A MOTORCYCLE FRONT FORK ASSEMBLY

Abstract: A motorcycle front fork assembly comprises substantially parallel fork legs (15), each leg being connected to an articulated linkage (16). The articulated linkage comprises a telescopic member (19) concentric with or adjacent to each fork leg (15) and axially movable substantially in the plane of the fork legs and a suspension link (17) having a first end for supporting a wheel spindle (24), a second end (22), and being pivotally connected between the two ends (22, 24) to the telescopic member (19). So that trail variations may be kept to the minimum, restraining means pivotally connect the second end (22) of the suspension link (17) to the leg (15) and allow translational movement of the second end (22) during axial movement of the telescopic member (19) in response to movement of the said first end (24). By simple experiment suitable dimensions may be determined to maintain trail variation substantially constant for any steering variation. The restraining means may be in the form of a link (18) pivotally connected between the second end (22) and the leg (15), or for example a slider or roller moving along a guide or track arranged between the second end (22) and the leg (15).

Description: Ti tl e : - A motorcycle front fork assembly This invention relates to a motorcycle front fork assembly- Improvements in motorcycle engine power and tyre "grip" in recent years have drawn attention to deficiencies inherent in the design of conventional front forks, which are the means commonly applied to support the front wheel of the vehicle and to steer, control and balance it. In particular the increase in performance arising from these improvement has highl ghted those factors that tend to cause steering instability of the kind that is termed "flutter" or "weave". This form of instability is characterised by a side-to-side oscillation of the front forks and wheel which in extreme circumstances can lead to structural damage or loss of control. The most significant factors that can induce this oscillation are believed to be a) changes in the steering geometry of the ehicle arising from suspension travel and b) torsicnal flexibility in the fork assembly. The overwhelming majority of motorcycles currently produced use the "telescopic" form of front fork. This comprises two parallel legs each formed as a pair of concentric tubular members having resilient suspension means acting between the tubular members. A wheel spindle is rigidly connected to, and extends between, the lower ends of the two legs. For there to be sufficient clearance between the tubular members of each leg to allow relative telescopic movement, there must also be some degree of relative lateral OMPI lA V/IPO movement, and therefore lateral stiffness is impaired. Further, because the tubular members of each telescopic leg are free to rotate relative to each other, it is not possible to exploit the torsioπal properties of the individual components of the fork. Thus, the torsional rigidity of the assembly as a whole is poor. A further defect is the friction developed by telescopic forks that often prevents smooth functioning over road irregularities; this condition is known as "stiction" and becomes more apparent as the forks are raked away from the vertical. However, a far more serious defect occurs with variations in steering geometry produced, e.g. by road irregularities. The variations in steering geometry are best illustrated with reference to the change in angle of the steering head axis. The angle of the steering head axis is specified at the normal weight of the vehicle when statically loaded, which , would generally be at one third of suspension travel at both wheels. However, t will be apparent that under practical road conditions the effective steering head angle can vary given that each wheel can move independently from full extension to full compression. Thus under heavy braki-ng, weight transfer will compress the front suspension fully whilst allowing the rear suspension to become fully extended, thus giving rise to a steeper head angle. Similarly, hard acceleration will cause the front suspension to become fully extended and the rear suspension to be fully compressed, thus giving rise to a shallower head angle. Road irregularities can produce a similar situation. Thus when the vehicle passes over a large bump in the road surface, first the front suspension and then the rear suspension will be compressed, thus causing changes in the effective head angle. This change in head angle can in itself cause disturbances in the steering characteristics of the vehicle, but the major consequence is to the castor effect of the front wheel which is normally termed "trail". Trail is best described as the amount the front wheel "trails" behind the steering head axis. In practice, trail is generally specified as the distance between the tyre contact