optimum crank arm length

Discussion in 'Cycling Equipment' started by Randyforriding, Nov 30, 2012.

  1. Randyforriding

    Randyforriding New Member

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    I've been using 195mm Zinn custom cranks for several thousand miles. These were actually shorter than Zinn's formula called for. I've recently had the chance to try 175mm and 172.5 mm cranks (not Zinn) on the same bike. I think I am now ready and able to make some judgements about crank arm length.
    One thing I noticed when I first started using the 195mm crank arms is that I got winded easier and long hard rides were more tireing. I figured this would go away when I became better adapted to the long cranks. To some degree this was true and I went back and forth between thinking the long cranks were an advantage and thinking they were a disadvantage.
    Now that I've been able to put some miles on shorter cranks on the same bike, I'm quite certain the 195s are too long.
    Here is what I think is going on: Though you are able to push with more force when your legs are near full extension, which works in favor of shorter cranks, I think there is a net gain in leverage with longer cranks. But I think the penalty (nothing is free) is that turning longer cranks takes more oxygen. That is, turning your legs in a big circle takes more oxygen than turning your legs in a small circle.
    If you were able to graph the two (leverage vs. heart rate) at some point the two graphs would cross at your optimal crank length. (Complicating this is that I think your optimum crank length standing is longer than your optimum length sitting.)
    For this reason, I think your VO2 max is probably as important in choosing a crank arm length as is your inseam measurement. That is, a short person with a good VO2 max might be able to use longer crank arms than a tall person with a poor VO2 max.
    I've read the article at powercranks.com regarding the Martin study and agree with most of what he says. I too have found that I can lower my handlebars with shorter cranks, giving me a more aero position, but I'm not quite ready to go to extremely short crank amrs. Maybe I still have a bias toward longer cranks, but I'm inclined to think that 177.5 is probably the best length for me and, if my memory serves me, I have an inseam of 94cm.
    Five years ago I would have said that most people are using crank arms too short for them. Now I would say that most people are probaly using crank arms that are about right or slightly too long.
    Randy
     
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  2. danfoz

    danfoz Well-Known Member

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    Those 195's must be murder in the corners.
     
  3. CAMPYBOB

    CAMPYBOB Well-Known Member

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    Yeah, but they make a great kickstand.

    Thanks for the post, Randy.
     
  4. Randyforriding

    Randyforriding New Member

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    You need to have a custom frame with higher bottom bracket. I have a Davidson custom. Toe overlap with the front wheel is an issue too.
     
  5. danfoz

    danfoz Well-Known Member

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    I wouldn't be so worried about the overlap but that high BB must make for "entertaining" high speed descents.
     
  6. Randyforriding

    Randyforriding New Member

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    With the longer cranks, your center of gravity is no higher than normal. Remember, you are adjusting the seat height so the leg has a slight bend with the pedal at the bottom of the stroke. Longer crank, lower seat. The bottom bracket is priobably only about a one to one and a half centimeter higher than what you have on your bike. I don't do crits, so no need for a high bottom bracket.
    With the shorter cranks, I've had to raise the seat accordingly, but haven't noticed any problems with descents. I live in Boulder County, Colorado where there are some really fast downhills. Haven't crashed yet. Came close when I blew a front tire coming down South Sait Vrain canyon. Always wondered what it would be like getting a flat in the front tire at speed. Now I know and it is pretty hairy.
    Randy
     
  7. danfoz

    danfoz Well-Known Member

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    Good point. The longer crank lowers the seat.
     
  8. dominikk85

    dominikk85 New Member

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    I actually don't think it makes a big difference. theoretically you have a longer lever but there are also studies that claim that shorter cranks produce higher power. match sprinters produce extreme power with 167.5s (because they don't want to bottom out on the track). road racers mostly use 177.5 I think.
     
  9. 531Aussie

    531Aussie Well-Known Member

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    For the regulars on here, look away now, coz I'm probably just gunna repeat myself........yet again. :) I'm posting because I had a couple of similar observations when I had long cranks.

    I'll keep it short. :)

    I'm 182cm, with long-ish legs (89cm inside leg) and size 43 Sidis. After accidentally putting myself in a blind crank length study about ten years ago* -- and finding that longer cranks felt faster when riding off the saddle up short hills -- I got a bit obsessed about the "secret weapon" that I thought long cranks were. I did lots of Googling; found the 'common' sites; did the formulas, and settled on trying some 180s *(10sp Dura-Ace), because that was the longest crank Shimano made. This was a much easier option than getting something custom.

    They felt great at first, and I really felt like I had a 'secret' advantage. Long story short: I persisted with the 180s for about a year (I bought another pair of 180mm 7800s, and a cheap pair of used 7700 Dura-Aces), but the position they put me in eventually annoyed me, and they hurt my knees a little, so I went back to mostly riding 172.5s. When I returned to shorter cranks, I probably would've been just as happy with 170s or 175s, but my local shop had a deal on 2 pairs of used 172.5s, so...

