5
Gears

Mechanical Principles

Gearing systems match your body’s strength and speed to the riding conditions. The bicycle is a machine for making your stride longer, but it needs to be adjustable. When you walk uphill, you take shorter steps than when going down—the human leg is infinitely adaptable between the shortest and the longest strides it can take. When you ride a single-speed bicycle, you have only one stride length. That is, for each half revolution of the pedals, which is a kind of stride, you travel a certain distance forward. This distance is controlled by two things—the size of the rear wheel and the drive ratio between chainwheel and sprocket. Before safety bicycles were invented in 1885, the usual bicycle was the “ordinary.” Its pedal cranks were attached directly to the front wheel, which the cyclist straddled. The bigger the front wheel, the further the cyclist went for each revolution, though the harder the effort. Those cyclists quickly found that the best size for a wheel was larger than a cyclist could straddle—the speed was limited by the length of the cyclist’s legs. He could buzz along with feet twirling like crazy, but not pushing very hard. Naturally, people bought bicycles as big as they could straddle, specifying them by the wheel diameter in inches—such as 54 or 60 inches.

The chain-drive safety bicycle fixes that problem by making the rear wheel turn faster than the pedals. Nowadays, the wheels of most bicycles are all very close to the same size, between 26 and 27 inches in diameter, but the chainwheel and sprocket sizes vary over a wide range. A large chainwheel and small sprocket make the rear wheel spin much faster, carrying you farther per pedal revolution—in effect, giving “long strides” for level roads. A small chainwheel and large sprocket make the rear wheel spin more slowly—maybe only as fast as the pedals, thus carrying you much less distance per pedal revolution—in effect, giving “short strides” for hill climbing.

Gear and Development Formulas

There are two systems for specifying “stride length.” The English system formula goes back to the ordinary system by specifying the equivalent wheel size:

gear = wheel diameter × chainwheel / sprocket

where wheel diameter is in inches, and chainwheel and sprocket sizes are in teeth.

Chainwheel and sprocket sizes are measured in “teeth”—just count them. If this formula calculates to 84 inches, for example, the feeling is equivalent to riding an ordinary with an 84-inch wheel diameter, a size that nobody could straddle but that is a useful gear for strong riders on level roads. Low gears, depending on conditions, are in the 20- to 50-inch range, middle gears 60- to 70-inch, and high gears 80- to 100-inch.

The metric system goes directly to the stride length by specifying how far you go on one revolution of the pedals, calling this value development, and specifying it in meters:

development = wheel circumference × chainwheel / sprocket

The wheel circumference is of course the wheel diameter multiplied by pi (3.14); here both diameter and circumference are measured in meters. Nominal circumferences are 2.14 meters for 27-inch wheels, 2.13 meters for 700C or tubulars, and 2.11 meters for 26-inch wheels.

Development generally runs from about 3 meters for low gears to 8.5 meters for high gears.

When speed and distance are counted in miles, the conversion from cadence (pedal revolutions per minute) to miles per hour is

mph = cadence × gear × 0.003

When speed and distance are counted in kilometers, the conversion from cadence to kilometers per hour is

kph = cadence × development × 0.06

Books on cycling used to contain pages of tables of gears for different setups. But, as explained later, there is no great need to calculate gears, and everyone now has a four-function calculator handy.

Hub Gear Formulas

The gearing system allows you to change chainwheel and sprocket sizes while riding. That is exactly what happens with a derailleur system, and you can see it, but in a hub gear the change is invisible. Therefore, a short digression on hub gears is in order. Three-speed hub gears have one direct ratio and two indirect ratios. The direct ratio is always 1:1, the rear sprocket driving the wheel directly. The indirect ratios are reciprocals of each other—3/4 and 4/3 are the only common ones. These are reciprocal ratios because the power goes through the gears one way for low and the opposite way for high. From the 1930s through the 1950s, both three- and four-speed hubs were available in a variety of ratios.

To calculate for the common three-speed, compute the middle (direct) gear from the wheel size, chainwheel teeth, and sprocket teeth, then multiply that number by 3/4 for low gear and by 4/3 for high gear.

Hub gears with 5, 7, 8, 9, and 14 gears are now available, but are uncommon. The ratios are available from the manufacturers.

Hub gears are used mostly for utility bicycles, which have a wide range of prices, and are most common in Holland and Germany.

