The Enigmatic Mystery of Gears

The Enigmatic Mystery of Gears

Many moons ago, people rode bicycles of all shapes and sizes. One of the early varieties was called a Penny-Farthing because it had a huge wheel at the front and a little teeny one at the back. This was, in part, because gears and freewheels were unknown and this configuration was at one time considered more efficient (believe that?). Anyway, because the pedals were welded to the front wheel, for one revolution of the pedals, the rider traveled a distance equivalent to the circumference of the big wheel (usually measured in inches). Big wheel equals large circumference; hard to pedal but a longer distance traveled. If the drive wheel were smaller, hence a smaller circumference, the Penny-Farthing would be easier to pedal but a shorter distance traveled for each revolution. Think about it. It’s true. Promise.

Why on earth do you care? When bicycle gears were finally invented, the original concept stuck but the arithmetic got distinctly more complicated. What gears do is to effectively combine in one device, the hard-to-pedal situation (high gears for a tailwind or going down hill) with the easy-to-pedal alternative (low gears for a headwind or going up hill or when you’re just plain pooped). Now for the only equation in this whole article. Another promise. The gear number (in inches) you are using at any particular time is the number of teeth on the front chainring divided by the number of teeth on your rear sprocket times the diameter of the back wheel (to be absolutely 100% correct, this equation should also take into account tire size and crank arm length but the extra accuracy so achieved is fairly minimal). Now you can forget that. Just remember that the high gears (hard to pedal) are big numbers, the low gears (easy to pedal) are little numbers.

On modern bikes, the number of gears you have, in theory, is the number of chainrings on the front (either two or three) times the number of sprockets on the back (usually 8, 9 or 10). For example a double chainring with a 9-speed cassette gives you a maximum of 18 gears. I lied, that’s actually another equation but it’s so easy, I cheated.

When you are zooming along with a tail wind, you need a high gear ……. big chainring and a SMALL rear sprocket. When you are struggling up the next hill you need a low gear ….... small chainring and a LARGE sprocket. If you want to convince yourself of that, look at the gear definition equation above.

However, life is not that easy. If we look at a typical road bicycle with two chainrings of 52 and 39 teeth and rear sprockets from 12 to 25 teeth (nine speed), the gear numbers look like this:

	12	13	14	15	17	19	21	23	25
52	114	106	98	92	81	72	65	60	55
39	86	79	74	69	61	54	49	45	41

This is a reasonably effective “all-round” set-up. The highest gear (the 52/12 combination) is 114 inches, the lowest (39/25) is 41 inches. Two things to note right away. If you wanted to start in your lowest gear and work your way up sequentially to the highest gear, you would need to switch back and forth between your front chainrings. The lowest gears are NOT all on the small chainring. The second thing is that when you are sold this as an 18-speed bike … its not really true as there is some duplication of gears ….. for example, the 39/17 combination is just about the same as the 52/23.

Life gets even more difficult because it is mechanically undesirable to run your chain from the biggest chainring to the biggest rear sprocket or from the smallest chainring to the smallest sprocket. This is because the chain then runs through the rear derailleur, the front chainrings and the rear cassette at a sharp angle, causing wear and reducing their life. Thus, in the example above, the 55 and 86-inch gears should generally not be used. That means that by not using these two combinations and eliminating duplication of gear combinations, your 18-speed bicycle is effectively a fourteen-speed machine or thereabouts. But that’s OK.

So when you buy a bicycle, should you care about what gears it comes with? Absolutely. If you are a self-contained camping tourist hauling oodles of gear up the hill from Vanderpool, you would want a triple chainset with ultra-low gears on the rear, something like this:

	13	15	17	19	21	24	27	30	34
52	106	92	81	72	65	57	51	46	40
42	85	74	65	58	53	46	41	37	33
30	61	53	47	42	38	33	29	26	23

This set-up has half a dozen or so lower gears than the first set-up we looked at and the lowest gear is “twice as low”. Some would argue that even on this configuration, the highest gears are too high. And before you scoff too much at the triple chainset (disparagingly called a ‘granny gear”), remember that Lance Armstrong’s US Postal Team have used them to good effect in European racing. Admittedly the hills were rather steep.

For a competitive or sports cyclist on flat course, he or she would have very closely spaced, relatively high gears:

	11	12	13	14	15	16	17	19	21
53	126	116	107	99	92	87	82	73	66
39	93	85	78	73	68	64	60	54	49

The lowest gear on this set-up is in the middle of the range for the ultra-tourist and the high gear is also 20% higher. Note that if a high-end component group were used, it would have a ten-speed rear cassette that would give even more closely spaced gears.

Whatever you eventually choose, you will need a low-enough first gear for you to comfortably climb hills, a high-enough top gear to meet your need for flat-out speed, even steps between ratios across the entire gear range and a smooth shifting progression that minimizes double shifting (having to change chainring and sprocket at the same time).

So now how do I use all these gears I have? That could easily be another article on its own but I will make a few generalizations.

I think that too much is made of constant cadence (keeping the number of pedaling revolutions per minute at a constant rate) though I definitely subscribe to the argument that there is a most effective cadence for a given situation. Many cycling aficionados insist that constant cadence is the most effective way to pedal but I personally find there are times when I want to spin in low gears and other times when I want to mash the higher gears. Without doubt, however, you will see beginners pedaling at a gazillion revolutions per minute and getting nowhere and others who are pushing gears so high, their legs seem to be almost motionless (and screwing up their knees into the bargain). Neither extreme is efficient but you do have considerable latitude within the “acceptable” cadence range (70 – 110 RPM for most riders with something around 90 being optimum for flat roads).

Even going up hills there is no universal “right” way to tackle the grade. You might choose to power up a short steep incline in a relatively high gear whereas on a long gradual climb you might want to sit back, arms on the tops of the bars and pedal up in a lower gear. There are actually several techniques for climbing, as well as specific approaches dependent on physiological factors (body size, muscle type, etc). In general, bigger cyclists tend to do better climbing seated, while smaller mountain goats tend to do better standing. The average cyclist should try riding the first 2/3 of a hill seated, spinning a smaller gear. Then the last 1/3 could be done in a slightly higher gear, while standing in the saddle.

And finally, remember three things. First, start off in a relatively low gear until you are well and truly warmed up. Second, experiment with different riding styles and techniques. Third, watch more experienced cyclists (who may or may not be stronger) in given situations and talk to them about why they are choosing a particular gear. Ignore the Smart-Alec answers but also keep in mind that all riders are different. What works for them, might not work for you.

And of course, if you want to read more there are oodles of websites out there. A couple to start with would be http://www.kenkifer.com/bikepages/touring/gears.htm and http://www.sheldonbrown.com/gearing/index.html