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  1. #1
    Senior Member mtnbud's Avatar
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    Does crank arm length matter?

    Howdy,

    I'm wondering if I should consider replacing the crank arms on the Craigslist find I've been messing with. It has 170 mm Sugino cranks. The cranks are in good shape.

    My mountain bike has 175 mm crank arms and my road bike has 172.5 mm cranks. I'm 5' 11" and have a 32" inseam. I replaced the seat and seat post and I am starting to dial in the fit. It's feeling pretty good, but I do think I notice the shorter crank arm length.

    Opinions? Have any of you had to change your cranks due to fit issues?

    I'm having fun with this bike. It's not worth it to me to buy new cranks, but I'm wondering if it's something I should be keeping my eyes open for on Craigslist.

    Thanks,

    Jeff
    “If You Open Your Mind Too Much Your Brain Will Fall Out”

  2. #2
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    I wouldn't bother. My bikes have crank arm lengths of 165 mm (tandem), 170 mm (touring and folder), and 175 mm (road) and I can't say that I really notice much difference between them. Yes, I do tend to pedal at a little higher cadence on the tandem - but I'm not sure that it makes me any less or more efficient than the lower cadence with longer cranks on my road bike.

  3. #3
    we be rollin' hybridbkrdr's Avatar
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    Some comments I read: longer crank arms might mean some knee soreness for some people and will mean less "leverage" for going faster. I mean if someone wanted to go fast with 52T front chainring and 11T cog in the rear, 170mm cranks would be faster than 175mm. I did read one comment of someone stated he felt liberated with 175mm cranks and wouldn't go back to 170mm.

    Part of me believes if going with what is recommended though. I was measured in a store at 83.5cm inseam (32.8 inches) and I'm 5'8-1/2"or 174cm so I'm guessing you measured manually and not on a machine. I've seen 172.5mm recommended for my inseam/height but since I have a bit of chondromalacia in my knees, I think I'm going to stick with 170mm. For whatever reason though, for my long legs, I often feel like longer crank arms would "let me breathe" although I don't want to risk installing 175mm or even 172.5mm on a bike.
    Feeling Good by David Burns

  4. #4
    Senior Member Western Flyer's Avatar
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    I think it is a matter of finding your sweet spot. With everything else being equal, the longer crank arms means a slower cadence, but longer and greater stretch of all the tendons going over your knees with each rev. Shorter cranks have your quads firing faster with less recovery time. I ride with a 170mm but at 6’1” should probably have longer cranks if I was going by the book. For me it is not something I would notice much on a day ride, but a few weeks out, riding most every day for six plus hours I think 170 is the right length for me. It is how you feel riding for the last hour of the day and getting back on your loaded bike in the morning that counts. Are you quads shot or your knees blowing up? If the answer is no to both, don’t worry about it.
    Life is a succession of lessons which must be lived to be understood.
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  5. #5
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    first off: if your current set up feels good for now keep it as is.

    in my recent experience: i've ridden several moderate to long tours 100 - 500 miles. i commute via bike and also race occasionally. all my bikes have 170mm cranks. i recently tried 175s on my touring bike for a little extra torque and oomph. but i had left knee pain for the first time on a tour to montreal. i gave myself some time to acclimate to 175s but ultimately went back to 170s. my bike guru friends extoll the virtues of longer cranks for extra power but i prefer to spin at easier gears but with less wear and tear on my knees. i'm right about 6' with a 33" bike inseam.

  6. #6
    Senior Member mtnbud's Avatar
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    Thanks for the insights and personal observations!

    Being used to the 172.5 mm on the road bike, the 170's do feel a little different. Knowing how many of you are using the 170 mm cranks is a great endorsement to stick with the 170 mm crank arms.
    “If You Open Your Mind Too Much Your Brain Will Fall Out”

  7. #7
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    Most people probably wouldn't notice unless it's an extreme change of length or they 5'2" like I am. So, just find what what feels good and go with it.

