FR 2950141 A1 20110318 FR 1003400 A 20100818 CLOCK FOR MOTORCYCLE This invention relates to a dock for a motorcycle. Motorcycles are a popular form of transport, which offer a freedom of movement not generally found in cars in today's urban traffic levels. However, the instrumentation found on a motorcycle instrument cluster tends to be somewhat limited compared to that generally found on a car; this is due to a lack of space. in particular, it is common for motorcycles to lack a dock. Obviously, it is useful to be able to tell the time whilst riding. According to a first aspect of the invention, there is provided a dock for a motorcycle, comprising a plurality of display elements switchable between first and second states, where the first state is visibly different to the second state, and a light transmissive carrier for the display elements, the display elements being arranged on the carrier so as to define a dock, the carrier being sized and shaped so as to be fixable to an instrument of the motorcycle such that the instrument can be viewed through the carrier and the display elements in at least one state of the display elements. This allows a dock to be displayed over the instrument, with the display elements forming the dock but preferably allowing the instrument to viewed through the display elements; in the preferred embodiment the instrument would be viewable through the display elements regardless of their state. Preferably, by sized and shaped, we mean that the carrier and the display elements can be fixed within a 5cm, 7.5cm or 10cm circle. Such a dock can conveniently be fitted to the tachometer of a motorcycle; some obscuration of the tachometer can be seen to be acceptable as the information conveyed by the tachometer is useful but not essential. The carrier or the display elements may be planar. 1 Given that the instrument may stil) be seen through the clock, the instrument may be a speedometer of the motorcycle, although that is less preferred. Typically, in the first and second states the display elements have differing light transmission levels; alternativefy, the in the first and second states the display elements may emit different levels of light. By "can be viewed", we may mean that the light transmission characteristics of the display elements in the relevant states is such that at least the outline of the elements of the instrument can be made out. Viewable may comprise at least 10%, 25% or 50% light transmission in the relevant state. For visibility's sake, the display elements may in the second state scatter more light in a diffuse fashion than in the first state. This aids visibility, as diffuse scattering can fead to improved contrast between the first and second states. The display elements may comprise liquid crystal (LC) elements. The LC elements may comprise LC material positioned between two light-transmissive electrodes. At least one of the electrodes may be segmented so as to define the shape of the display elements. The other electrode may be provided in common for all the display elements, and may cover the entire area of the carrier covered by the display elements. The electrodes may comprise indium tin oxide (ITO) or other suitable clear conductive materials. The display elements may comprise light emitting elements that emit light, in use, in one of the first or second states (preferably the second). This is useful when riding in the dark, although it is possible that the backlighting of the instrument would be sufficient. The light emitting elements may comprise an 2 electroluminescent phosphor, which may be sandwiched between a pair of drive electrodes. The dock may further comprise drive circuitry for the display elements. It may also comprise a power supply for the display elements and/or the drive circuitry. At least one of the drive circuitry and the power supply may be housed within a housing that can fitted to a sicle of the instrument. The power supply may comprise at least one of a battery and a solar power cell. According to a second aspect of the invention, there provided a display instrument for a motorcycle, comprising an indicator for a characteristic of the motorcycle and a dock according to the first aspect of the invention, in which the indicator is visible through the dock. There now follows, by way of example only, an embodiment of the present invention, described with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a motorcycle instrument panel according to an embodiment of the invention; Figure 2 shows an alternative dock graphic for use in the present invention; and Figure 3 shows a cross section through the overlay of the dock of Figures 1 25 or 2. Figure 1 of the accompanying drawings shows a motorcycle instrument panel 15 according to a first embodiment of the invention. As is common in the prior art, this comprises a speedometer 1, which indicates the speed of the motorcycle by 30 means of a needle 2 pointing at a scale 3. Whilst the scale is depicted in miles per 3 hour (mph) any other suitable scale, such as kilometres per hour (kph) could be used. Again as is common, the instrument panel comprises a tachometer 4, which indicates the rotationaf speed of the motorcycle's engine. This again comprises a needle 5, which points to a scale 6. The scale shown is in revolutions per minute (rpm), but again any suitable scale could be used. The instrument panel can also comprise informational lights, such as the indicator activation lights 7, which show when the motorcycle's directional indicators are illuminated, a neutral light 8, which shows when the neutral gear has been selected, and a high beam light 9, which shows when a high, rather than dipped, beam