JPH0635276B2 - Motorcycle casting rear arm - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a cast rear arm for a motorcycle.

[Prior art]

The aluminum rear arm of a conventional motorcycle is manufactured from an aluminum extruded tube. However, as is well known, it is difficult to weld an aluminum material having a high thermal conductivity, so that it must be turned to a skilled welding engineer, and automatic welding with high productivity by a robot is difficult. For this reason,
The rear arm made of aluminum extruded pipe was limited in productivity improvement, and the cost had to be increased.

Such a problem of welding can be solved by integrally molding by casting. By the way, when the rear arm is cast in this way, looking at the load applied to the rear arm, the cross member is mainly subjected to a torsional load, and the arm is mainly subjected to a vertical bending load. It is desirable to cast into a hollow structure with a closed cross-section surrounded by the wall surface of. However, a shell core is required to form a hollow structure with a closed cross section, and there is a problem that the mold cost is considerably high. In addition, when the shell core is used, it is necessary to remove the sand after casting.

[Object of the Invention]

An object of the present invention is to provide an aluminum-cast rear arm that can exhibit high rigidity against vertical bending loads while having an open cross-section structure that does not require a shell core.

[Structure of Invention]

The present invention that achieves the above object is such that a front cross member portion and left and right arm portions extending rearward from the left and right rear portions of the cross member portion are integrally cast from aluminum,
The arm portion has an open cross-section structure in which the upper surface and the left and right side surfaces are surrounded by wall surfaces and the lower side is opened, and a plurality of ribs extending over the left and right side surfaces are formed inside the arm portion at intervals in the longitudinal direction. It is characterized by.

〔Example〕

 The present invention will be described below with reference to the embodiments shown in the drawings.

FIG. 3 shows a motorcycle provided with a rear arm according to an embodiment of the present invention, 1 is a vehicle body frame, and 2 is an engine mounted on the vehicle body frame 1. A front wheel 4 is supported on the front part of the body frame 1 via a front fork 3, and a rear wheel 6 is supported on the rear part via a rear arm 5 of the present invention.
Is supported.

1A, 1B, 1C show the details of the rear arm 5,
A cross member portion 7 on the front side and a pair of left and right arm portions 8, 8 extending rearward from the left and right of the rear portion of the cross member portion 7.
It is made of aluminum and is integrally cast from aluminum. The cross member portion 7 is supported by the vehicle body frame 1 by the pivotal support portions 9, 9 at the front end, and the arm portions 8, 8 support the rear wheel 6 in the elongated holes 10, 10 at the rear end.

The cross member portion 7 is in the shape of a hollow box having a closed cross-sectional structure in which six outer peripheral surfaces are surrounded by wall surfaces. The hollow portion surrounded by the six faces is formed by inserting a shell core during casting. Therefore, sand removing holes 7a, 7a are provided on the lower wall surface of the cross member portion 7 so that the sand of the shell core can be extracted after casting.

On the other hand, the arm portion 8 has a hollow box shape with an open cross-section whose lower portion is open, although the upper surface and the left and right side surfaces are surrounded by wall surfaces. Further, a plurality of partition ribs 11, ... 11 are provided at predetermined intervals in the hollow portion. The ribs 11 are integrally formed on the left and right inner wall surfaces and the upper wall surface of the arm portion 8. Further, ribs 12, 12 along the longitudinal direction are integrally provided at the lower ends of the left and right outer wall surfaces of the arm portion 8.

Since the rear arm 5 having the above structure is formed by casting, it does not have low productivity as when welding an aluminum material. Moreover, since it is cast, it can be formed into any shape, and has a high degree of freedom in selecting the shape.

Further, among the above-mentioned constitutions, since the arm portion 8 has an open cross-section structure which is open on the lower side, it can be molded by casting, and the shell core is unnecessary. Therefore, the die cost is reduced and the sand removing work of the shell core is not required.

On the other hand, the arm portion 8 of the rear arm 5 is mainly subjected to a vertical bending load when it is mounted on a vehicle body for use. Due to the upward and downward bending loads, the open side of the arm portion 8 having the open cross-section structure tends to spread laterally. However, since the ribs 11 are provided inside the arm portion 8 so as to extend to the left and right side surfaces along the longitudinal direction, the deformation for expanding is prevented, and high rigidity is exhibited. To further enhance the rigidity effect,
As shown in FIG. 2, it is desirable that the cross section of the arm portion 8 be a vertically long rectangle.

Further, when the arm portion 8 is subjected to a vertical bending load as described above, expansion and contraction strains are generated in the upper layer side and the lower layer side, respectively, with respect to the horizontal neutral axis in the cross section, and the strain amount is It is known that the layer farther from the neutral axis is larger. And, in the case of the open cross-section structure in which the lower side is open like the arm portion 8, the neutral shaft is located at a position displaced upward from the center in the vertical direction. When the neutral axis is biased upward in this way, the amount of expansion / contraction strain of the lower surface becomes larger than the amount of expansion / contraction strain of the upper surface. growing. The large difference in the amount of expansion and contraction as described above is not preferable from the standpoint of strength because the cast product has a small resistance to expansion and contraction.

However, since the above-mentioned arm portion 8 is provided internally with the ribs 11 which are hung on both side surfaces, the neutral axis moves downward to a position close to the center, so that the upper and lower surfaces of the arm portion 8 are To reduce the difference in the amount of stretching strain. Therefore, the rear arm 8 can exhibit high strength even though it is a cast product.

Further, although the arm portion 8 has an open cross-section structure, it does not accumulate mud and the like inside because it is open at the bottom.

〔The invention's effect〕

As described above, the present invention includes the front cross member portion,
The left and right arm portions extending rearward from the left and right rear portions of the cross member portion are integrally cast from aluminum, and the arm portion has an open cross-section structure in which the upper surface and the left and right side surfaces are surrounded by wall surfaces and the lower portion is opened. A plurality of ribs extending over the left and right side surfaces are formed inside thereof at intervals in the longitudinal direction.

As described above, since the arm portion has an open cross-sectional structure in which the lower portion is opened, it can be molded by casting without using a shell core. Therefore, the mold cost is reduced,
It is possible to eliminate the sand removal work of the shell core.

Even if the arm section has an open cross section that is open on the lower side as described above, since a plurality of ribs are provided inside the arm section on both left and right sides, the bending load in the vertical direction applied to the arm section is increased. On the other hand, it is possible to prevent the deformation in which the lower portions of both side surfaces tend to expand to the left and right, and to exhibit high rigidity. Further, the rib allows the neutral axis of the expansion / contraction deformation in the cross section to be positioned substantially at the center, thereby reducing the difference in the expansion / contraction strain amount between the upper surface and the lower surface of the arm portion and exhibiting high strength.

Furthermore, since the arm portion is open on the lower side, it does not accumulate mud inside.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A, B, C show a rear arm according to an embodiment of the present invention, FIG. 1A is a plan view, and FIG. 1B is a side view taken along the line BB of FIG. Figure, Figure 1C is a bottom view, Figure 2 is that of Figure 1.
II-II arrow view, FIG. 3 is a side view of a motorcycle equipped with the above rear arm. 5 ... rear arm, 7 ... cross member part, 8 ... arm part, 11 ... rib.

Claims (1)

[Claims] 1. A front cross member portion and left and right arm portions extending rearward from the left and right rear portions of the cross member portion are integrally cast from aluminum, and the upper and left and right side surfaces of the arm member are wall surfaces. A cast rear arm for a motorcycle, which is surrounded by an open cross-section and has a plurality of ribs extending in the left and right side surfaces thereof at intervals in the longitudinal direction on the inside thereof.