Steering Wheel

The present invention relates to a steering wheel for a vehicle and, more particularly, to a steering wheel of the type that includes a hub section comprising first and second metallic hub members to be secured to a steering shaft in mutually axially aligned relationship with each other and a spoke core clamped between the first and second metallic hub members.

Japanese Utility Model Pre-Examination publication No. 59473/81 discloses a steering wheel of the type specified above. The steering wheel disclosed in the Japanese publication includes a pair of metallic annular hub members having mating surfaces shaped to define therebetween channels which receive radially inner ends of spoke cores for connecting the hub members to a rim section of the steering wheel. The metallic hub members are secured together by means of screws to fix the spoke cores to the hub members.

The steering wheel of the prior art, however, has problems that the ends of the spoke cores fixed to hub members tend to be loosened relative to the hub members and that the use of screws to secure the hub members together not only complicates the manufacturing steps due to the necessity for drilling screw holes in the hub members and screwing screws into the thus drilled screw holes but also increases the number of component parts of a steering wheel.

The present invention aims at providing a steering wheel of the type that is specified above and which is improved to prevent loosening of spoke cores relative to hub members and facilitate simplified assembling steps.

The steering wheel according to the present invention comprises a rim section including a rim core of a metal, a hub section including a metallic hub adapted to be secured to a steering shaft of a vehicle, and at least one spoke core interconnecting the metallic hub to the rim core. The metallic hub comprises a first hub member of a metal defining therein a first through-hole for the steering shaft and a second hub member of a metal defining therein a second through-hole for the steering shaft and disposed in axially aligned relationship to the first hub member. The first and second hub members have opposed surfaces between which a portion of the spoke core is disposed and fixed to the hub members. The first and second hub members are secured together either by press-fitting engagement between the hub members or by welding of the first and second hub members and the portion of the spoke core. The hub members are provided with means for positioning the hub members one to the other.

The features of the present invention pointed out above eliminate the necessity for the use of screws to secure the hub members and spoke core together and, therefore, advantageously reduce the number of component parts of a steering wheel and simplify the steps of manufacture of the steering wheel.

The above and other objects, features and advantages of the present invention will be made more apparent by the following description with reference to the accompanying drawings.

FIG. 1 is a plan view of an embodiment of a steering wheel according to the present invention;

FIG. 2 is an enlarged fragmentary sectional view of the steering wheel taken along line II--II in FIG. 1;

FIG. 3 illustrates hub members in perspective views;

FIGS. 4A and 4B illustrates in sectional views manufacturing steps of assembling hub members and spoke cores together and securing them together;

FIG. 5 is similar to FIG. 3 but illustrates a first modification to the hub members shown in FIGS. 1-4;

FIGS. 6A and 6B are similar to FIGS. 4A and 4B illustrate steps of assembling the modified hub members and spoke cores together and securing them together;

FIG. 7 is similar to FIGS. 3 and 5 but illustrates further modified hub members;

FIGS. 8A and 8B are similar to FIGS. 6A and 6B but illustrate steps of assembling the further modified hub members and spoke cores together and securing them together;

FIG. 9 is similar to FIG. 7 but illustrates still further modified hub members;

FIGS. 10A and 10B are similar to FIGS. 8A and 8B but illustrate steps of assembling the still further modified hub members and spoke cores and securing them together;

FIG. 11 is similar to FIG. 9 but illustrates hub members of another embodiment of the invention;

FIGS. 12A and 12B are sections taken along line XII--XII in FIG. 11 and illustrate steps of assembling the hub members shown in FIG. 11 and a spoke core together and securing them together;

FIG. 13 illustrates in perspective views hub members and a modified spoke core of a further embodiment of the invention;

FIG. 14 is similar to FIG. 2 but illustrates a still further embodiment of the present invention;

FIG. 15 illustrates in perspective views pair of hub members employed in the embodiment shown in FIG. 14;

FIGS. 16A and 16B illustrate steps of assembling the hub members shown in FIG. 15 and spoke cores together and securing them together;

