# Is there a standard crank arm length?



## jaybee64 (Jan 4, 2011)

I was wondering, when you buy a bike with a group like Ultegra/105/Tiagra, does it come with a standard crank arm length?

Also, how do you measure an arm? Is it from the pedal to the bolt on the arm?

I'm wondering how much difference there is between something like a 165mm, 170mm and 172.5mm arm length. The differences in those measurements seem so miniscule.

Thanks for schooling me on this.


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## cda 455 (Aug 9, 2010)

jaybee64 said:


> I was wondering, when you buy a bike with a group like Ultegra/105/Tiagra, does it come with a standard crank arm length?
> 
> Also, how do you measure an arm? Is it from the pedal to the bolt on the arm?
> 
> ...


Depending on the frame size of the bike. 58+cm frames have 175mm cranks.

It's measured from center-hole to center-hole on the crank. It's also printed on the back of the crank.


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## cda 455 (Aug 9, 2010)

Also, the formula I use to determine my crank size is:

18.5% of the length of my leg (From the top of the femur).


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## MR_GRUMPY (Aug 21, 2002)

It will say on the back of each arm.


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## Mel Erickson (Feb 3, 2004)

No, there isn't a standard length but larger frames typically come with longer crank arms and visa versa. Much has been written about optimal crank arm length but solid conclusions are scarcer than hens teeth. Most people can't tell the difference between 170 and 175, me included. Some claim they know right away.


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## jaybee64 (Jan 4, 2011)

Thanks for the info, much appreciated.


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## Mr. Versatile (Nov 24, 2005)

The difference between 170, 172.5 & 175 is indeed miniscule. You probably wouldn't notice a difference when riding.


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## Kerry Irons (Feb 25, 2002)

*Formulaic*



cda 455 said:


> Also, the formula I use to determine my crank size is:
> 
> 18.5% of the length of my leg (From the top of the femur).


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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## robdamanii (Feb 13, 2006)

I think one of the most important thoughts on crank length would be the possibility of front wheel overlap.

In some cases, that extra centimeter between 165 and 175 could give you the clearance you need.


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## Kerry Irons (Feb 25, 2002)

*Toe overlap*



robdamanii said:


> I think one of the most important thoughts on crank length would be the possibility of front wheel overlap.
> 
> In some cases, that extra centimeter between 165 and 175 could give you the clearance you need.


Why do you think that toe overlap is an issue. I have big feet and run my cleats all the way back. I've had toe overlap on bikes since the late '80s and maybe before (I don't remember) but I've never had an incident caused by my toe hitting my front tire.


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## robdamanii (Feb 13, 2006)

Kerry Irons said:


> Why do you think that toe overlap is an issue. I have big feet and run my cleats all the way back. I've had toe overlap on bikes since the late '80s and maybe before (I don't remember) but I've never had an incident caused by my toe hitting my front tire.


Had the occasional issue with 'cross frames. Not an earth shattering problem, just an observation.


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## Mapei (Feb 3, 2004)

Every time I start my bike out of my driveway, or I do the 270 degree turn from the Griffith Park Bike path onto the sidewalk of Western Avenue, one of my shoes scrapes against the front tire, causing me to momentarily lose my balance and curse "G***d**m toe overlap!"


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## cda 455 (Aug 9, 2010)

Kerry Irons said:


> 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.
> 
> ...


I see your points. 


However; There are general 'rule-of-thumbs' (IMHO). Two come to mind:

1) 5'-0" cyclist using a 180mm crankset vs. a cyclist 6'-2" (Miguel Indurain comes to mind). 

2) Using shorter Cranks for longer rides (50+ miles) vs. longer cranks on shorter rides (TT comes to mind). Saves the knees and is more energy-efficient.


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## Lookbiker (Sep 2, 2006)

Even if every cyclist had the exact same leg length and femur/tibia proportions, there would be many different crank lengths and equally heated debates on the correct length, Over the years I've used 170 and 172.5 and both seem to work fine for me.


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## rx-79g (Sep 14, 2010)

The old standard is 170 for road bikes. Longer became popular in just the last 20 years.

Longer cranks make you knees work harder - which is especially true if your legs are shorter. But the larger the pedal circle the greater change in angle your knees have to cope with on each revolution.

172.5 seem to now be standard on larger bikes, but I wouldn't automatically upgrade to larger cranks without some concrete reason (and crazy leg length formulas are not a reason) is just putting undue stress on your body for unclear reasons.

If there was a good formula, I think basing it on shin length makes much more sense than total leg length, since the lower leg moves much more than the top. Using Zinn's formula you end up needing a range of crank lengths between 155 and 195 just to serve the normal range of riders - nuts.


