Abstract:
A steering damper device attenuating the oscillations of the handlebar of a motorcycle during operation, and more particularly, a device capable of varying a damping force variable. The steering damping device prevents the adverse effect of oscillations of the handlebar that would otherwise occur when the load on the front wheel of the motorcycle becomes light. The device includes a rotary steering damper provided in a coaxial manner on a steering shaft. Either a damping force or zero damping force is produced by a variable valve arranged in the bypass passage communicating with a right liquid chamber and a left liquid chamber. The variable valve is controlled by a control device and generates a damping force in the steering damper only when the acceleration of the motorcycle as detected by the acceleration sensor exceeds a threshold value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-108981, filed on Apr. 6, 2001, the entire contents thereof are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a steering damper device for a motorcycle for attenuating oscillations of its handlebar during running operation, and more particularly a device capable of making a damping force variable. 
     2. Description of Background Art 
     In order to prevent any oscillation of a handlebar caused by a kickback or the like under external disturbance, it is well known in the art to provide a steering damper device generating a damping force against oscillation (as one example, refer to Japanese Patent No. 2,593,461). In addition, it is also well known in the art to provide a device generating a damping force only when it is required and making a damping force variable in other cases so as not to generate any surplus damping force. Examples include a device for controlling it in response to a steering angle and a running speed (refer to Japanese Patent Laid-Open No. Sho 63-64888), and a device for controlling it in response to a variation in a load of the front wheel (refer to Japanese Patent Publication No. Hei 7-74023). 
     As disclosed in Japanese Patent Publication No. Hei 7-74023, when a front wheel load is decreased, a kickback phenomenon may easily be produced, and either an inner pressure at a front fork or its stroke in this case detects the decreased front wheel load. However, usually a pressure in the front fork is changed in response to its inner temperature, making it difficult to perform an accurate sensing of the front wheel load. In addition, when the stroke of the front wheel is detected, the structure of detecting the stroke becomes complicated and the cost required for the entire vehicle body is increased. In view of this fact, it is desired to detect the front wheel load by a more accurate and simple detecting structure. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The present invention addresses the problems described above. In order to solve the above problems, the steering damper device of the present invention produces a damping force applied to a turning operation of a front wheel steering system supported at a vehicle body front segment. The value of the damping force is made variable, wherein an acceleration sensing means for sensing an acceleration of the vehicle body is provided, and only when the acceleration exceeds a predetermined threshold value, is the damping force generated at the steering damper. 
     This makes it possible to generate a damping force at the steering damper only when both a speed of the vehicle body and an acceleration of the vehicle body exceed the predetermined threshold values. In addition, it is also possible to cause the damping force of the steering damper to be changed in response to a speed and/or an acceleration of the vehicle body. 
     Further, it is also possible to detect a speed of the vehicle body in response to a gear position and the frequency of rotation of the engine, and to detect an acceleration of the vehicle body in response to an opening speed of the throttle. 
     In accordance with the present invention, when an acceleration of the vehicle body exceeds a predetermined threshold value, the load on the front wheel is decreased. When this occurs, a kickback may occur. However, with the present invention, it is possible to generate a damping force at the steering damper and to restrict the kickback. Further, with the present invention, since an acceleration of the vehicle body is not influenced by a temperature, detecting the acceleration is not as difficult as is the case with prior art devices. Thus, since a complex structure for use in detecting the acceleration is not required, the present invention provides for a more accurate and simple damping force control. 
     If in addition to the acceleration, the vehicle body speed is also applied as a controlling condition, a damping force is generated only when the vehicle body speed exceeds a predetermined threshold value. Thus, the controlling operation preventing against kickback becomes more accurate. This state is attained based on the fact that a relative large acceleration is apt to be generated at a low vehicle speed, and a relative low acceleration is apt to be generated at a high vehicle speed. Thus, by also considering the vehicle body speed as well as the acceleration, the present invention provides a means for correcting the damping force generated only by the acceleration. 
     Further, if the damping force is changed in response to either a vehicle body speed or a vehicle acceleration, or both of them, it is possible to generate an appropriate damping force of the steering damper responding to an operating state. 
