Patent Description:
In recent years, along with steels, aluminum alloys are widely used as the material of wheels for automotive vehicles.

Prior art document <CIT> discloses a process for the forming of wheel rims in metal alloy, wherein a cast blank undergoes a cold chip removal process by cutting its central area, its inner surface and its lateral surface in order to obtain a semi-finished work as an intermediate product including a precursory hub and an annular precursory rim which is then heated and plastically deformed by flow forming along its lateral surface to obtain an inner edge, an outer edge and a middle portion of the precursory rim with a defined machine allowance. In the flow forming process for the precursory rim of the intermediate product a first mandrel having a first abutting surface of the flow forming apparatus abuts an inner peripheral surface of the precursory rim of the intermediate product from a first direction along an axial direction of the intermediate product and a second mandrel of the flow forming apparatus having a second abutting surface abuts the inner peripheral surface of the precursory rim of the intermediate product from a second direction opposite to the first direction along the axial direction of the intermediate product. Than, rotating the intermediate product while pressing a roller of the flow forming apparatus against an outer peripheral surface of the precursory rim of the intermediate product. The rim thus obtained undergoes another cold chip removal process by cutting in order to work it down to the required size; the latter process may be preceded by solution and age hardening heat treatments.

<CIT> discloses a method of producing a wheel for cars that is made of an aluminum alloy. The production method disclosed in <CIT> includes: a gravity casting step of casting an intermediate work that includes a precursory disc and a precursory rim by gravity casting; and a plastic processing step of performing a plastic processing (which also involves thinning) for the precursory rim of the intermediate work to form a rim. Specifically, as the plastic processing at the plastic processing step, a flow forming process is performed.

After the gravity casting step, by performing a flow forming process for the precursory rim to form a rim, it becomes possible to reduce the weight of the rim, thereby allowing the weight of the entire wheel to be reduced. It is also possible to improve the material strength (mechanical properties) of the rim, in which solidification tends to be slow and defects are likely to remain.

The flow forming process is performed by, while rotating the intermediate product, pressing a roller against the rim of the intermediate product. If the spokes or the hub of the intermediate product is clamped, clamp scars will be left on the completed wheel.

In a wheel for straddled vehicles, unlike in a wheel for cars (namely, generic four-wheeled automobiles), both sides of the wheel (i.e., both the right side and the left side of the vehicle) are likely to be design surfaces. Therefore, clamp scars being left on the wheel will considerably compromise its commercial value.

The present invention has been made in view of the above problem, and an objective thereof is to provide a method of producing a wheel for straddled vehicles which does not leave a clamp scar on the wheel even if a flow forming process is performed for its intermediate product. According to the present invention said object is solved by a method of producing a wheel for straddled vehicles having the features of independent claim <NUM>. Preferred embodiments are laid down in the dependent claims.

Hereinafter, embodiments of the present teaching will be described with reference to the drawings.

With reference to <FIG>, an example of a wheel for straddled vehicles (hereinafter simply referred to as a "wheel") that is produced by a production method according to an embodiment of the present teaching will be described. <FIG> are a left side view and a right side view, respectively, schematically showing the wheel <NUM>. <FIG> is a plan view showing the wheel <NUM> from a radial direction.

As shown in <FIG> and <FIG>, the wheel <NUM> includes a hub <NUM>, a rim <NUM>, and a plurality of spokes <NUM>. The wheel <NUM> is made of a metal material. The hub <NUM>, the rim <NUM>, and the plurality of spokes <NUM> are monolithically formed.

The hub <NUM> is located in the center of the wheel <NUM>, and has an aperture (wheel-shaft insertion hole) <NUM>, into which a wheel shaft is to be inserted. A direction that is parallel to a center axis of the wheel-shaft insertion hole <NUM> may be referred to as the "axial direction" hereinbelow. Note that the specific shape of the hub <NUM> is not limited to what is illustrated in <FIG>, etc..

The rim <NUM> has an annular shape, and extends along the circumferential direction of the wheel <NUM>. The rim <NUM> has an inner peripheral surface 120a and an outer peripheral surface 120b. A tire is to be mounted on the outer peripheral surface 120b of the rim <NUM>.

