Recent Observation About Crank Length

Discussion in 'Cycling Equipment' started by Ted B, Sep 22, 2003.

  1. Ted B

    Ted B New Member

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    I'm a relative newbie with a background in high-intensity weight training. I've done extensive research into sizing, and components pursuant to planning my own ride from the ground up. At the present, I own two Bianchis:

    (1) 2001 Giro, 55cm, Mavic CXP21, Shimano 105
    (2) 2001 EV2 XL, 55cm, Mavic Open Pro, Chorus

    Other than the fact that the EV2 is an ultralightweight racing machine, both visually appear to have virtually identical geometry, and the seat positioning is the same (as per my adjustments).

    I've noticed that despite the inherent differences in frame materials and weight (which are AFAIK the only significant differents), I can ride the Giro faster over longer distances than the EV2. Puzzled, I've scratched my head trying to determine why. The interesting thing here is that the only real geometric difference between the two is the fact that the EV2 has 172.5mm cranks, and the Giro has 175mm.

    My inseam is 78.75cm (31"), which by most calculations equates to a 170cm crank. I find that I am extremely comfortable with the 175cm - more so than 172.5cm. My comfortable cadence at effort seems to average around 85-90rpm.

    My hunch is that while optimum crank lengths scale somewhat with leg length, a stronger leg of the same length will probably prefer a longer crank length. This works in my mind because in a free weight squat, the stronger leg of the same length will be able to squat lower with the same weight, which means the stronger leg will be able to exert more torque over a greater angular range of motion. Naturally, I am comparing legs of two people of same proportions and spinning ability, where one comes from a weightlifting background (like myself).

    I'd like to hear any relevant opinions as to my observations, as I cannot figure why else I am slower on the 'faster' bike, and more comfortable pedaling at effort on the 'slower' bike. Could the modest difference in crank length really make the difference?


    ???
     
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  2. percious

    percious New Member

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    Why not try swapping the cranks, and see if the 'faster' bike goes faster with the crank from the 'slower' bike? That way you can truely isolate the question at hand.

    -percious
     
  3. Ted B

    Ted B New Member

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    That is a logical suggestion, but I don't believe Shimano and Campy cranks are interchangable.
     
  4. DiabloScott

    DiabloScott New Member

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    Possibly some aerodynamic advantage to the Bianchi? Maybe the wheels - CXP21 is a semi-aero rim. Possibly a more comfortable seat that reduces fatigue?

    Could be the crank length difference I suppose, generally faster spinners like shorter cranks and stronger mashers like the longer ones so it could be that your leg strength is put to better use with longer cranks.
     
  5. mtndog

    mtndog New Member

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    An interesting theory. From what I've heard, crank length is based on personal preference. Although mountain bike cranks usually run longer for more torque on climbs. So, it would stand to reason that if you are a climber, longer cranks would be more suitable. I wonder what crank lengths Roberto Heras, Tyler Hamilton and Lance prefer? I would like to test your theory but it's an expensive proposition. I currently ride 172.5 on road and 175 mountain cranks and feel faster, stronger on the mountain bike. 5'8" with 31" inseam. The downside is cornering with longer cranks, though minimal it may make a difference in a crit. If you find out anything more, please post.
     
  6. mfallon

    mfallon New Member

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    Just one thing to consider here. I have been weight training on and off (more off than on lately) for many years and what you say about the stronger leg of comparable length being able to do a lower squat is true. However doing so puts a tremendous amount of more stress on the knees and I have messed up my knees doing full squats to the point that I can no longer even go parallel with any decent amount of weight. I have a 35in inseam and am using 175mm cranks arms with absolutely no knee problems (thankfully). I'm not saying the longer crank arms will lead to knee problems but that it is just something to consider in following your logic.

    Personally I think it's some what ironic that the heavier framed Shimano 105 is out performing the lighter framed Chorus. Should make for some interesting replies after seeing some of the other Shimano VS Campy threads:)

    Matt
     
  7. dhk

    dhk New Member

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    Could be the crank length, but remember we're only talking a 2.5mm difference.....that's 0.1 inch. The small weight differences in frame and gruppos shouldn't really matter much unless you're always climbing or sprinting. I'd suspect other factors, such as a difference in friction from the tires, wheels, chain or driveline.

    Dan
     
  8. lokstah

    lokstah New Member

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    Crankarm length calculations have always mystified the crap out of me. I have no concept of where to begin when I'm considering a new crankset (other than the feel of what I'm already using).

    Anyone have a good rule of thumb?
     
  9. rollers

    rollers New Member

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    It could but I'd consider another possibility. You didn't say what your weight is but I wonder if your own weight, geometry, and strength combined with the differences in stiffness between the bike frames causes you to perform better on one over the other.

    Kind of like saying that one bike is just better for you than the other one.
     
  10. PSR

    PSR New Member

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    Crank length is not related to total leg length (inseam), but only to the length of your femur (from your hip joint to your knee).

    Case in point: Most people 6'1" and under should ride 172.5 or even 170. Of course, this is a generalization. I, for example am 6'0" with a 33.8" inseam. Mathematically, not quite long enough for 175 mm cranks. However, I have long femurs for my leg length. Therefore, the extra 2.5 mm helps position the pedal under my knee more effectively.

    Incidently, I'm riding another bike while I'm getting parts together and building my new Waterford R-33. The "substitute" has 172.5 cranks and I can really feel the difference on climbs and sprints.

    Think about this as well: Because of the reduced diameter of the circle your pedal would travel in with a shorter crank, you will have to turn the crank slightly faster than with a longer crank. If you are a spinner, consider the shorter of the two, if you're a gear masher (I'm a bit of one myself), go long.
     
  11. rickt

    rickt New Member

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    I have been a competive cyclist for 34 yrs and an elite coach for 10 yrs. As a result of my eagerness to help my athletes I have used myself as a guinea pig to test some theories I have had relating to crank length. PSR hit the nail right on the head regarding crank length being relative to femur length not total inseam. I have an inseam of 79cm. But, using the fit kit method, I have a femur length of 35.5cm or 45%. This is relatively short compared to average. I found that (in road races) I couldn't rev much higher than 120rpm when under load following a peleton along a certain stretch of road. The gear being used was 53/12. I tried using smaller gears, 53/13, 53/14 with no improvement in speed. I kept getting blown away, I was using 170mm cranks. I changed to 165mm cranks. Instantly my peak cadence, along the same stretch of road, in almost identical conditions and over the same number of attempts, increased by 20% and instead of getting blown away I was easily staying with the peleton to the finish.
    I have tested this in several other areas (especially track) and even with other people, with the results always indicating the same answer. One other variable though is muscle attachment length. Mine appear to attach a long way down the bone, relative to the length of the femur, which reduces the capacity for speed of the knee, and therefore pedalling fast with long cranks. It does however increase the potential for strength. Hence I should have taken up weight lifting as a sport instead of cycling. But you can't let biomechanics stuff up your enjoyment of a great sport like cycling. In your case the frame should be no bigger than 51 or 52 cm centre to centre anyway. If the 55cm is c to c then it's too big. There is a lot more info I could add to explain all this but my typing skills are such that I can only do this for a short while before getting annoyed (typically aggressive track sprinter).
     
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