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How Critical Is Crank Length?

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  • 15-05-2011 2:32pm
    #1
    furiousox
    Registered Users Posts: 6,915 ✭✭✭


    Just wondering how critical crank length is?
    I'm 174cm (just under 5'9") and I'm using 172.5mm cranks, have been for years.
    Apparently if you're 5'10" or under you should be using 170mm cranks.
    So should I bother changing to 170mm, would I notice a discernible difference?

    CPL 593H



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    #2
    Lumen
    Registered Users Posts: 31,020 ✭✭✭✭Lumen


    No. If it aint broke don't fix it.


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    #3
    bcmf
    Closed Accounts Posts: 6,679 ✭✭✭bcmf


    some folk say its critical but srhorter cranks are supposedly better for those who spin and longer (as well as those with longer legs) ate better for mashers. But as Lumen says if you are not haveing any difficulties then stay with what you have


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    #4
    Murph100
    Registered Users Posts: 1,088 ✭✭✭Murph100


    +1 on what the lads have said.

    If you search the web you'll find proponents of crazy long cranks and silly short cranks battling it out with their various conflicting theories :rolleyes: ... but you'd be better off spending those wasted hours training instead.


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    #5
    Paul Kiernan
    Registered Users Posts: 2,161 ✭✭✭Paul Kiernan


    I'm building up a bike for my wife at the moment and unthinkingly ordered a 172.5 crank instead of a 170 (she's 5'5"). Am I going to change it? Not a chance in hell.

    Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................


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    #6
    anoble66
    Registered Users Posts: 1,096 ✭✭✭anoble66


    i moved from 172 to 170 crank arms, and I cannot tell any difference.


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    #7
    ednwireland
    Registered Users Posts: 14,315 ✭✭✭✭ednwireland


    anoble66 wrote: »
    i moved from 172 to 170 crank arms, and I cannot tell any difference.
    +1
    gone from 170 to 172.5 (on different bikes)

    switch between them cant tell the difference

    My weather

    https://www.ecowitt.net/home/share?authorize=96CT1F



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    #8
    Inquitus
    Registered Users Posts: 13,734 ✭✭✭✭Inquitus


    Paul Kiernan wrote: »
    Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................

    Depends what cassette you put on the back, with a 12/25 she might not be impressed!


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    #9
    Quigs Snr
    Registered Users Posts: 4,133 ✭✭✭Quigs Snr


    furiousox wrote: »
    Just wondering how critical crank length is?
    QUOTE]

    Apparently it's the girth that counts, or at least that's what I read in the missus's cosmo magazine.

    No seriously, very little difference between 170 and 172.5, it's very much a case of personal preference. I am 170cm and ride a 170mm crank, but I have ridden a 172.5 and felt no difference, my numbers (recorded between the two bikes over a period of months on an Imagic Turbo Trainer) show no difference at all in terms of power or speed, the only difference was that under pressure - HR over 160 - my cadence increased by an average of 1 rpm on the 170mm cranks, at a regular cruise steady there was no cadence difference. I did fatigue a little in the hip joint over long distances at a low threshold with the 172.5 but that could be unrelated to the crank length as it didn't happen enough for me to categorically state that was the reason.

    Most riders around your height would ride a 172.5 (notable exceptions are Sastre and Cavendish who ride 170).


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    #10
    Paul Kiernan
    Registered Users Posts: 2,161 ✭✭✭Paul Kiernan


    Inquitus wrote: »
    Depends what cassette you put on the back, with a 12/25 she might not be impressed!

    That's not a bad idea. I might whack on an 11-23 and take her for a little loop around Slieve Maan, Shay Elliot and Sally Gap. Then when I change it to an 11-28 she'll be over the moon:pac:.


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    #11
    velogirl
    Registered Users Posts: 182 ✭✭velogirl


    Very recently changed bikes and changed from a 172.5 to 170 cranks (quite by accident) and WOW what a difference it has made. Cadence went way up - has changed my pedalling style from a big gear pusher to a pedaller/spinner. Speed has increased also and climbing - although I pedal more is easier.

