VEHICLE WITH A SUSPENSION FOR A STEERABLE WHEEL

Abstract: A vehicle (10), such as a bicycle, is provided with a steering assembly (12) arranged on a frame (11) to be pivotable about a steering axis (S) and with a sprung suspension (13) for a wheel (14) steerable by the assembly. The suspension (S) comprises a leg (13a) carrying the wheel, a lower link (13b) coupling the leg to the frame and an upper link (13c) coupling the leg to the steering assembly. The leg and links together with the frame and steering assembly form a four-bar linkage permitting upward and downward travel of the leg and the leg is coupled to the frame by the lower link to be capable of turning with the steering assembly. The suspension is completed by a spring and damper unit (13d) which is arranged to resiliently resist the upward movement of the leg and is preferably connected to and acts between the lower link (13a) and the frame (11).

Description: VEHICLE WITH A SUSPENSION FOR A STEERABLE WHEEL The present invention relates to a vehicle and has particular reference to a suspension for a steerable wheel of the vehicle, especially a bicycle or other two-wheel vehicle. Spring suspensions for the steerable front wheels of motorcycles and, more recently, off- road and similarly specialised bicycles conventionally follow the principle of telescopic spring legs, which are simple in construction and function. However, accurate tolerances in the spacing and radial dimensions of the inner and outer elements of such legs are essential to ensure free sliding movement and this causes a significant increase in manufacturing costs. Moreover, telescopic legs are less suitable for lighter vehicles such as bicycles, as the sliding friction in the legs results in a relatively poor bump response. In off-road environments, functional efficiency can easily be compromised by ingress of dirt into the legs. To overcome these problems and provide a more capable suspension it is known to employ suspension systems based on linkages. In earlier designs of motorcycle systems, transmission of steering to the wheel carried by the suspension required auxiliary transmission elements, which had the undesirable consequence of adding complication and weight. In a more recent design incorporated in BMW (Trade Mark) motorcycles, a linkage is combined with telescopic front forks in such a manner that the forks are connected at a midpoint to the frame by a wishbone and at the top to a rotary element by a first spherical joint. The rotary element, which carries handlebars, is in turn connected by a second spherical joint to the motorcycle frame. Although this system provides a relatively uncomplicated method of transmission of turning motion to the wheel, the suspension still relies on telescopic forks and the system is thus not free of the disadvantages outlined above. A pure linkage system has been disclosed in United States patent specification No 5 441 291 in which, in the context of an off-road or rough-terrain bicycle, a forked front leg carrying a wheel is connected by an upper link and a lower link to a steering assembly joumalled in a head tube of the bicycle frame. A coaxial spring and damper unit is connected to and acts between the two links. Since both the links as well as the leg and the spring and damper unit are carried by the steering assembly, the steering inertia of the system is relatively high and all shock loads are borne by the steering assembly and transmitted to the junction of the head tube with the rest of the frame, which is a point of potential frame failure. The system design tends to oblige use of relatively short links, which in turn limit the available range of suspension travel. Other suspension systems utilising linkages are disclosed in United Kingdom (GB) patent specification 2 121 364 and European (EP) patent specifications 0 235 040, 0 238 768, 0 321 803, 0 468 138 and 0 621 171. In GB 2 121 364 and EP 0 235 040, a front wheel leg is coupled to a frame or other load-bearing structure of a motorcycle by upper and lower links forming, with the leg and structure, a four-bar linkage. Additional double links for transmission of steering input connect the top of the leg to handlebars. The resulting suspension system, in which the suspension travel and the steering functions are managed by different sets of linkages, incorporates numerous elements and pivot points, with consequent cost in both weight and complexity. A variant of this system is disclosed in EP 0 468 138, in which the upper link is omitted and in which two sets of double links connecting a front wheel leg to handlebars are geared together to produce, in effect, an expansible and compressible extension of a pivotable of the handlebars. The two sets of double links act to transmit steering input and to control suspension travel, but the essential geared coupling requires expensive gears, the vulnerable teeth of which have to accept almost the entire shockloading to which the suspension may be subjected. A modification also disclosed in EP 0 468 138 