Technical Field of the Invention

The present invention relates to roof racks for vehicles and in particular roof racks for use with motor cars.

Background Art

In order to enable goods to be carried on the roof of a vehicle, it is common practice to provide removable roof racks in the form of bars, usually two, which extend transversely across the roof of the car being secured at each end either by clamping to the roof gutter or to the roof itself.

Several different cross-sectional shapes may be used, for example circular, oval, elliptical or square cross-section. The cross-sectional shape should, however, provide sufficient structural integrity and aesthetic appeal.

It has previously been proposed by the applicant, for example, to provide a vehicle roof rack in which the bar being adapted to extend across the vehicle roof is of constant cross-section throughout its length and is shaped such that, when attached to a vehicle, it acts as an inverted aerofoil, as disclosed in WO91/10581.

When such roof racks are in place on a moving vehicle, however, considerable wind noise is generated which, in most cases, is directly proportional to the speed at which the vehicle is travelling. In severe cases, these vehicle roof racks may even alter the performance of the motor vehicle in terms of handling, fuel economy etc. due to the disturbance of the normally smooth air flow over the vehicle and the drag created by the roof rack.

In an effort to ameliorate the disadvantages of the prior art, it is proposed to provide a roof rack which offers a choice over the prior art and which, at least in

the preferred embodiments, produces less wind noise and drag while still providing sufficient load bearing capacity.

Disclosure nf the Invention In a first aspect, the present invention consists in a vehicle roof rack comprising a bar adapted to extend across the roof of a vehicle, each end of said bar being adapted to be secured to said vehicle, said bar being of constant cross-section throughout its length and shaped to allow smooth air flow over its surface, said bar having at least one air flow disturbance means adjacent a leading edge thereof.

As will be clear to persons skilled in the art, when air flows over an object a thin layer of air known as a boundary layer is formed adjacent the solid surface of the object. This boundary layer has distinctive flow characteristics as compared to the main surrounding air flow. The flow in a boundary layer may be "laminar" or "turbulent", depending upon the surface condition of the object and the speed of air flowing over the object. In laminar flow, the air flows over the surface relatively smoothly and moves in layers parallel to the surface of the object. Such air flow may be produced for example when air flows over a perfect aerofoil section. In turbulent flow, on the other hand, this regular motion is replaced by random air flow e.g., swirls, eddies etc. This usually occurs at the rearward or trailing surface of the object moving through the air and is particularly prevalent with objects of non-aerodynamic shape, e.g., roof rack bar of square cross-section.

In most cases, an object moving through the air has a boundary layer which near its leading or front portion is in laminar flow and which near its rearward or trailing portion is in turbulent flow. As an object passes through the air, the point at

which the boundary layer changes from laminar to turbulent flow is called the "transition point". The location of this transition point depends upon the size and shape of the object and the speed of the object passing through the air. A boundary layer in turbulent flow is usually substantially thicker than a boundary layer in laminar flow.

While in most cases a turbulent boundary layer better adheres to the surface of an object than a laminar boundary layer, in some cases, if the transition from laminar to turbulent flow is particularly severe, the boundary layer may in fact separate itself from the surface of the object flowing through the air. Such an expansion or separation of the boundary layer produces chaotic air flow leading to a partial vacuum on the trailing portion of the object thereby producing substantial wind noise and increasing drag on the object.

The present invention provides a vehicle roof rack, the bar of which has on its leading portion at least one air flow disturbance means adapted to produce turbulent air flow downstream thereof.

In this way, the point at which the boundary layer transforms from laminar to turbulent flow may be controlled as well as the severity with which the boundary layer transforms to turbulent flow.

Strict control over the transition point also avoids chaotic separation of the boundary layer from a surface bar and thereby reduces noise and drag produced by the roof rack passing through the air. Further, normally the position of the transition point on the surface of the bar would change with the speed of the vehicle producing an accompanying change in noise. By having a fixed point at which the boundary layer is transformed from laminar to turbulent flow no such change in noise will occur. In a first embodiment, said bar includes an air-flow

disturbance means on each of the upper and lower surfaces of said bar.

In another embodiment, said air-flow disturbance means comprises a boundary layer trip means adapted to transform a laminar flow of air on the surface of said bar to turbulent flow downstream thereof.

In a further embodiment, the air flow disturbance means is on the upper surface of said bar and comprises a turbulator rib means extending substantially along the entire length of said bar.

In still a further embodiment, the air flow disturbance means is on the lower surface of said bar and comprises a flared surface portion extending outwardly from a lower surface of said bar. In yet another embodiment, said bar cross-section comprises an abbreviated inverted aerofoil shape with a length to width ratio of about 3.

