Cycle information : 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.

Gear charts

Before you alter your gearing, it helps to be able to see the consequences of different combinations and to compare them with other cycles. To display the gear chart for your cycle and use it to choose your chainrings and sprockets...

	1.	Either measure the diameter of your rear wheel in millimetres, or estimate it from the ISO size of your rear tyre: wheel diameter = bbb + 2 × aa for a tyre marked 'aa-bbb' ( example for a tyre marked '28-622' : wheel diameter = 622 + 2 × 28 = 622 + 56 = 678mm )
	2.	Click on the nearest value that follows: ( 680 in the above example )


Wheel diameter (mm):
	400 410 420 430 440 450 460 470 480 490	600 610 620 630 640 650 660 670 680 690
	500 510 520 530 540 550 560 570 580 590	700 710 720 730 740 750 760 770 780 790

How to use the charts

The number of Gear inches shown for each chainring/sprocket combination indicates how easy or hard it will be to push that particular gear. Small values are easy to push and high values help you go faster. Use the chart to compare different combinations and to select those appropriate for your kind of cycling.

For derailleur gears:

Choose chainrings and sprockets that give your preferred top and bottom gears and a sensible spread inbetween.
Choose similar percentage jumps between your rear sprockets, not equal numbers of teeth. (For example, a shift of one tooth from 12t to 13t feels the same as two teeth from 24t to 26t - they both lower the gear by 8%.)

For hub gears:

Choose a chainring and sprocket that give your preferred middle gear. Multiply this by the high and low ratios for your particular hub in order to find your top and bottom gears.

Recommendations:

Many cyclists like their 'normal' gear to be around 65 inches, with an overall range from 30 to 95 inches.
Gears under 11 inches are too low for bicycles as it's difficult to balance at such slow speeds. But they can be used on tricycles and quadricycles, where balancing isn't a problem.
Gears over 100 inches are too high for general use on bicycles.
Choose the correct length cranks and never try to achieve a higher gear by using longer ones as this could damage your knees.

Technical

You may come across other ways of specifying cycle gearing:

Gear inches are normally used in english-speaking countries and are used in these charts. It has its origins in the 1890's and specifies the diameter, in inches, of the front wheel of an equivalent penny-farthing bicycle!

   gear inches  =  wheel diameter (ins)  ×    no. of chainring teeth  
                                            no. of rear sprocket teeth

Développment is used in french-speaking countries. It's the distance travelled on the ground, in metres, for one revolution of the cranks.

   développment  =  wheel diameter (m)  ×  π  ×    no. of chainring teeth  
                                                 no. of rear sprocket teeth

Velocity Ratio or Gain is not in common use, but it's more scientific than either of the above methods as it takes into account the important effect of crank length. It's a true ratio, so you can use any units as long as you don't mix them - a gain 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.

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

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Chris Bell, HIGHPATH ENGINEERING
Cornant, Cribyn, Lampeter, Ceredigion, SA48 7QW, Wales, UK