Metres Development

Bicycle gear calculations are based on the distance, in metres, that a bicycle travels for each turn of its 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 way up the road for each turn of the pedals – just what you need for tackling steeper 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

Summary: These are the distances that your bike will move forward for one revolution of the pedals.

Pedalling Cadence + Gear Chosen = Speed

A typical pedalling cadence for a competent older rider is around 75 RPM (the middle column below). We'll excuse ourselves for dropping to 50 RPM on steep hills. Downhill racers might be tempted to whizz the pedals around at 125 RPM for a few seconds (that's all they'll manage!) to lift the top speed to something truly scary.

When considering gears, remember: in the big hills, you'll need the low gears for 30 minutes at a stretch; you'll never genuinely need the high gears.

An aside about the mid-range gears: 18km/h appears in each column. If this is the speed you are comfortable cycling at, you should maintain that speed by 'spinning' the pedals at 75 RPM, not straining along at 50 RPM. Avoid pedalling too slowly (aim for 85 RPM but be satisfied with 75).

Perhaps, this is the time to suggest that you equip your bike with a computer that displays cadence (pedalling speed). Watch the cadence readout and keep it high by changing down a gear or two.

An Easier Way to Measure Circumference

The most obvious way of measuring the circumference of your wheel is by rolling it across the floor. Stick of strip of masking tape across the tyre so you'll know exactly when one revolution has been made. The slightest wobble off a straight line will result in a wrong measurement so best use a tiled floor (pencil marks come off more easily, too).

There is a far simpler way: measure the diameter of the wheel in the same way that you measure a person's height. Click photo for enlargement.

Multiply the wheel's diameter by pi to find its circumference – just use your calculator.

• 681mm (tyre diameter) x pi = 2140mm (circumference)

A Simple Gear Formula

tyre circumference x chainring / cassette = metres development

Example 1: A road bike with 'compact' 50—34 chainset (front) and 12—28 cassette (wheel):

• 2140mm x 34 / 28 = 2.6 metres : lowest gear
• 2140mm x 50 / 12 = 8.9 metres : highest gear
  
• circumference x chainring / rear sprocket = distance travelled per turn of pedals

Example 2: A race bike with 'standard' 53—39 chainset and 11—25 cassette:

• 2140mm x 39 / 25 = 2.9 metres : lowest gear
• 2140mm x 53 / 11 = 10.3 metres : highest gear
• circumference x chainring / rear sprocket = distance travelled per turn of pedals

This is way too high for all but the strongest. Forget it!

Example 3: A 'fixie' with 50-tooth chainset and 25-tooth rear sprocket:

• 2140mm x 50 / 25 = 4.3 metres : only gear
• circumference x chainring / rear sprocket = distance travelled per turn of pedals

Drawing by Wikipedia contributor Keith Onearth.