    Yeah nah, starting the down-stroke from such a high position with 180s eventually drove me nuts when riding seated (obviously), because I felt like I couldn't "get on top of", or "dominate" (I stole that word in this context from someone else :) ) the pedal, and was frequently getting off the seat to alleviate this feeling. I was constantly adjusting my saddle, looking for a comfortable position, but I never found it. I still loved stomping the long cranks off the saddle on short hills, or bridging gaps and getting out of fast corners in races, so I kept persisting.

    After about 9 or 10 months, I was getting some medial knee pain in both knees, so I did some rides of shorter cranks, in an attempt to alleviate the discomfort. Being back on shorter cranks was immediately much more comfortable, and not just because my knee pain went away, but because off the position: being relatively higher, I could really pound the pedals. So, I went back and forth between 180s and 175s for the next couple of months and, after being convinced that I was no slower on shorter cranks when riding seated (I was most likely faster, just coz I could get more aero), I ditched (and sold) all the 180s.

    I also believe (as I think you've suggested) that any benefits of longer cranks are balanced out by them being a little harder to push. In other words, when riding seated, while there's probably more leverage, starting the down-stroke with a sharper knee and hip angle presents a slight biomechanical disadvantage.

    I also had a strange sensation that, when I switched back to shorter cranks, my heart rate was a little lower for the same speeds, and my quads weren't working as hard. Go figure!

    Some more general points: Zinn's preferred formula of 21.6% is crazy! That puts me on ~192mm cranks!!!! I can't even get my head around that. I can't believe it's still out there. And it's not like there's a ton of pros using Zinn's formula and riding everywhere at 60km/h. Eh, whatever.

    Also, for anyone considering long cranks, you've gotta be in shape, because the slightest fat stomach will make them impossible to use. :) I'm serious. It's not just having your thighs hit the stomach, but it can even affect breathing. And if you have to ride totally 'unaero' to compensate, then what's the point?!

    Here's a twist: Having said all that, I did sometimes miss my long cranks at criteriums, so I bought 2 pairs of 177.5s. Ha. I often use these at fast criteriums where there's lots of off-the-saddle stomping.....especially when it's windy.


    * I'd been commuting on a couple of bikes with 170mm cranks, then, one day I dusted off my old Raleigh to ride to uni, totally forgetting that it had 175mm cranks on it. I was riding with jogging shoes and toe-clips which, due to the more 'arbitrary' way I used to flop my feet into the pedals, may be one reason I didn't instantly notice that I was pedalling a 1cm bigger circle. Anyway, on the regular short hills on my commute, up which I always rode off the saddle, I felt like Superman, and was using a gear or 2 higher than usual. I couldn't work out why I was flying! :) I even got off to check that the cassette was the same, and it was. I obviously eventually realized that it was the longer cranks, so......
     
  10. danfoz

    danfoz Well-Known Member

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    What size road racers? I haven't seen a 177.5 on a single bike sold in a shop - For example Specialized runs a 175 on their bikes from 58cm up to their largest frame size 64. Do you mean 172.5?
     
  11. alienator

    alienator Well-Known Member

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    This thread is one great example of how crank length formulas have no substance behind their calculations. The right crank length is an entirely based on a given individual, not some formula with vague foundations.
     
  12. 531Aussie

    531Aussie Well-Known Member

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    Yep, as someone else once said on another forum: it's more voodoo than science :)
     
  13. 531Aussie

    531Aussie Well-Known Member

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    Yes, I don't think many pros would be using that length, especially given that most of them tend not to be tall. I haven't researched it for a long time, but a few years ago, there were photos of Boonen, Ullrich and Backstedt using 177.5s, and that was about it at the time.
     
  14. danfoz

    danfoz Well-Known Member

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    V.Good post btw. May I attempt a finish of that sentence? So long, but not too long.

    I have no frame of reference (never having ridden longer cranks than 170- at 5'9" I'm not all that tall) but I think I grasp exactly what you are saying about needing to get out of the saddle to stay on top of the gear. My personal climbing style unfortunately requires staying on top of the gear while seated :)
     
  15. oldbobcat

    oldbobcat Well-Known Member

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    All big guys, well over 6' tall.

    Regarding Zinn's 21.6%, maybe his baseline was riders with 78.70 cm legs who happened to be happy with the 170s their bikes came with. I don't know.

    Merckx was a long-legged 6' tall, and he used 175s. Gimondi was a bit taller but more average-proportioned. He used 175s, occasionally 177.5s for time trials. Anquetil and LeMond were about 5'9 1/2" and they used 175s. Greg's V02 Max was through the roof.

    I've been using 175s since 1976 and see no reason to switch.
     
  16. alienator

    alienator Well-Known Member

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    I'm 6'1"--can't remember what my cycling inseam is--and use 175's. It's all about what you're comfortable with and plays nice with the way your body functions.
     
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