Different Crank Lengths

Discussion in 'Cycling Equipment' started by stiffback010, Jan 6, 2007.

  1. stiffback010

    stiffback010 New Member

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    Hi all,

    Ive just ordered a new groupset for a winter bike which I'm building .
    The only thing is , on my regular (best) bike I 'm riding with 172.5mm cranks , on the groupset I've ordered, they only have either 175mm or 170mm cranks.
    After giving them my height , (which is 5'10"), the bike shop advised me that the 175mm cranks may be more suitable for me.
    The thing is I've never really thought about crank length before and have no problem with the 172.5mm cranks on my bestest bike, so I was wondering , would having different crank sizes on my bikes have an adverse affect on my summer sportive riding ??
    Cheers all
     
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  2. alienator

    alienator Well-Known Member

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    Well, the right crank length is not a function of your height, but is a function, in part, of your leg length. Even then, there are other factors that are or can be more important.

    If you've been fine with 172.5 cranks, then stick with those. FWIW, at 5'10", I'd be surprised if 175mm cranks were right for you. I'm 6'1", with a 35" cycling inseam, and I use 175's.

    Also remember there is no universally accepted formula for figuring the right crank length. None.
     
  3. Retro Grouch

    Retro Grouch New Member

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    When I ordered my compact crankset, 172.5mm, which I was used to, wasn't available and nobody could tell me when it might become available. Since 175mm was immediately available, that's what I ordered.

    The difference in length is less than 1/8 inch. I assume there are people who say they can feel the difference, but I'm not one of them.
     
  4. stiffback010

    stiffback010 New Member

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    To be honest , I was thinking ,"how much of a difference could possibly 2.5mm make?"
    Don't get me wrong I totally value ALIENATOR'S advice I'm just a bit in the dark about it all.
    Stiffback
     
  5. nerdag

    nerdag New Member

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    As for Retro's repsonse - some people can feel a difference, other's can't.

    I'm 1.62m tall with a 76cm inseam, and most formulae I've seen recommend I use somewhere between 158mm and 167.5mm.

    I can't feel a difference going between 165 to 167.5 to 170, but I can feel a difference between 170 and 172.5.

    I currently ride 170s on all my bikes (2xroad, 1xMTB), but am considering putting on 165mm cranks on the next bike.

    I suspect that finding the ideal crank length for you is really going to be a process of trial and error.

    YMMV.

    n
     
  6. sogood

    sogood New Member

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    = Expensive. :p
     
  7. gclark8

    gclark8 New Member

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    What is your seat height?
    Measure from the centre of the bottom bracket to the centreline of the saddle.
    Divide this by 4 and that's the a good starting point for crank length.

    I am 685mm seat height and use 170mm cranks. I spin at 90-100 rpm and find longer not suitable.
    Little Jackie is 600mm seat height, her cranks are 152mm.
     
  8. sogood

    sogood New Member

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    That may be a good guide, but isn't it influenced by the initial crank length?

    Yes, yes, we are talking about potential differences of mm... :eek:
     
  9. Unbelievably

    Unbelievably New Member

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    Are there any advantages to having a shorter or longer crank?
    If so,
    What are they?
     
  10. gclark8

    gclark8 New Member

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    No, it is related to the length of the rider's leg. Make the bike and components fit the rider and not make the rider fit the bike.

    There can be other factors, I recently set up a road bike for a chap who had just both knees replaced. He has a limited range for his knee bend, so, I tried different crank lengths and seat heights. We found he could just make the circle with 160mm cranks. He is 6'2" and now a happy rider. ;)
     
  11. 531Aussie

    531Aussie Well-Known Member

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    'high spinners' like track riders prefer the smaller circle to get the revs up much quicker, but I reckon the best thing about shorter cranks is the relative higher saddle position. Witht he same length reach to the bottom of the pedal stroke, a 5mm shorter crank means the saddle is effectively 10mm through the top of the stroke. It's easier to push the crank at the top of the stroke when your higher over the pedal, with less knee flexion and less hip extension.

    Anyway, i could write a very boring book-length post on this because I was obsessed with it a while ago.:p About 2 years ago I got 180s (I'm 182cm with long legs: 89.5cm inseam), and have also used 165 (track bike), 170, 172.5 and 175. I currently use 172.5
     
  12. zorly

    zorly New Member

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    Some Criterium riders also claim they use the 172.5 (vs the 175) cranks for the little bit of added clearance available for pedaling through the corners.
     
  13. sogood

    sogood New Member

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    Well, sure about that, even at a theoretical level?

    If you had a 190mm crank (assume), the saddle height from BB would be less than if you had a 150mm crank to start with. A difference of 40mm. The reason why I think this may apply is that the saddle to pedal height is often referenced to the rider's heel to bum length.

    I just did your suggested measurement/calculation on my present setup (175mm crank) and came up with BB-Saddle of 710mm, starting crank length 177.5mm. And I am 5'8"/173cm. By emperical suggestion, shouldn't I be aiming at 170mm range? Based on your experience, how would you further advice?
     
  14. 531Aussie

    531Aussie Well-Known Member

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    this definitely happened, but I reckon it's less important these days because modern pedals are narrower and have much better clearance.

