Background Large road-going vehicles such as light and heavy goods vehicles ("trucks"), including articulated heavy goods vehicles, typically have poor aerodynamic characteristics. Design requirements, principally providing the greatest practical volume of load space for a given length and width of vehicle, and government legislated dimensional constraints tend to dictate cuboid shapes for both the load carrying space (i. e. trailer/semi-trailer in the case of an articulated vehicle) and cab unit (i. e. tractor unit of an articulated vehicle). This tends to give rise to a vehicle shape with high drag, which in turn leads to poor fuel consumption.

The measurable aerodynamic characteristic most commonly used to quantify the drag associated with a particular vehicle shape is drag coefficient (Cd), a non-dimensional measure that takes account of prevailing variable factors such as air pressure, density and wind velocity to more readily allow a direct comparison between different configurations of vehicle. More specifically: Cd = D/ (0. 5 x p x V x A) where: D = recorded drag force (N) p = density of air (kg/m3)

V = wind velocity (m/s) A = frontal area of vehicle (m2) A higher Cd value implies a vehicle with greater drag. Thus an improvement in the aerodynamic characteristics of a vehicle, giving rise to a reduction in drag, and therefore an improvement in fuel consumption, is reflected by a reduction of the Cd value. Whilst fuel savings are a function of many parameters, they will generally be between 30% and 50% of a percentage reduction in Cd value. Particularly for a business running a large fleet of vehicles over long distances, even a modest improvement in Cd value can therefore have a significant impact on overall fuel savings for the fleet.

A typical Cd value for a passenger car is about 0. 3. In comparison, Cd values for articulated goods vehicles are rarely better than about 0.5 and often much worse than this.

Attempts have been made to improve the aerodynamic shape of goods vehicles in the past. The shape of articulated vehicle seen in the model illustrated in Fig. 1 is not untypical of such attempts. An angled air deflecting component 102, referred to as a roof deflector or fairing, is mounted on the roof of the tractor cab 104 and a flared collar 106 extends from the rear edge of the cab 104 towards the trailer 108. These components are designed to deflect air above and around the trailer and in practice offer improvements in drag. It is also known to employ side skirts 110 along the lower sides of such vehicles. Fig. 1 shows a common layout for such side skirts 110 on the vehicle trailer 108. This results in a reduction in energy losses in the air due to turbulence and thus less drag is created. In tests conducted using the model of Fig. 1, the roof deflector 102 and collar 106 were found to reduce the Cd value of the vehicle by about 14% and the side skirts 110 reduced drag by about 8%.

It is a general aim of the present invention to provide further. improvements in fuel consumption by creating further improvements to the aerodynamic characteristics of such vehicles.

Summary of the Invention The present invention provides a number of bodywork components that can be employed individually or in any combination of two or more to improve the overall drag coefficient of a vehicle, in particular although not necessarily exclusively for articulated heavy goods vehicles.

The components have generally been developed with a view not only to reducing local drag but also to conditioning the airstream flowing around the vehicle. There is a benefit, for instance, in conditioning an airstream flowing around a cab unit (e. g. tractor unit of an articulated lorry) of a vehicle to provide a less turbulent airflow around the rear part of the vehicle (e. g. the trailer of an articulated vehicle). Likewise, similar benefits are obtained by conditioning airflow around the forward part of a vehicle trailer to provide less turbulent airflow around the rear part of the vehicle.

The various aspects of the invention set out below fall in to two principle categories: features of the design of the cab unit (e. g. tractor unit of an articulated lorry) ; and features of the design of the vehicle trailer (e. g. semi-trailer).

Cab Features In a first cab aspect, the present invention provides a roof fairing for a vehicle, the vehicle having a cab and a rear container part that protrudes upwardly above the cab, the roof fairing comprising a curved surface that extends from a front end adjacent a forward edge of the cab roof to a rear end adjacent a forward edge of the roof of the container part, the curvature of the surface being designed to result in a progressive increase in the pressure gradient of air flowing over the surface from its front end to its rear end.

Typically the surface of the fairing will have an elliptical curvature to achieve the desired pressure gradient profile.

It is particularly preferred that the fairing surface and the surface of the side walls (if provided) at the trailing edge of the roof fairing (i. e. closest to the container part) are substantially parallel to the adjacent surfaces (i. e. roof and sides) of the container part. This helps to establish a smooth airflow from the roof fairing over the container part.

