Lightweight Structures B.V.
Rotterdamseweg 380
2629 HG DELFT
The Netherlands
Tel +31 15 278 20 99
Fax +31 15 278 72 99
aldert.verheus@lightweight-structures.com
Chamber of commerce nr 27280593
Rotterdamseweg 380
2629 HG DELFT
The Netherlands
Tel +31 15 278 20 99
Fax +31 15 278 72 99
aldert.verheus@lightweight-structures.com
Chamber of commerce nr 27280593
| ColdFeather: lightweight composite isothermal trailer |
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1 Introduction In late 1994, the Dutch trade organisation for the coach building industry (FOCWA Cintec) initiated the development of a lightweight semi-trailer. The project, called ColdFeather, comprised the design, building and testing of an innovative composite trailer for transportation of conditioned products. The aim of this project was to improve the economic position of the Dutch coach building industry, by stimulating the use of new technologies. To realise this, a group of partners dedicated to this goal was formed to achieve the design and building of a prototype, proving the economic and technical feasibility of such an ultra light weight trailer by the use of composite materials. The main driving force for the development of a light weight trailer is the economy of the transport. Many trips are restricted by the maximum allowable weight of the vehicle, and not by the volume capacity of the trailer. A reduction of the trailer empty weight therefore directly increases the payload, improving the economy of the trip, and at the same time reducing the environmental impact per kilogram-kilometre. The market introduction of a light weight trailer, with an increased selling price, requires a thorough market study on the existing models, weights and their operating cost. This study, which was done by NEA (Rijswijk, the Netherlands), clearly showed a high interest of the transportation companies in such a product, given the relation of a 30% weigh reduction at an increased selling price below 20%. Chapter 5 deals in more detail with the economy of the trailer. This 30% reduction of the trailer empty weight had been the input of Lightweight Structures B.V. at the start of the project. Evidently great effort has been put into achieving this goal, which is the topic of chapters 2 and 3. Lightweight Structures B.V. has been responsible for the conceptual design, and the structural design of the composite trailer. The prototype was built by Burg (located in Pijnacker, the Netherlands) and was finished in February 1996, only 14 months after the project was initiated. Given the opportunity of a complete new design, the trailer aerodynamics, which have quite an impact on fuel consumption, were considered with great interest. This is discussed further in chapter 4. 2 Conceptual design of the lightweight composite trailer. The process of reducing the empty weight of isothermal semi-trailers is not new. Several sound structures have been developed in the past years. A clear trend can been found from these developments. In 1980, the average weight of an isothermal semi-trailer was 12,500 kg. In 1990 this was reduced to 9,200 kg, showing a reduction of 30 percent over a decade. At the start of the project discussed in this paper, an empty weight of 6,500 kg was thought to be technically and economically feasible. Besides the fact that this proved to be correct, the same reduction of 30 percent appears over a decade, presumed that most production models will have come to a similar state of technology within a few years. The structural concept by which this step was reached, along with other possible design concepts will be discussed below. Virtually all concepts for light-weight isothermal trailers start from the philosophy of using the box-shaped coachwork (or parts of it) as a load carrying element. In recent developments, especially the floor panel was included in the load transfer. Besides gaining weight, the internal height of the box can be increased when the some of the load carrying elements of the 'chassis' are enclosed into the floor panel. Along with this process, the idea of using the full height of the box to transfer the global bending moments, shear forces and torsional loads grew. It can even be stated that it is virtually impossible to uncouple the load carrying function of the bodywork and the chassis beams. This is caused by the rigid attachments between the chassis and the coach-work, which do not allow the two parts to deform individually. Any brainstorm session of structural designers on achieving successful load transfer by the coachwork will lead to a number of concepts. Key factor in all of the conceptual designs will be the introduction of concentrated loads, and how this can be reached at the lowest weight and production costs. It can be seen that the rear axle configuration of a semi-trailer offers the possibility of using the space in-between the suspension parts for structural items. This is even more so in the case of independent wheel suspension, which will not be further discussed here. These items may be a hollow box structure or a more or less massive thickened floor section. These structures allow for a load introduction of concentrated loads into a lightweight structure, and for spreading these loads in the longitudinal direction of the trailer. They do not however transfer the vertical loads from the hardpoints of the suspension to the side panels with great efficiency. Moreover, such structures reduce the available space below the trailer, which is often used for storage of pallets, the spare wheel, or other parts. At the upper coupler assembly, the height of the structure under the floor must be reduced to about 50 mm, which makes a lower box structure inefficient at this location. The load spreading function in the longitudinal direction of the trailer is therefore the main benefit of these type of structures. Another types of structure are shown in figures 1 and 2. The box basically consists of five panels and two rear doors, which transfer the global forces. Load introduction at the rear axles is done by means of a subframe, which consists of longitudinal and transverse beams. Such a subframe can be fabricated cost effectively and at low weight in steel or aluminium. The subframe transfers the vertical loads from the suspension to the side panels by means of the transverse beams. These loads are then evenly distributed over the total height of the side panels by means of vertical frames. Load transfer of the king pin to the side panels is done much the same way as is done currently with transverse beams incorporated into the floor panel. The material in his case however is carbon fibre / epoxy, which reduces the weight of such elements by more than 150 kg. The lowered floor concept holds interesting possibilities for an efficient structure. In this project, the subframe concept was further developed for its cost effective producibility and easy logistic handling of box, subframe, and suspension parts.  