The number of gears available on bikes continues to rise. A quarter-century ago twelve speeds was typical for a racing bike; now it’s twenty or thirty. The main reason behind this evolution is to help riders maintain the best cadence, but the key factor underlying the development is the prevalence of derailleur gears.
In the first place, only the derailleur system makes such a multiplicity of gears possible at all. A 30-speed hub-gear or gearbox would be nightmarishly complex and heavy.
Secondly, and more regrettably, derailleur gears create a degree of redundancy. More simply, having thirty gears on a bike doesn’t really mean that you have thirty usable gears.
To get 30 gears requires triple chainrings at the front and a 10-speed ‘block’ at the rear. However it is inadvisable to use, for instance, the smallest chainring and the smallest rear sprocket together as this leaves the chain running at a distinct angle between them, which radically increases wear and tear on all parts of the system.
In fact it is typically recommended to use only the seven or eight largest sprockets when using the small (inner) chainring, the middle eight sprockets when using the middle chainring, and the seven or eight smallest sprockets when using the ‘big ring’. This brings the number of fully usable gears down to 22 or 24.
Within these available gears there is also some inevitable duplication. For instance, a 52-tooth chainring used with a 26-tooth sprocket gives exactly the same (2:1) ratio as a 42 ring and 21 sprocket. Even if not duplicated exactly some ratios may be so close as to be indistinguishable.
Twin chainrings at the front create less severe chain angles than triple, but it is still not advisable to use the extreme ‘crossover’ ratios. There is still likely to be some duplication, though less so than with a triple.
This all means that the advantages of triple chainrings may not be quite as marked as you might think; you certainly don’t get a fifty percent increase in the number of usable gears, probably more like thirty percent. Triples do still have real advantages; they extend the range at the lower end, which is useful for climbing steep hills and/or toting heavy loads.
Also, the duplication of gears isn’t entirely a bad thing. For ordinary riding on flat terrain riders can mostly use the middle chainring, merely changing gear with the rear derailleur; with twin chainrings there’s often more need to switch between the two to get the right ratio.