The sides of the fairing are preferably closed by side walls extending between the fairing surface, the cab roof and the front wall of the container part. The junction of the fairing surface and each side wall is smoothly radiused.

In a second cab aspect, the invention provides a collar extending from the rear of the cab sides towards the trailer, the collar terminating adjacent the respective forward end of the container part side walls, an outer surface of the collar at its trailing edge being substantially in the plane of the respective container side wall.

In this way, the collar provides for a smooth transition between the cab sides and the trailer sides.

Preferably the spacing between the trailing edge of the collar and the front end of the container part is no more than 500mm, more preferably no more than 300mm and most preferably 150mm or smaller (in an articulated vehicle, this dimension is measured with the tractor and trailer in a straight line configuration).

In a third cab aspect, the invention provides a lower airdam for a cab unit, for example the tractor unit of an articulated heavy goods vehicle, the airdam extending across the width of the front, bottom edge of cab unit, wherein the airdam is profiled to extend closer to the ground at its centre than at its ends.

Air dams are in themselves known. They are used to moderate the airflow beneath the vehicle. They also serve to fair the front wheels of the vehicle.

The air dam of the present invention enables a majority of the width of the dam to be closer to the ground, giving greater control of airflow, whilst still providing sufficient ground clearance at its outer ends to avoid obstacles (e. g. curbs) when the vehicle is turning.

In a fourth cab aspect, the invention provides a cab chassis fairing for the tractor unit of an articulated vehicle, the fairing comprising a substantially flat panel extending along a side of the tractor unit adjacent the ground, the lower edge of the panel being curved such that the ground clearance is greater towards the centre of the panel.

Cab chassis fairings are known, but by profiling the lower edge of the fairing in this way dead air is dragged from below the floor plan of the tractor unit helping to balance the airflow proceeding under the trailer and preventing the air from damming up beneath the tractor unit behind the fairing.

In a fifth cab aspect, the present invention provides a cab chassis fairing for the tractor unit of an articulated vehicle, the fairing comprising a substantially flat panel extending along a side of the tractor unit adjacent the ground, the panel having one or more openings for venting air from beneath the trailer.

By venting the inside of the fairing in this way air is dragged from below the cab unit helping to reduce airspeed under the cab, reducing friction and pressure drag. Particular benefits can be obtained if this feature is arranged to reduce airflow speed directly in front of the rear (typically drive) wheels of the tractor unit. The openings are therefore preferably located forward of and close to these wheels. Additional ducting can be provided in board of the fairing panel to force airflow from below the tractor unit out of the openings.

The fairings of the fourth and fifth cab aspects, which are preferably featureless on their outer surface can also help provide a smooth transition for the airflow from the side of the tractor unit to the trailer side. This effect can be further enhanced by adapting the fairing to cover the rear wheels of the tractor unit.

Where the fairing covers the wheels, it is preferably arranged to allow visual inspection of the wheel nuts and tyre tread. Another preferred feature of the fairing if the wheels are covered is the inclusion of one or more brake cooling openings to allow brake heat to dissipate and preferably also to encourage a flow of air through the wheel cover into the area occupied by the brakes.

It is a more specific aim of some cab aspects of the present invention to reduce the drag created by the sharp edges of the windscreen and/or vehicle wing mirrors.

It is known already to use so called'A'-pillar deflectors, turning vane components that are mounted on the'A'-pillars, spaced slightly from the'A'- pillars themselves to improve the flow of air around the'A'-pillars. However, they are rarely used in practice.

It has been found, however, that by using'A'-pillar deflectors (turning vanes) in combination with one or more of the other cab aspects set forth above, further improvements to vehicle drag can be achieved. Such a combination is provided by a sixth cab aspect of the invention.

The turning vanes create an'effective larger radius'to the oncoming airflow. This reduces turbulence build up around the'A'-pillar itself. A smooth fully attached flow is promoted aft of the'A'-pillar leading to lower aerodynamic pressure drag and a better conditioned airflow over the cab collar (if present). The position and design of these deflectors is a function of an offset of the radial distance to the'A'-pillar, and the chord length is as large as possible while not creating any blind spots for the driver on either side of

the cab that may interfere with vision. The shape of the rear of the deflector may additionally be designed so as not to interfere with the opening articulation of the driver and passenger doors.

In a seventh cab aspect the invention provides a wing mirror for a vehicle, the wing mirror comprising a housing to support the mirror and means for mounting the housing adjacent the vehicle's'A'-pillar, the housing having an outer surface that has a shape derived from the outwardly facing surface of the'A'-pillar but that has a radius of curvature greater than that of the'A'-pillar.

