GB2336140A - Variable length bicycle crank - Google Patents

Abstract

The bicycle crank 6 is slidably received through a slot in a boss 8 that is fixed to the end of the crank axle. Fitted to the crank are cam followers 9a and 9b that are spaced apart by a distance equal to the diameter of a circular cam 7. The cam is positioned with its centre displaced from the axle centre so that when a downward force is applied to a pedal 10 the follower 9a moves around the cam lengthening the crank by causing it to slide through the slot until a maximum extension (Figure 3D), which is equal to the distance between the axle centre and the cam centre, is reached. After reaching this maximum extension the cam follower 9b abuts the cam and causes the length of the crank to reduce to a minimum (Figure 3H) on the pedal upstroke. Figures 4a and 4b show an embodiment wherein the crank is slotted and it slides over a fixed axle boss whilst the embodiment in Figure 5 comprises a telescopic crank and a guide rail (18) around the crank to maintain the follower in contact with the cam.

Description

2336140 VARIABLE CRANK This m'vention relates to an automatic variable

length crank, that is more efficient than the present fixed length type used on the bicycle or similarly propelled appliances, to drive the axle and the chain drive wheel. The variable length crank is designed to transmit the power, produced by the cyclists' legs into a more efficient dfivmg force, while the foot and pedal at the outer end of the crank follows a similar circular path and circumference to that of the fixed length crank.

The fixed length crank at present found on the bicycle has hardly changed since it was first used in the first bicycle, and the penny farthing Figure 1 shows a fixed crank 1 attached to each end of an axle 2, set at 180 degrees to each other, the axle is fixed to a chain drive wheel 3. The outer end of each crank is fitted with a pedal 4, this travels through a fixed circular path 5 and radial distance, when the pedals are driven round by the cychsfs legs. The downward force W applied to each pedal in turn moves each crank from top dead centre to bottom dead centre. The downward force on each pedal also moves the other crank from bottom dead centre to top dead centre, ready for the crank to repeat the cycle. The cyclist can then continue to apply power to each pedal in turn, to give a continual driving force to the axle. Each revolution of the crank and pedal is divided into two halves, the power stroke firom top dead centre to bottom dead centre and the second half, from bottom dead centre to top dead centre the non powered half Both the previous distances travelled by the pedal on the power and non power stroke are of the same length, which is half the circumference of a revolution that is made by the pedal about the centre of the axle.

Note:The item numbers in figures 1 to 5 are common to all. 1 of 13 Figure 1 continued. The downward stroke on the right hand side of the vertical centre Iffle, is the power stroke of one crank rotation. The upward part of the stroke to the left of the centre 1me is the return stroke, this transmits very little power to the axle. In the automatic variable length crank the pedals travel through a similar circumference, as that of the fixed crank, but the crank arm is longer on the power stroke and shorter on the return stroke.

The Automatic Variable Length Crank The variable length crank is designed to overcome the problems of just increasing the force moment of the crank on the axle by making the crank longer. The disadvantage of a longer fixed length crank is the circular distance the pedal would have to travel through during a full revolution. From a practical point a permanent long crank would also cause problems for most cyclists' legs when the pedal is in both the upper and lower vertical positions, also a long crank could cause the pedal to catch the ground when the cycle leans over during a turn.

Figure 2 This shows the mami component parts of this mivention and their layout.

a. b.

The variable length crank 6 (this slides through the axle end drive boss). Circular or almost circular shaped cam 7.

C. Fixed Axle end drive boss 8 slotted, enabling the crank to slide through 'm both directions.

d. Cam followers 9a & 9b, the lower one fitted to the crank at the opposite end to the pedal 9b. The second follower 9a, fitted to the crank at a distance from the first follower, equal to the outside diameter of the cam (this is so that tile cam can fit between the followers).

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g. h.

Pedals 10. Toothed wheel or chain drive wheel 3. Cycle frame 11. Rear wheel drive chain 12.

Figure 3. This illustrates the automatic variable length crank at eight different points during one complete rotation of the crank. Note: Only one crank is shown for clarity.

A. Shows the crank in the top vertical position ready to commence the down wards power stroke. The crank is not fixed to the axle as with the standard type, but is allowed to slide through the axle head boss 8 and its length is controlled automatically by the top 9a and lower 9b followers movm'g around the fixed cam 7. The cam centre 19 is positioned forward of the axle centre 20. The distance between the axle and the cam centres along with the cam diameter and its position in relation to the axle, will control the length the crank extends and contracts through a complete rotation. This shows a downward force is applied to the pedal the top follower 9a. moves round the cam gradually lengthening the crank.

