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Can you explain gearing to me?

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Can you explain gearing to me?

Old 08-30-12, 09:05 PM
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bianchi10
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Can you explain gearing to me?

I raced motorcycles then came to the world of cycling. both sports I have played with gearing by dropping a tooth or gaining. I understand the reasons to do so and how it effects the performance, but I have never been into engines or working on stuff myself so I guess I never really thought about how it works. I understand the reason of using a larger or smaller tooth cog or ring. I'm looking for a simple explination on how shifting from one cog to the next size cog, to the next cog, to the next cog makes it easier or harder.

my thoughts are that if you shift into your highest gear on the cassette (Biggest rear cog) the chain is tighter and the smaller the cog the more slack (is that right or do I have it reversed?). regardless if you go to a higher or lower gear, how does that make it easier to pedal?

I'm sure this is very simple, but I dont know the answer. I'm sure once it is explained, it will become very obvious and feel very stupid
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Old 08-30-12, 09:13 PM
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I don't think this has much to do with chain slack since the rear derailleur takes up the tension when you're on a small cog and releases that tension when you're on a large cog. This has more to do with the energy required to turn (mash) that cog over. I'm sure someone else can explain it better.
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Old 08-30-12, 09:23 PM
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Google is a good teacher. Here is just one site I found. I didn't try hard so there are probably better ones.

https://adventure.howstuffworks.com/o...g/bicycle4.htm
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Old 08-30-12, 09:34 PM
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Originally Posted by Mansram01 View Post
I don't think this has much to do with chain slack since the rear derailleur takes up the tension when you're on a small cog and releases that tension when you're on a large cog. This has more to do with the energy required to turn (mash) that cog over. I'm sure someone else can explain it better.

yes, slack was probably the wrong word to use. I guess I mean the chain is shorter when on a higher (larger rear cog) gear.
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Old 08-30-12, 09:37 PM
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It has to do with the number (ratio) of teeth when comparing the chainring and the cog. To make things easy, let's say that you had a 48 tooth chainring and (implausibly) a 48 tooth rear cog. For every revolution of the crankset/pedals, the rear cog would go around once, too (because there's an equal number of teeth). Make it a 24-tooth cog (while keeping the 48T chainring), and the rear will go around twice for every revolution of the crankset/pedals - you gain speed, but it's at the cost of using more effort. With a 12-tooth cog, the rear wheel would revolve 4 times for every revolution of the crankset, but it would take even more effort. Make sense?
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Old 08-30-12, 09:43 PM
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https://en.wikipedia.org/wiki/Mechanical_advantage

Power (how fast you're going) is always going to be equal to torque (how hard you push the pedals) X cadence (how fast you spin) no matter what gear you're in. When you shift into an easier gear, you don't need to push as hard but you have to spin faster to maintain your speed. One revolution of the crank is going to make the back wheel spin a shorter distance then when you were in the harder gear.
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Old 08-30-12, 09:45 PM
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best way I can explain it is your legs provide the energy to the crank, which turns the chain ring and then a cog on the rear cassette which turns the rear wheel.

When you start pedaling from a stop your human engine has an easier time turning the rear wheel at a higher RPM. Meaning you make more full pedal rotations to turn the rear wheel one time around. As you build up speed and increase RPM, you need to reduce the number of pedal RPMs to keep gaining speed. Among the many reasons this is required, one is that your human engine has a RPM limit at which point you are either transferring no energy to the rear wheel OR, you physically cannot maintain the very high RPM. The other is the momentum you are carrying. As you build up speed your gearing must increase to allow your legs the ability to put the energy into the rear wheel and then into ground. This is facilitated by reducing the number of pedal revolutions needed to turn the rear wheel one time around...

I'm not sure what I am saying in my head reads well... I'm more of a verbal teacher than a writer.
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Old 08-30-12, 09:45 PM
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Originally Posted by WhyFi View Post
It has to do with the number (ratio) of teeth when comparing the chainring and the cog. To make things easy, let's say that you had a 48 tooth chainring and (implausibly) a 48 tooth rear cog. For every revolution of the crankset/pedals, the rear cog would go around once, too (because there's an equal number of teeth). Make it a 24-tooth cog (while keeping the 48T chainring), and the rear will go around twice for every revolution of the crankset/pedals - you gain speed, but it's at the cost of using more effort. With a 12-tooth cog, the rear wheel would revolve 4 times for every revolution of the crankset, but it would take even more effort. Make sense?
that does help. It wasn't as "Simple" as I was thinking but still easy enough to understand when you explain it like that.I thought it was going to be such an easy explination that i was going to feel like I must have missed a couple important days in 2nd grade or something!

thanks for the help
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Old 08-30-12, 09:47 PM
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Originally Posted by bianchi10 View Post
that does help. It wasn't as "Simple" as I was thinking but still easy enough to understand when you explain it like that.I thought it was going to be such an easy explination that i was going to feel like I must have missed a couple important days in 2nd grade or something!

thanks for the help
I think that we covered this in 7th grade.

It's a little more complicated than that (the "yeah, but why is it harder?" bit actually has to do with the how far away from the center point of the hub the force is being applied [how far out from the center is the chain pulling, essentially], regardless of the number of teeth) but that's the gist of it.