Derailleur Gearing Systems

Back when only a few gears were possible on each bicycle, discussion of how to design the system was perennial among cyclists. Manufacturers provided a wide range of chainwheel and sprocket sizes so that each enthusiastic cyclist could design his own system according to how he liked to ride and to shift between gears. Technical development has now made that discussion moot. The development of rear clusters with many sprockets, now up to ten, has eliminated the need for much decision.

Mountain bikes use triple chainwheel sets; road bikes use triple or double. The cyclist looks at the road ahead—level, steep, or very steep—to decide which chainwheel to use. He then has a wide range of gears available from the many-speed rear cluster. Only rarely does he shift between chainwheels, and when he does so, he also has to make a big change at the rear. So this is done only at locations where it is necessary and the conditions are also favorable.

Chainwheels come in very few sizes to suit each manufacturer’s cranks. There is a range of rear cluster sizes, specified by the number of sprockets and the smallest and largest, so the cyclist can choose a cluster to produce generally high gears or generally low gears, but has no say in the steps between. With many steps available, the precise arrangement is immaterial.

Chain Sizes

With several choices of numbers of sprockets on each rear hub, there is a corresponding variety of chain widths and compatibilities.

Derailleurs

Derailleurs basically come in two kinds of sets. One kind is for double chainwheels for road bikes; the other is for triple chainwheels for both road bikes and mountain bikes. Obviously, the front derailleur has to be designed for either double or triple chainwheels. The rear derailleurs have to be different because triple chainwheels, with the small size of their small chainwheel, pull in or let loose more chain between largest and smallest chainwheels than do doubles. That greater amount of chain “take-up” requires that the rear derailleur have a longer cage (arm) to keep the chain tensioned at all times. There is another smaller difference in the largest sprocket that the derailleur will clear. Of course, a mountain bike derailleur that will clear the largest rear sprocket (34 teeth) will also work with a closer-geared cluster and double chainwheels; its disadvantage is that its shifting is not so crisp. Therefore, both long-cage and short-cage rear derailleurs are built for the different uses.

Shifting between Chainwheels

Suppose that you are using the big chainwheel and the hill has steepened so that you are using the largest sprocket, or the next one to it. And the hill will get steeper, or you will be getting tired, and you want to shift to a lower gear still. If you shift to the next smaller chainwheel and don’t shift the rear derailleur, you will be in a much lower gear, so that you have to slow down more than necessary and more than you desire. To get back to your desired speed, you need to shift up several sprockets on your rear cluster. To minimize this delay and loss of speed, you should practice shifting to a smaller chainwheel and simultaneously, or as quickly as possible, shift the rear derailleur up the required number of sprockets. This takes practice, but it will make your climbs smoother.

The same goes for shifting to a larger chainwheel, when you have to also shift to a larger sprocket. This usually takes place under easier conditions, but the shifts to larger chainwheel and larger sprocket are a bit more difficult to make. Again, practice smoothes out the double-shifting operation.

Using Your Gears Correctly

With modern gearing systems, the most important instruction is in using your gears correctly. You know what it feels like to change gears. In high gear, your feet turn slowly but you must push hard; in low gear, your feet spin around but you don’t have to push hard and you don’t go very fast. Somewhere in the middle is the gear that is best for the usual conditions. As the hills get steeper, you slow down, so you change to a lower gear to keep the leg effort within your strength and to keep up your pedal speed. On gentle downhills, or with the wind behind you, you speed up, so you change to a higher gear to prevent wasting your energy just spinning your feet, even though this change requires more force on the pedals. Obviously, there is one combination of foot speed and leg effort that best suits you under the given conditions, and you need to know how to shift to it.

You use your gears to adjust your pedal speed to the conditions—hills, fatigue, wind, traffic, and purpose.

Level Riding

Most beginners think that the slower they turn the pedals, the easier it is, so they ride on the level in highest gear at 12 mph. This is basically wrong. Muscles get tired both from producing power and from exerting a steady force without producing power. Low pedal speed means greater leg muscle force for the same power, so your legs get more tired from the hours of pushing hard on a slow-moving pedal than if they pushed less hard on a fast-moving pedal. The objective of using variable gears on a flat ride is to keep your feet turning fast enough so that the leg muscle force is not high, but the power output is. Power is force times speed, and power is what drives you along. You shift into higher gears on the level only when you get strong enough to go faster. You don’t shift into higher gears until your feet get to turning too fast for the gear you are using.

The high gear on your bike is probably around 100 inches. Most road-racing cyclists ride on the level in gears around 94 inches, and they move at about 26 mph. The junior racers are limited to a maximum gear of 85 inches, and fast juniors go over 25 mph on the average. They all go faster than you, and in lower gears. Learn from them.