C. The top follower 9a continues to extend the crank's length.

D. At this position the crank is at its maximurn length and the lower cam follower 9b starts to take over control from the top follower 9a.

E. The lower follower 9b now takes over to control the length of the crank as it passes through the second quarter of its rotation. At this stage the crank 6 is still steadily reducing its length.

B. F.

3 of 13 The position of the pedal 10 is now at its lowest position and it is now half way through one full rotation. The top follower at this position is at its maximum distance away from the circular cam 7. If it is required that the top and lower followers 9a & 9b should both be closer to the cam at all times this can be achieved by making a cam that has a more outwardly curved cam surface 'm its third and fourth quarters.

H. Between positions, 'G' and M' the length of the crank will reduce mi length to its shortest when the crank is honizontal. After this the crank will lengthen gradually until it reaches the start position W.

The object of the variable crank is to change the above parameters by lengthening the crank during the power stroke and by reducmg its length during the return stroke, therefore the power stroke is made more effective (length of crank times force applied). The return none power stroke has a shorter crank length. During the above crank rotation the end of the crank attached to the pedal will rotate through a similar circular path to that of a fixed crank. This is because the pedals appear to the cyclist's leg motion to rotate on a fixed length crank around the centre of the cam and not the axle.

Figure 4a shows the front elevation and the plan view of the crank and cam layout. The rear crank has been excluded from the front elevation for clarity. This shows an alternative crank 13, which is divided into two sections for part of its length, so the two parallel sides can slide over a fixed square or rectangular drive boss 14, on both ends of the axle 2.

Figure 4b shows the crank 'm the front elevation in figure 4a in four of its 4 of 13 positions as it turns the axle through one complete revolution. Each of the positions show the change mi length of the axle, this is marked with V. The following item nunibers and their descriptions are as follows: 9 a smgle follower, 18 an outer cam guide, 19 centre of cam, 20 axle centre, 21 the circle scribed by the fixed crank, 22 the circle scribed by the variable crank-

Figure 5. Another alternative variation, on the variable length crank, is one that changes its length by usmg a telescopic action 15, to alter its length between the fixed centre of the drive axle 16 and the centre bearing of the pedal 10a.

In both figures 4 and 5, the change in the effective crank length is controlled by a follower 9, rurtning round a cam and an outer guide rail 18. The guide prevents a smgle follower 9 firom straying away from the cam. When two followers are used 9a and 9b, the second follower will prevent the first follower firom moving to far from the cam 7 during each revolution.

The front elevation of the axle and variable length, chain drive wheel and cam in figures 4 and 5 do not show the far side crank, pedals and supporting cycle frame for clarity.

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Claims (8)

1. Claim 1. The Automatic Variable Length Crank and cam plate already describedunder the 'Automatic Variable Length Crank' sub title as described earlier and 'm figure 1, 2, and 3.
2. Claim 2. A variable crank and cam plate as described 'm clanin L, but fitted with bearings or friction reducing materials to reduce friction mi the variable crank, as it moves through the axles' fixed end boss.
3. Claim 3. A variable crank and cam plate as described in claim L, but with a cam of a non true circular shape, e.g. out of round.
4. Claim 4. A variable crank and guide cam as described in claim L, but the vaniable crank m'stead of it sliding through the fixed axle end boss, the crank arm can have a form which divides over part of its length, so that it forms a slot which can pass over a fixed axle end drive. This can be mi the shape of a rectangular or square section, thus the crank can slide over the axle end, but as the crank turns so will the axle.
5. Claim 5. A variable crank and cam as described 'm claim L but the crank will be fixed at the axle drive boss and the change 'm length will take place between the axle and pedal end by havmg a crank with a telescopic section.
6. 6 of 13 Claim 6. A variable crank and cam as described m claim L, but the cam is fitted with an outer parallel track, so it holds the follower agaffist the miner edge of the cam as it travels around the its parameter. Fittmig this type of cam will enable the sliding crank to operate with only one follower.
7. Claim 7. The alternative of using a cam, one where the follower/s run round the inside edge of a plate, where its centre in the shape of a cam is removed.
8. Claim 8. A variable crank and cam as described in claim L, but fitted with any combination of claims 1 to 7.

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