Making sense of gear ratios: Some mysterious numbers, and their uses, explained

With the ‘Ordinary Bicycle’, now more commonly known as the ‘penny-farthing’ or ‘high-wheeler’, one turn of the pedals equals one revolution of the driving wheel. The bigger the wheel, the further you travel. A bigger wheel was therefore equivalent to a bigger gear on a modern bike.

The size of the wheel gave a simple basis for comparison. The distance travelled is equal to the circumference of the wheel but, understandably enough, bicycles came to be described in terms of wheel diameter instead. The largest wheel size in regular use was about 60 inches.

As bikes driven by crank and chain developed, wheel size alone no longer allowed direct comparison. However, riders naturally still wanted to know how far they would travel for each turn of the pedals. In Britain and North America it became customary to describe bicycles in terms of their penny-farthing equivalent.

If you are not sure of BMX riding, You Can Check Out This Resource Here.

Let’s take a simple illustration. A 60-inch ordinary will travel approximately 188 inches for each turn of the pedals. Now consider a chain-driven bicycle with wheels of 30-inch diameter (this is not a modern standard size but makes the maths easier). The circumference of this wheel is 94 inches. If one turn of the pedals makes the wheels rotate once, it will travel 94 inches.

However, having a chain drive allows different drive ratios to be used. A 2:1 ratio means this bicycle will travel 188” per pedal-revolution – exactly the same as a 60-inch Ordinary. A 3:1 ratio moves the bike 282” per pedal-turn. This is the equivalent of a 90-inch Ordinary wheel.

Of course a 90” Ordinary would only be rideable by someone wearing stilts, but the system of describing gear ratios as the equivalent of Ordinary wheel sizes persists to this today. Cyclists, particularly in the English-speaking world, still refer to a 90-inch or 100-inch gear.

An alternative system was and is more favoured in Europe, using the actual distance travelled for each pedal-revolution. The French tern for this is développement. On an Ordinary bicycle this would be equal to the circumference of the wheel. Naturally this is usually expressed in metres and centimetres rather than inches.

The European system is arguably more logical, but either system, used consistently, allows comparison between different bikes even if they have different wheel sizes. More significantly, for many of us, it allows us to untangle the relationship between different gears on the same bike.

These systems allow us, for instance, to compare an Ordinary bicycle with a modern one. A 60-inch wheel was big for a ‘penny-farthing’ but 60 inches is in fact a low-to-middling gear ratio on a utility or touring bike.

Road bikes will have top gears well over 100 inches. Most of us might use this only on a fast downhill stretch but pro riders will use these ratios regularly when launching an attack, in a finishing sprint or in time-trials.

At the other end of the scale, really low gears, 30 inches and below, are useful for very steep hills and are commonly found on mountain bikes and touring bikes expected to carry heavy loads.

Thirty gears in Bike – or not: Making sense of multiplying ratios

The number of gears available on bikes continues to rise. A quarter-century ago twelve speeds was typical for a racing bike; now it’s twenty or thirty. The main reason behind this evolution is to help riders maintain the best cadence, but the key factor underlying the development is the prevalence of derailleur gears.

In the first place, only the derailleur system makes such a multiplicity of gears possible at all. A 30-speed hub-gear or gearbox would be nightmarishly complex and heavy.

Secondly, and more regrettably, derailleur gears create a degree of redundancy. More simply, having thirty gears on a bike doesn’t really mean that you have thirty usable gears.

To get 30 gears requires triple chainrings at the front and a 10-speed ‘block’ at the rear. However it is inadvisable to use, for instance, the smallest chainring and the smallest rear sprocket together as this leaves the chain running at a distinct angle between them, which radically increases wear and tear on all parts of the system.

In fact it is typically recommended to use only the seven or eight largest sprockets when using the small (inner) chainring, the middle eight sprockets when using the middle chainring, and the seven or eight smallest sprockets when using the ‘big ring’. This brings the number of fully usable gears down to 22 or 24.

Twin chainrings on bike
Twin chainrings

Within these available gears there is also some inevitable duplication. For instance, a 52-tooth chainring used with a 26-tooth sprocket gives exactly the same (2:1) ratio as a 42 ring and 21 sprocket. Even if not duplicated exactly some ratios may be so close as to be indistinguishable.

Twin chainrings at the front create less severe chain angles than triple, but it is still not advisable to use the extreme ‘crossover’ ratios. There is still likely to be some duplication, though less so than with a triple.

This all means that the advantages of triple chainrings may not be quite as marked as you might think; you certainly don’t get a fifty percent increase in the number of usable gears, probably more like thirty percent. Triples do still have real advantages; they extend the range at the lower end, which is useful for climbing steep hills and/or toting heavy loads.

Also, the duplication of gears isn’t entirely a bad thing. For ordinary riding on flat terrain riders can mostly use the middle chainring, merely changing gear with the rear derailleur; with twin chainrings there’s often more need to switch between the two to get the right ratio.