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Any difference in effort between gears with the same ratio?

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Any difference in effort between gears with the same ratio?

Old 09-09-06, 07:36 PM
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Any difference in effort between gears with the same ratio?

Hi,

Just wondering if there's any reason (other than psychological) that two sets of gears with the same product would feel different. For instance, 50-12 and 40-15. For some reason (most likely my cantankerous brain) the small chainring feels easier than the big one, even if it shouldn't. Am I imagining things?

Thanks,

Matt

PS: If I am totally misunderstanding how it all works, you can let me know that as well! But I believe that multiplying the above choices and getting 600 for each of them means that they should have the exact same mechanical advantage, at least in theory. Is that correct?
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Old 09-09-06, 07:44 PM
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You've got it backwards. Divide the smaller (cog) into the larger (chainring) number. The ratio there is what you want. The 40/15 will be easier to push since the final number is smaller
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Old 09-09-06, 07:46 PM
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Ratios require division, not multiplication...

53/25 = 2.12
39/19 = 2.05

Very close ratio, meaning nearly the same required effort...
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Old 09-09-06, 07:49 PM
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Good, Point
It is the ratio that is important, not the product. 50/15 would be equivalent to 40/12.
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Old 09-09-06, 07:52 PM
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In a race where fast shifts are important, you often choose your current gear based on where you want to shift to next. You need to be in your big ring before the sprint starts, etc.
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Old 09-09-06, 07:54 PM
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Then there's the big ring placebo effect, where one gear in the big ring feels faster than the gear of the identical ratio in the small ring.
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Old 09-09-06, 08:40 PM
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THAT'S what I'm talking about! Yes, I had my numbers backwards in the first post (sorry!), but is there such a thing? It sure seems like the same ratio, but in the big ring, feels faster/harder to climb, than in the small ring. Glad it's not just me!

Matt
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Old 09-09-06, 08:48 PM
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Well for some gear combinations you will end up with a much better chain line on the smaller/middle ring than on the larger ring. The better chain line will mean less friction and it will be slightly easier to push. Say 52-19 compared to 42-15. The 42-15 is a fraction higher but will most likely feel better due to a better chain line. Its possible to work the other way around sometimes too but much less likely due to the small/middle ring having a better average chain line to start with.

Regards, Anthony
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Old 09-09-06, 08:49 PM
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Originally Posted by DrPete
Then there's the big ring placebo effect, where one gear in the big ring feels faster than the gear of the identical ratio in the small ring.
Don't believe it is placebo Doc. My opinion. It can be explained by the energy expended to bend the sum of the chain links. I perceive a difference as do many others. Not enough to run X-chained when in shorter gears of course.
George

Last edited by biker7; 09-09-06 at 09:18 PM.
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Old 09-09-06, 09:00 PM
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I'm sure he meant 40/12 = 50/15.
I've had better feelings about the larger ring when it was close, but if you think about it long enough, you can probably have a favorite sock.
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Old 09-09-06, 09:11 PM
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I surmise you will acclerate faster on a bigger ring than a smaller ring even with the exact same ratio. Seems like with one pedal stroke you would push more chain through the drive train.
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Old 09-09-06, 09:18 PM
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Originally Posted by godspiral
I'm sure he meant 40/12 = 50/15.
I've had better feelings about the larger ring when it was close, but if you think about it long enough, you can probably have a favorite sock.
Or a favored pant leg ;-)...lol.
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Old 09-10-06, 12:21 AM
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Given the same gear ratio, the gear combination with the bigger cog on the rear will bit a tad more efficient, given how the chain links are discrete and not a continuous belt.
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Old 09-10-06, 12:29 AM
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yeah, Chester Kyle and/or Ed Burke had written a paper about the frictional losses of rotating the chain more when wrapped around smaller rings/cogs, and the larger amount of rotation of the links causes higher friction.

Question would be which has more friction, lateral flexing of the chain due to crooked chain-line, or rotating the links around the pins more...

In the absense of friction, both big/small-ring gear combos of similar ratios should be similar.

Last edited by DannoXYZ; 09-10-06 at 01:51 PM.
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Old 09-10-06, 12:37 AM
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I think it's all psychologial. 75 gear inches = 75 gear inches, same effort. I've experimented this on my fixed gear bikes using different ring/cog combos and I sure couldn't tell the diff...
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Old 09-10-06, 12:51 AM
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Yeah, any 'frictional difference' is probably close to zip from a realistic standpoint. But johnny made a good point about choosing a ring based on where to shift next. If you're coming up on a hill at 20 mph, you'd best be in the smaller chainring. If you're getting ready to sprint at that same 20 mph, it'd be wiser to go intot he larger chainring.
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Old 09-10-06, 09:49 AM
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The larger chain ring combo compared to a same ratio small ring combo is more mechanically favourable for the chain [I think] - but someone said it.. 75 inches = 75 inches.
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Old 09-10-06, 01:13 PM
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Originally Posted by Jhague
I surmise you will acclerate faster on a bigger ring than a smaller ring even with the exact same ratio. Seems like with one pedal stroke you would push more chain through the drive train.
And pull the wheel around less far for the given amount of chain pulled through. It's simple math. Same ratio equals the exact same travel down the road per pedal revolution.
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Old 09-10-06, 01:54 PM
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Ah yes... forgot about that. Even though torque & thrust at the contact-patch would be the same, the tension on the chain and torque at the crank is higher in the small-ring. You won't accelerate any faster, but will put more stress on the parts. My small-ring always wears out faster than the big-ring.
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Old 09-10-06, 03:54 PM
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Originally Posted by frischtr
Ratios require division, not multiplication...

