# What size crank arms?



## dndrich (Feb 21, 2012)

Gonna build me a bike. So, I am ordering components pretty soon. How do I determine the right crank arm length? I know most bikes come fairly standard of the shelf with 172.5 mm crank arms. I am Joe Average at 5'9" with standard looking femurs...

Thoughts here?


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## Nicole Hamilton (Sep 5, 2010)

dndrich said:


> I know most bikes come fairly standard of the shelf with 172.5 mm crank arms. I am Joe Average at 5'9" with standard looking femurs...


That could be a little long for you. I'm 5'9" also, but I need a 170 crank. It really depends on your proportions. You could either get a fitting or simply try different bikes. If the crank is too long, it'll feel a little like you're having to slide from one side to the other as you peddle, a bit like having the seat too high.


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## FBinNY (Jan 24, 2009)

In my experience, your height or leg length is less important than riding style. Spinners tend to prefer the small circle of shorter cranks, while slower rpm pedallers prefer the leverage that longer cranks afford. 

At 5'9'' I wouldn't go longer than 172, and maybe stick with 170, because an overly long crank is more of a problem than a short one. When a crank is too long, the difference between the pedal height at the bottom of the stoke and the top is too much, forcing you to compromise between knees too high at the top, or reaching for the pedals at the bottom. Watch a child riding their first adult sized bike and you'll see the problem right away.


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## MerlinAma (Oct 11, 2005)

Go here and link to the 2/2/12 newsletter. 

February, 2012 | Road Bike Rider

There is an article on "Perfect Circles" which will give you some food for thought.

The suggestion "your height or leg length is less important than riding style" is good advice. 

I would also suggest that terrain may play a factor. I always wanted slightly longer cranks for climbing. I'm not really sure that you can totally divorce crank arm length from gearing, but again that gets back to riding style too.


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## dndrich (Feb 21, 2012)

MerlinAma said:


> Go here and link to the 2/2/12 newsletter.
> 
> February, 2012 | Road Bike Rider
> 
> ...


Very interesting article. Food for thought. Presently I think my crank arms are too long. I have 175 mm arms. My left knee does a figure 8. I will consider all of this. I live in a hilly area, and climb all the time. Despite that I think that shorter may be better. Hmm.


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## wim (Feb 28, 2005)

FBinNY said:


> Spinners tend to prefer the small circle of shorter cranks, while slower rpm pedallers prefer the leverage that longer cranks afford.


Not disputing this, but it's interesting to contemplate that at the exact same leg speed (say, 1.00 meter/second), your cadence with the shorter crank will be slightly higher than your cadence with the longer crank. So it _could_ just be plain ol' physics instead of preference.


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## FBinNY (Jan 24, 2009)

wim said:


> Not disputing this, but it's interesting to contemplate that at the exact same leg speed (say, 1.00 meter/second), your cadence with the shorter crank will be slightly higher than your cadence with the longer crank. So it _could_ just be plain ol' physics instead of preference.


I think you have the horse pushing the cart. Most riders develop a cadence first, independent of leg speed. At the same RPM spinners have less leg movement, but I don't think it's a speed issue as much as less vertical travel. 

Slower rpm riders tend to ankle more effectively lowering their leg's vertical travel, and keeping the knees lower at the top of the stroke. 

But crank length isn't rocket science, it's a question of learned pedaling habits, and what feels "right" for every rider.


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## Jay Strongbow (May 8, 2010)

MerlinAma said:


> The suggestion "your height or leg length is less important than riding style" is good advice.
> 
> too.


I don't see it that way. I think it's about being able to be comfortable at the bottom of your stoke while at the same time having no issues getting over the top and that's about leg length not style.

Anyway OP. I'm 5 8 and use 172. That's fine but if I had it to do over I'd get 170. The reason is that when I'm where I want to be with my saddle set to be perfect at the bottom of my stroke I have a little dead spot at the top because the up and over is a little more leg lift than my natural motion. By the same token if I'm perfect at the top I need to toe point a little at the bottom. Its not a big deal at all but when I really concentrate on my peddling I can tell 172 is less than Ideal for me. I'm not sure cutting off 2 mm would cure it either but I'm certain it's a change in the right direction. I'm only an inch shorter than you but my legs are short for my height so you might not have the same issue. Then again I'm extremely flexible so who knows what that means here or how you'd feel on 172 vs 170.


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## Guest (Feb 26, 2012)

I've used 175, 170, and 165 at one time or another. I'm 5'7" with 32" cycling inseam.

I really didn't like the 175 at all, it was noticeably too long. Both the 170 and 165 are comfortable for me. there's probably a range of crank lengths a particular person will be comfortable with

I use 170 on my geared roadbike, 165 on my singlespeed/fixed gear bike. The choice to go short on that was both out of fear of pedal strike (when running fixed, which as it turns out I rarely do), and to make high-rpm spinning more comfortable since that's the only way I can go faster without multiple gears. On my road bike my overall average cadence is 97rpm (including climbs, starting from stop signs etc), and is usually near 110 when going fast, but never above 125 (41mph in 50/11). On my singlespeed though I will sometimes spin 150rpm+ for short stretches (>28.7mph in 39/16), and in those situations the shorter crank length is definitely noticeable.


