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Some questions about drivetrains and gear ratios

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Some questions about drivetrains and gear ratios

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Old 02-09-19, 12:07 PM
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krecik
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Some questions about drivetrains and gear ratios

Hi,

I'm new to SS/FG riding. I decided to start with an easy gear ratio until I feel more confident to increase it.

So here's the question if you start with a ratio of for example 40:20, and you increase it to 42:18, will you be able to use the same chain without adding or removing links? This also applies for any subsequent increments that will total 60 teeth, for example 44:16, 46:14, 48:12, etc...

Can you use the same chain so long as the new gear ratio totals the same nuber of teeth as the previous?

Really curious.

Kret
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Old 02-09-19, 12:39 PM
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fietsbob 
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Its Math ratios are math..


Can you use the same chain? you can try .. Riv Bike had the Quick beam .. a 2 cog flip flop hub 2 chainring bike ..

that was the idea turn wheel over and put chain on the other chainring..

the difference was not big because diameter differences is another factor ...
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Old 02-09-19, 02:03 PM
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krecik
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^
I get the feeling that you are answering me as if what I'm asking is obvious and it's not. It's not self evident that ratio is directly proportional to chain length which is why I'm asking the question. It could be the case that as you increment the ratio in the way that I said, the chain length required becomes bigger or lesser. I don't know how sprocket diameter affects the chain length, it might not be proportional, which is why I'm asking ! ! !

A flip flop hub only has 2 gear ratios, can't really take that as an answer if I'm planning to execute more conversions than 2.

So I guess I won't get a solid answer huh? Seems like people just ride their bikes. Maybe it's time to do the same and stop overcomplicating **** for myself.

Nvm my rambling.

Kret
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Old 02-09-19, 04:02 PM
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With only one reply so far why get so frustrated with us?

I can't really say yes or no to your question as I never give these things much thought. A well set up single cog system has an ability to adjust for even minor chain tension issues so I just do that regardless as a first step.

I suppose one could do the math and calculate the circumference differences. Andy
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Old 02-09-19, 04:41 PM
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If you pick cogs and chainrings that add up to the same number of total teeth, the wheel will move very little in the dropout or track end. (I am assuming you have a bike with either the horizontal rearward opening track end or near horizontal front opening droppout. If this is a derailleur bike with vertical dropouts, you have a much more demanding task that that bike would ve a poor choice for starting out fix gear. Single speed with a rear derailleur or "singulator" to take ou chain slack would work.

Yes, sticking to 60 links (or whatever) works. You still want a little wiggle room to get the chain slack right, but you don't need much.

On my old Mooney, I installed a triple chainring and have three fix gear cogs in back, each of which lines up exactly with its respective chainring. To get true mountain gearing, both uphill and downhill, I can run a 46,42,36 crankset and 13, 17 and 23 cogs (59 being that setup's magic number). (This was not done either easily or cheaply and I had no precedents to work from, But the end result is a blast!)

Ben
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Old 02-09-19, 05:05 PM
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Originally Posted by Andrew R Stewart View Post
With only one reply so far why get so frustrated with us?

I can't really say yes or no to your question as I never give these things much thought. A well set up single cog system has an ability to adjust for even minor chain tension issues so I just do that regardless as a first step.

I suppose one could do the math and calculate the circumference differences. Andy
The size of the rings should be pretty simple.

Add 2 teeth to one ring, and it makes the ring about 1" larger in circumference.
Subtract 2 teeth from one sprocket, and it makes the sprocket about 1" smaller in circumference.

The issue comes in that your chain doesn't wrap 100% around the chainring or rear sprocket.

It will wrap just over 1/2 the way around the chainring, and just under 1/2 the way around the rear sprocket.

One still should be pretty close with adding and subtracting 2 teeth (or 3 teeth or 4 teeth, etc).

It may depend a bit on the actual bike config. Is there at least a little adjustment? How much?