    Thought it was the new bike but now am of the opinion that it is the crank lenght as the new bike is very similar to the old bike. By the way I'm 5ft 7


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    #12
    Paul Kiernan
    Registered Users Posts: 2,161 ✭✭✭Paul Kiernan


    velogirl wrote: »
    Very recently changed bikes and changed from a 172.5 to 170 cranks (quite by accident) and WOW what a difference it has made. Cadence went way up - has changed my pedalling style from a big gear pusher to a pedaller/spinner. Speed has increased also and climbing - although I pedal more is easier.

    Thought it was the new bike but now am of the opinion that it is the crank lenght as the new bike is very similar to the old bike. By the way I'm 5ft 7

    You are taking the piss, aren't you?


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    #13
    feck sake lads
    Closed Accounts Posts: 2,889 ✭✭✭feck sake lads


    just drop or raise your saddle by same amount ,can't think who told me that;)
    leave well enough alone you'll be fine honest.


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    #14
    DonalK1981
    Registered Users Posts: 370 ✭✭DonalK1981


    I was looking for a new crankset, and if any of you guys are changing let me know. Looking for ultegra, just to match you know!


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    #15
    velogirl
    Registered Users Posts: 182 ✭✭velogirl


    No I'm not taking the piss.............. I am deadly serious. It made a huge difference


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    #16
    Paul Kiernan
    Registered Users Posts: 2,161 ✭✭✭Paul Kiernan


    velogirl wrote: »
    No I'm not taking the piss.............. I am deadly serious. It made a huge difference

    It's a difference of 2.5mm, a tenth of an inch! Now, I don't have much knowledge of these things but if you have any interesting stories on what a difference a tenth of an inch can make I'd just love to hear them.


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    #17
    velogirl
    Registered Users Posts: 182 ✭✭velogirl


    I'm not a mathematician - perhaps someone else would be more qualified to enlighten you on the technicalities of it. I just know what a difference it made.

    http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

    The above calculator might help someone.


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    #18
    Idleater
    Registered Users Posts: 6,236 ✭✭✭Idleater


    velogirl wrote: »
    I just know what a difference it made.

    I'm going to agree with this, I've gone from a 175 to a 172.5 after a bike fit and I much prefer the new size. When I go back to the old one it tends to niggle my bad knee a bit.

    Obviously there are two reasonably different bikes and groupsets involved, as well as the new one being the correct frame size which might have more influence, but it's the crank rotation that I seem to notice most. Both have the correct seat post measurement, the only difference being the length to handlebars being 10mm too long on the old.


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    #19
    C3PO
    Registered Users Posts: 3,746 ✭✭✭C3PO


    velogirl wrote: »
    No I'm not taking the piss.............. I am deadly serious. It made a huge difference

    I think it's only a very serious cyclist who could distinguish between 170mm and 172.5mm .... 2.5mm is really a tiny difference and certainly it couldn't change your whole riding style!
    I've got 170mm on my road bike, 172.5mm on my cross/winter trainer and 175mm on my mountain bikes and I can't feel any difference between them!


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    #20
    bcmf
    Closed Accounts Posts: 6,679 ✭✭✭bcmf


    About 18mnths I changed from 170 to 175. Yes it made a difference. I did have to adjust seat position a tad but found less cadence and found it better. Everyone is different to what kinda cadence they like and how their legs/knees react.


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    #21
    fat bloke
    Registered Users Posts: 5,825 ✭✭✭fat bloke


    RPL1 wrote: »
    I think it's only a very serious cyclist who could distinguish between 170mm and 172.5mm .... 2.5mm is really a tiny difference and certainly it couldn't change your whole riding style!
    I've got 170mm on my road bike, 172.5mm on my cross/winter trainer and 175mm on my mountain bikes and I can't feel any difference between them!


    If it was a negligible difference, then crank manufacturers wouldn't make them with those 2.5 mm stepped intervals.

    Put a carpet knife gently against the side of your face. Now push it in 2.5mm and tell me it's negligible.