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## TomH (Oct 6, 2008)

With a 175mm vs 170mm crank, your feet move ~31mm further, linearly, per rotation (the difference in circumference). At 80rpm, 2325mm more per minute (7.6 feet).. or 139,500mm more per hour. 457 feet further per hour. 

Since its a longer arm, its easier to push the same gear, but you are spinning around a larger circle. 5mm doesnt sound like much, but 7.6 additional feet per minute does.

I havent thought it out enough to come to a conclusion about how this added distance really impacts your body, or the amount of energy expended.. but thats the numbers.


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## rx-79g (Sep 14, 2010)

TomH said:


> With a 175mm vs 170mm crank, your feet move ~31mm further, linearly, per rotation (the difference in circumference). At 80rpm, 2325mm more per minute (7.6 feet).. or 139,500mm more per hour. 457 feet further per hour.
> 
> Since its a longer arm, its easier to push the same gear, but you are spinning around a larger circle. 5mm doesnt sound like much, but 7.6 additional feet per minute does.
> 
> I havent thought it out enough to come to a conclusion about how this added distance really impacts your body, or the amount of energy expended.. but thats the numbers.


_"Andy Pruitt, director of the Boulder Center for Sports Medicine andfit expert of many superstars, has a few other things to add. “Crank lengthformulas using femoral length or leg length are fine,” he says. “But ifyour style is mashing, use longer cranks, and if you are a spinner, shortenthem a bit. Mountain bike cranks should be a bit longer for that momentto get you over a rock. Use 2.5mm or 5mm longer for purely time trial usage,and vice versa for the track.” *Pruitt also warns that*, although a MarshallUniversity study showed that all participants regardless of body size wentfaster over short distances with each increase in crank length, *you canhurt yourself if you use cranks too long for your legs. In that case, hesays that the compressive and shear forces in the knee joints “go up exponentially,”due to the sharper knee bend.* (Compressive forces in the knee are stagnant,felt behind the knee. Shear forces are the result of fore-aft sliding ofthe condyles – cartilage-covered rounded femur ends – as they are rotatingon the soft meniscus – cartilage pad – atop the knee platform.) So, donot stray on the long side much beyond this proportionality relationship.*Cranks that are too short are not dangerous, however. You may forfeit somepower by not using your muscles as effectively, but you put less stresson your knees*."_

From Zinn's article on crank length: http://velonews.competitor.com/2003...-lennard-zinn-a-question-of-crank-length_5257

Leonard Zinn is a proponent of these formulas, but is 6'6" and does all his testing using his bikes and similar sized people. There is no good data out there to suggest a perfect formula, just anecdotal evidence about the benefit some riders have found, and the damage too-long cranks can do.

The idea of proportionlity is seductive, but sometimes misplaced. Tall people shouldn't use longer stems than short people or have bigger Q cranks. While a long crank arm might be useful to some people, you wouldn't select other tools purely based on proportions - like a hammer. Nor should you let a formula deprive you of an adequately long crank just because your legs are shorter.

170 is an old standard with known effects. Departing from it on little evidence and unproven theories is a potentially bad idea and unnecessary. Unless you're just gigantic, find something you like between 170 and 175, and favor shorter over longer.


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## Lookbiker (Sep 2, 2006)

I was reading that Cavendish uses 170mm cranks. He is 5' 10'' 

No power issues there.

Just use what works for you and don't treat these charts and formulas as the final word on the subject. Adding a few mm might not generate any more power for you but going longer might send a troublesome knee over the edge.


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## danl1 (Jul 23, 2005)

rx-79g said:


> _"Andy Pruitt, director of the Boulder Center for Sports Medicine andfit expert of many superstars, has a few other things to add. “Crank lengthformulas using femoral length or leg length are fine,” he says. “But ifyour style is mashing, use longer cranks, and if you are a spinner, shortenthem a bit. Mountain bike cranks should be a bit longer for that momentto get you over a rock. Use 2.5mm or 5mm longer for purely time trial usage,and vice versa for the track.” *Pruitt also warns that*, although a MarshallUniversity study showed that all participants regardless of body size wentfaster over short distances with each increase in crank length, *you canhurt yourself if you use cranks too long for your legs. In that case, hesays that the compressive and shear forces in the knee joints “go up exponentially,”due to the sharper knee bend.* (Compressive forces in the knee are stagnant,felt behind the knee. Shear forces are the result of fore-aft sliding ofthe condyles – cartilage-covered rounded femur ends – as they are rotatingon the soft meniscus – cartilage pad – atop the knee platform.) So, donot stray on the long side much beyond this proportionality relationship.*Cranks that are too short are not dangerous, however. You may forfeit somepower by not using your muscles as effectively, but you put less stresson your knees*."_
> 
> From Zinn's article on crank length: http://velonews.competitor.com/2003/11/bikes-tech/technical-qa-with-lennard-zinn-a-question-of-crank-length_5257
> 
> ...