The effect of a wing mirror in accordance with the invention is to increase the apparent radius of the'A'-pillar, improving control of the boundary layer to reduce turbulence and consequently reduce drag attributable to the'A'-pillar. Surprisingly, it has been found that the improvement offered by preferred embodiments of the wing mirror can result in a reduction of drag over and above that achieved by simply removing a vehicles wing mirrors altogether. That is to say, the improvements resulting from better control of the boundary layer around the'A'-pillar outweigh any increase in drag due to the presence of the wing mirrors.

The shape and curvature of the wing mirror surface are, as noted above, derived from the shape (and in particular curvature) of the'A'-pillar.

The optimal shape will vary with the'A'-pillar but can be derived through normal design steps. Typically, this will be done by first approximating the shape of the'A'-pillar to a curve. This can be done at various locations along the length of the pillar or, alternatively, a curve derived for one location (e. g. the top of the pillar) can be assumed for the complete length of the pillar. The curvature of the outer surface of the mirror is then generated as a multiple of this approximated shape of the'A'-pillar. The extent of the surface of the mirror and it's offset from the'A'-pillar are chosen to maximise the chord length of the mirror whilst taking into account the drivers eye lines (i. e. the need for the driver to be able to see the mirror glass and to see around the mirror.

The means for mounting the wing mirror housing on the vehicle are preferably one or more support arms which are themselves shaped to minimise drag. For instance they may be thin, gently curved elements which form upper and lower extensions of the outer surface of the wing mirror housing. In this way they may themselves contribute to the control of the boundary layer of the air flowing around the'A'-pillar.

It may be advantageous to use'A'-pillar deflectors in combination with a wing mirror in accordance with this aspect of present invention to give even greater reduction in drag. In this case, the'A'-pillar deflector could be mounted in a conventional manner, in-board of the inventive wing mirror.

In an alternative embodiment of the invention, an'A'-pillar deflector is formed integrally with the wing mirror so that these two elements can be straightforwardly mounted to the vehicle as a single component. For instance, the'A'-pillar deflector element can extend between and be supported by the wing mirror housing supports.

Trailer Features It has been found that appropriately designed side skirts can bring about a significant reduction in drag associated with the trailer.

In a first trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer), the side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground, the lower edge of the panel being curved such that the ground clearance is greater towards the centre of the panel.

By keeping the surface of the skirt as flat as possible and keeping any feature depths to a minimum, skin friction drag can be minimised.

The shape of the lower edge of the panel maximises the depth and therefore the area faired of the vehicle. This profiling of the lower edge also has the effect of drawing dead air from below the floor of the trailer. This

helps to balance the airflow proceeding to the rear of the trailer, preventing the air damming up beneath the trailer behind the fairing.

In a second trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer), the side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground, the panel having one or more openings for venting air from beneath the trailer.

By venting the inside of the fairing in this way air is dragged from below the trailer unit helping to reduce airspeed under the trailer, reducing friction and pressure drag.

In a third trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer), the side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground, the side skirt comprising a laterally extending wall adjacent a wheel or wheels of the trailer.

Preferably, the side skirt comprises a pair of such walls, one to the front and one to the rear of the trailer wheel or wheels.

The wall or walls most preferably extend parallel to the wheel axles. It is also preferable that they extend the full width of the wheel or wheels that they are adjacent to.

By exhibiting some control over the pressure extremes that exist around a rotating wheel, these lateral walls adjacent the wheel help control turbulence and drag created by its rotation.

In a fourth trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer) of an articulated vehicle, the side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground, the side skirt comprising a lateral wall at its front end (adjacent its connection to a tractor unit), this lateral wall being profiled in plan view substantially as an arc centred on the axis about which the trailer articulates (e. g. the trailer kingpin).

In this way, the forward end of the skirt can be close to the tractor unit rear end at the trailer sides, without compromising the articulation of the trailer.

Where the trailer is a semi-trailer, having legs to support its front end, lateral walls at the front of the side skirts can serve to fair the trailer legs.

In a fifth trailer aspect, the present invention provides trailer having a pair of side skirts, one on either side of the trailer, each side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground and comprising a lateral wall at its front end (adjacent its connection to a tractor unit), wherein the lateral walls extend in towards the centre of the trailer to leave a gap between them that defines a channel, the width of which is selected to control airflow underneath the trailer.

