Abstract:
A motorcycle having two drive chains. The gearbox output includes a left front sprocket and a right front sprocket. The rear wheel includes a left rear sprocket and a right rear sprocket. Left and right chains connect the respective front and rear sprockets. Power is transmitted to the rear wheel symmetrically. The swingarm is not subjected to asymmetric force that would twist it or pull it to one side, and can be made lighter. Swingarm flex can be better specified, making the swingarm a more useful suspension component, especially aiding the rear wheel while bent over in hard cornering over an irregular road surface.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   This invention relates generally to motorcycles, and more specifically to final drive chain solutions for them. 
   2. Background Art 
     FIG. 1  illustrates a conventional motorcycle  10  including a frame  12  holding an engine  14  which includes a gearbox  16 . The final output of the gearbox is a shaft  18  to which is coupled a front sprocket  20 . The front sprocket drives a chain  22 , which in turn drives a rear sprocket  24  which is coupled to a rear wheel  26 . The rear wheel is pivotably coupled to the frame or to the engine by a swingarm  28 . A shock/spring unit  30  supports the frame or engine on the swingarm. 
   Under acceleration, the engine is supplying tremendous torque through the front sprocket, and the top segment  22 T of the chain is under an extraordinary amount of strain, while the bottom segment  22 B of the chain is relatively slack. Because the chain is located to one side of the rear wheel, and thus closer to one side of the swingarm (the right side, as illustrated) than to the other side (Where the brake rotor is located), the pulling force of the top segment of the chain applies a very asymmetric load on the swingarm, tending to pull it rather strongly toward the chain side. 
   In order to avoid undesirable twisting and lateral flexing of the swingarm, the swingarm is made stronger, taller, thicker, and heavier. This increased mass reduces the acceleration of the motorcycle, reduces the ability of the rear wheel to track over bumps, and decreases the performance of the rear shock. Additionally, because the suspension components of the motorcycle are oriented in a substantially planar orientation with regard to the vertical, longitudinal plane of the motorcycle (i.e. the plane of the paper in  FIG. 1 ), and because motorcycles lean significantly over on their sides when cornering, during cornering the suspension components are not in an orientation conducive to tracking vertical bumps and irregularities in the road surface. A significant fraction of the motorcycle rear end&#39;s vertical tracking in corners should, ideally, be provided by flex in the swingarm itself. Unfortunately, this is directly at odds with the need to stiffen the swingarm enough to avoid chain-induced flex and twist. 
   What is needed, then, is an improved motorcycle which has a rear end in which the swingarm can be more readily utilized and tuned as a meaningful lateral suspension component. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention Which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
       FIG. 1  shows, in side view, a conventional motorcycle having a single chain on one side of its rear wheel. 
       FIG. 2  shows, in rear view, a motorcycle having dual rear chains, one on each side of the rear wheel. 
       FIG. 3  shows, in rear view, a portion of such a motorcycle, omitting the frame, seat, tail, tank, and front end, for visibility&#39;s sake. 
       FIG. 4  shows, in rear view with a partial cutaway, a motorcycle rear end according to one embodiment of this invention. 
       FIGS. 5 and 6  show one embodiment of a rear wheel such as may be utilized in the motorcycle rear end. 
       FIGS. 7 and 8  show the dual sprockets in in-phase and out-of-phase rotational alignment, respectively. 
   

   DETAILED DESCRIPTION 
     FIG. 2  illustrates one embodiment of a motorcycle  50  according to this invention. The motorcycle includes a right side chain  52  on a right side of the rear wheel assembly  54 , and a left side chain  56  on a left side of the rear wheel assembly. The swingarm  58  is adapted to accommodate the dual chains. 
     FIG. 3  illustrates a subset of the motorcycle  50  including an engine and gearbox assembly  60  which is adapted (not shown) to simultaneously drive both chains  52 ,  56 . In one embodiment, the gearbox includes a single output shaft which extends beyond both sides of the gearbox, for coupling to the left and right front sprockets. In another embodiment, the gearbox includes separate left and right output shafts. 
     FIG. 4  illustrates in closer detail the rear end assembly  70  which may be utilized in such a motorcycle. The rear end includes a swingarm  58  which accommodates dual chains, such as by including holes  72  on either side through which the upper segment of the chain (not shown) passes in its course from the dual front sprockets (not shown) to the dual rear sprockets  74 ,  76 . The rear sprockets are coupled to a hub  78  of the rear wheel  80 . Optionally, one or both sides of the hub may also be equipped with brake rotors  82 ,  84 . Optionally, a brake rotor may instead, or also, be coupled to the rim  86  of the wheel. Brake calipers are not shown, as their mounting will be well understood by the skilled designer in light of the teachings of this disclosure. The swingarm includes one or more mounts  88  for the rear shock, and mounts (not visible) for rotatably coupling to the frame or engine. The rear wheel is coupled to the swingarm by an axle  90 . 