patch and the steering head axis at ground level. As trail is of major importance in determining the steering characteristics of the vehicle, and in particular the self aligning or stabilising forces that contribute to the vehicle stability, any change in this value can have a serious effect on stability. Moreover it is believed that dynamic effects are generated as these changes in geometry inter-act with other forces produced by the machine's passage - principally gyroscopic precession, pendular mass and tyre reaction forces - to cause the steering oscillations previously referred to. Further, it is believed that lack of torsional rigidity in telescopic forks can increase this tendency, since it permits the wheels to travel further out of line than would be the case if the fork was of greater lateral stiffness (lateral stiffness if measured as the resistance provided by the fork assembly to sideways deflection of the front wheel at the lower point on the wheel rim, with the assembly held stationary at its upper end). From GB Patent Specification No. 744538 a motorcycle front fork assembly is known which comprises substantially parallel fork legs each leg being connected to an articulated l nkage comprising a telescopic member housing resilient suspension means and being concentric with or adjacent to each fork leg and axially movable substantially in the plane of the fork legs; a suspension link having a first end for supporting a wheel spindle, a second end, and being pivotally connected between the two ends to the telescopic member. The second end of the suspension link is attached to an extension of the fork leg. In order to allow movement of the suspension link, the telescopic member has to be able to pivot with respect to the fork leg and therefore, a special mounting is required. Consequently the telescopic member has no influence on the suspension path generated by wheel movement. The main disadvantage however, of such a construction is that trail varies considerably since movement of the wheel to provide suspension - as plotted at the wheel spindle - can only be an arc whose radius is determined by the length of the suspension link. Since this is limited by considerations such as ground clearance, and the weight and flexibility of the link and the extension on which it is pivoted, the resultant arc will always be of comparatively small radius. Such a small radius of operation will introduce trail variations and necessarily restrict the CMPI total suspension travel available. An object of the present invention is to provide a motorcycle front fork assembly in which trail variations are minimised, and preferably in which the lateral stiffness is improved. The characterising feature of the present invention is the pro ision of restraining means pivotally connecting the second end of the suspension link to the leg and allowing trans!ational movement of the second end during axial movement of the telescopic member in response to movement of the said first end. By simple experimentation suitable dimensions of the above component parts for any type of motorcycle may be derived whereby trail variation is substantially removed. Furthermore, the telescopic member may now be non-pivotally mounted thereby improv ng lateral stiffness. As will be explained below, it is possible to confer greater flexural stiffness to such a front fork assembly. The invention will now be described with reference to embodiments shown by way of example in the accompanying drawings, wherein: Figure 1 shows diagrammatically the variation of rake and trail experienced with known telescopic fork assemblies, Figure 2 shows diagrammatically the desired path of wheel spindle travel to achieve minimum trail variation, Figure 3 shows diagrammatically trail variation using a trail ng link suspension, Figure 4 shows in elevation part of a first embodiment of a motorcycle front fork assembly according to the invention, Figure 5 shows diagrammatically the embodiment of Figure 4 in place in a motorcycle, Figure 6 shows in elevation a second embodiment of a motorcycle front fork assembly according to the invention in place in a motorcycle, Figure 7 shows diagrammatically the embodiment of Figure 4, incorpo¬ rating a disc brake caliper assembly, and Figure 8 shows schematically a motorcycle front-fork assembly so as to illustrate suitable dimensions. Figures 9 and 9a show in elevation and part section, respectively, alternative restraining means. In Figure 1, a motorcycle has a telescopic front fork assembly 1 pivotable about a steering head axis 2 inclined to the horizontal datum line 3 by an angle of 60°. The horizontal datum line 3 is normally defined as the line passing through the tyre contact points 4, 5 of both wheels 6, 7. In a static condition the axis 2 is inclined at 30° to the vertical, and "trail" is the distance _t between the contact point 4 and the point 8 of