In a sixth trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer) of an articulated vehicle, the side skirt comprising substantially flat panels extending along the lower side of the trailer adjacent the ground to the front and rear of the wheels of the trailer, the side skirt further comprising a cover for the trailer wheels comprising a substantially flat panel arranged to provide a substantially continuous surface along the lower side of the trailer over the wheels.

Preferably, the wheel covers panels are arranged to allow visual inspection of the wheel nuts and tyre tread within a confined space at the trailer side, e. g. of less than 500mm. This allows these inspections to take place in trailer parks or yards for instance, where trailers are often parked very close to one another. This may be achieved, for example, by hinging the wheel cover at its lower edge so that its upper edge can be hinged out to allow a view of the wheel or wheels that it covers.

Another preferred feature of the wheel cover panels is the inclusion of one or more brake cooling openings to allow brake heat to dissipate and

preferably also to encourage a flow of air through the wheel cover into the area occupied by the brakes.

In a seventh trailer aspect, the present invention provides a side skirt for a trailer (e. g. a semi-trailer, the side skirt comprising a substantially flat panel extending along the lower side of the trailer adjacent the ground, a rear end of the side skirt tapering inwardly towards the centre of the trailer.

It is particularly preferred that this tapering surface of the side skirt is curved, the tapering curvature (and thus energisation of the trailer's aerodynamic wake) of the lower rear area of the side skirt being designed dependant on the upstream conditioning provided by the forward parts of the side skirt design.

In an eighth trailer aspect, the present invention provides a trailer (e. g. a semi-trailer) comprising a rear crash bar that is aerodynamically shrouded.

Such shrouding of the crash bar can be particularly beneficial in a trailer having side skirts s they result in more energetic air at the trailer rear, which in turn causes the local drag of the crash bar to increase. Simple aerodynamic shrouding, e. g. a shroud of ovoid cross-section, can bring about a substantial improvement.

It has also been found that trailer bogeys (i. e. one or more pairs of trailer wheels and their associated axle (s) and mounting structure) contribute significantly to trailer drag.

In a ninth trailer aspect, the present invention provides a trailer (e. g. a semi-trailer) having at least one bogey, the trailer further comprising a bogey axle shroud mounted in front (i. e. upstream) of the bogey and arranged to deflect air around the bogey.

In addition to serving to fair the bogey, the shroud also aids the overall slowing down of airspeed beneath the trailer (helping to reduce drag).

The bogey shroud preferably extends across substantially the whole width of the trailer. It may have the form of the bow of a boat, with a central point that extends further forward the its edges.

In preferred embodiments, where the trailer comprises vented side skirts, the shroud can conveniently also serve to guide air out through the skirt side vents.

The aerodynamic characteristics of the trailer's rear end have also been found to contribute significantly to drag. Aspects of the invention recognise that a curved and complex rear end shape can reduce drag.

In a tenth trailer aspect, the present invention provides a trailer (e. g. a semi-trailer) comprising sidewalls and a roof, a rear portion of the trailer roof curving downwardly towards the rear of the trailer.

Preferably this curving, which has the effect of creating a vertical taper to the rear of the trailer, commences at least one metre from the rear end of the trailer.

In an eleventh trailer aspect, the present invention provides a trailer (e. g. a semi-trailer) comprising sidewalls and a roof, rear end portions of the trailer sidewalls curving inwardly towards the rear of the trailer.

Preferably this curving of the sidewalls, which has the effect of creating a horizontal taper to the rear of the trailer, commences at least 300 millimetres from the rear end of the trailer.

Tapering the rear end of the trailer in the vertical and/or horizontal direction in this way increase the streamlining of the trailer rear end and results in a reduction in drag. Preferably the curvature and location if the tapering surfaces is chosen so as not to compromise the rear loading aperture size, the operation of the rear door of the trailer (e. g. a roller shutter door), or the overall useable load space.

Preferably buffers are mounted to the rear of the trailer. Such buffers are known and are used distribute impact forces over a large area (they are typically an elastomeric moulded component). As is known, the buffer is preferably continuous around the rear perimeter of the trailer. Uniquely, the outer surface of the buffer is preferably profiled to continue the aerodynamic taper of the roof and/or sidewall surfaces and the buffer is fixed to the trailer rear in a manner that minimises the interference with/distortion of this surface.

Advantageously, the top region of the may incorporates a'rain gully'to ensure any rain on the roof of the trailer is directed to the sides.

