Motorcycle and vehicle body frame

A vehicle body frame includes an upper frame, lower frame and a fastening member. The fastening member includes first and second bolts. The first bolt is provided to obliquely cross an axial line of a lower frame end connected to an upper frame end and to fasten the lower frame end to the upper frame end. The second bolt is provided to orthogonally cross the axial line of the lower frame end and fasten the lower frame end to the upper frame end.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on, and claims priority to, Japanese Patent Application No. 2011-233108, filed on Oct. 24, 2011, the contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a motorcycle and a vehicle body frame for use in a motorcycle.

2. Description of the Background Art

A vehicle body frame for use in a motorcycle has a loop shape and stores an engine inside. The vehicle body frame is generally produced as an integral form. When a motorcycle is produced, an engine is stored in the vehicle body frame. Therefore, the vehicle body frame is larger than the engine.

If a vehicle body frame is too large, operational feeling about the motorcycle is degraded. If the vehicle body frame is small, the weight of the motorcycle is reduced, and the vehicle body frame as a whole is positioned in the vicinity of the center of gravity of the motorcycle. Therefore, good operational feeling is obtained. Therefore, smaller vehicle body frames are preferable.

Furthermore, a vehicle body frame must have balanced rigidity (stiffness). If the motorcycle is an off-road type vehicle such as a motocrosser in particular, the motorcycle travels off-road or jumps. Therefore, external force in various directions is applied on the vehicle body frame. If the rigidity greatly varies against external force in various directions, operational feeling is degraded. If for example the torsional rigidity is extremely small as compared to the tensile rigidity and compression stiffness, the vehicle body frame is likely to flex against force in a torsional direction. Therefore, if force is applied in a torsional direction, the vehicle body frame flexes more than usual. This makes the motorcycle less maneuverable. In this way, if the rigidity against external force in various directions is greatly unbalanced, operational feeling is degraded. Therefore, it is preferable to reduce unbalance in rigidity against different kinds of external force.

The disclosure of JP-A 2008-222077 and the disclosure of JP-A 4-244492 suggest a vehicle body frame that allows rigidity and strength to be secured. The vehicle body frames disclosed by JP-A 2008-222077 and JP-A 4-244492 are partly detachable. By selecting a material for the detachable frame part, sufficient strength and rigidity can be secured for the vehicle body frame.

SUMMARY

In each of the vehicle body frames disclosed by JP-A 2008-222077 and JP-A 4-244492, however, rigidity against various kinds of external force would greatly vary at a connecting part between the detachable frame and the rest of the vehicle body frame. Therefore, good operational feeling is unlikely to be obtained.

It is an object of the present invention to provide a motorcycle that allows good operational feeling to be easily obtained.

A motorcycle according to some embodiments of the invention includes an engine and a vehicle body frame provided around the engine. The vehicle body frame includes an upper frame that includes a head pipe, a main frame that extends backward and obliquely downward from the head pipe, and a front frame that extends downward from the head pipe. The upper frame has a first upper frame end that is a lower end of the front frame and a second upper frame end that is a lower end of the main frame. The vehicle body frame also includes a lower frame provided between the first upper frame end and the second upper frame end. The lower frame includes a first lower frame end connected to the first upper frame end and a second lower frame end connected to the second upper frame end. The vehicle body frame includes a first fastening member that fastens a joined upper frame end of the upper frame and a joined lower frame end of the lower frame. The joined upper frame and lower frame ends are one of the first lower frame end and the first upper frame end, and the second lower frame end and the second upper frame end. The first fastening member includes a first bolt provided to obliquely cross a first axial line of the joined lower frame end, and to fasten the joined lower frame end to the joined upper frame end. The first fastening member further includes a second bolt provided to orthogonally cross the first axial line and to fasten the joined lower frame end to the joined upper frame end.

In the motorcycle according to the embodiment, the vehicle body frame can be divided into the upper frame and the lower frame. Therefore, the vehicle body frame as a whole can be smaller than a conventional frame. Furthermore, at a fastened portion for the upper frame and the lower frame, rigidity unbalance against external force applied in various directions can be reduced using the first fastening member. Therefore, good operational feeling is readily obtained.

DESCRIPTION OF THE EMBODIMENTS

Now, an embodiment of the present invention will be described in detail in conjunction with the accompanying drawings. In the drawings, the same or corresponding portions are designated by the same reference characters and their description will not be repeated. In the following description, the front, rear, left, and right refer to these directions as viewed from a rider seated on the seat.