utilises a single set of double links, with are coupled together by a cogged belt in a manner unlikely to be capable of accepting shockloading in the longer term without susceptibility to wear or breakage. In EP 0 621 171 , a front wheel leg is coupled to a frame by a lower link which, in terms of suspension travel, effectively forms a unit with the leg. The leg is further coupled to handlebars by double links which serve to transmit steering input and to control suspension movement and the layout of which not only imposes weight penalties but offers relatively poorly defined wheel location during suspension travel. On the other hand, EP 0 238 768 and 0 321 803 disclose suspensions with four-bar linkage systems in which both the vehicle structure and the vehicle steering assembly are integrated. In each system, however, the lower link is connected to the front wheel leg at its lower end, with the result that the link is not only lengthy, but also has to pass halfway across the wheel at one of its sides. In order to accommodate wheel deflection under steering, the lower link is significantly spaced from the wheel or otherwise suitably recessed. Apart from the severe constraint on available steering lock, the described linkages with such lengthy and laterally offset or recessed lower links are bulky and inevitably heavy if the lower links are sufficiently robust to withstand bending loads. Such linkages are usable only for vehicles with limited front wheel steering deflection. It is therefore the object of the invention to provide a vehicle equipped with a wheel suspension which combines the merits of a linkage system switch relatively simple and direct transmission of steering. Subsidiary objects are the avoidance of undue increase in steering inertia as a consequence of use of a linkage system and the attainment of a wider distribution of the loads arising from the action of the suspension. Other objects and advantages of the invention are indicated in or evident from the following description. According to the present invention there is provided a vehicle provided with a steering assembly arranged on a structure to be pivotable relative thereto about a steering axis and with a spring suspension for a wheel steerable by the assembly, wherein the suspension comprises a leg carrying the wheel, a lower link and an upper link connected with the leg above the wheel and coupling the leg to, respectively, the structure and the steering assembly, the leg and the links together with the structure and the steering assembly forming a four-bar linkage permitting upward and downward movement of the leg relative to the structure and the leg being coupled to the structure by the lower link to be movable with the steering assembly on pivotation thereof, and resilient means arranged to resist the upward movement of the leg. In such a suspension, the coupling of the lower link to the structure and the upper link to the steering assembly has the advantage of allowing an effective range of suspension travel in conjunction with relatively direct and light steering. The steering forces are transmitted to the wheel directly by way of the upper link, but without the negative consequence of feedback of suspension loads entirely into the assembly. The suspension loads are instead applied to the vehicle at two spatially distinct points, one at the steering assembly and one at the structure, which assists dissipation of the forces generated by the loads and reduces susceptibility to shear-induced failure at critical construction locations, such as the junction of the steering assembly and the structure. A related advantage is that downwardly directed loads exerted on the steering assembly are not transmitted directly and in their entirety through the leg to the wheel, but are largely fed through the lower link to the structure. For preference, the resilient means is connected to and acts between the lower link and the structure. The connection of the resilient means, which can comprise a spring and damper unit preferably of coaxial construction, to the structure has the consequence that the latter functions as a mount for the resilient means and thereby reduces the unsprung weight of the moving parts of the suspension linkage, i.e. leg and links, by comparison with known suspension systems. Similarly, the connection of the resilient means, together with the lower link, to the structure rather than to the steering assembly ensures that the steering inertia is not unduly increased by suspension components. This results in a lighter and more responsive action of the steering assembly. Preferably, the lower link is connected to the leg by connecting means enabling the movement of the leg with the steering assembly. The connecting means can be a universal joint, which is thus able to accommodate relative movement of the lower link and the leg about two mutually angled, for example mutually perpendicular, axes during upward and downward displacement of the leg and wheel relative to the structure to allow suspension travel