JPH08295278A - Front fork of motorcycle - Google Patents

Abstract

PURPOSE: To make improvements in durable strength in an inexpensive structure, in a front fork of a motorcycle. CONSTITUTION: A front fork 2 consists of an inner tube 6 supporting a front wheel shaft 4 free of rotation, two outer tubes 8, 8 fitted in this inner tube 6 free of sliding motion, and an underbracket 10 with two symmetrical hole parts 10a in which these outer tubes 8 and 8 are fitted. The outer tubes 8 is joined with the underbracket 10 by means of welding in a state of being inserted into the hole part 10a. The hole part 10a of the underbracket 10 is formed into burring form so as to be projected downward, while the outer tube 8 is fitted in this burring part 12. A lower end face of this burring part 12 and an outer circumferential surface of the outer tube 8 are welded together in a range of almost 90 degrees in a central angle at only both symmetrical side parts holding a central axis C of the outer tube 8 between.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a front fork of a motorcycle.

[0002]

2. Description of the Related Art Conventionally, as a front fork of a motorcycle, a telescopic front fork in which a spring is operated by an inner tube and an outer tube has been widely used. 4 and 5 show an example of a conventional telescopic fork.

As shown in FIGS. 4 and 5, this front fork a is mounted on a scooter b and holds an outer tube e by an under bracket g. The front fork a includes a pair of left and right inner tubes d and d that rotatably supports the front wheel shaft c at the lower end, and a pair of left and right outer tubes e and e slidably fitted to the inner tubes d and d. , Outer tube e, e
It is mainly composed of an under bracket g having two left and right hole portions f, f into which is inserted.

A stem shaft i, which is rotatably supported by a head pipe h, is welded to a substantially central portion of the under bracket g in the left-right direction. Also, the outer tube e is
As shown in FIG. 6, while being inserted into the hole portion f of the under bracket g, it is joined to the lower surface of the under bracket g by fillet welding all around.

The front fork a constructed in this way
In the scooter b having the above, the front wheel k can be steered via the outer tube e and the inner tube d by rotating the handle j (see FIG. 4) connected to the upper end of the stem shaft i. ing. The sliding of the outer tube e absorbs the impact from the road surface, and at the same time, the front wheel k can trace the road surface according to the steering wheel operation.

[0006]

An axial force acts on the outer tube e and the inner tube d of the front fork a which functions as described above, and a symbol m.
A bending moment indicated by (see FIG. 4) acts.

However, if the joint between the outer tube e and the under bracket g has a full circumference welded structure as in the conventional case, as shown in FIG. 5, the point where the maximum stress is generated by the bending moment m is the full circumference welded part. As a result, the durability strength of the joint part may decrease. Further, since the strength of the place where the maximum stress occurs is determined by the strength of welding, there is a problem that the cost becomes high if the durability strength is increased by the throat thickness or the like.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a front fork of a motorcycle which has an inexpensive structure and improves the durability strength of the joint portion of the outer tube. .

[0009]

The present invention has the following configuration to achieve the above object. That is, the present invention, a pair of left and right inner tubes that rotatably support the front wheel shaft at the lower end portion, and a pair of left and right outer tubes slidably fitted to the pair of left and right inner tubes,
Mainly composed of an under bracket having two left and right hole portions into which the pair of left and right outer tubes are fitted,
In a front fork of a motorcycle, wherein the outer tube is joined to the under bracket by welding while being inserted into the hole, the hole of the under bracket projects downward along an axial direction of the outer tube. Is formed into a burring shape, the outer tube is fitted into the burring portion, and the lower end surface of the burring portion and the outer peripheral surface of the outer tube are only left and right side portions sandwiching the central axis of the outer tube. In the above, the front fork of the motorcycle is welded in the range of the central angle of about 90 °.

[0010]

According to the present invention, since the outer tube is fitted in the burring portion, the structure of the joint portion of the outer tube, which is the location where the maximum stress is generated, is substantially the same as that of the double tube. Therefore, the section modulus at the location where the maximum stress occurs increases, the stress is relieved, and the durability of the joint portion increases.

Further, since the lower end surface of the burring portion and the outer peripheral surface of the outer tube are welded only at the left and right side portions sandwiching the central axis of the outer tube,
The welded portion exists only in the neutral axis direction (horizontal direction) related to the bending moment acting on the front fork. Therefore, the stress concentration on the welded portion can be avoided, and the durability strength can be further increased. Moreover, since the welding is performed in the range of the central angle of about 90 °, the length of the welded portion can be significantly shortened as compared with the conventional one, and the joining structure of the outer tube can be constructed at low cost while enhancing the durability strength. .

[0012]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a side view of the front fork of this embodiment,
2 is a front view of the outer tube joining portion according to the present embodiment, and FIG. 3 is a sectional view taken along line III-III in FIG.

