How does the gear shifting system of a mountain bike work and how do you choose the right gear ratio?

The gear shifting system of a mountain bike adjusts the riding speed by changing the position of the chain across different gear sets.

Working Principle of the Gear Shifting System
Components:
Front Derailleur and Rear Derailleur: Installed on the front and rear wheels, respectively, these are connected to the chain and gears through components such as guide pulleys, bases, and limit screws. The front derailleur moves the chain from one front chainring to another, while the rear derailleur shifts the chain between different rear cogs.

Gear Set: Comprised of the chainrings and the freewheel, the chainrings are mounted on the front and rear wheels driven by the chain, and the freewheel is fixed on the rear fork or frame. The chain moves between the front and rear cogs, allowing for gear changes.

Gear Control: Typically located on the handlebars of the mountain bike, the shifters control the derailleurs. The left shifter controls the front derailleur, and the right shifter controls the rear derailleur. Each shifter typically has multiple gear levels (e.g., low, medium, high). By combining different gear levels of both shifters, riders can switch between various gear combinations on the front and rear wheels.

Shifting Process:
When the rider rotates the shifters on the handlebars, the shifters send signals to control the movement of the front and rear derailleurs. The guide pulleys on the derailleurs, under the guidance of the pulley mount, move the chain from one gear to another. As the chain moves between different cogs, the gear combinations on both the front and rear wheels change, thereby adjusting the riding speed.

Choosing the Gear Ratios
Gear Count and Tooth Ratios: The number of gear ratios on a mountain bike refers to the number of available shift combinations, based on the number of teeth on the freewheel and chainrings. For example, with three chainrings in the front and eight cogs on the rear freewheel, the possible gear ratios would be 24-speed.
The gear counts available include 5, 6, 7, 8, 9, 10, 11, 18 (2×9 and 3×6), 20, 21, 22, 24, 27, 30, and 33-speed options.

Guidelines for Choosing Gear Ratios:
Riding Needs: For light mountain biking and urban cycling, an 18-speed or 21-speed gear system is suitable. For more technical mountain biking, including steep ascents and technical descents, a 24-speed or 27-speed system is recommended. Professional mountain bikers or competitive riders may prefer a 30-speed bike.

Terrain Conditions: On flat roads, higher gears should be selected for faster movement. On climbs, lower gears should be used to reduce the difficulty of pedaling. On descents, larger gears should be chosen to take advantage of speed.

Personal Ability and Preferences: Each rider's abilities and habits may vary, so the gear ratios can be adjusted based on personal preferences and riding style.