and also turning of the leg and wheel relative to the structure to allow directional change in response to a steering input. For preference, the leg is coupled to the structure by the lower link to be rotatable substantially about the steering axis in any setting of the linkage. Due to rotation of the leg, for steering purposes, substantially about the steering axis in any setting of the linkage the wheel carried by the leg can rotate about an axis which, disregarding compression or extension of the suspension, remains in substantially the same plane over the full range of steering deflection. The links can, for example, be designed with a torsional or telescopic capability or other form of compliance to counteract any slight change in steering feel otherwise detectable in settings of the linkage when the axis of rotation of the leg and the steering axis are non-coincident. In one preferred arrangement which accommodates the arcuate locus of the point of connection of the lower link to the leg, the axes are coincident simply in at least one setting of the linkage, which can be an end or intermediate setting. Preferably, the coincidence is in two predetermined settings, for example the two end settings or an end setting and intermediate setting. The particular relationship of the axes in the different settings of the linkage can be selected in accordance with preferred steering feel at different ride heights of the vehicle. In a preferred embodiment, the linkage is arranged to cause the wheel carried by the leg to travel at least initially on an arcuate path towards the structure during upward movement of the leg from a lowermost position thereof. This arrangement of the linkage offers the advantage that the wheel is able to move backwards with respect to the direction of forward travel of the vehicle to encourage the linkage to deflect against the resistance of the resilient means, thus to stimulate the spring action of the suspension in response to shock loading of the wheel. The backward relocation of the wheel represents wheel trail, i.e. rearward offset of the wheel-to-ground contact point relative to the point of intersection of the steering axis with the ground, which reduces wheel nervousness during forward motion. In the case of dive of the vehicle onto the suspension as in, for example, braking, the consequent change in angle of the steering axis displaces the ground intersection point rearwardly, which is matched by the rearward displacement of the wheel contact point under compression of the suspension so that wheel trail is maintained at a substantially constant value. In order to achieve a linkage geometry enabling such rearward movement of the wheel, the links are preferably arranged to extend convergently in direction towards the leg in the lowermost position thereof. In that case, the lower link is preferably longer than the upper link and preferably the lower link extends upwardly and the upper link downwardly towards the leg in the lowermost position thereof. The geometry resulting from this relationship of link relative angle, relative length and orientation with respect to the leg has the effect of causing the wheel to move upwardly on a defined arc with a progressively reducing component of rearward displacement. Variation of this movement can be achieved by variation of one or more of the parameters of the mentioned relationship. The movement can, if desired, be rectilinear instead of curvilinear. In an advantageous construction of the suspension components, the leg has relatively angled upper and lower portions which include an angle of less than 180° therebetween at a side of the leg remote from the structure. The lower portion of the leg can thus extend generally parallel to the steering axis, but with sufficient space for the upper link to extend between the steering assembly defining that axis and the free end of the angled-away upper portion of the leg. The upper portion in that case can join the lower portion in the region of the point of connection of the leg to the lower link. The leg preferably defines a fork receiving the wheel so as to enable support of the wheel axle at both ends, but a cantilever mounting of the axle is equally possible. Either or each of the upper link and the lower link can have the form of a wishbone connected at the free ends of its arms to the steering assembly in the case of the upper link or the structure in the case of the lower link. A balance of rigidity and lightness can be achieved if at least one of the leg, upper link and lower link is a hollow body. Expediently, the structure is a frame and the steering assembly includes handlebars, the vehicle being, for example, a pedal-driven two-wheel vehicle such as a bicycle or a motor- driven two-wheel vehicle such as a motorcycle or scooter. The vehicle can, however, be of any kind utilising a steerable wheel with sprung suspension, whether the wheel is at the front or at the back of the vehicle. An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic elevation of part