FIG. 17 is similar to FIG. 15 but illustrates hub members employed in a still further embodiment of the invention;

FIGS. 18A and 18B are sections taken along line XVIII--XVIII in FIG. 17 and illustrate the steps of assembling the hub members shown in FIG. 17 and a spoke core together securing them together;

FIG. 19 illustrates in perspective views hub members and a spoke core of a still further embodiment of the invention;

FIGS. 20A and 20B illustrate steps of assembling the hub members and spoke core shown in FIG. 19 and securing the together;

FIG. 21 a plan view of a still further embodiment of the steering wheel according to the present invention;

FIG. 22 is an enlarged fragmentary sectional view of the steering wheel shown in FIG. 21 taken along line XXII--XXII in 21; and

FIG. 23 in perspective views hub members of the embodiment shown in FIGS. 21 and 22.

Referring now to FIGS. 1-4B of the drawings, a steering wheel comprises a hub section 1, an annular rim section 2 and a pair of generally radially extending spokes 3 interconnecting the hub section and the rim section. The hub section 1 includes a metallic hub 5a adapted to be secured to a serrated end section 4a of a steering shaft 4 of a vehicle and to a tapered section 4b of the shaft 4. The spokes 3 include a pair of spoke cores 6a formed by lengths of metallic rods having generally arcuate central or intermediate sections which are so shaped as to form a generally circular bends 6a' at which the spoke cores 6 are secured to the outer periphery of the hub 5a. The spoke cores 6a have radially outer ends secured by welding to a rim core 7 of the rim section 2. The rim core 7 is covered with a covering 8 of a soft plastic material. The hub section 1 and the spokes 3 are respectively covered with outer coverings 9 and 10 of a plastic material.

The hub 5a comprises upper and lower hub members 12a and 14a secured together with the central bends of the spoke cores 6a firmly held therebetween, as will be best seen in FIGS. 3-4B. More specifically, the upper hub member 12a is annular and defines therein a serrated central through-hole 11 adapted to be engaged with the serrated end section 4a of the steering shaft 4. The lower hub member 14a is also annular and defines therein a tapered central through-hole 13 adapted to be engaged with the tapered section 4b of the steering shaft 4.

The lower end face of the upper hub member 12a directed to the lower hub member 14a is formed thereon with an annular engagement protrusion 15 extending around the serrated central through-hole 11 and having an outer diameter dimensioned to cause the engagement protrusion 15 to be contacted by the inner sides of the central bends 6a' of the spoke cores 6a. The upper surface of the lower hub member 14a directed to the upper hub member 12a is formed thereon with a similar annular engagement protrusion 16 extending around the tapered central through-hole 13 and having an outer diameter dimensioned to be contacted by the inner sides of the central bends 6a' of the spoke cores 6a. The annular engagement protrusions 15 and 16 project axially from the lower and upper surfaces of the upper and lower hub members 12a and 14a, respectively, and have annular end faces 15a and 16a in abutment engagement with each other, as best seen in FIG. 4B, so that a circumferential groove 17a is defined between the opposed surfaces 17 and 19 of the upper and lower hub members around the abutting annular engagement protrusions 15 and 16. The opposed upper and lower surfaces 17 and 18 of the circumferential groove 17a cooperate to grip therebetween the spoke cores 6a. For this purpose, these surfaces 17 and 18 are respectively provided with annular rows of radial serrations 18a which bite the round metallic rods of the spoke cores 6a to firmly hold them against movement relative to the hub members 12a and 14a. For this purpose, the upper and lower hub members 12a and 14a are made of a metallic material which is harder than the metallic material of the spoke cores 6a.

The upper hub member 12a is further provided with an annular projection 19a disposed between the annular engagement protrusion 15 and the central through-hole 11 and axially extending beyond the annular end face of the protrusion 15. The lower hub member 14a is provided with an annular recess 20a disposed between the central through-hole 13 and the annular engagement protrusion 16. The annular projection 19a and the annular recess 20a are dimensioned such that the projection 19a can be press-fitted into the recess 20a with the central bends 6a' of the spoke cores 6a held between the serrated gripping faces 17 and 18 of the upper and lower hub members 12a and 14a, as shown in FIG. 4A.

