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Discussion Starter · #1 · (Edited)
Gidday guys.

I have been riding 175mm cranks all my life. The first bike I bought had them, and not knowing any better, any other bike I bought had them, and bike shops just said "you are 6'1", it's what you need". Any groupset I bought had them, because it's what I though I needed.
Now that I am getting older, I am finding my knees are getting sore after a ride, so started researching via the good old internet and questioning if they were the right length.
With a 860mm inseam, all the research I did indicated 172.5 was the optimum length for me, so I took the plunge and got a set of Ultegra R8000, 52-36 in 172.5.
I really wasn't expecting to feel any difference with a change of only 2.5mm, and really thought I had just wasted my money, but wanted to give it a try.
Well, I was pleasantly surprised. As well as my knees feeling a lot better, it just felt smoother. My 175's didn't feel bad to turn in any way, but the 172.5's just feel smoother turning them.
Anyway, I know it's very subjective, and there are multiple formulas and opinions, and what worked for me, may not work for others, but just sharing my experience.

I wasn't expecting any noticeable difference, but I am glad I was willing to spend a relatively small amount of money to be a lot more comfortable on my bike. I didn't make any other setup changes.

Regards to all.
 

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Holy Cow, my knees just started hurting about a month ago. My inseam is way shorter than yours and running 172.5 I think. I'm ready to try anything before I stop riding, which I have thought about.
 

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Discussion Starter · #4 ·
you should have gone 170mm, it's even better
Yeah, I did think about 170, but as I said, pretty much all the "formulas" and opinions I could find steered me towards the 172.5. I may try them later, but I have only done 2 short rides (25 miles) with the 172.5's so far so I will run them for a while to get used to them before trying shorter again.
 

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Discussion Starter · #5 ·
Holy Cow, my knees just started hurting about a month ago. My inseam is way shorter than yours and running 172.5 I think. I'm ready to try anything before I stop riding, which I have thought about.
I reckon give 165 a try, or at most 170. I was surprised how much better I felt.
 

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For a few years, I'd develop iliotibial band syndrome (ITBS) in my left knee around mid season (fair weather rider here). My legs are slightly different lengths. After countless adjustments to seat and cleat positioning -- the final result of which helped a bit -- it was when I replaced my 172.5 with 170 that the left knee problem disappeared and has not returned. I highly recommend at least trying 170.
 

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Yeah, I did think about 170, but as I said, pretty much all the "formulas" and opinions I could find steered me towards the 172.5. I may try them later, but I have only done 2 short rides (25 miles) with the 172.5's so far so I will run them for a while to get used to them before trying shorter again.
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.
 

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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.
Performance is one thing. I believe @wettek is speaking from an entirely anatomical perspective. IOW, if something hurts, then you change something and it no longer hurts, as long as it doesn't complicate something else, the change is worthwhile IMO.
 
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Wettek , What type of cadence do you typically ride at ? I researched this a few years ago and never took the plunge to buy shorter cranks .I have always rode 172.5 and my research indicated I should be on 165 cranks , I am a spinner and find anything under 95 for a cadence feels like I am lugging . I too as I age find my knees aching more .
I would be interested in any other people to chime in on this topic . Thanks
 

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There are all kinds of formulas published / discussed out there. Some are based on femur length, some are on shoes size, overall leg length, overall height, ...etc. I plugged in my numbers and the crank length results ranged from 175mm to 170mm which is basically everywhere, which means nowhere. :rolleyes:

What did I do? I bought 3 different lengths and tried it myself over a period of about a year and settled with one that I felt the most comfortable with. It is an expensive process.
 

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For a few years, I'd develop iliotibial band syndrome (ITBS) in my left knee around mid season (fair weather rider here). My legs are slightly different lengths. After countless adjustments to seat and cleat positioning -- the final result of which helped a bit -- it was when I replaced my 172.5 with 170 that the left knee problem disappeared and has not returned. I highly recommend at least trying 170.
IT band syndrome won't be solved with cleat position or crank length change. It may alleviate tiny bit but if it got rid of it, it may have been something other that IT issue because the pain from it isn't only in pedaling moves.
 

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IT band syndrome won't be solved with cleat position or crank length change. It may alleviate tiny bit but if it got rid of it, it may have been something other that IT issue because the pain from it isn't only in pedaling moves.
Whatever it was presented as band syndrome, as diagnosed by me using illustrations i found online. May have been something else related to leg length differences. Whatever the case, I went from barely being able to ride short distances due to intense pain along the band on the outside side of my left knee... to being able to ride without any knee pain. The 170 cranks + cleat position + seat adjustments made all the difference in the world. For me.
 

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There are all kinds of formulas published / discussed out there. Some are based on femur length, some are on shoes size, overall leg length, overall height, ...etc. I plugged in my numbers and the crank length results ranged from 175mm to 170mm which is basically everywhere, which means nowhere. :rolleyes:

What did I do? I bought 3 different lengths and tried it myself over a period of about a year and settled with one that I felt the most comfortable with. It is an expensive process.
Just curious what is your height and inseam and what length did you settle on and what was the perceived difference between the 3? Cost is why I have never taken the plunge but I am seriously considering it .
 

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My inseam is 32" and have little longer than average femur length. Some formula suggested 175mm crank length. It wasn't comfortable for me. There is a slight difference in feel between 172.5mm and 170mm for me and I settled with 170mm. Just because it worked for me isn't a guarantee that it will work for someone with the same inseam measurement. I've seen a crank set with adjustable length but it wasn't for the bottom bracket of my bike so I had to buy 3 crank sets. If you buy used ones and sell the ones you don't want, it will be a smaller loss than trying this process with 3 new sets.
 

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My inseam is 30.5" and the calculation I have done put me at 167.5 crank which with the supply issues would be almost impossible to find . Research also shows I could go with a 165 .
My oldest bike which I still enjoy riding and put around 3k miles a year on could honestly use some new chainrings , so now may be the time to try this
 

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My inseam is 30.5" and the calculation I have done put me at 167.5 crank which with the supply issues would be almost impossible to find . Research also shows I could go with a 165 .
My oldest bike which I still enjoy riding and put around 3k miles a year on could honestly use some new chainrings , so now may be the time to try this
If I were you, I would try a 170mm crank for awhile and see how that works for you. Small changes are best.
 

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I rode 170's for over 20 yrs and decided to try 172.5's and found I preferred them and have gone with the 172.5's anytime I got a new crank. But I still have bikes with 170's and ride either length with no issues. So while I prefer 172.5, I can't feel the difference when riding 170. My cadence may be a bit higher on the 170's but I haven't really takin the time to compare.

It seems to me, in my case, that my preference is all in my head.

Kinda on the same note a friend picked up a bike and rode it for a number of months before giving it a good once over and when he got to the bottom bracket that one crankarm was 170 and the other was either a 172.5 or 175. He didn't realize till then that they were different.
 
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