    On my part, I definitely notice a difference. The socket for my right hand crank ended up damaged in a tumble and finding another 155 mm road chainset was near impossible. I tried 165 mm cranks until I could find something shorter. Huge mistake. I barely made it through 1 ride and had to be careful just flexing my knees to sit down in a chair for almost a week. Lucked out in finally finding a cycle shop who had 'left overs' of my out of production crankset and all was good again.

    But, my knees aren't particularly healthy. I think the extra 2 cm diameter of pedal rotation was too much flex. So, I learned what works for me.

  8. #8
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    I am the same size, 5'11, 32" inseam. This is pretty average male size and any commonly available crank will be OK, 170/172.5/175. These are only small increments that have small effects on pedalling style. I prefer to pedal rapidly so I prefer 170mm. I have a bike with 175 but use it for short range utility riding.
    For very short or tall riders , all of these common sizes are wrong.

  9. #9
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    mtnbud, Before I simplified my cycling life to just three bikes I had several road bikes with crank arms ranging from 165 mm to 175 mm. For me just a few minutes riding was all that I needed to be comfortable. My favorite size is 172.5 simply because it bridges the torque/cadence issues between the 170 mm and the 175 mm crank arms. At this time my road and touring bikes have 170 mm crankarms and the mountain bike has 175 mm crankarms, FWIW.

    Brad

  10. #10
    Senior Member geezerwheels's Avatar
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    Here's something to kick around -- from a reference found on Sheldon B's pages. However, it talks about producing maximum power--not about comfort or pain.



    Eur J Appl Physiol.
    2001 May;84(5):413-8.

    Determinants of maximal cycling power: crank length, pedaling rate and pedal speed.

    Martin JC, Spirduso WW.
    Source

    University of Utah, Department of Exercise and Sport Science, 250S. 1850E. Rm. 200, Salt Lake City, UT 84112-0920, USA. jim.martin@health.utah.edu

    Abstract

    The purpose of this investigation was to determine the effects of cycle crank length on maximum cycling power, optimal pedaling rate, and optimal pedal speed, and to determine the optimal crank length to leg length ratio for maximal power production. Trained cyclists (n = 16) performed maximal inertial load cycle ergometry using crank lengths of 120, 145, 170, 195, and 220 mm. Maximum power ranged from a low of 1149 (20) W for the 220-mm cranks to a high of 1194 (21) W for the 145-mm cranks. Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. Conversely, optimal pedal speed increased significantly with increasing crank length, from 1.71 m/s for the 120-mm cranks to 2.53 m/s for the 220-mm cranks. The crank length to leg length and crank length to tibia length ratios accounted for 20.5% and 21.1% of the variability in maximum power, respectively. The optimal crank length was 20% of leg length or 41% of tibia length. [emphasis added] These data suggest that pedal speed (which constrains muscle shortening velocity) and pedaling rate (which affects muscle excitation state) exert distinct effects that influence muscular power during cycling. Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.

  11. #11
    Still spinnin'..... Stealthammer's Avatar
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    I've ridden everything from 185mm to 155mm cranks on different bikes for different reasons, and I have found that most riders who swap out their 170mm cranks for 175mm (or any other exchange) generally only have pain or discomfort because they failed to make the other fit adjustments that the new crank required.

    These days my SS 29er mtb uses 177.5mm cranks because I use it mostly in tight technical riding and I like the extra leverage, my 26" mtb uses 175mm because I use it for all-around off-roading, my commuter mtb uses 170mm because I only use it exclusively on-road and I like to spin on it, one of my road bikes uses 170mm because I use it in group rides and the seem to work best for me and the other uses 172.5mm because it's more of a layed back distance ride, and my FG uses 165mm so I can spin like a madman on the downhills, and I barely notice a difference between them in actual use because I ride each bike differently.

