eBent Recumbent Cycling

A Bent Look at Self Contained Touring

Are Your Crank Arms Too Long?

with 10 comments

First Recumbent from Patent Pending Blog - 1902

Note: There is some feedback that calculated lengths using this tool are too long (3 MM from feedback). I would not take the calculated lenghth too literaly and lean toward shorter lenghts. I also think that recumbent riders should consider shorter cranks because of seating position.

I have been thinking that I should shorten my crank arms since I started having hip problems but I am not sure that the effect is significant. However, I do know that it will be expensive. A friend is going to let me ride his recumbent with shorter cranks before I proceed but I am prettty sure I will make the change.

So if I do change “What is the correct crank arm length?”.

Well to answer that question I found a neat little web page, Optimum Crank Arm Calculator.

This program by Machine Head Software calculates the optimum crank arm length base upon the riders inside leg measurement. This approach is based upon the technique is from The Racing Bike Book, By Dave Smith, Ben Searle & Steve Thomas (foreword by Sean Kelly), ISBN 1-85960-319-X. The book gives a table of recommended crank sizes in millimeters for inside leg measurements between 72 and 96 centimeters.

From this table of values the following relationship was developed:

Recommended crank length in millimeters = (1.25 * Inside Leg in cm ) + 65

I tried it out and with my very short for my height legs (29″ inseam) it recommends 157 mm cranks instead of the 170 mm I am using now.

See Cranks for more postings about Shorter Crank Arms, where to get them, how to install and remove cranks and Why I love them.

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Written by Roland

March 3, 2008 at 7:39 am

10 Responses

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  1. I think short cranks on a recumbent are something that has to be tried to be understood. It takes quite a few miles to really make a good evaluation and some people don’t give it a fair chance. Some riders may need an extended adjustment period because they don’t spin to begin with. You really can’t make good use of short cranks at low RPMs.

    I am using 153s and my x-seam is 42. The 153s seem really good and I think even shorter would be OK. After 2000 miles on the shorties, I switched to 165s and did not like them at all. I could really feel all the extra movement in my knees and hips.

    I also think that with short cranks you must have pedal retention (clips and straps, powergrips, clipless). High RPMs and platform pedals don’t go well together.

    macpublish

    March 3, 2008 at 8:07 am

  2. Thanks for the encouraging words.

    I agree with what you say. I expect to only find a subtle change but I am hoping the motion will be better for me.

    I don’t know how folks can ride a loaded recumbent without really spinning. My knees would never has lasted and I would have been able to climb a lot fewer hills.

    As you can see I am not a big fan of clipless pedals but I don’t think it makes any sense to ride with out some kind of foot constraint. Just the chance of having your foot slip off the pedal is enough to convince me. It is a lot easier for that to happen on a recumbent than on a diamond frame. Plus you lose the pull back power without foot constraints.

    Thanks for the comments … Roland

    Roland

    March 3, 2008 at 8:54 am

  3. I’m a former framebuider and fit junkie (no relation to book’s author that I’m aware of) and plugged my 36″ leg length into the calculator and came up with a 179.3 crank length. I ride 175s, 177.5s comfortably. Years ago when I was still young, strong and fit, I borrowed some 180s. They felt great for about a week, then it felt like I was pedaling squares. I put the 177.5s back on and was back to pedaling circles. So for me, the max I want to ride is a little under the calculator, so be a little cautious using online tools,they’ll get you close. I think there is a formula somewhere that uses thigh length rather than leg length. Depending on your individual physiology, this may be more accurate. I should add, I’m not a recumbent rider.

    Stevan Thomas

    March 4, 2008 at 11:13 am

  4. Thanks Stevan, I agree the calculated lenghts seem a little too long to me (I am at the other end of the distribution). I suspect that the formula used is too much of a simplification.

    I will be happy if people just think about crank length. It is expensive to try different lenghts for most folks so we usualy just accept what comes with the bike.

    Many folks probabaly use cranks that are too long but it doesn’t really become a problem for them until they get older.

    I also beleive that the recumbent riding position and the need to spin on a recumbent both lead to shorter cranks.

    However, my opinions are just speculation and have no emperical basis. Thanks for providing a real life data point.

    Roland

    March 4, 2008 at 11:51 am

  5. The online calculator spat out 166.6mm for me. I’m riding with the standard 170mm crankarms on my Slipstream.
    If I lived where the land was flat, I would be tempted to try shorter crankarms to see if the difference is worth it; however, here if there is a flat spot, someone came by with a Caterpillar and made it that way. There are hills here that are steep enough to defy my spinning a decent cadence even in the “Granny Gear.”
    Someone younger, lighter, and in better aerobic condition could do it; I’ll get there eventually.

    Charles

    March 4, 2008 at 3:11 pm

  6. Charles, I am thinking of going to 155 mm cranks for my Slipstream because I mostly ride in the Mountains. I know that shortening the cranks should decrease leverage but I expect to make up for that by spinning better.

    That is my theory and if you don’t like it, I have others … lol.

    Roland

    March 4, 2008 at 4:25 pm

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  9. The shorter cranks do make sense, and more for me because when I rode a diamond my spin easily topped a hundred. Got one question, my bike has a 20″ rear wheel. So far I have toured the distance on it. Is there really more drag with smaller wheels?

    Richard Harper

    September 28, 2008 at 8:26 pm

  10. I have a Bike Friday Sat-R Day with 16″ wheels and I have not found it any harder to ride than my bikes with 26″ wheels. It zips along, is very responsive and pedals easily. I also have a BikeE with 20″ wheels that I find a bit of a puller.

    Yes, tests show that bigger wheels with narrow high pressure tires roll more easily down a ramp but wheel diameter is a second order effect.

    At speeds above 12 MPH most effort is overcoming wind resistance. So the bike and rider’s wind profile becomes very important. Drive train design is also a very important factor.

    Recumbent designs are not as standardized as Diamond Frame designs so my only advice is ride the bike before you buy. But if you are a new recumbent rider and haven’t ridden many recumbents even this is not a good indicator. In that case you will have to rely on the advice of more experienced recumbent riders who do the type of riding you do.

    In any case, you should not think of your first recumbent as a life buy. What you want as a rider in a bike will change with time and you will start to know what type of design fits you. So buy an inexpensive and proven design first. Save the more expensive and specialized recumbent designs for later when you know better what you want.

    Roland

    October 3, 2008 at 12:38 am


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