headlight has been selected. In the panel of this embodiment of the invention, the tachometer 4 is provided with an overlay 10, which depicts the time using a digital display. The overlay is a four digit segmented display, where each of the four digits 11 comprises seven segments that can be activated separately. Seven-segment displays are common, and each digit 11 can be used to display any of the digits from zero to nine as is familiar from digital docks and watches. However, I have appreciated a novel use for such digital displays. The overlay 10 is positioned over the tachometer 4, and each of the segments of the digits 11 can be activated independently in two visibly different states, as will be discussed below. In either or both of the two states, the tachometer 4 is stil( visible through the overlay 10. Accordingly, a digital display of the time can be overlaid over the tachometer. The display is controlled by a control unit 12, which is attached by means of a self- adhesive pad 13 to a sicle of the panel 15. This control unit comprises a battery 4 (not shown) to provide power to the overlay 10. It also comprises a dock circuit (not shown), which keeps track of the time; such circuits are well known in the prior art. Finally, it comprises a drive circuit by means of which the segments can be selectively illuminated; generally, this will comprise a circuit by means of which a voltage can be applied between pairs of electrodes, each pair defining one of the segments; typically, an common "earth" electrode will be provided, with a segment electrode being provided for each segment. By applying an appropriate voltage to each segment electrode relative to the common electrode, the appropriate segment may be activated. In a second embodiment of the invention shown in Figure 2 of the accompanying drawings, rather than the classic digital display shown in Figure 1, a simulation of an analogue dock is provided. Equivalent integers to those of the embodiment of Figure 1 have been given the same reference numeral, raised by 20. In this embodiment, the overlay 30 is of the form of a circular disc 40, divided into 12 sectors. This is concentrically surrounded by an annular ring 41, which is divided into 12 sectors circumferentially co-extensive with the sectors of the disc 40. The sectors of the disc and of the ring together form the segments of the overlay 30. The sectors can be individually actuated. As such, the sectors of the disc 40 imitate the hour hand of a twelve-hour two-hand analogue dock, and so indicate the hour, in the manner of the small hand of an analogue dock. The sectors of the ring 41 indicate the minute of the time, to a five-minute accuracy, in the manner of the large hand of an analogue dock. The functioning of the overlays of either of the above two embodiments will now be explained with reference to Figure 3 of the accompanying drawings, although the numbering of Figure 1 is used. The overlay 10 comprises a flexible clear plastic substrate 50. On top of this is provided a clear electrode 52, which acts as an 5 electrode for all of the segments in common. It is formed of a material such as indium tin oxide (ITO). Over the electrode is provided the active material 54, which causes the segment to change appearance. One example for the active material would be a liquid crystal material, which either changes the polarisation of light passing therethrough dependent on an electric field applied to the material (in which case additional polarisers will be added above and below the active material 54), or acts to scatter light dependent upon such an electric field. Another option for the active material 54 would be a thin layer of electroluminescent phosphor, which emits light on application of an appropriate electric field. Above the active material 54, there is provided a patterned electrode 56. This is another transparent conductive electrode made of a material such as Indium Tin Oxide (ITO). The pattern of the electrode defines the segments 11 of the overlay 10, such that for each segment 11 there is provided a segment of the patterned electrode that is separately energisable. The section of active material (such as that highlighted at 55) sandwiched between the relevant section of the patterned electrode and the common electrode 52 will activate on energisation of the relevant section of the patterned electrode. The control unit 12 is also shown in more detail in Figure 3; only the detail used for driving the overlay 10 is shown. This comprises a battery 60 and a switch 62 in series. The switch is controlled by parts of the control unit 12 not shown that keep the time and decide which segments should be activated; a switch is provided for each segment 11. A voltage conversion circuit 64 may be provided if it is required to convert the battery 60 voltage to another voltage for the active material; for example, most batteries provide direct current at around 1.5 volts, whereas electroluminescent phosphors require approximately 200 volts alternating current (but at relatively low current). The negative terminal of the battery 60 and the 6 output of the switch 62 (or the voltage conversion circuit 64 if that is provided) are connected to the common electrode 52 and the patterned electrode 56 respectively. As such, the dock of either of these embodiments can be provided simply on a motorcycle instrument. It can be retrofitted to any motorcycle with a suitable instrument; given that most motorcycles have a circular instrument of at least 7.5 cm in diameter, most motorcycles will be suitable. Both the overlay 10 and the control unit 12 can be fitted using self-adhesives to the appropriate members, easily. 7