In order to help ensure that the maximum aerodynamic benefit is extracted from the tapered formation of the trailer rear, vortex generators are preferably formed in on the tapering surface of the roof and/or sidewalls.

These serve to delay separation of the airflow from the curved (cambered) surface. The preferred vortex generators have a"wishbone"formation.

In a twelfth trailer aspect, the present invention provides a trailer (e. g. a semi-trailer) comprising sidewalls, a floor and a roof, rear end portions of the trailer roof, floor and sidewalls extending rearwardly beyond a closure means (e. g. a roller shutter) at the rear end of the trailer. The cavity defined at the rear of the trailer in this manner has been found to reduce drag.

Preferred embodiments of the invention may employ one or any combination of two or more of the various cab and trailer aspects of the invention (and their preferred features) set out above.

The invention also provides a cab unit for a vehicle, preferably an articulate vehicle, comprising one or more of the cab aspects of the invention (and/or their preferred features) set out above.

The invention also provides a vehicle trailer comprising features in accordance with one or more of the various trailer aspects of the present invention (and/or their preferred features) set out above.

The invention also provides a vehicle having a cab comprising features in accordance with one or more of the various cab aspects of the present invention (and/or their preferred features) set out above.

The invention also provides a vehicle having a trailer comprising features in accordance with one or more of the various trailer aspects of the present invention (and/or their preferred features) set out above.

The invention also provides a vehicle having a cab comprising features in accordance with one or more of the various cab aspects of the present invention (and/or their preferred features) set out above and a trailer comprising features in accordance with one or more of the various trailer aspects of the present invention (and/or their preferred features) set out above.

Brief Description of the Drawings Embodiments of the invention are described below, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates a prior art articulated heavy goods vehicle ; Figure 2 shows an articulated heavy goods vehicle having tractor and trailer units incorporating features according to embodiments of various aspects of the present invention; Figure 3 is a rear three-quarter view of the tractor unit of the vehicle of Fig. 1; Figures 4a and 4b show (schematically) elements of the undercarriage bodywork of the trailer of the vehicle of Fig. 1; Figure 5 is a schematic plan view of a trailer underside employing features in accordance with an embodiment of the invention;

Figure 6 is a schematic view of the cambered rear end surfaces of a trailer in accordance with an embodiment of the invention; Figures 7a and 7c show vertical and horizontal sections through the cambered rear wall section of a trailer according to an embodiment of the invention, and Figures 7b and 7d show similar sections through a conventional trailer rear end; Figure 8 is a perspective view from the rear of a heavy goods vehicle employing aerodynamic features in accordance with an embodiment of the present invention; Figure 9 illustrates a pair of wing mirrors according to an embodiment of the present invention mounted on a cab unit (shown in a wire frame depiction) of an articulated vehicle; Figure 10 is a front view of the cab and wing mirrors of figure 9; Figure 11 is a side view of the cab and mirror of figure 9; and Figure 12 is a plan view of the cab and mirrors of figure 9.

Description of Embodiments Figures 2 onwards illustrate (as modelled in a CAD system) tractor and trailer units of articulated heavy goods vehicles in accordance with embodiments of the various aspects of the present invention.

The features of the tractor and trailer unit subject of the embodiments of the present invention are described in turn below. In a particularly preferred embodiment, the cab and trailer features described below are used in combination, but need not be. Other embodiments of the invention comprise one or more of the cab features and/or one or more of the trailer features. The wing mirror features described further below may be used on vehicles employing any of the cab and/or trailer features, but also have

independent merit and may be used, for instance, on vehicles that are otherwise conventional.

Cab Features Cab features in accordance with embodiments of the cab aspects of the present invention are described below by way of example with reference to Figures 2 and 3.

Roof Fairing The roof fairing 50 can be a single piece moulded component. In the illustrated embodiment it is formed as a unitary component along with the collar (see below) that can be retrofitted to the vehicle tractor unit.

Alternatively, the roof fairing could be formed as separate components. If incorporated during manufacture of the tractor unit, the roof fairing might alternatively be formed integrally with the cab walls.

As can be seen, the roof fairing has a substantially featureless, smoothly curved outer surface 52 that extends from a lower edge, flush with the edges of the cab roof, to sweep upwardly and rearwardly towards the trailer.

The shape of the fairing is designed to terminate as close as possible to the trailer unit without impeding its articulation. This is achieved in part by having side portions 54 of the fairing protruding less to the rear than the main, upper surface of the fairing.

