21 Gears!!! ... on a Bicycle!!!!. Whatfor?

21 gears!!!! That too on a bicycle!! What for? How do you manage to shift through so many gears? Is the typical reaction of the people curious enough to notice and quiz me about the gear shifter on my bike's handle bar.

The general opinion is that, this system is useless additional feature fitted just to raise the price of the cycle.

Many a times I take trouble to explain that one doesn't have to shift through all 21 gears one after the other (as is the case with motorbike) but choose the gear ratio appropriate to the terrain and the speed desired and that the gear system is very useful.

Here is a little gyan about the number of gears on the bicycle

The 21 gears is actually combination 3x7, where 3 is the number of cogs on the pedal crank and 7 is the number of cogs on the rear wheel. Similarly in 18 gear system there would be 3 front cogs and 6 rear cogs.

That is how a Cyclist has a choice of 21 (18) combinations.

But how does it help the cyclist? One needs to pedal anyway, isn't it? Questions and more questions follow.

So! Here is an explanation

High gear and low gear numbers In a 5 gear system numbered 1,2,3,4 and 5 the number 5 is the highest gear position and 1 is the lowest gear position

It is easiest to pedal and pick up from rest when in lowest gear (number 1) and toughest to pedal and pick up from rest when in highest gear (number 5). One shifts from low gear to high gear gradually increasing the speed. 

Detailed Explanation with little math and a little physics

Case 1

A cycle has a pair of cogs linked by a chain, one cog is on the pedal crank and the other fitted on the rear wheel.

Consider a cycle with front cog having 32 teeth and the rear cog having 16 teeth You divide the number of teeth on front cog with that on the rear cog i.e. 32/16 = 2

The ratio 2 indicates that when the front cog completes 1 rotation the rear cog executes 2 rotations

Case 2

Let us reverse the situation

Now the front cog has 16 teeth and the rear cog has 32 teeth, in this case, the ratio of number of teeth on front cog to that on the rear cog is 16/32 = ½ = 0.5

This means, when the front cog completes 1 rotation the rear cog has executed only half the rotation

Comparing the two cases

Case 1

1.High gear

2.Front cog executes less number of turns as compared to the rear cog

3.Effort (force) required to push the pedal is higher as compared to case 2

4.Covers larger distance per pedal stroke (Pedal cog at Low RPM)

5.Generates low TORQUE

6. Used when already in motion, to increase the speed.

Case 2

1. Low gear

2. Front cog executes more number of turns as compared to the rear cog

3. Effort required is half of that in case 1

4. Covers less distance per pedal stroke (Pedal cog at High RPM)

5. Generates high TORQUE

6. Used

a. To pick up from rest

b. To ride effortlessly at lower speeds

c. On climbs

d. On rough terrain where motion is at low speeds

 

A multi speed system provides various combinations of gears to suit the need of the cyclist in different riding conditions