# 170mm/175mm crank arms



## lambdamaster (Oct 6, 2009)

Hi all,
I have very recently purchased a 52" Specialized road bike with 170mm shimano cranks. I ride 17"/size medium mountain bikes with 175mm cranks (I am about 5'9, no idea on inseam but I was fitted to a 52cm at the shop), and I was wondering if I should consider 175mm arms for the road bike.I don't know how to tell if 170mm is the right length for me on the road bike.


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## Hank Stamper (Sep 9, 2009)

I don't really know squat about this but what I read was that rule of thumb is multiply your inseam (inches) by 5.48 to get a good guess in mms.
There are personal preference factors but generally speaking 175 is probably way to big for you.
I think that's the biggest shimano makes. If that was right for a 5 9 guy or a 52cm bike I doubt that would be the biggest they sell.


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## JCavilia (Sep 12, 2005)

Ride it for a while and see if it seems to be a problem. It's only 5mm. ----- Some people say it's crucial and they can sense tiny differences, and others say it's easy to get accustomed to anything in the normal range, and they can't really tell much difference. Personally, I'm in the latter camp. 

As for the MTB-to road conversion, even if 175's are "perfect" for you on the MTB, the slightly shorter cranks may be just fine on the road, where you tend to spin more.


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## PJ352 (Dec 5, 2007)

lambdamaster said:


> Hi all,
> I have very recently purchased a 52" Specialized road bike with 170mm shimano cranks. I ride 17"/size medium mountain bikes with 175mm cranks (I am about 5'9, no idea on inseam but I was fitted to a 52cm at the shop), and I was wondering if I should consider 175mm arms for the road bike.I don't know how to tell if 170mm is the right length for me on the road bike.


This is a question to ask at the shop where you were fitted. You haven't provided enough info for us to know, but for someone of your height to be fitted to a 52 cm tells me that you're proportioned towards long legs/ short torso (because the other way 'round you'd be running a _long_ stem), so you _might be _a candidate for longer cranks (not sure about 175's, though). This, of course, assumes you were correctly sized.

That said, I think of that adjustment (if needed) as more of a 'tweak', so none of this would stop me from riding with the current setup.


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## Mr. Versatile (Nov 24, 2005)

I’ve got 3 bikes. One has 175mm cranks, the other 2 have 170s. I can’t tell the difference even when jumping immediately from one bike to another. I forget who it was, but a long time, well respected poster on RBR accidently put 2 different crank lengths on his bike. He was overhauling his bike and didn’t look closely. He rode it that way for a year and only discovered it when he overhauled his bike again. He couldn’t tell the difference.

For additional info there are many crank length threads on these forums. Do a search and you’ll get more info than you ever thought possible


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## Kerry Irons (Feb 25, 2002)

*Not formulaic*



lambdamaster said:


> I have very recently purchased a 52" Specialized road bike with 170mm shimano cranks. I ride 17"/size medium mountain bikes with 175mm cranks (I am about 5'9, no idea on inseam but I was fitted to a 52cm at the shop), and I was wondering if I should consider 175mm arms for the road bike.I don't know how to tell if 170mm is the right length for me on the road bike.


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.


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## electech (Aug 7, 2009)

Being a novice, I have always wondered why in the world they don't just make one length anyway, especially if most can't tell any difference. What does it affect anyway?


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## tommyrhodes (Aug 19, 2009)

On a side note, I've got big feet and I tend to toe my front tire with my 175's. I always believed that switching to a smaller crack would decrease top speed while increasing my climbing ability. Is this no longer the common conception?


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## newmexrb1 (Aug 16, 2009)

electech said:


> Being a novice, I have always wondered why in the world they don't just make one length anyway, especially if most can't tell any difference. What does it affect anyway?


Leverage. Longer the lever the more the force applied is multiplied. 170 to 175 is what about 2% difference? Being that it is all part of the drivetrain, 2 percent is way less than the total 60 to 70 percent difference available in gear ratios on most 20 speed bikes. But it is part of the equation that relates speed to muscular force.

My guess is one _would _notice on a moderate grade at a cadence of 75 when going instantly from one crank to the other. Otherwise not. Lots of studies that purportedly prove that because of the way muscles work, there ain't cheating physics. Your max power output is unaffected by crankarm length, within reasonable limits.

General consensus best I can tell is if you have knee issues, go with a shorter arm. The logic is currently eluding me, but I have old knees so I went with 170 (in transit). You'll see many anecdotal reports of so and so resulted in huge improvements whether power, comfort, whatever--it seems completely reasonable that to keep things biomechanically similar, a longer legged rider needs a longer crank. Just look at the joint angles involved and the mechanical advantage of various muscle actions/insertions. If one is 10' tall spinning a 100mm crank, the angle of the knee will hardly vary. There must be an optimal range thru which the angle varies....Happily it doesn't seem to make a helluva lot of difference for most of us.


