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Discussion Starter · #1 ·
I bought my bike quite a few months back but without realizing the crank length is 172.5mm until one day I saw the marking when I overturn the bike for a wash. I’d have thought the standard stock length is 170mm.

I wish to get it right. I’m 5’ 5” (1.66m) and inseam is 29.5” (0.75m). I tried several web computation and got different results (varies from 165 to 170mm). Is there a preferred crank length for certain type of rides (such as sprint or endurance) or its strictly governed by ht and inseam. My rides are mostly endurance and with some climbs.

Anyway, being not so technically inclined, I wonder whether that minute 2.5mm does make any difference.
 

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Anyway, being not so technically inclined, I wonder whether that minute 2.5mm does make any difference.
It makes a difference on paper, but makes no difference whatsoever in terms of actual power output from an actual rider on an actual road.

It's true that at the same leg speed, cadence slows down very slightly with longer cranks because the crank circle is just a bit larger. But the tiny bit more leverage you get with a longer crank makes up for it.
 

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And there's no "right" in any objective sense. There are some traditional guidelines and various "rules," and in general riders with shorter legs prefer shorter cranks, but there's lots of individual variation in preferences. A lot of riders can't even detect a 5mm difference (someone on this or another forum once posted a story of inadvertently installing the wrong left arm and riding for months with 2 different lengths, never noticing) -- others seem sensitive to it.

Personally, I've ridden with lengths from 165 to 175, and while I might notice the difference when first switching, I quickly adjust and can't say whether one is better than the other.

So if you've been riding your bike and feel good, I'd ignore it.
 

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The reality is ... what ever length feels best to you. If you can, try both lengths and pick the one that feels best.

Leg length isn't the only determining factor, femur length is a big factor as well. You can have long legs, but short femurs, which might make a shorter crank a better option even though technically you go for a longer crank.

Some people prefer the higher leverage they get from the longer cranks, others prefer more room they get from shorter cranks.

With that said ... I have a 32.5" inseam (and am 5'11" tall) and I run 170mm cranks on both my road and TT bikes. My reasoning is this:

1) I have short femurs for my leg length.
2) Shorter cranks open my legs up a little, which opens up my hip angle more.
3) It allows me to raise my saddle, just that little bit, which helps with saddle to bar drop.
4) The shorter cranks move my pedals back a little, effectively increasing my STA without moving my saddle forward.
5) Shorter cranks just feel better to me :)

My wife on the other hand is the exact opposite of me ... she's 4'11" tall with a 28.5" inseam and very long femurs (longer femurs than I have). She would likely feel best on 170 - 172.5 mm cranks even though her inseam is 4" shorter than mine.

I've ridden bikes with 172.5 mm and 175 mm cranks ... and much prefer the 170's. I haven't see a drop off in power, if anything I saw an increase.

In the end ... try both if possible and go with the one that fits best.
 

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Is this your 1st road bike or have you been riding a shorter or longer crankset on another bike?

I ask because if you've been riding, say, a 170 and didn't know that you are now riding a 172.5, it doesn't seem to make much of a difference to you.
 

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I'm a little taller than you with a longer inseam and I like 170mm on my road bike and would try a 165 if I could.

The only way I can compare is that I had 175mm on my old mountain bike and would sometimes ride it for distance on the road. It felt like I was "thrashing" too much; I didn't have the same smooth circular motion that I got with the shorter arms.
My natural cadence is right around 90rpm and it is mostly flat around here.

As others have said; it is just what feels right to you. And it is probably not worth spending the money for a new crankset unless you find a cheap one on fleabay and sell your old one.
 

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Discussion Starter · #7 ·
Thks guys for all the replies. This my first road bike and there isn’t anything for me to compare. But the riding seems ok so far. Guess it’s a kind of psycological thing.

I reckon it’s similar to golf club. I recalled yrs ago Freddy Coupled (I think) was testing some golf club and he hit very well in one particularly club. When the grip was removed revealing the length of shaft, it was actually 5mm longer than his usual club length. Then after the hitting was terrible.
 
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