The upper surface 52 of the fairing has a substantially elliptical curvature to result in a gradual increase of the pressure gradient from its lower front edge to its upper rear edge adjacent the trailer, where the fairing surface is substantially parallel to the trailer roof 56.

In this way, as well as reducing drag associated with the tractor unit and the flat front face of the trailer, the fairing 50 also serves to better

condition that air flowing over it and around the trailer, giving improved air flow around the trailer itself.

Collar The collars 60 (one either side of the tractor unit) extend from the rear of the cab sides 62 of the tractor unit towards the trailer.

The profile of the rear edge 64 of each collar, and its rearward extent are designed to minimise the gap between cab and trailer without dramatically impeding the articulation of the trailer. Desirably, the gap is 150mm or less.

As with the roof fairing 50, the surfaces of the collars 60 are generally featureless, with the aim of providing a smooth, turbulence free, flow of air from cab side wall to trailer. The collar surfaces adjacent the rear edge are substantially in the plane of the trailer side walls. Again, this is done with a view to conditioning the air flow along the trailer sides.

LowerAirdam The lower airdam 70 is mounted beneath the front of the tractor unit and extends across substantially its full width. It serves to control airflow beneath the tractor unit (and hence to some extent the trailer) and also to fair the front wheels 72 of the tractor unit.

The airdam extends further forward (and optionally also closer to the ground) at its centre 74 than at its outer ends. This allows the clearance between dam and ground to be kept to a practical minimum whilst still allowing sufficient ground clearance, particularly at the outer ends 76 of the dam, to clear kerbs and other upwardly protruding obstacles when the tractor unit is turning.

Cab Chassis Fairing The cab chassis fairings 80 (one either side of the tractor unit) cover the sides of the tractor unit behind the cab.

The outwardly facing surface of the fairing is substantially featureless, with a view to maintaining a near turbulent free flow of air from the tractor unit sides along the sides of the trailer.

However, ducts 82 are provided in the fairings designed to encourage airflow to vent from underneath the tractor unit out to its sides. The duct takes the form of an open channel that tapers outwards from front to rear. This helps push the air out. The position of these venting ducts forward of the rear drive wheels 84 of the tractor unit serves to reduce the airflow speed directly in front of the wheels. This helps to reduce wheel drag as well reducing friction and pressure drag through a general reduction of the airspeed under the tractor unit.

The lower edge 86 of the fairing is shaped in an arc, with the ground clearance at the centre of the fairing being greater than at either end. This profile of the lower edge helps to prevent air from damming up under the tractor unit by allowing a controlled amount of air to bleed out sideways under the fairing.

Unusually, the fairing 80 covers the rear wheels 84 of the tractor unit, the wheels being covered by a hinged door panel 88. This panel is designed to provide as complete coverage as possible for the wheel areas, greatly reducing the associated drag. However, this arrangement gives rise to the problem of a severe reduction in cooling airflow to the brakes.

This is solved by the provision of cooling apertures/ducts 89 which are designed to take in an amount of fresh cool air to circulate in the wheel hub area at the front and expel (via external passive extraction) the hot air from the wheel hub area. To achieve this, a feature of these ducts is a small depression (of approximately 50mm in this example) allowing air to flow into the aperture followed, slightly further downstream, by a deeper extraction surface (around 60mm deep in this example) running from inside the wheel hub area out to the external surface of the door, panel.

As seen in the figures, in this example the brake cooling duct 89 is formed by a continuation of the features in the fairing wall that provide the ducts 82 for extracting air from below the tractor unit, but this need not necessarily be the case.

'A'-Pillar Deflectors Turning vanes 90, known as'A'-pillar deflectors are placed along the 'A'-pillar region of the cab unit. These turning vanes 90 create an'effective larger radius'to the oncoming airflow. This reduces turbulence build up around the'A'-pillar itself and smooth fully-attached flow is promoted aft of the 'A'-pillar leading to lower aerodynamic pressure drag.

The position and design of these deflectors 90 is a function of an offset of the radial distance to the'A'-pillar. They preferably extend over the maximum possible vertical length of the'A'-pillar. They are attached at the top to the cab roof and the bottom to the cab bodywork. Their chord is relatively small (preferably less than 100mm) in order to avoid the creation of blind spots for the driver and to avoid fouling the door articulation.

Trailer Features Figures 2 and 4 through 8 illustrate (by way of schematic CAD generated images) trailer units of articulated heavy goods vehicles. As discussed in more detail below, the trailer units include a number of aerodynamic features in accordance with embodiments of the trailer apsects of the present invention, aimed at reducing the trailer drag (and therefore improving fuel efficiency).

