The first quantity listed for each model is the overall range in percentage terms. If the highest gear, for example, is three times the lowest: that's a range of 300%. Such a gearbox could provide a range of gears from 25 to 75 inches, or from 33 to 100, or any other 3:1 range that's altogether higher, lower, or in-between. It's just a matter of choosing a number of teeth for chainwheel and sprocket that'll set this range at your desired level. I’ll describe exactly how to do that later.
Next comes a string of numbers in columns headed 1st, 2nd etc. If you put 1: in front of each of these numbers, those are the ratios of each internal gear. In most cases one of those numbers will be 1, indicating a 1:1, direct-drive gear, sometimes called normal. In that gear, which is usually somewhere near the middle of the range, the sprocket drives the hub directly, without the intervention of any internal gears, so it's also the most efficient gear to ride in. For some hubs you'll see 1.00 instead, which means the internal gears are involved, stepping down through one set of gears and up through another to give the same result: the hub turns at (near enough) the same speed as the sprocket and the size of that gear, in inches (or metres development), is given by the usual formula or by consulting a gear chart . Whether or not your hub has such a gear: first calculate direct-drive from the size of wheel and numbers of teeth on chainwheel and sprocket, or look it up a gear table. All of the gears can then be calculated from that gear, by multiplying with the numbers in the table below.
Hub-Gear Ratio Table
|Make & Model||Range||1st||2nd||3rd||4th||5th||6th||7th||8th||9th||10th||11th||12th||13th||14th|
|(Obsolete)||%||Ratio 1: …|
|Sturmey-Archer AW & Old Shimano 3-speed||178%||0.75||1||1.33|
|Sram/Sachs 3-speed & Shimano Inter-3||186%||0.73||1||1.36|
|Sturmey 5-speed Wide||256%||0.63||0.75||1||1.33||1.60|
|Sram Spectro S7||303%||0.57||0.67||0.80||1||1.24||1.48||1.74|
|Sram i-Motion 9||340%||0.54||0.62||0.73||0.85||1||1.17||1.38||1.61||1.84|
What if the bottom gear isn't low enough or top not high enough for your requirements? First you've got to decide which gear is most important to you. Divide the desired size of that gear by its ratio (the number in the table above) and the result is the size of direct-drive gear you'll need in order to get it. For example let's assume it's vital to have a bottom gear in the mid 20s (inches) to haul a week's groceries up the hill you live on top of and that you've got a Sram 7-speed hub. The ratio of 1st gear on that hub is 0.57, and 25 ÷ 0.57 = 44, so you want a 44in direct-drive. Top (7th) will be only 44 × 1.74 = 76in, but that may be high enough if you're content to freewheel down the hill! It's easiest to use a gear chart to choose a number of teeth for chainwheel and sprocket that'll provide your required direct-drive gear, but you can also do it by transposing the usual formula as follows: Sprocket = Chainwheel × Wheel ÷ Gear. Let's assume in the previous example that your bike has 26in wheels and a 38T chainwheel: in that case a sprocket with: 38 × 26 ÷ 44 = 22T is required.
Bottom-Bracket Gear Ratio Table
This table is used in a similar manner to the hub-gear ratio table.
|Make & Model||Range||1st||2nd|
|Schlumpf:||%||Ratio 1: …|