Abstract:
A suspension unit used in a pull type shock application is disclosed, whereby a main shaft passes completely through a damping fluid so that the shock absorber fluid is not compressed when the shock shaft is displaced. The shaft also acts on a compression spring by an additional piston on the shaft. A cylindrical outer housing provides two distinct air chambers. The pressurization of these air chambers alters the spring rate, preload of the suspension, and vehicle ride height.

Description:
FIELD OF THE INVENTION 
   This invention relates to motorcycles suspensions, and particularly to the suspension unit that includes the rear shock absorber, spring, damper and means for controlling vehicle ride height. 
   DISCUSSION OF RELATED ART 
   Typically the rear wheel of a motorcycle is held by a swingarm. The swingarm is held in the chassis by pivot bearings, allowing free movement of the swingarm around a pivot point on the motorcycle frame. The swingarm is then attached to a suspension unit, typically an arrangement of springs and dampeners. The wheel is free to follow road surface undulations as the vehicle traverses the road. The relative motion of the wheel and swingarm relative to the vehicle is controlled by the suspension unit. 
   A common method of mounting the suspension unit on custom motorcycles is to affix one end of the suspension unit to the lower portion of the swingarm and the other to the motorcycle frame near the engine. This is referred to as a “pull shock” type of mounting and results in a generally horizontal orientation of the suspension unit, allowing the suspension unit to be mounted out of the way underneath the transmission. As the rear wheel is forced up from the road, the swingarm extends the suspension unit, and as the wheel extends down towards the road the suspension unit compresses. 
   The pull shock mounting system is contrary to that of most other motor vehicles that vertically mount shock absorbers near the underside of the seat. Problems have arisen because most of the suspension units used in these under-pivot vehicles are based on these more-typical suspension units. 
   For example, standard automotive suspension units have a damper shaft inserted into a single fluid chamber. As a shock absorber of such a suspension unit is compressed, the shaft must displace the oil in the shock absorber. Typically a gas bag or some other means is provided to compensate for the change in volume of the oil chamber as the shock absorber shaft fills this volume. This means that as the dampener shaft compresses the fluid, the resistance experienced by the dampener shaft primarily results from the compressible gas bag, and not the dampening of the fluid. Conversely as the shock absorber extends the dampener compresses the fluid directly and so the resistance experienced by the dampener is relatively quite strong. 
   When such a shock absorber is installed in a pull shock type of mounting arrangement, and as the wheel compresses the shock absorber, the force from the dampener is quite large and as the wheel rebounds it is relatively light. As such, the motion of the wheel is opposite to what is expected in a standard automotive shock absorber, due to the inverse nature of the pull shock type of mounting. Therefore, using a standard shock absorber in a pull shock type of mounting results in the motorcycle feeling harsh over bumps and wallowing on normal road. 
   A further drawback of typical shock absorbers in a pull shock type of mounting includes the inconvenience and cost of adding separate gas chamber. Such gas chambers are also prone to deflating, causing the performance of such shock absorbers to fade over time. 
   Another difficulty is the ability to alter the spring dampening strength with compressed air. In the past this has required the addition of special air bags or other air chambers. The concept of using a through shaft damper is not unique, as it is commonly used on steering dampeners. Others have used a shaft encased in a hollow cylinder to pull on a spring via a piston attached to a shaft. This spring arrangement has not been combined with a through shaft dampener, however. 
   Therefore, there is a need for a suspension unit that does not require a gas chamber or a gas bag to compensate for compression of the fluid due to shaft motion. Such a needed device would allow for easy adjustment of the vehicle ride height and provide consistent damping in either direction through use of an adjusting spring, and would not require separate pressurization means. Such an improved suspension would be easy to manufacture, and would provide for simplified use. The present invention accomplishes these and other objectives and advantages. 
