlink combination.

Incindentally, it is interesting to note that as long ago as 1957, South Africa introduced an overall combination length of 21.95 m (72 feet) which was reduced to 20 m on 1 January, 1972 and on February 16, 1996 the 22 m was re-introduced!

It is of relevance to note that the origin of the Interlink combination was when, in the early 1950's, the English truck manufacturer Foden conceived the idea and built one. However, it was dropped because the permitted maximum gross was 32 tons and this could be achieved with a standard articulated vehicle.

Dimension Comparison
One of the problems experienced by operators conveying containers on Superlink combinations is the difficulty in accomodating three 6 m ISO containers within an overall length of 22 m when using an American normal control (bonneted) truck-tractor.

Using an American normal control truck tractor in the Interlink combination, the overall length is 423 mm over the 22 m permitted maximum. When an American normal control truck (freight carrier) is used in a truck-trailer combination intended for the conveyance of three 6m ISO containers, the standard wheelbase has to be changed to accomodate the 6 m container. Also, and underslung drawbar incorporating the extending facility when turning is necessary.

Consideration must also be given to the front overhang and wheelbase of the trailer to ensure that the front overhang does not exceed 5.8 m minus half the wheelbase.

When the difficulties of trying to comply with the permissable maximum overall length of 22 m for a combination intended for the conveyance of three 6 m containers are considered, it becomes apparent that the "horses for these courses" are conventional forward control vehicles, which present no problems in complying with the 22 m overall combination length.

There is a little chance of the South African authorities - or other countries in southern Africa - considering any increase in the 22 m since eight of the SADC countries are all on the 22 m maximum. Only Angola, Mozambique and Swaziland do not allow 22 m.

Payload Comparison
An analysis of the relative unladen mass of an Interlink combination and an equivalent truck / trailer combination reveals that the truck / trailer is over 2 000 kg lighter than the Interlink. Considering that the permissable maximum combination mass is 56 000 kg (Regulation 362D) the payload can be determined by deducting the unladen mass from the maximum gross mass.

Therefore, all other items being the same, the truck / trailer combination enjoys a plus 2000 kg payload advantage over the Interlink combination.

Based on a payload of 38 tons for the Interlink, an increase of 2000 kg gives the truck / trailer combination a 5.26 % payload advantage.

In pursuing the subject of payload, it must not be overlooked that the maximum achievable payload of any combination of vehicles is restriced by Regulation 362D which prescribes a maximum gross combination mass of 56 tons.

The sum of the maximum axle massloads of a seven axle combination is 7500 + 18000 + 18000 + 18000 = 65000 kg and in reducing the maximum to 56 tons, the potential payload is reduced by 9 tons.

Also, when loading ISO containers, it should be understood that the maximum gross mass of a 6 m ISO container is 24 tons. Therefore, if three 6 m containers, loaded to maximum gross mass, are loaded on an Interlink or truck / trailer combination, the payload would be 3 x 24 = 72 tons, which is almost double the legal payload of 38 tons. Therefore extra care must be exercised in checking the actual mass of each container loaded and in order to achieve the optimum, a maximum legal payload should be determined for each combination, whether it be an Interlink or a truck / trailer combination.

Cost Difference
According to the RFA Vehicle Cost Schedule (March 1999), the truck / trailer combination has a capital cost edge on the Interlink to the extent of approximately R12 000 on the purchase price, subject of course to discounts offered between buyer and seller.

The total annual cost of the Interlinks is R5.77 / km compared with R5.685 / km for the truck / trailer. A difference of only 8.5c / km. But, based on 140 000 km annual kilometres means R11 900 per year. It is emphasised that RFA obtain the input figures from the industry. Operators may wish to check their own comparative figures.

It is fully appreciated that there is a place in road transport for both the Interlink and the truck / trailer. But if general utilisation is considered, the truck / trailer must offer more flexibility.

For one, the 6 x 4 Freaight Carrier can be operated solo when the need arises, eg to convoy one 6 m container. Admittedly, the front section of an Interlink can do the same but there are two vehicles involved, there are eighteen tyres on the road instead of ten, and the 6 x 4 truck is more manoeuvrable and easier to "dock" than the arctic part of the Interlink.

Secondly, the rear semi-trailer of the Interlink can be operated as a conventional semi-trailer drawn by any suitable truck-tractor. The four axle trailer can do the same and is easier to "dock", and any suitable drawing vehicle can be used.

Turning Ability
Based on a turning radius of 12.8 m to the centre of the front axle of the truck-tractor, the radius to the outside of the raodway is 14.4 m and to the inside of the roadway 5 m providing for a road width of 9.4 m.

In the case of the truck / trailer combination, based on the same turning radius of 12.8 m for the truck, the radius to the outside of the roadway is 14.5 m and to the inside of the roadway 6.1 m providing for a road width of 8.4 m. This indicates that the truck / trailer combination requires a road width of 1 m less than the Interlink combination.

So which one is it ? Which do you choose ? Well, that's up to you but we hope that by putting the above points forward, we have demonstrated that there is a lot more to choosing a vehicle than merely going on length and horse-power.

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