When the upper and lower hub members 12a and 14a and the spoke cores 6a are assembled, these members are relatively placed in a manner shown in FIG. 4A and the upper and lower hub members 12a and 14a are forced toward each other with a pressing force of about 1-2 tons to press-fit the annular projection 19a into the annular recess 20a until the end faces 15a and 16a of the engagement protrusions 15 and 16 of the upper and lower hub members 12a and 14a are brought into abutment engagement with each other. When this position is reached, the radial serrations 18a on the gripping faces 17 and 18 of the groove 17a firmly bite the central bends 6a' of the spoke cores 6a to rigidly secure the spoke cores 6a to the upper and lower hub members 12a and 14a, as shown in FIG. 4B.

This embodiment of the invention is advantageous in that, because the serrations 18a on the gripping faces 17 and 18 of the circumferential groove 17a are substantially perpendicular to the axis of the arcuate central bends 6a' of the spoke cores 6a, the gripping faces of the groove 17a are effective to hold the spoke cores 6a against rotation about the axis of the hub 5a. In addition, because the arcuate bends 6a' of the two spoke cores 6a are disposed on the diametrically opposite sides of the outer periphery of the hub 5a, the spoke cores 6a are prevented from being dislodged from the hub 5a radially outwardly thereof.

In the described and illustrated embodiment of the invention, the spoke cores 6a are made of rods of a metal of a mechanical strength sufficient to withstand a load applied to the steering wheel. A preferred example of the metallic material of the spoke cores 6a is a steel having a Brinell hardness of from 80 to 100 under JIS (Japanese Industrial Standard). The upper and lower hub members 12a and 14a of the hub 5a are made of a metal harder than the metallic material of the spoke cores 6a. A preferred metallic material of the hub members 12a and 14a is a steel of a hardness higher than Brinell hardness of 120 under JIS. The height or axial dimension of each of the serrations 18a on the gripping surfaces 17 and 18 of the upper and lower hub members 12a and 14a may be from 0.3 to 1.0 mm and preferably 0.5 mm, while the diameter of the metallic rod from which the spoke cores 6a are prepared may have a diameter of from 9 to 10 mm.

In a modification shown in FIGS. 5-6B, the annular end faces 15a and 16a of the annular engagement protrusions 15 and 16 are respectively formed thereon with annular rows of radial serrations 31 which, when the upper and lower hub members 12a and 14a are forced toward each other from a position shown in FIG. 6A to a position shown in FIG. 6B to press-fit the annular projection 19a of the upper hub member 12a into the annular recess 20a in the lower hub member 14a, are brought into meshing engagement with each other to lock the upper and lower hub members 12a and 14a against relative rotation.

In a further modification shown in FIGS. 7-8B, the outer peripheral surface of the annular projection 19a of the upper hub member 12a is formed thereon with an annular row of axial serrations 33, while the inner prepheral surface of the annular recess 20a is formed thereon with an annular row of axial serrations 34 adapted to be in meshing engagement with the serrations 33 on the annular projection 19a of the upper hub member 12a when the upper and lower hub members 12a and 14a are forced toward each other from a position shown in FIG. 8A to a position shown in FIG. 8B. Thus, the axial serrations 33 and 34 are also effective to lock the upper and lower hub members 12a and 14a against relative rotation.

In a still further modification shown in FIGS. 9-10B, the projection 19a and the recess 20a are not annular but of complementary polygons so that the projection 19a can be press-fitted into the recess 20a to lock the upper and lower hub members 12a and 14a against relative rotation. In addition, the heights of the radial serrations 18a on the gripping surfaces 17 and 18 are increased radially outwardly so that the gripping surfaces 17 and 18 can more reliably grip the spoke cores 6a against radially outward removal from the hub 5a.

