61, an oil guide pipe 62, and a floating piston 63. As shown in FIG. 4, the knob 61 has a threaded head portion 612 screwed to the adjustment threaded hole 35 of the inner tube 30, and a body portion 614 connected with the threaded head portion 612 and inserted into the first oil chamber 33. Additionally, the knob 61 further has a hexagonal hole 616 recessed from the threaded head portion 612 toward the body portion 614 for engagement of a tool (such as a hex wrench, not shown). This ensures that when the knob 61 is driven by the aforementioned tool, it can be moved outward to rest against the outer stop ring 356 or inward to rest against the inner stop portion 352. As shown in FIGS. 2, 3, and 6, the oil guide pipe 62 is disposed in the inner tube 30, including a first guide end 622, a second guide end 624, and an oil channel 626 between the first and second guide ends 622, 624. The first guide end 622 is connected to a first oil guide member 64 screwed to the top end 31 of the inner tube 30 and provided with a first guide hole 642 communicating with the second oil chamber 34 and the guide channel 626. The second guide end 624 is penetrated through the top cover 53 and connected to a second oil guide member 65 located inside the inner cylinder 51 and provided with a plurality of second guide holes 652 communicating with the guide channel 626. In this embodiment, the floating piston 63 is disposed between the top cover 53 and the second oil guide member 65 and penetrated by the second guide end 624 of the oil guide pipe 62.

It can be seen from the above that when reducing a maximum upward stroke of the inner tube 30, first open the control valve 56 (as shown in FIG. 7) and press down the inner tube 30 to exceed its adjustable stroke, then close the control valve 56, temporarily positioning the inner tube 30. Next, rotate the knob 61 inward (as shown in FIG. 8), causing the oil O in the first oil chamber 33 to flow through the passage 316 into the second oil chamber 34. Then, the oil O flows from the second oil chamber 34 through the first guide hole 642 of the first oil guide member 64 into the oil channel 626 of the oil guide pipe 62 (as shown in FIG. 3). The oil O then flows through the oil channel 626 into the second guide holes 652 of the second oil guide member 65, and finally flows from the second guide holes 652 of the second oil guide member 65 into a third oil chamber 66 formed between the second guide end 624 of the oil guide pipe 62 and the floating piston 63 (as shown in FIG. 9). Thereafter, reopen the control valve 56 and release the inner tube 30. At this point, the inner tube 30 will automatically rise. During the upward movement, the oil O in the third oil chamber 66 pushes the floating piston 63 upward until the floating piston 63 comes into contact with the top cover 53, at which point the upward movement will stop. In this way, as shown in FIG. 10, the inner tube 30 is restricted by the third oil chamber 66 to complete the reduction of the maximum upward stroke.

After complete the reduction of the maximum upward stroke of the inner tube 30, if a rider lowers the height of the inner tube 30 to the minimum by using the pneumatic and hydraulic cylinder 50 after the ride, the next time the rider rides, simply opening the pneumatic and hydraulic cylinder 50 will cause the maximum upward stroke of the inner tube 30 to be limited by the third oil chamber 66, allowing the inner tube 30 to only rise to the height shown in FIG. 10. For the same rider, there will be no need to readjust the seat height, thus effectively saving operational time and increasing user convenience.

Conversely, to restore the original upward stroke, first open the control valve 56 and press down the inner tube 30 to exceed its adjustable stroke, then close the control valve 56. Next, rotate the knob 61 outward (as shown in FIG. 4), allowing the oil O in the second oil chamber 34 to flow back into the first oil chamber 33. Then, the oil O in the third oil chamber 66 flows through the second guide holes 652 of the second oil guide member 65 into the oil channel 626 of the oil guide pipe 62. The oil O then travels along the oil channel 626 and enters the second oil chamber 34 through the first guide hole 642 of the first oil guide member 64. Thereafter, reopen the control valve 56 and release the inner tube 30. At this point, the inner tube 30 will automatically rise until the second oil guide member 65 rests against the floating piston 63 (as shown in FIG. 6). This completes the restoration of the original upward stroke of the inner tube 30.

Please refer to FIG. 11, the main structure of the dropper seatpost 12 of the second embodiment is approximately the same with the first embodiment, but one of the differences therebetween is that the inner tube 70 omits the first oil chamber 33 and the second oil chamber 34, and the stroke adjustment assembly 80 uses an adjustment screw 84 to control the oil volume.

In this embodiment, as shown in FIG. 12, the inner tube 70 includes an adjustment hole 72 axially passing through the top end 71. Further, the stroke adjustment assembly 80 includes an adjustment screw 84 penetrated through the adjustment hole 72 and screwed to the first guide end 812 of the oil guide pipe 81. When reducing the maximum upward stroke of the inner tube 70, first detach the saddle clamping device 40, then open the control valve 56 (as shown in FIG. 7) and press down the inner tube 70 to exceed its adjustable stroke, then close the control valve 56, temporarily positioning the inner tube 70. Next, use a tool (such a screwdriver, not shown) to rotate the adjustment screw 84 through the adjustment hole 72 (as shown in FIG. 13), causing adjustment screw 84 to push the oil O in the oil channel 814 to flow through the second guide holes 822 of the second oil guide member 82 into the third chamber 86 (as shown in FIG. 14). Thereafter, reopen the control valve 56 and release the inner tube 70. At this point, the inner tube 70 will automatically rise. During the upward movement, the oil O in the third oil chamber 66 pushes the floating piston 63 upward until the floating piston 63 comes into contact with the top cover 53, at which point the upward movement will stop. In this way, as shown in FIG. 15, the inner tube 70 is restricted by the third oil chamber 86 to complete the reduction of the maximum upward stroke.