Overall Structure of Motorcycle

FIG. 1is a left side view of a motorcycle1. Referring toFIG. 1, the motorcycle1includes a vehicle body frame2, an engine3, a fuel tank4, a seat5, a front fork6, a front wheel7, a rear wheel8, a rear shock absorber12, and a link mechanism13.

The vehicle body frame2includes a head pipe21at a front end. The head pipe21is a tube that extends downward and obliquely forward. A handle11is rotatably attached to an upper part of the head pipe21. A pair of front forks6is provided on the left and right sides of the head pipe21. The pair of front forks6extends downward and obliquely forward. The front wheel7is attached to the lower ends of the pair of front forks6in a rotatable manner. The fuel tank4is provided above a front part of the vehicle body frame2. The seat5is provided above the vehicle body frame2and extends backward from an upper end of a rear part of the fuel tank4.

The vehicle body frame2is a so-called cradle type frame and loop-shaped. The engine3is stored in the vehicle body frame2. A pivot shaft9is provided at a rear part of the vehicle body frame2. A rear arm10is supported at its front end around the pivot shaft9so that it can swing up and down. The rear wheel8is attached rotatably at a rear end of the rear arm10.

The rear shock absorber12is provided behind the vehicle body frame2and under the seat5. The rear shock absorber12has its upper end attached to a bracket formed at an upper part of the vehicle body frame2. The rear shock absorber12has a lower end attached to the link mechanism13. The link mechanism13is attached to the rear arm10and a lower rear end of the vehicle body frame2. The rear arm10is attached so that it can swing up and down.

Structure of Vehicle Body Frame2

FIG. 2is a perspective view of the vehicle body frame2.FIG. 3is an exploded perspective view of the vehicle body frame2. Referring toFIGS. 2 and 3, the vehicle body frame2includes an upper frame20, a pair of lower frames24, and fastening members25and26. The fastening members25and26fasten the lower frames24to the upper frame20.

Structure of Upper Frame20

The upper frame20includes a head pipe21, a pair of main frames22, and a front frame23. Front ends of the pair of main frames are connected to a rear part of the head pipe21. The pair of main frames22extends obliquely downward and toward the back of the motorcycle1from the head pipe21. The main frames22are provided as they are aligned in the widthwise direction of the motorcycle1. A pair of trough holes229is provided at lower ends of the pair of main frames22. The pivot shaft9(seeFIG. 1) is inserted in the pair of through holes229.

The pair of main frames22includes ends221at their lower ends. The ends221are attached with the lower frames24. The end221is an end of the upper frame20, and therefore will be referred to as the “upper frame end221” in the following description.

A tubular cross frame260is provided between the pair of upper frame ends221. The cross frame260includes a pair of attachment members223. The attachment members223are provided apart from each other in the widthwise direction at the lower end of the cross frame260. A front end of the link mechanism13(seeFIG. 1) is provided between the pair of attachment members223and the pair of attachment members223is connected with the link mechanism13by a shaft.

An upper end of the front frame23is connected to a lower end of the head pipe21. The front frame23extends downward from the head pipe21. The front frame23includes a trunk frame231and a pair of branch frames232. The trunk frame231is provided above the front frame23and an upper end of the trunk frame231is connected to the head pipe21. The pair of branch frames232is provided under the front frame23. The pair of branch frames232is arranged in the widthwise direction of the motorcycle1and an upper end of each of the branch frames232is connected to a lower end of the trunk frame231. Each of the branch frames232includes an end237at the lower end. A lower frame24is attached to the end237. The end237is an end of the upper frame20and therefore will be referred to as the “upper frame end237” in the following description.

The front frame23further includes a pair of beam members233. The pair of beam members233is attached between a rear end of the trunk frame231and the pair of main frames22. The beam members233serve to reduce welding distortion that could be caused when the items are separate before being connected with bolts.

The lower frame24is provided between the upper frame end237and the upper frame end221.

The lower frame24extends in a front-back direction of the motorcycle1and its front end extends upward. The lower frame24has two lower frame ends241and242. The lower frame end241is provided at a front end of the lower frame24and extends upward. The lower frame end242is provided at a rear end of the lower frame24and extends backward.

The lower frame end241is attached to the upper frame end237. The lower frame end242is attached to the upper frame end221.