intersection of the extended axis 2 and the horizontal datum line 3. As the front wheel 6 travels over a bump or a hollow, the front fork assembly 1 is compressed or expands, respectively. The steering head angle, and hence the angle between the axis 2 and the vertical alters with suspension travel. Figure 1 shows the extent of variation of trail t-t 1 ' ' ' . Assuming t = 0, the percentage changes are: t' is 46% less than t; t' ' is 14% less than t; t*" is 14% more than t; t" " is 51% more than t. Figure 2 illustrates the path that the front wheel 6 of a typical motorcycle must follow with suspension movement to achieve substan¬ tially zero trail variations under the same conditions. The amount of compression and extension from the static condition is indicated by the arrows 9 and 10 respectively. As can be seen in this example, which is based on the same overall steering geometry and dimensions as Figure 1, the "ideal" wheel spindle movement is a shallow arc 11 having a centre 12 somewhat above and in front of the vehicle. Because of the length of radius 13 necessary to generate such a shallow curve 11 it would not be practicable to use a simple form of "trailing link" suspension, as shown in Figure 3, as the resulting arms and supporting structure would be too long to be rigid and would add excessive pendular mass to the steering system. Figure 3 shows the typical percentage variation in trail actually obtained with the trailing link suspension of Figure 3, with equal, simultaneous wheel movement. In Figure 2 a mean line 14 passed through the arc demonstrates that the "ideal" curve is inclined considerably further from the vertical than can be accommodated by merely re-positioning telescopic forks to achieve this effect. Even if this expedient could be used the forks would be raked far too much to be effective as a suspension medium. As has been mentioned, the frictional qual ties of telescopic forks become worse the further they are inclined away from the vertical - and therefore the more disadvantageously they are placed with regard to road shocks. It should be pointed out that the "ideal" curve need not be a circular arc, but may be irregular or parabolic, or even a substan¬ tially straight line. Figure 4 shows a first embodiment of the invention in which trail variation is minimal. A motorcycle front fork assembly includes a pair of parallel fork legs, only one leg 15 being shown. The leg 15 lies substantially in the same plane as the steering axis (see Figure 5). At the base of each leg is an articulated mechanism 16 comprising three members, i.e. a suspension link 17, a restraining link 18, and a telescopic member 19. In this embodiment the tubular member 16 is concentric with the fork leg 15, but other arrangements are possible whereby the telescopic member 16 is arranged adjacent to the leg 15 pro ided that the telescopic motion of movement of the member 16 is substantially in the plane of the fork leg 15. QMH A conventional suspension device (not shown) comprising a compression spring and damper unit to provide a resilient suspension medium is incorporated between the telescopic member 18 and the leg 15. However, alternative arrangements are possible whereby the suspension device is disposed differently or provided by different means for, for example by the compression of a gas or rubber compound. The suspension medium need not act through the telescopic member 19, but could by way of example be anchored at its lower end adjacent to the forward extremity of link 18 the wheel spindle, and anchored at its upper end by means of a lug or bracket to the outside of leg 15. , The suspension link 17 is connected to the telescopic member 19 by a pivot bearing 20. It can be seen that the suspension link is shaped as a bellcrank lever with a line connecting the ends 22, 24 lying above the pivotal connection 20 of the suspension " link 17 to the telescopic member 19. The restraining link 18 connects the suspension link 17 to the rear of the fork leg 15 by means of pivot bearings 21, 22 at either end of the retraining link 18 and a lug or reaction bracket 23 off-setting the bearing 21 from the central axis of the fork leg 15. At the forward extremity of each suspension link 17 a clamping arrangement is provided to locate rigidly and hold the wheel spindle 24. Thus, the wheel is rigidly held between the two legs of the fork, the dimensions of the wheel spindle and the clamping arrangement being so designed as to confer torsional rigidity. Thus the suspension link 12 are constrained to move together with wheel movement, whilst movement is resisted by the suspension medium disposed above the telescopic member or elsewhere. The suspension unit is omitted from the illustrations for clarity. In operation, movement of the wheel to provide suspension causes the suspension links 12 to exert an upward force on the telescopic members 19, whilst restraining