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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 09-13-17, 08:36 AM   #1
mtb_addict
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Big cog vs small--performance, durability

Big or small cog size?

Commuter bike with 26" tire.

Option 1: 38/16 (62 gear inches)?
Option 2: 40/17 (61 gear inches)?
Option 3: 42/18 (61 gear inches)?
Option 4: 44/19 (60 gear inches)?
Option 5: 46/20 (60 gear inches)?

Opt 1 use small cogs...Opt 5 use big cogs...what is the optimum choise and y?

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Old 09-13-17, 09:22 AM   #2
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My thought is that the bigger cogs would result in less stress on the chain for a given amount of torque. Also, a middle of the road setup gives you options if you want to try a slightly different ratio later on.
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Old 09-13-17, 09:40 AM   #3
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Sheldon Brown outlines the good and bad of larger vs. smaller gears:

Big gear pros...
- Slightly less friction
- Longer chain/sprocket life
- Less chain tension

Big gear cons...
- Slightly heavier
- Chainstay clearance my be a problem on some frames

Small gear pros...
- Slightly lighter
- More log-jumping clearance
- More chainstay clearance

Small gear cons...
- Rapid chain/sprocket wear
- Greater chain tension (increased likelihood of the axle slipping in the frame)

I don't think there's a big difference on any of these points. If you have a special situation (e.g., a mountain bike that needs lots of clearance for obstacles, or you're a real weight weenie) it's an easy choice. Otherwise, just go with whatever is readily available, or what you like the looks of.
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Old 09-13-17, 11:12 AM   #4
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I only recently learned this. Larger rings and cogs create a smoother and quieter drive train.

2.2.1 Chordal Action
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Old 09-13-17, 11:19 AM   #5
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38:16 is the 1:1 ; 11th gear on my Rohloff hub 26" wheel trekking bike , their steel hub cog, Surly Stainless steel chainring..

both can be flipped over to wear 2x as long.

Why? , because I had the hub cog, changed the crank, could have bought a 39t. but did not.




....
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Old 09-13-17, 11:53 AM   #6
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A smaller chainring or cog will wear more quickly, and a small cog will have more drag than a larger one. Also, a smaller chainring will result in higher chain tension, resulting in a faster wear. However, there is a point of diminishing returns where going larger yields minimal improvement.
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Old 09-13-17, 11:55 AM   #7
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My thought is that the bigger cogs would result in less stress on the chain for a given amount of torque.
This is determined entirely by the size of the chainring. The cog just goes along for the ride.
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Old 09-13-17, 11:58 AM   #8
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However, there is a point of diminishing returns where going latger yields minimal improvement.
Yes, given that your smallest rear cog option is a 16, I doubt the difference in efficiency is significant, or even measurable in the range of 16 to 20. The big tradeoffs seem to come when you get down in really small 10-12t rear cogs. So I'd tend to save the weigh/bulk and go small.

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Old 09-13-17, 01:20 PM   #9
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Theoretically the bigger cogs will run smoother and wear less quickly. Practically, you will not notice any difference on your commuter.
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Old 09-13-17, 01:38 PM   #10
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Yeah, as said above, big rings/cogs last longer. But, if you run 1/8" and start with new rings, cogs and chain, option 3, 42-18 will go many thousands of miles. I've run 42-17s, 44-17s and 48-19s. All work and there really isn't a lot to choose from. The realities of availability of rings and cogs and other factors count far more.

I run 42 or 43 on one bike so I can use big cogs for climbing and get as low a gear as possible (on a track standard 144 BCD crankset). My commuter is set up 44-17, really just "because". It is such a reliable set-up I see no reason to do anything beyond replace the chain every year or three and ring/cog/chain when the sharks teeth get established.

Now things will wear out faster using the more common 3/32" drive. (Probably by roughly the difference in chain inside width, ie 3/23" ve 1/8" or 75%. But 3/32s also costs roughly the same amount less probably so its probably a toss. I run 1/8" for reliability and resistance to throwing chains.)

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Old 09-13-17, 01:48 PM   #11
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Quote:
Originally Posted by TejanoTrackie View Post
"My thought is that the bigger cogs would result in less stress on the chain for a given amount of torque."

This is determined entirely by the size of the chainring. The cog just goes along for the ride.
Depends on how you're looking at it. If you're considering how much chain tension is generated for a given force applied to the pedals, then yes, that will vary depending on the chain ring size. But if you considering how much chain tension is needed to create a certain amount of torque to the hub shell then it's the cog size that matters. And since the OP is looking at a fixed gear ratio the chain ring and cog sizes will be proportional so it really doesn't matter.
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Old 09-13-17, 01:58 PM   #12
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Originally Posted by prathmann View Post
Depends on how you're looking at it. If you're considering how much chain tension is generated for a given force applied to the pedals, then yes, that will vary depending on the chain ring size. But if you considering how much chain tension is needed to create a certain amount of torque to the hub shell then it's the cog size that matters. And since the OP is looking at a fixed gear ratio the chain ring and cog sizes will be proportional so it really doesn't matter.
Nope. The force is applied at the cranks which is then transferred to the chain by the chainring. The tension in the chain does not magically change at the other end where it connects to the cog. This is called equillibrium.
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Old 09-13-17, 02:23 PM   #13
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Depends on how you're looking at it. If you're considering how much chain tension is generated for a given force applied to the pedals, then yes, that will vary depending on the chain ring size. But if you considering how much chain tension is needed to create a certain amount of torque to the hub shell then it's the cog size that matters. And since the OP is looking at a fixed gear ratio the chain ring and cog sizes will be proportional so it really doesn't matter.
At one time I had the same reasoning, but then I figured chain tension also depends on - is proportional to - how much chain is pulled through per revolution of the rear wheel.
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Old 09-13-17, 02:43 PM   #14
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At one time I had the same reasoning, but then I figured chain tension also depends on - is proportional to - how much chain is pulled through per revolution of the rear wheel.
I think you guys are confusing action and reaction. The chain tension is solely dependent on the force applied at the pedals and the mechanical advantage determined by the length of the crank arms and the radius of the chainring. This is the action and everything at the wheel end of things is reaction. Also, Newton's Third Law. As to chain tension, it is determined as follows:

