Steering device for a motor vehicle with four steered wheels

Abstract: In the proposed steering device a drive shaft is operationally connected to the front wheel steering mechanism and to a driven shaft by way of motion step-down means; the two shafts are provided with respective cranks, and with respective pins which engage in a link element operationally connected to steering levers for the rear wheels.

Description: The invention relates to a steering device for a motor vehicle with four steered wheels, which is able to vary the size and direction of the steering angle of the rear wheels relative to the steering angle of the front wheels as a function of the angle of rotation of the steering wheel. In motor vehicles with conventional steering devices applied only to the front wheels, lateral guide forces directed towards the interior of the curve are generated between the obliquely-disposed front wheels and the ground on turning the steering wheel. These forces give rise to a yawing torque which causes the motor vehicle to turn about its barycentre and start to change its direction. The rear wheels, which are aligned with the vehicle body, then also change direction so that respective lateral forces are generated between the rear wheels and the ground, by virtue of which the vehicle is able to undergo controlled direction change. Thus, after the transient curve-initiation stage the vehicle begins to take the curve under stable conditions, and follow the required path. Consequently, a motor vehicle begins to move uniformly along a curve only with a certain delay after the turning of the steering wheel. This delay, due to the fact that the lateral guide forces on the rear wheels are generated after those acting on the front wheels, is inevitable if the rear wheels remain aligned with the vehicle body and are unable to change direction relative to this body simultaneously with the front wheels. These considerations have led to the development of so-called integral steering devices which enable both the front wheels and rear wheels to be steered so that the lateral guide forces can arise simultaneously on all four vehicle wheels. Generally the known devices, which are mechanical, hydraulic or electrical, are structured so that the rear wheels are steered in phase with, ie. in the same direction as, the front wheels for small steering wheel turning angles and medium and high vehicle speeds. The result is that the vehicle becomes easier to handle because its directional stability is increased. With many of the known devices the rear wheels are steered in counterphase with, ie in the opposite direction to, the front wheels for large steering wheel turning angles and low vehicle speeds. This improves the manoeuvrability of the vehicle during parking because it reduces the minimum steering radius. A device of this type and having the features of the preamble of claim 1 is known from GB-A-2 130 986. In this known device the drive shaft and the driven shaft are rotatably connected to each other by respective gearwheels on which respective pins are eccentrically provided. To the pins respective link elements are pivotally connected, which are also connected to one another. One of the link elements is mechanically connected to the steering levers of the rear-wheels of the vehicle, so that on rotating the shafts, the mutual connection between the various elements provides the desired steering of the rear wheels. The known device is however rather complicated and bulky, hardly flexible and not without operational criticism. The object of the present invention is to provide an integral steering device which is of simple construction, reliable and safe in operation, and easily adapted to different performance requirements. This object is achieved by a steering device having the features claimed in claim 1. Preferably said motion transmission means are formed from a pair of motion step-down gearwheels, one rigid with the drive shaft and the other rigid with the driven shaft, and in mutual engagement. According to a preferred embodiment, said link element is connected at one end by a third pin to a rod supported in an axially slidable manner in the vehicle body and mechanically connected to said rear wheel steering levers. With the device according to the invention it is possible to simultaneously steer the front wheels and rear wheels so that for small steering wheel turn angles said wheels are orientated in the same direction or in phase, whereas for large steering wheel turn angles the rear wheels assume an opposite or counterphase orientation relative to the front wheels. The device is constructionally very simple and is safe in operation. Characteristics and