Abstract:
A front fork for a motorcycle in which an outer tube ( 1 ), an inner tube ( 2 ) which is inserted to slide freely in said outer tube ( 1 ), a damper cylinder ( 3 ) disposed on the bottom of said inner tube ( 2 ), a piston ( 4 ) accommodated in said damper cylinder ( 3 ), a piston rod ( 5 ) connected to said piston ( 4 ) and said outer tube ( 1 ), a spring ( 7 ) pushing said outer tube ( 1 ) and said piston rod ( 5 ) in an extending direction, a tubular guide ( 12 ) disposed on an upper end of said damper cylinder ( 3 ); and a free piston ( 15 ) disposed to displace freely in a vertical direction along said guide ( 12 ), said free piston ( 15 ) separating an upper reservoir ( 14 ) and a lower oil chamber ( 13 ) in said inner tube ( 2 ) and supporting an end of said spring ( 7 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a front fork for a motorcycle which functions as a fork and a shock absorber that absorbs shocks such as vibrations from a road surface. 
     BACKGROUND OF THE INVENTION 
     Jikkai-Hei 56-76588 published by the Japanese Patent Office in 1981 describes a front fork of a motorcycle in which an air pressure chamber and a hydraulic chamber are separated by a free piston. 
     The front fork comprises an inner tube inserted into an outer tube that is open at its upper end. A hollow rod that is open at its upper end is provided at the center of the lower end of the outer tube. A free piston is disposed above the hollow rod and slides on an inner periphery of the inner tube. The free piston is pushed downwardly by a spring and separates a reservoir in the top of the inner tube and a hydraulic chamber in the hollow rod. The reservoir is linked through a check valve provided on the free piston with the hydraulic chamber in the hollow rod. 
     When the front fork is compressed, an increase of the volume of entry of the inner tube causes the free piston to rise against the spring. When the front fork is extended, the check valve is opened and a volume of oil equal to the retraction of the inner tube is introduced into the hollow rod from the reservoir. 
     In this conventional front fork, the free piston operates vertically during extension and compression. In particular, during compression, the inner tube slides downward and at the same time the free piston rises while sliding on the inner tube. As a result, the amount of stroke of the free piston is the sum of the stroke of the inner tube and the stroke of the free piston. Thus the degree of stroke of the seal provided on the outer periphery of the free piston is large which has the undesirable consequence that the life span of the seal is reduced as a result. 
     As reserve oil from the outer periphery of the seal or the gap of the check valve seeps to the lower side of the free piston, the amount of oil on the lower section of the free piston increases and the position of the free piston displaces upwardly. Thus the possibility exists that the initial load on the spring will increase. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to increase the life span of the seal provided on the free piston. 
     It is a further object of the present invention to suppress undesirable increases in the initial load of the spring. 
     In order to achieve above the objects the present invention provides a front fork for a motorcycle. 
     The front fork for a motorcycle comprising an outer tube, an inner tube which is inserted to slide freely in the outer tube, a damper cylinder disposed on the bottom of the inner tube, a piston accommodated in the damper cylinder, a piston rod connected to the piston and the outer tube, a spring pushing the outer tube and the piston rod in an extending direction, a tubular guide disposed on an upper end of a damper cylinder and a free piston disposed to displace freely in a vertical direction along the guide, the piston separating an upper reservoir and a lower oil chamber in the inner tube and supporting an end of the spring. 
    
    
     The details as well as other features and advantages of the invention are set forth in the remainder of specification and are shown in accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged longitudinal sectional view of an essential part of a front fork according to a first embodiment of the present invention. 
     FIG. 2 is an longitudinal sectional view of the entire front fork. 
     FIG. 3 is similar to FIG. 1, but showing a second embodiment of the present invention. 
     FIG. 4 is similar to FIG. 1, but showing a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2 of the drawings, a front fork has an inner tube  2  which is connected to a wheel, and which is inserted into an outer tube  1  which is connected to a body. A damper cylinder  3  is disposed on the bottom of the inner tube  2 . A piston rod  5  is inserted through a piston  4  into the damper cylinder  3 . The upper end of the piston rod  5  is connected to an upper end of outer tube  1  through a cap  6 . The piston rod  5  and the outer tube  1  push each other through a spring  7  provided in the outer tube  1  in an extending direction. The front fork is of an inverted type having the structure outlined above. 