    As stated by some others here, I don't believe there is an "optimal length" for any rider or any type of riding, rather I believe that fitting the rest of the bike properly and buying the crankset that best suits your desired pedal speed/power output needs is more important. It is a matter of finding your sweet spot, but on each bike for the way you ride it, and getting the overall fit right regardless of what length crank you choose is more important than the crankarm length itself.
    Just your average 'high-functioning' lunatic, capable of passing as 'normal' for short periods of time.....

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  12. #12
    The Left Coast, USA FrenchFit's Avatar
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    Good posts. I've experimented between 170mm and 175mm on a few of my road bikes. I'm 6' and 32" in. I experience more leverage with the longer cranks, but less comfort over distance..and that is after resetting my fit for the difference. For bikes I'll spend the most time sitting on and use the longest distances, it's best with 170mm cranks for me. I also think my stroke is a little smoother with the shorter cranks. Hill climbing, the longer cranks are winners.

  13. #13
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    Quote Originally Posted by mtnbud View Post
    My mountain bike has 175 mm crank arms and my road bike has 172.5 mm cranks. I'm 5' 11" and have a 32" inseam. I replaced the seat and seat post and I am starting to dial in the fit. It's feeling pretty good, but I do think I notice the shorter crank arm length.

    Opinions? Have any of you had to change your cranks due to fit issues?
    I'm 5'9" with a 32" inseam, I've used 170mm, 172.5mm, and 175mm cranks. I, personally, don't feel much of a difference between the various lengths after the first few revolutions. Initially, 175mm cranks feel like they might put a bit more pressure on my knees than 170mm cranks... but I can't say that I notice the difference in how my knees feel at the end of the day.

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    It's more of an issue for shorter riders, as they are more likely to experience pain or injury from using cranks that are too long. Tall guys may not feel optimised pedalling in small circles, but this is unlikely to cause an injury.

    At 5'4", I've certainly found that 170mm cranks don't agree with my knees. I've experimented with everything from 150mm to 170mm and feel that 165mm cranks are the best compromise between the leverage of a long crank and the ease of spinning and avoiding injuries of a short crank. 160 is ok, but I find it much harder to get up hills.

  15. #15
    Senior Member robow's Avatar
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    Look at it this way, many of Shimano's cranks only come in 165-175 size. A difference of only 1 cm but yet that accommodates people that range in size from say 5 ft tall to 6.5 ft tall. Ball park it, find what's comfortable, and then worry about something more meaningful. And btw, there are very few individuals that can tell the difference in 2.5mm, the width of a nickle.

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    About five years ago I got a great deal on a left over model year demo road bike, some of its parts had been swapped. It had 39/53 175 mm crankset, being fairly hilly in our area - decided to swap the crank for a 34/50. Made the arrangements with LBS to let me know when they got a crankset from a swap or build.
    A couple weeks later, lbs said they had a set, we made the swap - a very nice ultega with 170 mm cranks. After riding a couple day, really missed the longer crank and the bike fit was different, just rely cramped for space - I'm about 6'3, the longer cranks just better.
    ride long & prosper

  17. #17
    Senior Member Western Flyer's Avatar
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    Quote Originally Posted by robow View Post
    Look at it this way, many of Shimano's cranks only come in 165-175 size. A difference of only 1 cm but yet that accommodates people that range in size from say 5 ft tall to 6.5 ft tall. Ball park it, find what's comfortable, and then worry about something more meaningful. And btw, there are very few individuals that can tell the difference in 2.5mm, the width of a nickle.

    With all the minutia that is picked over in the bike world it would seem the 6% difference between 165 and 175 cranks might be a big enough deal. Here is an article that calls for shorter crank arms, radically shorter. Now as I read the article it is making its case for full aero bikes in TT and triathlon racing situations, which don’t directly apply to very many loaded touring cyclists. But using Sheldon Brown type extrapolations it’s possible to make a good guess at relative crank arm lengths for different touring configurations. It would seem the more upright touring cyclist who has straight or trekking bars would be happier with longer cranks and someone like me with drop bars set an inch or more below the saddle and spends a good bit of time in the drop position might do better with shorter cranks.

