Patent Publication Number: US-11022204-B2

Title: Multiple gear range transmission

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a national phase filing under 35 C.F.R. § 371 of and claims priority to PCT Patent Application No. PCT/EP2017/057000, filed on Mar. 23, 2017, which claims the priority benefit under 35 U.S.C. § 119 of British Patent Application No. 1610691.6, filed on Jun. 20, 2016, and British Patent Application No. 1605063.5, filed on Mar. 24, 2016, the contents of each of which are hereby incorporated in their entireties by reference. 
     BACKGROUND 
     Some embodiments relate to a transmission, and in particular, though not exclusively, to a vehicle transmission for implementing gear shifts. 
     WO2014/0493171A1 describes a gearbox known as the Q-Shift gearbox, which is illustrated in  FIG. 1  herein. The gearbox shown has four selectable gear ratios and is suitable for use in car. Larger vehicles having a low power to weight ratio (e.g. trucks and lorries) typically require more selectable gear ratios than this. For a Q-Shift gearbox to have more selectable gear ratios, more gears are required to be coupled to the opposing transmission shafts. 
     SUMMARY 
     The Q-Shift gearbox becomes longer with increasing numbers of selectable gear ratios, which can be unpractical to implement in some vehicles due to limited space in which to house a transmission. Some embodiments are thus configured to address or resolve this and/or other issues. 
     Some embodiments provide a multiple gear range transmission including an input shaft, an output shaft and at least one bridging shaft, each carrying a plurality of drive members;
         wherein the bridging shaft is not connected to a torque input or output;   wherein drive members on the bridging shaft operatively cooperate with those carried on either the input shaft and/or output shaft so as to transfer load between the input shaft and the output shaft via a plurality of selectable load paths corresponding to a plurality of respective resultant gear ratios grouped in at least two gear ranges;   wherein each shaft has one or more drive members capable of being selectively rotationally fixed to the shaft;   wherein successive gear ratios up or down a gear range are selectable, using load paths that include different respective drive members from one of either the input or output shafts;   wherein successive gear ranges are selectable using load paths that include different respective drive members from the other of either the input or output shafts; and,   wherein a transition from a selected gear at the top or bottom of a selected gear range to the next available gear in the next available gear range involves selection of a new load path including drive members from the input and output shaft that are not involved in the load path for the currently selected gear.       

     As previously discussed, primary gearboxes that complete gear changes (gear shifts) without interrupting torque transmission or dipping a clutch are known and include, for example, the Q-shift gearbox referenced earlier. For a vehicle (e.g. heavy goods vehicle) requiring a wide gear range with lots of gear ratios, a simple two shaft gear box is impractical; however, the use of an additional range change gear box would involve declutching and interruption of torque transmission, which would lose some of the benefits of the primary gearbox. 
     In a transmission according to this first aspect, successive gear ratios up or down a gear range are selectable using load paths including different respective drive members from one of either the input or output shafts, which allows a gear change without interruption of torque transmission, as in the case of a Q shift type transmission. 
     Moreover, a transition from a selected gear at the top or bottom of a selected gear range to the next available gear in the next available gear range involves selection of a new load path including drive members from the input and output shaft that are not involved in the load path for the currently selected gear, which also allows a gear range transition without interruption of torque transmission. 
     In a possible embodiment, a load path feature characterizing a specific gear range will usually be a specific drive member selected on the output shaft, with drive members on that shaft being arranged in increasing size corresponding to respective higher gear ranges (for higher speeds). In that scenario, a specific drive member is selected on the input shaft in order to change gears within a gear range, with drive members on the input shaft also being arranged in increasing size corresponding to respective higher gears (for higher speeds) within a gear range. 
     Usually such a transmission includes an input shaft, an output shaft and only one bridging shaft arranged in a triangular arrangement (viewed end-on). 
     Usually, all the drive members on the input shaft are capable of being selectively rotationally fixed to the shaft. Likewise, usually all the drive members on the output shaft are capable of being selectively rotationally fixed to the shaft. 
     The present transmission is particularly for use in vehicles where torque needs to be transmitted in two opposed senses, so as to convey both engine acceleration and braking. Thus, all drive members capable of being selectively rotationally fixed to a shaft are preferably or advantageously fixed in a manner allowing torque to be transmitted in two opposed senses (as for any permanently fixed drive members). 
     Drive members that are capable of being selectively rotationally fixed to a shaft may be so fixed by engaging dog hubs (that rotate with the shaft) on either side of each such drive member, and wherein a forward driving dog hub for one drive member is mechanically coupled to a reverse driving hub for another said drive member (as in the case of the Q shift mechanism) such that those two dog hubs cannot be caused to engage such drive members simultaneously. This avoids lock-up during gear changes within a gear range. 
     Similarly, for any particular shaft (such as the input shaft and/or the output shaft) that includes a pair of such drive members at its respective ends, a forward driving dog hub for one of those drive members may be mechanically coupled to a reverse driving hub for the other of those drive members (for example by a rigid elongate coupling extending along the shaft) such that those two dog hubs also cannot be caused to engage those drive members simultaneously. This avoids lock-up during a transition between different gear ranges. 
     The bridging shaft must or should have at least one drive member capable of being selectively rotationally fixed to the shaft, so that it can be coupled or uncoupled from the bridging shaft, which member should usually be at one end of the bridging shaft. Ideally, the other end of the bridging shaft should usually have a similar drive member capable of being selectively rotationally fixed to the shaft. For bridging shafts with more than two drive members, it is possible for intermediate drive members disposed between the end drive members to be permanently fixed to the bridging shaft (e.g. since these need not be involved in gear range changes where disconnection from the bridging shaft is important). 
     In a possible embodiment, all the drive members of the input shaft and output shaft are selectively rotationally fixed to the shaft and a change in which one of the drive members is rotationally fixed to either the input shaft or the output shaft causes a change in the range of resultant gear ratios that can be selected, while gear ratios in each range are individually selected by changing which drive members are rotationally fixed to the other of the input or output shaft. Usually, a change in which one of the drive members is rotationally fixed to the output shaft causes a change in the range of resultant gear ratios that can be selected, while gear ratios in each range are individually selected by changing which drive members are rotationally fixed to the input shaft. 
     Respective drive members of the input shaft, output shaft and bridging shaft may merely form a simple spur gear train across those shafts. For example, respective end drive members of the input shaft, output shaft and bridging shaft may lie along a straight line to form a single aligned gear train across the shafts. This is possible for the smallest drive members (lowest gear in a range). For later gears (with larger drive members), the required increase in gear ratio may be achieved using more complex gear trains. For example, respective drive members of the output shaft and bridging shaft may form a compound gear train or planetary gear train across those shafts. 
     In particular, the bridging shaft may include at least one compound drive member including two differently sized parts that rotate together and that operatively cooperate with drive members on the input shaft and output shaft, respectively, thereby forming a compound gear train across the three shafts. Such compound drive members may be permanently fixed to the bridging shaft (e.g. for middle drive members) or selectively rotationally fixed thereto (e.g. for end compound drive members). 
     Ideally, the size and configuration of the drive members is selected such that, when progressing from the lowest gear to the highest gear in the transmission, all the steps in gear including transitions between gear ranges, are substantially equal steps. This means that the gears are truly sequential and none of the gears are redundant after a range change, as in the case of some related art range changing gear boxes. 
     In particular, the ratios of the selectable connections from the bridging shaft to the output shaft should preferably or advantageously be suitable to ensure similar ratio changes from input shaft to output shaft on a range change shift as for non-range change shifts. One of the connections from the bridging shaft to the output shaft may be a gear on the output shaft meshing with a gear on the bridging shaft which also meshes with a gear on the input shaft, but to provide the desired ratios other connections need to be either via an additional gear on the bridging shaft or another gearing connection for example a planetary gear connection. 
     For the avoidance of doubt, by input and output shafts is meant shafts in operative connection with a torque input and output of the transmission, respectively. The bridging shaft is an intermediate shaft lying between them (e.g. in bearings) that transfers a torque load between them and is not itself connected to a torque input or output. Drive members on each said shaft are arranged to operatively cooperate with those carried by at least one other said shaft for transferring load between a torque input and a torque output of the transmission in use but drive members on the input shaft and output shaft do not directly operatively cooperate with each other. 
     Preferably or advantageously, the drive members are selectively rotationally fixed to the shaft by the Q shift mechanism referenced below. In that mechanism, each drive member is temporarily fixed to rotate with its shaft by a pair of dog hubs that temporarily simultaneously engage with the opposed side faces of the drive member in respective positive and negative torque connections, so as respectively to convey engine acceleration and braking in the two opposed torque senses. The dog hubs are mounted such that they usually rotate with the shaft (e.g. by spline couplings) but are caused axially to slide to engage/disengage with the drive members by activation of a selection mechanism. Complementary projections on the dog hub faces and drive member side faces may engage to provide the positive and negative torque connections and may be shaped to allow gradual engagement (drawing in) and disengagement upon reversal of the torque connection. 
     Some other embodiments provide a transmission including a plurality of shafts each carrying a plurality of drive members, the drive members on each said shaft arranged to operatively cooperate with those carried by at least one other said shaft for transferring load between a torque input and a torque output of the transmission in use, the transmission being configured such that respective resultant gear ratios between the torque input and the torque output can be selected in use from each of a plurality of groups thereof by changing a load path between the torque input and the torque output, wherein each said group of selectable resultant gear ratios has a load path feature for transferring load between a pair of the shafts which is common to the selectable resultant gear ratios within that group. 
     Some other embodiments provide optional features and particulars of such a transmission. However, any feature mentioned above in connection with first aspect may also be incorporated in this second aspect. 
     According to some other embodiments, there is provided a vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments will now be described by way of non-limiting example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view of a known related art gearbox; 
         FIGS. 2 a  and 2 b    are schematic plan views of a transmission according to an embodiment, respectively showing the dog hubs hidden (for clarity) and present but in a neutral condition; 
         FIGS. 3 to 11  are schematic plan views of the transmission in  FIG. 2 b    shown respectively in first to ninth selectable gear ratio configurations; 
         FIG. 12  is a schematic plan view of a known gear and associated dog hub arrangement shown in a disengaged configuration; 
         FIG. 13  depicts the features in  FIG. 12  in exploded schematic view from different angles; 
         FIG. 14  is a schematic plan view of the arrangement in  FIG. 12  shown in an engaged configuration; 
         FIG. 15  is a schematic plan view of a transmission according to another embodiment; 
         FIG. 16  is a schematic line drawing of a transmission according to a further embodiment; 
         FIGS. 17 and 18  are schematic line drawings of a bridging shaft and an output shaft having planetary gear connections there between. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIGS. 2 a  and 2 b    show a vehicle transmission  10  according to an embodiment. For ease of understanding,  FIG. 2 a    shows the transmission with dog hubs removed, whilst  FIG. 2 b    shows the transmission in a neutral condition. 
