is 2.5mm really that much difference?

andy83
andy83 Posts: 1,558
edited September 2010 in Commuting chat
Looking to get a new chainset but the one i want I can only find in 170mm crank length but on my current bike have a 172.5 and it seems fine

Its a black 53/39 105 chainset if anyone knows of one for £75 id be grateful :)

Thanks in advance

Comments

  • Underscore
    Underscore Posts: 730
    My road bike is 172.5 and my commuter is 175. Can't say that I notice that much of a difference riding them, TBH. YMMV.

    _
  • itboffin
    itboffin Posts: 20,064
    Dont worry about it, it'll be fine.
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  • nich
    nich Posts: 888
    I went from 175 triple to a 170 single.

    Didn't really think about it at first. The only noticeable difference that I can tell is that I can lean into corners more without worrying about clipping the pedals - tho having said that it's a different bike, so the bb might be higher too.
  • cjcp
    cjcp Posts: 13,345
    I have 170mm on the crosser and 172.5 on my two road bikes. You may need to adjust your seat height just a tad.
    FCN 2-4.

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    "It stays down, Daddy."
    "Exactly."
  • bigmat
    bigmat Posts: 5,134
    cjcp wrote:
    I have 170mm on the crosser and 172.5 on my two road bikes. You may need to adjust your seat height just a tad.

    By about 2.5mm?! :lol:

    Seriously though, its not a lot. I have 175mm on my mountain bike and 172.5mm on both road bikes. I think the real difference relates to the impact on gearing - have a look at Sheldon Brown's gear calculator, those 2.5mm will give you a slightly bigger gear inch but outside of professionals, who really cares?!
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  • cjcp
    cjcp Posts: 13,345
    MatHammond wrote:
    cjcp wrote:
    I have 170mm on the crosser and 172.5 on my two road bikes. You may need to adjust your seat height just a tad.

    By about 2.5mm?! :lol:

    Oh, very good. There's a maths whizz in da howse. :)
    FCN 2-4.

    "What happens when the hammer goes down, kids?"
    "It stays down, Daddy."
    "Exactly."
  • greg66_tri_v2.0
    greg66_tri_v2.0 Posts: 7,172
    A few years back, some fettling disclosed that some earlier fettling had left me riding 170mm on the NDS and 172.5mm on the DS.

    Hadn't noticed.
    Swim. Bike. Run. Yeah. That's what I used to do.

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  • cjcp
    cjcp Posts: 13,345
    Greg66 wrote:
    A few years back, some fettling disclosed that some earlier fettling had left me riding 170mm on the NDS and 172.5mm on the DS.

    Hadn't noticed.

    Btw, been meaning to ask: how's th chronic muscle imbalance and limp these days? :P
    FCN 2-4.

    "What happens when the hammer goes down, kids?"
    "It stays down, Daddy."
    "Exactly."
  • andy83
    andy83 Posts: 1,558
    Thanks for the replies

    I have actually just checked my fixie and the cranks are 175 and i didnt even know.

    Ill be getting bigger gearing anyway so the shorter crank arm wont hurt

    CJCP decided to go for the black 53/39 on merlin thanks to your great explanation to a person not very clued up on ratios etc

    really pleased cos with selling a few bits off bike that were being replaced I will have ended up getting a whole 105 groupset (minus brake calippers as already had them) for around£130 which is quite good going
  • antfly
    antfly Posts: 3,276
    I went from 175mm to 170mm on my hardtail, by mistake, at first I noticed I was spinning quicker so I was using a higher gear and now I don't notice it at all.
    Smarter than the average bear.
  • bigmat
    bigmat Posts: 5,134
    andy83 wrote:
    Thanks for the replies

    I have actually just checked my fixie and the cranks are 175 and i didnt even know.

    Not a fixed rider myself, but isn't it better to have shorter cranks as you need to keep pedalling when leaning in round corners (so clearance more of an issue?)

    You obviously haven't had any problems though if you haven't even noticed...
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  • Clever Pun
    Clever Pun Posts: 6,778
    longer cranks give you more leverage going up hills

    but as mentioned with longer arms you'll need to lower the seat
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  • andy83
    andy83 Posts: 1,558
    MatHammond wrote:
    andy83 wrote:
    Thanks for the replies

    I have actually just checked my fixie and the cranks are 175 and i didnt even know.

    Not a fixed rider myself, but isn't it better to have shorter cranks as you need to keep pedalling when leaning in round corners (so clearance more of an issue?)

    You obviously haven't had any problems though if you haven't even noticed...

    well its my first fixie so makes no odds i dont know any different. Its great riding fixed when you havent got time for lots of miles as you cant hide. I love long rides on road bike but too easy to get into an easier gear. Fixed will be riden now until around March/April unless on sunny days with no chance of rain.

    When new groupset goes on road bike, along with new wheels it will seem like a new bike, i cant wait. hmmm 5 days of no work first week of october, i see a little tour coming on :)
  • biondino
    biondino Posts: 5,990
    I have 175mm on my roadie and 165mm on my fixed. The latter does feel a bit underpowered but it's perfectly rideable and as has been stated, advantageous when cornering. I'd think 2.5mm will make pretty much no difference, but you can of course blame it whenever you're feeling like your power output isn't quite what it should be.
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  • andy83
    andy83 Posts: 1,558
    thats what i thought but wanted to just double check anyway

    To be honest my road bike now has a 50/34 172.5 on a 9 speed setup, will be upgrading to 53/39 170 on 10 speed setup so hopefully shouldnt loose any power
  • essex-commuter
    essex-commuter Posts: 2,188
    Clever Pun wrote:
    but as mentioned with longer arms you'll need to lower the seat

    What have the length of your arms got to so with the height of the saddle, surely it's the length of the stem?

    :wink: <---- Take note!
  • Bugly
    Bugly Posts: 520
    MatHammond wrote:
    cjcp wrote:
    I have 170mm on the crosser and 172.5 on my two road bikes. You may need to adjust your seat height just a tad.

    By about 2.5mm?! :lol:

    Seriously though, its not a lot. I have 175mm on my mountain bike and 172.5mm on both road bikes. I think the real difference relates to the impact on gearing - have a look at Sheldon Brown's gear calculator, those 2.5mm will give you a slightly bigger gear inch but outside of professionals, who really cares?!

    ?how can it change your 'gear inches', the wheels, and cogs remain the same, it can (very) slightly change the torque you apply (but I bet you can only calculate and not ever feel it)
  • NGale
    NGale Posts: 1,866
    ooooo makes all the difference :wink::wink:

    sorry but someone had to bring it down to that level :roll:
    Officers don't run, it's undignified and panics the men
  • giner1961
    giner1961 Posts: 135
    have to say if all you chaps whom say you could not tell the difference of the 2.5mm then your position on the bike must be out or wrong, i changed to 175 some years ago from 172.5 and i felt the difference straight away took me 2 months to adapt.
    Tendons, hamstrings etc.
    As far as im aware crank lenth has no bearing on gear ratio, all it does is alter your pedaling cadance a 170 will spin more on the same gear as a 175, this is due to the crank lenth and not the gear ratio. also crank lenth is guided by the lenth of you thigh, so someone with short thighs would use a 170 and some one lke me with long thighs will use a 175.

    there said my bit, wating to be shot down.
  • Bugly
    Bugly Posts: 520
    actualy 170 will spin the same as 175 as the gearing is determined by the sprocket ratios and these are the same - the axle turns the same number of times for the same gear. The 175mm cranks will describe a longer path for the same revolution, the moment of inertia will be higher.

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