FIGS. 11-12B show another embodiment of the invention in which a pair of spoke cores, only one of which is shown at 6b, have their radially inner ends secured to a hub 5b which comprises a pair of upper and lower hub members 12b and 14b. The upper hub member 12b is formed therein with a serrated central through-hole 11, as in the preceding embodiment of the invention. The lower hub member 14b is also formed therein with a tapered through-hole 13, also as in the preceding embodiment. The distinction of the embodiment shown in FIGS. 11-12B is that the bottom surface of the upper hub member 12b and the upper surface of the lower hub member 14b are formed therein with diametrically extending grooves 36 each having a substantially semi-circular cross-section so that, when the upper and lower hub members 12b and 14b are assembled and secured together as shown in FIG. 12B, the grooves 36 in the upper hub member 12b cooperate with the grooves 36 in the lower hub member 14b to define a pair of radially extending channels 17b in which the radially inner ends of spoke cores 6b are received. The inner peripheral surfaces of the radial hole 17b are formed thereon with serrations 18b extending circumferentially of the axes of the radial holes 17b so that the channels 17b can firmly grip the radially inner ends of the spoke cores 6b. For this purpose, the hub members 12b and 14b are made of a material harder than the material of the spoke cores 6b, as in the preceding embodiment of the invention. The upper hub member 12b is further provided with a pair of generally square recesses 20b formed on the bottom surface of the upper hub member 12b and spaced at circumferentially substantialy equal intervals relative to the grooves 36. The upper surface of the lower hub member 14b is formed thereon with a pair of generally square projections 19b which are disposed and dimensioned such that, when the upper and lower hub members 12b and 14b are forced toward each other from a position shown in FIG. 12A to a position shown in FIG. 12B, the projections 19b are press-fitted into the recesses 20b to rigidly secure the two hub members and the spoke cores together so that the serrations 18b on the inner peripheral surfaces of the radial holes 17b bite the radially inner ends of the spoke cores 6b to firmly grip them again radially outward removal from the holes 17b.

FIG. 13 shows a further embodiment of the invention which is distinguished from the preceding embodiments in that this embodiment employs a spoke core 6c formed of a web of a metal and designed not to be covered with any decorative covering. In other words, the outer surfaces of the spoke core 6c are exposed when in use. The spoke core 6c is pinched between upper and lower hub members 12c and 14c which form a hub 5c. The bottom surface of the upper hub member 12c is formed therein with a serrated central through-hole 11, as in the preceding embodiments of the invention and, in addition, with a pair of substantially circular recesses 20c disposed in diametrically opposed relationship with each other. The upper surface of the lower hub member 14c is formed thereon with a pair of projections 19c disposed in diametrically opposed relationship with each other and on the opposite sides of a tapered through-hole 13 formed in the lower hub member 14c, as in the preceding embodiments of the invention. The projections 19c have peripheral surfaces formed with axial serrations 40. The spoke core 6c is formed therein with a central hole 39 for a steering shaft, not shown in FIG. 13, and with a pair of smaller holes 38 disposed on the opposite sides of the steering shaft hole 39. The recesses 20c in the upper hub member 12c, the projections 19c on the lower hub member 14c and the smaller holes 38 in the spoke core 6c are so dimensioned and positioned that, when the upper and lower hub members 12c and 14c are forced toward each other as in the preceding embodiments with the spoke core 6c placed between the upper and lower hub members 12c and 14c, the projections 19c extend through the smaller holes 38 in the spoke core 6c and are press-fitted into the recesses 20c to secure the three members together. In order to further securely fix the spoke core 6c to the upper and lower hub members 12c and 14c, the hub members are formed thereon with small pointed projections 18c which bite the spoke core 6c. For this purpose, the hub members 12c and 14c are formed from a material harder than the material from which the spoke core 6c is formed.

A still further embodiment of the steering wheel of the present invention is shown in FIGS. 14-16B in which parts similar to those of the embodiment shown in FIGS. 1-4B are designated by similar reference numerals followed by "d" to save repeated descriptions of similar parts of the steering wheel. The difference only will be described hereunder.

