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  1. #1
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    Crank Size and Flat Spots

    Hi all. I've read through the numerous topic about crank size both here as well as other sites around the web, along with having tried out a few of the formulas for determining appropriate crank size.

    With that out of the way, on to my problem. I've been professionally fitted on all my bikes, size 56 frames, 172.5 cranks. I'm ~6'2" (and shrinking ) and definitely in clyde territory at 220. While using SPD pedals, I would occasionally hit a flat spot with my left foot, with some more frequent pulling to the medial back of my knee. I've since recently switched to SPD-SLs and have noticed more frequent flat spots along with the continued tweak to my knee. I've done my best to account for stack height, having raised my saddle incrementally 3x.

    I have always had a short inseam in pants size for my height, but I know that's in no way a hard rule for determining crank length. With that, I looked into some of the more involved formulas for figuring crank length; apparently, most of them have me coming in with a shorter 170mm, which was suprising. So, I'd like to hear from some folks who may have experienced similar instances wherein their crank length is disproportionate to their height. Thanks!

  2. #2
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    I'm 6'2.5", and ride a 62cm, 172.5 crank. I would assume you have a very long stem to make a frame that small work for you. Also, if you are short-legged, that would probably result in needing a shorter crank as well. My inseam is 34"; if yours is closer to say 30", I would thing that a 172.5 crank is excessive, and really only good for climbing. I used to use 175mm cranks, and they were just too much for me.

    BTW, how "professional" was that fitting? From what I've seen, 90% of those offering "professional fittings" are pure hyperbola...
    I'm upping my standards;
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  3. #3
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    Quote Originally Posted by No Time Toulouse View Post
    I'm 6'2.5", and ride a 62cm, 172.5 crank. I would assume you have a very long stem to make a frame that small work for you. Also, if you are short-legged, that would probably result in needing a shorter crank as well. My inseam is 34"; if yours is closer to say 30", I would thing that a 172.5 crank is excessive, and really only good for climbing. I used to use 175mm cranks, and they were just too much for me.

    BTW, how "professional" was that fitting? From what I've seen, 90% of those offering "professional fittings" are pure hyperbola...
    Well, the fitting was done by a certified BG/Retul shop, so I guess it's as professional as it can be? It was very thorough, and I trust the guy who did it.

    My inseam is somewhere in the 32 range, although I can sometimes wear a 30 in pants, so it is definitely short. The 56 frame fit me better than the 58, so that's what I went with, and my stem is pretty short at 100. All of that is likely a lack of flexibility thing I guess, I'm pretty comfy on the bike as it is, and I suffer nothing in the way of issues with handling.

  4. #4
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    Quote Originally Posted by Wetworks View Post
    Well, the fitting was done by a certified BG/Retul shop, so I guess it's as professional as it can be? It was very thorough, and I trust the guy who did it.

    My inseam is somewhere in the 32 range, although I can sometimes wear a 30 in pants, so it is definitely short. The 56 frame fit me better than the 58, so that's what I went with, and my stem is pretty short at 100. All of that is likely a lack of flexibility thing I guess, I'm pretty comfy on the bike as it is, and I suffer nothing in the way of issues with handling.
    You obviously don't know your bicycling inseam: stand against a wall with your stocking feet 6 inches (15 cm) apart and with a book pushed firmly upward in your crotch. Measure the distance from the book spine to the floor. That is your bicycling inseam.

    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.

  5. #5
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    Quote Originally Posted by Kerry Irons View Post
    You obviously don't know your bicycling inseam: stand against a wall with your stocking feet 6 inches (15 cm) apart and with a book pushed firmly upward in your crotch. Measure the distance from the book spine to the floor. That is your bicycling inseam.

    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.
    It's a 34 according to your method. Other methods I have read state this isn't reliable due to limitations based on trochanter length and foot size, so there's that.

    Thing is, this isn't about me gaining performance. I've read all the studies and observations which state crank length really doesn't matter. As I stated in my OP, this is about a defect in my pedal stroke and comfort, both of which have taken a bigger hit as of late with a change in pedals. To that end, I am hoping someone of similar proportions (shorter legs) can tell me if they've experienced similar symptoms, and if so, did changing crank lengths help.

  6. #6
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    Just throwing this idea out there...if it is just your left side giving you issues, then buy a a left crank at 170mm and keep the right side at 172.5mm. If the "flat spot" goes away, then you know. One leg is usually slightly longer than the other--maybe it's your left leg. Check eBay for a matching NDS crank arm.

