Highpath Engineering : Cycle transmission notes : 10

Cycle transmission notes page 10

Choosing your gears

Large chainrings and small rear sprockets result in high gears for going fast. Small chainrings and large rear sprockets result in low gears for climbing hills.

If you are going to alter your gearing, it helps to be able to see the consequences of different combinations and to compare them with other cycles. Display the gear chart for your cycle and use it to choose chainrings and sprockets as follows:

Derailleur gears	Choose chainrings and sprockets that give your preferred top and bottom gears and a sensible spread inbetween.
Derailleur gears	Choose similar percentage jumps between your rear sprockets, not equal numbers of teeth. (For example, a shift of one tooth from 12t to 13t has the same effect as two teeth from 24t to 26t - they both lower the gear by 8%.)
Hub gears	Choose a chainring and sprocket that give your preferred middle gear. Multiply this by the high and low ratios for your hub in order to find your top and bottom gears.

Different riders like different gears, but you should bear in mind the following points:

Gear ratios under 0.8 are too low for bicycles as it's difficult to balance at such slow speeds. They can, of course, be used on tricycles and quadricycles.
Gear ratios over 7.5 are too high for general use on bicycles (7.5 has been used to break the World hour record!).
Many cyclists like their 'normal' gear to have a ratio of about 5.0, and an overall range from 2.5 to 7.0.
Never try to achieve a higher gear by using longer cranks as they could damage your knees.

Gear ratios - how to compare different gears

A gear ratio specifies how hard it is to push a particular gear. A ratio of 5 means that your cycle moves 5 metres along the ground for every metre you move the pedals, or 5 inches for every inch, or 5km for every km. Smaller ratios are easier to push and higher ones make you go faster.

Some people refer to this ratio as the 'Gain' and others call it the 'Velocity Ratio'. Other ways are also used to describe cycle gears, such as 'Gear inches' and 'Développment', but these ignore the effect of different crank lengths and are not so meaningful.

The easiest way to understand gear ratios is to print off a gear chart for your particular cycle (click here). Use it to compare different chainring/sprocket combinations and to select those appropriate to your kind of cycling.

How to calculate gear ratios

A gear ratio, or gain, is calculated as follows:

      gear ratio   =   wheel radius  ×    no. of chainring teeth  
                       crank length     no. of rear sprocket teeth

Use mm or inches but don't mix them! Alternatively you can calculate your wheel radius from its ISO tyre size (eg. 28-622) as follows:

      wheel radius (mm)   =   xx  +  yyy       for a tyre marked 'xx-yyy'
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