# be careful when changing crank length!



## osteomark (Jun 15, 2011)

I went from a FSA Vero 172.5mm to 175mm Tiagra and right knee is painful after 15mi on trainer. I talked to a guy who is certified FITS. He said becuase I'm only 5'8" and have a 54cm Trek 1.2 that I might even want 170mm crank. He told me to change back to the lesser quality FSA Vero 172.5mm. I guess the 2.5mm makes a whole lot of differenct. Getting fitted tomorrow night. Better to be accurately fitted than have nice components.


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## minutemaidman (Jun 14, 2010)

Fitted...yes. If youre 5'8" there MIGHT be a 1% chance, or so, that you need a 175 mm crank, or even a 172.5 mm. You would have to have some freak proportions. What made you think you could use a 175 mm crank effectively?


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## osteomark (Jun 15, 2011)

Being a newbie I never thought a few mm's would make a difference. Got a great deal on a used Tiagra and said " What the heck upgrade when you can especially if cheaply". Egg on my face:0
The LBS is swapping as I type. Probably should look for a good used 170mm now. Eh?


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## wim (Feb 28, 2005)

osteomark said:


> I guess the 2.5mm makes a whole lot of differenct.


Maybe, maybe not. For one, you have no proof that the 2.5 mm change is the actual cause of your knee pain. For the other, a few more sessions with the 175s could have made the knee pain go away due to adaptation. Sure, a 2.5 mm crank length change _could_ cause permanent knee pain, but only if you were pedaling at some sort of biomechanical limit with your 170s.

On the other hand, conventional wisdom says that at 5'8", the 170 mm crank is about right. But that's conventional wisdom. There are plenty of 5'8" riders out there who happily push 175s. They're not freaks.


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## AndrwSwitch (May 28, 2009)

I bought a bike from a friend recently that came with 172.5mm cranks. My usual size is 170mm. I was in a bit of a hurry and the bike is for commuting, so I wasn't being picky.

I noticed that I was scraping the toe clips on my pedals more when I was starting from a stop. That led me to look at the crank arm length. While I now think that I can also feel a difference, I'm no longer a good test subject because I also now know that the arms are a different length. I think it's telling that I figured it out based on a secondary thing.


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## osteomark (Jun 15, 2011)

I'll trust what this FITS specialist says and at least get back to the 172.5mm. I'm excited to see how badly I was set up since I have no clue on what any angles need to be. I'll tell you guys in 2 days. 
850miles on poor set up probably was not the smartest thing!


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## Kontact (Apr 1, 2011)

Longer cranks increase your leverage, and make you knees go through a greater range of motion. The conventional wisdom is not perfect because people's joint flexibility is variable. But there is no doubt that longer cranks are harder on your knees - it's just that if you legs are long, 170s might not be "hard enough".

Everything in cycling is a tradeoff. There isn't a single "more is better" part on the bike.


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## nightfend (Mar 15, 2009)

You know, if you buy longer crank arms, you need to adjust your saddle down a little to compensate.


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## osteomark (Jun 15, 2011)

putting the saddle down now makes the recovery leg hit the belly more. Your right there are trade offs


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## Zipp0 (Aug 19, 2008)

I use 170mm cranks on one bike and the other has 175s. Lots and lots of miles on both, and no issues yet. I guess I am one of the lucky ones who can get away with this sort of thing?


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## pmf (Feb 23, 2004)

I find this hard to believe. The difference between 172.5 and 175.0 in 2.5 mm. That's 0.25 cm. That's one tenth of one inch. Put another way, that's about a 1.5% difference. Can you really feel differences that tiny? I know I can't. I suspect it's something else.


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## osteomark (Jun 15, 2011)

Within 2 cranks I could tell there was something different and when I measured them it showed 2.5mm difference. I was in the same skeptical boat as you pmf, until I felt it. I've only been riding for 6months. I can't imagine what a highly tuned cycler would have felt. 
After 15miles on the trainer, I felt right knee pain. I've done 35milers on the road and 20milers on the trainer with no pain, so I switching back.


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## Mapei (Feb 3, 2004)

Some people are sensitive to crank length. Some ain't. As Kontact opines, joint flexibility probably plays a part, as does leg length. And keep in mind that you have to double the crank length difference because pedaling takes your foot up as well as down, and backwards as well as forwards.