    When i had my Shimano 600 pedals with toe-clips in the '80s, I ALWAYS hit the ground with 175mm cranks, but i didn't hit the ground last year with my new Ultegra pedals on the 180mm cranks (on the same bike)!! :eek: :)
     
  15. alpha2k

    alpha2k New Member

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    I'm 180cm tall with 82-83cm inseam. I tried 170, 172.5, 175 for some time and stick with 172.5 now.

    This article may help you on your decision:
    http://www.cptips.com/crnklth.htm
     
  16. carpediemracing

    carpediemracing New Member

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    crank length is a personal thing.

    I used to use 167.5's exclusively. I tried 165's, 170's, 172.5's, but at that time, with my fitness, postion, etc., the 167.5's worked best.

    I'm now heavier, stronger, and stopped focusing so much on my sprint (although I still do, I learned I don't need to aim for anything over 45 mph / 72 kph lol). I found that I could pick up significant speed by using longer cranks in 40-45 mph sprints in all conditions. I found this out by riding a mountain bike with 175 mm long cranks and learning a big effort on that bike netted me 33 mph on a slight rise. I was hard pressed to do the same on my 170 mm equiped road bike. I moved to 175's at that time (3 years ago) and have been doing great. I won a race series that I last won 10 years ago. I earned two state championships (in Cat 3's) in field sprints.

    I found the following with crank lengths:
    Longer cranks are good for:
    - shorter, big gear hills. For me, 6-7%, under 500 meters, and I fly up them - I can roll a 53x15-17 instead of a 53x19-21.
    - big gear resting in the field - rolling in a 12 or 13, waiting for something to happen.
    - being a bit more careless with gear selection (more torque = less need to be in optimal gear)
    - ideal cadence dropped about 30 rpm at first (110 to 80), raised back to within 5-10 rpm (100-105) within a few months.
    - max rpm dropped about 40 rpm - from 286 rpms on 170's to 246 rpms on 175's.

    Shorter cranks are good for:
    - very high top speed
    - spinning resting in field
    - pedaling clearance in turns
    - higher overall position on bike (resulting in lower bar relative to seat, gains a bit of power, some aero, and positions you better to launch big efforts).

    The kicker is that I ride a size S Giant, and I am about 5'7" tall. No one in their right mind would have ever recommended 175's for me but after I tried it, with an open mind, I decided to stay with them.

    hope this helps in your decision making
    cdr
     
  17. stiffback010

    stiffback010 New Member

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    Cheers ,
    this at least gives me the confidence to give 'em a try.
    If they're no good I'll just have to go back to the 172.5mm.
    Thanx for the advice , much appreciated.
    Stiffback
     
  18. Fraggle

    Fraggle New Member

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    Interesting information, you seem to know a lot about cranks. If you've got the time I have a somewhat related question for you.

    I ride 172.5 cranks and am in the process of changing from a 54/42 to 55/42 chainring set.

    The bigger chainring has allowed more options at high end speeds and that extra few km/h when decending. I spent 12 months with the 54/42 to make sure that my body has adjusted and cadence came back to normal range.

    My question is what difference is there between moving from say a 54 chainring to a 55 chainring as opposed to changing from a 172.5 crank to a 175 crank ? Excluding cost as an issue.
     
  19. Bigbananabike

    Bigbananabike Member

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    =============================================================
    Alien features is right - there are no formulas:eek:
     
  20. carpediemracing

    carpediemracing New Member

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    Changing chainrings doesn't affect your pedaling - it's no different than shifting into a slightly different gear. Just be aware of your gearing since you'll be using slightly higher gears when in the big ring.

    I use gear inches to compare gears - chainring/cog x 27". It's simple and easy to relate to the different gears. If you do the math, you'll see a one tooth cog difference in the 11-12T range is worth about 10%. A one tooth chainring difference is something like 1%.

    Without training your body to turn a bigger gear over, there won't be too much benefit to using the larger ring. There are some fine-tuning benefits to using a bigger big ring:
    1. Say your record TT effort involved turning a particular gear on a hill - say a 54x15 - and it is just unfeasible to use the next smaller cog (in this case, the 14T) or a similar gear in the small ring (39x11). A 55x15 would give you a slightly bigger gear, some mental motivation, and potentially about a 1% faster speed.
    2. If you have a downhill section in a race or sprint, and you find yourself aerobically maxed out, a larger gear will let you spin at a lower rpm. This might give you some extra reserves at the bottom or at the line if you are really flying.
    3. If you are looking to optimize your sprint against other racers and you find yourself unable to match your opponents (they beat you slightly but you are using the same gear), you can use a different chainring size to optimize your cadence - a larger ring if you prefer a lower cadence (power sprinter) or a smaller ring if you prefer higher cadence (track type sprinter).

    On the other hand, you'll find that changing your crankarm will seriously change your power and speed. A longer crank increases lower rpm power (you'll notice it most on standing climbs in big gears) and it decreases your speed (noticeable in your top end speed, esp in tailwind or downhill-ish sprints). Merckx famously changed his cranks from a 175 to 177.5 for a climbing stage in the Giro and lost his "snap". Unable to respond to sharp attacks, he almost lost the Giro that day.

    Just for reference, I used to regularly change my chainrings for races - 39/53, 42/53, 45/51, 45/53, 45/54. I don't have all the chainrings I'd like anymore but I'd use a 51, 53, 55 if I could.

    However, I rarely change crankarm length, and when I do (and the change seems okay), I'll stick with it for years.

    hope this helps,
    cdr
     
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