   SUMMARY OF THE INVENTION 
   The present invention is a device for controlling the motion of the rear wheel of a motorcycle relative to the motorcycle frame. The suspension unit has a shaft that runs completely through a fluid chamber, a piston attached to the shaft to create damping in the fluid chamber, and a spring piston to control both a mechanical spring and to compress air on both sides of the spring piston. The fluid chamber piston includes a valve means that allows the fluid in the fluid chamber to flow from one side of the fluid chamber piston to the other in a controlled manner, whereby the maximum flow rate of the valve means controls the amount of dampening of the suspension unit. The suspension unit also provides an adequate structural for mounting the device to a motorcycle swingarm or other attachment point. 
   Although the use of air cylinders is well known, combining an air cylinder with a through shaft dampener and an external spring is unique. Given the significant drawback associated with the prior art it will become apparent that this new invention offers many significant advantages. The present device does not require a gas chamber or a gas bag to compensate for compression of the fluid due to shaft motion. Further, the present device allows for adjustment of the vehicle ride height and provides consistent damping in either direction through use of an adjusting spring, and does not require separate pressurization means. Such an improved suspension is relatively easy and inexpensive to manufacture, and provide for simplified use. Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view of the invention, illustrating the invention as mounted to a swingarm of a motorcycle; and 
       FIG. 2  is a cross-sectional view of the invention, taken generally along lines  2 — 2  of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention embodies a number of features and innovations that make it useful as a suspension unit.  FIG. 1  illustrates how the suspension unit is mounted in a motorcycle  2 . The suspension unit  1  is attached to the swingarm  5  below a swingarm pivot  15  at a swingarm mount  4 . As such, compression of a rear wheel  20  causes an extension of the suspension unit  1 , and extension of the wheel away from the motorcycle  2  causes a compression of the suspension unit  1 . 
     FIG. 2  shows the internal construction of the suspension unit  1 , which comprises a shaft  25  having a connection fitting  30  at one end, which is a means for attaching the suspension unit  1  to a suspension mount  3  of the motorcycle  2 . The suspension mount  3  may be any number of methods known to those skilled in the art. A clevis is typically used, but a stud post or other means may work just as well. It may also be noted that the suspension unit  1  works equally well with the connection fitting  30  attached to either the swingarm mount  4  or the main motorcycle mount  3 . 
   At the other end of the shaft  25  is a spring piston  35 . Typically there is a spring piston seal  36  on the spring piston  35  to restrict the transfer of fluid or gas past the spring piston  35 . 
   Centrally located along and around the shaft is the damping piston  40 , which may include a damping piston seal or bushing  65 . The damping piston  40  is surrounded by an inner damping tube  45  which encases a non-compressible fluid (not shown) surrounding the damping piston  40 . 
   The shaft  25  passes completely through the inner damping tube  45 , and as such there is no displaced fluid as the shaft  25  strokes through the damping fluid. The damping fluid is contained inside the inner damping tube  45  by an inner shaft seal  50  at one end and an outer shaft seal  55  at the other end. The damping piston  40  typically includes one or more valve elements  60  for controlling the resistance of the damping piston  40  as it moves thru the damping fluid. 
   The damping piston  40  creates two separate fluid chambers in the inner tube  45 . A rebound damping chamber  70  is formed between the inner shaft seal  50  and the damping piston  40 , and a compression damping chamber  75  is formed between the damping piston  40  and the outer shaft seal  55  ( FIG. 2 ). 
   An outer body  80  encases the spring piston  35  and includes an outer tube cap  85  at an end opposite to the shaft connector  30 . An outer body connector  90  provides a means for connecting the outer body  80  to the suspension mounting point  4 . The outer body connector  90  may be integrally formed with the outer body  80  or the outer tube cap  85 . The means for mounting the outer body connector  90  to the suspension mounting point  4  may be any number of methods known in the prior art. A clevis is typically used, but a stud post or other joint may work just as well. 
   One advantage of the present invention is that it makes no difference whether the outer body connector  90  is attached to the suspension mounting point  4  and the shaft connector  30  is attached to the main motorcycle mount  3 , or if the outer body connector  90  is attached to the main motorcycle mount  3  and the shaft connector  80  is attached to the suspension mounting point  4 . The outer body  80  includes a spring support  95  opposite the outer tube cap  85 , the spring support  95  for supporting a main spring  100  that lies between the spring piston  35  and the spring support  95 . The main spring  100  may be of a coil type or other flexible material, such as an electrometric cylinder. 
