Background Art Various geared (transmission) mechanisms are known at present. Most of them, including automatic gear boxes, have a predetermined number of gear speeds. It happens in practice that a gear box having these gear speeds (stages) does not always allow maximum energy utilization. Often a different gear stage would be more favorable which cannot be changed to with a discontinuous gear box.

This problem, for example in the case of bicycles, is solved by a high number of gear speeds. Modern bicycles are commonly equipped with 18 to 24 gear stages. Nevertheless, at full load it is often too complicated to change to a different gear stage. Moreover, the necessity of using maximum propelling force of a rider at the moment of maximum repulsion of the vehicle, and incomplete utilization of his energy for propelling the vehicle reflects in low dynamic properties and considerable difficulty, as well as in the driver's fatigue when propelling the vehicle. As concerns automatic gear boxes for bicycles, none of them has been used so far.

There have been many attempts to modify the transmission mechanism, as well as the propelling mechanism for bicycles. For example, EP-0 88448 A2 describes a propelling mechanism and a transmission mechanism for lever propelled bicycles, where the gear change is obtained by moving a connecting rod along an arm to change the gear. Individual gear changes are obtained by manual actuation which is a disadvantage of the solution. EP-0 29055 describes a bicycle pedal which is firmly joined to an arm. The angle between the pedal and the arm is greater than 90°. The above given solution does not allow to perform uniform movement of the arm, and the movement is transmitted to the whole wheel, including the pedals and the cyclist. The non-uniformity of the movement is an accompanying feature of any lever gears.

The published CS patent application No. PV 3881-92 describes an angular bicycle which is propelled by pedal arms with a transmission system. It has a disadvantage that it allows the use of only one or, as a maximum, of two gear stages.

Contrary to the above given solution CS pat. 269 402 describes a four-joint lever mechanism which enables better energy utilization of the cyclist. It has a disadvantage that it contains additional elements (levers, joints) which contribute to higher friction and energy loss.

CS utility model No. 1281 describes a spring transmission of the force for a vehicle, propelled by a rider, which makes use of a swinging double-arm lever. A change of the propelling force transmission to the driving wheel is performed continuously by any displacement of the point of application of the force along the arm of the double-arm lever. A disadvantage is the very necessity to change the point of application of the force, i. e. the force does not act along one trajectory, but the trajectory changes.

The object of the present invention is the minimization of the non-uniform movement for lever transmissions, and to create a lever transmission mechanism having the possibility to change the gears continuously.

Disclosure of Invention Most of the above mentioned disadvantages are eliminated by a spring- tensioned gear (transmission) mechanism for changing gears for a vehicle, propelled by a force, according to the present invention, the nature of which consists in that it comprises at least one lever which is at one of its ends rotary attached to the vehicle frame and at its other end rotary joined to a draw rod which is rotary joined to a driving element. The driving element is at its central axis rotary joined to a delimiter which is rotary attached to an axis, passing the center of the driven small wheel, and the delimiter is further rotary joined to a spring tensioning device, equipped with a spring, which device is rotary attached to the vehicle frame. Rotational movement of the driving element is transmitted to the driven small wheel by means of a movement transmitter which is either a belt or a geared assembly. The spring-tensioned transmission mechanism is further equipped with at least one free wheel.

The spring-tensioned transmission mechanism may also consist of a pair of levers, draw rods, driving elements, delimiters, driven small wheels and movement transmitters, situated in mirror arrangement on both sides of the vehicle frame. The levers may be rotary joined with each other through a lever movement synchronizer, and the lever movement synchronizer consists of arms, rotary joined to a rocking lever. The pair of driving elements may also be firmly joined to each other; in that case also the driven small wheels are joined to each other.

The spring-tensioning device is preferably equipped with a spring. It has been found that it is preferred when the spring-tensioned transmission mechanism according to the present invention is provided with a blocking element and the levers are rotary joined to each other through a lever movement synchronizer which consists of arms, rotary joined to a rocking lever.

An advantage of a spring-tensioned gear mechanism according to the present invention consists in that in this way a simple automatic gear box arises with continuous gear changing, with minimized non-uniform movement which non-uniform movement is usual for lever bicycles, known so far. The arc which is followed by the lever is moderate, and this favorably reflets in low energy loss.

When changing a gear, problems are eliminated which are common when transferring the chain from a small wheel to another small wheel in the case of classical gear change at bicycles. Also the necessity of gear on both bicycle wheels is eliminated. The influence of man on the gear adjusting is eliminated- the gear choice is not performed by the rider according to his subjective need, but it is set automatically to the optimum value as a response to the magnitude of the acting force. A further advantage consists in the possibility of better torsional moment utilization.

Moreover, the structure of the spring-tensioned gear mechanism according to the present invention is simple not only from the structural point of view, but it is also less demanding from the material point of view (it does not require such highly resistant materials as"classical"gear boxes do).