links 18 are under a corresponding tensile load. As the telescopic members 19 slide in each leg 15 the angular relationship between the three members 17, 18 and 19 changes in such a manner as to confer a motion represented by the line 25 at the wheel spindle 24. The characteristics of this motion are determined by the distance _a between the wheel spindle 24 and the pivot bearing 20, the distance _b between the pivot bearings 20 and 22, the length £ of the restraining link 18, the "offset" _d of the pivot bearing 21 from the centre longitudinal axis of the leg 15, and the vertical displacement _e of the pivot bearing 20 from a line drawn between the pivot bearing 22 and the wheel spindle 24. The wheel spindle motion 25 in this embodiment may be substantially straight over the major portion of its travel, or be arcuate in character with its centre at a large radius J and lying to the rear and below the front wheel . Thus it is possible to select experimentally (see below with reference to Figure 8) a set of dimensions for the three moving members 16, 17 and 18 of each fork leg to produce a line 25 that approximates to the mean zero-trail line 14 illustrated in Figure 2. Moreover by selecting alternative dimensions it is possible to produce a motion 25 to approximate the mean zero trail line for any particular motorcycle geometry regardless of steering head angle, trail, wheelbase, suspension travel, etc. Further it is possible by selecting different dimensions for the moving element to confer alternative wheel spindle motions other than the zero-trail curve 25 if desired. For example a special require¬ ment may exist to minimise wheelbase variations, whereby this object can be achieved by selecting the appropriate dimensions for the moving components whereby the motion generated at the wheel spindle will be substantially vertical . In the embodiment shown in Figure 6 the links are arranged so that the wheel spindle 24 is located to the rear of the fork leg whilst the restraining link 18 connects the link 17 and the fork leg 15 at the front. Here the line connecting the ends 22, 24 of the suspension link 17 shaped as a bellcrank lever lies below to pivotal connection 20 of the suspension link 17 to the telescopic member 19. The geometry of this arrangement WIO is such that the curve 25 thus generated can share substantially the same path as the ideal zero-trail curve 11 illustrated in Figure 2, or be a substantially straight line approximating to the mean line 26 shown in Figure 6. The principle of operation of this embodiment is identical with that of Figures 4 and 5 in that movement of the wheel spindle 24 moves the suspension link 17 and thus induces a sliding motion of the telescopic member 19. The geometry of these components plus that of the restraining link 18 produces a motion of the desired characteristics, this being related to the dimensions of the moving members 16, 17 and 18 and their relationship to the fork leg 15. In both of the embodiments described above the suspension medium need not necessarily be associated directly with the telescopic member 19, although convenience may indicate that this is a preferred arrangement. Because the load path from fork leg 15 to wheel spindle 24 is conveyed via two paths, i.e. through the suspension link 17 via the restraining link 18 and via the telescopic member 19, torsional rigidity of each fork leg is maintained. Thus flexural rigidity of the complete assembly is improved in comparison with a telescopic fork where each fork leg is unable to convey torsional loads. Further, because the dimensions of fork leg 1 are independent of the dimensions of the other components these may be proportioned to confer great stiffness. Thus whilst it is unusual for telescopic fork stanchions to exceed 40 mm in diameter the fork legs 15 can be much larger, for example 60 mm in diameter, with a correspon¬ ding increase in stiffness. Moreover, because the bearing surface provided for the telescopic member 19 is internal to fork leg 15, its external profile need not be circular and can be constructed in a manner and to a profile designed to confer maximum stiffness. Another advantageous feature is the mechanical advantage or leverage exerted by the suspension link 17 upon the telescopic member 19. By arranging (consistent with the other objects of the assembly) for the pivot point 20 to be some distance from the wheel spindle 24, an element of mechanical advantage is introduced that results in a differential in movement between those points with suspension travel. Thus in the example illustrated for en ery 10 cm of movement the spindle 24 along the path 25, the point 20, and thus the telescopic member 19, moves only 6 cm. Changes in the geometry of the members 17, 18 and 19 may enhance or reduce this effect. The consequences of this differential are twofold. Firstly the effective 'unsprung' weight of the telescopic