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What effect do longer crank arms have?

100%100% Posts: 236
edited August 2008 in Workshop
The title says it all really!

I've got a demo bike and I'm pretty sure the cranks are slightly longer than those on my current bike - 172.5 vs 170.

It's the same frame size. I'm just wondering what difference the longer cranks will make? My belief is that I will find it slightly harder to maintain a high(er) cadenance.



  • cougiecougie Posts: 22,512
    Theoretically - yeah. In practice - I doubt you would notice.
  • a12tfpa12tfp Posts: 24
    I have been running 175 cranks and tried a 170.
    Not sure the step down is very wise.
    Many have commented that it's too much of a leap and I relly did notice it. A bit odd too!
    Much of it is to with torque, and after the first explaination and not really following the physics, I can but offer up a Google search and you can see what it relates to yourself.
    Stick with wha you know although a 172.5 might better suit you if you're a 30 inch inside leg or slightly shorter from some of hte comments I got from a similar post elsewhere.
    Dog-slow everywhere
  • redddraggonredddraggon Posts: 10,862
    a12tfp wrote:
    I have been running 175 cranks and tried a 170.
    Not sure the step down is very wise.

    Going "shorter" shouldn't be a problem - after all track riders have been known to use 165mm rather than longer cranks so they get more clearance.
    I like bikes...

  • 100%100% Posts: 236
    Thanks for the replies - I would have thought I wouldn't have noticed a slight different either. Maybe I need to remeasure the cranks on the cross bike I normally ride.

    I'm a 30 inside leg or even 29 again a remeasure - a proper bike fitment may be in order.

    I've moved the saddle forward and raised it a little on the demo bike and that feels better for me.

    The reason I asked though is despite the bigger gearing on the demo bike, I feel like I can almost spin the 48 ring on my cross at higher cadenance more comfortably than the 39 ring on the demo bike.

  • dennisndennisn Posts: 10,601
    I switched from 170's to 172.5's(more out of curiosity and needing a new crank than anything else) and can honestly say that I didn't notice any difference.

    Dennis Noward
  • DoobzDoobz Posts: 2,800
    When I went for Posture analysis with ex pro cyclist, we talked a bit about crank lengths. He advised that usually longer crank lengths are used by people with longer femurs (usually good climbers). He also said this was one part of a bike shops "fitting" process that they usually get wrong or miss out completely.
  • Its worth being particular over this issue if youve had any injuries in the past-knee for example-as over extension of recovered(but scarred) tissue might cause recurrence.
    I recently built a bike and dwelt on this a bit, for this exact reason.
    Spesh Works Roubaix '10
    28 Charolais and counting.
  • satanassatanas Posts: 1,303
    Perceptions vary greatly. Some people won't notice 5mm difference and others will get injuries from it. The reality is that longer cranks mean that you will use higher gears and a lower cadence, and vice versa; this may or may not be a good thing. The best summary of how to figure out what crank length is okay for you I have ever seen is here: ... tters07-29

    I like 172.5mm cranks and use them except on the fixie which has 170mm. For me 175mm inhibits my spin more than I like, 178 is okay for climbing but worthless everywhere else, 180 is worthless everywhere and very uncomfortable, and 165mm means no power. It is almost certain that YMMV. Also, it may take a little while to tweak your position after you change crank length.
  • DustineDustine Posts: 184
    I just installed some 165mm cranks on my fixed, after running 170mm hitherto. First ride out i didnt notice any difference over the 10 miles. Next time i paid more attention, and felt that normal spinning along is easier and smoother, but going uphill is somewhat harder due to the lesser leverage. On the flat its like i have dropped a couple of teeth from the chainring; on the hills i have to get out of the saddle more. My geared bike runs 172.5mm cranks, and i havent yet tried going straight from one to the other. But i will give that a go sometime soon.
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