The fastening member25fastens the lower frame end241to the upper frame end237. The fastening member26fastens the lower frame end242to the upper frame end221. Hereinafter, in the vehicle body frame2, a part fastened by the fastening member25will be referred to as the “front fastened portion” and a part fastened by the fastening member26will be referred to as the “rear fastened portion.”

Positioning of Engine3in Vehicle Body Frame2

As described above, the vehicle body frame2is divided into the upper frame20and the lower frames24. When an engine is stored in an integrally formed vehicle body frame, the vehicle body frame is large enough to insert the engine inside.

On the other hand, the vehicle body frame2can be smaller than the conventional integrally formed vehicle body frame. As described above, the vehicle body frame2is divided into the upper frame20and the lower frame24. As shown inFIG. 4, when the engine3is stored in the vehicle body frame2, the engine3is attached to the upper frame20. More specifically, the engine3is attached to a bracket250that extends downward from the main frame22. At the time, the lower frame24is not attached to the upper frame20, and therefore the engine3can be attached easily to the upper frame20. A part of a rear end of the engine3attached to the upper frame20is provided between the pair of main frames22. Therefore, the main frame22has a part that overlaps the engine3in a side view.

After the engine3is attached to the upper frame20, the lower frame24is attached to the upper frame20as shown inFIG. 5. As described above, the lower frame24is fastened to the upper frame20by the fastening members25and26. At the time, the lower end of the engine3is provided between the pair of lower frames24. Therefore, the lower frame24has a part that overlaps the engine3in a side view.

As described above, the vehicle body frame2has the part that overlaps the engine3in a side view. The vehicle body frame2is smaller than an integrally formed vehicle body frame. Therefore, the vehicle body frame2can be more lightweight than conventional frames, and the entire vehicle body frame2can be placed in the vicinity of the center of gravity of the motorcycle1. This improves the operational feeling.

Rigidity of Vehicle Body Frame

The rigidity (stiffness) of the vehicle body frame2affects operational feeling and riding comfort for a rider. As for a motorcycle such as a motocrosser that travels off-road in particular, the vehicle body frame2receives external force in a plurality of directions. At the time, as the fastened portions (front fastened portions and rear fastened portions) have less varying rigidity against various kinds of external force, better operational feeling and riding comfort can be obtained. The fastening members25and26serve to reduce rigidity unbalance at the fastened portions against various kinds of external force. Now, how the front fastened portions and the rear fastened portions of the vehicle body frame2are arranged will be described in detail.

Front Fastened Portions

Arrangement of Upper Frame End237and Lower Frame End241

FIG. 6is an exploded perspective view of a front fastened portion in the vehicle body frame2.FIG. 7is a perspective view of front fastened portions. Referring toFIG. 6, the upper frame end237is provided at a lower end of the branch frame232. The upper frame end237has an end surface234at a lower end. The end surface234is a plane and inclined with respect to an axial line L237of the upper frame end237. A front end234F of the end surface234is provided above a rear end234R of the end surface234. Therefore, the normal line N234to the end surface234extends forward and obliquely downward.

The lower frame end241is provided at a front end of the lower frame24. The lower frame end241has an end surface243. The end surface243is a plane and inclined with respect to an axial line L241of the lower frame end241. A front end of the end surface243is provided above a rear end of the end surface243. Therefore, the normal N243to the end surface243extends backward and obliquely upward.

As shown inFIG. 7, the end surface243is in contact with the end surface234upon fastening. Therefore, the end surface234is opposed to the end surface243.

The upper frame end237further has through holes235and236. The through hole235is provided to obliquely cross the axial line L237. More specifically, the through hole235is formed between the end surface234and a rear surface of the upper frame end237and extends in the direction of the normal N234. Therefore, the through hole235has an opening each at the end surface234and the rear surface of the upper frame end237. Upon fastening, the axial line L237is provided coaxially with the axial line L241of the lower frame end241. Therefore, upon fastening, the through hole235is provided to obliquely cross the axial line L241.

The through hole236is provided between the side surfaces of the upper frame end237. Therefore, the through hole236is provided to orthogonally cross the axial line L237of the upper frame end237. Therefore, upon fastening, the through hole236is also provided to orthogonally cross the axial line L241. The through hole236orthogonally cross the through hole235. An inner surface of the through hole235is internally threaded.

Note that the term “orthogonal” herein not only means exact 90 degree intersection but also includes a range of ±10 degrees from 90 degree intersection. The term “orthogonal” preferably refers to a range of ±5 degrees from 90 degrees, more preferably a range of ±2 degrees from 90 degree.