     Transmission  10  has an input shaft  12 , a bridging shaft  14  and an output shaft  16  arranged parallel to each other. The input shaft  12  carries first to third gears  18   a ,  20   a ,  22   a  with progressively increasing diameters from left to right. The bridging shaft  14  carries primary gear  18   b , secondary gear  20   b   1 , tertiary gear  20   b   2  and quaternary gear  22   b  which is a compound gear. The quaternary gear  22   b  has a first portion  22   b   1  with a smaller diameter than a second portion  22   b   2 . The output shaft  16  carries first to third gears  18   c ,  20   c ,  22   c  with progressively increasing diameters from left to right. A resultant gear ratio between torque input and output features  13 ,  15  of the transmission  10  can be changed by selectively causing different combinations of gears to be rotationally fixed to the respective shafts  12 ,  14 ,  16  so as to selectively change a load path between the torque input and output features. Respective gear sizes and tooth numbers are selected such that progressive shifts between separate selectable gear ratio configurations ( FIGS. 3 to 11 ) result in similar step variations in the overall resultant gear ratio between the torque input and output features  13 ,  15 . 
     In more detail, the primary gear  18   b  on the bridging shaft  14  meshes with the first gear  18   a  on the input shaft, and also the first gear  18   c  on the output shaft  16 . The secondary gear  20   b   1  on the bridging shaft  14  meshes with the second gear  20   c  on the output shaft  16 . The tertiary gear  20   b   2  on the bridging shaft  14  meshes with the second gear  20   a  on the input shaft  12 . The first (reduced diameter) portion  22   b   1  of the quaternary gear  22   b  on the bridging shaft  14  meshes with the third gear  22   c  on the output shaft  16 , whereas the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  meshes with the third gear  22   a  on the input shaft  12 . 
     All of the aforementioned gears except both the secondary gear  20   b   1  and the tertiary gear  20   b   2  on the bridging shaft  14  are mounted on their respective shafts by a bearing. Such a bearing could include a combination of plain thrust washers and a needle roller bearing, thus causing the gears to be axially and radially located on their respective shafts but free to rotate relative thereto. The secondary  20   b   1  and the tertiary gear  20   b   2  on the bridging shaft  14  are permanently rotationally fixed thereto e.g., by welding. 
     No load path exists between the torque input and output features  13 ,  15  in  FIG. 2 b    because in this configuration the transmission  10  is in a neutral condition. In this condition the gears selectively rotationally fixed to respective shafts include the third gear  22   a  on the input shaft  12  and the first gear  18   c  on the output shaft  16 . When torque is applied to the torque input feature  13  (e.g. from an engine or motor upstream from the transmission  10  in a vehicle powertrain) the input shaft  12  and thus the third gear  22   a  are drivingly rotated. Torque is transferred directly through the quaternary gear  18   b  on the bridging shaft  14 , which is free to rotate relative thereto, to the third gear  22   c  on the output shaft  16 . However, the third gear  22   c  is not rotationally fixed relative to the output shaft  22   c  and so does not transfer torque to the output shaft  16  upon being drivingly rotated. Similarly, when torque is applied to the torque output feature  13  (which could occur in a negative torque condition e.g. if a driver lifts their foot off the vehicle throttle) the output shaft  16  and thus the first gear  18   c  are drivingly rotated. Torque is transferred directly through the primary gear  18   b  on the bridging shaft  14 , which is free to rotate relative thereto, to the first gear  18   a  on the input shaft  12 . However, the first gear  18   a  is not rotationally fixed relative to the input shaft  12  and so does not transfer torque to the input shaft  12  upon being drivingly rotated. 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress from neutral ( FIG. 2 b   ) to the first selectable gear ratio configuration of the transmission  10  ( FIG. 3 ). 
     Looking at  FIG. 3 , in the first selectable gear ratio configuration of the transmission  10  the gears selectively rotationally fixed to respective shafts include: the first gear  18   a  on the input shaft  12 ; the primary gear  18   b  on the bridging shaft  14 ; the quaternary gear  22   b  on the bridging shaft  14 ; and the third gear  22   c  on the output shaft  16 —remembering that the secondary and tertiary gears  20   b   1 ,  20   b   2  on the bridging shaft are permanently rotationally fixed thereto. In this configuration when torque is applied to the torque input feature  13  the input shaft  12  and thus the first gear  18   a  are drivingly rotated. Due to the meshing engagement with the primary gear  18   b  on the bridging shaft torque is transferred thereto, which causes the bridging shaft  14  to rotate. The quaternary gear  22   b  on the bridging shaft  14  thus rotates, transferring torque to the third gear  22   c  on the output shaft  16  via a meshing engagement therewith; whereby torque is transferred through the output shaft  16  to the torque output feature  15  for rotatably driving components downstream in the vehicle powertrain. 
     The load path defined by the particular combination of gears rotationally fixed to the respective shafts in  FIG. 3  is labelled L 1 . Second to ninth respective combinations of gears rotationally fixed to the different shafts—giving rise to second to ninth selectable gear ratio configurations of the transmission  10 —are illustrated in  FIGS. 4 to 11 . Respective load paths defined by the different combinations of gears rotationally fixed to the shafts are labelled L 2  to L 9 . 
     Remembering that only the secondary gear  20   b   1  and the tertiary gear  20   b   2  are permanently fixed to the bridging shaft  14 , the other gears can be selectively rotationally fixed to the shaft on which they are mounted using the technique described in WO2014/049317A1; the contents of which are incorporated herein by reference. In particular as taught from page 9, line 12 to page 12, line 16 of WO02014/049317A1 a gear is selectively rotationally fixed to a shaft by moving dog hubs either side thereof into engagement with the gear. For completeness a general summary of the technique described in WO2014/049317A1 is as follows. 
       FIG. 12  shows a close up of a gear  100  carried by a shaft  102 , like the first gear  18   a  carried by the input shaft  12  in  FIG. 2 b    for instance.  FIG. 13  is an exploded view of the components in  FIG. 12  shown from different angles to illustrate the various features thereof. Now with reference to these drawings the gear  100  is mounted on the shaft  102  between two splined portions  104 ,  106 . First and second selector members  108 ,  110  (otherwise referred to as dog hubs) are mounted on the splined portions  104 ,  106  on opposite sides of the gear  100 . This is achieved by meshing a toothed portion  112  of each dog hub  108 ,  110  with the respective splined portions  104 ,  106 . The dog hubs  108 , no can be caused to slide along the shaft  102  but are rotationally fixed to it. 
     Respective ramp features  114  extend circumferentially around each side face of the two dog hubs  108 ,  110 . In the embodiment illustrated three such ramp features, each terminating in a steeped end portion  114   a , extend circumferentially around each side face of the two dog hubs in either a clockwise or anti-clockwise direction. Corresponding ramp features  114  extend circumferentially around each side face of the gear  100 . In the embodiment illustrated three such ramp features  114 , each terminating in a steeped end portion  114   a , extend circumferentially around each side face of the gear  100  in a direction opposite to that of the immediately facing ramp features  114  on an adjacent dog hub  108 ,  110 . 
     When the dog hubs  108 ,  110  are axially separated from the gear  100  as in  FIG. 12 , the gear  100  and shaft  102  can be rotated relative to each other. However, the gear  100  can be rotationally fixed to the shaft  102  by moving the dog hubs  108 ,  110  along the shaft  102  into engagement with the gear  100  as in  FIG. 14 , specifically by biasing the dog hubs  108 , no towards the gear  100  to be engaged. Continuing with reference to  FIG. 14 , twisting the first dog hub  108  and the gear  100  in a first direction relative to each other while in engagement (e.g. by rotating the shaft  102  in the direction denoted A and thus causing the first dog hub  108  to rotate in direction A also; or alternatively by driving rotation of the gear  100  and causing it to be rotated in the direction denoted B) causes one such component to drive rotation the other due to the inter-engaging steeped end portions  114   a  of the interlocking ramp features  114 . However, upon twisting the first dog hub  108  and gear  100  in the opposite direction relative to each other while in engagement, this will cause the ramped portions  114  of the respective components to ride over each other such that the first dog hub  108  and gear  100  are urged apart. 
     The forgoing similarly applies to the second dog hub  108  and ramp features  114  on the opposite side of the gear  100 , although they are configured the other way around. In particular twisting the second dog hub  110  and gear  100  in a first direction relative to each other while in engagement (e.g. by rotating the input shaft  12  in the direction denoted B and thus causing the second dog hub  110  to rotate in direction B also; or alternatively by driving rotation of the gear  100  and causing it to be rotated in the direction denoted A) causes one such component to drive rotation of the other due to the inter-engaging steeped end portions  114   a  of the interlocking ramp features  114 . However, upon twisting the second dog hub  110  and gear  100  in the opposite direction relative to each other while in engagement, this will cause the ramped portions  114  of the respective components to ride over each other such that the second dog hub  110  and gear  100  are urged apart. 
     A fuller description of the dog hubs used in the transmission  10  is now provided. 
     Looking back at  FIG. 2 b    the input shaft  12  carries first to fourth dog hubs  32   a ,  34   a ,  36   a ,  38   a . Moreover, these dog hubs are rotationally fixed relative to the input shaft  12  but can be moved along the length thereof as heretofore described. Depending on the position of the various dog hubs along the input shaft  12 : the first dog hub  32   a  can transfer torque between the input shaft  12  and the first gear  18   a ; the second dog hub  34   a  can transfer torque between the input shaft  12  and either the first or second gear  18   a ,  20   a ; the third dog hub  36   a  can transfer torque between the input shaft  12  and either the second or third gear  20   a ,  22   a ; and the fourth dog hub  38   a  can transfer torque between the input shaft  12  and the third gear  22   a.    
     The bridging shaft  14  carries first to fourth dog hubs  32   b ,  34   b ,  36   b ,  38   b . Moreover, these dog hubs are rotationally fixed relative to the bridging shaft  14  but can be moved along the length thereof. Depending on the position of the various dog hubs along the bridging shaft  14 : the first and second dog hubs  32   b ,  34   b  can each transfer torque between the bridging shaft  14  and the primary gear  18   b ; and the third and fourth dog hubs  36   b ,  38   b  can each transfer torque between the bridging shaft  14  and the quaternary gear  22   b  (which, it will be recalled, is a compound gear). 