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## pmf (Feb 23, 2004)

osteomark said:


> Within 2 cranks I could tell there was something different and when I measured them it showed 2.5mm difference. I was in the same skeptical boat as you pmf, until I felt it. I've only been riding for 6months. I can't imagine what a highly tuned cycler would have felt.
> After 15miles on the trainer, I felt right knee pain. I've done 35milers on the road and 20milers on the trainer with no pain, so I switching back.


I guess we're all different. I swapped out a 172.5 set of cranks for 175 set a few years ago and could tell absolutely no difference. And this was after riding on 172.5 for many (12+) years. The thickness in the soles of biking shoes differs by more than 2.5 mm. I still think it's something else, but if the 172.5's cause you no pain, I guess stick with them.


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## pianopiano (Jun 4, 2005)

I switched from a 172.5mm crankarm to a 175 at the beggining of this year. I meant to make a very small adjustment to my saddle height to compensate, but then I simply forgot to. I've put more than 3,000 kilometers on the 175's without any problems whatsoever.


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## Creakyknees (Sep 21, 2003)

My road bike has 175's, my cx bike has 172.5's, my track bike has 165's... I never notice any difference. 

Your mileage may vary.


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## baker921 (Jul 20, 2007)

I'm sceptical too if you are riding a bike in the real world. Stopping, starting, climbing moving your weight around, adjusting your hand and backside positions, all best practice. I suppose if you are able to tell the difference in crank length, it is most likely to be on a trainer where you able to hold the same position for an extended period of time.


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## wim (Feb 28, 2005)

osteomark said:


> I'll trust what this FITS specialist says!


What is a FITS specialist?


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## Kontact (Apr 1, 2011)

pmf said:


> I find this hard to believe. The difference between 172.5 and 175.0 in 2.5 mm. That's 0.25 cm. That's one tenth of one inch. Put another way, that's about a 1.5% difference. Can you really feel differences that tiny? I know I can't. I suspect it's something else.


2.5mm of crank equals 5mm larger pedal circle. People can easily feel a 1cm difference in stem length, why wouldn't they be able to feel a half cm difference in something their legs are doing 5400 times an hour?


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## Mr. Scary (Dec 7, 2005)

pmf said:


> I find this hard to believe. The difference between 172.5 and 175.0 in 2.5 mm. That's 0.25 cm. That's one tenth of one inch. Put another way, that's about a 1.5% difference. Can you really feel differences that tiny? I know I can't. I suspect it's something else.


It's a 5mm diametric change, there is a difference. BTW, I'm 5'8" with 33" inseam and I run 175mm cranks on all of my bikes. Gilberto Simoni is 5'7" or so and he also ran 175s when he raced, your body will adapt...


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## Special Eyes (Feb 2, 2011)

pmf said:


> i find this hard to believe. The difference between 172.5 and 175.0 in 2.5 mm. That's 0.25 cm. That's one tenth of one inch. Put another way, that's about a 1.5% difference. Can you really feel differences that tiny? I know i can't. I suspect it's something else.


+1 !!


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## medimond (Apr 26, 2009)

I'm 5'8.5, long torso, short legs. 

My old road bike had 172.5 mm cranks, the new one has 170 mm ... the change is slight, but I can feel it. 

On my MTB I swapped the cranks from 175 to 170 mm ... completely obvious, I can spin smoother now on the bike.


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## bigbill (Feb 15, 2005)

I'm in the military and deploy for 6-8 months. During that time, the only riding I get is on spin bikes with 170mm cranks with a wide Q-factor. When I return and get on my road bike with it's 175mm cranks and more narrow Q-factor, my knees complain for a week or so until I re-adapt. Crankarms come down to leg length and whether you're spinning or torquing over.


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## mtnroadie (Jul 6, 2010)

I switched from 172.5 to 170 on two of my bikes, i still have 172.5 on another bike and i notice a huge difference in pedaling efficiency and spining. I will probably get another 170mm for my third bike and sell the 172.5.

I am conviced that crank difference makes a big difference.


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## orange_julius (Jan 24, 2003)

osteomark said:


> putting the saddle down now makes the recovery leg hit the belly more. Your right there are trade offs


Yes, but I think the maximum extension length is the likely cause of your grief. You exert the greater bulk of your power when you are pushing down on the pedals, to where you then hit the bottom of the pedal stroke. 