The lower frame end241further has two through holes245and246similarly to the upper frame end237. The through hole245is formed between the end surface243and a front surface of the lower frame end241. A depression248is formed at the front surface of the lower frame end241. The through hole245is provided between the end surface243and the depression248. Upon fastening, the through hole245is provided coaxially to the through hole235and obliquely crosses the axial line L241.

The through hole246is provided to orthogonally cross the axial line L241. Therefore, upon fastening, the through hole246is parallel to the through hole236.

Structure of Fastening Member25

The fastening member25includes bolts B1, B2and B5and plate type brackets253and254. The bolts B1, B2and B5are externally threaded. The bolt B1is inserted into the through holes235and245and fastens the lower frame end241to the upper frame end237. Upon fastening, the bolt B1is provided to obliquely cross the axial line L241. The axial line L241passes through the bolt B1. In this example, the axial line of the bolt B1is included in the same virtual plane as the axial line L241.

In the positional relation between the vehicle body frame2and the engine3, if the side where the engine3is provided in the vehicle body frame2in a side view is defined as “inner side,” the bolt B1is inserted into the through holes235and245from the outer side to the inner side of the vehicle body frame2. This makes it easier for an operator to insert the bolt B1during the manufacture of the motorcycle1.

The attachment bolt B5is used to attach the brackets253and254to the lower frame end241. The bolt B2further fastens the brackets253and254attached to the lower frame end241to the upper frame end237. Therefore, the brackets253and254are attached to the upper frame end237and the lower frame end241by the bolt B2and the attachment bolt B5.

The bracket253is provided in contact with a side surface of the upper frame end237and a side surface of the lower frame end241upon fastening as shown inFIG. 7. Referring toFIG. 6, the bracket253has through holes253A and253B. The through hole253A is provided above the through hole253B. The through hole253A is provided coaxially with the through hole236upon fastening, and the through hole253B is provided coaxially with the through hole246.

The bolt B2is inserted in the through holes253A and236and fastens the bracket253to the upper frame end237. At the time, the bolt B2is provided to orthogonally cross the axial line L241. The attachment bolt B5is inserted in the through holes253B and246and attaches the bracket253to the lower frame end241. At the time, the attachment bolt B5is provided to orthogonally cross the axial line L241. Furthermore, the bolts B2and B5are provided to orthogonally cross the bolt B1.

The bracket254is provided on a side surface opposite to the side surface where the bracket253is provided. The bracket254has through holes254A and254B. The through hole254A is provided above the through hole254B. The through hole254A is provided coaxially with the through hole236, and the through hole254B is provided coaxially with the through hole246. The through holes254A and254B are both internally threaded. The bolt B2is inserted in the through hole254A and fastens the bracket254to the upper frame end237. The attachment bolt B5is inserted in the through hole254B and attaches the bracket254to the lower frame end241.

The bracket254further has a through hole254C. The through hole254C is provided behind the through holes254A and254B at a rear end of the bracket254. The fastening member25further includes a bolt B6. As shown inFIG. 5, the bolt B4is inserted in the through hole254C and fastens the engine3to the bracket254.

As described above, as for the front fastened portions, the bolts B1, B2, and B5fasten the lower frame end241to the upper frame end237. At the time, the bolt B1is provided to obliquely cross the axial line L241. The bolts B2and B5are provided to orthogonally cross the axial line L241.

As described above, the rigidity of the front fastened portion against force in different directions is unlikely to vary by the function of the bolts B1, B2and B5provided in two different directions. The bolt B1is provided to obliquely cross the axial line L241, and therefore the compression stiffness, tensile rigidity, and bending stiffness of the front fastened portion are improved by the bolt B1. Furthermore, the bolts B2and B5are provided to orthogonally cross the axial line L241, and therefore, the torsional rigidity and bending stiffness of the front fastened portion are improved by the bolts B2and B5. Therefore, the rigidity of the front fastened portion is unlikely to vary against external force applied in different directions. Now, this will be described in detail.

Referring toFIG. 8, the frame specimen S1had a rectangular cross section and a fastened portion in the center. The frame specimen S1included frames S11and S12, and the frames S11and S12were fastened in the same manner as that of the front fastened portion inFIG. 7. More specifically, the frames S11and S12were coupled by a bolt B11, a pair of bolts B12, and a pair of brackets P1. The bolt B11was provided to obliquely cross the axial line of the frame specimen S1and fastened the frame S12to the frame S11. The pair of bolts B12secured each of the pair of brackets P1to one and the other side surfaces of the frames S11and S12. The pair of bolts B12was provided to orthogonally cross the axial line of the frame specimen S1.