    As an aside, I switched to 145mm cranks (I'm about a foot shorter than you) and feel that my spinning is smoother.
    Wake me up when it's alarm green.

  7. #7
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    Quote Originally Posted by Tachycardic View Post
    Just throwing this idea out there...if it is just your left side giving you issues, then buy a a left crank at 170mm and keep the right side at 172.5mm. If the "flat spot" goes away, then you know. One leg is usually slightly longer than the other--maybe it's your left leg. Check eBay for a matching NDS crank arm.

    As an aside, I switched to 145mm cranks (I'm about a foot shorter than you) and feel that my spinning is smoother.
    I actually have considered that, but I'm concerned about possible other issues arising.

    If I decide to go ahead and do this, I intend to get a power meter as well. Just want to be sure about what I choose as far as the cranks. I'll just have to go to the shop and ask for a refit with a special eye on my pedal stroke.

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    They only way you are going to know if different crank length is going to work is to try it.

    I'm not a physical therapist, but I would strongly suggest NOT riding with disparate crank lengths. Don't forget if one crank is longer or shorter, it's not just affecting extension at the bottom, it's also affecting compression at the top - It might be ok to do this as a short test, but I wouldn't recommend it for any long term solution without consulting with a physical therapist who is trained to work with cyclists (they are sadly rare).

    You can sometimes find cranksets sans chainrings on ebay fairly cheap - I've bought them this way.

    BTW, I'm 6' 5" and but also have a 34" cycling inseam - I ride a 61cm bike.

    I've always ridden 175mm cranks on my road bikes and 180mm on my short lived MTB. I've actually considered giving 180 a try. Not for any reason other than it just *seems* like it would be more efficient for me. I've seen videos of me riding and it looks like I'm pedaling a tricycle or something. Cost (only available in Dura Ace) and concerns about skipping a pedal have kept me on 175s

  9. #9
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    Quote Originally Posted by Migen21 View Post
    They only way you are going to know if different crank length is going to work is to try it.

    I'm not a physical therapist, but I would strongly suggest NOT riding with disparate crank lengths. Don't forget if one crank is longer or shorter, it's not just affecting extension at the bottom, it's also affecting compression at the top - It might be ok to do this as a short test, but I wouldn't recommend it for any long term solution without consulting with a physical therapist who is trained to work with cyclists (they are sadly rare).

    You can sometimes find cranksets sans chainrings on ebay fairly cheap - I've bought them this way.

    BTW, I'm 6' 5" and but also have a 34" cycling inseam - I ride a 61cm bike.

    I've always ridden 175mm cranks on my road bikes and 180mm on my short lived MTB. I've actually considered giving 180 a try. Not for any reason other than it just *seems* like it would be more efficient for me. I've seen videos of me riding and it looks like I'm pedaling a tricycle or something. Cost (only available in Dura Ace) and concerns about skipping a pedal have kept me on 175s
    That gave me a legit lol!

    I raised the seatpost another 2mm or so over my previous line, seemed to help a bit actually. I'm going to stick with the 172.5 for the time being and invest in a left-sided power meter. Worst comes to worst I can sell it all and start over.

  10. #10
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    What? 34" inseam are not short legs, sorry. I would suggest to shift to a lower gear (more spin) and work on your technique!
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    Quote Originally Posted by Migen21 View Post
    I've seen videos of me riding and it looks like I'm pedaling a tricycle or something.
    hehe, you have to post those videos?

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    I'm 6'2" and ride a 59 or 60cm frame depending on the manufacture. What I always use is a 58cm top tube with a 140mm stem, with 5" of drop. I started out with 172.5's but long ago, changed to 175's.
    If your opinion differs from mine, ..........Too bad.
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    Quote Originally Posted by MR_GRUMPY View Post
    ...... I started out with 172.5's but long ago, changed to 175's.
    You know, I tried 175's, but kept on slipping my rear tire when climbing. I could rotate tubulars a full inch......
    I'm upping my standards;
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  14. #14
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    Quote Originally Posted by duriel View Post
    What? 34" inseam are not short legs, sorry. I would suggest to shift to a lower gear (more spin) and work on your technique!
    Which is why inseam should never be the sole measurement utilized. I'm long-torsoed, average arms and smallish legs. Trust me, looking at me trying to stand over the top tube of a 58 Tarmac was pretty sad.

