Gears without Tears

Having lots of gears is very fashionable, but they add weight and complexity. In this extract from his book MIKE BURROWS asks: do we really need them?

Certainly if a vehicle's power source works well under all normal combinations of load and speed (like steam engines) there is no need for variable gears. But if the engine works best over a relatively restricted output range (like petrol engines and most cyclists) gears can be a great boon.

Back in 1901 Henry Sturmey, a great champion of the bicycle but also founding editor of Autocar magazine, put it this way:

“... the human being is not unlike the internal combustion motor. It has been found absolutely indispensable for the latter that variable speed gears should be provided, simply because the engine cannot develop more than a given amount of power, and if its speed is reduced unduly owing to the heaviness of the drive, its power declines so rapidly that it cannot take the ascent, and although the human motor possesses far more elasticity than the petrol engine, its limits are very soon reached, and a variable speed gear is the correct thing for the rider who desires to combine the maximum of distance and speed with the minimum of exertion.”

We therefore need gears to help match our limited power output to widely varying load conditions. Then we can cope better with rough surfaces and smooth, ascents and descents, headwinds and following winds, load-carrying and travelling light, racing or just ambling along.

Primary Drive

Before looking at variable gearing itself, we need to take a quick look at the primary drive train. In other words, how the rider's power gets delivered to the back wheel.

Most early bicycles had the pedal cranks connected directly to the front wheel. The ultimate form of the front driver was the graceful and very efficient Ordinary or High Bicycle, often known by its derogatory Victorian slang name “Penny Farthing”.

Step-up Drive - and How We Measure It

The development of the bicycle chain enabled use of smaller driving wheels. Drive was fed to the rear wheel from cranks mounted on the frame. By using a relatively large chainwheel on the crank shaft and a much smaller one on the rear hub, the drive was “stepped up”. The rider could therefore travel much further for each turn of the pedals.

The step-up gear was expressed in terms cyclists of the time could understand - the equivalent Ordinary front wheel diameter. So, a 48 tooth chainwheel driving a 24 tooth rear sprocket on a 28” wheel gave a gear of 48/24 x 28 = 56”. This system is still used in the English-speaking world.

An alternative system originated in France, le dévelopment. This is the distance in metres the bicycle travels for one turn of the pedals. You can convert the British figure to the metric pretty accurately by multiplying by 0.08. Conversely a development figure can be converted to the British system by multiplying by 12.5.

The Virtues of Single-Speed

Let's look briefly at single-speed chain transmission.

Why? Because its light, cheap, reliable and as efficient as it possibly could be. It's also used by the majority of cyclists worldwide - people in third world and developing countries who use their bikes day in, day out as primary transport and for load carrying. It's cost-effective and it works.

Single-speed transmissions can be fixed-wheel or free-wheel. We are all used to being able to stop pedalling and freewheel along. Virtually all bikes today let you do this. This is because of a major breakthrough in Victorian cycle technology that we all take for granted, the automatic free-wheel.

But with single-speed chain drive you can, if you wish turn the clock back 100 years and have a fixed-wheel. In this case the rear sprocket is rigidly screwed to the hub, so that the pedals must revolve whenever the wheels do. Some experienced cyclists prefer this, because it gives them more control, particularly on winter roads. It also enables them legally to dispense with a rear brake (provided the sprocket has a locking ring fitted), as they can slow the rear wheel by back-force on the pedals.

Single-speed fixed-wheel is the standard for most track racing. It can also be a great boon for some disabled riders. Special fixed-wheel conversions of hub gears and derailleurs have sometimes been made for such riders.

Cyclists who are torn between fixed and free-wheel can buy a Fixed-free Drive, which enables a choice at the click of a trigger.

Another once popular option is a double-sided or reversible hub. This has a different sized sprocket on each side. To change from one to the other, the rider has to stop, take out the wheel, turn it over and reinstall it. The sprockets can be fixed or free-wheel, or one of each. At the expense of oily hands and some inconvenience they give the rider a choice of two speeds or drive modes while adding little weight and no efficiency losses. But does anyone today make such hubs?

Apart from track racing use, single-speed transmission today is almost always combined with a free-wheel. Standard uses on production bikes are for very young children's machines, basic roadsters, and lightweight compact folders such as the Cresswell Micro Lite. But occasionally individualists go against current fashion for multiple gears and retro-fit single speed. There are even a few mountain bikers who have decided that a low-geared single-speed (say 52”) suits them better than a 21-speed derailleur.

Variable Gears

Despite the merits of single-speed transmission, for most purposes most of us prefer a choice. But how many speeds do we need? How much difference should there be between each speed? How high should the highest be, and how low the lowest?

How Many Speeds?

As early as 1902, when most variable gears offered a choice of only two speeds, it was technically possible to create a 42-speed transmission. Today it is entirely feasible to have an 84-speed bike without using any specially made components. Even cheap off-the-peg bikes commonly have 21-speeds. Yet some of the greatest rides have been done by cyclists using between one and four speeds. Is it a case of more speeds the better or what?

The first thing to note is that when we speak of 21 speeds we are not talking about 21 distinct choices useable in sequence. We'll look at this point in more detail when we discuss derailleurs below. What is more important than the number of gears is the difference between each and the overall range.

How Widely Spaced?

Common sense suggests that evenly spacing the gears is a good idea. Hence it might be thought that a spacing of 10” difference is good for touring or mountain biking. This could give a spread of 25, 35, 45, 55, 65, 75, 85 and 95”. However, this ignores the fact that humans tend to perceive differences as a proportion of what they last experienced. Therefore it is better to work on a percentage increase for each gear shift. We would do better to suggest a maximum 20% increase, which would give a progression such as 25, 30, 36, 43, 52, 62, 75 and 90”. Whilst the overall spread is almost the same as with the 10” rule of thumb, the steps would feel much more evenly spaced - despite the spacing being 5” at one end of the range and 15” at the other.

For racing purposes much closer spacing is required to allow subtle fine-tuning of the rider's output within a narrower speed range. Percentage increases in the order of 7 to 10% would be appropriate.

Utility riders, for whom consistency of speed is less important than the overall gearing range, typically cope quite easily with increases in the order of 33% - as provided by a Sturmey-Archer three-speed.

Incidentally a 33% increase gives a 25% decrease when changing down again. Likewise a 20% increase gives a 17% decrease, and a 10% increase gives a 9% decrease. In other words, the closer the spacing, the less proportionate difference between upshifts and downshifts.

How Low And How High?

As for the range, it's horses for courses. As a starting point, single-speed utility bikes tend to have a gear around 64”. British three-speed utilities traditionally offer something like 48, 64 and 85”. However, this is really too high for many riders. It would be better to drop the range to something more like 41, 55, 74”. This is easily done by fitting a larger rear sprocket.

Tourers benefit from a wide range, but again, it's better to get the low gears right. You can choose to freewheel down the hills but you always have to pedal up them - or get off and push. A bottom gear of about 20” is desirable for loaded touring in hilly terrain, whereas the top would typically be around 90”. It's worth noting, however, that when Colin Martin successsfully cycled from the UK to Australia in 1970 his highest gear was only 68”. He found it quite adequate.

Road racers will typically be more concerned about the high gears than the low. Top might be something like 115” and the bottom anything from 40” upwards.