advantages of the invention are described hereinafter with reference to Figures 1 to 6 which show a preferred embodiment of the invention by way of non-limiting example. Figure 1 is a diagrammatic representation of an integral steering device according to the invention; Figure 2 is in enlarged longitudinal section through a detail of the device on the line II-II of Figure 1, with some components shown in full view for greater clarity; Figure 3 is a section through the same detail on the line III-III of Figure 2, with some components shown in full view for greater clarity; Figure 4 is a graphical representation of the variation in steering angles obtainable with the device of Figure 1; Figures 5 and 6 are views similar to that of Figure 3 but to a smaller scale and with the link element in its respective maximum travel positions. Figure 1 shows the front wheels 10 and the rear wheels 11 of a motor vehicle. The reference numerals 12 and 13 indicate the wheel axes, the figure not showing the suspensions which connect the hub assemblies of the wheels 10 and 11 to the vehicle body, which is also not shown. The steering mechanism for the front wheels 10 comprises steering levers 14 fixed to the hub assemblies of the wheels 10 and hinged to lateral tie bars 15, which are connected by Ball joints 17 to a rack 16. The rack 16 is supported axially slidable in the vehicle body and engages a pinion 18 rigid with the steering column 19 and steering wheel 20. Suitable toothing 16a of the rack 16 also engages a gearwheel 22 to obtain a desired step-down ratio. The gearwheel 22 is rigid with a longitudinal drive shaft 23 which is rotatably supported at 24 in the vehicle body. A further drive shaft 25, clearly visible in Figure 2, is rotatably connected with the drive shaft 23 by means of splines 21. A pinion 26 is rigid with the drive shaft 25 and engages a gearwheel 27 rigid with a driven shaft 28. A step-down motion transmission is therefore formed between the shafts 25 and 28. Cranks 29 and 30 are rigid with the shafts 25 and 28 and are provided at their ends with pins 31 and 32, these therefore being eccentric to said shafts 25 and 28. Specifically, the pins 31 and 32 have different eccentricities with respect to the axes of their shafts 25 and 28, the pin 31 associated with the drive shaft 25 having a greater eccentricity than the pin 32 associated with the driven shaft 28, the crank 29 therefore having a greater crank radius than the crank 30. The shafts 25 and 28 are rotatably supported by rolling-contact bearings 33 and 34 in a casing 35, which is fixed to the vehicle body. The casing 35 is formed from a cap 36, a substantially annular wall 37 and a cover 38, which are fixed together by bolts such as those indicated by 39 in Figure 2, passing through bored logs such as those indicated by 40 in Figure 3. A link element 41 is provided with a longitudinal substantially rectangular slot 42 with which the pins 31 and 32 slidingly engage by way of respective bushes 43 and 44. Each bush 43 and 44 comprises externally a pair of flat faces 45 and 46 respectively, visible in Figure 3, which form sliders for guiding the axial sliding of the pins 31 and 32 in the slot 42 of the link element 41. Spacers 47 and 48 are formed integrally with the bushes 43 and 44, and a further spacer 49 and a snap ring 50 axially retain the pin 31 in the slot 42. The link element 41 is provided at one end with two arms 51 containing holes 52. A rod 53 is gripped between the two arms 51, and for this purpose comprises two external flat opposing faces indicated by 54. A pin 55 axially retains the rod 53 between the two arms 51 by means of two snap rings 56. The rod 53 is supported so that it can slide axially, by way of plain bearings 57, in tubes 58 rigid with the annular wall 37 of the casing 35, as shown in Figure 3. As shown in Figure 1, the rod 53 is connected by ball joints 59 to tie bars 60 which are pivoted to steering levers 61 fixed to the hub assemblies of the rear wheels 11. When the motor vehicle enters the curve, the driver turns the steering wheel 20, for example to the right in the direction of the arrow c of Figure 1, or to the left, according to the direction of the curve, and correspondingly the front wheels 10 steer in the same direction of rotation as the steering wheel 20. The relationship between the angle of rotation of the steering wheel 20 and the steering angle of the wheels 10 depends on the step-down ratio between the pinion 18 and rack 16. In the diagram of Figure 4, in which the horizontal axis represents the angle of rotation α v of the steering wheel 20 and the vertical axis represents the steering angle α r of the wheels, the straight line "a" shows indicatively