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I have interested in walking bicycle. For the concept is just bicycle that you don't cycling pedal but stepping or striding to drive wheel

Which leads me to think that is it better if we use linear or ellipse motion of leg to transmit to bicycle

The closest mechanic I could think is piston crankshaft like a car. It convert linear motion to rotation

How disadvantage of the mechanic compare to cycling motion?

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    $\begingroup$ Just Google "linear pedal bicycle" -- there are plenty of examples. They are usually based on wrapping a cable or chain around a drum that's mounted to a ratchet (one-way drive mechanism) of some sort. $\endgroup$ – Dave Tweed Jan 16 '16 at 14:56
  • $\begingroup$ @DaveTweed Sorry to forget mention that. I have seen that kind but it not like piston rotor but a chain drive. So I am curious why we never use piston crankshaft $\endgroup$ – Thaina Jan 16 '16 at 15:32
  • $\begingroup$ Let me see if I have this right. Instead of putting your feet directly on the pedals of a conventional bicycle crank, you want to have a separate set of connecting rods that go to a set of pedals that move linearly? What possible advantage would that have? It would just add weight and friction. $\endgroup$ – Dave Tweed Jan 16 '16 at 16:39
  • $\begingroup$ @DaveTweed Not that. I mean the way they implement linear conversion is like a pulley or drive gear rod. What I mean is I wonder why they don't use piston style which is more circular. To let it drive the bicycle gear and chain is the same $\endgroup$ – Thaina Jan 16 '16 at 16:51
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    $\begingroup$ How can you be "more circular" than a pulley or drive gear? $\endgroup$ – Dave Tweed Jan 16 '16 at 16:59
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The biggest problem with a "two cylinder" crankshaft is that there is a dead spot in the torque curve whenever either "piston" is at TDC or BDC.

When you have your feet directly on the pedals of a conventional circular crank, you actually have the ability to apply force over greater than a 180° arc — by a combination of flexing your ankles and pushing forward/backward with your thigh muscles — which eliminates the dead spot.

If you try to use a mechanical connecting rod with a crankshaft, you need some other way to eliminate the dead spot. This typically involves using more than two pistons, or by incorporating a large flywheel into the mechanism. None of this is practical or desirable on a bicycle.

I have seen toy vehicles for small children that use such a mechanism. but invariably, they need to put their feet down on the ground from time to time to push themselves out of the dead spot.

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