Referring toFIGS. 9A and 9B, the frame specimen S2had a cross section whose shape and area were the same as those of the frame specimen S1. The frame specimen S2had a fastened portion in the center. The frame specimen S2included frames S21and S22. As shown inFIG. 9B, a tapered groove was formed at a rear end of the frame S21. A tip end of the frame S22had a tapered shape so that it was fitted into the groove of the rear end of the frame S21. The tip end of the frame S22was fitted into the groove of the rear end of the frame S21and fixed to the rear end of the frame S21by the pair of bolts B12. The pair of bolts B12was provided to orthogonally cross the axial direction of the frame specimen S2.

FIG. 10Ais a plan view of the frame specimen S3andFIG. 10Bis a side view. Referring toFIGS. 10A and 10B, the frame specimen S3had a cross section whose shape and area were the same as those of the frame specimen S1. The frame specimen S3had a fastened portion in the center. The frame specimen S3included frames S31and S32. Referring toFIG. 10B, a rear end of the frame S31had a step and a lower part of the rear end extended behind an upper part. A front end of the frame S32had a step and an upper end of the front end extended ahead of a lower part. The rear end of the frame S31is placed on the front end of the frame S32to form the frame specimen S3. The pair of the bolts B12was provided in a portion where the rear end of the frame S31and the front end of the frame S32are placed on each other and the frames S31and S32were fastened by the bolts B12. The bolts B12were provided to orthogonally cross the axial line of the frame specimen S3.

Referring toFIG. 11, the frame specimen S4was a tube stock whose cross section had the same shape and area as those of the frame specimen S1and had no fastened portion.

The frame specimens S1to S4all had the same length and outer size.

The frame specimens S1to S4were subjected to a tensile test and a compression test with respect to the axial direction of each frame specimen and their tensile rigidity and compression stiffness were obtained. Furthermore, the frame specimens S1to S3were subjected to a three point bending test and a cantilever bending test with respect to the axial direction of the bolts B12and their three point bending stiffness and cantilever bending stiffness were obtained. Furthermore, a three point bending test and a cantilever bending test were carried out in a direction orthogonal to the bolts B12, so that three point bending stiffness and cantilever bending stiffness in the direction orthogonal to the bolts B12were obtained.

The frame specimen S4was subjected to a three-point bending test and a cantilever bending test in an arbitrary direction (hereinafter as the “first direction”) orthogonal to the axial direction of the frame specimen S4and bending stiffness with respect to the first direction was obtained. The three-point bending stiffness and cantilever bending stiffness in the first direction were defined as the three-point bending stiffness and cantilever bending stiffness with respect to the lengthwise direction of the bolt B12. A three-point bending test and a cantilever bending test were carried out with respect to a direction (second direction) orthogonal to the axial direction of the frame specimen S4and the first direction and bending stiffness with respect to the second direction was obtained. The three-point bending stiffness and cantilever bending stiffness in the second direction were defined as three-point bending stiffness and cantilever bending stiffness with respect to the direction orthogonal to the bolt B12. Furthermore, the frame specimens S1to S4were each subjected to a torsion test and the torsional rigidity was obtained.

Test results for the frame specimens S1to S4are given inFIG. 12.FIG. 12is a graph showing rigidity values obtained from each of the tests for the frame specimens S1to S4(compression stiffness, tensile rigidity, the three-point bending stiffness in the lengthwise direction of the bolt B12, the cantilever bending stiffness in the lengthwise direction of the bolt B12, the three-point bending stiffness in the direction orthogonal to the bolt B12, and the cantilever bending stiffness in the direction orthogonal to the bolt B12) when the rigidity (stiffness) of the frame specimen S4in each test is 100. InFIG. 12, the solid line represents the result for the frame specimen S4. The broken line represents the result for the frame specimen S1. The chain dotted line represents the result for the frame specimen S2. The chain double dotted line represents the result for the frame specimen S3.