The principle components that contribute to the reduction in drag are: - undercarriage sidepods 2,4 (also referred to as side skirts or undercarriage fairings) fore and aft of the trailer rear bogey 6; - trailer wheel covers 8;

- a trailer bogey fairing 10; and - a modified trailer rear end 12.

These elements are discussed in turn below and may be used alone, or more preferably in combination to reduce trailer drag.

Undercarriage Sidepods-Front & Rear Sections The trailer illustrated in Figs 2,4 and 5 has front and rear sidepods 2,4 mounted along its side adjacent the ground. Similar pods would be mounted on the other side of the trailer. Each side pod is designed as a box-like structure, which attaches as smoothly and seamlessly as possible to the underside of the trailer unit at the approximate level of the loading floor. The surfaces are substantially flat and featureless and the external walls are aligned with the maximum width of the trailer, for the most part.

End walls 14,16, 18 (best seen in figs 4 and 5) of the sidepods, adjacent the trailer wheels 20, are designed to lie as close as practically possible to the adjacent wheel (s) and are of the same width or span as the wheel. This controls the amount of air the wheel is exposed to and thus reduces related turbulence and drag.

Ducts 22 (shown in fig 2) are designed to encourage airflow to vent from underneath the trailer out to its sides. The duct takes the form of an open channel that tapers outwards from front to rear. This helps push the air out. The flow of air through these ducts is also aided by the design of the Rear Bogey Fairing 10 (see below) which helps guide air to the ducts on either side of the trailer.

The ground clearance is increased in the centre 24 of the front sidepods 2 (seen most clearly in fig 2), preferably by more than 200mm. This allows the trailer sidepods to clear dips and bumps in the road. The front sidepod is designed such that the depth increases again in side view in the

areas around the wheels to increase faired area of the wheels and reduce ground clearance locally.

Figure 5 illustrates the plan view shape of the front-end wall 14 of the front sidepod 2. This has been generated by an articulation study with aim of getting the outer wall of the pod to extend as far forward as possible in order to reduce the gap between the tractor and trailer bodies, increasing streamlining, without compromising articulation of the trailer.

The rear sidepod sections 4 taper inboard extensively towards the rear of the trailer, as best seen in fig 5. The curvature of the outer wall 26 of these pods is the result of a continuously progressive curvature to ensure pressure recovery stability and thus flow attachment.

As mentioned above, the front wall 18 of the rear sidepod is as close as possible to the rear face of the trailer wheels 20 and approximately the same width. The side view profile is generated to again ensure maximum depth for fairing coverage but is inflected to avoid very low clearances at the rearward part 28 as this is a common damage area due to backing into low walls etc.

The rear under-run bar (or crash bar) 30 is profiled or faired to produce an elliptical section. The benefits of an elliptical or ovoid section are that the incoming flow angle of the approaching air is often far from the horizontal and steady at the rear of the vehicle. An elliptical section allows a more universal, but effective fairing shape in view of the different approach angles.

Wheel Covers The wheels of the trailer illustrated in Fig. 2 are covered by a door panel 8. This panel is designed to provide as complete coverage as possible for the wheel areas, greatly reducing the associated drag. However, this arrangement gives rise to the problem of a severe reduction in cooling airflow to the brakes.

This is solved by the provision of cooling apertures/ducts 32 which are designed to take in an amount of fresh cool air to circulate in the wheel hub area at the front and expel (via external passive extraction) the hot air from the wheel hub area. To achieve this, a feature of these ducts is a small depression (of approximately 50mm in this example) allowing air to flow into the aperture followed, slightly further downstream, by a deeper extraction surface (around 60mm deep in this example) running from inside the wheel hub area out to the external surface of the door cover.

The door 8 hinges from the bottom to enable quick inspection of tyre tread and wheel nuts, from above, when parked very close other trailers in trailer yards. The door panel is also quickly removable in order easily to change wheels should that be necessary.

Trailer Bogey Fairing As seen best in figure 4 the trailer bogey fairing 10 is designed primarily to shroud the axle regions and components of the trailer bogey 6.

Air is encouraged, by means of a subtle snowplough shape, to flow towards the sides of the trailer and out the sides around the front of the wheel area or the ducted vent in the front sidepod (where present).