Pruitt's opinion is one, but it's a bit of a generalization. While the increase in flexion in the stroke could be troublesome for certain individuals with selected knee problems, others would stand to gain from the proportionately less force of a longer arm. Riding much-too-short cranks has it's own problems, as the larger rider will tend to push more 'through the bottom' of the stroke, which is both inefficient and stressing on the knee in it's own way. And while I have a lot of respect for the good Dr, it's worthwhile to note that several fitting experts that are _medical_ doctors rather than doctors of education take issue with several of his conclusions. 

And for all that, it's waaay overthought. As Sheldon Brown notes, we're all different sizes, but all use the same staircases. And the loads we put on our knees on the stairs are much higher than average loads when riding. Shorter folks adapt to the same stairs tall folk do, and curiously enough, it's taller folk that tend to have more knee problems. Oh, and the typical riser height on a staircase is ~ 7-3/8", or 187mm. 

I'm not advocating for long cranks, just saying that there's nothing definitive here.


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## rx-79g (Sep 14, 2010)

danl1 said:


> Pruitt's opinion is one, but it's a bit of a generalization. While the increase in flexion in the stroke could be troublesome for certain individuals with selected knee problems, others would stand to gain from the proportionately less force of a longer arm. Riding much-too-short cranks has it's own problems, as the larger rider will tend to push more 'through the bottom' of the stroke, which is both inefficient and stressing on the knee in it's own way. And while I have a lot of respect for the good Dr, it's worthwhile to note that several fitting experts that are _medical_ doctors rather than doctors of education take issue with several of his conclusions.
> 
> And for all that, it's waaay overthought. As Sheldon Brown notes, we're all different sizes, but all use the same staircases. And the loads we put on our knees on the stairs are much higher than average loads when riding. Shorter folks adapt to the same stairs tall folk do, and curiously enough, it's taller folk that tend to have more knee problems. Oh, and the typical riser height on a staircase is ~ 7-3/8", or 187mm.
> 
> I'm not advocating for long cranks, just saying that there's nothing definitive here.


Just a thought - one can achieve "proportionally less force" by shifting. Crank arm length should always be about pedal stroke, not gear ratio.

But I do agree with your comments. I'm mainly speaking out against the "long crank craze", because I don't think there is any good data about the negative effects of changing what has been a long standard. I think it pays to be conservative with one's knees.


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## TomH (Oct 6, 2008)

I didnt mean the impact of the change in angle from a larger crank, I was referring to the impact that traveling a larger circumference circle, albeit easier, has on endurance/power expended. If you're tall enough to push a 175 *comfortably*, arent you just looking at the difference in leverage vs circumference of your foot circles? 

It kinda ties in with the high cadence style of riding.. Shorter cranks mean your legs are moving less per rotation. For whatever reason, I suck at spinning a high cadence.. in that case, wouldnt it make sense to run the longest comfortable crank?


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## rx-79g (Sep 14, 2010)

TomH said:


> I didnt mean the impact of the change in angle from a larger crank, I was referring to the impact that traveling a larger circumference circle, albeit easier, has on endurance/power expended. If you're tall enough to push a 175 *comfortably*, arent you just looking at the difference in leverage vs circumference of your foot circles?
> 
> It kinda ties in with the high cadence style of riding.. Shorter cranks mean your legs are moving less per rotation. For whatever reason, I suck at spinning a high cadence.. in that case, wouldnt it make sense to run the longest comfortable crank?


Not necessarily. You can ride at 75 rpm with any crank length by shifting to a higher gear. Does making a bigger circle with your foot make you more efficient or powerful? If your legs don't like moving quickly, why would making the distance they have to move be advantageous?


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## danl1 (Jul 23, 2005)

rx-79g said:


> Just a thought - one can achieve "proportionally less force" by shifting. Crank arm length should always be about pedal stroke, not gear ratio.


At least until one runs out of shifts. That's actually why I switched to longer cranks back in the day: I lived at the top of an awful hill, componentry wasn't as diverse as it is today, and that small bit of extra leverage was just enough to get me up and over without stalling out (and my knees rather liked me for it.)

Obviously, a minor consideration, and I'm tall enough that going to 175's was moving from too-small to correct, not from OK to too-large for may size. But yeah, gear development is the last reason to make that switch.