You may even be ok to do 1 or 2 difference between the front and rear.

I have an older Campagnolo shifter that I still need to get setup. The design is for an indexed long slot rear dropout. One loosens the rear quick release, shifts up or down up to about 6 teeth, and re-locks the rear wheel. All wile RIDING. No chain tensioner. I.E. it all functions like a single speed that can be shifted.
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Old 02-10-19, 06:56 AM
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^ @79pmooney @CliffordK

Cheers guys, just what I was hoping to hear. To answer both posters, yes I have a typical track/fixie frame with horizontal dropouts, 35mm deep so plenty of room for adjustment of slack chain. I was hoping I could increment my gear ratio acording to need without adding or removing links, so it looks like I'll be able to do that.

In response to the bikes you guys described, those sound like some pretty wacky machines. I like it! It's cool to hear people get creative with their gear.

I had an idea pop into my mind the other day too but it seems like it was already done before me, to install a flip-flop hub on the back and two geared crank arms on either side, that way you wouldn't need to flip the wheel, just whip the chain from one side to the other.

Kret
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Old 02-10-19, 08:13 AM
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Originally Posted by krecik View Post
^ @79pmooney @CliffordK

Cheers guys, just what I was hoping to hear. To answer both posters, yes I have a typical track/fixie frame with horizontal dropouts, 35mm deep so plenty of room for adjustment of slack chain. I was hoping I could increment my gear ratio acording to need without adding or removing links, so it looks like I'll be able to do that.

In response to the bikes you guys described, those sound like some pretty wacky machines. I like it! It's cool to hear people get creative with their gear.

I had an idea pop into my mind the other day too but it seems like it was already done before me, to install a flip-flop hub on the back and two geared crank arms on either side, that way you wouldn't need to flip the wheel, just whip the chain from one side to the other.

Kret
Yes it has been done, but the low gear side only works as a starting gear, and really only useful in standing start track events. Here's how it works to the best of my memory.It requires the use of a specially made hub with left and right hand cog threads and a left threaded freewheel.
The fixed cog on the right side is only partially threaded on the hub at the starting gate, and for the first few strokes of the pedal, the power is transferred to the hub at the higher mechanical advantage of the larger cog.
The fixed cog on the right, being driven faster than the freewheel on the left tightens on to the hub until it tightens against the shoulder, and takes over from the freewheel which then, well, freewheels.
This setup was used around 40 years ago by Canadian cycling champ Jocelyn Lovell. In this ancient video, you can see this unique drive train at a couple of spots, although you have to look close, and it's not discussed in the video.
https://www.cbc.ca/archives/entry/jo...l-its-been-fun
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Old 02-10-19, 10:14 AM
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So you have about 1" of adjustment, if you can get your chain right,

You could probably do what you want with say a 3-speed freewheel,

14/16/18

+ a double chainring: (40/42)

To give you ratios: 14/42, 16/40, 16/42, 18/40

https://www.ebay.com/itm/FRENCH-MADE...S/163525016515

https://www.ebay.com/itm/3-Speed-Fre...S/312447627144

To get that much difference squeezed in, you might need a half-link for your chain depending on how it all fits.

Perhaps a few other permutations.
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Old 02-10-19, 11:55 AM
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Originally Posted by krecik View Post
also applies for any subsequent increments that will total 60 teeth, for example 44:16, 46:14, 48:12, etc...

Can you use the same chain so long as the new gear ratio totals the same number of teeth as the previous?

)
The short answer to this question is NO, partially for the reasons Clifford described and partially because of the change in angle of the chain between the ring to the cog. The bigger answer though is there's usually enough slack to accommodate a few different cogs with a few more or less teeth. And the even bigger answer is you're thinking about this wrong, because if you are considering changing chain rings and changing FG cogs, you're going to get involved in some time consuming labor that will make chain length adjustments seem quick and easy. I suggest finding the crank you like, use whatever chainring it has on it, and then just experimenting with different cogs - that's what almost everyone does.

