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Thread: shorter cranks?

  1. #1
    Senior Member LuckySailor's Avatar
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    shorter cranks?

    Recently, I downloaded an app called Size My Bike at the suggestion of my training coach. I have ordered a Trek 520 for some upcoming tours, and am making alot of component changes to suit what I want. The model that I took for a 50 km (about 32 miles) test ride was a 57 cm (23") frame and 175 cranks. I now have the frame and have made alterations so its mine. However the cranks have not been installed yet so there is the ability to change out if I decide to. I am 5'11", 33" inseam. The SMB app, suggests that I should be riding a 50 cm (19") frame with 165 cranks. Last evening, I was riding my mountain bike on my trainer, and when I am in an aero position, my legs are coming up to my chest, and actually rotating out at the top. I measured my crank length on the MTB and its 180. (yea, holy crap!) love the bike though. The 175 cranks on the test bike felt comfortable to me, but I didn't have the bike on the trainer, nor did I have aero bars it when I took it for a test ride. My concern is that when I build up the new Trek, that my knees might be rotating out at the top or hitting my chest or I won't be able to fully get into an aero position because the cranks are too long at 175. The difference between 165 and 175 is about an half inch. Factor in the the 180 crank to a 165, that's 3/4". The SMB app was pretty well bang on with the MTB except for that my cranks are 180-everything else was spot on or within millimeters. Coach's experience was that it was spot on with his bike(s) and he paid $300 for custom bike fittings. LBS did a basic bike set up for me to test ride the 520. Sorry to be so long hopefully I have provided enough info. Thank you for your thoughts.

  2. #2
    Senior Member Number400's Avatar
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    The 165's will definitely improve your aero position and open you up a bit. It feels so weird for me coming from a 172.5. It takes some getting used to and you feel a little weaker with the loss of leverage.

  3. #3
    Senior Member LuckySailor's Avatar
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    Quote Originally Posted by Number400 View Post
    The 165's will definitely improve your aero position and open you up a bit. It feels so weird for me coming from a 172.5. It takes some getting used to and you feel a little weaker with the loss of leverage.
    How long have you been with the new cranks? Why the switch?

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    I have a 30" inseam and I went with 165s. I found them to be much better on my knees, although you do find yourself shifting gears a bit more due to having less torque with each stroke. You may try 170s for less of a change if you have an old pair laying around, or check the coop, it may be worthwhile to try them before you make your big purchase.

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    Senior Member Number400's Avatar
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    Quote Originally Posted by LuckySailor View Post
    How long have you been with the new cranks? Why the switch?
    I got the 165's as luck of the draw for a particular build. I use 172.5 on my road bikes but have experimented with 175's. For me, longer cranks feel faster but comparing times, they all work out the same despite feeling different. I also ran short cranks on my too small framed TT bike and it made a huge difference with comfort and breathing.

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    I'm 5' 9" on a 54cm Madone with 165mm crank arms (down from 172.5.) I have short legs for my height (30.5" inseam) and definitely feel more comfortable with the shorter crank arms. I did notice a slight difference in leverage with the smaller crank arms, but my speeds have actually gone up and my breathing has opened a bit more; cadence is generally higher too. If you're riding a 57cm and have a 33" inseam, I'd probably go with 170mm crank arms.
    2015 Norco Threshold SL Ultegra | 2012 Trek Madone 6.9 SSL w/Di2

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    coprolite fietsbob's Avatar
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    No Apps, no cell phone , so I'm fine riding everything from 170 ~ 180 with no issues for 40 years +.

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    Quote Originally Posted by LuckySailor View Post
    Recently, I downloaded an app called Size My Bike at the suggestion of my training coach. I have ordered a Trek 520 for some upcoming tours, and am making alot of component changes to suit what I want. The model that I took for a 50 km (about 32 miles) test ride was a 57 cm (23") frame and 175 cranks. I am 5'11", 33" inseam....... The SMB app, suggests that I should be riding a 50 cm (19") frame with 165 cranks.
    Unless I'm missing something here, how can a rider your size need a 50cm frame?
    Mal

  9. #9
    Senior Member LuckySailor's Avatar
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    Quote Originally Posted by zx9rmal View Post
    Unless I'm missing something here, how can a rider your size need a 50cm frame?
    Its telling me that for a mountain bike frame. For a performance bike its suggesting slightly less than 22"-which is in the ballpark. Depends how much seatpost you want exposed. That and with 165 cranks. BTW, my mountain bike which I bought 10 years ago and truly love, is an 18" frame. True enough the seatpost is quite long. But I have always felt comfortable on this bike on the road or off. Has 180 cranks. Anyhow, it works for me. All this means is its up to the individual rider, and there is more tahn one way to skin a cat! Its another perspective, and only the rider can determine whether it is right or wrong-but its a hell of alot cheaper than 2or300 bucks.

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