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## Kerry Irons (Feb 25, 2002)

*Some facts*



dndrich said:


> Gonna build me a bike. So, I am ordering components pretty soon. How do I determine the right crank arm length? I know most bikes come fairly standard of the shelf with 172.5 mm crank arms. I am Joe Average at 5'9" with standard looking femurs...
> 
> Thoughts here?


The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
=============================
Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.

So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.

Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing.

All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.
=============================
Jim Martin tests as reported in VeloNews: 16 bike racers of various heights doing maximal sprint power tests of under four seconds duration on cranks of 120, 145, 170, 195, and 220mm showed no statistical difference between crank lengths. Seat height to the pedal was maintained throughout, but fore-aft saddle position and handlebar height were not readjusted with crank length changes, despite variations with crank length of pedal-to-knee relationship and saddle-to-bar drop. This also led to Martin’s assertion that he could see no point to positioning the knee over the pedal spindle.

Further Martin tests showed no statistical relationship between metabolic cost and either pedaling rate (RPM) or crank length, using nine trained cyclists riding 145, 170 and 195mm cranks who pedaled at 30-, 60-, and 90 percent of their lactate threshold at 40, 60, 80 and 100 RPM. On the contrary, power output and pedal speed (pedaling rate times crank length), accounted for over 98 percent of the variation in metabolic cost.

In another test, Martin had 10 racers perform a 30-second maximal sprint on 120mm and 220mm cranks at 135RPM for the 120mm and 109RPM for the 220mm. he found that, while the rate of fatigue was less for longer cranks, the fatigue per revolution was identical. This led him to suggest that track sprinters, rather than spinning at high RPM, should select the gear at or just below the one at which they produce maximum power output. The higher gear, as fatigue per revolution would be constant, would get the rider to the finish sooner, as fatigue would take more time to set in.

Finally, Martin’s studies of pedaling technique indicated that regional cyclists had “better” pedaling mechanics than elite cyclists. It indicated that elite riders pull up less on the pedals on the backstroke and push down harder on the downstroke.


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## wim (Feb 28, 2005)

FBinNY said:


> I think you have the horse pushing the cart. Most riders develop a cadence first, independent of leg speed. At the same RPM spinners have less leg movement, but I don't think it's a speed issue as much as less vertical travel..


Sorry I wasn't clear. By "leg speed" I meant the velocity at which the foot travels around the circle. _If that velocity is unchanged_, shorter cranks will always produce a higher cadence, longer cranks a lower cadence. This has nothing to do with "developing" something, it just happens.


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## FBinNY (Jan 24, 2009)

wim said:


> Sorry I wasn't clear. By "leg speed" I meant the velocity at which the foot travels around the circle. _If that velocity is unchanged_, shorter cranks will always produce a higher cadence, longer cranks a lower cadence. This has nothing to do with "developing" something, it just happens.


I understood you and we agree about the concept. It's just that I believe that the cadence habit is more ingrained than the leg speed habit, so it's cadence first, and more or less speed to make up the difference in circumference. 

But it's just as possible that it's the opposite. I guess we'd need an exercise physiologist to render an authoritative opinion.


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## Nicole Hamilton (Sep 5, 2010)

PhotonFreak said:


> On my road bike my overall average cadence is 97rpm (including climbs, starting from stop signs etc), and is usually near 110 when going fast, but never above 125 (41mph in 50/11). On my singlespeed though I will sometimes spin 150rpm+ for short stretches ...


Oh, to be young again.


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## wim (Feb 28, 2005)

FBinNY said:


> It's just that I believe that the cadence habit is more ingrained than the leg speed habit, so it's cadence first, and more or less speed to make up the difference in circumference.


After a visit to my book shelf, looks right you're right and my egg-before-the-chicken comment has little or no merit. The French rider and coach Cyrille Guimard is quoted in Bernard Hinault's cycling book thus: 

_They often say that a young rider, even if he's tall, should use 170 mm because he doesn't need big gears. I think exactly the opposite. It's when you're young that you acquire the technique of pedaling, repeated hundreds of thousands of times. Therefore, you should use the cranks that are best for your height from the start of your career and spin as if it were nothing. [...] The question of gears has nothing to do with it. The point is to learn to spin._


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## looigi (Nov 24, 2010)

With respect to cadence being ingrained: I laid off cycling for 3 years or so. When I came back my avg cadence according to Garmin was in the low 70s. In two years of regular riding it increased linearly to the low 90s now. I don't know what my avg cadence used to be but I imagine it was probably near 90 when I was riding regularly. This mirrors what I've seen in others, that is people who don't ride a lot or frequently tend to have low cadence and as they gain experience, their cadence tends to increase significantly. 

