h a l f b a k e r y"Not baked goods, Professor; baked bads!" -- The Tick
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On my bicycle the cranks are fixed at 180º to each other. I was getting bored riding up a long slow incline today and I wondered about shifting the angle. On my bike the cranks fit onto square tapers on the ends of the shaft, and so I thought it would be a simple matter to put them in phase to make a
frog bike. But instead of all the faff of getting out a crank-puller and unding the nut etc. wouldn't it be great if there was a handlebar-mounted lever that disengaged one of the cranks, allowing you to spin with your feet and select a new crank angle?
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// On my bicycle the cranks are fixed at 180º to each other. // |
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Your bike is not unusual in this regard. I do a lot of cycling and I've never ever wanted to change the phase angle between the cranks. |
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But sticking with this, how about decoupling the cranks completely and having two chainrings, one either side of the bottom bracket, driving two freewheels on the rear hub? Completely pointless but it might be fun for several seconds. |
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You may find, at least in the case of a freewheel, that cycling in frog formation is rather difficult. This could be solved by adding an additional lever which locked the freewheel, or more desirably, kept the freewheel but engaged a large coil spring to the bottom bracket creating a reciprocating drive train. |
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I have seen a real bicycle with the axis of rotation of the cranks offset to that of the chainring, connected by a type of yoke which gives variable leverage throughout the rotation. It was probably a gimmick. |
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// decoupling the cranks completely and having two chainrings, one
either side of the bottom bracket, driving two freewheels on the rear
hub? // |
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That's OK for the down (power) stroke, but how would the pedal return
on the up stroke ? This would just result in both cranks dropping to
BDC and staying there. |
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There are elliptical sprockets available which give variable gearing
through one crank rotation. |
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//That's OK for the down (power) stroke, but how would the pedal return on the up stroke ? // |
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I was assuming the cyclist was wearing pedal cleats. |
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Or old, rusty, gummed-up freewheels that only freewheel when you stamp backwards, but otherwise have enough friction to keep the pedals turning? |
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90° out-of-phase would give a smoother power curve, if your feet are well attached so you can power on "up" as well as "down" (although with a bit of practice, well-attached feet could give smooth power whatever the crank phase, I guess...). |
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An Octalink bottom bracket would allow 45
degree steps, so it would be easy to test. |
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