The most important difference of the embodiment shown in FIGS. 14-16B from the preceding embodiments is that a central arcuate potion 6d' of a spoke core 6d is secured to upper and lower hub members 12d and 14d of a hub 5d by welding. Compared with the embodiments shown in FIGS. 1-13, therefore, the hub members 12d and 14d are not provided with any projections and recesses to be press-fitted together to secure the upper and lower hub members.

More specifically, the upper and lower hub member 12d and 14d have annular engagement protrusions 15d and 16d having annular end faces 15d' and 16d' to be disposed in abutment engagement with each other, as shown in FIG. 16B, so that an annular circumferential groove 17d' is defined between the upper and lower hub members 12d and 14d. The heights of the annular protrusions 15d and 16d are determined such that the width of the circumferential groove 17d', as measured in the axial direction of the hub 5d, is slightly smaller than the diameter of a metallic rod of the spoke core 6d. As an example, the spoke core 6d may be prepared from a metallic rod having diameter of from 9 to 10 mm which is greater, by from 0.3 to 1.0 mm, than the width or axial dimension of the circumferential groove 17d'.

When the upper and lower hub members 12d and 14d and the spoke core 6d are secured together, these members are first placed relative to each other in a manner shown in FIG. 16A; Namely, the hub members 12d and 14d are placed with the annular protrusions 15d and 16d axially aligned and being in contact with an inner side of a central arcuate bend 6d' of the spoke core 6d. Then, electrodes of a resistance welding machine (not shown) are applied to the upper and lower hub members 12d and 14d and are moved toward each other to force the hub members toward each other while an electric voltage is applied across the electrodes. Because the spoke core 6d is in electrical contact with the upper and lower hub member 12d and 14d, a welding current passes through the contacting portions of the upper and lower hub members and the spoke core so that these portions are fused. The pressure applied to the upper and lower hub members by the welding electrodes continues to move these members toward each other until the annular end surfaces 15d' and 16d' of the annular protrusions 15d and 16d are brought into abutment engagement with each other, as shown in FIG. 16B, whereby welding is completed.

During the welding operation, the stroke of the welding electrodes is not fixed but kept freely adjustable to assure that the pressure applied to the upper and lower hub members is adjustable. In addition, the welding current is adjusted during the welding. However, it will be appreciated that, because the spoke core 6d is in electrically conductive contact with the upper and lower hub members 12d and 14d over a very limited areas in the initial stage of the welding, the electrical resistance at these contacting areas is relatively high to insure that the contacting portions of the hub members and the spoke core are heated and fused while the hub members are continuously urged toward each other. Thus, the fused portions of the hub members and the spoke core are deformed to increase the areas of contact between these members with a resultant decrease in the electrical resistance at these contacting areas and thus decrease in the generation of heat at the contacting portions of the hub members and the spoke core until the annular end surfaces 15d' and 16d' of the annular engagement protrusions 15 d and 16d of the upper and lower hub members 12d and 14d abut each other to mechanically stop the movement of the hub members toward each other. In other words, the annular end surfaces 15d' and 16d' of the annular engagement protrusions 15d and 16d act as stops to properly position the upper and lower hub members when the welding is to be stopped.

As will be seen from the foregoing description, when the welding operation is conducted, the upper and lower hub members 12d and 14d are positioned in alignment with each other by the contact of the annular engagement protrusions 15d and 16d with the arcuate central bend 6d' of the spoke core 6d. In addition, the abutment engagement between the annular end surfaces 15d' and 16d' of the annular protrusions 15d and 16dassures that the hub 5d formed by the welded hub members 12d and 14d has a precisely controlled axial dimension regardless of whether the welding electrodes are new or not. Thus, the welding electrodes can be used all over their operative lives. In addition, the upper and lower hub members 12d and 14d can easily be prepared because the annular abutment protrusions 15d and 16d can easily be formed by machining one end face of each of the hub members.