     The output shaft  16  carries first to fourth dog hubs  32   c ,  34   c ,  36   c ,  38   c . Moreover, these dog hubs are rotationally fixed relative to the output shaft  16  but can be moved along the length thereof. Depending on the position of the various dog hubs along the output shaft  16 : the first dog hub  32   c  can transfer torque between the output shaft  16  and the first gear  18   c ; the second dog hub  34   c  can transfer torque between the output shaft  16  and either the first or second gear  18   c ,  20   c ; the third dog hub  36   c  can transfer torque between the output shaft  16  and either the second or third gear  20   c ,  22   c ; and the fourth dog hub  38   c  can transfer torque between the output shaft  16  and the third gear  22   c.    
     As hinted earlier when discussing the arrangement depicted in  FIG. 14 , it is not usually the case that a dog hub drivingly rotates a gear because in some instances a gear drivingly rotates a dog hub. To explain this in more detail reference is made to  FIG. 3 . In a positive torque condition (i.e. when torque from an engine or motor is being transferred through the transmission  10  between the torque input feature  13  and the torque output feature  15  to components downstream in the vehicle powertrain) the first dog hub  32   a  on the input shaft  12  drivingly rotates the first gear  18   a . Moreover, torque transferred from the torque input feature  13 , through the input shaft  12  for causing the first dog hub  32   a  to rotate, which drivingly engages and thus causes rotation of the first gear  18   a . However, turning attention to the third gear  22   c  on the output shaft  16  for instance, this gear drivingly rotates the fourth dog hub  38   c . Moreover, torque is transferred from the quaternary gear  22   b  to the third gear  22   c  on the output shaft  16  which drivingly engages and so rotates the fourth dog hub  38   c , which subsequently causes rotation of the output shaft  16  and the torque output feature  15 . 
     A dog hub in engagement with a gear that is loaded only when the transmission is in a positive torque condition (i.e. when torque from an engine or motor is being transferred through the transmission  10  between the torque input feature  13  and the torque output feature  15  to components downstream in the vehicle powertrain) can be referred to as forward driving dog hub. Similarly a dog hub in engagement with a gear that is not loaded when the transmission is a positive torque condition can be referred to as a reverse driving dog hub, whereas such a dog hub only becomes loaded when the transmission is in a negative torque condition (e.g. when torque is being transferred through the transmission  10  from the torque output feature  15  to the torque input feature  13 ). To illustrate this by example, with continued reference to  FIG. 3  the first dog hub  32   a  on the input shaft  12  and the fourth dog hub  38   c  on the output shaft  16  are forward driving dog hubs because they are loaded only when the transmission  10  is in a positive torque condition. However, the second dog hub  34   a  on the input shaft  12  and the third dog hub  36   c  on the output shaft  16  are reverse driving dog hubs because they are loaded only when the transmission  10  is in a negative torque condition. In the respective selectable gear ratio configurations illustrated in  FIGS. 3 to 11 , forward driving dog hubs are denoted “F”. 
     A suitable technique for manipulating respective dog hubs and causing them to move into specific positions depending on a gear ratio to be selected is described between page 12, line 17 to page 15, line 32 of WO2014/0349317A1. Such a technique could be used to manipulate the dog hugs on the separate shafts of the transmission  10  into specific positions depending on a gear ratio to be selected, thereby giving rise to the aforementioned respective load paths ( FIGS. 3 to 11 ). It is however acknowledged that various other techniques could be used to manipulate dog hubs into required positions depending on a gear ratio to be selected as mentioned on page 19, lines 10 to 18 of WO2014/049317A1. 
     The particular movements required to be made by the dog hubs of the transmission  10  in order to progress through the various selectable gear ratios will now be described. 
     Neutral to First 
     To progress from the neutral condition ( FIG. 2 b   ) in which no load path exists between the torque input and output features  13 ,  15  to the first selectable gear ratio configuration ( FIG. 3 ): the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are to be separated from engagement with the third gear  22   a ; and the first and second dog hubs  32   c ,  34   c  on the output shaft  16  are to be separated from engagement with the first gear  18   c . Subsequently the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be moved into engagement with the first gear  18   a ; the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are to be moved into engagement with the primary gear  18   b ; the third and fourth dog hubs  36   b ,  38   b  on the bridging shaft  14  are to be moved into engagement with the quaternary gear  22   b ; and the third and fourth dog hubs  36   c ,  38   c  on the output shaft  16  are to be moved into engagement with the third gear  22   c . Thereafter torque is transferred along the load path L 1  between the torque input feature  13  and the torque output feature  15  when the input shaft  12  is rotatably driven in a forward rotational direction (i.e., when the transmission  10  is in a positive torque condition). In a reverse torque condition however, torque is transferred in the reverse direction along load path L 1 . 
     In more detail, when the first selectable gear ratio configuration ( FIG. 3 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the first and second dog hubs  32   a ,  34   a  rotationally fixed thereto&gt;the first dog hub  32   a  acting as a forward driving dog hub drivingly rotates the first gear  18   a &gt;torque is transferred to the primary gear  18   b  on the bridging shaft  14  via the meshing engagement therewith&gt;the first dog hub  32   b  acting as the forward driving dog hub is rotatably driven by the primary gear  18   b &gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the first dog hub  18   b &gt;the fourth dog hub  38   b  thus rotates with the bridging shaft  14  due to being rotationally fixed thereto&gt;the fourth dog hub  38   b  acting as a forward driving dog hub rotatably drives the quaternary gear  22   b &gt;torque is transferred to the third gear  22   c  on the output shaft  16  via the meshing engagement therewith&gt;the fourth dog hub  38   c  acting as a forward driving dog hub is rotatably driven by the third gear  22   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  38   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the second dog hub  34   a  on the input shaft  12 , the second and third dog hubs  34   b ,  36   b  on the bridging shaft  14  and the third dog hub  36   c  on the output shaft  16  all act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 1 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     First to Second 
     A change in the overall combination of gears rotationally fixed to the respective shafts is required in order to progress to the second selectable gear ratio configuration of the transmission  10  ( FIG. 4 ). Moreover to progress from the first to the second selectable gear ratio configuration: the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be separated from engagement with the first gear  18   a ; the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are be separated from engagement with the primary gear  18   b ; and the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be moved into engagement with the second gear  20   a . However, the third and fourth dog hubs  36   b ,  38   b  on the bridging shaft  14  are to remain in engagement with the quaternary gear  22   b ; and the third and fourth dog hubs  36   c ,  38   c  on the output shaft  16  are to remain in engagement with the third gear  22   c . Subsequently torque is transferred along the load path L 2  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 2  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the first selectable gear ratio configuration ( FIG. 3 ) to the second selectable gear ratio configuration ( FIG. 4 ) while the transmission  10  is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the first selectable gear ratio configuration ( FIG. 3 ) the second dog hub  34   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the first gear  18   a  into engagement with the second gear  20   a . When it engages and synchronises with the second gear  20   a  then the second dog hub  34   a  acts as a forward driving dog hub and so drivingly rotates the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that when torque is being transferred along the first load path L 1  the second gear  20   a  on the input shaft  12  will be rotating slower than the first gear  18   a —and so the second gear  20   a  will be rotating slower than the input shaft  12  itself. Thus, when the second dog hub  34   a  is urged against the second gear  20   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the second gear  20   a.    
     Following this the second gear  20   a  will rotate at the same speed as the input shaft  12 , thus enabling the third driving dog hub  36   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. The second gear  20   a  on the input shaft  12  has a bigger diameter than the first gear  18 . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the second gear  20   a  is rotationally fixed to the input shaft  12  compared to when only the first gear  18   a  is rotationally fixed thereto. 
     When the second dog hub  34   a  begins to act as the forward driving dog hub of the second gear  20   a , torque begins flowing along the load path L 2  (see  FIG. 4 ) between the torque input feature  13  and the torque output feature  15 . Since the first dog hub  32   a  on the input shaft  12  and the first and second dog hubs  32   b ,  34   b  on the bridging shaft  12  are not loaded when torque starts flowing along load path L 2  they are able to be moved out of engagement with the first gear  18   a  and the primary gear  18   b.    
     In more detail, when the second selectable gear ratio configuration ( FIG. 4 ) has been engaged, positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the second and third dog hubs  34   a ,  36   a  rotationally fixed thereto&gt;the second dog hub  34   a  acting as a forward driving dog hub drivingly rotates the second gear  20   a &gt;torque is transferred to the tertiary gear  20   b   2  on the bridging shaft  14  via the meshing engagement therewith&gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the tertiary gear  20   b   2 &gt;the fourth dog hub  38   b  thus rotates with the bridging shaft  14  due to being rotationally fixed thereto&gt;the fourth dog hub  38   b  acting as a forward driving dog hub rotatably drives the quaternary gear  22   b &gt;torque is transferred to the third gear  22   c  on the output shaft  16  via the meshing engagement therewith&gt;the fourth dog hub  38   c  acting as a forward driving dog hub is rotatably driven by the third gear  22   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  38   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the third dog hub  36   a  on the input shaft  12 , the third dog hub  36   b  on the bridging shaft  14  and the third dog hub  36   c  on the output shaft  16  all act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 2 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the first to second selectable gear ratio configuration gives rise to a step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the first to second selectable gear ratio configuration could give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the second selectable gear ratio configuration ( FIG. 4 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the first selectable gear ratio configuration ( FIG. 2 b   ). 
     Second to Third 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the third selectable gear ratio configuration of the transmission  10  ( FIG. 5 ). Moreover to progress from the second to the third selectable gear ratio configuration: the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be separated from engagement with the second gear  20   a ; the third and fourth hubs  36   b ,  38   b  on the bridging shaft  14  are be separated from engagement with the quaternary gear  22   b ; and the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are to be moved into engagement with the third gear  22   a . However, the third and fourth dog hubs  36   c ,  38   c  on the output shaft  16  are to remain in engagement with the third gear  22   c . Subsequently torque is transferred along the load path L 3  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 3  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the second selectable gear ratio configuration ( FIG. 4 ) to the third selectable gear ratio configuration ( FIG. 5 ) while the transmission  10  is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission  10  is in the second selectable gear ratio configuration ( FIG. 4 ) the third dog hub  36   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the second gear  20   a  into engagement with the third gear  22   a . When it engages and synchronises with the third gear  22   a  then the third dog hub  36   a  acts as a forward driving dog hub and so drivingly rotates the third gear  22   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that when torque is being transferred along the second load path L 2  the third gear  22   a  on the input shaft  12  will be rotating slower than the second gear  20   a —and so the third gear  22   a  will be rotating slower than the input shaft  12  itself. Thus, when the third dog hub  36   a  is urged against the third gear  22   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the third gear  22   a.    
     Following this the third gear  22   a  will rotate at the same speed as the input shaft  12 , thus enabling the fourth dog hub  38   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. The third gear  22   a  on the input shaft  12  has a bigger diameter than the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the third gear  22   a  is rotationally fixed to the input shaft  12  compared to when only the second gear  20   a  is rotationally fixed thereto. 