The plurality of spokes <NUM> connect the hub <NUM> and the rim <NUM>. More specifically, the plurality of spokes <NUM> connect an outer periphery of the hub <NUM> and the inner peripheral surface 120a of the rim <NUM>. Although the example shown illustrates the wheel <NUM> as having ten spokes <NUM>, the number of spokes <NUM> is not limited to ten. Although the example shown illustrates that two adjacent spokes <NUM> are united at the hub <NUM> side, the configuration of the spokes <NUM> is not limited thereto.

With reference to <FIG>, a method of producing the wheel <NUM> will be described. <FIG> is a flowchart showing an example of a method of producing the wheel <NUM>.

First, by using a gravity casting technique, an intermediate product (hereinafter referred to as a "workpiece") <NUM>' that is made of a metal material (e.g., an aluminum alloy) is provided (Step s1). The workpiece <NUM>' may suitably be formed by a gravity casting technique, for example. Specific shapes for the workpiece <NUM>' are shown in <FIG>. <FIG> are a left side view and a right side view, respectively, schematically showing the workpiece <NUM>'. <FIG> is a plan view showing the workpiece <NUM>' as viewed from a radial direction. <FIG> are a cross-sectional view taken along line 8A-8A' and line 9A-9A', respectively, in <FIG>.

As shown in <FIG>, the workpiece <NUM>' includes a precursory hub <NUM>', a precursory rim <NUM>', and a plurality of precursory spokes <NUM>' connecting the precursory hub <NUM>' and the precursory rim <NUM>'. The precursory hub <NUM>' is a portion to become the hub <NUM> of the wheel <NUM>. Similarly, the precursory rim <NUM>' is a portion to become the rim <NUM>, and the precursory spokes <NUM>' are portions to become the spokes <NUM>.

The precursory hub <NUM>' is located at the center of the workpiece <NUM>'. In the example shown, the wheel-shaft insertion hole <NUM> is not formed in the precursory hub <NUM>' yet. The precursory rim <NUM>' has an annular shape, and extends along the circumferential direction of the workpiece <NUM>'. The precursory rim <NUM>' has an inner peripheral surface 120a' and an outer peripheral surface 120b'. The plurality of precursory spokes <NUM>' connect the precursory hub <NUM>' and the precursory rim <NUM>'. More specifically, the plurality of precursory spokes <NUM>' connect an outer periphery of the precursory hub <NUM>' and the inner peripheral surface 120a' of the precursory rim <NUM>'.

Then, a flow forming process is performed for the precursory rim <NUM>' of the workpiece <NUM>' by using a flow forming apparatus (Step s2). An example of a flow forming apparatus to be used at Step s2 is shown in <FIG>. The flow forming apparatus <NUM> shown in <FIG> includes: a first mandrel <NUM> and a second mandrel <NUM>; a rotating mechanism <NUM> to rotate the first mandrel <NUM> and the second mandrel <NUM>; and a roller <NUM> to be pressed against the outer peripheral surface 120b' of the precursory rim <NUM>' of the workpiece <NUM>'.

Hereinafter, between the first mandrel <NUM> and the second mandrel <NUM>, the first mandrel <NUM> being located relatively low will be referred to as the "lower mandrel", whereas the second mandrel <NUM> being located relatively high will be referred to as the "upper mandrel".

The rotating mechanism <NUM>, which includes a motor, is placed on a pedestal <NUM>. The lower mandrel <NUM> is attached to a leading end of the rotating mechanism <NUM>. On a roof <NUM>, a mandrel moving mechanism <NUM> to move the upper mandrel <NUM> up or down is placed. The upper mandrel <NUM> is attached to a leading end of the mandrel moving mechanism <NUM>. On a wall <NUM>, a roller moving mechanism <NUM> to move the roller <NUM> in the up-down direction, the right-left direction, or the front-rear direction is placed. The roller <NUM> is supported, in a rotatable manner, by an arm <NUM> that is attached to a leading end of the roller moving mechanism <NUM>.

