Crank length...

differentstrokes87
differentstrokes87 Posts: 140
edited July 2013 in Road buying advice
I'm looking to order a Rose Pro Carbon-RS, today hopefully so it's here in time for my holidays and the last query is which crank length to opt for.

My cross bike has 172.5mm cranks but the Rose sizing guide suggests a 170mm would be more suited to my size at a fraction under a 30" inseam. Having never had a bike fit I'm unsure which is better for me.

I've done a bit research via Google but seem to be getting mixed answers, some say that the smalles size would be better and others suggest just to stick with what I'm comfortable with.

Can anyone offer advice?
Planet X XLS 2013
Planet X London Road 2015

Comments

  • drlodge
    drlodge Posts: 4,826
    Doubt it'll make much difference, I'm 5'9" with 31"+ inseam and prefer 172.5mm cranks but there's no much difference to the 170mm cranks I was using before. If you're happy with 172.5mm cranks I'd stick to them.
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  • diamonddog
    diamonddog Posts: 3,426
    I'm a 30'' inseam and have 170 cranks also wouldn't longer cranks give a lower seatpost length. Some sites on t'internet will give you a formula to work it out but this might cause you more confusion.
  • Markjaspi
    Markjaspi Posts: 729
    drlodge wrote:
    Doubt it'll make much difference, I'm 5'9" with 31"+ inseam and prefer 172.5mm cranks but there's no much difference to the 170mm cranks I was using before. If you're happy with 172.5mm cranks I'd stick to them.

    Ditto, but I have dropped my seat post by just under 5 mm too.
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  • styxd
    styxd Posts: 3,234
    Stick to what you're comfortable with is the answer. If you're comfy on 172.5mm then go with them. Although you may also be comfy on 170mm
  • jameses
    jameses Posts: 653
    I have 175mm on one bike, 172.5mm on another. I can't tell the slightest bit of difference when riding.
  • cougie
    cougie Posts: 22,512
    I really don't think there is a measurable difference.
  • jameses
    jameses Posts: 653
    cougie wrote:
    I really don't think there is a measurable difference.

    2.5mm? :wink:
  • StillGoing
    StillGoing Posts: 5,211
    JamesEs wrote:
    cougie wrote:
    I really don't think there is a measurable difference.

    2.5mm? :wink:

    But it isn't 2.5mm difference is it? It's 5mm and to get the length correct at the bottom or top means it will be out by 10mm at the opposite end. The length difference isn't the issue though, what is is the angles it creates at the top and bottom of the stroke. With a longer crank the rider has to have the seat lower to avoid over extending at the bottom of the stroke. At the top of the stroke they will be over compressing preventing getting power down easily. With too short cranks the seat is raised to create the right extension at the bottom of the stroke but the leg isn't reaching the correct angle at the top of the stroke.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • jameses
    jameses Posts: 653
    philthy3 wrote:
    JamesEs wrote:
    cougie wrote:
    I really don't think there is a measurable difference.

    2.5mm? :wink:

    But it isn't 2.5mm difference is it? It's 5mm and to get the length correct at the bottom or top means it will be out by 10mm at the opposite end.

    You make a good point with the rest of your post (although as mentioned above, I personally can't tell the difference between 175 and 172.5 cranks when riding), but I don't understand where you're getting that 10mm from. The crank length (which is the radius of the pedalling circle) will be 2.5mm longer, so surely the total difference in diameter of the pedalling circle will only be 5mm?
  • jonomc4
    jonomc4 Posts: 891
    You boys forgot the circumference of a circle rule? Pie D

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