FR 3125995 A1 20230210 FR 2208041 A 20220803 TILTING VEHICLE A tilting vehicle includes: a steering mechanism (31) that has two handle grips (311) arranged along an imaginary auxiliary line (L); a tilting mechanism (32) that has two upright linking bars (322), and upper and lower crossbars (321, 323) pivoted to the upright linking bars (322); a power unit (4) that includes a crankshaft (42) extending along a rotational axis (R); and an air cleaner unit (5) that is disposed between the tilting mechanism (32) and an upright longitudinal plane (S3) overlapping the rotational axis (R), and that is disposed in a predetermined zone (Z) bounded by a horizontal top plane (S1) overlapping the auxiliary line (L), two upright side planes (S2) respectively intersecting the upright linking bars (322) and extending along a front-rear direction, and a horizontal bottom plane (S4) extending horizontally and overlapping the rotational axis (R). [FIG. 3] TILTING VEHICLE The disclosure relates to a vehicle, and more particularly to a tilting vehicle having two tilting front wheels. Referring to , a conventional motorcycle 1 includes a vehicle body 10, a rear swing arm 11 connected to the vehicle body 10, a rear wheel 12 rotatably connected to the rear swing arm 11, and an air cleaner 13 mounted next to the rear wheel 12 and disposed at a height level between the vehicle body 10 and the rear swing arm 11. Since the air cleaner 13 is exposed to the external environment and disposed next to the rear wheel 12, foreign objects such as dirt particles may easily accumulate on the air cleaner 13 and affect the performance thereof. In addition, for existing tilting vehicles such as ones equipped with two tilting front wheels, the above-mentioned configuration (i.e., the air cleaner 13 being disposed next to the rear wheel 12) becomes less appropriate considering the differences in spatial arrangements of the components surrounding the air cleaner or the rear wheels. In such cases, the air cleaner becomes more susceptible to damages caused by foreign objects and may need to undergo maintenance more frequently to achieve stable operation. Therefore, an object of the disclosure is to provide a tilting vehicle having two tilting front wheels and an air cleaner unit that can alleviate at least one of the drawbacks of the prior art. According to the disclosure, a tilting vehicle includes a chassis, a steering unit mounted to the chassis, and including a steering mechanism that has two spaced-apart handle grips arranged along an imaginary auxiliary line, a power unit mounted to the chassis and disposed behind the steering unit, and an air cleaner unit mounted in the chassis. The tilting vehicle is characterized in that the steering unit further includes a tilting mechanism and two front wheel mechanisms. The tilting mechanism is connected to the steering mechanism, and has two upright linking bars, each of which has top and bottom end segments, an upper crossbar which has opposite end segments respectively pivoted to the top end segments of the upright linking bars, and a lower crossbar which is disposed under the upper crossbar, and which has opposite end segments respectively pivoted to the bottom end segments of the upright linking bars. The front wheel mechanisms are connected to the upright linking bars of the tilting mechanism, respectively. The power unit includes an engine case, and a crankshaft that extends transversely and rotatably through the engine case along a rotational axis. The air cleaner unit is disposed in a predetermined zone bounded by a horizontal top plane that overlaps the auxiliary line, two upright side planes that respectively intersect the upright linking bars and that extend along a front-rear direction, and a horizontal bottom plane that extends horizontally and that overlaps the rotational axis, and is disposed between the tilting mechanism and an upright longitudinal plane that overlaps the rotational axis. Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment with reference to the accompanying drawings, of which: is a fragmentary side view of a conventional motorcycle having an air cleaner; is a side view of an embodiment of a tilting vehicle according to the disclosure; is a fragmentary side view of the embodiment, to be viewed in conjunction with for illustrating an air cleaner unit mounted in a chassis of the embodiment; is a fragmentary front view of the embodiment, illustrating a steering unit of the embodiment; is a perspective view illustrating the air cleaner unit of the embodiment; and are fragmentary cross-sectional views of the air cleaner unit, to be viewed in conjunction with for illustrating a main body and an air inlet component of the air cleaner unit; and is a fragmentary rear view of the embodiment, illustrating relative positions of the air cleaner unit and a tilting mechanism of the steering unit. Referring to FIGS. 2 and 3, an embodiment of a tilting vehicle according to the disclosure includes a chassis 2 made of a rigid material, a steering unit 3 mounted to the chassis 2, a power unit 4 mounted to the chassis 2 and disposed behind the steering unit 3, and an air cleaner unit 5 mounted in the chassis 2. Referring to FIGS. 3 and 4, the steering unit 3 includes a steering mechanism 31, a tilting mechanism 32 connected to the steering mechanism 31, and two front wheel mechanisms 33 connected to the