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The ins and outs of crank sizing. Help required....

bristolpetebristolpete Posts: 2,255
edited August 2010 in Workshop
Hi ,

I was wondering if someone could shed any light on something that has sort of blinded my common sense and I cant see the wood for the trees.

To advise, I ride a 54 Pinarello Dogma which has a tob tube of 555. I run it with a 100mm stem which I like as I am not a small 5' 9" guy, neither am I big. 14 stone in fact. However, the seat tube of which Pinarello measure is 54 which seems fine. For reference I cannot stand riding compact geometry, I just cant ride it so the taller seat tube suits me.

However, I feel that I have dropped the seat post a tad too much and sometimes feel like I am riding on the top tube though I am not. It is exasperated by the fact I also ride different bike as my commuter/hack/winter machine.

So I was thinking, in order for me to keep the saddle slightly higher I could change my crankset from 172.5 to 175 but soon realised the issue is relative as although I am higher on the top of the pedal stroke, I would in turn be the same distance at the bottom, pushing the pedlal further away. So, my other option is to change it to a shorter crank, perhaps 170mm which will bring the pedal closer to my seat clamp and allow me plenty of leverage on the down stroke. trouble is that a couple of people have told me that shorter cranks give you less power climbing, but I wonder if I would notice it...?

Can anyone shed any light on this, other than me staring at a ruler wondering how on earth such small measurements make that much difference ! Even if I had dropped a frame size we are talking tiny amounts but do they really make that much difference to a layman rider like me?

Appreciate any advice here.



  • bill57bill57 Posts: 454
    Probably better if you said what size you are, rather than what size you aren't. I don't think I would ever consider changing crank length as a way of correcting an ill-fitting bike - the differences are too small to make any serious difference.
  • Monty DogMonty Dog Posts: 20,614
    FWIW compact frame geometry makes no difference to how a bike handles - all still leave the saddle, cranks and bars in the same position.
    Bike set up is determined by the rider, not the bike - I'd try and replicate the same position on all your bikes - having different ones can compromise your performance.
    Crank length is as much influenced by your own pedalling style - whilst shorter cranks give you less leverage, if you can spin more smoothly in a lower gear then you may find it can improve you climbing. Only you can tell what difference it makes and can only really be determined by actually try riding a pair
    Make mine an Italian, with Campagnolo on the side..
  • crankycrankcrankycrank Posts: 1,830
    Agree 100% with Monty Dog. Changing crank length to get proper frame fit is all wrong. How close to the top tube is your saddle now and why is this a problem? Sounds as though unless you're willing to accept that the saddle seems too close to the top tube you would have to get smaller frame and adjust stem length accordingly or get a frame with different geometry. Not a pleasant option with a Dogma. :(
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