With the workpiece <NUM>' being externally fitted on the lower mandrel <NUM> and the upper mandrel <NUM>, a flow forming process is performed. At this time, the lower mandrel <NUM> and the upper mandrel <NUM> are coupled in such a manner that the upper mandrel <NUM> will rotate as the lower mandrel <NUM> rotates. For example, the flow forming process may be performed in a state where a portion the lower mandrel <NUM> is fitted in a portion of the upper mandrel <NUM>.

The lower mandrel <NUM> has a surface (hereinafter referred to as the "first abutting surface") 21a that abuts with the inner peripheral surface 120a' of the precursory rim <NUM>' of the workpiece <NUM>' from one direction (e.g., from below herein) along the axial direction of the workpiece <NUM>'.

The upper mandrel <NUM> has a surface (hereinafter referred to as the "second abutting surface") 22a that abuts with the inner peripheral surface 120a' of the precursory rim <NUM>' of the workpiece <NUM>' from the opposite direction (e.g., from above herein) along the axial direction of the workpiece <NUM>'.

As shown in <FIG> and <FIG>, the inner peripheral surface 120a' of the precursory rim <NUM>' of the workpiece <NUM>' includes: a first region R1 to abut with a first abutting surface 21a of the lower mandrel <NUM>; and a second region R2 to abut with a second abutting surface 22a of the upper mandrel <NUM>. Each of the first region R1 and the second region R2 includes at least one stepped portion (protrusion) <NUM> protruding inwardly along the radial direction of the workpiece <NUM>'. In the example shown, each of the first region R1 and the second region R2 includes a plurality of (i.e., two or more) stepped portions <NUM> that are formed discretely along the circumferential direction of the workpiece <NUM>'.

In the configuration illustrated herein, the stepped portions <NUM> in the first region R1 and the stepped portions <NUM> in the second region R2 overlap one another when viewed along the axial direction; however, the positioning of the stepped portions <NUM> is not limited thereto. Alternatively, the stepped portions <NUM> in the first region R1 and the stepped portions <NUM> in the second region R2 may be shifted from one another when viewed along the axial direction (i.e., so that the stepped portions <NUM> in the first region R1 and the stepped portions <NUM> in the second region R2 are staggered along the circumferential direction). Although an example has been illustrated where essentially the entire inner peripheral surface 120a' of the precursory rim <NUM>' consists of the first region R1 and the second region R2, the positioning of the first region R1 and the second region R2 is not limited thereto. Alternatively, only a portion of a half of the inner peripheral surface 120a' along the axial direction (i.e., a lower half in <FIG>) may be the first region R1, while only a portion of another half of the inner peripheral surface 120a' along the axial direction (i.e., an upper half in <FIG>) may be the second region R2.

In <FIG>, the shapes of the lower mandrel <NUM> and the upper mandrel <NUM> are shown simplified. <FIG> and <FIG> show specific examples of the lower mandrel <NUM> and the upper mandrel <NUM>. <FIG> is a side view schematically showing the lower mandrel <NUM> and the upper mandrel <NUM>, illustrating a state where the lower mandrel <NUM> and the upper mandrel <NUM> are coupled (more specifically, a guide pin <NUM> of the lower mandrel <NUM> is inserted in an aperture of the upper mandrel <NUM>). <FIG> is a cross-sectional view schematically showing a state where the workpiece <NUM>' is clamped by the lower mandrel <NUM> and the upper mandrel <NUM>.

As has already been described, the lower mandrel <NUM> has the first abutting surface 21a, which abuts with the inner peripheral surface 120a' of the precursory rim <NUM>' from below; and the upper mandrel <NUM> has the second abutting surface 22a, which abuts with the inner peripheral surface 120a' of the precursory rim <NUM>' from above. Each of the first abutting surface 21a of the lower mandrel <NUM> and the second abutting surface 22a of the upper mandrel <NUM> is shaped so as to correspond to the plurality of stepped portions <NUM> of the workpiece <NUM>'. Specifically, each of the first abutting surface 21a of the lower mandrel <NUM> and the second abutting surface 22a of the upper mandrel <NUM> includes a plurality of engaging portions (recesses) <NUM> to engage with the plurality of stepped portions <NUM>.