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## cda 455 (Aug 9, 2010)

TomH said:


> With a 175mm vs 170mm crank, your feet move ~31mm further, linearly, per rotation (the difference in circumference). At 80rpm, 2325mm more per minute (7.6 feet).. or 139,500mm more per hour. 457 feet further per hour.
> 
> Since its a longer arm, its easier to push the same gear, but you are spinning around a larger circle. 5mm doesnt sound like much, but 7.6 additional feet per minute does.
> 
> I havent thought it out enough to come to a conclusion about how this added distance really impacts your body, or the amount of energy expended.. but thats the numbers.


Very interesting perspective.

I just started riding my wife's Cannondale hardtail with 170mm cranks ( My bike has 175mm cranks) and I've noticed I haven't had any deadspots spinning 170mm vs. spinning my 175mm.


And just to throw it out for discussion; Since I've a Big Mig fan (Miguel Indurain), if I ever buy a dedicated road bike for doing TT or short races I would definitely use 180mm cranks (As well as a 55+T chainring) like he did to take advantage of the leverage as well as the advantage of the leg power I have.


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## cxwrench (Nov 9, 2004)

Lookbiker said:


> I was reading that Cavendish uses 170mm cranks. He is 5' 10''
> 
> No power issues there.
> 
> Just use what works for you and don't treat these charts and formulas as the final word on the subject. Adding a few mm might not generate any more power for you but going longer might send a troublesome knee over the edge.


Cav is 175cm, 5'9", and probably uses 170 because he's raced on the track so much. his sprint doesn't produce huge watts, most likely because he sprints very low, and his leadout is usually very fast. his acceleration is brutal, so you'd think he'd show more max wattage but he's around 1400. low, aero, and he doesn't weigh much.


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## muskat (Jan 9, 2011)

Your legs will feel the difference.


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## divest (Feb 8, 2010)

I just switched to 172.5 from 170 and the things that I have noticed are: I get noticeably more force when climbing, esp when climbing out of the saddle (feels much more natural and efficient) 

In terms of spinning I felt more comfortable spinning at 110-115 rpm with the 170 cranks, I feel like more of a masher with the longer cranks but I've only had them for a few weeks and I think I'll adapt quickly in my base period coming up next month.

The reason I switched is because I have big feet for my height.


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## black_box (Jun 7, 2008)

danl1 said:


> As Sheldon Brown notes, we're all different sizes, but all use the same staircases. And the loads we put on our knees on the stairs are much higher than average loads when riding. Shorter folks adapt to the same stairs tall folk do, and curiously enough, it's taller folk that tend to have more knee problems. Oh, and the typical riser height on a staircase is ~ 7-3/8", or 187mm.


With your feet on opposite ends of this 187 total crank diameter, that would be a 93.5mm crankarm. If you take steps two at a time, it would be 187mm. 93.5mm is well under any crank we'd see on a bike and I would guess that taller folks have more knee issues due to their extra weight (same with larger dogs and their hips).

Stairs could be a nice comparison though. Ascend a few flights of stairs one day using one step at a time and with a comfortable pace and see how you feel. Then do them two at a time with half the cadence so you finish at the same time? Technically, you've done the same amount of work in the same time, so equal power output. You could then try it with a comfortable cadence for 2-stepping and then double it for the singles. Then compare your perceived effort after the 4 runs.


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## rx-79g (Sep 14, 2010)

black_box said:


> With your feet on opposite ends of this 187 total crank diameter, that would be a 93.5mm crankarm. If you take steps two at a time, it would be 187mm. 93.5mm is well under any crank we'd see on a bike and I would guess that taller folks have more knee issues due to their extra weight (same with larger dogs and their hips).
> 
> Stairs could be a nice comparison though. Ascend a few flights of stairs one day using one step at a time and with a comfortable pace and see how you feel. Then do them two at a time with half the cadence so you finish at the same time? Technically, you've done the same amount of work in the same time, so equal power output. You could then try it with a comfortable cadence for 2-stepping and then double it for the singles. Then compare your perceived effort after the 4 runs.


It wasn't intended as a direct mechanical comparison between steps and cranks, but a general comparison that we all use standardized things like stairs, hammers, door knobs despite different body sizes.


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## black_box (Jun 7, 2008)

rx-79g said:


> It wasn't intended as a direct mechanical comparison between steps and cranks, but a general comparison that we all use standardized things like stairs, hammers, door knobs despite different body sizes.


I figured, I just thought it was interesting and my mind wandered. Why not 220mm steps? It seems when one-size-fits all we want 187mm steps. When 3 sizes fit all, we want 170, 172.5, and 175mm cranks. If everyone can be put onto one set of steps, why bother with different cranks at all? If taller people have more problems with their knees, shouldnt they have shorter cranks to reduce the strain?


