Bicycle gearing
A cyclist's legs produce power optimally within a narrow pedalling speed range, or cadence.
As in other types of transmissions, the gear ratio is closely related to the mechanical advantage of the drivetrain of the bicycle.
On single-speed bicycles and multi-speed bicycles using derailleur gears, the gear ratio depends on the ratio of the number of teeth on the crankset to the number of teeth on the rear sprocket (cogset).
For a bicycle to travel at the same speed, using a lower gear (larger mechanical advantage) requires the rider to pedal at a faster cadence, but with less force.
Gear inches and metres of development also take the size of the rear wheel into account.
Some cyclists choose to fine-tune the gearing to better suit their strength, level of fitness, and expected use.
When buying from specialist cycle shops, it may be less expensive to get the gears altered before delivery rather than at some later date.
While long steep hills and/or heavy loads may indicate a need for lower gearing, this can result in a very low speed.
The epicyclic gears used within hub gears have more scope for varying the number of teeth than do derailleur sprockets, so it may be possible to get much closer to the ideal of consistent relative differences, e.g. the Rohloff Speedhub offers 14 speeds with an average relative difference of 13.6% and individual variations of around 0.1%.
3-speed hub gears may have a relative difference of some 33% to 37%;[5] such big steps require a very substantial change in pedalling speed and often feel excessive.
The number of gears for such a derailleur equipped bike is often stated simplistically, particularly in advertising, and this may be misleading.
The outer ranges only have 7 ratios rather than 8 because the extreme combinations (largest chainring to largest rear sprocket, smallest chainring to smallest rear sprocket) result in a very diagonal chain alignment which is inefficient and causes excessive chain wear.
There are a few other relatively uncommon types of gear change mechanism which are briefly mentioned near the end of this section.
Most hybrid, touring, mountain, and racing bicycles are equipped with both front and rear derailleurs.
This arrangement provides much more scope for adjusting the gear ratio to maintain a constant pedalling speed, but any change of chainring must be accompanied by a simultaneous change of 3 or 4 sprockets on the cogset if the goal is to switch to the next higher or lower gear ratio.
In the 1960s the term was used by salespeople to refer to then current 10-speed bicycles (2 chainrings, 5-sprocket cogset), without any regard to its original meaning.
The nearest current equivalent to the original meaning can be found in the Shimano Megarange cogsets, where most of the sprockets have roughly a 15% relative difference, except for the largest sprocket which has roughly a 30% difference; this provides a much lower gear than normal at the cost of a large gearing jump.
[12] This general arrangement is suitable for touring with most gear changes being made using the rear derailleur and occasional fine tuning using the two large chainrings.
[11] The small chainring (granny gear) is a bailout for handling steeper hills, but it requires some anticipation in order to use it effectively.
Changing gears is accomplished by using your foot to tap a button protruding on each side of the bottom bracket spindle.
Pinion GmbH introduced in 2010 an 18 speed gearbox model, offering an evenly spaced 636% range.
The Pinion system is well suited for mountain bicycles due to its wide range and low gravity center suitable for full-suspension bikes, but it is still somewhat heavier than derailleur-based drivetrain.
An encyclopedic overview can be found in Chapter 9 of "Bicycling Science"[20] which covers both theory and experimental results.
Factors which have been shown to affect the drive-train efficiency include the type of transmission system (chain, shaft, belt), the type of gearing system (fixed, derailleur, hub, infinitely variable), the size of the sprockets used, the magnitude of the input power, the pedalling speed, and how rusty the chain is.
Some experiments have used an electric motor to drive the shaft to which the pedals are attached, while others have used averages of a number of actual cyclists.
There is little independent information available relating to the efficiency of belt drives and infinitely variable gear systems; even the manufacturers/suppliers appear reluctant to provide any numbers.
[21] Derailleur efficiency is also compromised with cross-chaining, or running large-ring to large-sprocket or small-ring to small-sprocket.
200 W will drive a typical bicycle at 20 miles per hour (32 km/h), while athletes can achieve 400 W, at which point efficiencies "approaching 98%" are claimed.
