Just like in the cars and two wheelers, bicycle gears helps the rider to optimise his efforts for speed and power. This helps to achieve the goal with least amount of efforts – allowing him to go stronger, faster and higher.
What are gears?
Generally in a fixed speed (non-geared) cycles, one rotation of pedals produces about 1.5 rotation of the rear wheel. This remains a fixed ratio. The disadvantage of a fixed speed bicycle is that getting to higher speeds or climbing hills becomes more energy intensive. Gears help to change the ratio of pedal rotation to rear wheel rotation.
Structure of gears
Because the chain remains the same – the teeth geometry in the front wheel and the rear wheel remains the same. However in the front usually there are two and in mountain bikes – three rings to vary the number of teeth. Similarly in the rear wheel the sprocket (the set of small rings with teeth over which the chain grips) there are between 5 and 10 rings with varying number of teeth. Shifting the chain with various combination of the front chain ring and rear sprocket gives different gear ratios.
How gears work
As in vehicles, lower gear (1st gear) gives you maximum power but minimum speed and vice versa the highest gear (4th or 5th gear) gives you minimum power but maximum speed. You always start your vehicle in the 1st gear – because a stationary vehicle requires maximum power. As the vehicle gathers speed, you change gears sequentially to higher gear to achieve higher speed at low engine revolution.
Similarly in bicycling, the lower gear propels the bike forward with minimum power but the speed of pedalling is higher and the bike speed is lower. Once the bike gets into motion, you keep changing the gear to progressively higher gear. This leads to lower pedalling speed but pedalling becomes slightly harder and the speed of the bike increases.
The mechanism of gear shifting, various technologies involved, gear adjustment and technique of gear changes will be covered in future.
Simple understanding -
Suppose the chain is on the biggest cog of the sprocket in the rear wheel which has 25 teeth. At the same time the chain is in the largest chain ring in the front, which has 50 teeth. This gives ratio of teeth of 2:1 (50 teeth to 25 teeth) – i.e, for every one revolution of the front chain ring (one revolution of the pedal) the rear wheel will rotate twice. Or in other words the work required to rotate the rear wheel once is about half the pedal.
Now if the gear is changed so that the chain engages the smallest cog at the back (10 teeth), the gear ratio changes to 10 teeth to 50 teeth, or 5:1. Now for each revolution of the front chain ring (one revolution of pedal), the rear wheel rotates five times. Initially one rotation of pedal did work of two rotation of rear wheel. The same pedal now does the work of 5 rotations. Obviously the one pedal which produced two rotations of the rear wheel is now producing 5 rotations of the rear wheel. This is higher work. But since more rotations of the rear wheel are produced, it becomes possible to achieve higher speed (because there is a limit to how fast you can keep on pedalling). This requires higher force on the pedal making it more difficult to pedal. But once the bike gathers speed, due to the factor of inertia of a moving object, the power required to pedal goes down – hence it becomes more efficient to bike with lower pedalling speed.
|Higher pedalling speed (Low gear)||Lower pedalling speed (High gear)|
|Load on heart||More||Less|
|Bike speed (same gear)||Lower||Faster|
When buying a cycle
The gear ratios on mountain bike and road bike vary slightly to accommodate specific use of both. Road bikes are meant for speed and largely flat terrain with gradual changes in inclinations. Whereas mountain bikes are meant for off road biking, facing rough roads and steep and sudden inclines. Therefore road bikes have larger gear ratios compared to mountain bikes. Hence its easier to climb a hill on a mountain bike but slower on a flat terrain. Conversely its more difficult to climb a hill on a road bike but faster on a flat terrain.
Bike shops generally advertise bikes as 16 speeds and 21 speed etc. Basically this means is number of chain rings in the front – multiplied with – number of cogs in the back sprocket. So if your cycle has two chain rings in the front and 5 cogs in the sprocket – it’s a 10 geared bike. However unless at a professional level, these many speeds are not needed. Anything more than 8 gears usually works fine.