    On one hand when I am out of the saddle grinding up a steep dirt Forest Service road 175 cranks would be great. On the other hand when I have my forearms resting on the hoods as I am peddling into a steady head wind my knees hit my gut if I lean too far down with my 170 cranks. (Yes, losing a few pounds would be a better solution than shortening the cranks.) But it is ultimately finding what works best for the individual rider in most touring situations.
    Life is a succession of lessons which must be lived to be understood.
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  18. #18
    coprolite fietsbob's Avatar
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    It matters if you ride a banked velodrome track, But..
    Since you are not seeking to be the next winner of the world championship bike race,
    No it does not matter

    I'm normal sized,5'9", use 180 - 170 interchangably.

    performance coaches , high 100 RPM cadence riders .. will help you idealize the Length..

    Like at licenced to Race Camp.. USCF, Colorado.
    Last edited by fietsbob; 01-24-13 at 05:39 PM.

  19. #19
    Collector of Useless Info
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    6'3" with a 35" cycling inseam. I honestly can't tell the difference between 175 cranks and 170 cranks. The bikes they're on are different materials and geometries so they feel different generally, but the pedaling seems exactly the same.

  20. #20
    Senior Member robow's Avatar
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    Quote Originally Posted by Western Flyer View Post
    With all the minutia that is picked over in the bike world it would seem the 6% difference between 165 and 175 cranks might be a big enough deal.
    It's not a 6% spread. The OP is 5'11", so rule out the smallest length and now he can choose between either a 170 or 175. If he can't make up his mind and the 172.5mm is available, then go with that and he'll be set. But he should be careful as to make sure his socks don't compress after peddling a while or that couple of mm will throw everything off.

  21. #21
    In the right lane gerv's Avatar
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    Quote Originally Posted by robow View Post
    But he should be careful as to make sure his socks don't compress after peddling a while or that couple of mm will throw everything off.
    Good point

  22. #22
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    5'9 with 30" inseam. Most of my bikes have 170mm which works fine for me. I had 2 bikes with 175. I stopped riding them because they gave me knee pain, consistently. The knee pain wasn't awful and didn't appear until after I rode maybe 35 miles, but it was consistent and only on the bikes with 175. If I recall it was only in left knee, probably because my legs arent exactly same length.

    One is a 1993-ish Univega Via Carisma hybrid 54cm. It still has the 175mm so i dont ride the bike. (It has a biospace triple crank that I would be happy to trade for 170mm.)

    The other is a 1998 Jamis Aurora 57cm touring bike I swapped out the 175 for a 170mm triple crank, from same era, and I am much more comfortable on the bike.

    I have never tried any sizes other than 170mm or 175mm but 170mm works fine for me so I am sticking with that.

    I decided i am not going to needlessly incur pain or injure my knees so I wont ride anything but 170 anymore.

  23. #23
    The Rabbi seely's Avatar
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    I might have an excellent 170 Biopace triple for trade. Would you be interested? I need 175's.
    commuter turned bike mechanic turned commuter (also a Velocity USA employee, but this is my personal account)

  24. #24
    Senior Member robow's Avatar
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    GaryinLA,
    though it may be the crank length, many riders developed knee problems with the BioPace chainrings, I for included, and they have long been discontinued as a commonly found chainring. I realize that some might love them but most didn't and hence why Shimano dropped them so quickly.

  25. #25
    Certified Bike Brat Burton's Avatar
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    Quote Originally Posted by robow View Post
    GaryinLA,
    though it may be the crank length, many riders developed knee problems with the BioPace chainrings, I for included, and they have long been discontinued as a commonly found chainring. I realize that some might love them but most didn't and hence why Shimano dropped them so quickly.
    Elliptical chainrings were marketed before Shimano introduced Biopace and those other designs apparently did cause knee issues. The Biopace design was completely different and I think you'll find Shimano dropped them for other reasons altogether.

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