A SHOCK ABSORBER FOR A MOTOR VEHICLE

This invention relates to shock absorbers for motor vehicles as well as motor vehicles utilising such shock absorbers.

Shock absorbers are used in motor vehicles to absorb or dampen road induced vibrations caused by uneven road surfaces, thus providing comfort and safety to driver and passenger or rider in the case of a motorcycle or motor scooter. Shock absorbers also bear vehicle loads.,

A typical shock absorber comprises a hydraulic cylinder operatively disposed with respect to a helical compression spring pre-compressed to a predetermined load value and disposed over the hydraulic cylinder. The spring may be disposed over the hydraulic cylinder between the lower spring seat and upper spring seat of the shock absorber. Sometimes the hydraulic cylinder itself is referred to as the "shock absorber" since it is the cylinder and fluid contained within it that absorbs energy arising from the vibration of the vehicle.

The spring is compressed to a predetermined load value having regard to parameters such as the vehicle weight and load acting on the vehicle as well as the desired shock absorption capacity of the shock absorber. During motion of the vehicle, the spring is subjected to frequent compression and expansion due to road induced vibrations. This creates shear stresses in the spring, leads to spring fatigue and may cause premature spring failure thereby reducing the life of the shock absorber. Spring failure during motion of the vehicle may also lead to serious accidents.

The capacity of the shock absorbers to absorb or dampen road induced vibrations and bear vehicle load depends on various factors including the spring diameter, spring length and spring characteristics. This is also necessary for rider comfort. Reduction in length of a spring leads to a reduction in the number of turns for a given spring wire diameter and mean coil diameter and given spring stroke and, consequently, harder suspension and reduced rider comfort. The overall length of the shock absorber depends on the pre-compressed length of the spring. Minor length adjustment of the spring, may be possible by adjusting the position of the lower spring seat. However, in the usual case where the shock absorber is located close to the motorcycle body the chain case on one side and muffler or silencer on the other side of the vehicle are generally required to have depressions to provide sufficient clearance for proper location of the lower spring seat of the shock absorbers. Fabrication of such depressions creates additional manufacturing costs as additional manufacturing is required.

Further, the consequential limitation in the orientation of the silencer, increased irritating noise due to motorcycle chain wearing against the depression in the chain case and reduced aesthetics of the vehicle are problems with such arrangements.

An alternative to such arrangements is mounting of a shock absorber away from the vehicle body. Such an arrangement requires additional reinforcement which adds to the cost of a vehicle.

It is therefore an object of the invention to provide a shock absorber which has improved reliability and life.

Another object of the invention is to provide a shock absorber comprising pre compressed springs of reduced length.

Another object of the invention is to provide a shock absorber which allows increased ride comfort; optimally with an arrangement which may be conveniently mounted on the motorcycle without cost inducing additional fabrication operations.

With these objects in view, the present invention provides a shock absorber comprising a spring combination having first and second spring elements having capacity to store energy from vehicle vibrations, said first and second spring elements being operatively disposed to share the load due to road induced vibrations. In this way, the resultant shear stresses resulting from compression and expansion of the spring elements during motion of the vehicle are within and may be well below, permissible limits. Operative disposition of the springs may involve binding together of the springs.

Each spring element, there may be more than two, may have differing spring rate, spring rate of springs within the combination being selected to achieve the desired degree of rider comfort. The spring elements may be dual or plural rate springs. The spring rates may be selected such that stiffness of the spring combination is the same as for a single spring, yet with a shorter spring length than for a comparable single spring arrangement. Furthermore, the reduced spring length allows a greater clearance from a chain case of a motorcycle and enables fabrication of depressions, and consequent wear and noise to be reduced or avoided.

Such a shock absorber may comprise a hydraulic cylinder or damper body, with the spring combination taking the form of a pair of helical springs pre-compressed to a predetermined load value. The springs may include an inner spring disposed over the hydraulic cylinder and an outer spring disposed over the inner spring. Each end of each spring is located in a spring seat. The pre

compression of both springs may be adjusted by adjusting position of a spring seat, preferably the lower spring seat for the spring.

One end of the outer spring, for example the lower end, may be located on an outer locator, this being -in the preferred case - located on the lower spring seat. Similarly, one end of the inner spring - again conveniently the lower end - is located against an inner locator located on the lower spring seat.

The shock absorber may comprise a spacer provided between the springs for preventing friction and wear through relative motion of the springs. This spacer may be located in one of the spring seats, for example the upper spring seat. The shock absorber is applicable to any vehicle but is particularly well suited for two wheel vehicles such as motorcycles and motor scooters or three wheel vehicles. The shock absorber may be conveniently included within a rear wheel suspension of such a vehicle.

The characteristics of the springs making up the spring combination of the shock absorber may be selected with the object of reducing vehicle manufacturing cost. As assembled spring length is less for given rider comfort and acceptable maximum shear stress, more space may be made available for routing of a motorcycle silencer. Clearances between chain case and shock absorber may also allow fabrication operations to be simplified since it is no longer necessary to manufacture other components of the motorcycle to accommodate the shock absorber, for example through fabrication of depressions in the chain case and so forth.

