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Gear inches, gain ratio, etc - does it matter how we gear it?

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Gear inches, gain ratio, etc - does it matter how we gear it?

Old 05-06-19, 10:43 AM
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MEversbergII
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Gear inches, gain ratio, etc - does it matter how we gear it?

Something I had been wondering for a while about bike gearing - is there any functional difference in different combinations of front and rear rings and sprockets if they achieve the same number of inches (or gain ratio, or whichever metric you use)?

If I have two almost identical bikes that both are in, say, 54", with the only difference is one uses a smaller chain ring and a larger rear sprocket while the other uses a larger chain ring and a smaller rear sprocket. Both have the same number of inches, but achieve it differently.

I would suppose that the larger front chain ring would be more difficult to start pedaling from a start, but I'm not sure if that's actually a factor of the chain ring or the fact that usually if I'm in the larger ring I'm already geared up higher than an ordinary start.

Thoughts? Does it actually matter?

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Old 05-06-19, 11:06 AM
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If the wheel size is the same on both bicycles and the combination of big to small or small to big gives the same amount of gear inches then the effort will be the same.
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Old 05-06-19, 11:23 AM
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It doesn't matter.
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Old 05-06-19, 11:45 AM
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Well the smaller sprockets in the back tend to wear faster than larger ones, especially if they are aluminium or titanium
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Old 05-06-19, 11:55 AM
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Only thing I can think of in terms of difficulty pedaling would be if the torque differed--if one of the combos involved cross-chaining, it would be harder to pedal than the other.
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Old 05-06-19, 11:55 AM
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Originally Posted by MEversbergII View Post

If I have two almost identical bikes that both are in, say, 54", with the only difference is one uses a smaller chain ring and a larger rear sprocket while the other uses a larger chain ring and a smaller rear sprocket. Both have the same number of inches, but achieve it differently.
Your example is incorrect. Your bike with a "smaller chain ring and a larger rear sprocket" could not possibly be the same gear inch as your other bike with a "larger chain ring and a smaller rear sprocket". The second bike would absolutely be a higher gear, and a higher gear inch.

Now, in a comparison between two differing systems where the final gear inch IS actually the same, the one with the larger rings/sprockets will be more efficient. Not by much, but it has been measured on test equipment.
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Old 05-06-19, 12:04 PM
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Gear inches and gain ratios are just social constructs. Beyond very trivial differences in efficiency, there's no real need for most riders to concern themselves with it.
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Old 05-06-19, 12:04 PM
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Originally Posted by AlmostTrick View Post
Your example is incorrect. Your bike with a "smaller chain ring and a larger rear sprocket" could not possibly be the same gear inch as your other bike with a "larger chain ring and a smaller rear sprocket". The second bike would absolutely be a higher gear, and a higher gear inch.

Now, in a comparison between two differing systems where the final gear inch IS actually the same, the one with the larger rings/sprockets will be more efficient. Not by much, but it has been measured on test equipment.
There's also the issue that on small-small that fewer teeth are carrying the load, so they'll wear out sooner, but for this thought experiment it's not an issue.
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Old 05-06-19, 12:05 PM
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Originally Posted by AlmostTrick View Post
Your example is incorrect. Your bike with a "smaller chain ring and a larger rear sprocket" could not possibly be the same gear inch as your other bike with a "larger chain ring and a smaller rear sprocket". The second bike would absolutely be a higher gear, and a higher gear inch.

Now, in a comparison between two differing systems where the final gear inch IS actually the same, the one with the larger rings/sprockets will be more efficient. Not by much, but it has been measured on test equipment.
I just assume the incorrect stuff you talk about in the first paragraph is a result of typos, and OP meant to say Smaller/smaller and Larger/larger.

Stuff in the second paragraph--I didn't know that. Is there an explanation? Doesn't that make the most efficient combos cross-chained ones? That doesn't seem right.
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Old 05-06-19, 12:09 PM
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In practice, it may matter if you select large/large versus small/small combinations to get the same gear ratio. More teeth in contact with the chain means that the loading is spread across more surface area, so wear (on the teeth) could be slower. I would venture to guess that heavy torque riders might wear in components faster, so this effect might be more pronounced than with, say, light spinners.

When possible, it's good to select gear ranges that complement each other with the minimum of overlap.
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Old 05-06-19, 12:29 PM
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Originally Posted by livedarklions View Post
Doesn't that make the most efficient combos cross-chained ones? That doesn't seem right.
Studies I've read showed chain line increases friction less than small sprockets do. (modern chains are pretty "flexible") I will post link(s) if I have a chance, or maybe others will.
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Old 05-06-19, 12:33 PM
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The mechanical advantage for identical tooth ratios of front chainring/rear cog is identical. You'll go exactly the same distance per rotation of the crank. Your cadence at the same speed will be identical.

But, if you find two front/rear combinations that are nearly identical, there appears to be a slight advantage in using the larger sizes( this would be on both front and back), for frictional purposes. The dominant frictional factor appears to be the small cog, so the degree of chain bend and wrap seems to be the driving frictional issue.

That works for, say, 52/17 vs 43/14. But at some point, the chainline being more twisted matters. For me (Dura Ace and Ultegra 11 speed) I only use the smallest 8 or so rear cogs with my big (53) front ring. I also feel (pure conjecture here) that the smaller front chainring is more tolerant of cross-chaining, and I can do my two smallest cogs no problem.