When the lower mandrel <NUM> and the upper mandrel <NUM> are rotated by the rotating mechanism <NUM>, the workpiece <NUM>' rotates accordingly. As the roller <NUM> is pressed against the outer peripheral surface 120b' of the precursory rim <NUM>' of the workpiece <NUM>' in this state, the flow forming process is accomplished.

The workpiece <NUM>' after the flow forming process is shown in <FIG> and <FIG>. <FIG> are a left side view and a right side view, respectively, schematically showing the workpiece <NUM>' after the flow forming process. <FIG> is a plan view showing the workpiece <NUM>' after the flow forming process as viewed from a radial direction. As shown in <FIG> and <FIG>, the flow forming process has caused plastic deformation of the precursory rim <NUM>'; more specifically, the precursory rim <NUM>' has been stretched along the axial direction while being also thinned.

Next, a heat treatment is performed. More specifically, a solution treatment (Step s3), a quenching process (Step s4), and an artificial aging process (Step s5) are performed in this order. This series of processes may sometimes be called a T6 heat treatment. The solution treatment may be performed at <NUM> for <NUM> hours, for example. The quenching process may be performed through water-cooling at <NUM>, for example. The artificial aging process may be performed at <NUM> for <NUM> hours, for example.

Thereafter, a cutting process is performed (Step s6). Through the cutting process, the wheel-shaft insertion hole <NUM> and the like are formed, and also their dimensions are adjusted. At this time, the plurality of stepped portions <NUM> on the inner peripheral surface 120a' of the precursory rim <NUM>' are removed. In this manner, the wheel <NUM> is obtained. Note that the wheel-shaft insertion hole <NUM> does not need to be formed via a cutting process. For example, at step S1, a workpiece <NUM>' that already has a wheel-shaft insertion hole <NUM> may be provided.

As described above, in the method of producing the wheel <NUM> according to an embodiment of the present teaching, within the inner peripheral surface 120a' of the precursory rim <NUM>' of the intermediate product (workpiece) <NUM>' provided, each of the region (first region) R1 to abut with the lower mandrel (first mandrel) <NUM> and the region (second region) R2 to abut with the upper mandrel (second mandrel) <NUM> includes at least one stepped portion <NUM> protruding inwardly along the radial direction of the workpiece <NUM>'. Because of such a stepped portion(s) <NUM> being provided on the inner peripheral surface 120a' of the precursory rim <NUM>', the torque from the lower mandrel <NUM> and the upper mandrel <NUM> being rotated by the rotating mechanism <NUM> can be sufficiently transmitted to the workpiece <NUM>'. In other words, the stepped portion(s) <NUM> enables clamping of the workpiece <NUM>'. Since this makes it unnecessary to rely on the precursory hub <NUM>' or the precursory spoke <NUM>' for the clamping of the workpiece <NUM>', it becomes possible to produce the wheel <NUM> in such a manner that clamp scars will not be left on the hub <NUM> and the spoke <NUM>. For example, since the at least one stepped portion <NUM> including a clamping surface is removed at the step of performing a cutting process, no clamp scars will be left on the final product (i.e., the wheel <NUM> for straddled vehicles).

Note that the number of stepped portions <NUM> in each of the first region R1 and the second region R2 is not limited to what is illustrated in <FIG>. It suffices if at least one stepped portion <NUM> is provided in each of the first region R1 and the second region R2.

Another exemplary configuration of the stepped portion <NUM> is shown in <FIG>. In the example shown in <FIG>, only one stepped portion <NUM> is provided in each of the first region R1 and the second region R2. Such a configuration also allows the wheel <NUM> to be produced without leaving clamp scars on the hub <NUM> and the spoke <NUM>.

Still another exemplary configuration of the stepped portion <NUM> is shown in <FIG>, <FIG>. In the example shown in <FIG>, in each of the first region R1 and the second region R2, one stepped portion <NUM> is formed along the entire circumference along the circumferential direction of the workpiece <NUM>'. In this case, an upper surface (i.e., a surface of the workpiece <NUM>' facing outward along the axial direction) 121u of the stepped portion <NUM> has an annular shape, this upper surface 121u functioning as a clamping surface. Specifically, clamping of the workpiece <NUM>' is achieved by utilizing a frictional force occurring between the upper surface (clamping surface) 121u of the stepped portion <NUM> and the abutting surface (i.e., the first abutting surface 21a of the lower mandrel <NUM> or the second abutting surface 22a of the upper mandrel <NUM>) of each mandrel pressed against the upper surface 121u.

