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I'm a road and mtb regional racer about to have a go at my first road endurance nationals very soon.

I'd like some educated scientifically validated answer on a matter that has been on my mind ever since I got a new bargain race bike that unfortunately came with a compact crankset. I actually went from 39/53 + 12/28 10speed to 34/50 + 11/28 11speed.

On paper I'm a lot better off with the new setup, since I'm using the exact same spacing on the cassette with the benefit of the 11t cog i didn't have, which, even though doubled with a compact gearing upfront, ends up giving me a lot more headroom in bottom and even a tad bit more top speed.

My coach has always insisted that even on same gearing ratios, when compared, a larger crank ring with a smaller cassette cog end up giving you higher leverage and ultimately better power transfer or whatever mechanical benefit. Last Saturday, trying out hard climbs on fresh legs with about same gear ratios on the 50 vs my 34, actually felt less efficient on the 34 for some reason, mind you same cadence, intensity, gradient.

What's your take on all this? If the math says gear ratios are same, are the actual ring and cog sizes irrelevant? Is it actually beneficial to switch to my standard crank due to increased leverage?

Thanks a lot, lads!

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    I would put it down to placebo effect. You want the 50/34 to be more efficient because your a "real" cyclist who can push 53 all day, not a wimp on a compact with a piddly 50. As they say, its not the size that matters, its how you use it. – mattnz Jun 15 '15 at 9:32
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    @mattnz might be placebo effect indeed, but you're judging me and my genuine objective interest solely based on your apparent predesposition on racers feeling all mucho and wanting big gears to feel superior. Does not apply in this case, I'm that guy that coasts at 100 cadence and climbs at 80-90, I like to spin my legs and for this reason the new compact setup. But I also need to know what's actually mechanically optimal. – maninak Jun 16 '15 at 9:28
  • Sorry - I re-read my comment and agree it could have been better worded ....I was not meaning you personally, just the roadie community as a whole. (MTB's don't debate cranks, they have other, equally immeasurable things to focus their attention on) – mattnz Jun 16 '15 at 21:36
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    A gear ratio is a gear ratio. For a given ratio measured in "gear inches" (which is the diameter of the wheel on an equivalent penny-farthing) you get the same effect regardless of the specific sprockets you use. The only other factor when comparing two bikes is the length of the crank arms. (But note that a "larger crank ring with a smaller cassette cog" is not the same gear ratio -- you'd only get the same ratio if both sprockets were made larger or smaller in sync.) – Daniel R Hicks Jun 20 '15 at 12:12
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While it's true that a larger chainring will have slightly less friction and thus slightly more efficiently, it's an extremely negligible amount, that you wouldn't be able to feel. More importantly, are your crankarm lengths different on the new crankset than the old? That makes a much bigger difference. For example, if you've went from 175's to 170's, that will be a noticeable difference that could more likely explain what you're feeling.

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  • Nope, never ridden anything other than 172.5mm crank arms. – maninak Jun 23 '15 at 17:36
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Talking about science, there are two factors here:

  • moment of inertia - smaller and lighter cogs in a compact crankset mean smaller moment of inertia, which in turn means that you need smaller force to obtain the same acceleration. In this aspect a compact crankset gives a slight advantage. Note: this parameter is practically insignificant when it comes to maintaining pedaling speed - it's all about acceleration.
  • drivetrain efficiency - I've already written a post on it somewhere, but in short: with the same gear ratio drivetrain efficiency improves with larger rear cog (lighter gear at the back). Since with a compact crankset you need relatively smaller cog at the back for the same gear ratio, a it will have a slight disadvantage.

To see the overall effect, we'd have to make calculations for each crankset, power input and gear ratio separately, but I'd argue that drivetrain efficiency will be a little more important here (it's due to friction force, which is there all the time), so a compact crankset could be a "slower" option. However the differences in both cases will be barely noticeable on the road, and even less noticeable off-road. Or at least placebo effect can give a feeling of much larger difference than even extreme cases of the above factors - if you didn't know which one you're using, you probably wouldn't be able to feel the difference. So unless you're fighting a Marginal Gains war in a world tour peleton, it's not a thing worth worrying about.

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    For his test, the moment of inertia would have been the virtually identical as he was using the same crank set (The chain moving between rings being the only change). – mattnz Jun 15 '15 at 20:22
  • @mattnz You're right, in this experiment the moment of inertia is the same and plays no role at all. I focused more on the standard-compact crankset comparison and hence the note on moment of inertia. – Slovakov Jun 15 '15 at 21:20
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    @Slovakov Thank you for your response. Can you further explain why, on your second point, a smaller cog is slightly disadvantageous? Or possibly a link to that former post of yours? If the gains are marginal for any kind of matter I'm not going to get in the hassle since there are bigger matters affecting my performance to work on, but I always want to be in the know and aware of them, being that CS guy that is always curious about how stuff works. – maninak Jun 16 '15 at 9:37
  • @maninas I'm not sure if the physical cause of decreased efficiency with smaller cogs was examined. One theory that appears quite likely is that when chain is held by more teeth it decreases load on links and thus less energy is lost due to chain stretching. But it's only one of theories and I don't know if any of them has been prooved. Here's the link to my previous post with some more references: bicycles.stackexchange.com/questions/26040/… – Slovakov Jun 16 '15 at 11:20
  • The moment of inertia of the drive train is trivial and can be ignored outside of the laboratory. – Daniel R Hicks Jun 20 '15 at 12:14
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The larger chainring reduces drive chain friction - so is more efficient. This article from cyclingtips lists a study which demonstrated this

You can see the effects of this in the peleton where riders like Froome are favouring assymetric rings to give them periods of greater efficiency and reduced torque during pedalling

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    Asymmetric rings introduce too many variables to be considered as an example i.e. dynamic gear ratio phasing, which by definition constantly changes the leverage anyway. – maninak Jun 16 '15 at 15:14

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