Referring toFIG. 12, the rigidity (stiffness) values in the tests for the frame specimen S1were close to corresponding rigidity values for the frame specimen S4. Therefore, the graph shape for the frame specimen S1was approximate to a regular heptagon similarly to the frame specimen S4. On the other hand, the frame specimen S2had a significantly higher stiffness value than the frame specimen S4in the three-point bending test for the orthogonal direction to the bolt b12. The torsional rigidity value was also high. Therefore, the graph shape for the frame specimen S2was an irregular shape different from a regular heptagon. The frame specimen S3had a significantly lower three point bending stiffness value with respect to the orthogonal direction to the bolt B12than that of the frame specimen S4, and the torsional rigidity value was significantly higher than the frame specimen S4. Furthermore, the tensile rigidity value and the compression stiffness value of the frame specimen S3were significantly lower than those of the frame specimen S4. Therefore, the graph shape for the frame specimen S3was an irregular shape different from a regular heptagon.

As can be understood from the test results described above, at the front fastened portion of the vehicle frame S1, rigidity values against external force applied in various different directions are unlikely to be unbalanced and the front fastened portion has similar stiffness to that of a tube stock. Therefore, a rider can maneuver the motorcycle1with the same feeling as if an integrally formed vehicle body frame is employed. In other words, the motorcycle1can provide improved operational feeling and riding comfort.

In addition, as shown inFIG. 6, the end surface243of the lower frame end241is inclined with respect to the axial line L241. If the fastened portion between the upper frame member273and the lower frame end241has a step like the one at the fastened portion at the frame specimens S31and S32shown inFIGS. 10A and 10B, a cross section greatly changes at the fastened portion. If the cross section greatly changes, force is not transmitted smoothly across the fastened portion, which degrades the operational feeling. In this example, the end surface243is inclined with respect to the axial line L241. Therefore, a cross section of the lower frame end241and the upper frame237at the fastened portion gradually changes from the tip end. Therefore, stress is smoothly transmitted across the fastened portion, so that the operational feeling is improved.

Furthermore, as shown inFIG. 6, the bolt B1is inserted in the through hole245having an opening at the end surface243of the lower frame end241and provided in the direction of the normal N243to the end surface243. Therefore, the bolt B1can firmly fasten the end surface243to the end surface234.

Furthermore, the brackets253and254are attached to the lower frame end241by the attachment bolt B5. The bolt B2then fastens the brackets253and254attached to the lower frame end241to the upper frame end237. At the time, the attachment bolt B5extends in the same direction as the bolt B2and is provided to orthogonally cross the axial line L241. Therefore, the torsional rigidity and the bending stiffness of the fastening members improve.

Furthermore, the engine3is attached to the bracket254. The bracket254fastens the upper frame20, the lower frames24and the engine3in a single location. In the vehicle body frame, as the number of fastened portions increases, the stiffness becomes more unbalanced. If the engine3is not attached to the bracket254and is fastened to another bracket provided in another location in the vehicle body frame2, the number of fastened portions in the vehicle body frame2increases. Since the bracket254serves to reduce the number of fastened portions in the vehicle body frame2, unbalance in the stiffness of the vehicle body frame2can be reduced and the operational feeling improves.

In this example, the brackets253and254were used for the front fastened portion. However, one of the brackets253and254may be used and the other does not have to be used. In such a case, the lower frame end241is fastened to the upper frame end237by the bolts B2and the bracket used.

In this example, the bracket253or254is attached to the lower frame end241by the attachment bolt B5. However, the bracket253and254may be attached to the lower frame end241by a method other than using the attachment bolt B5. For example, the brackets253and254may be welded to a side surface of the lower frame end241. Alternatively, the brackets253and254may be formed at the lower frame end241by cutting processing.

The brackets253and254may be attached to a side surface of the upper frame end237instead of the lower frame end241. In this case, the brackets253and254are fastened to the lower frame end241by the attachment bolts B5. In short, a bracket may be attached to one of the upper frame end237and the lower frame end241and fastened to the other by the bolts.

Details of Rear Fastened Portion

As for the rear fastened portion, the same method applied to the front fastened portions is employed.FIG. 13is a side view of a rear fastened portion. Referring toFIG. 13, the rear fastened portion includes the lower frame end242, the upper frame end221, and the fastening member26. The lower frame end242is provided at a rear end of the lower frame24. The upper frame end221is provided at a lower end of the main frame22.

FIG. 14is a side view of the lower frame end242and the upper frame end221in the rear fastened portion.FIG. 15is an exploded perspective view of the rear fastened portion. Referring toFIGS. 13 to 15, the fastening member26includes a bolt B3, a bolt B4, a nut N1, and a bracket222. The fastening member26fastens the lower frame end242to the upper frame end221.