     When the third dog hub  36   a  begins to act as the forward driving dog hub of the third gear  22   a , torque begins flowing along the load path L 3  (see  FIG. 5 ) between the torque input feature  13  and the torque output feature  15 . Since the second dog hub  34   a  on the input shaft  12  is not loaded when torque flows along load path L 3  it can be moved out of engagement with the second gear  20   a . Furthermore, since torque is transferred directly via the quaternary gear  22   b  and not along the bridging shaft  14 , the third and fourth dog hubs  36   b ,  38   b  are not loaded when torque is transferred along the load path L 3  and so are able to be moved out of engagement with the quaternary gear  22   b.    
     In more detail, when the third selectable gear ratio configuration ( FIG. 5 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the third and fourth dog hubs  36   a ,  38   a  rotationally fixed thereto&gt;the third dog hub  36   a  acting as a forward driving dog hub drivingly rotates the third gear  22   a &gt;torque is transferred to the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14  via the meshing engagement therewith&gt;this causes the first (reduced diameter) portion  22   b , of the quaternary gear  22   b  to rotate also&gt;torque is transferred to the third gear  22   c  on the output shaft  16  via the meshing engagement therewith&gt;the fourth dog hub  38   c  acting as a forward driving dog hub is rotatably driven by the third gear  22   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  38   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the fourth dog hub  38   a  on the input shaft  12  and the third dog hub  36   c  on the output shaft  16  both acts as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 3 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the second to third selectable gear ratio configuration gives rise to another step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the second to third selectable gear ratio configuration could also give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the third selectable gear ratio configuration ( FIG. 5 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the second or first selectable gear ratio configuration ( FIG. 4 ). 
     Third to Fourth 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the fourth selectable gear ratio configuration of the transmission  10  ( FIG. 6 ). Moreover to progress from the third to the fourth selectable gear ratio configuration: the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are to be separated from engagement with the third gear  20   a ; the third and fourth hubs  36   c ,  38   c  on the output shaft  16  are be separated from engagement with the third gear  22   c ; the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be engaged with the first gear  18   a ; the first and second dog hubs  32   b ,  34   b  on the bridging shaft  12  are to be engaged with the primary gear  18   b ; and the second and third dog hubs  34   c ,  36   c  on the output shaft  16  are to be engaged with the second gear  20   c . Subsequently torque is transferred along the load path L 4  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 4  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the third selectable gear ratio configuration ( FIG. 5 ) to the fourth selectable gear ratio configuration ( FIG. 6 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the third selectable gear ratio configuration ( FIG. 5 ) the fourth dog hub  38   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the third gear  22   a , which is enabled due to the fourth dog hub  38   a  being a reverse driving dog hub and so not loaded in a positive torque condition of the transmission  10 . Subsequently the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are moved into engagement with the first gear  18   a ; the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are moved into engagement with the primary gear  18   b ; and the second and third dog hubs  34   c ,  36   c  on the output shaft  16  are moved into engagement with the second gear  20   c ; whereby the third dog hub  36   c  on the output shaft  16  can be separated from the third gear  22   c  while positive torque is being transferred along the load path L 3  due to its also being a reverse driving dog hub. Torque will thus begin flowing along load path L 4  instead of L 3 . 
     Subsequently the third dog hub  36   a  on the input shaft  12  and the fourth dog hub  38   c  on the output shaft  16  will no longer be loaded due to torque flowing along load path L 4  and so can be removed from engagement with the third gear  22   a  on the input shaft  12  and third gear  22   c  on the output shaft  16 . 
     More specifically during an upshift from the third selectable gear ratio configuration ( FIG. 5 ) to the fourth selectable gear ratio configuration ( FIG. 6 ), only after each of the forward driving dog hubs of the fourth selectable gear ratio configuration (in other words, all of such forward driving dog hubs) have synchronised with the gears into which they are respectively moved into contact with will torque start flowing along load path L 4  instead on L 3 . Such forward driving dog hubs being: the first dog hub  32   a  on the input shaft  12 ; the first dog hub  32   b  on the bridging shaft  14 ; and the third dog hub  36   c  on the output shaft  16 . Now with regards to the reverse driving dog hubs of the fourth selectable gear ratio configuration ( FIG. 6 ), when a forward driving dog hub thereof synchronises with its respective gear as described then the corresponding reverse driving dog hub will fall into interlocking engagement with that gear as well with minimal backlash. In this manner the reverse driving dog hubs of the fourth selectable gear ratio configuration are engaged, such dog hubs being: the second dog hub  34   a  on the input shaft  12 ; the secondary dog hub  34   b  on the bridging shaft  14 ; and the secondary dog hub  34   c  on the output shaft  16 . 
     To avoid a lockup condition of the transmission  10  while upshifting from the third to fourth selectable gear ratio configuration, the reverse driving dog hub of the third gear  22   a  on the input shaft  12  (i.e. the fourth dog hub  38   a ) must or should be disengaged from contact with the third gear  22   a  before the forward driving dog hub of the first gear  18   a  on the input shaft  12  (i.e. the first dog hub  32   a ) is engaged with the first gear  18   a . One way of achieving this is to provide a mechanical connection between the first dog hub  32   a  and the fourth dog  38   a  on the input shaft  12  such that if one moves to the left or right so does the other and vice versa. In this manner both cannot be engaged at the same time because moving say the first dog hub  32   a  to the right in order to engage it with the first gear  18   a  on the input shaft  12  moves the fourth dog hub  38   a  also to the right but away from the third gear  22   a . It is appreciated that other ways of achieving the same effect are possible and will be apparent to those of ordinary skill in the art having read the foregoing disclosure. 
     It does not matter in which order the forward driving dog hubs of the fourth selectable gear ratio configuration move into contact and synchronise with the first gear on the output shaft  18   a , the primary gear  18   b  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16  respectively. However as mentioned previously, only after each such forward driving dog hub has synchronised with the gear into which it is moved into contact with will torque start flowing along load path L 4 . 
     The order of synchronisation mentioned above is likely to differ depending on the specific configuration of the transmission  10 . For instance, different mechanisms used to manipulate changes in dog hub positions could cause the forward driving dog hubs of the fourth selectable gear ratio configuration to come into contact with respective gears at slightly different times. Furthermore, differences in the extent of friction provided by bearings coupling gears to shafts could cause the forward driving dog hubs of the fourth selectable gear ratio to synchronise with respective gears at slightly different times 
     Continuing with the extent of friction experienced by bearings, in some transmissions  10  bearings could experience enough friction such that even when gears are not engaged by dog hubs these gears experience a torque and are essentially dragged by the shaft on which they are mounted to rotate. This could cause some gears, even when not engaged by dog hubs, to rotate at substantially the same speed as the shaft on which they are mounted unless forced otherwise e.g. by another gear in mesh therewith that is rotating at another speed. In other transmissions  10  however the friction experienced by bearings could be less so that gears essentially free-wheel unless engaged by dog hubs. The degree of friction experienced by bearings affects the degree of synchronization required when implementing gear shifts. For instance in shifting between the third and fourth selectable gear ratios, it will be apparent from  FIG. 5  that the first gear  18   a  on the input shaft  12 , the primary gear  18   b  on the bridging shaft  14  and the first gear  18   c  on the output shaft are only coupled to their respective shafts by the bearings mounting them thereto—they are not rotationally coupled to the shafts by dog hubs. As such, if the friction provided by such bearings is sufficiently low and the third gear ratio configuration ( FIG. 5 ) is engaged for a sufficient length of time then the essentially freewheeling gears will slow down. As a result, when subsequently upshifting into the fourth selectable gear ratio configuration ( FIG. 6 ) a higher degree of synchronization will be required by the first dog hub  32   a  on the input shaft  12  and the first dog hub  32   b  on the bridging shaft  14  than had the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14  been rotating closer to the rotational speeds of such shafts due to the friction provided by the bearings on which they are mounted. 
     Requiring less synchronisation as heretofore described provides that smoother gearshifts are achievable, so improving vehicle and driver passenger comfort. 
     More specific details of an exemplary upshift operation between the third and fourth selectable gear ratio configurations is now provided. 
     Since the first gear  18   a  on the input shaft  12  is free to rotate relative to the input shaft  12  while the transmission  10  is in the third selectable gear ratio configuration ( FIG. 5 ), upon shifting to the fourth gear ratio configuration ( FIG. 6 ) the input shaft  12 —and thus the first dog hub  32   a  rotationally fixed to it—will be rotating faster than the first gear  18   a . Therefore, in a positive torque condition of the transmission  10 , as the first dog hub  18   a  is urged against the first gear  32   a  it will catch up (synchronise) with that gear and so become loaded and begin drivingly rotating the first gear  18   a ; in other words, it will act as a forward driving dog hub. 
     After this the first gear  18   a  will rotate at the same speed as the input shaft  12 , thus enabling the second dog hub  34   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. Due to the meshing engagement with the first gear  18   a  on the input shaft  12 , the primary gear  18   b  on the bridging shaft  14  is drivingly rotated. The first dog hub  32   b  on the bridging shaft  14  acting as a forward driving dog hub will thus become drivingly engaged by the primary gear  18   b  as it rotates, after which the bridging shaft  14  will be caused to rotate at the same speed as the primary gear  181 ). This enables the second dog hub  34   b , acting as a reverse driving dog hub, to fall into interlocking engagement with the primary gear  18   b  as in  FIG. 14 . 
     The secondary gear  20   b   1  on the bridging shaft  14  has a bigger diameter than the first (reduced diameter) portion  22   b   1  of the quaternary gear  22   b . Moreover the difference in gear ratio between: i) the interface between the secondary gear  20   b   1  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16 ; and ii) the interface between the first (reduced diameter) portion  22   b , of the quaternary gear  22   b  on the bridging shaft  14  and the third gear  22   c  on the output shaft  16 , is such that for a given rotational speed of the bridging shaft  14  the second gear  20   c  on the output shaft  16  is driven faster than the third gear  22   c . As such the second gear  20   c  will be caused catch up (synchronise) with and drivingly rotate the third dog hub  36   c  which will thus become loaded and act as a forward driving dog hub, whereby torque will subsequently flow along the load path L 4  (see  FIG. 6 ) between the torque input feature  13  and the torque output feature  15 . 
     After this the second gear  20   c  on the output shaft  16  will rotate at the same speed as the output shaft  16 , thus enabling the second dog hub  34   c  on the output shaft  16  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. Since the third dog hub  36   a  on the input shaft  12  does not sit in the load path L 5  it is no longer loaded when torque begins flowing along load path L 4  and so can be moved out of engagement with the second gear  20   a . Furthermore, since the output shaft  12  will be rotating faster than the third gear  22   c , the fourth dog hub  38   c  is pushed out of engagement therewith because of the heretofore described ramped portions of these components being caused to ride over each other. 