As shown in FIGS. 1 and 2, the present embodiment is a front fork 2 of a scooter (an example of a motorcycle) 14, which is a pair of left and right inner tubes 6, which rotatably support a front wheel shaft 4. 6 and outer tubes 8, 8 slidably fitted to the inner tubes 6, 6.
And an under bracket 10 having two left and right hole portions 10a, 10a into which the outer tubes 8, 8 are fitted, and the outer tube 8 is joined by welding. As shown in FIG. 1, a fork cover 16 is provided on the side of the inner tube 6, and the fork cover 16 is fastened to the inner tube 6 with a screw member or the like.

As shown in FIG. 1, the under bracket 10 has a substantially U-shape that opens rearward. As shown in FIG. 2, the under bracket 10 has a stem at a substantially central portion in the left-right direction on the upper surface of the upper wall 10u. The lower end of the shaft 18 is welded.
The stem shaft 18 is rotatably supported by a head pipe 20 provided at the front end of the vehicle body frame,
The front wheels 22 (see FIG. 1) can be steered.

As shown in FIG. 2, the lower wall 10d of the under bracket 10 is provided with a pair of left and right holes 10a into which the outer tube 8 is fitted, and each hole 10a.
Is formed as an inner peripheral surface of the burring portion 12 which protrudes downward from the lower surface of the lower wall 10d along the outer tube axial direction. The burring portion 12 is, for example, a hole expansion (burring process) from a circular hole blank by a press die.
Is processed into a desired shape.

The outer tube 8 is fitted into the hole 10a until the upper end surface of the outer tube 8 contacts the lower surface of the upper wall 10u of the under bracket 10. Burring part 1
As shown in FIG. 3, the lower end surface of 2 and the outer peripheral surface of the outer tube 8 are fillet welded in a range of a central angle of about 90 ° only on both left and right sides sandwiching the central axis C of the outer tube 8. As a result, there is no welded portion in the bending moment axial direction (vehicle front-back direction) related to the bending moment M (see FIG. 1), while the left and right welded portions formed within the range of the central angle of about 90 ° are It exists in the direction of the neutral axis where bending stress due to tensile and compressive forces does not occur.

In this embodiment, as shown in FIG.
Although the lower surface of the upper wall 10u of the under bracket 10 is also welded to the upper end surface of the outer tube 8, the welded portion on the upper end surface of the outer tube 8 is welded in the same manner as the welded portion shown in FIG. Alternatively, it may be welded around the entire circumference.

According to the present embodiment having the above-mentioned structure, since the outer tube 8 is fitted in the burring portion 12, the maximum stress generation point (see FIG. 1) becomes substantially the same as the double tube. Therefore, the section modulus at the joint portion of the outer tube 8 is increased, the stress is relieved, and the durability strength is increased.

Further, since the lower end surface of the burring portion 12 and the outer peripheral surface of the outer tube 8 are welded only at both left and right sides sandwiching the central axis C of the outer tube, the welding is performed only in the neutral axis direction related to the bending moment M. Since the portion is present, the stress concentration in the welded portion can be avoided and the durability strength can be improved.

The center angle is approximately 9 without welding over the entire circumference.
Since it is welded at two places in the range of 0 °, the length of the welded portion can be greatly shortened compared to the conventional one, and the joint structure of the outer tube 8 can be constructed at low cost while enhancing the durability strength. Becomes

The present embodiment is a preferred embodiment of the present invention, and the technical scope of the present invention is not limited to this embodiment.

[0022]

As described above, according to the present invention, in the front fork of the motorcycle, the durability strength of the joint portion of the outer tube can be improved with an inexpensive structure.

[Brief description of drawings]

FIG. 1 is a side view of a front fork of the present embodiment.

FIG. 2 is a front view of an outer tube joint portion according to the present embodiment.

3 is a sectional view taken along line III-III in FIG.

FIG. 4 is a side view of a scooter equipped with a conventional front fork.

FIG. 5 is a front view of a conventional front fork.

6 is a sectional view taken along line VI-VI in FIG.

[Explanation of symbols]

 2 Front fork 4 Front wheel shaft 6 Inner tube 8 Outer tube 10 Under bracket 10a Hole 12 Burring C C Outer tube center axis

Claims (1)

[Claims] 1. A pair of left and right inner tubes that rotatably support a front wheel shaft at a lower end thereof, a pair of left and right outer tubes slidably fitted to the pair of left and right inner tubes, and a pair of left and right outer tubes. In a front fork of a motorcycle, which is mainly composed of an under bracket having two left and right hole portions into which a tube is fitted, and the outer tube is welded to the under bracket while being inserted into the hole portion. The hole portion of the under bracket is formed in a burring shape so as to protrude downward along the axial direction of the outer tube, and the outer tube is fitted into the burring portion, and the lower end surface of the burring portion is formed. The outer peripheral surface of the outer tube is a left and right side portion sandwiching the central axis of the outer tube. In only front fork of the motorcycle, characterized in that welded in a range of central angle of approximately 90 °.