Yes, you are also right that adding crank length and lowering your seat changes your leg angles at the top of the stroke, but at that point you are exerting much less power.


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## wim (Feb 28, 2005)

orange_julius said:


> Yes, you are also right that adding crank length and lowering your seat changes your leg angles at the top of the stroke, but at that point you are exerting much less power.


That depends on the rider. Some riders learn to ease off on pedal force as the pedal approaches bottom dead center. They do that to avoid expended so-called unused force, which is force applied where it can't possibly contribute to forward propulsion. Force applied at bottom dead center (what popular literature calls "maximum leg extension") is unused force.

Point of all this is that pedal force application is not uniform across all riders, with some bringing maximum force to the pedal very early in the stroke. Once these new force and force-vector measuring pedals become popular, more riders will be able to see their pedaling forces more clearly.


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## Mike T. (Feb 3, 2004)

osteomark said:


> I went from a FSA Vero 172.5mm to 175mm Tiagra and right knee is painful after 15mi on trainer. I talked to a guy who is certified FITS. He said becuase I'm only 5'8" and have a 54cm Trek 1.2 that I might even want 170mm crank. He told me to change back to the lesser quality FSA Vero 172.5mm. I guess the 2.5mm makes a whole lot of differenct. Getting fitted tomorrow night. Better to be accurately fitted than have nice components.


Your evidence is totally anecdotal. I regularly go from 165mm cranks on my track bike to 170 and 172.5 on my road bikes (and 170 on my current MTB) and went to 175 on my old MTB for years with zero knee problems. So there is a whole 10mm difference. Sure 2.5mm could have "hurt" your knee but it's doubtful that this was the cause and it's very unlikely to affect anyone else.


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## alexp247365 (Dec 29, 2009)

Some people are very sensitive to adjustments (myself included.) You'd be surprised at how much of a difference a few millimeters can make to either improve, or worsen your pedaling efficiency and endurance.


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## osteomark (Jun 15, 2011)

OK I'm back to the 172.5mm FSA Vero. Tonights the fitting. 
What would be my benefit to going back to the 175mm cranks? Faster speeds because they are longer?


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## wim (Feb 28, 2005)

osteomark said:


> Faster speeds because they are longer?


If that would be true, everyone would be on the longest cranks possible.

Although studied and written about by scientists, amateur scientists and kooks for many years, no one's ever been able to prove that crank length has any predictable effect on power output. What's clear is that people feel better pushing one crank length as opposed to another. But even that is subject to adaptation: it's entirely possible that after 6 weeks (not just 15 mi) on 175s, you and your knees would feel just great.


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## AndrwSwitch (May 28, 2009)

Since nobody has posted it yet,

Crank Length – Does Size Really Matter?

(The Pez Cycling article looking at three studies on the effects of messing with crank arm length.)

My university's library is being pissy right now so I can't read the references.

The bottom line for the main study referenced in the article was that with appropriate gearing, anything in the 145-195mm range is fine. Most of us don't even have access to cranks outside the 165-185 range; getting outside the 170-180 range is pretty difficult.

There are some problems with the studies. I think when people ask why something in cycling hasn't been determined scientifically, there are two really big reasons - science is difficult and expensive to do with any kind of rigor, and humans are incredibly variable test subjects, with a ton of variables, so isolating anything is really difficult. My physics teacher liked to make fun of astronomy papers that had error bars the full height of the graph; I think a lot of athletic performance stuff is just as bad. The article's author notes that to do good aerobic testing with different crank arm lengths requires a reasonable period of training and adaptation to the new arm lengths. Isolating crank arm length is also pointed out as a bad method because the leverage is changed, so gearing has to be changed too, and people compensate by changing cadence.


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## wim (Feb 28, 2005)

AndrwSwitch said:


> The article's author notes that to do good aerobic testing with different crank arm lengths requires a reasonable period of training and adaptation to the new arm lengths.


I think that's exactly why there's so much speculation and so little hard science about this. Who has the inclination, time and money needed to do such a long-term study? It's like these saddle height studies which have people ride one saddle height for 20 minutes and another saddle height for the next 20 minutes. Basically useless.


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## cda 455 (Aug 9, 2010)

I have a 33" inseam and I plan to up my crank length from 175mm to 180mm on my next road bike build.