With such an abbreviated inverted aerofoil shape, said air flow disturbance means is preferably a rib which in cross-section includes a semi-circular portion with a radius of about 0.375mm. It is preferable that this rib is about 0.75mm high and said flare portion preferably extends from said lower surface by about 0.75mm. Brief Description of Drawings In order that the nature of the present invention may be more clearly understood, preferred embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which:-

Fig. 1 is a perspective view of a vehicle roof rack in accordance with a first embodiment of the invention adapted for mounting directly to the roof of a motor car;

Fig. 2 is a similar perspective view showing a vehicle roof rack in accordance with a second embodiment of the present invention adapted for attachment to the gutters of a motor car; and

Fig. 3 is a cross-sectional view of a rack bar of Figures 1 and 2 according to yet another embodiment of the present invention. Industrial Applicability A vehicle roof rack according to the invention is shown in Fig. 1. The rack comprises a bar 10 adapted to extend transversely across the roof of a motor vehicle and end members 11 and 12. The end members 11 and 12 are intended for attachment directly to the roof of a vehicle by any appropriate means such as screws, bolts, pop rivets etc. The roof rack shown in Fig. 2 is substantially identical to that shown in Fig. 1 apart from end members

13 and 14 which are intended for attachment to the roof gutter of a car. In a particular embodiment, end members 11, 12, 13 or

14 may be attached to the vehicle by means of an over-centre clamping action. By turning locking means 19, a rotatable tang within the respective end member locks the end member to both the car and bar 10. This reliably locks bar 10 to the automobile and prevents unauthorised removal or tampering with the roof rack.

Fig. 3 shows a cross-section of the bar 10 according to an embodiment of the present invention. Bar 10 in this embodiment is shaped such that when attached to a vehicle it acts as an inverted aerofoil. However, the present invention may be applied to any roof rack bar of constant cross-section. As bar 10 moves through the air, air hitting its leading edge 15 is separated into an upper stream, flowing over the upper surface 16 and a lower stream, flowing under the lower surface 17. The lower surface is longer than the upper surface 16 and therefore the lower stream travels faster than the upper surface creating a pressure differential acting downwardly on the bar in a manner inverse to that of an aeroplane wing. In other words, the bar section is shaped to act as an

inverted aerofoil.

In this case, the bar also comprises attachment means 18 for facilitating attachment of various articles to the bar, as outlined in WO91/10581. Portions of attachment means 18 which are not in use may be covered by a suitable means to ensure smooth air flow over the upper surface 16 of the bar 10.

Bar 10 also comprises air flow disturbance means adjacent its leading portion. In this case the air flow disturbance means comprise boundary layer trips in the form of a longitudinally extending turbulator rib means 20 on the upper surface 16 and a flared surface portion 21 on the lower surface 17 of the bar 10.

When in position on a vehicle, the roof rack operates as follows. Air is firstly incident on leading edge 15 of bar 10 and then flows along upper surface 16 and lower surface 17. As the air progresses along the upper and lower surfaces, it strikes turbulator rib means 20 and flared surface portion 21 respectively. These air disturbance means transform the laminar boundary layer of air on the upper and lower surfaces of the bar 10 to turbulent flow downstream thereof. As previously mentioned, a turbulent boundary layer better adheres to the surface of bar 10 than a laminar boundary layer. Since the air flowing downstream of turbulator rib means 20 and flared portion 21 is turbulent, there is reduced separation of the boundary layer near the trailing portion of bar 10 and therefore both wind noise and drag on the bar 10 are reduced. As would be expected, the size and effect of turbulator rib means 20 and flared portion 21 depends to a great extent on the actual size of bar section 10.

It is preferable that bar section 10 is somewhat shorter and wider than a true aerofoil. For example, a true aerofoil may have a length, parallel to the direction

of air flow, of 200mm and a width, perpendicular to the air flow, of 20mm or a ratio of length to width L/W = 10. Such a true aerofoil shape does not provide sufficient load bearing capacity to the roof rack and, in addition, is somewhat unsightly when attached to a vehicle.

In a particular embodiment of the present invention, bar section 10 comprises an "abbreviated" aerofoil with a length of 90mm and a width of 31mm or L/W = 3. In such a case, turbulator rib means 21 comprises a longitudinally extending rib of semi-circular cross-section with a radius of 0.375mm. Similarly, with such a bar 10, flared portion 21 extends from the lower surface by 0.75mm.

It will be clear to persons skilled in the art that various types of air flow disturbance means may be used on the leading edge portion of bar 10 to produce turbulent flow downstream thereof. For example, a leading surface portion of bar 10 may be formed with a corrugated structure to alter the air flow downstream thereof.

Comparative tests have shown that when compared to roof racks presently on the market, the roof rack of the present invention may be up to lOdB quieter. In addition, wind noise generated by the roof rack of the present invention does not increase with the speed of the vehicle to the same extent as presently marketed roof racks. From the foregoing it should be apparent that the invention encompasses an advantageous advance in the art or at least a commercial alternative to the prior art. Further, it should be clear that the invention may be embodied in other specific forms without departing from the spirit or scope of the invention or the essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.