In the case where such a stepped portion <NUM> (a stepped portion <NUM> being formed along the entire circumference along the circumferential direction of the workpiece <NUM>') are provided in each of the first region R1 and the second region R2 of the inner peripheral surface 120a' of the precursory rim <NUM>', there is an advantage in that the workpiece <NUM>' can be easily set to the flow forming apparatus <NUM>.

On the other hand, as was illustrated in <FIG>, etc., when a plurality of stepped portions <NUM> are formed discretely along the circumferential direction of the workpiece <NUM>', the torque from the mandrel (the lower mandrel <NUM> or the upper mandrel <NUM>) can be received on a partial side surface (a side surface oriented in the circumferential direction of the workpiece <NUM>') of each stepped portion <NUM>, so that not only the upper surface 121u of each stepped portion <NUM> but also the partial side surface functions as a clamping surface.

In the case where a plurality of stepped portions <NUM> are provided on the inner peripheral surface 120a' of the precursory rim <NUM>', the workpiece <NUM>' can be prevented from slipping against each mandrel. Moreover, a smaller amount of metal material is needed to form the workpiece <NUM>' than in the case where one stepped portion <NUM> is formed continuously along the entire circumference.

Each of the first abutting surface 21a of the lower mandrel <NUM> and the second abutting surface 22a of the upper mandrel <NUM> is preferably shaped so as to correspond to at least one stepped portion <NUM> of the workpiece <NUM>'. Specifically, as is illustrated, each of the first abutting surface 21a of the lower mandrel <NUM> and the second abutting surface 22a of the upper mandrel <NUM> preferably includes at least one engaging portion <NUM> to engage with at least one stepped portion <NUM>.

In the step of providing the workpiece <NUM>', the workpiece <NUM>' can be formed from a metal material by a gravity casting technique. Employing a gravity casting technique provides an advantage of reducing the weight of relatively large wheels because it is easy to form a hollow structure by utilizing a core.

As the metal material serving as the material of the workpiece <NUM>', an aluminum alloy can be suitably used, for example.

Moreover, as in the above-described production method, Step s3 of performing a solution treatment after the flow forming process of Step s2, Step s4 of performing a quenching process after Step s3, and Step s5 of performing an artificial aging process after Step s4 may further be included. This series of heat treatment processes allows the mechanical properties of the wheel <NUM> made of an aluminum alloy to be adjusted (e.g., so that it increases in tensile strength, proof stress, and hardness).

Preferable compositions for the aluminum alloy to be used as the material of the wheel <NUM> will be described.

Preferably, the aluminum alloy contains silicon (Si) in an amount of not less than <NUM> mass% and not more than <NUM> mass%, magnesium (Mg) in an amount of not less than <NUM> mass% and not more than <NUM> mass%, and aluminum (Al) and inevitable impurities as a balance.

When the percentage content of Si is <NUM> mass% or more, castability (e.g., melt fluidity) can be adequately improved. The influence of the percentage content of Si on the melt fluidity is disclosed in <NPL> (hereinafter "Non-Patent Document <NUM>"), for example. Non-Patent Document <NUM> states that, for example, given a constant pour point, lowest fluidity results when the percentage content of Si is <NUM> to <NUM> mass%. On the other hand, if the percentage content of Si is too high, toughness may lower. When the percentage content of Si is <NUM> mass% or less, a decrease in toughness can be suppressed.

Moreover, since Mg yields a deposition (Mg2Si) with Si, when the percentage content of Mg is <NUM> mass% or more, tensile strength and proof stress can be improved. However, if the percentage content of Mg is too high, elongation may lower. When the percentage content of Mg is <NUM> mass% or less, a decrease in elongation due to Mg can be kept at an insignificant level.

Thus, an aluminum alloy having the aforementioned composition has good castability, as well as good tensile strength and proof stress. Good castability makes it easier to realize a complicated shape (e.g., the shape of a vehicle wheel) or a thin-walled shape, even by using a gravity casting technique. Goode tensile strength and proof stress make it possible to provide a sufficient strength even in the case of a complicated shape or a thin-walled shape.