Last edited by WhyFi; 08-30-12 at 09:56 PM.
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Old 08-30-12, 09:54 PM
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It's not directly related to numbers of teeth. It's the size or diameter of the cog. A force of 10 lbs on a cog with a radius of 1ft gives you 10 ftlbs. But a force of 10 lbs on a cog of radius 2 ft gives you 20 ftlbs, i.e. twice as much torque. In a similar fashion if you have 10 lbs of force available, it will be easier to turn a cog that is 2 ft in radius rather than 1 ft in radius (so long as the resistance of the cog is the same in both cases). Combining these two effects into bicycle situation means that you can apply the same force at the pedal and get more torque at the back wheel if you put a small radius cog on the front and a large radius cog on the back.
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Old 08-30-12, 10:04 PM
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hahaha, I'm sorry. I know that this is plain english but my brain exploded and turned numb after the 3rd sentence.
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Old 08-30-12, 10:12 PM
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to add to what 009jim said above: Gearing is all about ratios. Lets say your front ring is 36t and your cog is 36t. Ratio is 1:1 meaning one rev of the pedal is one rev of the wheel. Let's say you have a 50t ring and a 10t cog. Ratio is 5:1. One turn of the pedal equal 5 turns of the wheels. This takes a lot more force to turn.
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Old 08-30-12, 10:12 PM
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Instead of thinking of it as a cassette or gear, think of it as a lever. A larger gear has a longer span, or lever, from the axle to the outer edge of the gear where the chain pulls on it, so when you pedal, the chain is, in effect, pulling on a longer lever which gives you more power for the hills but less speed.

Of course the opposite is true for the smaller gears on the cassette, there you are pulling a shorter lever which gives you less mechanical advantage power wise for hills, but the ability for more speed.

Last edited by RobertL; 08-30-12 at 10:25 PM.
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Old 08-30-12, 10:20 PM
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Here - put away the cogs for a second and think about levers. Walk over to an open door, and push it at the outside edge of the door, by the knob. Easy, right? Now push it from the middle of the door, halfway between the hinge and the knob. Harder, yeah? But you don't have to push it as far to get it to swing closed. Now push it 3/4 of the way towards the hinge - even harder effort is required, but you don't have to push it more than a few inches for the outside edge of the door to travel a pretty big arc. Same thing with the rear wheel of the bike. The closer to the hub (hinge) you apply your effort (the pulling chain) the harder it is, but the farther the outside (rim/door knob) travels.
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Old 08-30-12, 10:31 PM
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Originally Posted by WhyFi View Post
Here - put away the cogs for a second and think about levers. Walk over to an open door, and push it at the outside edge of the door, by the knob. Easy, right? Now push it from the middle of the door, halfway between the hinge and the knob. Harder, yeah? But you don't have to push it as far to get it to swing closed. Now push it 3/4 of the way towards the hinge - even harder effort is required, but you don't have to push it more than a few inches for the outside edge of the door to travel a pretty big arc. Same thing with the rear wheel of the bike. The closer to the hub (hinge) you apply your effort (the pulling chain) the harder it is, but the farther the outside (rim/door knob) travels.
you just get me lol, I needed it drawn out in crayon on a BIG white piece of paper! That makes sense!
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Old 08-30-12, 10:36 PM
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the ratios are more important. the closer the better, and high or low, depends on the rider, course, and competition.
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Old 08-31-12, 08:52 AM
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Did you skip high school physics?
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Old 08-31-12, 09:01 AM
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Originally Posted by SCochiller View Post
Did you skip high school physics?
I would bet that most here understand that there's a trade-off between distance and effort when changing gears. I would bet that a much smaller percentage here understand well enough to explain why (particularly in regard to effort), though.
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Old 08-31-12, 09:11 AM
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https://sheldonbrown.com/gears/
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Old 08-31-12, 09:15 AM
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You, just like your motorcycle's engine works best at powering the bike within a certain speed range (rpm).

The gears are the means to attempt to keep the engine within its most efficient range.
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Old 08-31-12, 09:46 AM
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Originally Posted by 009jim View Post
It's not directly related to numbers of teeth. It's the size or diameter of the cog.
For a fixed chain pitch (1/2" for bicycles), number of teeth dictates pitch diameter, making the two synonymous for any calculation.
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Old 08-31-12, 10:18 AM
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Old 08-31-12, 10:21 AM
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Originally Posted by surgeonstone View Post
glad someone else thought to link it.
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Old 08-31-12, 12:45 PM
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Originally Posted by RollCNY View Post
For a fixed chain pitch (1/2" for bicycles), number of teeth dictates pitch diameter, making the two synonymous for any calculation.
His statement is still more accurate. Your statement makes a couple of assumptions that aren't always true: it assumes that there are no missing teeth (like some of the previous SRAM cassettes) and it assumes circular (Q-rings are not circular hence the 'easy' and 'hard' parts of the stroke).

Originally Posted by RaleighSport View Post
glad someone else thought to link it.
Why? I didn't see anything there that answers his question; knowing what gear inches you're dealing with in any particular combo does nothing to explain why it's easier to spin a 53/24 combo vs 53/12.
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Old 08-31-12, 12:51 PM
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Originally Posted by WhyFi View Post
His statement is still more accurate. Your statement makes a couple of assumptions that aren't always true: it assumes that there are no missing teeth (like some of the previous SRAM cassettes) and it assumes circular (Q-rings are not circular hence the 'easy' and 'hard' parts of the stroke).



Why? I didn't see anything there that answers his question; knowing what gear inches you're dealing with in any particular combo does nothing to explain why it's easier to spin a 53/24 combo vs 53/12.
With the other answers already in this thread, the other information outputs and also including the GI are useful. Thanks for 41ing me though.
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