    My technique is considered pretty solid, no issues with knee tracking, no bouncing in the seat until I hit about 110 RPMs. I'm just not all that flexible anymore, lol. That said, smaller cranks would make my spin better. Nonetheless, I am sticking with 172.5 and will look into tweaking my fit more and more.

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    If the back of your knee is hurting, you should not have raised your saddle - you should have lowered it.

    IMHO, you need to work on technique, not throw money at it. I would try pedaling drills.

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    Quote Originally Posted by crit_boy View Post
    If the back of your knee is hurting, you should not have raised your saddle - you should have lowered it.

    IMHO, you need to work on technique, not throw money at it. I would try pedaling drills.
    I don't disagree that my technique could use some work. The back of my knee isn't the classic "seat is too high" problem, it manifests itself on hard efforts (think headwinds) and almost always at around the 9-10 o'clock position (left knee for reference).

    Keep in mind that this is also the leg that experiences flat spots on easy spins, hence the reason I think the crank may be too long, which would then negate what I just said and validate your advice. Ugh.

  17. #17
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    it doesn't really address Wetworks questions, but I thought this new video from GCN on the crank lengths the pros use was pretty interesting.

    Adam Hansen's comments in particular interested me.


  18. #18
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    A little history with another opinion.

    Dave Moulton's Blog - Dave Moulton's Bike Blog - Crank*Length
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  19. #19
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    Quote Originally Posted by Wetworks View Post
    Keep in mind that this is also the leg that experiences flat spots on easy spins...
    What exactly do you mean by "flat spots?" I realize you consider it a defect in your pedal stroke. Do you mean a momentary loss of resistance or increase of resistance from the pedal?

  20. #20
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    Quote Originally Posted by Wetworks View Post
    I don't disagree that my technique could use some work. The back of my knee isn't the classic "seat is too high" problem, it manifests itself on hard efforts (think headwinds) and almost always at around the 9-10 o'clock position (left knee for reference).

    Keep in mind that this is also the leg that experiences flat spots on easy spins, hence the reason I think the crank may be too long, which would then negate what I just said and validate your advice. Ugh.
    Your knee is feeling the stress in the recovery stage of hard efforts, are you pulling up on your pedals?
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    Next time, try gluing the tubular! Also, there's a hole in your rim. Put that stem thingy through the hole!
    Quote Originally Posted by No Time Toulouse View Post
    You know, I tried 175's, but kept on slipping my rear tire when climbing. I could rotate tubulars a full inch......

  22. #22
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    Quote Originally Posted by SwiftSolo View Post
    Next time, try gluing the tubular! Also, there's a hole in your rim. Put that stem thingy through the hole!
    Oh, they were glued-with mastik1. But it was hot....

    Yeah, I used to buy threaded stems, and put the dork nut on the rear for that reason. few come with threaded stems today.....
    I'm upping my standards;
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  23. #23
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    Quote Originally Posted by wim View Post
    What exactly do you mean by "flat spots?" I realize you consider it a defect in your pedal stroke. Do you mean a momentary loss of resistance or increase of resistance from the pedal?
    Yes, a momentary loss of resistance wherein my foot kind of clunks the pedal forward, sort of like when you do 1 legged cadence drills with your non-dominant foot, but this is during low resistance.

    Quote Originally Posted by velodog View Post
    Your knee is feeling the stress in the recovery stage of hard efforts, are you pulling up on your pedals?
    No, on the hard effort, first part of the downward stroke. I pull up a bit at times, but not always.

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    Wetworks,

    Have you had the knee diagnosed?

    I had almost the exact same 'feeling' in my left knee - kind of a strange catch/hiccup right as you go over the top of the pedal stroke. Not really painful, just a strange feeling - sometimes the knee would be a little sore after a particularly long or hard ride.

    In my case, it turned out to be a small tear in the miniscus - not worthy of surgery - and not really otherwise treatable. The orthopedic surgeon who reviewed the MRI said if it gets worse to come back. I asked him what kind of exercise I could do - he said 'go ride your bike'.

    Anyway, it's always a good idea to get a diagnosis so you know what you are dealing with.

  25. #25
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    Quote Originally Posted by Wetworks View Post
    No, on the hard effort, first part of the downward stroke. I pull up a bit at times, but not always.
    OK, I got a bit mixed up. When I think crank I always think as if looking at the drive side of things, 1-4 o'clock=power, 7-11o'clock =recovery. Left or right side doesn't matter. That way I don't have to do clock conversions in my head.

    Thinking that it was happening on the recovery phase is why I was concerned that you may have been pulling up, stressing the joint.
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