Currently many varying types of bicycle transmissions exist, from archaic single speeds to modern multi-speeds. In general, the types of transmissions can be categorized in three ways: manually shifted, automatic, or a hybrid manual-automatic.
Manual bicycle transmissions operate by means of a cable or lever, which when activated change the gear higher or lower, as desired by the cyclist. This relatively simple and older design allows for maximum control by the cyclist, however, it requires constant vigilance, to ensure that the transmission is in the optimal gear. Additionally, it can be cumbersome in cities where frequent stops and starts are required.
Subsequently, to alleviate these problems, a variety of automatic transmissions were designed. Starting from rest these designs engaged the lowest gear and upon acceleration of the bicycle to higher speeds, would automatically to the middle and then the highest gear. This automatic shifting obviated the need to select gears manually. As an additional feature some of the designs allowed for a hybrid manual-automatic shift control, by which the cyclist could manually select gears, if desired.
U.S. Pat. Nos. 4,229,997; 3,701,292; 3,69,690; and 3,592,081 disclose many of the early embodiments of automatic three speed hubs. In most, the automatic shifting feature is accomplished by mean of centrifugal weights, which shift gears as the speed of the bicycle increases.
In more recent years, new designs of transmission have become available. Currently three types are known to exist: fluid drive, centrifugal, and torque-responsive. Each design has particular benefits and limitations associated with it.
Fluid drive transmissions, commonly used in automobiles, utilize the drag of a fluid contained within the transmission to transfer power. An example of this applied in bicycles is U.S. Pat. No. 7,059,618, which describes a transmission that utilizes the principle of fluid drag to transfer power from the cyclist to the bicycle. It achieves this by means of a fluid filled chamber containing a stator assembly, which can alter the drag of the fluid between the inner and outer walls of the chamber. As the input torque increases, the stator vanes are forced away from the outer shell, thereby creating a larger clearance and more slippage between the fluid and the outer chamber wall. The increase in slippage effectively changes the mechanical advantage between the input and output torque, and, in the above situation, effectively acts as a “shift” into a lower gear. Correspondingly, if the input torque is reduced, the operation performs in reverse as a “shift” into a higher gear. Unfortunately, this system has several drawbacks. First, the system requires continuous input torque to operate effectively and, if the input torque is varied, could up-shift at an inopportune time, i.e. in the middle of a long hill. Secondly, the transmission would be difficult to maintain, as the entire assembly would need to be drained to access the mechanical components, in addition to other disassembly. Alternatively, the fluid could leak out during operation and render the bicycle useless, after only a small amount of fluid is lost. Finally, this system would require extensive retrofitting to be equipped on a standard bicycle.
The second category of automatic transmissions, torque responsive transmissions, come in variety of configuration, however all operate by selecting the gear ratio based on the input torque from the cyclist. There are two main types of torque responsive transmissions, v-belt pulley systems, such as U.S. Pat. No. 4,781,663, or a spiral and spring system, as described in U.S. Pat. No. 3,769,848 or application Ser. No. 13/165,807. The v-belt pulley systems operate by means of a v-belt connected to several pulleys, which are of variable size. As the input torque increases, the tension on the v-belt causes the pulleys to effectively change in size, by means specific to each embodiment, and thereby change the gear ratio. The main drawback of this invention, is that it requires a constant torque input or it will return to a “low torque, high speed” configuration. This means the transmission will have effectively up-shifted every time the cyclist pauses in or varies his rhythm. Several methods of “holding” the gear have been devised, but all require manual input from the rider to be activated. The second type of torque responsive transmissions uses a spiral and spring to select the appropriate gear. In lace of the v-belt and pulleys, these use a helical shaped or “spiral” shaft through which the input torque is transmitted to push a mechanism, specific to each embodiment, against a spring and as the spring is compressed, the transmission shifts gears. With this invention, the spring will be at maximum compression when the maximum input torque is applied and through varying means, the transmission will shift to the lowest gear ratio. As the input torque decreases, the spring will force the mechanism toward its zero input torque position and in so doing up-shift. These designs still suffer from a lack of gear “holding,” and will up-shift without a continuous torque input, though an automatic “hold” device is described in U.S. App. U.S. 2012/0329589 A1. However, this “hold” device only activates when the rider is traveling uphill.