I might even bump up my MTB crank length from 175mm to 185mm since I'm a masher anyway.


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## ls1togo (Aug 14, 2009)

*went longer too!*

Maybe I'm odd..but...being 5'9'' with a thirty inch inseam on a 55 cm Lemond B/A, I always felt slightly "cramped" with a 172.5 crank arm...I switched to a 175 and feel much more natural...jus' my two cents....


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## wim (Feb 28, 2005)

cda 455 said:


> I might even bump up my MTB crank length from 175mm to 185mm since I'm a masher anyway.


Keep in mind that to turn the same cadence with 185 cranks than you did with 175 cranks in the same gear you could just increase leg speed a bit. Of course, if you don't increase leg speed after the crank swap, your cadence would drop a little. So becoming a bit more of a "masher" after going to longer cranks does happen. But it doesn't have to.


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## AndrwSwitch (May 28, 2009)

wim said:


> I think that's exactly why there's so much speculation and so little hard science about this. Who has the inclination, time and money needed to do such a long-term study? It's like these saddle height studies which have people ride one saddle height for 20 minutes and another saddle height for the next 20 minutes. Basically useless.


Saddle height's another good one. I rode for years with too high a saddle, and had knee problems. When I saw a fitter and he lowered it, I was pretty resistant and it didn't feel good at first. But lo and behold, I adapted and was able to start riding for longer periods of time again. It took a few rides.

I was thinking about the issue of adaptation - for one, I don't think we even know what a reasonable period is. That's actually a whole study on its own. Imagine trying to design and adequately control it to filter out day-to-day inconsistencies in performance, and make the performance change from changing crank length big enough to measure. You'd also need a group of cyclists with good enough form to be representative, good enough fitness to be relatively stable, and who were willing not to screw up your data by riding their own bikes, with their usual cranks, during the period of the study. Yeow.


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## Hanks (Sep 30, 2011)

*Just Maybe*

The trainer is set up with too much tension. You haven't compared the two on just a normal As a newbie, I'm not sure I'd draw any conclusions after a 800+ mile ride either. (Most likely, you will be hurting).

Hope it works out for you,

Hank


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## cda 455 (Aug 9, 2010)

wim said:


> Keep in mind that to turn the same cadence with 185 cranks than you did with 175 cranks in the same gear you could just increase leg speed a bit. Of course, if you don't increase leg speed after the crank swap, your cadence would drop a little. So becoming a bit more of a "masher" after going to longer cranks does happen. But it doesn't have to.


Good points. 

I figured, when I'm climbing while on a trail with my MTB my cadence is way down and inconsistent compared to when I'm climbing on my road bike on a hill road, I can use more leverage on my MTB.


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## Mapei (Feb 3, 2004)

Mike T. said:


> Your evidence is totally anecdotal. I regularly go from 165mm cranks on my track bike to 170 and 172.5 on my road bikes (and 170 on my current MTB) and went to 175 on my old MTB for years with zero knee problems. So there is a whole 10mm difference. Sure 2.5mm could have "hurt" your knee but it's doubtful that this was the cause and it's very unlikely to affect anyone else.


Ah, but ultimately it's the anecdotal that counts. Statistics may give you a general idea of what's going on but it's you yourself, an individual and an anecdote, that sits on that bike, spinning the cranks. And why do the component makers make different sized cranks anyway? They'd be saving money if they didn't have to make them in various sizes. They certainly wouldn't be charging less if they only made one size.


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## AndrwSwitch (May 28, 2009)

The idea that crank arm length matters is very intuitive. That doesn't make it right, but it does mean that a manufacturer that didn't pander to that idea would lose market share to one that did.

I bet if it mattered a lot, we would see a lot more options in that 145-195 range that one of the studies found had good power, not just in the very narrow range in which we do see them. Of course then we'd probably also need different gearing, people would start demanding different bottom bracket drops...


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## cda 455 (Aug 9, 2010)

AndrwSwitch said:


> The idea that crank arm length matters is very intuitive. That doesn't make it right, but it does mean that a manufacturer that didn't pander to that idea would lose market share to one that did.
> 
> I bet if it mattered a lot, we would see a lot more options in that 145-195 range that one of the studies found had good power, not just in the very narrow range in which we do see them. Of course then we'd probably also need different gearing, _*people would start demanding different bottom bracket drops...[*_/QUOTE]
> 
> ...