     When the fourth selectable gear ratio configuration ( FIG. 6 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the first and second dog hubs  32   a ,  34   a  rotationally fixed thereto&gt;the first dog hub  34   a  acting as a forward driving dog hub drivingly rotates the first gear  18   a &gt;torque is transferred to the primary gear  18   b  on the bridging shaft  14  via the meshing engagement therewith&gt;the first dog hub  32   b  acting as the forward driving dog hub is rotatably driven by the primary gear  18   b &gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the first dog hub  32   b &gt;the secondary gear rotationally fixed to the bridging shaft  14  thus rotates&gt;torque is then transferred to the second gear  20   c  on the output shaft  16  via the meshing engagement therewith&gt;the third dog hub  36   c  acting as a forward driving dog hub is rotatably driven by the second gear  20   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  36   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle power train. 
     In the just described positive torque condition the second dog hub  34   a  on the input shaft  12 , the second dog hub  34   b  on the bridging shaft  14  and the second dog hub  34   c  on the output shaft  16  act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 4 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     It will be noted that a shift from the third to fourth selectable gear ratio configuration involves a block shift along the input shaft  12  and a single shift along the output shaft  16 . The difference in gear ratio between: i) the interface between the secondary gear  20   b   1  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16 ; and ii) the interface between the first (reduced diameter) portion  22   b   1  of the quaternary gear  22   b  on the bridging shaft  14  and the third gear  30   c  on the output shaft  16 , is equivalent to three step changes in resultant gear ratio described up to now (i.e. three times the step in resultant gear ratio caused by shifting from the first to second or second to third heretofore described gear ratio configurations). However this coupled with the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft, provides that upon shifting from the third to fourth gear ratio configuration of the transmission  10  the change in resultant gear ratio experienced between the torque input and output features  13 ,  15  is equivalent to a single step change in gear ratio configuration (i.e. equivalent to the step change caused by shifting from 1 st  to 2 nd , or 2 nd  to 3 rd  as heretofore described). 
     Moreover, in some embodiments a shift from the third to fourth selectable gear ratio configuration could give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     Due to the increase in rotational speed of the output shaft  16  following a shift into the fourth selectable gear ratio configuration ( FIG. 6 ), for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the third or any other previous selectable gear ratio configuration. 
     Fourth to Fifth 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the fifth selectable gear ratio configuration of the transmission  10  ( FIG. 7 ). Moreover to progress from the fourth to the fifth selectable gear ratio configuration: the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be separated from engagement with the first gear  18   a ; the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are to be separated from engagement with the primary gear  18   b ; and the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be moved into engagement with the second gear  20   a . However, the second and third dog hubs  34   c ,  36   c  on the output shaft  16  are to remain in engagement with the second gear  20   c . Subsequently torque is transferred along the load path L 5  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 5  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the fourth selectable gear ratio configuration ( FIG. 6 ) to the fifth selectable gear ratio configuration ( FIG. 7 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the fourth selectable gear ratio configuration ( FIG. 6 ) the second dog hub  34   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the first gear  18   a  into engagement with the second gear  20   a . When it engages and synchronises the second gear  20   a  the second dog hub  34   a  acts as a forward driving dog hub and so drivingly rotates the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that when torque is being transferred along the first load path L 4  the second gear  20   a  on the input shaft  12  will be rotating slower than the first gear  18   a —and so the second gear  20   a  will be rotating slower than the input shaft  12  itself. Thus when the second dog hub  34   a  is urged against the second gear  20   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the second gear  20 . 
     Following this the second gear  20   a  will rotate at the same speed as the input shaft  12 , thus enabling the third dog hub  28   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. The second gear  20   a  on the input shaft  12  has a bigger diameter than the first gear  18 . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the second gear  20   a  is rotationally fixed to the input shaft  12  compared to when only the first gear  18   a  is rotationally fixed thereto. 
     When the second dog hub  34   a  begins to act as the forward driving dog hub of the second gear  20   a , torque begins flowing along the load path L 5  (see  FIG. 7 ) between the torque input feature  13  and the torque output feature  15 . Since the first dog hub  32   a  on the input shaft  12  and the first and second dog hubs  32   b ,  34   b  on the bridging shaft  12  are not loaded when torque flows along the load path L 5  they are able to be moved out of engagement with the first gear  18   a  and the primary gear  18   b.    
     In more detail, when the fifth selectable gear ratio configuration ( FIG. 7 ) has been engaged, positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the second and third dog hubs  34   a ,  36   a  rotationally fixed thereto&gt;the second dog hub  34   a  acting as a forward driving dog hub drivingly rotates the second gear  20   a &gt;torque is transferred to the tertiary gear  20   b   2  on the bridging shaft  14  via the meshing engagement therewith&gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the tertiary gear  20   b   2 &gt;the secondary gear  20   b   1  on the bridging shaft  14  thus rotates&gt;torque is transferred to the second gear  20   c  on the output shaft  16  via the meshing engagement therewith&gt;the third dog hub  36   c  acting as a forward driving dog hub is rotatably driven by the second gear  20   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  36   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the third dog hub  36   a  on the input shaft  12  and the second dog hub  34   c  on the output shaft  16  act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 5 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the fourth to fifth selectable gear ratio configuration gives rise to a step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the fourth to fifth selectable gear ratio configuration could give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the fifth selectable gear ratio configuration ( FIG. 7 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the fourth or any other previous selectable gear ratio configuration. 
     Fifth to Sixth 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the sixth selectable gear ratio configuration of the transmission  10  ( FIG. 8 ). Moreover to progress from the fifth to the sixth selectable gear ratio configuration: the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be separated from engagement with the second gear  20   a ; the third and fourth hubs  36   b ,  38   b  on the bridging shaft  14  are be moved into engagement with the quaternary gear  22   b ; and the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are to be moved into engagement with the third gear  22   a . However, the second and third dog hubs  34   c ,  36   c  on the output shaft  16  are to remain in engagement with the second gear  20   c . Subsequently torque is transferred along the load path L 6  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 6  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the fifth selectable gear ratio configuration ( FIG. 7 ) to the sixth selectable gear ratio configuration ( FIG. 8 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the fifth selectable gear ratio configuration ( FIG. 7 ) the third dog hub  36   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the second gear  20   a  into engagement with the third gear  22   a . When it engages and synchronises with the third gear  22   a  the third dog hub  36   a  acts as a forward driving dog hub and so drivingly rotates the third gear  22   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that when torque is being transferred along the fifth load path L 5  the third gear  22   a  on the input shaft  12  will be rotating slower than the second gear  20   a —and so the third gear  22   a  will be rotating slower than the input shaft  12  itself. Thus, when the third dog hub  36   a  is urged against the third gear  22   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the third gear  22   a . Following this the third gear  22   a  will rotate at the same speed as the input shaft  12 , thus enabling the fourth dog hub  38   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. 
     Due to the meshing engagement with the third gear  22   a  on the input shaft  12 , the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14  is drivingly rotated. The fourth dog hub  38   b  on the bridging shaft  14  acting as a forward driving dog hub will thus become drivingly engaged by the quaternary gear  22   b  as it rotates, after which the bridging shaft  14  will be caused to rotate at the same speed as the quaternary gear  22   b . This enables the third dog hub  36   b , acting as a reverse driving dog hub, to fall into interlocking engagement with the other side of the quaternary gear  22   b  as in  FIG. 14 . 
     The third gear  22   a  on the input shaft  12  has a bigger diameter than the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the third gear  22   a  is rotationally fixed to the input shaft  12  compared to when only the second gear  20   a  is rotationally fixed thereto. 
     When the third dog hub  36   a  on the input shaft  12  begins to act as the forward driving dog hub of the third gear  22   a , and the fourth dog hub  38   b  on the bridging shaft  14  begins acting as the forward driving dog hub of the quaternary gear  22   b , torque begins flowing along the load path L 6  (see  FIG. 8 ) between the torque input feature  13  and the torque output feature  15 . Since the second dog hub  34   a  is not loaded when torque flows along load path L 6  it can be moved out of engagement with the second gear  20   a.    
     In more detail, when the sixth selectable gear ratio configuration ( FIG. 8 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the third and fourth dog hubs  36   a ,  38   a  rotationally fixed thereto&gt;the third dog hub  36   a  acting as a forward driving dog hub drivingly rotates the third gear  22   a &gt;torque is transferred to the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14  via the meshing engagement therewith&gt;the fourth dog hub  38   b  acting as the forward driving dog hub is rotatably driven by the quaternary gear  22   b &gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the fourth dog hub  38   b &gt;the secondary gear rotationally fixed to the bridging shaft  14  thus rotates&gt;torque is then transferred to the second gear  20   c  on the output shaft  16  via the meshing engagement therewith&gt;the third dog hub  36   c  acting as a forward driving dog hub is rotatably driven by the second gear  20   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the third dog hub  36   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the fourth dog hub  38   a  on the input shaft  12 ; the third dog hub  36   b  on the bridging shaft  14 ; and the second dog hub  34   c  on the output shaft  16  all act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 6 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the fifth to sixth selectable gear ratio configuration gives rise to another step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the fifth to sixth selectable gear ratio configuration could also give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the sixth selectable gear ratio configuration ( FIG. 8 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the fifth or any other previous selectable gear ratio configuration. 
     Sixth to Seventh 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the seventh selectable gear ratio configuration of the transmission  10  ( FIG. 9 ). Moreover to progress from the sixth to the seventh selectable gear ratio configuration: the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are to be separated from engagement with the third gear  20   a ; the third and fourth hubs  36   b ,  38   b  on the bridging shaft  14  are be separated from engagement with the quaternary gear  38   b ; the second and third dog hubs  34   c ,  36   c  on the output shaft  16  are be separated from engagement with the second gear  34   c ; the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be engaged with the first gear  18   a ; and the first and second dog hubs  32   c ,  34   c  on the output shaft  16  are to be engaged with the first gear  18   c . Subsequently torque is transferred along the load path L 7  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forward rotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 7  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the sixth selectable gear ratio configuration ( FIG. 8 ) to the seventh selectable gear ratio configuration ( FIG. 9 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the sixth selectable gear ratio configuration ( FIG. 8 ) the fourth dog hub  38   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the third gear  22   a , which is enabled due to the fourth dog hub  38   a  being a reverse driving dog hub and so not loaded in a positive torque condition of the transmission  10 . Subsequently the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are moved into engagement with the first gear  18   a ; and the first and second dog hubs  32   c ,  34   c  on the output shaft  16  are moved into engagement with the first gear  18   c ; whereby the second dog hub  34   c  on the output shaft  16  can be separated from the second gear  20   c  while positive torque is being transferred along the load path L 6  due to its also being a reverse driving dog hub. Torque will thus begin flowing along load path L 7  instead of L 6 . Subsequently each of the third dog hub  36   a  on the input shaft  12 , the third and fourth dog hubs  36   b ,  38   b  on the bridging shaft  14  and the third dog hub  36   c  on the output shaft  16  will no longer be loaded due to torque flowing along load path L 7  and so can be removed from engagement with the third gear  22   a  on the input shaft  12 , the quaternary gear  22   b  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16 . 