how the steering angle of the front wheels 10 varies with the angle of rotation of the steering wheel 20. This steering angle α r can reach a maximum of 30°, for a steering wheel rotation angle α v of about 600°. With the described device, movements of the rack 16 result in corresponding rotations of the drive shafts 23 and 25 (in the direction of the arrow d if the steering wheel is turned in the direction of the arrow c) and, in the opposite direction, of the driven shaft 28. The consequent angular movements of the respective cranks 29 and 30 and of their pins 31 and 32 induce oscillations of the link element 41 and consequent sliding of the rod 53 about its neutral position (arrows e). The curve "b" of Figure 4 shows the relationship between the steering angle of the rear wheels 11 and the steering angle of the steering wheel 20, and thus also of the front wheels 10. Specifically, the rear wheels 11 are made to steer in phase with, ie in the same direction as, the front wheels below a predetermined angle of rotation of the steering wheel 20, for example 360°, whereas said rear wheels 11 are made to steer in counterphase with, ie in the opposite direction to, the front wheels 10 above said predetermined angle of rotation of the steering wheel 20. This steering angle can attain a maximum of 2°-4° in phase for a steering wheel rotation angle of about 360°, whereas it can reach a maximum of 5°-10° in counterphase for a steering wheel rotation angle of about 600°. In Figure 3 the link element 41 is shown in its neutral position with the steering wheel 20 also in the neutral position. Their relative positions are set using suitable reference marks, not shown. The link element 41 is shown in Figure 5 in its position of maximum phase travel from its neutral position, and in Figure 6 in its position of maximum counterphase travel from its neutral position. The illustrated device is of very simple construction and is particularly reliable and safe in operation. It will be apparent that the illustrated device is easily adapted to different performance requirements, in that different relationships between the rear wheel steering angle and the front wheel steering angle can be obtained by varying the step-down ration between the shafts 25 and 28, and/or providing cranks 29 and 30 with different crank angles.

Claims: A steering device for a motor vehicle with four steered wheels, comprising at least one drive shaft (25) rotatably supported in the motor vehicle body and operationally connected to the steering mechanism (16,18) for the vehicle front wheels (10) by first motion transmission means (16a, 22), said drive shaft (25) being further connected by second motion transmission means (26,27) to a driven shaft (28), the drive shaft (25) and the driven shaft (28) being operationally connected to the steering levers (61) for the vehicle rear wheels (11) through a link member (41) mechanically connected with said levers (61), characterised in that each of the two shafts (25,28) is provided with a respective eccentric pin (31,32) and in that said pins (31,32) slidably engage a longitudinal slot (42) in the link member (41), the link member (41) being connected to the steering levers (61) by means of a rod (53) supported in an axially slidable manner in the vehicle body. A steering device as claimed in claim 1, characterised in that said second motion transmission means (26, 27) are formed from a pair of step-down gearwheels, one (26) rigid with the drive shaft (25) and the other (27) rigid with the driven shaft (28), and in mutual engagement. A steering device as claimed in claim 1, characterised in that the eccentric pins (31, 32) are disposed with different eccentricity relative to the axis of their respective shafts (25, 28). A steering device as claimed in claim 3, characterised in that the pin (31) associated with the drive shaft (25) has greater eccentricity than the pin (32) associated with the driven shaft (28). A steering device as claimed in one of the preceding claims, characterised in that said eccentric pins (31, 32) are made rigid with their respective shafts (25, 28) by means of respective cranks (29, 30). A steering device as claimed in claim 1, characterised in that said pins (31, 32) bear respective bushes (43, 44) provided externally with a pair of flat faces (45, 46), to act as guide sliders for said pins (31, 32). A steering device as claimed in claim 1, characterised in that said link element (41) is provided at one end with two arms (51) which are connected by a third pin (55) to the rod (53).

Inventors: VISCONTI MARCO, VISCONTI, MARCO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1989
Main classification: B62D7
IPC codes: B62D7