In frames of that type, there is normally no significant possibility of movement of the hub of the rear wheel relative to the frame with the aim of adjusting the tension of the chain because the tension of the chain is auto- matically adjusted by the derailleur mechanism which is specially designed, among other things, to recover the excess chain for each selected transmis- sion ratio by means of resiliently sprung wheels.

This type of bicycle is without doubt the most common and the frames dedicated thereto are manufactured in a multiplicity of versions and in all de- sired sizes.

However, the case of bicycles without gears or with gears integrated and concealed in the hub of the rear wheel is different. In that case, in order to permit adjustment of the chain tension it is usual to provide that the axle of the wheel hub is accommodated in an adjustable position inside slotted holes which are elongated in the direction of the desired adjustment. By moving the axle of the wheel hub along those slots, it is possible to vary the tension of the chain as desired. However, frames for bicycles of that type are distributed in a limited number of forms and sizes, which is without doubt less extensive than the previous type.

On the other hand, bicycles having gears integrated in the wheel hub are greatly valued by hirers because the gear mechanism is easy to use, even for non-experts.

The principal technical task of the invention is to provide a chain- tensioner for bicycles which is capable of permitting adjustment of the chain tension in bicycles in which the hub is constrained on the frame without any substantial possibility of movement in a direction capable of significantly vary- ing the tension of the chain, also permitting such adjustment without making use of derailleurs.

Within the framework of this task it is a principal object of the invention to provide a chain-tensioner capable of optimising transmissions with gears in- tegrated in the wheel hub on frames arranged to accommodate derailleur gears.

Not last object of the invention is to provide a chain-tensioner for bicy- cles which is rapid and easy to use and can be produced at low cost.

This task and these objects are achieved by the invention by means of a chain-tensioner for bicycles which is produced in accordance with the ap- pended claims.

The features and advantages of the invention will become clearer from the following detailed description of a preferred embodiment thereof which is illustrated by way of non-limiting example with reference to the appended drawings in which: - Figure 1 is a perspective view from one side of a bicycle having a chain- tensioner according to the invention; - Figure 2 is a perspective view from the opposite side of the bicycle of Fig- ure 1.

In the drawings, referring to a bicycle generally indicated 1 and shown only partially, 2 distinguishes a rear wheel having a wheel hub 3 constrained by means of a wheel axle 4 and nuts 5 in a drop-out 6 of a frame 7. Figure 1 also shows a sprocket 8 which is keyed onto the hub 3 and on which is en- gaged a chain 9 having a normally active pass 10 and a return pass 11.

In relation to the frame 7, the drawing shows two frame elements 12 which converge at a junction 13 in which the drop-out 6 is arranged and from which extends a downwardly directed lug 14 which is arranged for the fitting of a derailleur which is not shown and which is not present in the bicycle pro- duced in accordance with the invention.

The constraint with which the axle 4 is held in the drop-out 6 is such as not to permit relative movements in a direction capable of significantly varying the tension of the chain. This can, however, be remedied by adopting a chain- tensioner 20 according to the invention. The chain-tensioner 20 comprises an arm 21 having a first and a second end 22,23 which are arranged longitudi- nally opposite each other. The first end 22 is constrained pivotally on the frame, at the location of the lug 14 for the attachment of the derailleur, by means of a threaded fastener 25. A first and a second wheel 26,27 are mounted on the second end 23 of the arm and at the location of an intermedi- ate portion thereof, respectively.

The first wheel 26 and the second wheel 27 are operatively engaged on the return pass 11 of the chain 9 with the first wheel engaging on the side of the chain remote from the active pass 10 (or from below the return pass 11) and with the second wheel engaging on the side of the chain facing the active pass 10 (or from above the return pass 11). Preferably, the wheel 26 has a di- ameter which is substantially larger than the diameter of the wheel 27. For ex- ample, the wheel 26 is of the type having 13 teeth and the wheel 27 is of the type having 10 teeth.

The two wheels 26,27 are mounted idly on respective pins 29 which can be locked in an adjustable position by means of nuts 30 in slots 31 ex- tending transversely to the longitudinal direction of the arm 21. A helical spring 32 is active between the arm 21 and the frame 7 in order to urge the

chain-tensioner 20 into engagement with the return pass of the chain and to pivot the arm 21 towards the active pass 10 of the chain 9.

The chain-tensioning effect is promoted in the invention both by the re- silient urging exerted by the spring 32, and by the adjustment of the wheels 26,27 relative to the slots 31.

The invention so described may be modified and varied. For example, there is provision for the arm 21 to be produced in two rod-shaped portions which are articulated to each other, for example, at the location of the pin of the wheel 27. It is also provided that the number of wheels may be increased.