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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?

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    $\begingroup$ What have you searched for so far? Just so we don’t duplicate effort and have you say "I already found that"… $\endgroup$
    – Solar Mike
    Commented May 13, 2022 at 19:12
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    $\begingroup$ I think this is almost a weird non-question because the only other alternatives are water and air craft. Space is out because there is no reaction mass. Water drag prevents high speeds while aircraft have the issue of energy required to stay aloft unless lighter than air craft is used. But the large surface area of a lighter-than-air craft makes the drag higher than a ground craft. Unless what the OP has in mind are alternative land vehicles but it's pretty difficult to think of alternative ways to human-power a ground vehicle. $\endgroup$
    – DKNguyen
    Commented May 13, 2022 at 19:57
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    $\begingroup$ I am thinking of alternative land vehicles. And really I want to understand if and WHY the bicycle is the the theoretically most efficient. For example, why not a tricycle? Why not a unicycle? I have updated my question to include “why”. $\endgroup$
    – Eoin
    Commented May 13, 2022 at 22:16
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    $\begingroup$ Tricycle adds losses for no benefit. Unicycle is less aerodynamic… $\endgroup$
    – Solar Mike
    Commented May 13, 2022 at 22:40
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    $\begingroup$ @SolarMike Riding behind a modified car doesn't really count in my book. Not only does it block aero drag, it creates a bit of a vacuum. Two different kinds of records. One is only human powered, the other is added by a motorized vehicle. $\endgroup$
    – Eric S
    Commented May 14, 2022 at 18:51

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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?

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  • $\begingroup$ Well, any competitive design will have a complete fairing & cockpit to minimize air drag, so the motion of the ugly sack of mostly water matters only in the efficiency of power transfer to the wheel(s). $\endgroup$
    – Carl Witthoft
    Commented May 16, 2022 at 14:30
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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.

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