On a bit different note, here's an interesting study, FWIW: http://www.plan2peak.com/files/32_article_JMartinCrankLengthPedalingTechnique.pdf


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## dndrich (Feb 21, 2012)

*Wow!*

I am the OP, and I am delighted by the tenor of this discussion. I had no idea what a hornet's nest I kicked with my original question! Keep it up guys and gals. Based on what I have heard here, I think I will start with 170, and see how it goes. As a recreational cyclist, that will probably meet my needs. My current cranks are 175, and they just feel a bit too long, especially with the upstroke.

This is a great forum.


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## Kodi Crescent (Aug 3, 2011)

I'm 5'8" and had 175's and switched to 170's. More comfortable for me, but meaningless for you as our bodies and needs are not the same. But you should give the 170's a try.


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## sneakyracer (Dec 1, 2007)

dndrich said:


> Gonna build me a bike. So, I am ordering components pretty soon. How do I determine the right crank arm length? I know most bikes come fairly standard of the shelf with 172.5 mm crank arms. I am Joe Average at 5'9" with standard looking femurs...
> 
> Thoughts here?


I would not think much of it. Just get the 172.5mm cranks. If you were very short or very tall then I would tell you to get a different size. That not being the case just make sure you are properly fitted on your bike. That does make a significant difference in comfort and overall performance/enjoyment. MUCH more than a 2.5mm difference in crank length. 

I am 6'-2" and use 165mm on my fixed gear and 175mm on my mtb and road bikes. I have not had any issues with either in thousands of miles of riding.


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## Nicole Hamilton (Sep 5, 2010)

sneakyracer said:


> I would not think much of it. Just get the 172.5mm cranks. ... I am 6'-2" and use 165mm on my fixed gear and 175mm on my mtb and road bikes. I have not had any issues with either in thousands of miles of riding.


Uhm, he's 5'9". You're 5" taller. You're big enough you could reach anything. Are you really in a position to advise what crank arm he needs? At least I'm the same height and even I admit that while 170 works for me and 172.5 is too long (and I know that from trying it), his proportions could be different from mine. They're definitely different from yours.


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## Seneb (Sep 29, 2009)

FWIW, I'm 5'8" and ride 172.5 cranks and have been for 20 years. I spoke with a professional fitter (he teaches Specialized employees and has worked with top pro teams) on Saturday about cranks and he said that it doesn't matter at all. It's whatever makes you comfortable and nobody can tell you what crank length you should ride. The discussion came up because I mentioned that most stock bikes my size come with 170s and the next size bikes have 172.5s. He told me about a study that tested a range of crank lengths from way outside the normal range (something like 140 to 200) and the variance was minimal. I wish I knew where the study came from, but unfortunately don't. I'm guessing that I like the 172.5 because that's what my first road bike came with, I didn't know different lengths existed, and it was what became normal to me.


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## dndrich (Feb 21, 2012)

Seneb said:


> FWIW, I'm 5'8" and ride 172.5 cranks and have been for 20 years. I spoke with a professional fitter (he teaches Specialized employees and has worked with top pro teams) on Saturday about cranks and he said that it doesn't matter at all. It's whatever makes you comfortable and nobody can tell you what crank length you should ride. The discussion came up because I mentioned that most stock bikes my size come with 170s and the next size bikes have 172.5s. He told me about a study that tested a range of crank lengths from way outside the normal range (something like 140 to 200) and the variance was minimal. I wish I knew where the study came from, but unfortunately don't. I'm guessing that I like the 172.5 because that's what my first road bike came with, I didn't know different lengths existed, and it was what became normal to me.


That is what it is sounding like. I will likely go with 170.


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## Kerry Irons (Feb 25, 2002)

*Wishing*



Seneb said:


> He told me about a study that tested a range of crank lengths from way outside the normal range (something like 140 to 200) and the variance was minimal. I wish I knew where the study came from, but unfortunately don't.


You don't have to wish - you could read my post.


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## Nicole Hamilton (Sep 5, 2010)

Seneb said:


> I'm guessing that I like the 172.5 because that's what my first road bike came with, I didn't know different lengths existed, and it was what became normal to me.


Possibly. Or maybe you chose that bike because even though you didn't know much about bikes, you at least knew that one felt best. And maybe the reason was because you really are more comfortable on 172.5 arms.


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## Trek2.3 (Sep 13, 2009)

I'm 5'4" and I can ride with 170mm crank arms but the pedaling is more comfortable with 165's.

Like everything about bicycles, for some 5mm matters while for others 20 mm doesn't.

It's people at the edges (5'4" vs 6'4") who are most likely to feel a difference. If you are 5'9"-5'10" you can ride almost anything.


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## PlatyPius (Feb 1, 2009)

The correct crank length (for me) is:

a) which one is on sale
b) Which one they have in stock

I've ridden 165, 170, 172.5, 175, and 180mm cranks. Between 170 and 175, I can't tell a difference. 165 feels a little too short and 180 feels a little too long. The others, I don't care which I get - so I look for daily deals or specials from my distributors.


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