Trailer Rear End Assuming proper fore-body conditioning of the air flow, it is possible to create a tapering flow field to reduce wake size (and thus drag). Figure 6 shows how the top 40 and sides 42 of the rear end of a trailer can be cambered (tapered) inwardly. In this example, the top section has a greater camber as the available drop is greater. The flow exits the surface with an inclination that'fills'the wake in. The surface is a smooth and continuously progressive curvature, which provides a stable pressure recovery.

Mechanical design and packaging, including significant re-design of the standard roller shutter location has allowed the shape of this surface without compromising internal load space (see figure 7).

The rear impact rubber stops (not shown) are profiled to continue the streamlining to the furthest point rearward on the trailer. They also act to spread impact loads over a greater area when transmitted to the trailer rear frame. This will decrease impact damage at the rear.

The overall tapering effect reduces the rear-projected area and thus drag by virtue of reducing the surfaces acted upon by the base or wake drag of the vehicle.

The overhang 44 of the tapered rear end compared with the roller shutter 46 in it's closed position (see fig 8), is designed to take advantage of an aerodynamic cavity effect. This effect manipulates vortex roll-up from the trailing edge of the vehicle and increases wake or base pressure, thus reducing drag.

In order to minimise, and preferably ensure no premature flow separation from the roof surface, specifically designed vortex generators 48 are employed (see figure 6). The vortex generators create opposing, twin vortices which mix fast and slow air close to the roof surface. This helps the boundary layer and thus general airflow stay attached to the trailer roof. The vortex generators are a basic wishbone shape in plan view. The side view profile of the vortex generators is specific to the cambered surface they sit upon.

The side tapering section camber is less than the roof camber and is determined by the mechanical packaging requirements of the structural design of the rear upright posts defining the end of the trailer as illustrated by fig 7b.

Mirrors Referring to Figures 9 onwards, a pair of wing mirrors 120 according to an embodiment of the invention are mounted one either side of the tractor cab of an articulated heavy goods vehicle ("truck"). The trucks windscreen 122 and side windows 124 are separated by vertically extending'A'-pillars 126 which, as can be seen in the drawings, are sharply radiused in horizontal cross-section.

Looking at one of the wing mirrors 120 in more detail (the other is identical, save from being oppositely handed), a housing component 130, which forms the main body of the wing mirror, supports and houses the mirror 132 itself. The orientation of the mirror within the housing can be adjusted between limits to give the driver of the truck the desired view. Upper and lower support arms 134 mount the wing mirror to the truck cab adjacent its respective'A'-pillar 126.

The outer surface of the housing 130 (i. e. the surface facing away from the'A'-pillar) has a shape derived from the shape of the'A'-pillar 126, especially the cross-sectional shape. However, the radius of curvature of this surface is greater, preferably at least 1.3 times the approximated radius of the 'A'-pillar.

The outer surface of the housing 130 is spaced from the'A'-pillar 126 and its location is a function of maximising span or chord length within the eye-sight constraints for the driver.

As can be seen, the support arms 134 form extensions of the outer surface of the mirror housing 130. They have a thin, plate form and curve in gently to respective upper and lower mounting points on the cab, above and below the'A'-pillar 126. They are shaped to maximise the effective length of the wing mirror without impeding the driver's view past the wing mirror to any significant extent.

Experimental Results Cab and Trailer Features In tests using a 1/3 scale model, the components described above fitted to the cab (tractor unit) have been shown to provide a significant reduction in overall drag of about 16-20% wind-averaged.

The components fitted to the trailer unit have been shown to independently provide a significant reduction in overall drag of about 20-25% wind-averaged.

In combination, a vehicle fitted with both the cab and trailer components would be expected to return a reduction in Cd of about 40% wind-averaged in comparison with a standard cab/trailer combination, equating to a very significant fuel saving, particularly across a large fleet of vehicles.

Mirrors Similarly, in tests using a 1/3 scale model, the wing mirrors described above have been shown to provide a significant improvement in overall drag compared with a truck fitted with conventional wing mirrors. Indeed, as illustrated by the results in the table below, use of these wing mirrors can offer an improvement over running the truck without wing mirrors at all. % of base Cd Truck with conventional wing mirrors (base) 100.00 Truck with wing mirrors removed 96.46 Truck with wing mirrors according to 95.35 embodiment This improvement in drag factor (Cd) of over 4.5%, brought about by using wing mirrors according to an embodiment of the present invention, is likely to give a fuel saving in the region of about 1.5 to 2% which across a large fleet of trucks travelling long distances equates to a substantial annual cost saving.