How to choose your shifter wisely
Indexed shifters vs. friction shifters
The vast majority of modern shifters (for road and MTB) are indexed, meaning that each ‘click’ of the gear shifter moves a precise length of gear cable, equivalent to one gear shift (assuming the gears are correctly adjusted). Older models used friction shifters, where the rider had to learn exactly how much to move the shift lever in order to change gear.
Road shifters vs. MTB shifters
There are a number of different types of shifter, depending on manufacturer and transmission system, but the biggest difference is between road and MTB shifters, or more accurately between shifters designed to be mounted on bikes with drop handlebars (‘racing’ bikes), and those designed for bikes with ‘normal’ handlebars. The former use a combined brake- and gear-shifter unit, part of which (the ‘hood’) is also intended as a hand rest while cycling while the latter are positioned under the handlebars just inside the grips, where they can be activated with thumb and forefinger.
NOTE: A properly-indexed gear system is a pleasure to use but requires regular maintenance and adjustment. One of the primary reasons for degradation in shifting performance is cable quality. Cheap cables will quickly stretch and/or corrode, affecting the adjustment of the gearing, while cable housings can also become clogged with mud and water. Use only good-quality cables, lubricate frequently and if you are regularly riding in poor conditions change your cables every once in a while or consider a set of fully sealed cables to keep out the elements.
When choosing a new set of road or MTB shifters (for upgrade or replacement) be sure to match them with your existing transmission – an 11-speed rear setup will need an 11-speed right shifter, etc. It is advisable not to try to mix and match transmission parts from different manufacturers as they may not be compatible. Different makers use varying standards in their systems (e.g. the amount of cable actuation, or the length of cable pulled per individual shift, varies according to manufacturer) so replace like with like.