The shock absorber of the invention may be more fully understood from the following description of preferred embodiments thereof described with reference to the accompanying drawings in which:

Figure 1 is a side cross-sectional view of a shock absorber in accordance with one embodiment of the present invention;

Figure 2 is a side cross-sectional view of a shock absorber in accordance with a second embodiment of the present invention;

Figures 3a to 3d provide wheel deflection to load characteristics of four types (zl) to (z4) of real wheel suspension,

viz: (zl) shock absorber with a conventional single spring having stiffness 1.91 kg/mm;

(z2) shock absorber incorporating the present invention with its spring combination' configured for the same rider comfort and load capacity as (zl);

(z3) shock absorber with a conventional single spring with lower stiffness of 1.48 kg/mrii (a so-called "comfort" setting); and

(z4) shock absorber incorporating the present invention with its spring combination configured for the same rider comfort and load capacity as (z3).

Figures 4a and 4b indicate spring rate curves for the single spring of shock absorber (zl) and the single spring of shock absorber (z3);

Figures 5a to 5c indicate spring rate curves for the outer and inner spring elements and spring combination of the shock absorber (z2) of the invention and for which the wheel deflection to load characteristic is shown in Figure 3b;

Figures 6a to 6c indicate spring rate curves for the outer and inner spring elements and spring combination of the shock absorber (z4) of the invention and for which the wheel deflection to load characteristic is shown in Figure 3d; and

Figure 7 is a view of a motorcycle incorporating a shock absorber in accordance with one embodiment of the present invention.

Figure 8 is a view of a motorcycle with a conventional single spring shock absorber.

Referring now to Figures 1 and 7, there is shown the location and design of a shock absorber 1 utilised as part of the rear wheel suspension of a motorcycle 100. The shock absorber 1 for rear wheel 102 comprises a damper body 2 comprising a hydraulic cylinder 4 which absorbs vibrations caused by driving motorcycle 100 along an uneven road. Hydraulic cylinder 4 includes a piston 8 and piston rod 9 which moves through the cylinder 4 in accordance with the motion of motorcycle 100 along the road. Piston rod 9 is locked into position by lock nut 14. The hydraulic fluid is an oil of desired density and. viscosity for the suspension application and the working portion 4a of the cylinder 4 is sealed by oil seal washer 10. Dust cover 13 also seals cylinder 4 against ingress of dust or dirt. The shock absorber ύ

Si also includes a spring combination comprising a pair of springs 17 and 18. Springs 17 and 18 are helical compression springs pre-stressed to a predetermined load value having regard to parameters such as vehicle weight and load on the vehicle and desired shock absorption capacity of the shock absorber 1.

The spring elements are springs. Spring 17 is an inner spring disposed over hydraulic cylinder 4. Spring 18 is an outer spring disposed over hydraulic cylinder 4 and inner spring 17. Springs 17 and 18

are operatively disposed relative to each other and bind, during use, to share load acting on motorcycle 100 during its travel along the road.

Outer spring 18 is located against an outer locator 19 located on lower spring seat 5. Inner spring 17 is located against inner locator 20 also located on the lower spring seat 5. Shock absorber 1 is} mounted to the vehicle body (not shown) by fastening means secured through upper and lower eyelets 15 and 16.

Hydraulic cylinder 4 is provided with an adjustable lower spring seat 5 which accommodates the lower ends 17a and 18a of springs 17 and 18. Lower spring seat 5 is provided with a number of cam profiles 6 located at different heights enabling different settings for the lower spring seat 5. In this way, pre-compression may be adjusted to achieve a desired level of rider comfort. Hydraulic cylinder 4 is also provided with a pair of lugs, one lug 7 of which is shown in Figure 1.

Adjustment of the height of the lower spring seat 5, spring - precompression and desired rider comfort, is achieved by rotating or turning the lower spring seat 5 on the hydraulic cylinder 4 which forms part of an adjustment means for spring stiffness of the springs 17 and 18. Lower spring seat 5 slides up or down on the hydraulic cylinder 4 against the lugs 7 and engages with a cam profile 6 to raise or lower the height of the lower spring seat 5 on the hydraulic cylinder 4 as required.

The upper ends of the springs 17 and 18 are seated in upper spring seat 11 which is fitted to the piston rod 9. A compression bump stop 12 fitted to piston rod 9, it absorbs shock from any jerky movement of the piston 8.

The shock absorber 201 of Figure 2 differs from the shock absorber 1 of Figure 1 in that a spacer 21 is provided between the springs 17 and 18. Spacer 21 is located in the upper spring seat 211 of the shock absorber 201. Spacer 21 prevents contact and wear by rubbing action of springs 17 and 18 against each other. Spacer 21 is therefore ideally made of a low friction polymer such as nylon or polypropylene. A spacer may also be provided between the hydraulic cylinder 4 and the inner spring 17 to prevent contact and wear by rubbing action of spring against the hydraulic cylinder.