member 19 is reduced in direct proportion to the ratio of this differential, and thus the response to road shocks of the suspension is improved in that the overall inertia of the assembly is thereby reduced. Secondly the upward force exerted by the bearing 20 upon the telescopic member 19 is increased, again in direct proportion to the differential of movement. This upward force assists in overcoming the resistance to iritial movement, i.e. "stiction" OMPI commonly experienced in telescopic fork elements. Another advantage of the assembly according to the invention is illustrated diagra atical y in Figure 7. As a consequence of the pivotal inter-connection of the principle parts of the assembly it is possible to provide a further feature that is beneficial . In Figure 7, a disc brake comprises a disc brake caliper assembly 27 which is pivotally mounted on the wheel spindle 24 and acts upon a disc 28. The caliper assembly 27 is restrained from rotating about the spindle 24 by a torque arm 29, which is connected pivotally at one end at pivot 30. The other end of the torque arm 29 is anchored pivotally on the fork leg 15 at mounting 31. Under braking, the caliper assembly 27 is subjected to a force tending to turn it in the direction of wheel rotation (counter clockwise in Figure 7), and this force is transmitted along the torque arm 29 to act upon the fork leg 15. Thus braking forces are arranged to exert an upward force on the legs 15, and thereby a correponding downward force on the suspension link 17 and the telescopic member 15. By this means the braking forces tend to resist compression of the suspension and thereby compensate for the weight transfer that causes additional loads to be thrown on the front suspension. By suitable arrangement of the pivot points it is possible in this manner to prevent the suspension from being fully compressed by weight transfer and to leave the normal wheel travel unaffected to deal with road irregularities. Other arrangements of the same principle mechanism are possible without departing from the scope of the invention. Moreover by arrangement of the geometry of the linkage mechanism it is possible to produce this opposition of braking and weight transference forces by rigidly attaching the brake caliper to the suspension link 17. Thus, the disc brake caliper is subjected to a force tending in the same direction as wheel travel and thereby acting upon the suspension link 17 to rotate it in such a sense as to oppose suspension compression under weight transference. The following two examples give suitable dimensions of the features of the motorcycle front assembly so as to provide a substantially constant trail. These dimensions have been ascertained experimen¬ tally for representative production motorcycles. Dimensions for other motorcycles may also be ascertained by simple experiment. EXAMPLE 1 - 400cc off-road motorcycle Rake / Steering angle = 30° Trail = 127mm Wheelbase = 1455mm Front suspension travel = 241mm Rear = 254mm Front wheel diameter = 685.8mm Rear wheel diameter = 584mm Suspension link 17: a = 260mm f = 426mm e = 105mm OMPI EXAMPLE 1 - (Cont/d) Restraining link 18: c s 195mm d s 120mm Telescopic member 19: excursion 20-20' = 173mm Wheel spindle 24: excursion 24-24' = 334mm Leverage Ratio s 1.93:1 Radius _R = 1168mm Angle (β ) of mean line 26 producing zero trail variation = 50.5° EXAMPLE 2 - l . OOOcc Sports/Touring motorcycl e Rake / Steering angle = 26° Trail = 89mm Wheelbase = 1500mm Front suspension trave " 1 = 127mm Rear suspension travel = 94mm Front wheel diameter = 584mm Rear wheel diameter = 584mm Suspension link 17: a = 120mm f = 165mm e = 35mm Restraining link 18: c = 75mm d = 22mm Tel escopic member 19: excursion 20-20' 55mm Wheel spindle 24: OMP EXAMPLE 2 - ( Cont/d) excursion 24-24 ' = 153mτπ Lev erage Ratio = 2.78: 1 Radius R s 297.6mm Angle (β) of mean line 26 producing zero trail variation = 42° It will be apparent that the restraining link 18 may be replaced by any other restraining means to constrain movement of the point 22 in a translational direction as shown in Figure 8. The restraining means may for example be in the form of a roller and track mechanism as shown in Figures 9 and 9a. A roller 22* is constrained to move within a track 31 carried by a bracket or arm 23' fixed to the leg 15. Other restraining means will be apparent, e.g. the roller 22' may be replaced by a trunnion slidable along a track rigidly fixed to the leg 15. Alternatively, a cylinder-piston assembly could be connected between the point 22 and a carrier fixed to the leg 15. The essential feature of any suitable restraining means is that ts pivotal and translational movement forces any movement of the wheel spindle 24 to produce a corresponding axial movement only of the telescopic member 19 via the pivotal connection 20, the dimensions being such that trail variation is substantially eliminated.