     The upper end of the piston rod  1  is attached to the cap  6  and is secured to be tightened by a lock nut. 
     A cushioning rubber member  9  and a spring seat  10  are provided on the lower side of the cap  6 . The cap  6  through the spring seat  10  supports the upper end of the spring  7 . 
     A spring guide  11  is disposed on the upper outer peripheral section of the piston rod  5 . The downward displacement of the spring guide  11  is prevented by a stopper  8  such as, i.e., a snap ring provided on the piston rod  5 . 
     The inner periphery of the spring  7  is guided by the outer periphery of the spring guide  11  that prevents the generation of a buckling. 
     Referring to FIG. 1, a tubular guide  12  is provided on the upper section of the damper cylinder  3  and a free piston  15  is free to displace vertically between the guide  12  and the inner tube  2 . The free piston  15  separates the oil chamber  13  and the reservoir  14 . 
     The guide  12  may be directly connected to the upper end of the damper cylinder  3 , as shown in FIG.  1 . The guide  12  may be screwed or welded to the upper section of a tubular adapter  16  which is screwed to the upper end of the damper cylinder  3 . 
     The guide  12  comprises a hollow mount  17  connected to an upper end of the damper cylinder  3  through the adapter  16 , a step support  18  for the free piston is formed on to the mount  17 , and an inner hollow tubular body  19  is formed at a lower part to the support  18  and is disposed along the piston rod  5 . 
     The free piston  15  comprises an annular main body  15   a , a tubular section  15   b  disposed from the main body  15   a , a seal  20  provided on the outer periphery of the main body  15   a  and a seal  21  provided on the inner periphery of the tubular section  15   b.    
     The inner periphery of the free piston  15  contacts the outer periphery of the inner tubular body  19  via the seal  21  and the outer periphery of the free piston  15  contacts the inner periphery of the inner tube  2  via the seal  20 . 
     The main body  15   a  of the free piston  15  has a limited downward stroke as the free piston  15  abuts with the support  18  provided with a step  18   a.    
     An annular or slit-shaped gap  23  which communicates with the oil chamber  22  of the damper cylinder  3  and the reservoir  14  is formed between the inner tubular body  19  forming the guide  12  and the piston rod  5 . A horizontal through hole  24  that communicates with the gap  23  and the reservoir  14  is formed on the upper section of the inner tubular body  19 . 
     A check seal  25  is provided on the inner side of the support  18 . The check seal  25  prevents oil leaks from the oil chamber  22  to the reservoir  14  and allows oil flow to the oil chamber  22  through the gap  23  from the reservoir  14  when a negative pressure exists in the oil chamber  22 . 
     When the front fork operates, the piston  4 , the piston rod  5  and the outer tube  1  rise during extension of the front fork and oil from the upper oil chamber  22  in the damper cylinder  3  flows into the lower oil chamber  27  through the damping valve  28  and the port  26  of the piston  4 . Oil equal to the volume of retraction of the piston rod  5  is introduced into the lower oil chamber  27  from the oil chamber  13  and the damping valve  28  generates a damping force. At this time, the free piston  15  displaces downward due to a force of the spring  7 . 
     The piston  4  is depressed during compression of the front fork and oil from the lower oil chamber  27  is introduced into the upper oil chamber  22  through the check valve  29  and the port provided on the piston  4 . Oil equal to the entry of the piston rod  5  is introduced into the oil chamber  13  from the lower oil chamber  27 . Thus the free piston  15  is raised against the force of the spring  7  by the volume of introduced oil. 
     The free piston  15  displaces upwardly along the outer periphery of the inner tubular body  19  through the seal  21  and the inner periphery of the inner tube  2  through the seal  20 . The degree of slide experienced by the seals  20 ,  21  at this time results only from the stroke of the free piston  15  which displaces due to the oil introduced into the oil chamber  13  from the lower oil chamber  27 . As the degree of slide is low, it has little effect on the overall operation of the front fork and thus the life span of the seals  20 ,  21  is not adversely affected. 
     The check seal  25  allows oil flow to the oil chamber  22  from the reservoir  14  when a negative pressure exists in the oil chamber  22  by rapid extension and compression operations. When a greater amount of oil than necessary has accumulated in the oil chamber  13  under the free piston  15  due to oil leaks from the seals  20 ,  21 , the free piston  15  is raised accordingly. As a result, an inner gap  30  of the tubular body  15   b  opens the through hole  24  and oil with the added pressure of the oil chamber  13  relieves into the reservoir  14  through the through hole  24 . In this way, it is possible to prevent variations in the initial load of the spring  7  since the pressure acting on the free piston  15  is released. 
