Land vehicles – Wheeled – Occupant propelled type
Reexamination Certificate
1998-08-19
2001-01-16
Schwartz, Christopher P. (Department: 3613)
Land vehicles
Wheeled
Occupant propelled type
Reexamination Certificate
active
06173982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to transmissions used typically in multi-wheeled cycles such as bicycles, and more particularly to mechanisms that allow for smooth shifting of such transmissions between gears of varying ratio.
2. Description of Related Technology
Multi-geared bicycle transmissions have been known for many years. Most bicycles achieve multiple gear ratios by utilizing an arrangement of sprockets on a front sprocket assembly, commonly called the chain ring or chainwheel, and a rear sprocket assembly, called the freewheel or cassette. Typically, the chainwheel comprises two to three sprockets of varying diameter. The freewheel assembly typically comprises five to seven sprockets, also of varying diameter, but each smaller than the diameter of the sprockets on the chainwheel. More recently, freewheels with as many as 9 sprockets have been introduced. A drive chain coupling the chainwheel and the freewheel make up the drive train. As the chain is coupled to different sprockets on both the chainwheel and freewheel assemblies, a different effective drive ratio of the transmission is obtained. Each combination of chainwheel/freewheel sprockets in use by the drive chain represents a different gear ratio, and a different gear. Obviously, the more sprockets on both the chainwheel and the freewheel, the greater the number of available gear ratios, and, consequently, the greater number of gears.
For almost as many years, manufacturers and cyclists alike have recognized the need for efficient, smooth shifting from one gear to another, all the while maintaining a positive drive connection during the shifting process. The standard front and rear derailleur, which comprise the shift mechanism of choice for the vast majority of multi-geared bicycles, forces the chain on a holding sprocket toward an adjacent or receiving sprocket until a tooth of the adjacent sprocket catches the chain. The drive chain thus becomes entrained around a different combination of front and/or rear sprockets. Because gears are defined by the ratio of the front holding sprocket to the rear sprocket, the shift from one holding sprocket to an adjacent sprocket of different diameter results in a different gear ratio on the drive chain.
Several systems for effecting such changes of gear ratio are present in the prior art. One such system involves mechanisms for indexing the shift control such that either a front or rear derailleur moves laterally (e.g., transverse or perpendicular to the rotational plane of the sprocket) in response to a predetermined amount of movement of a shift lever, thereby causing the drive chain to move laterally upward or downward among the sprockets in the free wheel cluster (or chainwheel). This type of mechanism is described, for example, in U.S. Pat. No. 3,972,247, “Bicycle Shift Mechanism”.
A second prior art system involves a complex lateral displacement of the rear derailleur in relation to the sprockets in a freewheel cluster. In these types of solutions, the rear derailleur starts in a first position in which the drive chain is entrained around one of the cogs in a freewheel and around the wheels of the rear derailleur. Movement of the shift control forces a lateral movement of the rear derailleur to a second position such that the trailing portion of the drive chain that is entrained around the wheels in the rear derailleur becomes aligned with a separate sprocket in the freewheel cluster. This lateral movement, together with a change in the angular position of the rear derailleur with respect to the bicycle frame, forces the drive chain to “jump” from one freewheel sprocket to another.
One embodiment of the aforementioned system is shown in U.S. Pat. No. 5,358,451, “Devices Enabling Shifting of Gears on Bicycles” issued to Lacombe, et al. Lacombe teaches a means of assisting the lateral movement of the rear derailleur along an axle slidably engaged within a housing having multiple control teeth, each tooth being adapted to permit lateral movement corresponding to one shift in the free wheel cluster.
Another embodiment of this type of mechanism is disclosed in U.S. Pat. No. 5,152,720, “Bicycle Transmission” issued to Browning, et al.
However, the foregoing prior art systems fail to address two fundamental shortcomings of modem, multi-geared cycle transmissions; specifically, 1) the high degree of redundancy and overlap between the gears; and 2) the inability to shift gears under a heavy load. To appreciate the significance of these problems, it is necessary fully to understand the operation of the drive train.
The diameter, or size, of the chainwheel and the freewheel sprockets is measured by the number of “teeth” on the sprocket. Chainwheels typically include sprockets having from 36 to 53 teeth. Freewheels include sprockets having from 11 to 34 teeth. Thus, a typical maximum and minimum gear ratio for bicycle drive trains with these sizes of chainwheels and freewheels is 4.82 (e.g., 53/11) for a maximum, and 1.06 (36/34) for a minimum.
Bicycle gearing is typically measured in “gear inches”. This concept is derived from early “high-wheeler” bicycles, the gearing for which was measured based on the diameter of their front wheel. A high-wheeler with a 50-inch wheel diameter was said to be geared at 50 gear inches. A bicycle with a 25 inch wheel diameter and a two-to-one gear ratio in the drive train achieves the same forward motion with each pedal revolution as does the 50-inch high-wheeler, and thus is also said to be geared at 50 gear inches. Gear inches are computed by multiplying the drive wheel diameter by the effective gear ratio on the drive train. Using the maximum and minimum drive train gear ratios set out above, and using the normal diameter of a bicycle wheel (27 inches), the minimum gear inches would be approximately 28.6, and the maximum gear inches would be 130.1.
Experienced cyclists are well aware of the significance of gear inches. The greater the number of gear inches (i.e., the “taller” the gear), the more difficult the pedaling due to lower mechanical advantage. Conversely, the lower the number of gear inches (i.e., the “shorter” the gear), the easier it is to pedal. Depending on the gradient of the terrain (and the direction of the cyclist's travel with respect to the gradient), selecting the appropriate gear is not an insignificant task. Pushing a tall gear on the order of 130 inches would be quite difficult going up hills or even on flat terrain, and is consequently usually reserved for pedaling at high rates of speed down steep hills. Pushing a short gear of 28 inches (the so-called “granny” gear) is much easier and is useful in going up steep inclines while carrying baggage on the bicycle. Between these two extremes, there is a virtual continuum wherein a wide selection of gearing is possible. Gear inches of between 30 and 60 are likely adequate to accommodate most hills, reserving the shorter gears for the steeper hills. Gear inches between 60 and 90 are appropriate for travel on level ground, with the taller gears reserved for fast pedaling at high speeds. Gear inches above 90 are appropriate primarily for traveling down hills.
While the frequency and magnitude of gear changes varies in accordance with both the terrain and the degree of rider fitness, most riders prefer a gear change if their pedaling speed, or cadence, increases or decreases by more than ten percent. Gears spaced apart by seven to ten gear inches allow the rider to adjust the gearing up or down, and still maintain a cadence within the desired ten percent variance. Gears spaced closer than seven gear inches produce such a minimal impact on pedaling efficiency that most riders will not notice the difference between adjacent gears. Gears spaced further apart than ten inches will create a larger jump than is comfortable.
To achieve an optimum overall gear range and to provide adequate gear spacing, a common practice in recent years has been to include as many gears as possible on the bicycle in order to accommodate the widest range
Bartz C. T.
Gazdzinski & Associates
Schwartz Christopher P.
LandOfFree
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