Originally Posted by krecik View Post
I have a typical track/fixie frame with horizontal dropouts,
You don't have horizontal dropouts, you have track ends. Might as well learn the jargon. Welcome to the club.
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Old 02-10-19, 12:09 PM
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Originally Posted by DiabloScott View Post
You don't have horizontal dropouts, you have track ends. Might as well learn the jargon. Welcome to the club.
If it is a "track frame", then it has "track ends".

If it is a 1960's/1970's conversion, then it may well have long horizontal dropouts.

Anything else, and it is anybody's guess.
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Old 02-10-19, 12:21 PM
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Not fixed but there are 2 speed kick back, without or with coaster brakes, so you have a starting off gear and a once you get going , higher gear..
Sturmey-Archer | S2 Silver No cables , Other than the Brakes...


and a cable shifted gear hub which is using a fixed driver and has 3 speeds , direct and 2 reduction gears..

The S3-X ... ( I don't see it on the current site so it may be limited to what i already Out there .. in shops and online owner resales)

& abundant additional cable shifted non fixed internal gear hubs

not street fixie trendy ,but imminently practical for many decades to come..


Now If you are competing on a Velodrome - track that's another story..











....

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Old 02-11-19, 03:16 PM
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The practical answer is yes. The more nuanced answer is that for a given gear ratio, the closer the cog and the chainring are in size, the more extra chain length you'll have if you keep the same chain, but it is a very small amount. For any kind of bike with horizontal dropouts or track ends, you can just slide the rear wheel to adjust.

Why does having a chainring and cog closer in size give you more chain length? Because the chain travels along a hypotenuse that gets steeper as the chainring gets bigger and the cog gets smaller. If they were both the same size, the chain would travel on a horizontal line. Can you picture it? Imagine the bike from the driveside, like a bike glamour shot, not from above. The chain runs in a straight line, across the top and bottom, when the chain ring and cog are the same size. That would be the shortest route for a chain to take for a given gear ratio.

For those who are still with me, there is a useful part of this math. It's when you're trying to run a frame with vertical dropouts as a fixie or singlespeed. Can't quite get the chain tension dialed? Tighten it by using a larger chainring and smaller cog for the given gear ratio. Or loosen it by doing the reverse -- this is the kind of tiny adjustment that often can make the impossible vertical dropout work.
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Old 02-11-19, 05:25 PM
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Originally Posted by ljsense View Post
The practical answer is yes. The more nuanced answer is that for a given gear ratio, the closer the cog and the chainring are in size, the more extra chain length you'll have if you keep the same chain, but it is a very small amount. For any kind of bike with horizontal dropouts or track ends, you can just slide the rear wheel to adjust.

Why does having a chainring and cog closer in size give you more chain length? Because the chain travels along a hypotenuse that gets steeper as the chainring gets bigger and the cog gets smaller. If they were both the same size, the chain would travel on a horizontal line. Can you picture it? Imagine the bike from the driveside, like a bike glamour shot, not from above. The chain runs in a straight line, across the top and bottom, when the chain ring and cog are the same size. That would be the shortest route for a chain to take for a given gear ratio.

For those who are still with me, there is a useful part of this math. It's when you're trying to run a frame with vertical dropouts as a fixie or singlespeed. Can't quite get the chain tension dialed? Tighten it by using a larger chainring and smaller cog for the given gear ratio. Or loosen it by doing the reverse -- this is the kind of tiny adjustment that often can make the impossible vertical dropout work.
And sometimes you just get lucky. I have two IGH conversions with vertical dropouts that I am running without tensioners.
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Old 02-11-19, 07:23 PM
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I have 3 chainrings (44,45,46) and 3 cogs (16,17,18), which gives me ~64-76" with 1-2" steps, and I can run all combinations with the same chain. It's a great setup for getting started to figure out what gear you like. I start at 44x18 in the spring and work my way up as my fitness improves.
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