   A top out spring  105  may be fitted between the spring piston  35  and the outer tube cap  85 . The top out spring  105  may be of a coil type or other flexible material, such as an elastomeric cylinder. A bottom out spring  110  may also be included to augment the main spring  100 . The bottom out spring  110  is positioned between the spring piston  35  and the spring support  95 . The load path between the bottom out spring  110  and the spring support  95  may pass thru the inner damping tube  45 . The bottom out spring  110  begins to experience compression only when the shaft  25  is mostly extended from the outer body  80 . The bottom out spring  110  may be of a coil type or other flexible material, such as an electrometric cylinder. 
   A main chamber air inlet  115  is provided to allow the introduction of pressurized gas into a main chamber  117 . The main chamber  117  is formed between the spring piston  35  and the spring support  95  and related structures. A top out chamber air inlet  120  is provided to allow the introduction of pressurized gas into a top out chamber  125 , which is formed between the spring piston  35  and the outer tube cap  85 . The spring piston seal  36  helps to maintain a pressure differential between the main chamber  117  and the top out chamber  125 . 
   There are four modes of operation of the present invention. The first mode is when the wheel  20  hits a bump or other road surface (not shown) which causes the wheel  20  to compress towards the seat of the motorcycle  2 . Because the suspension unit  1  is mounted below the pivot point  15 , such movement of the wheel  20  causes the shaft  25  of the suspension unit  1  to extend out from the outer body  80 . When the suspension unit  1  extends the main spring  100  is compressed, and the bottom out spring  110  may also be compressed, depending upon the severity of the impact to the wheel  20 . The gas (or fluid) in the main chamber  117  is also compressed. The fluid in the compression-damping chamber  75  is forced past the damping piston  65  through the valve element  60  and into the rebound-damping chamber  70 . The valve element  60  may be adjusted to provide the proper damping action. 
   The next mode of operation is the rebound mode. Typically, mode one compression damping is very light compared to the rebound mode. The rebound mode is active when the wheel  20  encounters a dip in the road and the wheel  20  falls away from the motorcycle  2 . In this instance the suspension unit  1  is compressed, which allows the main spring  100  and the top out spring  110  to extend. The volume of the main chamber  117  is also increased, thus causing the pressure of the gas in the main chamber  117  to drop. This in turn reduces the force of the gas in the main chamber  117  against the spring piston  35 . Conversely the gas in the top out chamber  125  is simultaneously compressed, causing the force within the top out chamber  125  to increase. The top out spring  105  may also be compressed in the event the compression force of the shaft  25  into the outer body  80  reaches a predetermined level. The combination of these forces prevents a sudden and undesirable “top out” of the spring piston  35  forcefully contacting the outer tube cap  85 . 
   The third mode of operation occurs when compressed gas is introduced into the main chamber air inlet  115 . This pressure exerts a force against the spring piston  35 , causing the suspension unit  1  to contract. Because this is a pull type shock application, the contraction of the shock causes a seat (not shown) of the motorcycle  2  to rise with respect to the rear wheel  20 . As such, the ride height of the motorcycle  2  may be controlled by the introduction of pressure to the main chamber  117 . 
   The forth and final mode of operation occurs when compressed gas is introduced into the top out chamber air inlet  120 . This pressure exerts a force on the spring piston  35  that causing the suspension unit  1  to expand, whereby the contraction of the shock causes the seat of the motorcycle to lower with respect to the rear wheel  20 . In this way the ride height of the motorcycle may be controlled by the introduction of gas into the top out chamber  125 . 
   The effect of simultaneously increasing the pressure in both the top out chamber  125  and main chamber  117  is to increase the net spring force of the suspension unit. Thus by controlling the pressure in the main and top out chambers  117 , 125  independently one can adjust both the ride height and spring rate of the motorcycle. 
   While a particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.