During running condition of a vehicle fitted with the shock absorber 1 or 201 of the invention* both the inner and outer springs 17 and 18 bear the vehicle load and are subjected to frequent compression and expansion due to road induced vibrations. The load due to road induced vibrations and resultant shear stresses resulting from the compression and expansion of springs 17 and 18 during motion of motorcycle 100 are shared by the springs. Therefore, spring fatigue and chances of premature spring failure are reduced. Even if one of springs 17 and 18 fails during motion of the

vehicle, the shock absorber 1 or 201 can still function with one spring, at least for some time. The risk of both springs 17 and 18 failing simultaneously are very low to negligible, as the springs 17 and 18 are selected for expected running conditions. Therefore, the life and reliability of shock absorber 1, 201 is improved.

The degree of sharing of shear stresses and vehicle load may be achieved by appropriate selection of parameters like spring diameter and length and spring characteristics. At the same time, the' effective length of the pre-compressed springs may be reduced without reducing the characteristics of the shock absorber 1, 201 and riding comfort.

Referring now to Figures 3 to 6, the characteristics and advantages of shock absorber 1, 201 of the invention may be demonstrated comparatively with single spring shock absorbers of the prior art. Figure 3a to 3d provide wheel deflection to load characteristics ("wheel rates") as described above. The wheel rates overlap for both single and dual spring shock absorbers set at the same stiffness value.

Figures 4a and 4b show the spring rate curves for springs set at initial stiffness 1.91 kg/mm (zl) and 1.48 kg/mm (z3), the "comfort" setting for a single rider condition on a two wheel vehicle.

The design characteristics of springs 17 and 18, according to the invention, set at the two stiffness values 1.48kg/mm and 1.91 kg/mm, as well as for a single spring shock absorber are reported in Table 1 below:

Figures 5a to 5c respectively show the spring rate curves for outer spring 18 (Fig 5a) and inner spring 17 (Fig 5b), with the spring combination (z2) having the same stiffness as for the conventional single spring (zl) described above, that is a stiffness of 1.91 kg/mm. The springs 17 and 18 are dual rate springs. The spring rates for inner spring 17 are greater than for the outer spring 18.

Figures 6a to 6c respectively show the spring rate curves for outer spring 18 (Fig 6a) and inner spring 17 (Fig 6b), with the spring combination (z4) having the same

stiffness of 1.48 kg/mm as for the conventional single spring (z3) at "comfort" setting as described earlier.

The shock absorber with spring combination, formed by inner and outer springs 17 and 18 were then compared in performance with the single spring shock absorber, when set to the two stiffness values 1.48 kg/mm and 1.91 kg/mm. The results are provided in Table 2:

The advantages of the spring combination (z2) over the conventional spring (zl), for the same rider comfort and load capacity at spring stiffness 1.91 kg/mm, may be seen from the reduced spring assembled length (A), down 20% (that is, a reduction from 217 mm to 174.8 mm). The distance B, between the lower spring seat 5 and bottom eyelet 15, is increased 55% with benefit in terms of avoidance of fabrication of depressions in the

chain case to accommodate the spring seat. In other words, the chain case may be fabricated with a configuration independent of the design parameters of a shock absorber 1,201 mounted proximate to the component with consequential reduced manufacturing cost. This evident from Figure 7, which depicts a motorcycle fitted with a rear shock absorber incorporating the invention. Figure 8 depicts a motorcycle fitted with a conventional single spring shock absorber, where a depression 50, as marked, has been provided on the chain case. Such a depression is absent on the chain case in the motorcycle with the invention.

There is also increased space available for the routing of the silencer in comparison with prior art shock absorbers allowing motorcycle design to accommodate the silencer in the increased space available using the spring combination shock absorber 1,201 of the present invention. The weight of spring material is also reduced by 5% and the maximum stress by 4.3%, allowing further advantage in terms of reduced spring material cost and usage.

If the single spring (z3) is compared even with the spring combination (z4) at the comfort setting of 1.48 kg/mm, there is still reduced assembled spring length, greater clearance of shock absorber to chain case and reduced maximum stress so the benefits remain apparent. The significant benefits also accrue for the comparison between spring combination and single spring set at the comfort stiffness setting of 1.48 kg/mm.

Further, if the single spring were configured to deliver more rider comfort (z3), the spring rate would be increased by 38%. The spring combination for the same rider comfort (z4) would result only in increased spring weight of 19%. Therefore, the spring combination is more efficient in terms of lesser usage of materials. Thus, a shock absorber 1, 201 providing significantly greater rider comfort is possible, at lower manufacturing cost than for a single spring shock absorber.

The dimensions, material properties and performance characteristics of the two configurations of the invention (that is, z2 and z4) are examples. The settings of spring stiffness (1.91 kg/mm and 1.48 kg/mm) are two examples for a known motorcycle.

It is possible to achieve similar results for different stiffness settings suitable for different motor vehicles.

Modifications and variations to the shock absorber and vehicle of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.