fundamentals

Cycling laws and regulations come and go but the laws of nature don't change, so cyclists and bikes must ride with them. Most of the fundamentals of science actually facilitate cycling and make it possible, although at least one does, in fact, make it harder. The key is to use science and technology to minimize the disadvantages and maximize the benefits so that cycling is easier. It must be reasonably easy because interest in cycling is booming to the point where bicycle sales far outstrip those of any other vehicle type. Cycling is not only a massively popular way to travel but has also become a political touchstone because of the impact that it can have on the environment, society, and the individual. Where once world leaders waved imperiously from the steps of jetliners, they are now eager to be seen on a bike. So, health, safety, climate, and other issues fundamental to human existence are brought into this chapter, which lays down a broad, smooth track for the journey ahead.

What are the forces acting upon a bicycle?

Am I not alone on my bike?

There are four external forces that every cyclist must work with or against—gravity, air resistance, rolling resistance, and friction—and a fifth effect, referred to as inertia. None of them can be utterly vanquished (and it would not necessarily be desirable to eradicate them completely). However, it is wise to understand what you're up against so that you can minimize the negative consequences and harness the positive.

Gravity is the force that gives weight to matter. The Earth pulls everything to itself with a gravitational acceleration of about 32 ft/s2 (9.8 m/s2). In fact, gravity makes cycling possible by pressing the bike to the ground, but it makes riding uphill harder. Descending is made easier by the pull of gravity, but you never get back all the energy you put into climbing the same hill.

Air resistance generally works against the cyclist. The planet's gravity is strong enough to hold a blanket of air some 62 miles (100 km) thick to the Earth's surface. Cyclists couldn't breathe without it, but must push it aside continually to make progress. This same force can be helpful, too, if you've got a fair tailwind.

Rolling resistance results from the fact that, when a tire comes into contact with a road, both tire and road deform a little. The road and the tire do not spring back with the same energy that deformed them, with some energy always lost to heat. This has the effect of a resisting force.

Friction helps to move the bike forward by maintaining contact between tire and road. However, friction in the bearings of the bicycle's drivetrain—from the pedals through to the chain, gears, and hubs—can absorb up to 5 percent of the cyclist's energy.

Riders must also overcome inertia, which is not a force at all, but an innate property of matter—its resistance to any change in its state of motion. A bicycle's motion won't change if there are no forces acting on it.

Forces and inertia

Inertia

The principle of inertia is a way of saying that an object doesn't change its motion unless there is a force acting on it. The bigger the force, the greater the change in motion (in speed or direction). Steep hills, strong winds, muscular legs, and powerful brakes overcome inertia to the greatest degree. Mass determines how big the effect will be—under a particular force, a heavy bike will change its motion more slowly than a light model. Likewise, a rider who loses weight will accelerate more quickly than their former, fatter self.

Gravity

The Earth subjects bike and rider to a gravitational force that would make them accelerate downward at approximately 32 ft/s2 (9.8 m/s2) if they weren't supported by the ground. Gravity makes cycling uphill harder, but without it you couldn't cycle at all—it keeps the bike on the ground and the rider on the saddle.

Air resistance

A cubic foot of dry air at 68°F (20°C) at sea level weighs about 0.076 lb (0.034 kg). When the cyclist and the atmosphere meet head on, some of a rider's energy is lost to pushing this air out of the way. If the difference in their speeds is more than about 9 mph (15 km/h) on a flat road, this becomes the biggest drain on the rider's energy.

Friction

The friction between the tire and the road surface is crucial for forward motion. Without it the wheel would spin on the spot, as if on ice. However, friction in the bearings of the bicycle's power train drains energy into wasted heat and noise.

Rolling resistance

Bike tires deform under the weight of bike and rider as the rubber comes into contact with the road surface. Because the tire doesn't spring back with quite the same energy as it is deformed, this shape changing absorbs a small amount of the energy which, in the main, has been put into the system by the cyclist pressing on the pedals. A hard tire on soft ground suffers from similar rolling resistance, although this time it's the ground that deforms, once again absorbing the rider's energy.

equipment: the bicycle

The modern bicycle is the result of two centuries of refinement, propelled by better understandings of science and technology. Bike builders and designers have repeatedly worked out novel ways to use established materials and to incorporate new materials to supplement or replace the old. Yet a cyclist from the late nineteenth century would have little difficulty in recognizing today's bicycle because the silhouette of frame, wheels, saddle, and handlebars has remained largely unaltered. They may be alarmed by the slender saddle or amused by the 27 gears, but they would certainly be reassured by the familiar chain and spokes.