Fig. 1 Structural design concept for a semi-trailer  Fig. 2 Another structural design concept for a semi-trailer  Fig. 3 The finally chosen structural ColdFeather concept 3 Structural design of the sandwich panels and subframe Sandwich panels The design of the sandwich panels is governed by various load cases: vertical accelerations, longitudinal accelerations during braking, high speed cornering, low speed manoeuvres, etc. The floor panel is further subjected to high local loads from pallet lifts and fork lifts. Other considerations are impact loads both on the inside and outside of the side panels. For reasons of insulation, the side panels will be of a sandwich structure of which the minimum (45 mm) and maximum (50 mm) thickness is governed by legislation. Remaining parameters are the foam and facing materials and facing thickness. In this application, aramid fibre reinforced facings offer some important benefits over other isotropic and anisotropic materials: given a proper load introduction, the loads are such that a very low facing thickness will suffice. The low density of aramid laminates along with the good impact properties in this case offer a good, lightweight solution. At a thickness of about 1 mm, aramid reinforced facings offer an acceptable impact resistance in combination with a PVC foam core. The use of the relatively expensive PVC core material is further governed by the required mechanical properties of the foam. The foam has an important function in preventing local failure modes of the sandwich panel such as wrinkling. The development of PU foams with higher mechanical properties may prove an acceptable alternative in the near future. Fibre tailoring of the facings will further reduce the weight of the panels. The side panels will have a relatively high amount of +/- 45 degree fibres compared to the roof and floor panels. The foam core of the side panels is replaced by wood or any other stronger core material at locations where mechanical joints or high impact loads occur, such as the lower edges. The frames, which carry normal stresses at load introduction points of the side panels can be fully incorporated. Wooden beams or aluminium extrusion will transfer the loads successfully. To ensure a good thermal insulation at the lowest possible weight, in this case a narrow sandwich beam was used, with uni-directional carbon fibre facings on a high density PVC foam core. Apart from being the lower flange of the box-type bending beam, the floor panel is loaded directly by the weight of the payload. The floor panel is therefore also designed as a sandwich panel, which transfers the distributed load to the side panels. PVC foam is used as a prime core material to ensure good thermal insulation. The compression strength however of this foam is not sufficient for the high local loads from the fork lifters. Therefore, a double sandwich structure with a high strength top core material was applied. Both a high grade balsa or a high grade aluminium honeycomb core fulfils the requirements, but any other high compressive strength material may be applied with a weight penalty. Corrosion resistance or water absorption could both be reasons to switch to other materials. Special profiles and inserts are included in the floor panels to mount the subframe and the trailer support legs to. Subframe The subframe consists of two longitudinal and four transverse steel beams, welded together. The suspension parts are welded and bolted onto this subframe, which is a state of the art technology. The subframe introduces the vertical forces into the frames of the side panels. Other functions of the frame are equally important: during low speed manoeuvring, high transverse forces are transferred from the axles to the subframe. The subframe introduces these loads into the floor panel. Braking forces are also introduced into the floor panel by the subframe. Integration of the coupler assembly into the sandwich floor structure In order to ensure sufficient internal height for the payload, the inner floor level of most semi-trailers is about 1,300 mm above road level. With a coupler assembly height of 1,150 mm, only 150 mm remains to transfer the high vertical loads from the king pin to the chassis or, as in this case, to the side panels. This 150 mm includes the floor panel thickness itself. It is obvious that any solution will show an integration of the floor and the required structural elements of the coupler assembly. It is more a matter of chassis and coach logistics weather a chassis strip is found under the box at the coupler assembly. The required transverse beams are nowadays usually incorporated into the floor. This concerns heavy steel beams with a typical height of about 100 mm. The remaining height is used to place a multiplex floor on. In this concept, a carbon fibre reinforced epoxy beam structure was developed, replacing the steel beams and profiles. The main beams are placed in he transverse direction. A high degree of fibre tailoring is possible in this application, which further reduces weight compared to the steel counterpart, but which also limits the use of the relatively expensive carbon fibre parts. Despite of this optimisation, this part will be the most expensive one in terms of price per kilogram. It will therefore be applied only if the cost can be brought to an acceptable level. This can be done with automated production techniques, such as resin transfer moulding, and by producing at higher rates. 