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...