The third category of automatic transmissions are centrifugally governed. In general, these types of transmissions work by harnessing the rotational inertia of the bicycle wheels. This is achieved by having a particular component of the system rotate at a speed proportional to the bicycle wheels. The centrifugal force created by this is then used to induce the movement of the gear selection mechanism, and subsequently lead to a change in gear ratio. Two designs have evolved using this principle; automatic hear hubs and automatic transmissions within the multi-stage sprocket bracket assembly. The first type, automatic hubs, such as U.S. Pat. No. 4,229,997, are comprised of an epicyclical gear assembly, a plurality of centrifugal weights, and an overrunning clutch. The rotational motion of the bicycle wheels causes the weights to be forced away from the centerline of the hub and, upon reaching a given speed, the weights are far enough from the centerline to engage the gear set and thereby increase the gear ratio. Once the speed of the bicycle wheels surpasses the speed of the gear set, the overrunning clutch will disengage the gear set. Alternatively, if the bicycle wheel speed decreases the weights will, by use of springs, be pulled toward the centerline of the hub and disengage from the gear set. There are several distinct disadvantages with this type of automatic transmission. First, the entire assembly must be contained within the bicycle wheel hub, and is therefore constrained in size. This constraint has meant that all previous designs have been limited to no more than three speeds. Secondly, the assemblies and various mechanisms are complex and rely on many small pieces, making manufacture difficult and expensive.
The second type of centrifugal transmissions move the control system from the rear wheel hub to the either the front sprocket assembly or both the front and rear sprocket assemblies. An example of the latter is disclosed in U.S. Patent Application Publication No. US 2007/0213150 A1. In that patent application, a transmission system which comprises a rotatable shaft, a plurality of centrifugal weights pivotally connected to each other and to a star-shaped collar, a collar assembly that includes the above star-shaped collar and a rear derailleur is disclosed. The rotational motion of the bicycle wheels causes the centrifugal weights to rotate and as the rotational speed of the centrifugal weights increases, the increased centrifugal force causes the centrifugal weights to flare outward. As the centrifugal weights flare outward, the collar assembly is then forced to move along the rotatable shaft, and consequently the chain guide of the rear derailleur derails the drive chain from a larger diameter sprocket to an adjacent smaller diameter sprocket, thereby resulting in an “up-shift” to a higher gear ratio as the speed of the bicycle increases. As the bicycle speed decreases, the reverse occurs. However, this design and other similar to it, have some drawbacks. First, the system is complex and would require retrofitting any onto any conventional bicycle. Second, the designs still require the use of a chain and sprocket system, which are exposed during operation. This is not limited to the last type of automatic transmission, as, all of the above inventions require the use of chain and sprocket drive assemblies. Due to the lack of shielding, chain and sprockets are known to become tangled clothing, or are prone to oxidation through exposure to the elements. Further, the chains require frequent lubrication and are prone to stretching from normal use, which results in the need for more frequent maintenance.
Previously, attempts have been made to eliminate the chain and sprocket system using shafts. Earlier attempts simply replaced the chain and sprockets with a shaft, a disclosed in Publication EP 0105949 B1, which used a shaft to drive a multi-speed rear hub. A later design, disclosed in U.S. Pat. No. 6,685,205, utilized an inline transmission, in place of a rear hub. In the disclosed invention, the transmission is comprised of an input shaft, an output shaft and an epicyclical gear train. The input shaft is rotated by a bevel gear set located in the pedal assembly. The input shaft, in turn rotates the gear train, with the cyclist manually selecting the gear ratio. The gear train operates by using the input shaft to drive the sun gear, which in turn drives a plurality of planetary gears attached to an axle in sets. The axle is allowed to rotate freely. Each planetary gear set is surrounded by a free rotating ring gear. To select a certain gear ratio the cyclist, by means of a cable and shift key, fixes the ring gear, which forces the planetary gears to rotate the sun gear on the output shaft at the given gear ratio. The output shaft in turn rotates the rear wheel by means of a bevel gear set. This design, while eliminating the chain and sprocket, still has disadvantages. First, the transmission is not automatic and requires manual gear selection. Second, as the gear cannot “stretch” under load, when the transmission shifts gears the cyclist will be jarred.
Thus, there is still a need for a shaft driven automatic bicycle transmission, that has more than three gear ratios, is simple to manufacture, and requires no cyclist input, to select the optimum gear.
Therefore, it would be desirable to have a simple, inline, shaft driven automatic bicycle transmission, which is easy to manufacture and efficiently transmits power to the rear bicycle wheel, without cyclist input.
Therefore, it is an objective of this invention to provide an apparatus that overcomes the inadequacies of the prior art devices and provides an improvement that is a significant contribution to the advancement of automatic bicycle transmissions. Another object of this invention is to provide for a modular transmission, which may be modified to contain as many gear ratios as the cyclist desires.