     During maximum compression, the free piston  15  compresses the cushioning rubber member  9  with the upper end of the spring guide  11 . A load from the cushioning rubber member  9  is applied to the free piston  15  and the oil chambers  22 ,  27  in the damper cylinder  3  and the oil chamber  13  act as a shock absorbing oil chamber and create a soft shock absorbing action during maximum compression with the assisting absorbing force of the cushioning rubber member itself. 
     Since the inner sides of the free piston slide along the guide  12  and the guide  12  is fixed, thus the seals  20  of the free piston  15  which are fitted to the guide  12  and are not fitted to the piston rod  5  slide only through a stroke of the free piston  15 . Since the amount of sliding is small, the life span of the seals is improved. 
     A through hole  24  is provided in the guide  12  and the inner gap  30  is provided in the free piston  15 . Thus oil which has accumulated in the oil chamber  13  can be relieved to the reservoir  14  through the inner gap  30  and the through hole  24 . Therefore it is possible to prevent variations to the initial load of the spring  7  by reducing the pressure applied to the spring  7  through the free piston  15 . 
     As the reservoir  14  and the oil chamber  13  are separated by the free piston  15 , air from the reservoir  14  can be prevented from leaking into the oil chamber  13  and damping force can therefore be stabilized. Furthermore since the free piston  15  is pressed by the spring  7  and the oil chamber  13  and the oil chambers  22 ,  27  in the damper cylinder  3  are pressurized by the free piston  15 , a response time of damping force is improved and shocks proximate to maximum compression are absorbed to the degree to which the oil chambers  22 ,  27  are pressurized. 
     Since the check valve  25  is provided which allows unidirectional flow with the reservoir  14  and the damper cylinder  3 , when a slight negative pressure exists in the damper cylinder, oil may be drawn into the cylinder  3  to stabilize damping force. 
     Next referring to FIG. 3, a second embodiment of the invention will be described. However since all components other than the groove of the guide are the same as the above-described embodiment, similar components are designated by the same numerals as FIG.  1 . 
     The guide  12  of the second embodiment comprises the hollow mount  17  connected to an upper end of a damper cylinder  3 , a support  18  of the free piston formed at the mount  17 , an inner tubular body  19  formed at the top of the support  18  and disposed along the piston rod  5 , and the outer tubular body  19   a  disposed in contact with the inner tube  2 . 
     The free piston  15  slides freely between the inner periphery of the outer tubular body  19   a  and the outer periphery of the inner tubular body  19  through the seal  20 ,  21 . Since it is possible for the free piston  15  to slide without reference to the inner tube  2 , the seals  20  of the free piston  15  which are not fitted to the inner tube  2  slide only through a stroke of the free piston  15 . Since the amount of sliding is small, the life span of the seal  20  is improved. Other operations are the same as those shown in FIG.  1 . 
     Next referring to FIG. 4, a third embodiment of the present invention will be described. The guide  12  of the front fork according to this embodiment comprises the hollow mount  17  connected to an upper end of the damper cylinder  3 . The support  18  for the free piston is formed at the mount  17 . The outer tubular body  19   b  is disposed in contact with the inner tube  2 . 
     The inner periphery of the free piston  15  slides freely on the outer periphery of the piston rod  5  and the outer periphery of the free piston slides on the inner periphery of the outer tubular body  19   b . Furthermore a check seal  25  is provided which allows oil flow to the upper oil chamber  22  in the damper cylinder  3  from the reservoir  14  in the mount  17  in the guide  12 . 
     In the present embodiment, excess oil accumulated below the free piston  15  escapes to the reservoir  14  when the outer periphery of the seal  20 ,  21  rise to a position which separates from the top end of the inner tubular body  19   b . The operation of the relief mechanism is the same as those shown in FIG.  1  and FIG.  3 . Other operations and effects are the same as those shown in FIG.  1 . 
     The entire contents of Japanese Patent Application P10-199925 (filed Jul. 15, 1998) are incorporated herein by reference. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of embodiments described above will occur to those skilled in the art, in light of the above teaching. 
     The scope of the invention is defined with reference to the following claims.