a wheel unit (10) having a wheel (101) and a front fork (11) with which the wheel (101) is rotatably connected, an insertion hole (111) defined through a top of the front fork (11) and communicating with an opening (112) and a through hole (113) defined in an underside of the front fork (11), the opening (112) facing the wheel (101);

a first tube (12) having a first end thereof securely inserted into the insertion hole (111), a first passage (121) and a second passage (122) defined axially in the first tube (12), a second tube (13) having a first end connected to a second end of the first tube (12) and a first pivot (131) located on a side of the second tube (13), the second tube (13) including a first hole (132) and a second hole (133), the first hole (132) communicating with the first passage (121), the second hole (133) communicating with the second passage (122), a third tube (14) having a first end connected to a second end of the second tube (13) and having a second pivot (141) on a side thereof, the first and second pivots (131, 141) being pivotably connected to each other, the third tube (14) including a third hole (142) and a fourth hole (143), the third hole (142) communicating with the first hole (132) and the fourth hole (143) communicating with the second hole (133), a fourth tube (15) having a first end connected to a second end of the third tube (14), the fourth tube (15) including a third passage (151) and a fourth passage (152) defined axially therethrough, the third passage (151) communicating with the third hole (142) and the fourth passage (152) communicating with the fourth hole (143), the first and third passages (121, 151) communicating with each other and the second and fourth passages (122, 152) communicating with each other, a fifth tube (16) connected between the fourth tube (15) and a handle (17), the handle (17) having a first control slot (171) defined in a side thereof, a second control slot (173) defined in a top thereof and a third control slot (176) defined in an underside thereof, the first, second and third control slots (171, 173, 176) communicating with each other, the fifth tube (16) engaged with the third control slot (176), a first recess (172) defined in a lower portion of the first control slot (171), a second recess (174) and a third recess (175) respectively defined in the handle (17) and located on two ends of the second control slot (173);

a measuring unit (20) including a base (21), a top cover (22), a counter (23), a first arm (24) and a second arm (25), the base (21) connected to the front fork (11) and including a control slot (213) which faces the through hole (113) of the front fork (11), the top cover (22) mounted onto the base (21) and a counter (23) located in the base (21), the counter (23) including a rotary portion (233) which is rotated when the wheel (101) rolls, the first arm (24) located in the base (21) and having a first control member (241) which is located in the control slot (213) and the through hole (113), the first control member (241) having an end received in the second passage (122) of the first tube (12), the second arm (25) located in the base (21) and driven by the first arm (24), the second arm (25) initializing the counter (23);

a brake unit (30) having a first brake rod (31), a brake key (32) and a second brake rod (33), the first brake rod (31) received in the third passage (151) of the fourth tube (15) and a first link (311) connected to a top of the first brake rod (31), a first contact section (312) connected to a lower end of the first brake rod (31) and located in the third hole (142) of the third tube (14), the brake key (32) extending through the first control slot (171) of the handle (17) and having a third pivot (321) which is pivotably engaged with the first recess (172) of the handle (17), a fourth pivot (322) located at a distance from the third pivot (321) and the first link (311) pivotably connected to the protrusion (322), the second brake rod (33) received in the first passage (121) of the first tube (12) and a top of the second brake rod (33) received in the first hole (132) of the second tube (13) and contacting the first contact section (312), a brake pad (331) connected to a lower end of the second brake rod (33) and located in the opening (112) of the front fork (11) so as to brake the wheel (101) when the first brake rod (31) is moved to push the second brake rod (33) downward, and

an initialization unit (40) including a first initialization rod (41), an initialization key (42), a pivotal member (43) and a second initialization rod (44), the first initialization rod (41) received in the fourth passage (152) of the fourth tube (15) and a second link (411) connected to a top of the first initialization rod (41), a second contact section (412) connected to a lower end of the initialization rod (41), the second contact section (412) located in the fourth hole (143) of the third tube (14), the initialization key (42) located in the handle (17) and having a fifth pivot (421) which is engaged with the second recess (174), the initialization key (42) including a button (422) on a top thereof and the pivotal member (43) having an action end (431) which is located in contact with an underside of the button (422), the pivotal member (43) including a sixth pivot (432) which is engaged with the third recess (175) in the handle (17), the pivotal member (43) including a seventh pivot (433) which is connected with the second link (411), the second initialization rod (44) received in the second hole (133) of the second tube (13) and contacting the second contact section (412), an initialization section (441) connected to a lower end of the second initialization rod (44) and being in contact with the first control member (241) of the first arm (24).