     More specifically during an upshift from the sixth selectable gear ratio configuration ( FIG. 8 ) to the seventh selectable gear ratio configuration ( FIG. 9 ), only after each of the forward driving dog hubs of the seventh selectable gear ratio configuration (in other words, both of such forward driving dog hubs) have synchronised with the gears into which they are respectively moved into contact with will torque start flowing along load path L 7  instead on L 6 . Such forward driving dog hubs being: the first dog hub  32   a  on the input shaft  12  and the second dog hub  34   c  on the output shaft  16 . Now with regards to the reverse driving dog hubs of the seventh selectable gear ratio configuration ( FIG. 9 ), when a forward driving dog hub thereof synchronises with its respective gear as described then the corresponding reverse driving dog hub will fall into interlocking engagement with that gear as well with minimal backlash. In this manner the reverse driving dog hubs of the seventh selectable gear ratio configuration are engaged, such dog hubs being: the second dog hub  34   a  on the input shaft  12  and the first dog hub  32   c  on the output shaft  16 . 
     To avoid a lockup condition of the transmission  10  while upshifting from the sixth to seventh selectable gear ratio configuration, the reverse driving dog hub of the third gear  22   a  on the input shaft  12  (i.e. the fourth dog hub  38   a ) must or should be disengaged from contact with the third gear  22   a  before the forward driving dog hub of the first gear  18   a  on the input shaft  12  (i.e. the first dog hub  32   a ) is engaged with the first gear  18   a . Ways of achieving this have already been discussed in connection with upshifting between the third and fourth selectable gear ratio configurations. 
     It does not matter in which order the forward driving dog hubs of the seventh selectable gear ratio configuration move into contact and synchronise with the first gear  18   a  on the input shaft  12  and the first gear  18   c  on the output shaft  16  respectively. However as mentioned previously, only after both forward driving dog hubs have synchronised with the gear into which they are moved into contact with will torque start flowing along load path L 7 . The order of synchronisation is likely to differ depending on the specific configuration of the transmission  10  and factors affecting this have already been discussed in connection with upshifting between the third and fourth selectable gear ratio configurations. 
     More specific details of an exemplary upshift operation between the sixth and seventh selectable gear ratio configurations are now provided. 
     Since the first gear  18   a  on the input shaft  12  is free to rotate relative to the input shaft  12  while the transmission is in the sixth gear ratio configuration ( FIG. 8 ), upon shifting to the seventh gear ratio configuration ( FIG. 9 ) the input shaft  12 —and thus the first dog hub  32   a  rotationally fixed to it—will be rotating faster than the first gear  18   a . Therefore, in a positive torque condition of the transmission  10 , as the first dog hub  18   a  is urged against the first gear  32   a  it will catch up (synchronise) with that gear and so become loaded and begin drivingly rotating the first gear  18   a ; in other words, it will act as a forward driving dog hub. 
     After this the first gear  18   a  will rotate at the same speed as the input shaft  12 , thus enabling the second dog hub  34   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. Due to the meshing engagement with the first gear  18   a  on the input shaft  12 , the primary gear  18   b  on the bridging shaft  14  is drivingly rotated. The primary gear  18   b  is free to rotate relative to the bridging shaft  14  and so torque is transferred via the primary gear  18   b  to the first gear  18   c  on the output shaft  16 . 
     The primary gear  18   b  on the bridging shaft  14  has a bigger diameter than the secondary gear  20   b   1  on the bridging shaft  14 . Moreover the difference in gear ratio between: i) the interface between the primary gear  18   b  on the bridging shaft  14  and the first gear  18   c  on the output shaft  16 ; and ii) the interface between the secondary gear  20   b   1  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16 , is such that for a given rotational speed of the input shaft  12  the first gear  18   c  on the output shaft  16  is driven faster in the seventh selectable gear ratio configuration ( FIG. 9 ) compared to the rotational speed at which the second gear  20   c  on the output shaft  16  is driven in the sixth selectable gear ratio configuration ( FIG. 8 ). As such, upon selecting the seventh selectable gear ratio configuration ( FIG. 9 ) the first gear  18   c  will be caused to catch up (synchronise) with and drivingly rotate the second dog hub  34   c  which will thus become loaded and act as a forward driving dog hub, whereby torque will subsequently flow along the load path L 7  (see  FIG. 9 ) between the torque input feature  13  and the torque output feature  15 . 
     After this the first gear  18   c  on the output shaft  16  will rotate at the same speed as the output shaft  16 , thus enabling the first dog hub  32   c  on the output shaft  16  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. Since each of: the third dog hub  36   a  on the input shaft  12 ; the third dog hub  38   b  on the bridging shaft  14 ; and the fourth dog hub  38   b  on the bridging shaft  14  will subsequently no longer be loaded when torque flows along the load path L 7 , they are able to be removed from engagement with the third gear  22   a  on the input shaft  12  and the quaternary gear  22   b  on the bridging shaft  14 . Furthermore, when torque starts flowing along load path L 7  the output shaft  16  will start rotating faster than the second gear  34   c  so the third dog hub  36   c  is pushed out of engagement therewith due to the heretofore described ramped portions of these components being caused to ride over each other. 
     When the seventh selectable gear ratio configuration ( FIG. 9 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the first and second dog hubs  32   a ,  34   a  rotationally fixed thereto&gt;the first dog hub  32   a  acting as a forward driving dog hub drivingly rotates the first gear  18   a &gt;torque is transferred to the primary gear  18   b  on the bridging shaft  14  via the meshing engagement therewith&gt;torque is then transferred to the first gear  18   c  on the output shaft  16  via the meshing engagement therewith&gt;the second dog hub  34   c  acting as the forward driving dog hub is rotatably driven by the first gear  18   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the second dog hub  34   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle power train. 
     In the just described positive torque condition the second dog hub  34   a  on the input shaft  12  and the first dog hub  32   c  on the output shaft  16  act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 7 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     It will be noted that a shift from the sixth to seventh selectable gear ratio configuration involves a block shift along the input shaft  12  and a single shift along the output shaft  16 . The difference in gear ratio between: i) the interface between the primary gear  18   b  on the bridging shaft  14  and the first gear  18   c  on the output shaft  16 ; and ii) the interface between the secondary gear  20   b   1  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16 , is equivalent to three step changes in resultant gear ratio (i.e. three times the step in resultant gear ratio caused by shifting from the first to second, second to third, third to fourth etc. heretofore described gear ratio configurations). However this coupled with the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , provides that upon shifting from the sixth to seventh gear ratio configuration of the transmission  10  the change in resultant gear ratio experienced between the torque input and output features  13 ,  15  is equivalent to a single step change in gear ratio configuration (i.e. equivalent to the step change caused by shifting from 1 st  to 2 nd , 2 nd  to 3 rd , 3 rd  to 4 th  etc. as heretofore described). 
     Moreover, in some embodiments a shift from the sixth to seventh selectable gear ratio configuration could give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     Due to the increase in rotational speed of the output shaft  16  following a shift into the seventh selectable gear ratio configuration ( FIG. 9 ), for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the sixth or any other previous selectable gear ratio configuration. 
     Seventh to Eighth 
     A change in the overall combination of gears rotationally fixed to the respective shafts is required in order to progress to the eighth selectable gear ratio configuration of the transmission  10  ( FIG. 10 ). Moreover to progress from the seventh to the eighth selectable gear ratio configuration: the first and second dog hubs  32   a ,  34   a  on the input shaft  12  are to be separated from engagement with the first gear  18   a , whereas the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be moved into engagement with the second gear  20   a ; and the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are to be engaged with the primary gear  18   b . However, the first and second dog hubs  32   c ,  34   c  on the output shaft  16  are to remain in engagement with the first gear  18   c . Subsequently torque is transferred along the load path L 8  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forwardrotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 8  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the seventh selectable gear ratio configuration ( FIG. 9 ) to the eighth selectable gear ratio configuration ( FIG. 10 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the seventh selectable gear ratio configuration ( FIG. 9 ) the second dog hub  34   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the first gear  18   a  into engagement with the second gear  20   a . When it engages and synchronises with the second gear  20   a  the second dog hub  34   a  acts as a forward driving dog hub and so drivingly rotates the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that when torque is being transferred along the seventh load path L 7  the second gear  20   a  on the input shaft  12  Will be rotating slower than the first gear  18   a —and so the second gear  20   a  will be rotating slower than the input shaft  12  itself. Thus, when the second dog hub  34   a  is urged against the second gear  20   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the second gear  20   a.    
     Following this the second gear  20   a  will rotate at the same speed as the input shaft  12 , thus enabling the third driving dog hub  36   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. The second gear  20   a  on the input shaft  12  has a bigger diameter than the first gear  18 . Moreover the difference in gear ratio between: i) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 ; and ii) the interface between the first gear  18   a  on the input shaft  12  and the primary gear  18   b  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the second gear  20   a  is rotationally fixed to the input shaft  12  compared to when only the first gear  18   a  is rotationally fixed thereto. 
     While the second and third dog hubs  34   a ,  36   a  on the input shaft are moved into engagement with the second gear  20   a  as described above, the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are moved into engagement with the primary gear  18   b.    
     Thus, when the second dog hub  34   a  begins to act as the forward driving dog hub of the second gear  20   a , torque begins flowing along the load path L 8  (see  FIG. 10 ) between the torque input feature  13  and the torque output feature  15 . Since the first dog hub  32   a  on the input shaft  12  is not loaded when torque flows along load path L 8  it is able to be moved out of engagement with the first gear  18   a.    
     In more detail, when the eighth selectable gear ratio configuration ( FIG. 10 ) has been engaged, positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the second and third dog hubs  34   a ,  36   a  rotationally fixed thereto&gt;the second dog hub  34   a  acting as a forward driving dog hub drivingly rotates the second gear  20   a &gt;torque is transferred to the tertiary gear  20   b   2  on the bridging shaft  14  via the meshing engagement therewith&gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the tertiary gear  20   b   2 &gt;the first dog hub  32   b  thus rotates with the bridging shaft  14  due to being rotationally fixed thereto&gt;the first dog hub  32   b  acting as a forward driving dog hub rotatably drives the primary gear  18   b &gt;torque is transferred to the first gear  18   c  on the output shaft  16  via the meshing engagement therewith&gt;the second dog hub  34   c  acting as a forward driving dog hub is rotatably driven by the first gear  18   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the second dog hub  34   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle power train. 