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## Mapei (Feb 3, 2004)

I'm sensitive to crank arm length. I ride 52 cm bikes. My coffee bike has 170's and it sometimes makes me feel a bit like a hamster. My mountain bike has 175's. I appreciate the leverage in sticky situations, but sometimes the cranks make me feel as if I'm doing deep knee bends. (Okay, I'm exaggerating, but there's still more than a grain of truth here.) My modern, go-to racing bike has 172.5's. They're just right...


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## newmexrb1 (Aug 16, 2009)

Exactly, every one has a range of motion that works best with their muscles and anatomy--some are real sensitive to it, others not. But if hamstering, go up a gear? Whats the diff unless you're so attuned to a certain chainring/sprocket set that it never feels right. Don't mean to challenge your perception, just understand it...


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## Mapei (Feb 3, 2004)

newmexrb1 said:


> Exactly, every one has a range of motion that works best with their muscles and anatomy--some are real sensitive to it, others not. But if hamstering, go up a gear? Whats the diff unless you're so attuned to a certain chainring/sprocket set that it never feels right. Don't mean to challenge your perception, just understand it...


It's not the gearing that engenders the hamstering or clodhopper effect. It's the size of the circle that I must pedal through. The low point in the revolution will always be the same, of course, no matter what the length of the crank arm may be. But the other three cardinal points will all feel (as well as be) different. The diameter of the pedal stroke affects the amount of flex my knees must negotiate, forward, backward, and upward. It affects the leg angle at which you apply most of your power. Turning to another point, because KOPS will vary, it will alter saddle positioning, which in turn alters front-back position on the bicycle.

Certainly, the centimeter difference in the diameter of the pedal stroke between the 170 and 175 cranks doesn't seem like a lot. But remember, on a bicycle, small differences can be significant. Bicyclists and bicycle makers agonize over centimeter differences in top tube lengths. Half-degree differences in head tube angle are enough to alter handling characteristics. Riders fine-tune saddle height. They swap stems. They experiment with bar drop.

Finally, is it possible that shorter people are more sensitive to these variables than taller folks? After all, the taller the rider, the lower the percentage of change will be when these measurements are altered.


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## newmexrb1 (Aug 16, 2009)

Mapei said:


> It's not the gearing that engenders the hamstering or clodhopper effect. It's the size of the circle that I must pedal through. The low point in the revolution will always be the same, of course, no matter what the length of the crank arm may be. But the other three cardinal points will all feel (as well as be) different. The diameter of the pedal stroke affects the amount of flex my knees must negotiate, forward, backward, and upward. It affects the leg angle at which you apply most of your power. Turning to another point, because KOPS will vary, it will alter saddle positioning, which in turn alters front-back position on the bicycle.
> 
> Certainly, the centimeter difference in the diameter of the pedal stroke between the 170 and 175 cranks doesn't seem like a lot. But remember, on a bicycle, small differences can be significant. Bicyclists and bicycle makers agonize over centimeter differences in top tube lengths. Half-degree differences in head tube angle are enough to alter handling characteristics. Riders fine-tune saddle height. They swap stems. They experiment with bar drop.
> 
> Finally, is it possible that shorter people are more sensitive to these variables than taller folks? After all, the taller the rider, the lower the percentage of change will be when these measurements are altered.


I'm with you on all points--you expressed the reasons for potential differences in perception very well.


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## wim (Feb 28, 2005)

Mapei said:


> The low point in the revolution will always be the same, of course, no matter what the length of the crank arm may be..


That's true for all those who change saddle height by the crank length difference when going to longer or shorter cranks. But as an exception, there are riders who are very sensitive to the _high_ point in the revolution, with some of them not changing saddle height at all, others splitting the crank difference in half and applying that dimension to the saddle height change. These exceptions, of course, just confirm what you say in your excellent post about the significance of small differences.


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## PJ352 (Dec 5, 2007)

I'll add a +1 for your efforts as well. Very well (and clearly) put. This topic reminds me of dropping the pebble in still water and watching the ripple effects, as it relates to bike fit. A mm change here, 2 mm's change there..


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## lambdamaster (Oct 6, 2009)

I guess it's more about feel than anything??
I will ask the shop if they have any bikes with longer arms that fit me to try out.


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## waldo425 (Sep 22, 2008)

At that difference you wont really notice much of anything. I go from 175s to 160s all the time during the road season and never have a problem. It just takes a bit of time and going "hey that feels funny" and then you get used to it. But thats just my experience really and I can adapt pretty quick to new bikes and gear.


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## Mr. Versatile (Nov 24, 2005)

??160s??


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