     In addition, when a vehicle body speed is detected in response to both a gear position and the engine rotation frequency and an acceleration is detected in response to a throttle opening speed, it is not necessary to use an acceleration sensor or a vehicle body speed sensor exclusively. When both the vehicle body speed and the vehicle acceleration are considered, an appropriate damping force of the steering damper can be more accurately generated. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a perspective view for showing a motorcycle to which the preferred embodiment is applied; 
     FIG. 2 is a side elevation view for showing a vehicle body front segment to indicate a part of the steering damper device; 
     FIG. 3 is a top plan view for showing the part of the steering damper device; and 
     FIG. 4 is a view for showing a schematic structure of the steering damper. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, one preferred embodiment of the present invention will be described as follows. FIG. 1 is a perspective view for showing a motorcycle to which the preferred embodiment of the present invention is applied. FIG. 2 is a side elevation view for showing a vehicle body front segment structure having a steering damper arranged. FIG. 3 is a top plan view for showing the vehicle body front segment structure. FIG. 4 is a view for showing a schematic structure of a steering damper. 
     In FIG. 1, the upper part of front forks  2  supporting a front wheel  1  at their lower ends is connected to the front part of a vehicle body frame  3  and the upper part can be turned by a handlebar  4 . A fuel tank  5  is supported on the vehicle body frame  3 . Reference numeral  6  denotes a seat, reference numeral  7  denotes a rear cowl, reference numeral  8  denotes a rear swing arm and reference numeral  9  denotes a rear wheel. 
     The steering damper will be described next. As shown in FIGS. 2 and 3, a steering damper  10  is arranged between a top bridge  11  having the handlebar  4  fixed thereto and the front end of the vehicle body frame  3 . The top bridge  11  is a member integrally assembled with a steering shaft  14  (indicated by a center line) supported at a head pipe  13  by a lower bottom bridge  12  being held therebetween. The top bridge  11 , bottom bridge  12  and steering shaft  14  are integrally turned. 
     Each of the upper segments of a pair of right and left front forks  2  is supported at the top bridge  11  and the bottom bridge  12 , respectively. The head pipe  13  is a pipe-like segment integrally assembled with the front ends of the vehicle body frames  3 . The vehicle body frames  3  in a pair of right and left segments extend from the head pipe  13  in a rearward direction (FIG.  3 ). A steering lock  15  is arranged in front of the head pipe  13  and its lock is unlocked with a key  16 . 
     The steering damper  10  of the preferred embodiment is a hydraulic damper for preventing the kickback. It is comprised of a main body  17  and a lid  18 . The steering damper is fastened with a bolt  20  to a nut segment arranged at a hub  21  on the top bridge  11 . The main body  17  and the lid  18  are also integrally assembled with the bolt  20 . Numeral  22  denotes a nut for connecting the top bridge  11  with the upper end of the steering shaft  14 . 
     A damper shaft  23  is arranged within the steering damper  10  with its axis being directed in an upward or downward direction as shown in FIG.  2 . The lower end of the damper shaft  23  extends out of the main body  17  in a downward direction and integrally formed with the front end of the arm  24 . The damper shaft  23  is coaxially arranged at the steering shaft  14 . 
     The arm  24  is bent in a crank-like shape as viewed in a side elevation view and extends in a forward or rearward direction at the center of the vehicle body as viewed in a top plan view. The front end of the arm extends upwardly toward the upper part of the nut  22  and integrally assembled with the damper shaft  23  projecting into the steering damper  10 . The rear end of the arm  24  forms forked ends  25  and a hub  26  of the vehicle body frame  3  is fitted to the forked ends. 
     The hub  26  is arranged at the central part of a bracket  27  to project upwardly. The bracket  27  at its both right and left ends is fixed with bolts  28  to a hub  29  arranged at a central part of the front end of the front wheel  1 . A stay  30  projecting from the front end of the fuel tank  5  is fastened together on the hub  29  by bolts  28 . 
     FIG. 4 shows schematically a structure of the steering damper  10 . A fan-like liquid chamber  32  expanding in a rearward direction is arranged inside the steering damper  10 . A damper shaft  23  is positioned at its essential position, and an inner part of the liquid chamber  32  is divided into a right liquid chamber  34  and a left liquid chamber  35  by a wing-like segment  33  extending rearwardly integrally from the damper shaft  23 . 
     The extremity end of the wing-like segment  33  forms a sliding surface and slidably contacts with the inner surface of an arcuate wall  36  of the liquid chamber  32 . Non-compressive liquid such as oil or the like is sealingly filled in the right liquid chamber  34  and the left liquid chamber  35 . The right liquid chamber  34  and the left liquid chamber  35  are connected by a bypass passage  37 . A variable valve  38  is arranged at the midway part of the bypass passage  37 . The variable valve  38  has a metering passage for use in generating a damping force and the variable valve can meter the metering passage by changing a sectional area of the metering passage. However, the variable valve  38  is not restricted to such a structure as above and various types of well-known structure can be employed. 