FIGS. 17-18B show modified hub members 12e and 14e of a still further embodiment of the invention similar in structure to the embodiment described with reference to FIGS. 11-12B. Thus, the parts of the embodiment similar to those shown in FIGS. 11-12B are designated by similar reference numerals followed by "e". The difference of the embodiment shown in FIGS. 17-18B is that the hub members and spoke cores (only one of which is shown at 6e) are secured together by welding, as in the embodiment shown in FIGS. 15-16B. More specifically, axially opposed end surfaces 15e' and 16e' of the hub members 12e and 14e are formed therein with radial grooves 36e which, when the hub members are assembled and welded together as shown in FIG. 18B, cooperate to define radially extending channels 17e having substantially circular cross-sections and substantially smooth inner peripheral surfaces. The inner diameter of each hole 17e, however, is slightly smaller than the diameter of a metallic rod from which the spoke cores 6e are prepared. The opposed surfaces 15e' and 16e' of the upper and lower hub members 12e and 14e are provided with pairs of substantially square recesses 20e and projections 19e to be engaged with each other when the upper and lower hub members are assembled to precisely position the two hub members in the circumferential direction thereof.

When the upper and lower hub members 12e and 14e and the spoke cores 6e are welded, these members are placed relative to each other in a manner shown in FIG. 18A. Then, welding electrodes of a resistance welding machine (not shown) are applied to the hub members to conduct a resistance welding in such a manner as discussed in connection with the embodiment shown in FIGS. 14-16B until the upper and lower sides of the radially inner ends of the spoke cores 6e are welded to the inner surfaces of the radial grooves 36e. It will be noted that, because the upper and lower hub members 12e and 14e are urged toward each other by the welding electrodes, the axially opposed end surfaces 15e' and 16e' of the upper and lower hub members 12e and 14e are ultimately brought into abutment engagement with each other when the spoke core 6e and the hub members 12e and 14e are welded together. Thus, the abutment engagement between the end surfaces 15e' and 16e' are effective to assure that a hub 5e formed by the two hub members thus welded has a precisely controlled axial dimension.

A still further embodiment shown in FIGS. 19-20B is somewhat similar in structure to the embodiment described with reference to FIG. 13 in that a hub 5f is connected with a spoke core 6f which is formed of a web of a metal and is designed not to be covered with any decorative covering; namely, the spoke core 6f has its outer surfaces to be exposed when in use. However, the spoke core 6f is secured to the hub 5f by welding. More specifically, the hub 5f comprises upper and lower hub members 12f and 14f, as in the preceding embodiments. The upper and lower hub members 12f and 14f are formed therein with a serrated central through-hole 11f and a tapered central through-hole 13f, respectively, for the purpose already made apparent. The bottom end surface of the upper hub member 12f is formed with a substantially square engagement recess 20f disposed coaxially with the central through-hole 11f and having a bottom face 20f'. This recess 20f is surrounded by a plurality of concentric circumferential projections or ridges 34f formed on the bottom surface of the upper hub member 12f. The upper end surface of the lower hub member 14f is formed thereon with a substantially square engagement protrusion 19f disposed coaxially with the central through-hole 13f and having a top face 19f'. The protrusion 19f is designed to be press-fitted into the square recess 20f in the upper hub member 12f and is also surrounded by a plurality of circumferential projections or ridges 35f formed on the upper surface of the lower hub member 14f. The spoke core 6f is formed therein with a substantially square central hole 39f of a size which allows the square protrusion 19f to extend therethrough.

The square engagement recess 20f and protrusion 19f are dimensioned such that, when the protrusion 19f is press-fitted into the recess 20f until the top face 19f' of the protrusion 19f is contacted by the bottom face 20f' of the recess 10f, as shown in FIG. 20B, an annular space 17f is defined between the bottom surface of the upper hub member 12f and the upper surface of the lower hub member 14f. As an example, the width or axial dimension of the annular space 17f is smaller, by from 0.3 to 1.0 mm, than the thickness of the spoke core 6f which preferably is 4 mm.