    It's all relative bud. If the girl found an appreciable difference, who are you to tell her otherwise?:rolleyes:


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    #22
    user111
    Registered Users Posts: 5 ✭user111


    Paul Kiernan wrote: »
    I'm building up a bike for my wife at the moment and unthinkingly ordered a 172.5 crank instead of a 170 (she's 5'5"). Am I going to change it? Not a chance in hell.

    Of more concern is that she's used to a triple and I got her a compact and there's going to be hell to pay when she hits that first big hill and goes looking for her granny .................

    longer crank arm will give you better force, so it will be easier to push. small saddle height adjustments will be recommended.
    What fr.derailleur&r.derailleur&cassette do you have atm ?


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    #23
    Lumen
    Registered Users Posts: 31,020 ✭✭✭✭Lumen


    Crank length obviously matters to some degree since it has an effect on gearing and joint angles.

    2.5mm doesn't sound much that but for a given effective saddle height an extra 2.5mm on the cranks will raise your knee by 5mm at the top of the stroke.

    People with knee and back problems will fuss over 5mm of saddle height so I don't think it's unreasonable to be fussy about 2.5mm of crank length, if you're a princess-and-the-pea kind of person.

    I still wouldn't change it unless I had a reason to, or if I was well outside normal recommendations for my leg length.


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    #24
    Hermy
    Moderators, Society & Culture Moderators Posts: 11,314 Mod ✭✭✭✭Hermy


    fat bloke wrote: »
    Put a carpet knife gently against the side of your face. Now push it in 2.5mm and tell me it's negligible.

    But according to Lumen you would have to push it in 5mm to get the full effect.:)

    Genealogy Forum Mod



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    #25
    kumate_champ07
    Closed Accounts Posts: 7,150 ✭✭✭kumate_champ07


    feck sake lads wrote: »
    just drop or raise your saddle by same amount ,can't think who told me that;)
    leave well enough alone you'll be fine honest.

    the pedals move in a circular motion so it would be like moving your saddle back 2.5mm also, but forward 2.5mm at the opposite part of the crank revolution. With 2.5mm more, youre making a bigger circle


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    #26
    kumate_champ07
    Closed Accounts Posts: 7,150 ✭✭✭kumate_champ07


    Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.


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    #27
    C3PO
    Registered Users Posts: 3,746 ✭✭✭C3PO


    Lumen wrote: »
    2.5mm doesn't sound much that but for a given effective saddle height an extra 2.5mm on the cranks will raise your knee by 5mm at the top of the stroke.

    I'm probably going to regret this but please explain that to me - I can't see how 2.5mm on the crank raises your knee by 5mm? I can see that the circumference of the circle increases by 5mm but the radius also?


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    #28
    Beasty
    Administrators, Social & Fun Moderators, Sports Moderators Posts: 75,598 Admin ✭✭✭✭✭Beasty


    kumate_champ07 wrote: »
    Ive heard of people doing TT's pusing a big gear with 180mm cranks, the extra leverage apparently helps.
    Indurain used 190mm for his hour record, although the geometry on TT bikes will be significantly different anyway

    You normally are restricted on the track - 165mm in Manchester, which is 10mm shorter than my road bike, although I never notice (the fact I am riding fixie I guess overshadows something like that)


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    #29
    Lumen
    Registered Users Posts: 31,020 ✭✭✭✭Lumen


    RPL1 wrote: »
    I'm probably going to regret this but please explain that to me - I can't see how 2.5mm on the crank raises your knee by 5mm? I can see that the circumference of the circle increases by 5mm but the radius also?

    It seemed plausible when I wrote it.

    ELWUZekN.jpg


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    #30
    C3PO
    Registered Users Posts: 3,746 ✭✭✭C3PO


    Lumen wrote: »
    It seemed plausible when I wrote it.

    ELWUZekN.jpg

    I'll get you to explain it to me on Friday at the kids spin!!


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    #31
    TinyExplosions
    Registered Users Posts: 6,860 ✭✭✭TinyExplosions


    Lumen wrote: »
    It seemed plausible when I wrote it.

    ELWUZekN.jpg

    I think it's a little off, as the centre of the circles should be in the same place, meaning that a 2.5mm difference in crank length = a 2.5mm shortening at the top of the stroke (unless my idea of math is very wrong)


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