As has already been described, as the amount of Mg added to the aluminum alloy is increased, tensile strength and proof stress may improve, but elongation (toughness) tends to lower. When a flow forming process is performed for the product (intermediate product) after casting (e.g., gravity casting), the metallographic structure changes through plastic deformation (or more specifically, the metallographic structure becomes stretched out to create a metal flow) at the site(s) which have been subjected to the flow forming process; as a result, elongation improves, thus to compensate for a decrease in elongation that is ascribable to the increased amount of Mg added. Therefore, an aluminum alloy having the aforementioned composition can be suitably used for vehicle wheels that are produced through a combination of a gravity casting technique and a flow forming process.

If the percentage content of Cu is too high, elongation may lower. Therefore, the percentage content of Cu is preferably <NUM> mass% or less.

The percentage content of Fe is preferably <NUM> mass% or less. If the percentage content of Fe is too high, toughness may lower. When the percentage content of Fe is <NUM> mass% or less, a decrease in toughness that is ascribable to Fe can be suppressed. From the standpoint of suppressing a decrease in toughness, it is more preferable that the percentage content of Fe is <NUM> mass% or less.

The percentage content of Ti is preferably <NUM> mass% or less. If the percentage content of Ti is too high, a decrease in toughness may be caused. When the percentage content of Ti is <NUM> mass% or less, a decrease in toughness that is ascribable to Ti can be suppressed.

When the aluminum alloy further contains Na in an amount of not less than <NUM> mass% and not more than <NUM> mass%, or Sr in an amount of not less than <NUM> mass% and not more than <NUM> mass%, modification (microstructuring of the eutectic Si phase) can be suitably achieved.

Now, results of studying influences of changes in the percentage content of Si, Mg, etc., in the aluminum alloy on tensile strength, proof stress, and elongation (toughness) will be described. In the following, any Example in which Si is not in the range of not less than <NUM> mass% and not more than <NUM> mass% and any Example in which Mg is not in the range of not less than <NUM> mass% and not more than <NUM> mass% is indicated as "Reference Example" for ease of understanding.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM> to <NUM> and Reference Examples <NUM> to <NUM>. As for elongation, the case where a flow forming process was not performed for the sample (indicated as "w/o FF") and the case where a flow forming process was performed for the sample (indicated as "w/ FF") are also shown. Also, for Examples <NUM> to <NUM> and Reference Example <NUM>, their relationship between the percentage content of Si and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Si and elongation is shown in <FIG>. In the following, a preferable tensile strength of <NUM> MPa or more and a preferable proof stress of <NUM> MPa or more are assumed, and a preferable elongation (in the case of not performing a flow forming process) of <NUM>% or more and a preferable elongation (in the case of performing a flow forming process) of <NUM>% or more are assumed.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM> to <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Reference Example <NUM>, where the percentage content of Si is <NUM> mass% (i.e., above <NUM> mass%), elongation is insufficient and toughness is poor, as can be seen from Table <NUM> and <FIG>.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM> to <NUM> and Reference Examples <NUM> and <NUM>. Also, for Examples <NUM> to <NUM> and Reference Examples <NUM> and <NUM>, their relationship between the percentage content of Mg and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Mg and elongation is shown in <FIG>.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM> to <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Reference Example <NUM>, where the percentage content of Mg is <NUM> mass% (i.e., below <NUM> mass%), tensile strength and proof stress are insufficient, as can be seen from Table <NUM> and <FIG>. Moreover, in Reference Example <NUM>, where the percentage content of Mg is <NUM> mass% (i.e., above <NUM> mass%), elongation is insufficient, as can be seen from Table <NUM> and <FIG>.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM>, <NUM> and <NUM>. Also, for Examples <NUM>, <NUM> and <NUM>, their relationship between the percentage content of Cu and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Cu and elongation is shown in <FIG>.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM> and <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Example <NUM>, where the percentage content of Cu is <NUM> mass% (i.e., above <NUM> mass%), elongation is smaller than in Examples <NUM> and <NUM>, as can be seen from Table <NUM> and <FIG>.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM>, <NUM> and <NUM>. Also, for Examples <NUM>, <NUM> and <NUM>, their relationship between the percentage content of Fe and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Fe and elongation is shown in <FIG>.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM> and <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Example <NUM>, where the percentage content of Fe is <NUM> mass% (i.e., above <NUM> mass%), elongation is smaller (i.e., toughness is lower) than in Examples <NUM> and <NUM>, as can be seen from Table <NUM> and <FIG>.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM>, <NUM> and <NUM>. Also, for Examples <NUM>, <NUM> and <NUM>, their relationship between the percentage content of Ti and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Ti and elongation is shown in <FIG>.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM> and <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Example <NUM>, where the percentage content of Ti is <NUM> mass% (i.e., above <NUM> mass%), elongation is smaller (i.e., toughness is lower) than in Examples <NUM> and <NUM>, as can be seen from Table <NUM> and <FIG>.