     A control device  40  controls metering of the variable valve  38 . The control device  40  is a microcomputer, or the like. This control device  40  controls the variable valve  38  in response to each of the sensing signals of an acceleration sensor  41 , vehicle body speed sensor  42 , throttle sensor  43 , engine rotation frequency sensor  44  for an engine and gear position sensor  45  or the like. A metering of the variable valve  38  is changed to adjust a damping force under a predetermined condition. 
     A method for controlling damping force of the steering damper  10  performed by the control device  40  can be classified into two cases. In the first case, the damping force is generated when a predetermined threshold value of acceleration is exceeded. In the second case, a determination is made as to whether a vehicle body speed exceeds the threshold value. If so, the damping force generated by sensing the acceleration alone is changed based on the vehicle speed. Thus, it is possible to change the damping force in response to both an acceleration and a vehicle body speed. Further, the throttle sensor  43  can be used in place of the acceleration sensor  41 , and the rotation frequency sensor  44  for the engine and the gear position sensor  45  can be used in place of a vehicle body speed sensor  42 . 
     Each of the aforesaid sensors is a well-known sensor. The acceleration sensor  41  is a G sensor for sensing a vehicle body acceleration. This sensor is installed at an appropriate location in a vehicle. The vehicle speed sensor  42  detects a vehicle speed in reference to the frequency of rotation of the output sprocket of an engine, or the like. The throttle sensor  43  detects a degree of opening of the throttle arranged in an intake passage, the rotation frequency sensor  44  of the engine detects the frequency of rotation of the crank shaft, and the gear position sensor  45  detects a present gear position in the transmission. 
     Next, the operation of the present preferred embodiment will be described. When the acceleration detected by the acceleration sensor  41  is below a predetermined threshold value, it is judged that there is a less possibility that a kickback will occurs. Therefore, the control device  40  does not generate any damping force at the steering damper  10 . Further, since there is no increase in a handlebar load, a smooth and comfortable steering operation can be attained. In turn, when the value exceeds a predetermined threshold value, the front wheel load becomes light, and a kickback may easily occur, the control device  40  changes the variable valve  38  to its metering side to meter the bypass passage  37 , increases a flowing resistance of liquid and increases a damping force. 
     As a result, a liquid flow between the right liquid chamber  34  and the left liquid chamber  35  is restricted to restrict a turning of the steering shaft  14 , thus further restricting the possibility of a kickback. With the present invention, it is possible to eliminate the influence of temperature as found in the prior art front wheel load sensing operation, and the sensing operation can be carried out by a less complex structure. 
     As a result, with the present invention, the turning difficulties of the steering shaft  14  and the steering system caused by the kickback are restricted. 
     At this time, the vehicle speed is also monitored in response to a sensing signal of the vehicle body speed sensor  42 . In this situation, the damping force is generated only when both the vehicle acceleration and the vehicle body speed each exceed predetermined threshold values. As such, control can be provided more accurately, since both the acceleration and the speed of the vehicle are considered. It is well known that acceleration of the vehicle and possibility of kickback can easily occur easily when the vehicle is running at low speeds. On the other hand, acceleration and the possibility of kickback is reduced when the vehicle is running at a high speed. In this latter situation, the front wheel load can be held more accurately. Thus, using the a vehicle body speed to adjust the damping force that was produced by the sensing of the vehicle acceleration alone provides for a more accurate control of the damping force. 
     Additionally, a damping force is not only generated when the value exceeds a threshold value as described above. The damping force can also be changed in a multi-stage manner or continuous manner in response to either one of an acceleration or a vehicle body speed or both of them. If such an arrangement is applied, a more appropriate control over the steering damper  10  corresponding to the operating state becomes possible. 
     Further, an acceleration can be calculated in response to a sensing signal of the throttle sensor  43  for sensing a variation of a degree of opening of the throttle. Also, a vehicle body speed can be calculated in response to sensing results of the rotation sensor  44  for sensing the engine rotation frequency and the gear position sensor  45  for sensing a gear position. In such an arrangement, it is not necessary to use the acceleration sensor  41  or the vehicle body speed sensor  42  exclusively. Both a vehicle body speed and a vehicle body acceleration can be detected accurately, making it possible to generate an appropriate damping force of the steering damper. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.