tilting mechanism 32. The steering mechanism 31 includes a steering mechanism 31 that has two spaced-apart handle grips 311 arranged along an imaginary auxiliary line (L). The handle grips 311 are adapted to be operated by a rider's hands by rotation, such that the tilting mechanism 32 is driven to synchronously drive the front wheel mechanisms 33 to perform tilting and/or steering. The tilting mechanism 32 has two upright linking bars 322, each of which has top and bottom end segments, an upper crossbar 321 which has opposite end segments respectively pivoted to the top end segments of the upright linking bars 322, and a lower crossbar 323 which is disposed under the upper crossbar 321, and which has opposite end segments respectively pivoted to the bottom end segments of the upright linking bars 322. The front wheel mechanisms 33 are connected to the upright linking bars 322 of the tilting mechanism 32, respectively, and define therebetween a passageway (H) that is spatially communicated with the air cleaner unit 5. By virtue of the aforementioned configurations, the upper crossbar 321, the upright linking bars 322, and the lower crossbar 323 are pivotally connected to one another and cooperatively form a quadrilateral shape that enables the tilting of the front wheel mechanisms 33. In addition, the passageway (H) between the front wheel mechanisms 33 allows maintenance or repair of the air cleaner unit 5 to be performed without having to disassemble the front wheel mechanisms 33. The power unit 4 includes an engine case 41, a crankshaft 42 that extends transversely and rotatably through the engine case 41 along a rotational axis (R), and a cylinder 43 that is disposed in the engine case 41 and that is spatially communicated with the air cleaner unit 5. Referring to FIGS. 5 to 7, the air cleaner unit 5 includes a main body 51, a cleaner medium 52, and an air inlet component 53. The main body 51 defines an inner space 510, and is formed with an upward-facing inlet port 511 that is spatially communicated with the inner space 510. The cleaner medium 52 is detachably disposed in the inner space 510. The air inlet component 53 is connected to the main body 51 at the inlet port 511 and extends upwardly. Specifically, the main body 51 is further formed with two exhaust ports 512 that intercommunicate spatially with the inner space 510 of the main body 51 and the cylinder 43, and that is disposed above the cylinder 43. The air inlet component 53 is configured as an airflow guiding structure that is integrally connected to the main body 51 as one piece, and has an air collecting tube 531 that is spatially communicated with the inlet port 511, and two intake tubes 532 that are connected to left and right sides of the air collecting tube 531, respectively. Each of the intake tubes 532 is formed with a suction port 530 being open in a direction that is inclined downwardly and rearwardly. Referring further to FIGS. 7 and 8 in conjunction with , the suction port 530 of each of the intake tubes 532 is disposed above the lower crossbar 323 of the tilting mechanism 32 relative to the ground, and is disposed substantially at the same height as the upper crossbar 321 of the tilting mechanism 32. Since the inlet port 511 is facing upwardly and away from the ground, chances of heavy dirty particles, water droplets, and other foreign objects entering the inlet port 511 are reduced. In addition, since the suction ports 530 are open in a direction that is inclined downwardly and rearwardly, and are arranged on the left and right sides of the air collecting tube 531, an air intake thereof is optimized and stabilized in terms of its intake volume and direction (i.e., from bottom to top). Furthermore, the position of the air cleaner unit 5 relative to the upper crossbar 321 and the lower crossbar 323 may ensure that the suction ports 530 are disposed substantially at the same height as the upper crossbar 321 of the tilting mechanism 32, and foreign objects moving rearwardly during travel can be greatly kept from entering the suction ports 530, thereby reducing the workload of the cleaner medium 52 and thus extending the lifespan of the cleaner medium 52. Referring to in conjunction with , the air cleaner unit 5 is disposed in a predetermined zone (Z) bounded by a horizontal top plane (S1) that overlaps the auxiliary line (L), two upright side planes (S2) that respectively intersect the upright linking bars 322 and that extend along a front-rear direction, and a horizontal bottom plane (S4) that extends horizontally and that overlaps the rotational axis (R). Furthermore, the air cleaner unit 5 is disposed above the cylinder 43 and between the tilting mechanism 32 and an upright longitudinal plane (S3) that overlaps the rotational axis (R). By virtue of the configuration of the exhaust ports 512 of the air cleaner unit 5 being disposed above the cylinder 43, the air cleaner unit 5 is kept at a certain height above the ground, and the air cleaned by the air cleaner unit 5 may enter the cylinder 43 effectively from the exhaust ports 512 of the air cleaner unit 5. Additionally, since the air cleaner unit 5 is disposed in the predetermined zone (Z) and in front of the rotational axis (R) of the crankshaft 42, the air cleaner unit 5 is allowed to be arranged in a spatial position that is appropriate for various types of tilting vehicles that are equipped