I read a study that measured the efficiency of locomotion for various species on the planet. The condor used the least energy to move a kilometer. And, humans came in with a rather unimpressive showing, about a third of the way down the list. It was not too proud a showing for the crown of creation. So, that didn’t look so good. But, then somebody at Scientific American had the insight to test the efficiency of locomotion for a man on a bicycle. And, a man on a bicycle, a human on a bicycle, blew the condor away, completely off the top of the charts.
This is how Steve Jobs recounted his inspiration for referring to a computer as a “bicycle for the mind”.
So what is it about bicycles, and similar small, low-speed vehicles (what I’m going to call “low kinetic energy vehicles” or “LKEVs”), that makes them so energy efficient?
Efficiency of Locomotion
It’s important to nail down exactly what we mean by efficiency, which turns out to be a bit more tricky than it would seem.
Not to get all “Webster’s defines”, but a physicist’s definition of efficiency is the fraction of the energy used by a system that gets converted into useful work. A problem arises from the fact that the useful work done by starting from Point A and stopping at Point B is, from a physics standpoint, zero. The kinetic energy at both ends of the trip is zero, and we’ll assume both points are at the same elevation so that we can ignore potential energy due to gravity.
That leaves us with the conclusion that real-world vehicles—which have to use some amount of energy to overcome aerodynamic drag, rolling resistance, and inertia—are all zero percent efficient.
But while we can’t make useful absolute statements of vehicle efficiency, we can make relative comparisons in the context of a particular trip. And relatively speaking, LKEVs blow the doors off the competition when it comes to how little energy they need to complete the sorts of trips they are suitable for. And it comes down to the factors that make an LKEV an LKEV: low speed and light weight.
The biggest contributor to the efficiency of LKEVs is that they don’t go very fast.
A cyclist in a “tops” position on a bike has a net aerodynamic drag area (CdA for you engineers) of around 0.73 square meters, which by way of comparison, is about the same as a Hummer H3. Keep in mind that this is an absolute number, and not corrected for size or number of passengers. Even in full aero position, that number only drops down to about 0.21, which is still achievable by something that works vaguely like a car, or by a Hummer carrying four people.
So a Hummer going 30 kilometers per hour (19 mph), or a human on a bike going 30 kilometers per hour is going to use around 250 watts to overcome air resistance alone.
But the nasty thing about aerodynamic drag is that the power required to go a certain speed goes up with the cube of speed. To go 120 kilometers per hour (75 mph) takes not four times as much, but 64 times as much power as it takes to go 30 kilometers per hour.
(It’s worth noting that you are also going faster, so it takes correspondingly less time to complete your trip. That’s why energy consumption only goes up with roughly the square of speed.)
The next factor is that LKEVs weigh very little relative to cars.
A typical bicycle might weigh a tenth as much as its passenger, whereas a very light car (by modern standards) will probably outweigh its driver by a factor of at least 10.
So depending on the number of passengers in the comparison car, LKEVs enjoy a weight advantage of 50 to 200 times.
Much of a car’s weight is to allow for the aforementioned high speeds. You need to be able to produce dozens of times as much energy as it would take to go slower, and you need a much stronger structure to give the passengers any hope of surviving a collision. You also need to protect them from the wind, insects, and small rocks. Protection from inclement weather also becomes much more important once the apparent wind reaches hurricane speeds.
But a lot of the weight is a consequence of a vicious cycle, where in a larger vehicle needs a larger engine, which in turn needs larger brakes to stop, and a heavier chassis to support these components, which requires a larger engine, and so on.
The most obvious consequence of this excess weight is that it takes more energy to accelerate it, as well as more energy to go uphill.
A secondary consequence is that it takes more energy to overcome the drag of tires on pavement (particularly the large tires used by heavy cars) as more force is applied to them.
Many people argue that bicycles are particularly efficient because they’re human-powered, but this is only true so long as a cyclist is using energy they otherwise would have exerted at the gym, or that is stored as body fat. In that case the energy used for biking is free, or better than free (from a health perspective).
But when looked at in terms of well-to-wheels efficiency, a human eating a modern diet does quite poorly: Only about 3.5% of the energy that goes into producing food ends up where the rubber meets the road.
So the extent that human power is a factor in the efficiency equation, the sign could be positive or negative depending on the assumptions you use.
One element solidly in favor of supplementing or replacing human power with electrical power is that for many westerners, arriving at their destination drenched in sweat can be socially and professionally iffy. And a single hot shower can easily use as much energy as hundreds of kilometers of low-speed electric travel.
All that said, one can’t really talk about efficient transportation by looking exclusively at vehicle efficiency. Some kinds of trips simply aren’t practical to make with certain types of vehicles.
There is research that suggests that people generally won’t tolerate a commute that takes longer than about 30 minutes each way. So for bicycles to be a viable commuting option, the trip must be at most around 15 km each way.
Unfortunately, many North American cities have been built around the ubiquitous availability of cars, and commutes of 50 km or more are commonplace, simply because the time they take at 100 kilometers per hour is still tolerable.
This pattern sets up a sort of vicious cycle of car dependence: You need a car to make almost every trip, which means building lots of big roads to move cars around, and lots of parking to store them (at both ends of every trip), which means everything needs to be built farther apart to make room for all of that infrastructure, which in turn means that most trips are long enough to all but require a car.
So once again, land use and public policy have played (and are playing) a huge role in defining the efficiency of the transportation network.
In a perfect world (at least from a transportation efficiency perspective), we would try to spark a virtuous circle of living in denser neighborhoods where car ownership is unnecessary or even kind of a hassle. More people would make trips by walking and biking, encouraging businesses to locate themselves in such a way that they are walkable and bikeable.
One of the most pressing problems in countries and regions that were developed after the widespread deployment of automobiles is how to get from our current land use to something more sustainable.