of a vehicle, in particular a bicycle, embodying the invention and showing a wheel suspension of the vehicle fully extended; Fig. 2 is a view similar to Fig. 1 , but showing the suspension fully compressed; and Figs. 3A, 3B and 3C are diagrams schematically illustrating the settings of different components of the suspension in fully extended, intermediate and fully compressed states of the suspension. Referring now to the drawings there is shown in Fig. 1 part of a bicycle 10 comprising a frame 11 , which includes an obliquely extending down tube 11a and a sleeve-like head tube 11 b connected to the down tube 11a, and a steering assembly 12, which includes a steering tube 12a joumalled in the head tube 11b to be pivotable about a steering axis S and handlebars 12b fixed to the steering tube 12a. The frame 11 and steering assembly 12 carry a sprung suspension 13 for a front wheel 14 rotatable on a wheel axle 14a. The suspension 13 comprises a leg 13a mounting the axle 14a at the lower end of the leg, a lower link 13b connected at one end to the down tube 11a of the frame 11 and at the other end to the leg 13a intermediate its ends, an upper link 13c connected at one end to the steering tube 12a of the steering assembly 12 and at the other end to the upper end of the leg 13a, and a coaxial spring and damper unit 13d connected at one end to the lower link 13b intermediate the ends thereof and at the other end to the down tube 11a of the frame 11. The connections of the lower link 13b to the down tube 11a, the upper link 13c to the steering tube 12a and the leg 13a, and the unit 13d to the lower link 13b and down tube 11a are each provided by a bearing, for example a sealed ball bearing, or an axle shaft or bush enabling relative pivotal movement of the interconnected components about a respective horizontal axis. The connection of the lower link 13b to the leg 13a is provided by a sealed spherical joint enabling universal relative movement of the interconnected components. The thus interconnected leg 13a and links 13b and 13c co-operate with sections of the down tube 11a, head tube 11b and steering tube 12a to form a four-bar linkage permitting upward and downward movement of the wheel 14 between a lowermost position determined by full extension or droop of the suspension 13 as depicted in Fig. 1 and full compression or bump of the suspension as depicted in Fig. 2. Compression of the suspension is resisted by spring force supplied by the spring and damper unit 13d, and extension of the suspension under the spring force is damped by damping action of the same unit. The universal joint connecting the leg 13a to the lower link 13b permits turning of the leg and wheel 14 together with the steering assembly 12, the turning force being transmitted to the leg directly by way of the upper link 13c. The centre of rotation of the spherical joint has an arcuate locus during movement of the suspension and this locus is preferably arranged to coincide with the steering axis S in the end settings of the suspension. However, it may equally well be arranged to coincide with the axis S in, for example, the fully compressed setting and in a setting intermediate the fully compressed and fully extended end settings, such as an intermediate setting associated with a normal ride height of the bicycle frame as determined by the sprung weight inclusive of a nominal rider weight and the resistance offered by the spring force of the unit 13d. The leg 13a has a forked lower portion receiving the wheel 14 and a forked upper portion angled relative to the lower portion to extend away from the steering assembly 12 and offer space for the upper link 13c, the two portions being part of a hollow body of fabricated construction. The lower link 13b consists of a hollow wishbone provided at the free ends of its arms with journal eyes for bearings producing the pivot connection to the down tube 11a and at a spacing from the free ends of the arms with further journal eyes for an axle bush producing the pivot connection to the spring and damper unit 13d. At the united ends of its arms the wishbone has one of two co-operating components of the spherical joint, the other component of which is mounted on the leg at the junction of its relatively angled upper and lower portions. The upper link 13c consists of a hollow wishbone provided at the free ends of its arms with journal eyes for bearings producing the pivot connection with the steering tube 12a and at the united ends of its arms with a receptacle for axles of bearings producing the pivot connection to the top of the leg 13a. Details of the above-described constructions of the leg and links inclusive of pivot connections are not shown in the drawings, as such constructions are merely examples of diverse possibilities. The operation of the spring suspension and additional aspects of the layout of the four-bar linkage are evident from the diagrams forming Figs. 3A to 3C, in which the reference numerals used in Figs. 1 and 2 to designate constructional elements are employed