Let:

F = force applied to pedal

ECL = perpendicular distance from pedal force to centerline of bottom bracket, which is the same as the crankarm length at the 3 and 9 o'clock positions

T = chain tension

CR = chainring radius

Applying static equillibrium (sum of the moments equals zero):

F x ECL - T x CR = 0

Solving for T:

T = F x ECL / CR

So, it is seen that chain tension is inversly proportional to the chainring radius, which is why a smaller chainring will result in a higher chain tension.
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Old 09-13-17, 02:51 PM   #15
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True, but if there's no load on the rear wheel, then there's no tension on the chain. So it's slightly more complicated. The chain tension is ultimately related to how much power you can deliver at a given cadence.
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Old 09-13-17, 03:06 PM   #16
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Quote:
Originally Posted by TejanoTrackie View Post
I think you guys are confusing action and reaction. The chain tension is solely dependent on the force applied at the pedals and the mechanical advantage determined by the length of the crank arms and the radius of the chainring. This is the action and everything at the wheel end of things is reaction. Also, Newton's Third Law. As to chain tension, it is determined as follows:

Let:

F = force applied to pedal

ECL = perpendicular distance from pedal force to centerline of bottom bracket, which is the same as the crankarm length at the 3 and 9 o'clock positions

T = chain tension

CR = chainring radius

Applying static equillibrium (sum of the moments equals zero):

F x ECL - T x CR = 0

Solving for T:

T = F x ECL / CR

So, it is seen that chain tension is inversly proportional to the chainring radius, which is why a smaller chainring will result in a higher chain tension.
You can look at the cog also, since the forces are the same.

I'm just saying a different way to visualize it. At a given gear ratio, twice the CR radius means twice the chain is pulled. It means the same thing, same energy but two lengths (of chain moved) => (since W=FxD) half the force on the chain => half the tension.

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Old 09-13-17, 03:32 PM   #17
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True, but if there's no load on the rear wheel, then there's no tension on the chain. So it's slightly more complicated. The chain tension is ultimately related to how much power you can deliver at a given cadence.
Sure. There has to be a reaction at the wheel end to create chain tension. So if you spin up the wheel in a bike stand, then you won't get a lot of tension. On the road you get a dynamic inertial reaction in the form of acceleration, plus wind resistance, such that all of your applied pedal force gets consumed to create the maximum chain tension possible.
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Old 09-13-17, 04:35 PM   #18
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OP: Should I use big cogs or small cogs?
SSFG:
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Old 09-13-17, 07:27 PM   #19
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Quote:
Originally Posted by TejanoTrackie View Post
" Originally Posted by prathmann
Depends on how you're looking at it. If you're considering how much chain tension is generated for a given force applied to the pedals, then yes, that will vary depending on the chain ring size. But if you considering how much chain tension is needed to create a certain amount of torque to the hub shell then it's the cog size that matters. And since the OP is looking at a fixed gear ratio the chain ring and cog sizes will be proportional so it really doesn't matter."


Nope. The force is applied at the cranks which is then transferred to the chain by the chainring. The tension in the chain does not magically change at the other end where it connects to the cog. This is called equillibrium.
As I said, that's one way of looking at it. But it's also perfectly legitimate to consider that in order to maintain a certain bike speed and overcome drag from rolling and air resistance one must apply a corresponding amount of torque to the rear hub shell. That torque is created by the chain tension acting at a lever arm distance given by the cog radius, i.e. Torque at hub = Chain tension x cog radius
or Chain tension = desired hub torque / cog radius. So the chain tension required to overcome drag at any specific speed is inversely proportional to the cog radius.

Neither of the two ways of looking at the situation is 'wrong' and both views lead to the same practical conclusion, i.e. that minimum chain tension will be given by the largest ring and cog combination.
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Old 09-13-17, 08:45 PM   #20
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I only recently learned this. Larger rings and cogs create a smoother and quieter drive train.

2.2.1 Chordal Action

UjjwalRane explains it very well.

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Old 09-14-17, 08:18 AM   #21
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Chordal action also explains why some chain slack is necessary in any SSFG drivetrain, regardless of how perfectly round and concentric the chainring and cog are.
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Old 09-14-17, 09:06 AM   #22
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OP: Should I use big cogs or small cogs?
SSFG:
I think this is the only post I actually read and LOLed.

Go ride your bikes.
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Old 09-14-17, 09:37 AM   #23
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I only recently learned this. Larger rings and cogs create a smoother and quieter drive train.

2.2.1 Chordal Action
It has always been very noticeable on geared bikes when the chain is on the small 11 and 12t cogs. Now I know the effect has a name
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Old 09-14-17, 12:45 PM   #24
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We aren't thinking about this in the 3rd dimension.

If we apply what we know already to the diagram below, we can figure out the relationship between the cogs.

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Old 09-14-17, 01:04 PM   #25
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I'm sorry @Unkle Rico , but that diagram fails to account for the Coriolis effect.
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