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## dcgriz (Feb 13, 2011)

osteomark said:


> I went from a FSA Vero 172.5mm to 175mm Tiagra and right knee is painful after 15mi on trainer. I talked to a guy who is certified FITS. He said becuase I'm only 5'8" and have a 54cm Trek 1.2 that I might even want 170mm crank. He told me to change back to the lesser quality FSA Vero 172.5mm. I guess the 2.5mm makes a whole lot of differenct. Getting fitted tomorrow night. Better to be accurately fitted than have nice components.


Dont be in a hurry to change the cranks. Sometimes knee pain comes from doing too much too soon. Furthermore, it should take more than 15 miles to cause any knee pain when changing from 172.5 to 175 assuming everything else is set relatively right. 
If I was you I would try moving the cleats back about .5 cm and see what happens.


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## Lookbiker (Sep 2, 2006)

Raced on 172.5 and 170 cranks and didn't have any problems but that's just me. An increase in q-factor is quickly noticed by my knees, however. Leg length is a factor but how your bones, feet, and joints track/align is also critical.


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## osteomark (Jun 15, 2011)

OK. I was waaaay out of wack with my bike. An awesome experience with the FIST specialist.
The biggest things were my seat was too low. went BB to top of seat of 71cm to 74cm. the spacers (did not even know they were present!) on the stem were moved. Two from the bottom were put above the stem so now only one spacer. So that dropped the handle bars 4 cm. My seat went back 1.5cm and tilted up in the front a bit. My balance was too forward and now I'm more center balanced. My arm will like that a lot.
Dying to get out and ride now.
Thanks a bunch all.


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## wim (Feb 28, 2005)

osteomark said:


> went BB to top of seat of 71cm to 74cm. the spacers (did not even know they were present!) on the stem were moved. Two from the bottom were put above the stem so now only one spacer. So that dropped the handle bars 4 cm.


To an experienced rider, those would be huge changes that he'd make in small steps over a period of time. But you seem to be at the beginnig of your cycling career, so your body will probably adapt pretty quickly to the new settings. Just go easy for the first few days.

For what it's worth: with your 5'8" height, the 74 cm saddle height seems strangely high to me. But perhaps you have long legs in relation to your upper body.


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## Mike T. (Feb 3, 2004)

wim said:


> For what it's worth: with your 5'8" height, the 74 cm saddle height seems strangely high to me. But perhaps you have long legs in relation to your upper body.


I'm 5' 8" and I just measured my BB to seat height - 74cm dead on. And I'm nothing special for leg length or other weird body dimensions.


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## wim (Feb 28, 2005)

Mike T. said:


> I'm 5' 8" and I just measured my BB to seat height - 74cm dead on. And I'm nothing special for leg length or other weird body dimensions.


Then I'm the weird one and am imposing my weirdness on others: 5'10" and 72 cm saddle height here.


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## medimond (Apr 26, 2009)

5'8.75 and 70 cm saddle height. Short legs, long torso.


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## Mike T. (Feb 3, 2004)

wim said:


> Then I'm the weird one and am imposing my weirdness on others: 5'10" and 72 cm saddle height here.


Do you waddle like a duck? 

I must admit thought that in my younger days I was 5' 9" and due to (I'll assume) spinal deterioration I'm now an inch shorter. I'll also assume that my leg length didn't change.


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## osteomark (Jun 15, 2011)

OK. Now I'm just going to crawl under a rock now and die! I was contemplating not even letting the world know this. I went to clean the Tiagra crank and said "Hey, I'll just measure the cranks again"
170mm!!!!!!!! And sure enough it's stamped on the cranks 170mm
So I call the FIST specialist in a panic and asked if the fitting I had with the 172.5mm was in anyway at the limits of measurement or component adjustment. He said I really need to be with the 172.5mm.
I really was almost ready to vomit thinking I just changed the cranks back and fitted, only to reverse it all. 
I guess I lucked out. I guess I can use the Tiagra BB at least if I want a 172.5mm Tiagra crank. 
I think I'm just staying put with the FSA Vero until it craps out.


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## AndrwSwitch (May 28, 2009)

LOL. You still changed it by 2.5mm.

I'm going to give you something else to overthink.