to designate the corresponding bars, including the reference combination 11a, 11 b, 12a for the bar represented by parts of the frame and steering assembly. For the sake of clarity, the spring and damper unit has been omitted from Figs. 3A to 3C, but the points of connection with the relevant elements of the linkage are indicated by centre lines. Similarly, for consistency with Figs. 1 and 2 three of the four bars are shown as solid bodies, but the following description of relationships considers the links merely as straight lines between the respective connecting points. In the fully extended state of the suspension or linkage shown in Fig. 3A, the lower link 13b is angled upwardly and the upper link 13c angled downwardly towards the leg 13a, as indicated in each instance by an associated dashed horizontal datum line. The two links thus extend convergently in direction towards the leg 13a. In addition, the lower link 13b is longer than the upper link 13c. The loci of the centre points of the pivot connections of the lower and upper links to the leg during travel of the suspension are represented by dashed arcs, as is the locus of the axle 14a - and hence the wheel 14 - during that travel. The locus of the centre point of the connection of the lower link with the leg is coincident with the steering axis S at the ends of the respective arc. On upward deflection of the leg 13a to produce partial compression of the suspension, which may equate with the normal loaded state of the bicycle in use, the suspension is compresses into a setting such as that shown in Fig. 3B, in which the lower and upper links 13b and 13c are approximately parallel. In the transition from the state of Fig. 3A to that of Fig. 3B, the parts of the arcs described by the centre points of the connections of the links to the leg are so positioned on the respective notional circles containing those arcs that the point associated with the lower link moves comparatively steadily towards and that associated with the upper link initially moves away from and subsequently increasingly towards an arbitrary vertical reference plane to the left of the arcs. As a consequence, the leg 13a is tipped in such a manner during upward travel that the axle 14a travels not only upwardly, but also towards the reference plane, i.e. the wheel moves towards the frame 11 or counter to the direction of forward travel of the bicycle. This rearward movement of the wheel assists the tendency of the suspension to compress and, through maintenance of constant wheel trail, enhances the handling feel of the bicycle. As apparent from the reduced spacing of the indicated connection points of the spring and damper unit 13d, the unit is compressed during the upward travel of the leg and wheel and thereby generates a spring force progressively opposing the suspension compression. Finally, in the fully compressed state of the suspension as illustrated in Fig. 3C, the lower and upper links 13b and 13c extend divergently towards the leg 13a. The centre points of the connections of the links to the leg have, in the transition from the state of Fig 3B to that of Fig. 3C continued to move towards the mentioned reference plane, with the rate of movement of the centre point associated with the upper link accelerating to overtake that of the centre point associated with the lower link due to the shorter length of the upper link and consequently smaller radius of the described arc. The consequence is progressive decrease in the rate of rearward movement of the wheel. Through variation in the relative lengths of the lower and upper links and respective angles relative to the horizontal in a selected, for example fully extended, setting of the suspension, the upward and downward travel characteristics of the wheel can be varied to meet specific requirements. The leg 13a together with the wheel can be readily turned by the steering assembly 12 in any setting of the suspension. The slight departure of the centre of rotation of the spherical joint connecting the lower link 13b to the leg 13a may manifest itself as slight tilting of the wheel during turning, but any influence on the feel of the steering when the steering assembly is turned can be virtually eliminated by, for example, a torsional compliance or the like incorporated in the upper link. In addition to the already-mentioned advantages, the use of rotary bearings results in lower stiction and friction by comparison with linear bearings commonly used in conventional telescopic fork systems and facilitates sealing against ingress of contaminants. The suspension action is unaffected by wheel hub width tolerances by comparison with telescopic forks, in which excess or reduced hub width may lead to non- alignment of the fork legs and consequently an increase in stiction and friction. Moreover, whereas shock forces which are out-of-line with the leg axis can induce binding in the case of telescopic forks, the linkage suspension described in the foregoing is able to accept such forces largely without such problems.

