Crank lenght?

Discussion in 'Cycling Training' started by RHR38, Oct 9, 2009.

  1. RHR38

    RHR38 New Member

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    Ok, 'subject' is short sized (5'2,7"/136 lbs) racing cyclist, U19 first year next season. Inseam lenght is 32,4". Pretty good legspeed and decent performance (=FTP310, 5 min 360, 1 min 585 and + 220 rpm on free rollers good relaxed straight line). Now with 170 cranks, but wondering if switch to 172,5 cranks is worth something?
     
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  2. swampy1970

    swampy1970 Well-Known Member

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    It'll lighten your wallet significantly but I wouldn't change crank length for increments of at least 5mm.

    Draw a line that's 17cm and then draw another that's 17.25 and figure if you really are going to get a whole bunch of advantage from that .25 cm.

    FWIW I used to love my 185mm cranks (TA Alize).
     
  3. jhuskey

    jhuskey Moderator

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    I agree it is such a small amount of measurment. Just turn the 170 faster and you will see results.:D
     
  4. frenchyge

    frenchyge New Member

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    62.7" total height and 32.4" inseam, with over 5 W/kg at FTP already?

    Is this subject a real person, or something you are building out of spare parts dug up from former cycling greats? :p:D
     
  5. RHR38

    RHR38 New Member

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    Subject is a real person.

    I have paper in front of me that suggests that inseam between 30,7-33,0 inch takes 170 and inseam between 31,8-34,2 inch takes 172,5 cranks. Now these suggestions are overlapping and thats's what I'm wondering here.

    Started wondering it also, because some complete bike setups i looked lately, suggest already 172,5 crank in sizes suitable for this subject.

    As it's about time to start next training season, timing for crank size change would be perfect now IMHO as there's plenty of time to teach legs and muscles to different circles/angles without problems aso. I've heard some stories how people have run into knee problems, when doing shoe/crank changes in the middle of tough training season.
     
  6. dkrenik

    dkrenik Member

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    No "paper" or on-line calculator can take into account an individual's flexibility (or lack there of), position on the bike, functional and structural asymmetries, and such. All of these factors (and probably more) affect "optimum" crank arm length.

    It sounds like your "subject" is already about where they need to be (judging by their power numbers - yikes!). I also agree with the others in that there's no point in changing crank arms for a 2.5mm difference in length.

    Dave
     
  7. RHR38

    RHR38 New Member

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    2,5 mm longer cranks w/ same FTP is 10 second faster time in 25K timetrial, right or wrong?

    10 seconds and seven athletes in top level race is not a news, I think. Even in junior level.
     
  8. dkrenik

    dkrenik Member

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  9. daveryanwyoming

    daveryanwyoming Well-Known Member

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    +1 Dave, for the same power and the same CdA (Crr, weight, etc.) you'll get the same time whether riding 160mm cranks, 190mm cranks or anything in between.

    Crank lengths (and cadence) is a means to generate your sustainable power but it's the power that determines how fast the bike goes down the road.

    -Dave
     
  10. Piotr

    Piotr New Member

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    Same FTP, who is faster, the guy riding in 53x17 or the guy riding in 53x16? :D
     
  11. swampy1970

    swampy1970 Well-Known Member

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    If 17 sprocket guy is pedalling significantly faster then it's a no brainer, but it's still the guy who's in the 13 that's going to win. :p
     
  12. Bikeridindude

    Bikeridindude New Member

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    I have a riding partner that is about your size and he switched from 170's to 172.5's earlier this year. He thought it would be the magic little pill that made him superman. Of course it wasn't. He was excited about it at first, but now that the season's over he's looking to get rid of the 172.5's. Sorry, I have no scientific information.
     
  13. RHR38

    RHR38 New Member

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    Yeah, must admit I got pretty damn far with that statement; watt = watt afterall.

    Another speculation..:D Same strenght endurance (hahaa..) and longer cranks = smaller average pedal force = less lactate / crank revolution = higher sustainable power = more speed w/ same effort. Looked at it at analyticcycling. Something like 1,5% difference, it's so small difference between these two sizes that I'm wondering too if it's worth nothing, but then again..

    Flexibility, functional and structural asymmetries are no problem. TBO no full blown tt bike has not been used so far, only road bike with ergostem, clipons, tt helmet and skinsuit..clearly below average wheels (no disc wheel aso). Posion on tt is quite good, prolly reason why tt results are good too compared others w/ super hardware.

    No magic pill thoughts about it. We know it takes more quality work to make fast faster, but in a way competition is hard = small things matter and in another way it's still a long walk to go.
     
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