     In the just described positive torque condition the third dog hub  36   a  on the input shaft  12 , the second dog hub  34   b  on the bridging shaft  14  and the first dog hub  32   c  on the output shaft  16  all act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 8 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the seventh to eighth selectable gear ratio configuration gives rise to a step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the seventh to eighth selectable gear ratio configuration could give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the eighth selectable gear ratio configuration ( FIG. 10 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission  10  was in the seventh or any other previous selectable gear ratio configuration. 
     Eighth to Ninth 
     A change in the overall combination of gears rotationally fixed to respective shafts is required in order to progress to the ninth selectable gear ratio configuration of the transmission  10  ( FIG. 11 ). Moreover to progress from the eighth to the ninth selectable gear ratio configuration: the second and third dog hubs  34   a ,  36   a  on the input shaft  12  are to be separated from engagement with the second gear  20   a , whereas the third and fourth hubs  36   a ,  38   a  on the input shaft  12  to be moved into engagement with the third gear  22   a ; and the third and fourth dog hubs  36   b ,  38   b  on the bridging shaft  14  are to be moved into engagement with the quaternary gear  22   b . However, the first and second dog hubs  32   b ,  34   b  on the bridging shaft  14  are to remain in engagement with the primary gear  18   b  and the first and second dog hubs  32   c ,  34   c  on the output shaft  16  are to remain in engagement with the first gear  18   c . Subsequently torque is transferred along the load path L 9  between the torque input and output features  13 ,  15  when the input shaft  12  is rotatably driven in a forwardrotational direction. In a reverse torque condition torque is transferred in the reverse direction along load path L 9  between the torque output and input features  15 ,  13 . 
     Variations in dog hub positions required to change from the eighth selectable gear ratio configuration ( FIG. 10 ) to the ninth selectable gear ratio configuration ( FIG. 11 ) while the transmission is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ) will now be described in more detail. While the transmission is in the eighth selectable gear ratio configuration ( FIG. 10 ) the third dog hub  36   a  on the input shaft  12  (acting as a reverse driving dog hub) is moved from engagement with the second gear  20   a  into engagement with the third gear  22   a . When it engages and synchronises with the third gear  22   a  the third dog hub  36   a  acts as a forward driving dog hub and so drivingly rotates the third gear  22   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that when torque is being transferred along the load path L 8  the third gear  22   a  on the input shaft  12  will be rotating slower than the second gear  20   a —and so the third gear  22   a  will be rotating slower than the input shaft  12  itself. Thus, when the third dog hub  36   a  is urged against the third gear  22   a  it catches up (synchronises) with and begins to act as the forward driving dog hub for that gear and so becomes loaded by drivingly rotating the third gear  22   a.    
     Following this the third gear  22   a  will rotate at the same speed as the input shaft  12 , thus enabling the fourth dog hub  38   a  to fall into interlocking engagement therewith as in  FIG. 14  with minimal backlash. The third gear  22   a  on the input shaft  12  has a bigger diameter than the second gear  20   a . Moreover the difference in gear ratio between: i) the interface between the third gear  22   a  on the input shaft  12  and the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14 ; and ii) the interface between the second gear  20   a  on the input shaft  12  and the tertiary gear  20   b   2  on the bridging shaft  14 , is such that for a given rotational speed of the input shaft  12  the bridging shaft  14  will be caused to rotate faster when the third gear  22   a  is rotationally fixed to the input shaft  12  compared to when only the second gear  20   a  is rotationally fixed thereto. 
     While the third and fourth dog hubs  36   a ,  38   a  on the input shaft  12  are moved into engagement with the third gear  22   a  as described above, the third and fourth dog hubs  36   b ,  38   b  on the bridging shaft  14  are moved into engagement with the quaternary gear  22   b.    
     Thus, when the third dog hub  36   a  on the input shaft  12  begins to act as the forward driving dog hub of the third gear  22   a , torque begins flowing along the load path L 9  (see  FIG. 11 ) between the torque input feature  13  and the torque output feature  15 . Since the second dog hub  34   a  on the input shaft  12  is not loaded when torque begins flowing along load path L 9  it can be moved out of engagement with the second gear  20   a.    
     In more detail, when the ninth selectable gear ratio configuration ( FIG. 11 ) has been engaged positive torque is transferred through the transmission  10  as follows. The torque input feature  13  is rotatably driven in a forward rotational direction (e.g. by an engine or motor)&gt;the input shaft  12  operatively connected to the torque input feature  13  thus rotates at the same speed&gt;this causes rotation of the third and fourth dog hubs  36   a ,  38   a  rotationally fixed thereto&gt;the third dog hub  36   a  acting as a forward driving dog hub drivingly rotates the third gear  22   a &gt;torque is transferred to the second (larger diameter) portion  22   b   2  of the quaternary gear  22   b  on the bridging shaft  14  via the meshing engagement therewith&gt;the fourth dog hub  38   b  acting as the forward driving dog hub is rotatably driven by the quaternary gear  22   b &gt;this causes the bridging shaft  14  to rotate due to being rotationally fixed relative to the fourth dog hub  38   b &gt;the first dog hub  32   b  thus rotates with the bridging shaft  14  due to being rotationally fixed thereto&gt;the first dog hub  32   b  acting as a forward driving dog hub rotatably drives the primary gear  18   b &gt;torque is then transferred to the first gear  18   c  on the output shaft  16  via the meshing engagement therewith&gt;the second dog hub  34   c  acting as a forward driving dog hub is rotatably driven by the first gear  18   c &gt;this causes the output shaft  16  to rotate due to being rotationally fixed relative to the second dog hub  34   c &gt;which causes the torque output feature  15  to rotate at the same speed due to being operatively connected to the output shaft  16 &gt;whereby the torque output feature  15  rotatably drives components downstream in the vehicle powertrain. 
     In the just described positive torque condition the fourth dog hub  38   a  on the input shaft  12 , the second and third dog hubs  34   b ,  36   b  on the bridging shaft  14  and the first dog hub  32   c  on the output shaft  16  act as reverse driving dog hubs; meaning that they are not loaded. However, in a reverse torque condition torque is transferred in the reverse direction along the load path L 9 , in which case the reverse driving dog hubs become loaded and the forward driving dog hubs are no longer loaded. 
     A shift from the eighth to ninth selectable gear ratio configuration gives rise to another step variation in resultant gear ratio between the torque input and output features  13 ,  15 . In some embodiments such a shift from the eighth to ninth selectable gear ratio configuration could also give rise to a variation of 1.23:1 in resultant gear ratio between the torque input and output features  13 ,  15 . 
     An increase in rotational speed of the bridging shaft  14  following a shift into the ninth selectable gear ratio configuration ( FIG. 11 ) results in a corresponding increase in rotational speed of the output shaft  16 —and thus the torque output feature  15 . As a result, for a given engine or motor speed the vehicle can be caused to travel at a greater speed than when the transmission was in the eighth or any other previous selectable gear ratio configuration. 
     Additional Information 
     Since upshifts in gear ratio can occur while the transmission  10  is in a positive torque condition (i.e. while power is being transmitted along the vehicle powertrain via the transmission  10 ), this means that upshifts can occur without loss of vehicular driving power. An advantage of this is that faster vehicular acceleration can take place, compared to vehicles that require a clutch to be engaged/disengaged to implement upshifts. 
     Down shifts are completed by the same mechanism of movement heretofore described but in reverse and while negative torque is being transmitted (i.e. when torque is being transferred through the transmission  10  from the torque output feature  15  to the torque input feature  13 , which can occur when a driver lifts their foot off the vehicle throttle for example. 
     In the heretofore described transmission  10  the input shaft  12 , bridging shaft  14  and output shaft  16  each extend along the same notional plane. In other words, a notional straight line perpendicular to the input shaft  12  will extend through the bridging shaft  14  and the output shaft  16  also. However, in some embodiments the input shaft  12 , bridging shaft  14  and output shaft  16  may not extend along the same notional plane, in which case they could be arranged in a triangular orientation making the input shaft  12  closer to the output shaft  16  like in  FIG. 15 . 
     Details are here provided of one suitable mechanism for causing appropriate dog hub movements to occur for implementing gear ratio shifts in the heretofore described transmission  10 . In this mechanism the dog hub movements required to implement gear ratio shifts are controlled by shift shafts similarly as described between page 12, line 17 to page 15, line 32 of WO2014/049317A1 already referred to. Moreover, each of the input shaft  12 , bridging shaft  14  and output shaft  16  is associated with its own respective shift shaft for causing movements of the dog hubs thereon when the associated shift shaft is rotated. In such an arrangement the shift shaft for the input shaft  12  is indexed round by 30 degrees on each gear ratio shift. The shift shaft for the output shaft  16  is linked to the shift shaft for the input shaft  12  by an indexing mechanism which causes the shift shaft for the output shaft  16  to rotate 30 degrees on each third indexing of the shift shaft for the input shaft  12 . The shift shaft for the bridging shaft  14  is also geared to the shift shaft for the input shaft  12  so as to rotate at three tenths the rate of that shaft. This provides that the shift shaft for the bridging shaft  14  has ten positions corresponding to neutral and the first to ninth selectable gear ratio configurations. Each of the three shift shafts mentioned here carries shift barrels with the desired groove patterns to cause the appropriate movements of dog hubs desired to implement shifts between neutral and the first to ninth selectable gear ratio configurations. Previously it was mentioned that in order to avoid a lockup condition of the transmission  10  a mechanical connection could be provided between the first and fourth dog hubs  32   a ,  38   a  on the input shaft  12  so that their respective movements are equal and opposite. In embodiments including such a mechanical connection, the shift shaft for the input shaft  12  has one less shifting barrel. For example, a shifting barrel may not be provided for controlling movements of the first dog hub  32   a , whereas a shifting barrel may be provided for controlling movements of the fourth dog hub  38   a . Nevertheless, the first dog hub  32   a  is caused to move with the fourth dog hub  38   a  due to the mechanical connection therebetween, so enabling respective gear ratio configurations to be selected whilst minimising the possibility of gearbox lockup. In other embodiments a shifting barrel may be provided for controlling movements of the first dog hub  32   a  but not the fourth dog hub  38   a.    