Claims: AMENDED CLAIMS [received by the International Bureau on 11 January 1984 (11.01.84); original claims 1 to 10 replaced by amended claims 1 to 7] 1. A motorcycle front fork assembly comprising sub¬ stantially parallel, fork legs (15), each leg being connected to an articulated linkage (16) comprising a telescopic member (19) concentric with or adjacent to each fork leg (15) and axially movable substantially in the plane of the fork legs; a suspension link (17) having a first end for supporting a wheel spindle (24), a second end (22), and being pivotally connected between the two ends (22, 24) to the telescopic member (19) at a pivot (20) displaced in a substantially vertical plane from a line joining the two ends (22, 24); and restraining means (18) pivotally connecting the second end (22) of the suspension link (17) to the leg (15) to allow translational movement of the second end (22) during axial movement of the telescopic member (19) in response to movement of the said first end (24), characterised in that the second end (22) and the restraining means (18) are arranged rearwardly of the fork legs (15) and the pivot (20) lies below the said line joining the two ends (22, 24), the arrangement being such that, in use, trail variation is minimised under pitching conditions. 2. A motorcycle front fork assembly comprising sub¬ stantially parallel fork legs (15), each leg being connected to an articulated linkage (16) comprising a telescopic member (19) concentric with or adjacent to ' each fork leg (15) and axially movable substantially in OMPI - n - the plane of the fork legs; a suspension link (17) having a first end for supporting a wheel spindle (24), a second end (22), and being pivotally connected between the two ends (22, 24) to the telescopic member (19) at a pivot (20) displaced in a substantially vertical plane from a line joining the two ends (22, 24); and restrai¬ ning means (18) pivotally connecting the second end (22) of the suspension link (17) to the leg (15) to allow translational movement of the second end (22) during axial movement of the telescopic member (19) in response to movement of the said first end (24), characterised in that the second end (22) and the restraining means (18) are arranged fαrwardly of the fork legs (15) and the said pivot (20) lies above the said line joining the two ends (22, 24), the arrange¬ ment being such that, in use, trail variation is minimised under pitching conditions. 3. A motorcycle front fork assembly according to. claim 1 or 2, wherein the restraining means comprise a link (18) pivotally connected to the second end (22) and to the leg (15). 4. A motorcycle front fork assembly according to claim 1 or 2, wherein the restraining means comprise a sliding or rolling member (22* ) movable along a guide or track (31) arranged between the second end (22) and the leg (15). 5. A motorcycle front fork assembly according to any one of the preceding claims, characterised in that the suspension link (17) is shaped as a bellcrank lever. 6. A motorcycle front fork assembly according to any one of the preceding claims, wherein a front wheel (28) is clamped between the said first ends (24) of the suspension links (17), characterised in that a disc brake caliper (27) is rotatably mounted on the front wheel (28) and is pivotally connected via a torque arm (29) to the leg whereby application of the brake generates forces opposing movement of the articulated linkage (16) to minimise or eliminate effects of weight transference during braking. 7. A motorcycle front fork assembly as claimed in claim 6, characterised in that the disc brake caliper (27) is rigidly attached to the suspension' link (17). __ C ?I__

Inventors: HAYNES TERENCE ANTHONY, HAYNES, TERENCE, ANTHONY
Applicants: HAYNES TERENCE ANTHONY, HAYNES, TERENCE, ANTHONY
Application year: 1983
Main classification: B62K25
IPC codes: B62K25