Q-factor.


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## jtyy (Sep 28, 2011)

I am 5.6 feet and have a 31.9 inch inseam length. I run 177.5mm dura ace 7800 cranks on my roadie and they work really well especially on climbs and there is very little difference from my previous 170mm cranks. Though, my cadence dropped alittle compared to my previous crank. However, I am sticking to this crank for now =).


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## Anthony3 (Aug 29, 2011)

jtyy said:


> I am 5.6 feet and have a 31.9 inch inseam length. I run 177.5mm dura ace 7800 cranks on my roadie and they work really well especially on climbs and there is very little difference from my previous 170mm cranks. Though, my cadence dropped alittle compared to my previous crank. However, I am sticking to this crank for now =).


Wow those are very large for your height.


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## Guest (Dec 17, 2011)

jtyy said:


> I am 5.6 feet and have a 31.9 inch inseam length. I run 177.5mm dura ace 7800 cranks on my roadie and they work really well especially on climbs and there is very little difference from my previous 170mm cranks. Though, my cadence dropped alittle compared to my previous crank. However, I am sticking to this crank for now =).


Interesting. I'm the same height and inseam (within <0.2 inch), currently on 170. 

I looked at some of the websites linked from this thread talking about crank length. Some of the formulas say I should be running 165mm, others say I should be running 175mm... for now I figure I'll just stick with the average of the two suggestions 

If I ever do change crank lengths, it will almost certainly be to go shorter. I sometimes feel pretty cramped when pedaling riding down in the drops as low as possible into headwinds (ie knees bumping into my torso on each pedal stroke) I figure reduced range of motion of the legs would help with this.


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## latman (Apr 24, 2004)

Slightly off topic but i loaned a bike to a visiting friend one summer and when i got it back I rode it a fair bit and got sore knees , it turned out he had slid the saddle back an inch and i was adapted to a more fwd position over the BB.


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## McJim (Dec 14, 2011)

Longer cranks give you more leverage. Shorter cranks allow you to spin faster (assuming you're strong enough to wind the gear up to spinning speed!) I use 175 on MTB, 175 on Road bike and 165 on Track bike. This is because I need to be able to hit high rpms on the track to compensate for the lack of gears. I think the cause of the knee pain may have been the lack of a seat height adjustment to compensate for the crank length change. I try to keep the distance from pedal spindle to butt :ciappa: fairly constant from bike to bike.


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## jtyy (Sep 28, 2011)

Yup, my seat height is about 74 cm actually even running with 177.5 mm cranks.


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## osteomark (Jun 15, 2011)

McJim,
I also think it was from the seat height being 3cm too low and 1.5cm too forward. When I went from 172.5mm to170.mm, I was lengthening my leg by 2.5mm which was the correct move but the cranks would have had to have been a lot shorter to fully correct the leg length. That would have been an inappropriate thing to do since all my other measurements were so skewed. So in the end I'm glad I'm still with the 172.5mm for the leverage and it appears that is the length I really need.
It's snowing here in CNY so I guess it's onto the trainer today.
Thanks


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## wim (Feb 28, 2005)

osteomark said:


> I also think it was from the seat height being 3cm too low and 1.5cm too forward. When I went from 172.5mm to170.mm, I was lengthening my leg by 2.5mm


This makes no sense to me. In your original post, you said you went from "a FSA Vero 172.5mm to 175mm Tiagra and right knee is painful after 15mi on trainer." You implied that a 2.5 mm crank length change made your knee hurt. (mi = miles or minutes?).

There was no mention of a saddle height change in that post. If you did not lower your saddle by 2.5 mm after that 2.5 mm crank length change, you "lengthened your leg," to use your words. If you did lower your saddle by 2.5 mm, your leg extension stayed the same at bottom dead center.

It's entirely possible that you were 3 cm too low. But you need to take that 2.5 mm crank length change out of your speculations about the knee pain.


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## outcast7 (Nov 17, 2011)

I am a rider with longer legs, and I typically run 175 cranks on my road bikes. On the track however, I run 170s which are considered the typical size for my height. To me it isn't really an issue of the cranks "fitting". Any crank within a reasonable range can be made to work for any rider. What is important is that the length you ask your legs to extend from the saddle to the pedal remains constant. When setting up saddle height and setback this is important to take into account.


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