When you travel at 15 mph, you should be in a gear of 74 inches or lower, and you should be pedaling at least 70 pedal revolutions per minute. A better gear for 15 mph is 63 inches for 80 pedal rpm. Riding like this makes your legs supple and relaxed instead of stiff and tired, develops endurance so the miles go easier, keeps a reserve of power in your legs for the short hills and sprints, and prepares you for going faster in higher gears later on.

Hills

When approaching a hill, consider how steep it is. When you have had some experience, you will be able to estimate which chainwheel would suit you best for the climb. If one specific chainwheel will probably provide the gears for the whole climb, then shift onto it at the bottom.

By the same reasoning, keep up your pedal speed on hills. Don’t let the hill slow down your pedals. When you feel the first grade, turn on enough more leg force to maintain your pedal speed. You will travel just as fast, but it will take more effort. When the hill gets steeper, the effort gets too high and you cannot maintain your speed without getting tired too rapidly. Before you get tired, reduce the bike speed (and hence the demand for power) but keep up the pedal speed by shifting down one gear. Shift to maintain pedal speed, not to lower the force. Keep in the sit-down-and-twirl mode until you are down to the bottom gear. This is the best way to climb long hills.

If the hill is short, like a freeway overpass or a short steeper section of a longer hill, stand up and drive hard. The faster you go over a short hill, the less speed you lose. Stronger racers use this kind of hill to break up or separate the opposition—it is really just a sprint, but in a place where “wheel sucking” (riding in the draft of a faster rider without ever doing your share) helps least.

Long Rides

You have only so much sprinting in you per day. Get sprinted out and you are reduced to average effort for all the rest of that day. So on long rides, conserve your sprinting power. Be extremely conscious of the hills and winds and continually assess whether you should change gear. I expect that in rolling country, an expert cyclist rarely rides a quarter mile in the same gear. When I ride double centuries with younger riders who are stronger than I am, I maintain the same average speed by careful shifting at every grade or wind shift. I never get sprinted out, but I am always working at my maximum long-term power with the optimum combination of pedal speed and pedal force.

Wind

Wind is just like hills. Change gears for wind as it affects you. For instance, when you ride along a river in a canyon on a twisty road, the headwind may hit you on every right curve and leave you on every left curve. Shift gears every curve to match. On my club’s criterium course, I ride one gear higher on the downwind side than on the upwind side. With the wind behind you, it is a different story—you can ride in 104-inch gears or higher and go as fast as your feet will turn.

Traffic

In heavy traffic, speeds are moderate but always changing. Therefore, ride in medium gears, so you have a reserve of quick acceleration. Riding in high gears in traffic simply tires you by making you strain to accelerate every time the traffic opens up, and leaves you blocking traffic as well. I ride about 77 inches in faster heavy traffic, and change down to 70 inches if it slows down. Always change up or down as your speed changes.

When you stop in traffic, don’t get stuck in high gear. Brake harder than you need to, shift down to a good starting gear, then brake to the stop.

Warming Up

When you begin a ride, start in lower gears to “warm up.” As your legs become comfortable, shift to higher gears. Don’t start in high gears at a low pedal speed—it only tires you out at the start.

If you coast on long downhills in cool or cold weather, never turn on the power suddenly at the bottom. Cold knees are very subject to cartilage damage, so first pedal downhill if you can match the bike’s speed in high gear, even if you don’t really go any faster by pedaling. If you can’t match the bike’s speed, get your legs turning fast near the bottom, and apply a gentle driving force the moment the speed drops enough for your legs to catch up to pedaling speed. If the grade goes up, don’t sprint in high gear just because you are refreshed and going fast. Keep using gentle force and let the speed drop until you can change into a gear low enough to protect your knees. When they get warmed up again, increase the speed and the gear.

Thinking

All the information presented in this chapter might seem like too much to remember. It is a lot, but it is worth it. Cycling rewards you for doing it right. Even the ride to work becomes fascinating when you practice doing it as well as you know how. Mind, body, and bike are all tuned up and flying, and you arrive with a sense of well-being and optimism. Going homeward is the same—the cares of the day slip away as the miles go by.

Proper shifting is one of the first things you neglect when you get tired. You find yourself stuck in high gear at a stop sign, or pushing too hard and going too slow on a hill because you didn’t shift down, or even making rough and noisy shifts instead of instantaneous and silent ones. These are the effects of weariness—but because these mistakes make you wearier still, it is important to train yourself to pay attention to your cycling technique. The best technique takes you the most miles with the least effort at the same speed.