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ABSTRACT The use of suspension on off-ride bicycles has proliferated in the past few years. Although such a proliferation may be partially attributed to a fad, bicycle suspension does have technical merits. Bicycles equipped with suspension result in increased rider comfort, enhanced wheel contact and control, and less net rolling resistance. The primary design consideration for suspension systems is to design the system so that it responds to bumps but does not respond to rider induced forces. If the suspension responds to the rider's forces, it may be absorbing valuable energy which could otherwise be helping to propel the rider and bike faster. In addition, such a suspension results in a strange, uncomfortable ride. To fulfill this design goal, free body analysis was done on the bicycle and suspension system. By examining the forces on the bicycle and suspension, frame configuration and pivot placement(the two primary factors that affect suspension performance) of the suspension swingarm were determined. The next design consideration was what type of suspension component should be used. The accepted suspension component used on most production bicycles is a coil spring and oiled damper combination, but for our experimental purposes, a tension band was chosen as the suspension component. This provided strength and size adjustability, low cost, and simplicity. A full-scale working prototype bike designed with these principles was built and superficially tested.



BICYCLE A bicycle, also known as a bike, pushbike or cycle, is a pedal-driven, humanpowered, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist or a bicyclist. Bicycles were introduced in the 19th century and now number about one billion worldwide, twice as many as automobiles. They are the principal means of transportation in many regions. They also provide a popular form of recreation, and have been adapted for such uses as children's toys, adult fitness, military and police applications, courier services and bicycle racing. The basic shape and configuration of a typical upright bicycle has changed little since the first chain-driven model was developed around 1885.[2] However, many details have been improved, especially since the advent of modern materials and computer-aided design. These have allowed for a proliferation of specialized designs for particular types of cycling. The invention of the bicycle has had an enormous impact on society, both in terms of culture and of advancing modern industrial methods. Several components that eventually played a key role in the development of the automobile were originally invented for the bicycle, including ball bearings, pneumatic tires, chain-driven sprockets, and spoke-tensioned wheels.

DYNAMICS A bicycle stays upright while moving forward by being steered so as to keep its center of gravity over the wheels. This steering is usually provided by the rider, but under certain conditions may be provided by the bicycle itself. The combined center of mass of a bicycle and its rider must lean into a turn to successfully navigate it. This lean is induced by a method known as countersteering, which can be performed by the rider turning the handlebars directly with the hands[12] or indirectly by leaning the bicycle. Short-wheelbase or tall bicycles, when braking, can generate enough stopping force at the front wheel to flip longitudinally. The act of purposefully using this force to lift the rear wheel and balance on the front without tipping over is a trick known as a stoppie, endo or front wheelie.Bicycle transmissions have continuously

evolved for over a century and although they are deceptively simple, they are extremely difficult to improve . One might suppose that we are nearing the end of the development process. Not so. The last decade has seen the rapid acceptance of index shifting, the grip shift, 8 speed cassettes, the Mavic electric rear derailleur and other innovations. Amazingly, there is still one important avenue that has been left almost entirely unexplored - the automatic bicycle transmission. Browning Research, near Seattle, Washington, is probably the first company to successfully solve the formidable problems associated with fully automatic shifting. The basic mechanical system behind the Browning transmission was invented in 1974 by Bruce W. Browning and developed by Bruce and his sons Marc, David, Paul and Chris. This unique shifter uses a hinged sprocket sector which can swing either out or in to guide the chain to the next sprocket. See Figure 1 and Figure 2.

Fig 1.

Fig 2.

Figure 1. The elements of the Browning three-speed transmission are shown on the left. On the right is a Browning four speed rear cluster.

Figure 2. The left hand view shows a front Browning three-speed chain ring shifting down from the large sprocket to the middle sprocket. The right view shows the chain shifting up from the middle sprocket to the large sprocket. Note in both views the chain is in driving engagement with both sprockets at the same time.

THE SPROCKET CLUSTERS The three front chainwheels on the bike I rode were 48, 38 and 30, and the four speed rear cluster was 12, 17, 23, and 32 which gave a gear range from 26 to 110 inches in 12 steps, see Table I. Swinging sectors are included on all but the smallest gears. On the 12 tooth sprocket and the 30 tooth front chainwheel, no swinging sectors are necessary since some of the teeth are shortened, and the adjacent sprocket sector is able to swing over into the chain line and pick up the chain when shifting up. When a sector moves either out or in, it positively guides the chain to the next sprocket like a ramp on a freeway, or a railroad switch guiding a train from one rail to another. The sector provides a helical path for the chain to follow, and maintains positive contact during the switch. The shifts are therefore smooth and flawless. Shifts are timed so the swinging sectors can be moved without chain interference and they require very little force. At present a limited number of gear sizes have been made for the prototypes, but this selection can be expanded upon manufacture. To assure the reliability of the shifting system, the Brownings have built testing machines that have subjected the transmission system components to thousands of hours of service under various loads while undergoing millions of cycles. Any failures have been analyzed with high speed videos and corrected, until the system is now superbly reliable.

THE GEAR SELECTOR Figure 4. The front and rear gear selectors in neutral, up-shift, and down shift positions

Front and Rear Cams in Neutral Position

Cams in Switch Up Position, From Smaller to a Larger Sprocket

CAMS IN SWITCH DOWN POSITION From Larger to a Smaller Sprocket

To change gears, a pawl is displaced by a selector cam, and all of the sectors are hinged either outward or inward depending upon which cam groove is followed. See Figure 4. Once the chain is derailed, the pawl is released and a small spring returns the sectors to the neutral position. The rider provides all of the force necessary to displace the cam and complete the shift, so the gear selector requires very little power. Without the computer power drain, one 9 volt battery will complete from 80,000 to over 100,000 shifts. A small reversible electric motor is contained within the selector. A 50 millisecond pulse accelerates the motor and an inertial hammer strikes a trip mechanism. A latch and key system then locks the cam in either of its two switching positions. Once the shift has started, the trip mechanism, then releases the latch and holds the cam in its neutral position.

Forces between the rider, bike, and ground were analyzed using free body diagrams and applying laws of statics and dynamics. The analysis was done for a seated rider. The rider's temporary foot force at the pedal results in several temporary forces on the system: chain tension, traction force at the rear wheel contact, and an increase in the vertical force at the rear wheel due to weight transfer when the rider accelerates: all these forces can excite the suspension. Aerodynamic drag and rolling resistance were neglected because of their relatively small magnitudes. In our analysis, the rider was idealized as applying only a force on the downward stroke (no force exerted on the upstroke). All the rider-induced forces that may excite the suspension are all functions of the pedaling force. Thus when the pedaling force is greatest, during uphill climbing and sprinting, the suspension is most likely to be activated. First we consider a conventional suspension design. The net temporary forces that result from the rider's cyclic pedaling motion, shown below, must not excite the