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It seems to me that bicycles have proven themselves to be the fastest human-powered vehicle in common usage. By “human-powered” I mean it has no fuel or power source other than the inputs given by a human, i.e. no engines, no sails, no clever use of gravity. I wonder if this is a function of the design of the bicycle or just a coincidence. In other words, is the bicycle the fastest possible human-powered vehicle, or is just that no-one has built a faster vehicle? Are there faster human-powered vehicles that I don’t know about?

For standards, lets say we are talking about locomotion on a level (no incline) road surface.

If the bicycle is the fastest, can someone give a technical reason why this should theoretically be so? For example, why not a tricycle, unicycle, or something without wheels, perhaps?

Eoin
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There are two reasons your conjecture is probably true, as follows.

First, best power transfer from leg muscles to the rear wheel of a bike occurs when the impedances of the leg mechanism and the pedal crank mechanism are matched. This is done with careful selection of the crank arm lengths and the gear drive ratio. Bike transmissions are designed to do exactly that.

Second, best speed occurs with maximum power and minimum drag. Note that a bike by itself presents a smaller profile to the oncoming air than a human body does which means the drag part of the relationship is going to be determined in large part by the human on the bike: making the bike smaller is not going to allow it to travel faster, so you work on the human part instead.

Bike designers have worked that out, too by putting the human into a crouched position on the frame so as to minimize the human's profile.

These two factors mean that it is probably true that a well-designed bike is the fastest way for a human to travel under their own power.

niels nielsen
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To understand why a bike wins, consider the needs and the costs of maximizing the power output of a human body and minimizing losses.

Human body cannot move continuously. Joints rotate but need to rotate in the opposite direction after. If one were to move in spurts like this, the drag losses would be much greater. Bike casette helps smooth out the speed.

There is an additional alternating moment to cancel out too. As legs pump, there is need for a twisting moment to maintain orientation. You may see cyclists swing their bikes left and right between their legs sometimes in order to reduce the effects of their pumping legs. Bikes can impart this moment to the ground via their two wheels (two points of contact).

Supposing a person unicycle managed to somehow cancel this while travelling fast, it would involve a lot of movement of mass (flailing arms to cancel pumping legs?) which in turn cause air movements that make it inefficient compared to the bike simply relying on the normal force from the road.

Knowing this, what else might show promise? - Would you care to design a unicycle or hamster wheel incorporating a bike casette, and a mechanism that allows it to be powered via symmetric movements (think person running like a cheetah), sacrificing steering and control for energy efficiency and hence speed?

Abel
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