There are hundreds of parts on a bicycle, the majority exposed to full view. On the most functional bikes they each serve a mechanical purpose. Without doubt, the most important part is the frame, often described as the heart of the bicycle by people whose grasp of anatomy should disqualify them from medical practice. It is, actually, more akin to the skeleton, holding everything together, supporting many of the components and also the rider. If the metaphor is to be continued, the front fork can reasonably be equated to a limb, articulated at the headset.

The major components

Variations on a themeBicycles come in scores of varieties, with differences in design, components, and materials to optimize the balance between function and cost. Every one is an assembly of parts, each of which can be changed in line with the owner's desires, with replacements that may be mass-manufactured or handcrafted.

The wheels define it as a machine that is able to translate the work by the rider into motion along the ground, preferably forward. Even a Neanderthal would agree that the wheel is the most beautiful of human inventions, while kicking himself for not having thought of it first. The wheel has been fundamental in human development and its application in the bicycle is both elegant and minimalist, giving the bike its extraordinary efficiency.

The saddle's role of seating the rider comfortably while allowing the legs to move freely has determined its unique appearance as a distorted reflection of the body parts it supports. The lines of handlebars, however, are as varied as handshakes, their contours a compromise between the need for an appropriate degree of bike control and the flailing, versatile geometry of the rider's arms.

Pedals, cranks, chainsets, gears, and brakes are diverse in design and detail. They will continue to be refashioned as the market, marketeers, and technologies evolve. Only a handful of each year's fresh ideas will endure. There will always be the frame, wheels, saddle, and handlebars. For all the other hundreds of components and their various configurations, only the fittest will survive.

How efficient is cycling?

Why is it easier to ride than to walk?

Walking is fine and running is faster but cyclists go farther, quicker, for longer. Cycling is the most efficient way for a human being to use their own energy for propulsion. The bicycle is an extraordinary machine capable (at best) of converting 98.6 percent of the cyclist's pedaling effort into spinning the wheels, while those who stride along with their feet on the ground waste a third of theirs.

The average human walking speed is about 3 mph (5 km/h). At this speed, humans can travel for hours. Nevertheless, the walking movement is wasteful and inefficient. At every single step, the knee of the grounded leg bends and flexes, lowering the entire body for a moment before levering it back up to normal height. While this is going on, the spine bends a little, the hips twist, arms move to and fro, and the swinging motion of the other leg stops with an energy-absorbing impact when the foot strikes the ground. In fact, compared to an "ideal" walking machine, we are only about 65 percent efficient. In other words, in walking we lose about one-third of our energy to effects other than forward motion. No engineer would be proud of such an inefficient vehicle.

The bicycle, however, forces us to adopt a pose that maximizes the use of our calories. A walker moving at a particular speed could, if they rode a bike, travel three times faster without having to increase their effort at all, because the two-wheeled machine is phenomenally efficient. There are two reasons for this remarkable improvement. The first is the way the bike is configured to make us use our bodies more efficiently. The second is that the machine acts as a lever, multiplying the distance our feet travel around the bottom bracket with each pedal stroke into the much greater distance the tires travel along the ground.

Energy efficiency

Saving on fuelIf a cyclist and a pedestrian expend the same amount of power, the efficiency of a bicycle means the cyclist will be traveling three times as fast. At an average walking pace, the walker uses more than six times the amount of metabolic energy above the resting level compared to the cyclist. Running is four times as greedy of energy as riding and, at normal riding speeds, humans on foot simply drop off the scale.

Speed records

Faster and fartherThe plain fact is that the world's fastest runners cannot keep up with a cyclist. Usain Bolt ran at 23.35 mph (37.58 km/h) in the 2009 Berlin World Championships, but that was for less than 10 seconds. Speed skaters, too, trail behind cyclists—even Jeremy Wotherspoon, who set a world record of 32.87 mph (52.89 km/h) over a 547-yard (500-meter) course. Thanks to the brilliant mechanical advantage of the bicycle, top cyclists can quickly outstrip both to achieve 40 mph (64 km/h) over similar distances. And no athlete could run the 35.03 miles (56.37 km) that Chris Boardman rode in one hour at the Manchester (UK) velodrome in 1996. In 2009, Sam Whittingham reached 82.82 mph (133.28 km/h) riding the Varna Tempest, a recumbent bike in an aerodynamic shell. When it comes to human-powered travel, nothing beats cycling.

