Variable transmission gear, particularly for bicycles

Variable transmission gear particularly for bicycles comprising a transmission element such as a chain or a belt which is in engagement with a driving and a driven rosette, one rosette consisting of two parts adapted to be mutually shifted along straight guidings, the axis of said guidings being parallel and passing beyond the axis of supporting shaft.

BACKGROUND OF THE INVENTION 
The invention relates to a variable transmission gear with a transmission 
element such as a chain or belt, which transmission element is in 
engagement with a driving and a driven rosette, one rosette connected with 
a carrier from which or to which a torque is transmitted, whereby said 
rosette is formed by segments guided slidably with respect to the carrier. 
At actually known embodiments of variable transmission gears the rosette 
consists of a number of parts which in case of a change of the 
transmission ratio are radially shifted with respect to or from the center 
of the rosette. Radial guiding means have to be provided for the shifting 
of individual parts of the rosette and also additional means are used, 
usually a spindle mechanism of an additional plate with a spiral groove, 
into which individual parts of the rosette are engaging. These 
arrangements have the drawback of an application of a large number of 
components of the transmission gear, whereby they are relatively expensive 
both in construction and in manufacture. 
Another drawback of these variable transmission gears is the limiting of 
the extent of changes of the transmission ratio due to the limit of length 
of guidings of segments by a central supporting bolt. 
It is thus impossible to obtain at these mentioned variable transmission 
gears the advantages of oval rosettes where in the course of a single 
revolution of the supporting shaft the length of the power arm on the 
rosette is cyclically changing, so that it is possible to synchronize the 
maximum power generated by the cyclist with the maximum transmission ratio 
in the course of the one revolution of the shaft as is the case at systems 
known by the name of "biopace". 
At other known constructions of variable transmission gears the individual 
segments are arranged rotatably on a carrier. The rotational arrangement 
of segments is more demanding on an accurate guiding of segments and a 
lower stiffness is mostly caused thereby. Another drawback of rotational 
arrangements at a variable transmission gear is the circumstance that by 
this construction a constant prior required adjustment of the crank with 
respect to a chosen point on the rosette within the whole range of gear 
shifting is not achieved. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a variable transmission gear 
of simple design with a small number of component parts where the 
advantage of oval rosettes would be obtained, whereby at any change of the 
transmission ratio the adjustment of the position of the crank with 
respect to the chosen position of the rosette would remain constant. 
According to this invention, the rosette is formed by two segments which 
are by means of a carrier kinematically linked by straight guidings, the 
axis of which guidings passing beyond the axis of the supporting shaft, 
whereby the axis of said guidings are parallel. The segments are at their 
mutually facing parts provided with parting surfaces, which are at least 
partly parallel with the axis of the guidings and are in sliding contact 
either mutually or with a straight sliding surface formed on the 
supporting shaft. A sliding couple formed by a guiding element and a 
guiding groove can be arranged between the segment and the carrier and a 
control couple formed by a control bolt and a control groove can be 
arranged between the segment and the control plate. 
An advantage of the variable transmission gear according to this invention 
is its simplicity of construction and the possibility to utilize the 
"biopace" system while maintaining a constant adjustment of the crank with 
respect to the chosen position of the half axle of the rosette. Another 
advantage is the possibility of a wide range of variations of the 
transmission ratio due to the arrangement of guidings of segments beyond 
the axis of the supporting shaft and the possibility of an increase of the 
stiffness of segments which are in mutual engagement or in engagement with 
the supporting shaft.

DETAILED DESCRIPTION 
With reference to FIGS. 1, 2, and 3 a rosette is shown in engagement with a 
chain 12 is formed by two segments 2 arranged between a carrier 7 and a 
control plate 10. The carrier 7 is supported on the supporting shaft 5 non 
rotatably, whereas the control plate 10 is mounted rotatably on a sleeve 
of the carrier 7. The segments 2 are on mutually facing places provided 
with parting surfaces 4 which are partly in contact with a guiding prism 
11 which is formed by a part of the sleeve of the carrier 7. Guidings 3 
are provided in the segments 2, with guiding elements 6 firmly connected 
with the carrier 7 being in engagement with said guidings 3. Control pins 
9 are fixed to the segments 2 engaging into a control groove 8 of the 
control plate 10. The parting surfaces 4 of the segments 2 are in case of 
a minimum transmission ratio bearing against each other, and in case the 
segments 2 are distant from each other, parts of their surfaces which are 
parallel with the guidings 3 are in contact with the supporting shaft 5 or 
with the guiding prism 11. All guidings 3 of segments 2 are paralleled and 
their axis are passing beyond the axis of the supporting shaft 5. 
The axial distance between the rosette 1, the carrier 7 and the control 
plate 10 is secured by distance plates 14 and by a safety rings 13 
arranged on control pins 9 
FIG. 4 shows a control device formed by a control lever 15 rotatably 
supported on a holder 17 fixed to the frame 18 of the bicycle. The control 
lever 15 is connected to a Bowden cable 19 and is maintained in its 
starting position by a return spring 16. 
In an alternative embodiment shown in FIGS. 5 and 6, the parting surface 4 
of the segments 2 are arranged differently. 
According to the alternative embodiment indicated in FIGS. 7, 8, and 9 
nearly the whole parting surface 4 of both segments 2 is parallel with the 
guiding 3 and the difference against above described constructions is that 
in case of a change of the transmission ratio, the distance of extreme 
points on the circumference of segments is changing whereas at the earlier 
mentioned constructions the distance of opposite central points of the 
circumference of segments 2 are changed. The control plate 10 and the 
rosette 1 are rotatably supported directly on the supporting shaft 5. The 
control pins 9 engage at this alternative also into the guiding 3 of the 
carrier 7 and contributes to the guiding of segments 2. The parting 
surfaces 4 of both segments 2 are mutually in sliding contact. At all 
mentioned embodiments it is possible to provide the control groove in the 
segments of the rosette and the control pin in the control plate. 
In case the supporting shaft 5 is rotated, the carrier 7 takes along by way 
of the guiding element 6 and the guiding 3 the rosette 1 and furthermore 
by way of the control pin 9 and the control groove 8 the control plate 10. 
In case the ratio of the transmission has to be changed, the control lever 
15 is pressed against the circumference of the control plate 10 by pulling 
the Bowden cable 19, due to what the control plate 10 is braked and the 
rosette 1 is turned with respect to the control plate 10. Due to this 
turning, the control pin 9 is shifted in the control groove 8 from its 
position more close to the axis of the supporting shaft 5 to a position 
more distant from the axis of the supporting shaft 5. The segment 2 are 
thus removed from each other to a more distant position whereby the 
segments 2 are guided by means of guiding 3 and the guiding element 6 and 
by parts of parting surfaces 4 which are in contact either with the 
guiding prism 11 or with the supporting shaft 5. The increase of the 
transmission ratio is finished by release of the control lever 15, 
preferably by contact of the control pin 9 to the external end of the 
control groove 8. A different transmission ratio from a minimum to a 
maximum can be thus achieved by different braking of the control plate 10. 
In case the transmission ratio has to be reduced, the control plate 10 is 
again braked by pull of the Bowden cable 19, pressing thereby the control 
lever 15 against the control plate 10. The supporting shaft 5 has to be 
turned against its original direction of rotation, so that the control pin 
9 is shifted in the control groove 8 so that it is brought to a position 
more close to the center of the supporting shaft 5. The control pins 9 
thereby take along the segment 2 toward a position corresponding to the 
minimum transmission ratio.