4 Aerodynamic optimisation for fuel economy During the course of the project, valuable support was given by DAF, especially in the field of truck and trailer aerodynamics. Figure 3 shows a picture of a more or less ideal aerodynamic situation. This situation was integrated into the design as far as possible. Given a maximum use of the trailer length, the suggested slope at the rear end of the trailer could not be implemented. Other than that, the suggested radii and side skirts were included. The radii of 60mm and 150mm for the upper and front edges respectively require a special structural layout of these edges, which potentially reduce the strength of such a joint compared to sharp edge. The lower edges of the trailer, which are highly loaded, are therefore fabricated a with sharp edge. Together with the side skirts, these edges provide a smooth outer surface, which gives an optimal aerodynamic solution. The effect of such actions in terms of the drag coefficient were calculated by DAF. The combined effect of the side skirts and rounded edges will vary between 7% and 15%. The exact number will not only depend upon the lay-out of the skirts and the value of the radii, but also on the aerodynamic performance of the truck. In case of a truck with a poor aerodynamic lay-out, the effect is the largest. The effect on fuel consumption is usually 40% of the drag reduction. This implies that a 10% reduction of drag will save 2,667 litres of fuel per year in case of long distance transportation (200,000 km). Apart from the economic benefits, the environmental effects are obvious. Fuel consumption is the prime factor in an environmental life cycle assessment of semi-trailer transportation. Large diesel engines still have a relatively poor reputation as far as exhaust gasses are concerned, especially after they have been used for several years. Obviously, the weight reduction itself will cut down on fuel consumption as well, as will be shown briefly in the paragraph below.  Fig. 4 Aerodynamically ideal semi-trailer lay-out 5 Economic evaluation A successful market introduction of an ultra light semi-trailer can be achieved only if the extra investment will pay back within a reasonable time limit. For a majority of the entrepreneurs in the transportation business a pay-back period of only 2 to 4 years is acceptable, although the economic life time of a semi-trailer is 10 to 12 years. All financial effects of the exploitation of the light-weight composite trailer were calculated by NEA. After a short overview of the market size, the main effects will be briefly discussed here. International isotherm transportation by Dutch companies amounts to 665 million kilometres by 5,100 vehicles. Of these vehicles, 2,580 are of the semi-trailer type with a cooling unit. A conservative estimate of the amount of trailers which will greatly benefit from and increased payload is 1,540 items for the Dutch companies only. The main positive effects of the use of a light-weight semi-trailer are: increased payload and fuel economy. A small effect is found from the decreased road tax. The main negative effect is the higher investment, resulting in an interest penalty and a higher depreciation. A small effect can be found from extra insurance fees. All of the above effects are easily quantified exept for the effect of the increased payload. There is not a single relation between the increased payload and the increased earnings. This relation depends upon the way that the freight cost are calculated, e.g. price per kilogram, price per volume or price per unit. The transportation companies which were interviewed during the course of this project give very different insights in these numbers. As an average value, it was found that if 1% more load is carried, a 0.5% increase of earnings will follow. Given the fact that an increased payload will not always result in an increased mission weight, this results seems poor. It can be shown however, that a break-even point can be reached within a time limit of 2.5 to 4 years (against 1995 fuel prices and labour costs!), depending on the purchase price of the new semi-trailer and the exact numbers on increased earnings. Given the average value of EUR 4,480 increased earnings per year, an increased trailer price of EUR 11,210 or even EUR 17,940 is acceptable for a pay-back period of 2.5 and 4 years respectively. Without elaborating on the costs of such a lightweight trailer produced in series, in 1995 it was stated that an increased selling price of EUR 11,210 was well feasible. Naturally, since 1995 the worldwide economy drastically changed: fuel prices are increasing, a small number foreign trailer and coach builders is getting a dominant position and Chinese manufacturers will enter the European market (ColdFeather's builder Burg was purchased by Chinese CIMC in February 2006). Innovation is getting even more important! 6 Concluding remarks The initiative of the Dutch trade organization for the coach building industry (FOCWA Cintec) and the Dutch Ministry of Economical Affairs for the development of a light weight composite semi-trailer has proven to be very valuable. It was shown that is technically and economically feasible to produce and exploit such a trailer with an empty weight of 6,500 kg. Several concepts were studied for structural efficiency and production cost. The final concept ColdFeather consists of a load-carrying box structure of five composite sandwich panels and a steel subframe, which can be produced at an acceptable cost level. Since fuel economy of the trailer is the prime factor of the life cycle assessment, weight reduction and aerodynamic performance are very important design parameters. Both weight and drag coefficient were reduced considerably. The 6,490 kg ColdFeather prototype was presented at the International Road Transport Show (Amsterdam RAI, February 1996). It has been used by a Dutch transportation organization since its introduction. In 2004 the ColdFeather was sold to a Russian transportation company. A number of projects in the Dutch coach and trailer industry has been carried out since by FOCWA Cintec and Lightweight Structures B.V.. One of the recent projects is the development of the GIGA trailer. Please do not hesitate to contact us in case you are interested in our capabilities! 7 References 1. Economische haalbaareid van composiet opleggers in isothermvervoer, NEA Transportonderzoek en -opleiding, Rijswijk, oktober 1995 |