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Trying to learn more about bike fit and gearing, and having trouble understanding the effect of crankarm length on gearing. In an article on bicycling.com it states

"The relative jumps between gears stay the same, but the overall range will feel slightly easier to pedal, or smaller, with shorter cranks and harder with longer ones."

It makes sense to me that crankarm length would have an effect on how hard it feels to spin a certain ratio, but instinctively I would assume the opposite effect described here -- that a longer arm would give more leverage and make the overall range feel lower.

https://www.bicycling.com/bikes-gear/components/what-you-need-to-know-about-crankarm-length

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    The physics say short cranks produce less torque so will need to run a lower gear / higher cadence to maintain the same power output, opposite to this. If you look at the rider - shorter cranks tend to allow the rider to spin smoother, so for untrained riders with poor stroke, this will lift cadence and be more efficient. Crank length is the one remaining aspect of cycling that's 'one size fits all' - Bikes fit is tuned to the mm, but cranks come just two common sizes - 170mm for MTB and 175mm for road. – mattnz Mar 27 '18 at 19:56
  • Another common size nowadays is 172.5mm. for road-bikes that when compared to 175mm allows either for a lower BB and better aerodynamics due to a more compact frontal surface or with a standard BB-height for a deeper cornering angle. – Carel Mar 28 '18 at 15:20
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    @mattnz: "just two common sizes - 170mm for MTB and 175mm for road* which isn't entirely true, because cranks come in many different sizes from 155mm to 180mm for one example if you look at Spécialités TA's website. – Carel Mar 28 '18 at 15:28
  • All mainstream brands (Shimano, Sram, Campy, FSA) at least make 170, 172.5, 175 in all models now. – Syl-bonk Mar 28 '18 at 16:11
  • @Carel - By 'common' I mean when buying a new bike, your average LBS does not look at you lie your an alien if you ask to have a different sized crank from the one installed on the bike. – mattnz Mar 28 '18 at 22:17
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I think the article you reference is in error. Obviously a longer crank arm gives more leverage, requiring less pedaling force to move the bike but requiring a faster pedaling cadence the pedal to moved faster around a larger circle (same angle/unit time, more distance/unit time). The rider's leg must therefore extend further and more quickly.

  • A pedalling cadence of 80 RPM for instance will produce 244.7 RPM on the rear wheel with a 52/17 gear ratio and 30.8km/h with a roadbike, independently from the length of the crank. With 170mm cranks each foot will travel 42.7m per minute and 43.96m with 175mm cranks. The angular movements of hip, knee and ankle joints are smaller with shorter cranks because the circle described by each foot is smaller, requiring a lesser extension/contraction of the active muscles of the legs. – Carel Mar 28 '18 at 17:58
  • Anyone care to explain the downvote? I don't think my answer is incorrect, just short. – Argenti Apparatus Mar 28 '18 at 18:28
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    There's no influence on the cadence. The cadence is all a matter of gear ratio.The lesser pedalling force doesn't come up in the calculation. If you keep the same gear with either short or long cranks the RPM of the rear wheel is the same. – Carel Mar 28 '18 at 19:17
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    Ahhh yeah, you are correct. I guess what I meant to say was faster pedal speed – Argenti Apparatus Mar 28 '18 at 19:20
  • Although with a shorter crank you might not be able to run the 52/17 gear of the example because of the lesser leverage and you'll have to use a larger rear cog and a higher cadence to go at the same speed. – Carel Mar 29 '18 at 20:14
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The difference between a 170mm and a 175mm crank is about 3%. That will also be the difference of the leverage, so probably barely perceptible.

However the impact they may have on the pedalling will depend on anatomical differences, mainly on the length of legs as a shorter crank reduces the circumference of the pedal circle which in return affects the angles of the joints in hips, knees and ankles. The general recommendation is that shorter legs ask for shorter cranks. Taller riders will need longer cranks. A serious bike-fitter should find the best crank-length for the individual rider.

  • 5 in 170 is about 3%, not 1% – Chris H Mar 31 '18 at 12:24
  • @ChrisH: You're right. – Carel Mar 31 '18 at 13:49
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Shorter cranks increase gearing 3% for every 5mm shorter. Mine are 150mm and I struggle to get low enough gears I have 12 to 32 cassette and even 32 is not low enough, my granny ring is 42 teeth. Shorter is better with no loss of power, because a bent knee is the weakest, try doing half squats instead of full ones. Also you get less injury's and pain with shorter cranks

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    I'm confused. Struggling, i.e., over-exerting yourself by being over-geared, is exactly what leads to injuries. You say that your shorter cranks give you less injury and pain but that you're constantly struggling with being over-geared, which seems to be pretty close to a contradiction. – David Richerby Jun 10 '18 at 10:38
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All the responses answer the question, but if you need help understanding what the ratios mean as they relate to riding, I would recommend heading over to Sheldon Brown's gear calculator. It is very simple to use, just select your crank length, wheel size, chain rings and cassette, and it spits out a table of ratios for each combination.

In particular, I would suggest finding your gain ratio using the calculator. He invented the formula, and in my opinion, it is the superior method of using bicycle gear ratios in the real world. He describes it in an example he gives:

This number is a pure ratio, the units cancel out. I call this a "gain ratio" (with thanks to Osman Isvan for suggesting this term.) What it means is that for every inch, or kilometer, or furlong the pedal travels in its orbit around the bottom bracket, the bicycle will travel 5.58 inches, or kilometers, or furlongs.

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