with a front wheel tilting mechanism equivalent to the present embodiment. Moreover, since the air cleaner unit 5 is disposed behind the tilting mechanism 32 and at a certain distance from the front wheel mechanisms 33, chances of foreign objects entering the air cleaner unit 5 are further reduced, which ensures a stable operation of the air cleaner unit 5. In summary, by virtue of the relative positions of the air cleaner unit 5 and the tilting mechanism 32, the present embodiment of the tilting vehicle according to the disclosure is suitable for various types of vehicles in reducing chances of foreign objects entering the air cleaner unit 5 as well as facilitating air intake of cylinder 43 of the power unit 4 from the air cleaner unit 5, thereby ensuring stable operation of the air cleaner unit 5 and extending the service life of the air cleaner unit 5. In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiment. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects, and that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure. A tilting vehicle including a chassis (2), a steering unit (3) mounted to said chassis (2), and including a steering mechanism (31) that has two spaced-apart handle grips (311) arranged along an imaginary auxiliary line (L), a power unit (4) mounted to said chassis (2), and disposed behind said steering unit (3), and an air cleaner unit (5) mounted in said chassis (2), said tilting vehicle being characterized in that: said steering unit (3) further includes a tilting mechanism (32) that is connected to said steering mechanism (31), and that has two upright linking bars (322), each of which has top and bottom end segments, an upper crossbar (321) which has opposite end segments respectively pivoted to said top end segments of said upright linking bars (322), and a lower crossbar (323) which is disposed under said upper crossbar (321), and which has opposite end segments respectively pivoted to said bottom end segments of said upright linking bars (322), and two front wheel mechanisms (33) that are connected to said upright linking bars (322) of said tilting mechanism (32), respectively; said power unit (4) includes an engine case (41), and a crankshaft (42) that extends transversely and rotatably through said engine case (41) along a rotational axis (R); and said air cleaner unit (5) is disposed in a predetermined zone (Z) bounded by a horizontal top plane (S1) that overlaps the auxiliary line (L), two upright side planes (S2) that respectively intersect said upright linking bars (322) and that extend along a front-rear direction, and a horizontal bottom plane (S4) that extends horizontally and that overlaps the rotational axis (R), and is disposed between said tilting mechanism (32) and an upright longitudinal plane (S3) that overlaps the rotational axis (R). The tilting vehicle as claimed in Claim 1, further characterized in that said air cleaner unit (5) includes a main body (51) that defines an inner space (510), and that is formed with an upward-facing inlet port (511) spatially communicated with said inner space (510), a cleaner medium (52) that is detachably disposed in said inner space (510), and an air inlet component (53) that is connected to said main body (51) at said inlet port (511) and that extends upwardly. The tilting vehicle as claimed in Claim 2, further characterized in that said air inlet component (53) of said air cleaner unit (5) has an air collecting tube (531) that is spatially communicated with said inlet port (511), and at least one intake tube (532) that is connected to said air collecting tube (531) and that is formed with a suction port (530). The tilting vehicle as claimed in Claim 3, further characterized in that said suction port (530) of said at least one intake tube (532) is open rearwardly. The tilting vehicle as claimed in any one of Claims 3 and 4, further characterized in that said suction port (530) of said at least one intake tube (532) is open in a direction that is inclined downwardly and rearwardly. The tilting vehicle as claimed in Claim 3, further characterized in that said air inlet component (53) of said air cleaner unit (5) has two intake tubes (532) that are connected to left and right sides of said air collecting tube (531), respectively. The tilting vehicle as claimed in any one of Claims 3 and 6, further characterized in that said suction port (530) of said at least one intake tube (532) is disposed above said lower crossbar (323) of said tilting mechanism (32). The tilting vehicle as claimed in any one of Claims 3 and 6, further characterized in that said suction port (530) of said at least one intake tube (532) is disposed substantially at the same height as said upper crossbar (321) of said tilting mechanism (32). The tilting vehicle as claimed in Claim 1, further characterized in that said front wheel mechanisms (33) define therebetween a passageway (H) that is spatially communicated with said air cleaner unit (5). The tilting vehicle as claimed in Claim 2, further characterized in that said power unit (4) further includes a cylinder (43) that is disposed in said engine case (41), said main body (51) of said air cleaner unit (5) being further formed with at least one exhaust port (512) that spatially intercommunicates with said inner space (510) of said main body (51) and said cylinder (43), and that is disposed above said cylinder (43).