53/25 = 2.12
39/19 = 2.05

Very close ratio, meaning nearly the same required effort...
I've never paid any attention, but I have an engineering friend who always chooses the combination with the larger sprockets (in this example 53/25). His logic is that the greater lever arms result in less chain tension and consequent wear.
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Old 09-10-06, 04:11 PM
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Originally Posted by Retro Grouch
I've never paid any attention, but I have an engineering friend who always chooses the combination with the larger sprockets (in this example 53/25). His logic is that the greater lever arms result in less chain tension and consequent wear.
I agree for precisely the chain tension reason (and I am also an engineer), except that on my bike 53/25 is a very bad chainline (big front/biggest rear). I might ride in 53/25 for only a brief moment between other big chainring/cog selections, say like the last 25 yards up a hill, to avoid two double shifts close together in time.
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Old 09-10-06, 06:51 PM
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From:
https://www.ihpva.org/pubs/HP52.pdf
A newsletter featuring an article about bicycle transmission efficiency.


Derailleur gears
On the other hand, factors affecting
the efficiency of derailleur gears
become clear by examining the curves
in figures 10 and 11. For example, a
12-tooth sprocket seems to cause inefficiency.
In the Shimano 27-speed, gears
4, 9, 15, 18, and 24 have the lowest
efficiency. The two gears with the lowest
efficiency of the 15 tested, both
use a 12-tooth sprocket. The gears with
12-tooth sprockets (18, 24 and 27) have
an average efficiency of 91.2%, while
those involving 16-tooth sprockets (11,
20 and 25) have an average efficiency
of 93.5%.
Other gears
In the Browning, the 12-tooth sprockets
averaged 92.1% efficiency, while
the gears involving a 17-tooth sprocket
averaged 92.9%. The two lowest efficiencies
of the 12 gears tested had
12-tooth sprockets (gears 9 and 12).
Apparently the sharp angle of chain
link bend in the 12 causes increased
friction compared to larger sprockets.
So it appears that larger gears than 12
are necessary for efficient operation.
When there is a choice of gear ratios
that are close, cyclists should choose
the gearing combination with larger
diameters [8].
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Old 09-11-06, 07:02 AM
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Assuming you compare identical ratios, there are a few minute differences :

1. Difference in "efficiency" as stated above. Basically, it lies in the fact that the perfect cog would be round and a 11-teeth cog isn't round. So the chain jumps up and down between each cog.
The effect would be more important on tiny cogs like those found on a Shimano Capreo freehub. The smallest cog is 9-teeth.

2. Difference in chainline
Assuming you've got a triple with 44-33-22 chainrings and 12-13-14-16-18-21-24-28-34 (these numbers will work well in the example).
Using the 22/12, 33/18 or 44/24 will give you exactly the same development (3,96 m or 49,5 gear-inches).
In the first case, you'll have a very oblique chainline, with the chain rubbing against the 33 and a loose derailleur which won't keep the chain tightly onto the 12.
In the second case, you'll get an almost perfect chainline.
In the third case, you'll get a quite acceptable chainline, but the derailleur spring will be fairly tight, which adds a bit of resistance.

3. Difference in wear
Wear occurs for two reasons: an oblique chainline means an uneven traction force onto the pins between each chain link. The more oblique the chain, the faster it will wear out, especially if you ride in bad weather.
But also, the tension in the chain is higher on smaller rings. So while the end result of all the above three ratios is the same, there is twice more tension in the chain when riding in 22/12 than in 44/24. Twice the tension in the chain and half the number of teeth on the ring and cog mean much faster wear and tear.


Points 1 and 3 work against the more compact drivetrains we find nowadays. A 52/42/30 crankset with a 14/34 freewheel or cassette will last longer than a 46/36/26 crankset with a 11/28 cassette. But the compact drivetrain has a few advantages: less weight, wider ratios available and faster shifts. All in all, don't expect drastic differences.
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Old 09-11-06, 12:15 PM
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I used to try the smaller gears with the same ratio on the track bike to reduce weight but on a road bike I think the chain line makes the biggest difference, then the placebo effect that the big ring is faster.
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Old 09-11-06, 12:44 PM
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All of this brings to my attention that SOME OF YOU have been using the small ring

I won't tell you again.
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