Brief Description of the Drawinqs The spring-tensioned geared mechanism will be explained in more detail by means of pictures on the accompanying drawings in which: Fig. 1 shows a side view of a"one-sided"spring-tensioned transmission mechanism, Fig. 2 shows a side view of a spring-tensioned transmission mechanism with pairs of individual elements, Fig. 3 shows an alternative of the movement transmitter, Fig. 4 shows a top view of the transmission mechanism from Fig. 2, Fig. 5 shows the principle of the spring-tensioning device, Fig. 6 shows the lever movement synchronizer, and Fig. 7 A, B, C, D show the principle of the gear changing.

Detailed Description of the Embodiments The spring-tensioned gear mechanism (further described as SGM), shown in Fig. 2, consists of a pair of levers 1,1'which are at one of their ends rotary attached to the vehicle frame, and at the other end rotary joined to a pair of draw rods 2,2'which are also rotary joined to a pair of driving elements 3,3'. The driving elements 3,3' (in this case wheels) are at the central axis rotary connected to the delimiters 4, 4'which are rotary attached to an axis, passing the center of driven small wheels 5,5'. The delimiters 4,4'are further rotary joined to at least one spring-tensioning device 10 (for the structure of the spring- tensioning device see Fig. 4), equipped with a spring 11, which is rotary attached to the vehicle frame 13. Transmission of the rotational movement of the driving elements 3,3'to the driven small wheels 5,5'is ensured by means of the movement transmitters 12,12'-in this case belts-but it is possible to use also other known means (for example a geared belt, geared wheels, and so on).

When a force F (for example the cyclists foot) starts to act on lever 1 of the SGM in the direction to the bottom dead center of the lever (Fig. 7A), the lever starts rotating in the given direction. The movement is transmitted from the lever 1 through the draw rod 2 to the driving element 3 which reacts as follows: it sinks along a path, determined by the delimiter 4 until the forces are balanced, while the delimiter 4 pivots around the axis which passes the center of the driven small wheel 5 (Fig. 7B); it rotates around its own axis which passes the place, where the driving element 3 is rotary joined to the delimiter Fig. 7C); it rotates around its own axis and simultaneously it sinks along a path which is determined by the delimiter 4.

It holds the following rule: the more the driving element 3 sinks along the path, determined by the delimiter 4 (whereby at the same time the angle between the longitudinal axis of the lever 1 and the longitudinal axis of the draw rod 2 decreases), the less it pivots around its own axis (Fig. 7A-C). If the delimiter 4 would allow the driving element 3 to sink to a position in which the angle between the longitudinal axis of the lever 1 and the longitudinal axis of the draw rod 2 would become 0°, the driving element 3 would not react at all on the movement of the lever 1 (Fig. 7D).

As mentioned above, SGM according to the present invention includes at least one free wheel 7. The movement, transmitted from the driving element 3 to the driven small wheel 5 by means of the movement transmitter 12, is then further transmitted in a known manner by means of the free wheel 7 (for example to the rear wheel of the bicycle).

One of possible arrangements of the free wheel is shown in Fig. 1. Further possibilities include its arrangement on additional geared wheels, on a modified driving element 3 or, alternatively, such an arrangement that the ratchet of the free wheel 7 is located on the modified driving element 3, and the pawls of the free wheel 7 are located on the modified draw rod 2. Then the driven small wheels 5,5'may be firmly joined with the rear boss 6.

The spring-tensioning device 10 with the spring 11 which device may be preferably equipped with a blocking element (not shown) to lock the currently set gear draws the driving element 3 upwards along a path, determined by the delimiter 4. It is required that both the right and left driving elements 3,3'must move simultaneously along the paths, determined by the delimiters 4,4'. This is achieved by the structure and attachment of the spring-tensioning device 10 or, alternatively, by a fixed joint of the delimiters 4,4'with an axis which is rotary attached to the frame, and on which both driven small wheels 5,5'rotate.

By means of the spring 11 of the spring-tensioning device 10 which spring may be, for example, a spiral spring (Fig. 5), in combination with geared assembly relatively uniform spring-tensioning can be achieved, and simple blocking can be achieved by means of a blocking element which blocking element is the spring-tensioning device 10 which is preferably equipped, together with an element to control the blocking, with, for example, a wire.

Fig. 6 shows the lever movement synchronizer, by means of which the levers 1, 1'are rotary joined with each other. The synchronizer consists of the arms 8, 8', rotary joined with the rocking lever 9. The synchronizer may also consist of a sheave, rotary attached to the frame, and of a connecting wire of mutually interconnected levers. The driving elements 3,3'transmit the movement to the driven small wheels 5,5'by means of the movement transmitters 12,12' (by a V- belt, a geared belt, a link belt or gearing).

Moreover, the SGM may be equipped with a flexible or a hard top and bottom stop for the levers 1,1', as well as with top and bottom stops for the delimiters 4,4'which determine the angle of the positions (top and bottom dead center) which can be adjusted. If necessary, the SGM may be modified for manual control-by adding manual actuators for the control of the movements of the delimiters 4,4' (a screw mechanism, simple hydraulics).

The levers 1,1'may be further joined in a known manner to a crankshaft, and in this case the force F is transmitted to the levers not from the cyclists foots, but from the crankshaft.