The lower frame end242has an end surface243at a rear end. The end surface243is a plane and inclined with respect to an axial line L242of the lower frame end242. The lower end of the end surface243is positioned behind the upper end of the end surface243. The lower frame end242has through holes245and247. The through hole245is formed between the end surface243and a depression246formed at a lower surface of the lower frame end242. Therefore, the through hole245has an opening each at the end surface243and the surface of the depression246.

The through hole247is formed between the side surfaces of the lower frame end242and is provided to orthogonally cross the axial line L242.

The upper frame end221has an end surface224at a lower end. The end surface224is provided opposed to the end surface243. The upper frame end221has a non-penetrating hole225. The non-penetrating hole225extends backward and obliquely upward from the end surface224. Upon fastening, the non-penetrating hole225is provided coaxially with the through hole245. The inner surface of the non-penetrating hole225is internally threaded.

At the lower end of the upper frame end221, a bracket222is attached adjacent to the end surface224. In this example, the bracket222is integrally attached to the lower end of the upper frame end221. The bracket222extends forward and has a through hole227. Upon fastening, the through hole227is provided coaxially with the through hole247.

The bolt B3is inserted into the through hole245and the non-penetrating hole225and fastens the lower frame end242to the upper frame end221. Upon fastening, the bolt B3is provided obliquely to the axial line L242. The axial line L242passes through the bolt B3. In this example, the axial line of the bolt B3and the axial line L242are included in the same virtual plane.

The bolt B3is inserted into the through hole245and the non-penetrating hole225from the outer side to the inner side of the vehicle body frame2. This makes it easier for an operator to insert the bolt B3during the manufacture of the motorcycle1.

The bolt B4is inserted into the through holes247and227. At the time, the tip end of the bolt B4protrudes from the through hole247. The nut N1is attached to the tip end of the bolt B4protruding from the through hole247. The bolt B4and the nut N1fasten the upper frame end242to the upper frame end221. The bolt B4is provided to orthogonally cross the axial line L242.

Using the bolts B3and B4, unbalanced rigidity at the rear fastened portion against external force in different directions is reduced. Using the bolt B3, the rear fastened portion has improved compression stiffness, tensile rigidity, and bending stiffness. Using the bolt B4, the rear fastened portion has improved torsional rigidity and bending stiffness. This improves operation feeling.

According to the above-described embodiment, as shown inFIG. 5, the main frame22has a part that overlaps the engine3and the lower frame24has a part that overlaps the engine3in a side view. However, in the vehicle body frame2, either the main frame22or the lower frame24may overlap the engine3. Even in this case, the vehicle body frame2is smaller than the conventional integrally formed vehicle body frame.

According to the above-described embodiment, the vehicle body frame2includes the fastening members25and26. However, the vehicle body frame2may have one of the fastening members25and26. For example, the vehicle body frame2may include the fastening member25and another fastening member different from the fastening member26may be attached at the rear fastened portion. In this way, stiffness unbalance in the vehicle body frame2can be reduced to some extent.

According to the above-described embodiment, the end surfaces243and223are planes. However, the end surfaces243and223do not have to be planes.

According to the above-described embodiment, the fastening member26includes the nut N1. However, the fastening member26does not have to include the nut N1. In this case, the inner surfaces of the through holes227and247are internally threaded.

According to the above-described embodiment, the bracket222is integrally attached to the upper frame end221. However, the bracket222may be attached to the upper frame end221by a different method. For example, the bracket222may be attached to the upper frame end221by an attachment bolt.

Alternatively, the bracket222may be attached to the lower frame end242instead of the upper frame end221. In this case, the upper frame end221has a side surface in contact with the bracket222and the side surface has a hole (that may be either a through hole or non-penetrating hole) provided coaxially with the through hole227. The bolt B4is inserted into the through hole227and a hole formed in the upper frame end221and the lower frame end242is fastened to the upper frame end221.

According to the above-described embodiment, the upper frame20includes a pair of main frames22. However, only one of the main frames22may be provided. Only one of the upper frames24may be provided. The front frame23does not have to have branch frames.

While preferred embodiments of the present invention have been described above, it is to be understood that the embodiments disclosed herein are illustrative and not restrictive. The above-described embodiment can be carried out in various modified forms without departing the scope and spirit of the present invention.