21 gears on a cycle!!! What for?

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21 gears on a cycle!!! What for?

Postby ajay890 » October 31st, 2007, 11:04 pm

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 a 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.

Image

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

Image


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
Last edited by ajay890 on May 11th, 2008, 11:52 pm, edited 1 time in total.
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Postby deepak_majipatil » November 1st, 2007, 12:09 pm

nice one Ajay :D
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Postby SreeHarsha » November 2nd, 2007, 10:43 am

yeah, good insight.
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Postby Old Man » November 2nd, 2007, 10:47 am

Me from the old school of thought, i feel cycling is going the F1 way. would be happy with a single speed bike, not that i dont enjoy the geared ones though.
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Postby sspal » November 13th, 2007, 7:15 pm

now that was the answer for which i always used to scratch my head..

thanks to ajay he save me becoming bald.....
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Postby balu » November 14th, 2007, 8:47 am

My perspective on gears is that as you get older they get more indispensable! I cannot imagine not riding a bike with multiple speeds - esp the slower speeds. 20 years back it was ok to simply stand and mash my knees but not any more...

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Postby abhi » November 15th, 2007, 9:16 pm

:) Ajay, thanks for taking the effort to put that up!
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would post More tech stuff

Postby ajay890 » November 18th, 2007, 9:58 am

anyone wanting to know more technical stuff about gears, just post your query on the forum and I vl glad to answer.
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Postby pranavnegandhi » May 11th, 2008, 9:29 pm

Thank you very, very much Ajay.

I've been using a single speed bicycle and struggling hard to get my head around the physics of multi-geared systems without any real-life example to guide me.

This explanation really made things click and fall into place. I don't think I'll be forgetting this principle for a while now.

.p
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Missing Diagrams added

Postby ajay890 » May 11th, 2008, 11:55 pm

Thanks Pranavnegandhi!!

Uploaded those missing diagrams
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Re: 21 gears on a cycle!!! What for?

Postby alyrza » December 25th, 2008, 9:07 pm

informative!
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Re: 21 gears on a cycle!!! What for?

Postby prakritij » December 30th, 2008, 3:19 pm

Thanks.....quite informative
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Re: 21 gears on a cycle!!! What for?

Postby nishant333 » February 4th, 2009, 10:30 am

Good piece of info for knowing the basics of gears..!!
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Re: 21 gears on a cycle!!! What for?

Postby muralisk » September 2nd, 2009, 11:53 am

Thanks for the information, quite useful for newbies.
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Re: 21 gears on a cycle!!! What for?

Postby mkovuri » October 7th, 2009, 9:41 pm

Very informative, and thanks. Just needs a small correction though. The effort required in case 2, shouldn't that be one-fourth of case 1 (0.5 versus 2)?

Also it looks like there could be some more help from you. I heard that you use only certain combinations of gears between front and rear. For example, when front is on 1, you may go up to 2 or 3 (and not beyond) on the rear to avoid the chain moving in an angle. Is that right?

Also you may want to clarify the term speed as against number of gears. We get to hear some bicycles mentioned as 10 speed, 7 speed etc. How does this relate to number of gears? I was thinking, if it was 27 gear bike, it is equivalent to 9 speed. Am I right?

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