The frictional differences are at most a few percent, though, and cadence has a pretty big impact on your physiological efficiency, so probably just adjust the gears to get the cadence you want/are comfortable with, without hearing any complaining noises from your driveline (that is, avoid big/big). If it matters (e.g if you are a TdF racer) then I suppose that they'd put you on a dynamometer and use calimetry to see which gear is best.

https://www.bikeradar.com/features/f...oper-shifting/

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Old 05-06-19, 01:24 PM
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Originally Posted by ThermionicScott View Post
Gear inches and gain ratios are just social constructs. Beyond very trivial differences in efficiency, there's no real need for most riders to concern themselves with it.
I think this probably applies to most, but I have one very, Very, VERY steep hill that I can't get up with one of my bikes, but I can with the other one. If I was going to buy a bike with the understanding that I MUST be able to ride the steep hill then I had better consider the gear inches of what works and the gear inches of what doesn't!
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Old 05-07-19, 07:58 AM
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Originally Posted by AlmostTrick View Post
Studies I've read showed chain line increases friction less than small sprockets do. (modern chains are pretty "flexible") I will post link(s) if I have a chance, or maybe others will.
Thanks!

I pretty much live in the high gears where there isn't overlap, so it's all pretty academic to me.
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Old 05-07-19, 09:12 AM
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Chain efficiency comes into play here. Larger sprockets means less friction with the chain links rotating less as they run through the system. And supposedly, more chain tension, meaning more torque applies to the rear wheel.
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Old 05-07-19, 09:36 AM
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Originally Posted by friday1970 View Post
Chain efficiency comes into play here. Larger sprockets means less friction with the chain links rotating less as they run through the system. And supposedly, more chain tension, meaning more torque applies to the rear wheel.
The first half is correct, but a larger ring/sprocket combo results in LESS chain tension because the lever arm is longer on the gear. The lower chain tension plays into why a larger ring/sprocket combo is more efficient.

In the end, the torque on the rear wheel is the same, disregarding the losses. These losses are also fairly small compared to your power output though. They are measurable but definitely marginal gains.
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Old 05-07-19, 12:34 PM
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I have wondered this for years and played with different ring size combos front and rear giving the same gear inch and swear the smaller ring up front climbs easier than having a larger ring up front. It should not make a difference, however it seems that it does for me.
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Old 05-07-19, 12:47 PM
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Originally Posted by caloso View Post
There's also the issue that on small-small that fewer teeth are carrying the load, so they'll wear out sooner...
That actually is a factor in my shifting - why wear out the small cogs prematurely?
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Old 05-07-19, 01:00 PM
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There's a limit. If you're running 10/30 then the gearing is identical to running 30/90. However, throwing a chain three times the weight around a 90t chainring at the same speed will be causing increased rolling resistance due to the increased centrifugal forces whilst doing so.
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Old 05-07-19, 01:11 PM
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Some old racers I knew used to say that for a given gear ratio, small/small is better for fast pace changes and large/large is better for steady efforts. Is that true? Who knows...
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Old 05-09-19, 05:16 PM
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I am more likely to ride in my 52/20 than 42/16 even though they are basically the same gear. Bigger chainring and sprocket have more chain wrap and just feels better.
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Old 05-09-19, 08:11 PM
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Originally Posted by caloso View Post
There's also the issue that on small-small that fewer teeth are carrying the load, so they'll wear out sooner, but for this thought experiment it's not an issue.
However, here, there are lots of short hills, so I spend a lot of time on the larger cogs. I try to use the small-small when I can so the larger cogs don't wear out too quickly. Otherwise, my 12, 13 & 14 would almost never get used.
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Old 05-09-19, 10:44 PM
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it matter because ....... the straighter the chain line the more efficient the drive train will be , even if its the same ratio or size , you want your chain line strain as can be in the most used gear and or 1 or two cogs next to it . id live to see a drive train use one chain ring that moves over with a simple hydrolic action so you can run 1x without loosing power
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Old 05-10-19, 06:48 AM
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Originally Posted by Teamprovicycle View Post
it matters because ....... the straighter the chain line the more efficient the drive train will be(
With the derailleur chains of ~100 years ago, this was probably true. As other's on this thread have already mentioned, with modern chains efficiency loss - if any - due to chain line is so small it disappears in the noise of the test.

Fun fact: early derailleur gearing systems (in the 1890s) moved the (2!) cogs side to side under the straight chain line.
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Old 05-10-19, 08:18 AM
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Something I had been wondering for a while about bike gearing - is there any functional difference in different combinations of front and rear rings and sprockets if they achieve the same number of inches (or gain ratio, or whichever metric you use)?
Yes there is. There IS SOMETHING very different about gearing ratio results - depending very much on the size of the front sprockets. Size matters!

Let's start by examining what happens to gear ratio results when using a 48 tooth sprocket. If this sprocket is used with a wide range 10-spd cassette - perhaps a 12-36 teeth range the resulting gear ratios would span a 4-to-1 ratio for high gear - all the way down to 1.333-to-1 ratio for low gear. If the front sprocket choice is 52 teeth - then the same cog selections result in 4.333 and 1.444-to-1.

If you can imagine that there are eight other gear ratio variations between these two extremes - AND - you review the ratio-variation between each individual cog's ratio - you will realize that ALL the gear ratios on the smaller sprocket are closer together than the big sprocket. (often called gear "steps")

And the point I am trying to make about the selection of sprocket size and the resulting ratios is that depending on each rider's personal power-to-weight ratio - there IS an optimal gear ratio "step" range for each rider.

However, in modern times - with everyone choosing multi-sprocket chain sets - there are so many gear possibilities few riders notice or care. In practice - one of the few times I would expect this gear "theory" to be important would be a big guy riding a loaded touring bike in mountainous territory. (never needing double shifts under load)

Typically other aspects of bicycling power trains are more often noted - such as making sure that adequate gear ratio ranges keep a rider in the saddle no matter the load or grade. And just as importantly the optimal crank arm length and seat position for cycling terrain at hand.

Harrumph.

Last edited by Richard Cranium; 05-10-19 at 02:34 PM.
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