     The number of selectable gear ratio configurations of the heretofore described transmission  10  could be increased by the inclusion of further gears along the input shaft  12  and meshing gears on the bridging shaft  14 ; the output shaft  16  however still having three gears. Any such additional gears included on the input shaft  12  (e.g. between the heretofore mentioned second and third gears  20   a ,  22   a ) would have a similar configuration thereto and would thus be rotationally couplable to the input shaft by dog hubs either side thereof. Whereas any additional gears included on the bridging shaft  14  (e.g. between the heretofore mentioned tertiary and quaternary gears  20   b   2  and  22   b ) would mesh with a respective additional gear on the input shaft  12  and would also be rotationally fixed to the bridging shaft  14  like the heretofore mentioned secondary and tertiary gears  20   b   1 ,  20   b   2  fixed thereto. For example, six gears along the input shaft  12  would give a 6×3 transmission configuration having eighteen different selectable gear ratio configurations (whereby in the expression 6×3 the number 6 pertains to the number of gears on the input shaft  12  and the number 3 pertains to the number of gears on the output shaft  16 ). Simpler transmission configurations however are also possible for example a 4×2 configuration having four gears on the input shaft  12  and two on the output shaft  16 . Furthermore, other transmission configurations for example 4×4 having sixteen selectable gear ratio configurations are also possible by using further dog hubs and gears on the bridging shaft  14 . 
       FIG. 16  is a schematic line drawing of an example 4×2 version of the transmission. The transmission  100  has an input shaft  102 , a bridging shaft  104  and an output shaft  106 . The gears  108 ,  110 ,  112 ,  114  carried by the input shaft  102  can be rotationally fixed thereto by dog hubs in a similar manner to that heretofore described. The left and right most dog hubs on the input shaft (one being a forward driving dog hub and the other a reverse driving dog hub) are shown as being mechanically coupled so that they cannot be caused to engage the gears  108  and  114  simultaneously. The first gear  116  on the bridging shaft  104  can be rotationally fixed thereto by dog hubs in a similar manner to that heretofore described. The second, third, fourth and fifth gears  118 ,  120 ,  122 ,  124  are rotationally fixed to the bridging shaft  104 . As for the output shaft  106 , a first gear  126  carried thereby can be rotationally fixed thereto by dog hubs either side thereof. Furthermore, a second gear  128  carried by the output shaft  106  can be rotationally fixed thereto by dog hubs either side thereof. A forward driving dog hub and a reverse driving dog hub of the first/second gear pair  126 ,  128  are shown as being mechanically coupled so that they cannot be caused to engage the gears  126 ,  128  simultaneously. By changing the overall combination of gears of the transmission  100  rotationally fixed to the shafts  102 ,  104 ,  108  the load path, and so the resultant gear ratio, between a torque input  130  and a torque output  132  of the transmission (operatively coupled to the input and output shafts  102 ,  106  respectively) can be changed. Moreover, when only the first gear  126  of the output shaft  106  is rotationally coupled to the output shaft, respective resultant gear ratios from a first group thereof can be selected by changing the combination of gears rotationally fixed to the input and bridging shafts  102 ,  104 . However, when only the second gear  128  of the output shaft  106  is rotationally coupled to the output shaft, respective resultant gear ratios from a second group thereof can be selected by changing the combination of gears rotationally fixed to the input and bridging shafts  102 ,  104 . 
     Upon inspecting  FIGS. 3 to 5 , progressing through the first to third selectable gear ratios requires a change in the gears fixed to the input shaft  12  and the bridging shaft  14  to achieve the desired variations in load path in order to give rise to the different selectable gear ratios. However, no such change occurs in the gear fixed to the output shaft  16 . This similarly occurs while progressing through the next three selectable gear ratios (i.e., the fourth to sixth gear ratios) and again while progressing through three selectable gear ratios after that (i.e. the seventh to ninth gear ratios). Shifts between: i) the third and fourth gear ratio configurations; and ii) the sixth and seventh gear ratio configurations can be referred to as range change shifts because they cause a change in the range of gear ratios that are selectable by movements of dog hubs on the input and bridging shafts  12 ,  14 . As a result, the 1 st  to 3 rd  heretofore described selectable gear ratio configurations can be thought of as a first range of selectable gear ratios. Similarly, the 4th to 6th heretofore described selectable gear ratio configurations can be thought of as a second range of selectable gear ratios. Thus the 7 th  to 9 th  heretofore described selectable gear ratio configurations can be thought of as a third range of selectable gear ratios. 
     Although the shaft denoted  12  in the drawings has up now been described as an input shaft in operative connection with a torque input of the transmission  10 , in other embodiments the transmission  10  could be connected the other way around inside a vehicle such that the shaft denoted  12  in the drawings is instead an output shaft in operative connection with a torque output of the transmission  10 . The same correspondingly applies to the shaft denoted  16  in the drawings. 
     Up to now the respective gear connections between the bridging shaft  14  and the output shaft  16  have been described in the context of pairs of gears carried thereby being in mesh with one another. However, it is envisaged that in other transmission embodiments one or more such gear connections between the bridging shaft  14  and the output shaft  16  could include a planetary gearset. In other words, torque could be transferred between the bridging shaft  14  and the output shaft  16  in use via respective planetary gear set arrangements, depending on which was engaged at the time. Advantageously the ability of a planetary gearset to provide a larger ratio than is practical with a single pair of spur gears (for example a single planetary gear stage can readily provide a ratio from 3:1 up to 5:1) increases the range of resultant gear ratios achievable between the torque input feature  13  and output feature  15  in use, which is useful in heavy vehicles requiring high output torque such as tanks. 
     For example,  FIG. 17  illustrates a bridging shaft  204  and an output shaft  206 , whereby torque is able to be transferred there between via at least one planetary gear set in some instances. In particular a first spur gear  208  mounted on the bridging shaft  204  can transfer torque to the output shaft  206  via a 1.2:1 gear ratio connection, for example, by virtue of another spur gear  209  in mesh therewith when such gears are rotationally fixed to the shafts  204 ,  206  they are mounted on. A second spur gear  210  mounted on the bridging shaft  204  can transfer torque to the output shaft  206  via a different gear ratio connection (e.g. a 3:1 connection) by virtue of a planetary gear set  211  when such gear  210  and the output of the planetary gear set  211  are rotationally fixed to the shafts  204 ,  206  they are mounted on. A third spur gear  212  mounted on the bridging shaft  204  can transfer torque to the output shaft  206  via yet a different gear ratio connection (e.g. a 9:1 connection) by virtue of a planetary gear train  213  (which includes two or more planetary gear sets in series with one another) when such gear  212  and the output of the planetary gear train  213  are rotationally fixed to the shafts  204 ,  206  they are mounted on. 
     How the spread/range of selectable gear ratios can be increased in some embodiments should now be apparent. Nevertheless, it is here stated that in the example embodiment described with reference to  FIGS. 2 to 11  the spread/range of selectable gear ratios is (1.23) 8 =5.24. However, the provision of one or more planetary gear sets between the bridging shaft  14  and output shaft  16  as described with reference to  FIG. 17  could increase the spread/range of selectable gear ratios, making the transmission even more suitable for vehicles with low power to weight ratio (e.g. heavy trucks and main battle tanks). 
     One way of achieving the foregoing is illustrated in  FIG. 18 , wherein components of an exemplary planetary gearset  211  and planetary gear train  213  are shown. In the embodiment illustrated the sun gear  211   a  is arranged to receive torque from the spur gear  210  and to transfer it to an output  211   b  via planet gears  211   c  and a ring gear  211   d  that is restricted from rotating. The planetary gear train  213  includes two similarly configured planetary gear sets in series with one another. 
     It will be appreciated that dog hubs provided on the output shat  206  and located on either side of the output  211   b  to the planetary gearset  111  can be used to rotationally couple it to the output shaft  206  in a similar manner to that heretofore described. 
     Similarly, dog hubs provided on the output shaft  206  and located on either side of the output to the planetary gear train  213  can be used to rotationally couple it to the output shaft  206  in a like manner. 
     In the heretofore described transmission  10 , the gears able to be selectively coupled to the bridging shaft  14  include the primary gear  18   b  and the quaternary gear  22   b  (which is a compound gear). However, in other embodiments the compound quaternary gear  22   b  could instead be replaced by two separate gears that are respectively able to be rotationally fixed to the bridging shaft  14  in a manner appropriate to enable the required changes in gear ratio between the torque input and output  13 ,  15  to be implemented. Furthermore: i) the order of respective gears carried by the bridging shaft  14  (i.e. all gears carried thereby including those permanently fixed thereto); and ii) the respective gear ratios between those gears and gears carried by the input and output shafts  12 ,  16 ; are free to be chosen by those of ordinary skill in the art in order to enable whatever step changes in gear ratio between the input and output shafts  12 ,  16  are required to be implemented in use. 
     In general the heretofore described transmission  10  can be described as a transmission  10  including a plurality of shafts  12 ,  14 ,  16  each carrying gears  18   a ,  20   a ,  22   a ,  18   b ,  20   b   1 ,  20   b   2 ,  22   b ,  18   c ,  20   c ,  22   c  for transferring load between a torque input  13  and a torque output  15  of the transmission which is configured such that respective resultant gear ratios between the torque input and the torque output can be selected in use from each of a plurality of groups thereof (1 st -3 rd ; 4 th -6 th ; 7 th -9 th ) by changing a load path between the torque input and the torque output, wherein each said group of selectable resultant gear ratios has a load path feature for transferring load between a pair of the shafts which is common to the selectable resultant gear ratios within that group. The 1 st  to 3 rd  selectable gear ratio configurations define a first group of selectable gear ratios, wherein the interface between the quaternary gear  22   b  on the bridging shaft  14  and the third gear  22   c  on the output shaft  16  is the common load path feature for each of the 1 st  to 3 rd  selectable gear ratio configurations ( FIGS. 3 to 5 ). The 4 th  to 6 th  selectable gear ratio configurations define a second group of selectable gear ratios, wherein the interface between the secondary gear  20   b   1  on the bridging shaft  14  and the second gear  20   c  on the output shaft  16  is the common load path feature for each of the 4 th  to 6 th  selectable gear ratio configurations ( FIGS. 6 to 8 ). The 7 th  to 9 th  selectable gear ratio configurations define a third group of selectable gear ratios, wherein the interface between the primary gear  18   b  on the bridging shaft  14  and the first gear  18   c  on the output shaft  16  is the common load path feature for each of the 7 th  to 9 th  selectable gear ratio configurations ( FIGS. 9 to 11 ). 
     It will be appreciated that whilst various aspects and embodiments of the presently disclosed subject matter have heretofore been described, the scope of the presently disclosed subject matter is not limited to the embodiments set out herein and instead extends to encompass all arrangements, and modifications and alterations thereto, which fall within the spirit and scope of the presently disclosed subject matter.