Pedal power

Wheel radius to crank length ratioThe pedal as a lever gives a mechanical advantage which, in its simplest form, can be calculated as the ratio of the wheel radius to the crank length. As an approximate example, a direct-drive High Wheeler with a wheel radius of 30 in (75 cm) and cranks 6 in (15 cm) long amplifies the distance moved by the ratio 30/6 = 5. That is, with one revolution of the pedals, the big wheel will travel 15 ft (471.25 cm) along the ground, five times greater than the 3 ft (94.25 cm) the Victorian rider's foot has traveled around the axle. The real magic of modern bikes is that they have gears, so the rider can choose their preferred ratio at any time.

What is the most efficient design?

Which type of bike should I choose?

While most bicycles use the same basic design, there are many variations on the theme, each with a different level of efficiency. Most have pedals, a chain, brakes, a saddle, and handlebars; many have gears. Those are about the only common areas; like most adaptable creatures, bicycles have evolved into a multiplicity of designs, each excelling in a particular niche.

Some designs are conceived to be efficient for a single purpose. A track bike, with its stiff frame and no brakes, may not be very comfortable but at a velodrome it allows the rider to reach very high speeds. A mountain bike has suspension that smooths rough terrain, but its fat, heavy tires prevent it from breaking any speed records. A hybrid may be equally at home commuting in the city and roaming the trails. The key to choosing a bicycle is to identify what area of efficiency is your key criterion. That decision will dictate the choice of wheels and frame shapes.

The vast majority of bike frames rely on engineering's favorite shape—the triangle. It's the strongest two-dimensional geometric shape, which is why the main part of the frame is, or approximates to, a triangle. Admittedly, the top tube may not be horizontal and a short section of head tube may add a fourth side a few inches long, but designers try to get as close as possible to a triangle because it is the most stable and rigid structure for a given mass of material. In a triangle, the three fixed tubes share the transmission of forces between them when put under pressure or stretched. The triangle can be deformed only by changing the length of one of its sides or by breaking it.

Variations on this triangular principle can be seen at the core of each style of bike, playing a key part in the relative power requirement and comfort of each model. The cycling science of frame triangles is more fully explored (see here).

Frame and power

Power requirementsThis graph demonstrates how the power efficiency of different styles of bike becomes more significant at the top end of the speed curve. At speeds under 6 mph (10 km/h), the Dutch-style upright bicycle only needs a little more pedal power from its rider than does the sleek velo-raptor. As soon as the rider gets cranking, however, the efficiency of the upright slumps while the racing bikes start to deliver on what they were built for: speed. If you want to save energy at speed, then you have to sacrifice comfort and carrying capacity.

Seated rider

Standing rider

The power of trianglesThe triangle frame excels at the transmission of forces without distortion. When a rider sits on the saddle, the two adjacent sides (the seat tube and the seat stay) share that load, while the opposite side (the chain stay) is put under tension. When the rider stands up, the down tube and seat tube are stretched (and the bottom bracket supports their weight through the pedals), while the top tube is compressed and stops the bike from collapsing in the middle.

science in action

conservation of energy

Energy cannot be created or destroyed—that is one of the fundamental concepts of physics. This is demonstrated every time you ride. The nutrients in the food and drink that you consume are metabolized to nourish your body and power your bike. Riders convert the chemical energy in food and drink into mechanical energy to propel them forward. Other energy conversions less useful to cyclists also take place—making them hot, moving the air around them, and creating sound from the chain and the tires on the road. If it was possible to measure all of the output energies and add them together, they would equal the energy stored, then consumed and expended, during that ride. The total energy put into the cyclist has been converted, but it is never lost.

Unfortunately, it is very hard to prove this empirically because cyclist and bike are not an isolated system as far as physics is concerned. There are other energy impacts that cannot be excluded—for example, gusts of wind, the roughness of the road, and what the rider ate for dinner the previous evening. Also, there are factors that cannot be readily quantified, such as fingernail growth and eyelid blinks. Yet the law of the conservation of energy remains true—energy may be converted from one form to another, but in total it cannot be created or destroyed.