Claims: CLAIMS 1. A vehicle provided with a steering assembly arranged on a structure to be pivotable relative thereto about a steering axis and with a spring suspension for a wheel steerable by the assembly, wherein the suspension comprises a leg carrying the wheel, a lower link and an upper link connected with the leg above the wheel and coupling the leg to, respectively, the structure and the steering assembly, the leg and the links together with the structure and the steering assembly forming a four-bar linkage permitting upward and downward movement of the leg relative to the structure and the leg being coupled to the structure by the lower link to be movable with the steering assembly on pivotation thereof, and resilient means arranged to resist the upward movement of the leg. 2. A vehicle as claimed in claim 1 , wherein the resilient means is connected to and acts between the lower link and the structure. 3. A vehicle as claimed in claim 1 or claim 2, wherein the resilient means comprises a spring and damper unit. 4. A vehicle as claimed in any one of the preceding claims, wherein the lower link is connected to the leg by connecting means enabling the movement of the leg with the steering assembly. 5. A vehicle as claimed in claim 4, wherein the connecting means comprises a universal joint. 6. A vehicle as claimed in any one of the preceding claims, wherein the leg is coupled to the structure by the lower link to be rotatable substantially about the steering axis in any setting of the linkage. 7. A vehicle as claimed in any one of the preceding claims, wherein the leg is rotatable about an axis coincident with the steering axis in at least one setting of the linkage. 8. A vehicle as claimed in claim 7, wherein the at least one setting of the linkage is one of its end settings or a setting intermediate its end settings. 9. A vehicle as claimed in claim 7 or claim 8, wherein the axis of rotation of the leg is coincident with the steering axis in two predetermined settings of the linkage. 10. A vehicle as claimed in any one of the preceding claims, wherein the linkage is arranged to cause the wheel carried by the leg to travel at least initially on an arcuate path towards the structure during upward movement of the leg from a lowermost position thereof. 11. A vehicle as claimed in claim 10, wherein the links are arranged to extend convergently in direction towards the leg in the lowermost position thereof. 12. A vehicle as claimed in claim 11 , wherein the lower link is longer than the upper link. 13. A vehicle as claimed in claim 11 or claim 12, wherein the lower link extends upwardly and the upper link downwardly towards the leg in the lowermost position thereof. 14. A vehicle as claimed in any one of the preceding claims, wherein the leg has relatively angled upper and lower portions which include an angle of less than 180° therebetween at a side of the leg remote from the structure. 15. A vehicle as claimed in claim 14, wherein the upper portion joins the lower portion in the region of the point of connection of the leg to the lower link. 16. A vehicle as claimed in any one of the preceding claims, wherein the leg defines a fork receiving the wheel. 17. A vehicle as claimed in any one of the preceding claims, wherein the upper link has the form of a wishbone connected at the free ends of its arms to the steering assembly. 18. A vehicle as claimed in any one of the preceding claims, wherein the lower link has the form of a wishbone connected at the free ends of its arms to the structure. 19. A vehicle as claimed in any one of the preceding claims, wherein at least one of the leg, upper link and lower link is a hollow body. 20. A vehicle as claimed in any one of the preceding claims, wherein the structure is a frame and the steering assembly includes handlebars. 21. A vehicle as claimed in any one of the preceding claims, wherein the vehicle is a pedal-driven or motor-driven two-wheel vehicle.

Inventors: WARD ADRIAN ROGER, WHYTE JON FRANK ROSS, WARD, ADRIAN, ROGER, WHYTE, JON, FRANK, ROSS
Applicants: ATB SALES LTD, WARD ADRIAN ROGER, WHYTE JON FRANK ROSS, ATB SALES LIMITED, WARD, ADRIAN, ROGER, WHYTE, JON, FRANK, ROSS
Application year: 2000
Main classification: B62K25
IPC codes: B62K25