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Bicycling basics
So how many speeds? The full range of any gearing system is actually achieved by having a number of set speeds: particular ratios spread (ideally) evenly across the range, between which you can shift as your bike speed changes. You’ll generally have little choice if using commercial systems, but jumps of around 15% or so are usually considered sufficiently finely spaced. Any jump over 25% or so feels a bit ‘wide’, jarring to the legs which suddenly have to make a big jump in pedalling speed. Almost all of the commercial systems stick within these limits.
How gears are measured Gear ranges are sometimes mentioned in bike reviews, and can usually be worked out from the manufacturer’s specification if not. It’s worth knowing what the figures mean. A touring bike, for example, might have a range of 21" to 104". Those measurements are in ‘gear inches’, most easily visualised as the diameter of penny-farthing wheel which would go the same distance per turn of the pedals. But most cyclists just go by experience: anything below 30 is a low touring gear, and anything over 100 is a tailwinds-only overdrive. Practical cyclists will choose a range to match their riding. If you commute over flat terrain, 40-90" will normally be plenty.Tow heavy trailers, or add some hills, and you’d prefer a low of 30" or less.The same applies to carrying children in child seats or pulling child trailers – the extra weight means you’ll need the lower gears. At this point I could go into the maths of it all, but I don’t really have space and I’d rather write about something more interesting. If you’re keen to learn more look up bicycle gear ratios on the internet, for example at Sheldon Brown’s website: www.sheldonbrown.com.