Bicycle Gears Explained
  -->Metres Development – metres travelled per pedal revolution.
-->Pedalling Cadence – pedalling r/min x gear selected = speed.
-->Tyre Circumference – an easy way to measure it.
-->Derailleur Gears – simple formula for calculating metres development.
-->Hub Gears – not-so-simple formula for calculating metres development.
-->Gain Ratio – distance bike travels : distance pedals moves.
-->Gear Inches – archaic system for penny-farthing high wheelers.
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How bicycle gears are measured…

Bicycle gear calculations are based on the distance, in metres, that a bicycle travels for each turn of the pedals. This distance is commonly called 'metres development' – a rather odd name for a logical and useful way of measuring gears.

Low gears will move your bike a very short distance for each turn of the pedals – just what you need for tackling steep inclines at slow speed.

High gears will move you three or even four times as far – assistance from gravity or a tail wind will probably be needed!

For practical purposes, the extremes are:

  • 2 metres : lowest gear
  • 9 metres : highest gear

Unless you are riding a folding bike (I do!), the circumference of your wheels will be just over two metres. Later, I'll help you measure tyre circumference – accurately and without tears – but for now we'll assume that our bicycles roll forward 2.1 metres for each wheel rotation.

Gears on a 27-speed hybrid…

The hire bikes that Jen and I rode down the Danube had triple chainrings (50—40—30) and nine-sprocket cassettes (12—30).

In our granny gears with smallest chainring (30) and largest sprocket (also 30) our wheels turned at the same speed as the cranks – and the bikes moved forward 2.1m (the tyres' circumference) for each turn of the pedals. Our granny gear was 2.1 metres. We didn't need anything that low on the towpath to Vienna!

Most of the time, we selected the middle chainring (40) and a middle sprocket (20) so that the wheels made two rotations for each turn of the pedals. 4.2 metres (twice the tyres' circumference).

Metres Development Formula

tyre circumference x chainring / cassette = metres development

Granny Gear: 30-tooth chainring and 30-tooth rear sprocket:

  • 2100mm x 30 / 30 = 2.1 metres
  • diameter x chainring / rear sprocket = distance travelled

Midrange: 40-tooth chainring and 20-tooth rear sprocket:

  • 2100mm x 40 / 20 = 4.2 metres
  • diameter x chainring / rear sprocket = distrance travelled

Top Gear: 50-tooth chainring and 12-tooth rear sprocket:

  • 2100mm x 50 / 12 = 8.75 metres
  • diameter x chainring / rear sprocket = distrance travelled

Gears & Speed…

A casual cyclist will probably turn the pedals around at about 60 r/min – once per second. The distances shown above are, therefore, how far you travel in one second.

More about gears and speed in the section on pedalling cadence.

2013-03-26 19:29

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