1. Field of the Invention
The present invention relates to a shock-absorbing frame, and more particularly to a shock-absorbing frame for a bicycle, wherein the distance between the drive chain wheel and the driven chain wheel is kept at a constant, so that the chain is mounted between the drive chain wheel and the driven chain wheel rigidly and stably without detachment so as to protect the rider's safety.
2. Description of the Related Art
A conventional shock-absorbing frame for a bicycle in accordance with the prior art shown in FIGS. 1–3 comprises a first body 1, a second body 2, a rear fork 3, a shock-absorbing device 4, and two levers 5. The first body 1 includes a head tube 103, a top tube 102, a bottom tube 104, and a seat tube 106. The seat tube 106 has a lower end formed with a pivot tube 105. The lower end of the seat tube 106 is provided with a protruding connecting seat 108 located adjacent to the pivot tube 105. The second body 2 is pivotally mounted on the first body 1 and includes two arms 202 each having a first end pivotally mounted on the connecting seat 108 by a first pivot shaft 204 and a second end snapped onto a wheel axle 702 of a rear wheel 7 of the bicycle. The rear fork 3 includes two fork arms 302 each having a first end pivotally mounted on the second end of a respective one of the arms 202 of the second body 2 and a second end formed with a connecting post 304. The shock-absorbing device 4 includes a cylinder 402 and a piston rod 404 retractably mounted on the cylinder 402 and has a first end pivotally mounted on a connecting block 1042 on the bottom tube 104. Each of the two levers 5 has a first end pivotally mounted on the connecting post 304 of a respective one of the fork arms 302 of the rear fork 3, a mediate portion pivotally mounted on the seat tube 106 by a second pivot shaft 502 and a second end pivotally mounted on a second end of the shock-absorbing device 4. A drive unit 8 includes a driven chain wheel 802 mounted on the wheel axle 702 of the rear wheel 7, a rotation shaft 806 mounted on the pivot tube 105, a drive chain wheel 804 mounted on the rotation shaft 806, and a chain 803 mounted between the drive chain wheel 804 and the driven chain wheel 802.
As shown in FIG. 2, when the seat tube 106 is subjected to a downward force from a rider's weight, the front wheel 6 is subjected to a reaction from the ground, so that the first body 1 is rotated about the first pivot shaft 204, while the rear wheel 7 is subjected to the reaction from the ground, so that the second body 2 is rotated about the first pivot shaft 204. At this time, the shock-absorbing device 4 exerts a force on the two levers 5 to drive the rear fork 3 to press the wheel axle 702 of the rear wheel 7, thereby forming a force balance state.
As shown in FIG. 3, when the bicycle is ridden on a rugged road, the front wheel 6 and the rear wheel 7 are subjected to the reaction from the ground, so that the first body 1 and the second body 2 are rotated about the first pivot shaft 204. At this time, the shock-absorbing device 4 is compressed to reduce the vibration due to rotation of the rear wheel 7, thereby providing a shock-absorbing effect.
However, when the first body 1 and the second body 2 are rotated relative to each other, the wheel axle 702 of the rear wheel 7 and the driven chain wheel 802 are moved relative to drive chain wheel 804 about the first pivot shaft 204, thereby reducing the distance between the drive chain wheel 804 and the driven chain wheel 802, so that the chain 803 mounted between the drive chain wheel 804 and the driven chain wheel 802 easily becomes loosened or detached, thereby causing inconvenience and danger to the rider.