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## rx-79g (Sep 14, 2010)

black_box said:


> I figured, I just thought it was interesting and my mind wandered. Why not 220mm steps? It seems when one-size-fits all we want 187mm steps. When 3 sizes fit all, we want 170, 172.5, and 175mm cranks. If everyone can be put onto one set of steps, why bother with different cranks at all? If taller people have more problems with their knees, shouldnt they have shorter cranks to reduce the strain?


Really, most everyone DID use 170s until fairly recently, and longer cranks do create more strain. So the answer to your question is not that it isn't a bad idea, but that cyclists are always grabbing for the newest widget, and crank length is another thing to play with.

I'm not saying it is pointless, or that someone who's 6'2" with 172.5s is straining their knees more than a someone 5'6" on 170s. But right now it's free-for-all with little data on the benefits or negative effects.


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## orlin03 (Dec 11, 2007)

*I like choices*

As a 6'1" guy with long legs for his height, when it was time to upgrade the crank on my roadbike, I went with a 175. The difference was almost nonexistent, until I hit the hills; climbing seemed stronger, and out of the saddle efforts seemed especially improved & balanced. Fast forwarding to my first TT build, I almost went with another 175, but decided to dig deeper and research this decision. I ended up going with the opposite approach and went with 170 cranks, with the idea that holding a higher cadence would be easier, and that finding a lower position on the bike would be more comfortable (since the crankarm comes up lower at the top of it's travel). It was also a good choice for me. 

Since I liked two very different cranks, I swapped the cranks on both bikes to test the differences. Truthfully, I prefer the 170 on both bikes for spinning at high speeds/high outputs; my typical 40k race-pace cadence is above 95, with 97-105 feeling optimal. The 175s, after the saddle height had been lowered, made me feel more crowded by my knees (edit: it just felt harder to "get on top" of the gear with the 175s). At normal-fairly fast riding speeds and cadences between 90-95, I couldn't tell a difference between the cranks. Everytime I got out of the saddle, though, I felt most comfortable on the 175s regardless of the bike I was on; I simply felt like I had a more complete stroke. Seated climbing was not as different as I previously thought, but when the incline got really steep and my cadence dropped, I did feel a bit better with the longer arms; I can't say which one would've been better after a day of repeated long climbs though. Basically, I'm happy with the 175 arms on my roadbike, though if roadracing was my main focus, maybe I'd think differently. I definitely wouldn't want them on my TT bike for my riding style; I really wish I had the opportunity to test a smaller crank to see what would happen if I went even further.

I believe that having choices is a good thing. Try matching your decision to the school of thought that most fits your own perceptions, riding style, and intended use.


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## Cycling for Cancer (Nov 9, 2010)

*Stronglight adjustable crank arm length*

came with my Aerozine X-13 crankset with ceramic BB bearings. I rode with the adjustment set for 170mm for a couple of weeks, then changed it to 175mm. Finished the season at 175mm (at least 1000 miles). I did not mind the extra motion of the knees, in fact my quads kind of appreciated the extra "excursion" and I could feel different parts of my quad developing in response to the longer crank length. I also appreciated the longer length on climbs. Perhaps a minor difference, but noticeable.

I am 5' 7" with a normal sized torso and relatively shorter legs (29" inseam). Typical ride 20+ (median ride length 37.8 miles, longest ride 50 miles). Overall I liked the longer crank length, it allowed me to use my leg strength more effectively and did not seem to hurt my knees.


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## chas0039 (Jun 26, 2007)

One of the most important things for me is to set up the correct seat setback from the pedal with the thigh horizontal. This in turn is effected by the length of the pedal and then, in turn, I need to adjust the stem length to get the overall spacing correctly set up, from seat back to bar. On some frames, the top bar length really limits what I can do, given that the Brooks saddles I use only go so far back. A change of a few mm in pedal length can make a difference and I have found some common frames where I really couldn't use 170mm and still get the whole thing to fit right.

I may be wrong, but I think a lot of people don't really look closely enough at the seat to pedal length when they set things up. At least for me, this is an important set up measurement.


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## chas0039 (Jun 26, 2007)

delete, I keep getting double postings for some reason. Sorry


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## Camilo (Jun 23, 2007)

Mapei said:


> Every time I start my bike out of my driveway, or I do the 270 degree turn from the Griffith Park Bike path onto the sidewalk of Western Avenue, one of my shoes scrapes against the front tire, causing me to momentarily lose my balance and curse "G***d**m toe overlap!"


Slow learner!

(ducking and running...)


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