   In one embodiment, the teeth of the left and right rear sprockets may be aligned at same angular positions, as shown. In another embodiment, the teeth of the left and right rear sprockets may be differently aligned, such as if they are one half tooth offset from each other, such that they engage their respective chain rollers at different times; this may in some instances reduce vibration or harmonic effects transmitted to and through the swingarm. In some embodiments, the left and right chains may be identical. In other embodiments, the chains may, for example, be of different pitch, such as a 520 chain and a 428 chain; the mechanic may choose this setup if, for example, two 428 chains would not together be strong enough, but two 520 chains would together be more than sufficiently strong and too heavy. 
   Similarly, it is not necessarily the case that both sprockets be identical. In some embodiments, the mechanic may choose to set up the motorcycle with a 16:48 sprocket combination on the left side, and a 15:45 sprocket combination on the right side (with both sides having a 1:3 sprocket ratio). 
   Although the rear end is illustrated with the sprockets being in mirror-image identical locations with respect to the central plane of the rear wheel, in other embodiments they may be at different locations. In some embodiments, it may be desirable or acceptable to use one or both rear sprockets as brake rotors, by mounting brake calipers over them and their chains, as long as the brake system can handle the chain lubrication which would tend to contaminate the pads, calipers, and rotors/sprockets. 
     FIGS. 5 and 6  illustrate one embodiment of a rear wheel  80  according to this invention, in a perspective view and a radial view, shown with a portion of the rim  86  cut away for better visibility of the more pertinent components. The wheel includes a hub  78  connected to the rim, such as by spokes  92 . The hub includes mounts  94  for left and right sprockets (not shown), including a left sprocket mount  94 L and a right sprocket mount  94 R. In one embodiment, the sprocket mounts simply comprise the machined ends of the hub, including threaded holes for accepting sprocket mounting bolts (not shown) or, alternatively, protruding captive sprocket mounting bolts (not shown). The hub end faces may optionally also have a contoured face formed to mate with the sprockets in a positive engagement, such as with the five-sided protrusion shown. 
   The hub may optionally include brake rotor mounts  96 , including a left rotor mount  96 L and/or a right rotor mount  96 R. In one embodiment, the rotor mounts comprise extrusions extending radially outward from the central body of the hub, as shown. In one embodiment, the rotor mounting surfaces are offset inward from the sprocket mounting surfaces. The rotor mounts may include threaded holes for accepting rotor mounting bolts (not shown), or other suitable coupling means. 
     FIG. 7  illustrates the dual rear sprockets in an in-phase alignment. The teeth  74 T of the left rear sprocket  74  and the teeth  76 T of the right rear sprocket  76  are aligned in a same rotational position, such that the left and right teeth will engage the rollers of their respective chains (not shown) at the same time. 
     FIG. 8  illustrates the dual rear sprockets in an out-of-phase alignment. The sprockets are in different rotational alignments, such that the left rear sprocket&#39;s teeth  74 T and the right rear sprocket&#39;s teeth  76 T will engage the rollers of their respective chains at different times. The sprockets are shown one half tooth out of phase. The front sprockets (not shown) will generally need to be similarly out of phase. 
   CONCLUSION 
   While the invention has been illustrated with reference to chains, it could equally well be applied to belt-driven motorcycles, or to a motorcycle having a belt drive on one side and a chain drive on the other side. Belts are generally referred to as running on pulleys (either toothed or smooth), while chains are generally referred to as running on sprockets. Pulleys and sprockets are generically “wheels”; however, to avoid confusion with the word “wheel” being a specific reference to the wheel upon which the tire is mounted, I will use the term “sprocket” to generically refer to either sprockets or pulleys. I will use the term “drive sprocket” to refer to the sprocket which is powered by the gearbox (the “front sprocket” in common usage), and the term “driven sprocket” to refer to the sprocket which is coupled to the rear wheel (the “rear sprocket” in common usage). I will use the term “flexible band” to generically refer to either a chain or a belt which connects a driven sprocket to a drive sprocket. 
   When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated. 
   The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown. 
   Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.