When the spoke core 6f is secured to the upper and lower hub members 12f and 14f, the three members are assembled in a manner shown in FIG. 20A; namely, the upper and lower hub members are assembled so that the spoke core 6f is disposed between the two hub members and the engagement protrusion 19f extends through the central hole 39f in the spoke core 6f. Then, a pair of welding electrodes of a resistance welding machine (not shown) are applied to the upper and lower hub members 12f and 14f and are moved toward each other to urge the hub members toward each other while a welding voltage is applied across the welding electrodes, as in the welding operation already described above. Thus, a welding current flows through the circumferential ridges 34f of the upper hub member 12f and the upper surface of the spoke core 6f and through the lower surface of the spoke core and the circumferential ridges of the lower hub member 14f so that these portions are fused. The upper and lower hub members 12f and 14f are urged toward each other until the top face 19f' of the engagement protrusion 19f is engaged by the bottom face 20f' of the engagement recess 20f as shown in FIG. 20B. When this position is reached, the welding operation is completed. It is again noted that the abutment engagement between the top surface 19f' of the engagement protrusion 19f and the bottom face 20f' of the recess 20f is effective to assure that the hub 5f formed by the thus welded upper and lower hub members 12f and 14f can be given a precisely controlled axial dimension.

A still further embodiment of the steering wheel according to the present invention is shown in FIGS. 21-23 wherein the parts of the embodiment similar to those of the first embodiment shown in FIGS. 1-4B are designated by similar reference numerals followed by "g". The steering wheel of this embodiment includes a pair of spoke cores 6g formed by metallic rods each having opposite radially outer ends welded to a rim core 7g and a substantially straight intermediate or central portion secured to a hub 5g which is composed of a pair of hub members 12g and 14g best shown in FIG. 23. More specifically, the upper hub member 12g has a bottom end surface 17g formed thereon with an axial protrusion 15g through which a cerrated central through-hole 11g for a steering shaft 4 extends. The outer peripheral surface of the axial protrusion 15g is substantialy cylindrical excepting flat surface portions 15g". The protrusion 15g has a generally oval end face 15g'. The lower hub member 14g has an upper end face 18g in which an engagement recess 20g is formed to receive the engagement protrusion 15g on the upper hub member 12g. For this purpose, the recess 20g is dimensioned and shaped such that the protrusion 15 can be press-fitted into the recess 20g so that the press-fitted engagement between the protrusion 15g and the recess 20g holds the two hub members against axial dislodgement and against rotation. The recess 20g has a bottom face 20g' in which a tapered through-hole 13g for the steering shaft 4 is open. The axial dimension of the protrusion 15g and the depth of the recess 20g are determined such that, when the protrusion 15g is inserted into the recess 20g until the end face 15g' of the protrusion 15g is engaged with the bottom face 20g' of the recess 20g, the bottom face 17g of the upper hub member 12g and the upper face 18g of the lower hub member 14g define therebetween an annular space of an axial dimension slightly smaller than the diameter of the metallic rod from which the spoke cores 6g are prepared. The upper and lower hub members 12g and 14g and the spoke cores 6g are welded together as in the preceding embodiments so that these members are secured together, as shown in FIG. 22.

Referring to FIG. 22, the lower hub member 14g has a lower portion which is surrounded by a lower cover 10g of a molded plastic material. The lower cover 10g includes a pair of axial projections only one of which is designated by 10g-1. Each projection 10g-1 is so disposed as to engage the radially outer side of the central part of one of the spoke cores 6g to prevent the spoke core 6g from being disconnected from the hub 5g even if the welded portions were broken. The lower cover 10g further includes a second set of four axial projections only one of which is designated by 10g-2. A mounting member 41 formed from a metallic plate is secured to the top surface of the upper end face of the hub 5g and extends radially outwardly therefrom. Each of the second set of projections 10g-2 has an upper free end secured to the mounting plate 41 by a screw 44 so that the lower cover 10g is fixed to the hub 5g.

In the embodiments described with reference to FIGS. 14-23, the upper and lower hub members may be formed from either the same metallic material or different metallic materials. For example, the upper hub members may be formed from mild steel or stainless steel, while the lower hub members may be formed from a hard steel to facilitate easy removal of the lower hub members from associated tapered portions of steering shaft.

Moreover, the upper hub members may alternatively be formed therein with tapered through-holes, while the lower hub members may alternatively be formed therein with serrated through-holes.