Table <NUM> shows the compositions and measurement results of tensile strength, proof stress, and elongation for aluminum alloys of Examples <NUM>, and <NUM> to <NUM>. Also, for Examples <NUM> to <NUM>, their relationship between the percentage content of Na and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Na and elongation is shown in <FIG>. Also, for Examples <NUM>, <NUM>, and <NUM> to <NUM>, their relationship between the percentage content of Sr and tensile strength and proof stress is shown in <FIG>, and their relationship between the percentage content of Sr and elongation is shown in <FIG>.

As can be seen from Table <NUM> and <FIG>, in Examples <NUM>, and <NUM> to <NUM>, preferable levels are attained in all of tensile strength, proof stress, and elongation. On the other hand, in Example <NUM>, where neither Na nor Sr is substantially contained, elongation is smaller than in Examples <NUM>, and <NUM> to <NUM>, as can be seen from Table <NUM> and <FIG> and <FIG>. This is presumably because modification (microstructuring of the eutectic Si phase) is not suitably achieved.

Claim 1:
A method of producing a wheel (<NUM>) for straddled vehicles, the wheel (<NUM>) including: a hub (<NUM>); an annular rim (<NUM>); and a plurality of spokes (<NUM>) connecting the hub (<NUM>) and the rim (<NUM>), the method comprising:
step A of providing an intermediate product (<NUM>') including a precursory hub (<NUM>'), an annular precursory rim (<NUM>'), and a plurality of precursory spokes (<NUM>') connecting the precursory hub (<NUM>') and the precursory rim (<NUM>'), the intermediate product (<NUM>') being made of a metal material;
step B of performing a flow forming process for the precursory rim (<NUM>') of the intermediate product (<NUM>') by using a flow forming apparatus (<NUM>),
abutting an inner peripheral surface (120a') of the precursory rim (<NUM>') of the intermediate product (<NUM>') from a first direction along an axial direction of the intermediate product (<NUM>') by a first mandrel (<NUM>) of the flow forming apparatus (<NUM>) having a first abutting surface (21a);
abutting the inner peripheral surface (120a') of the precursory rim (<NUM>') of the intermediate product (<NUM>') from a second direction opposite to the first direction along the axial direction of the intermediate product (<NUM>') by a second mandrel (<NUM>) of the flow forming apparatus (<NUM>) having a second abutting surface (22a);
rotating the intermediate product (<NUM>') by rotating the first mandrel (<NUM>) and the second mandrel (<NUM>);
pressing a roller (<NUM>) of the flow forming apparatus (<NUM>) against an outer peripheral surface (120b') of the precursory rim (<NUM>') of the intermediate product (<NUM>');
the inner peripheral surface (120a') of the precursory rim (<NUM>') of the intermediate product (<NUM>') provided at step A includes a first region (R1) to abut with the first abutting surface (21a) of the first mandrel (<NUM>) and a second region (R2) to abut with the second abutting surface (22a) of the second mandrel (<NUM>), characterized in that
each of the first region (R1) and the second region (R2) of the inner peripheral surface (120a') of the precursory rim (<NUM>') includes at least one stepped portion (<NUM>) protruding inwardly along a radial direction of the intermediate product (<NUM>').