# Crank arm length vs rider size



## ride4fun (Feb 5, 2005)

Hi all,

I just picked up a new 56cm Specialized Roubaix Comp triple and noticed the crank arm length is 172.5mm, and all my other bikes are 175mm. I am 6 foot tall with 34.5 inch inseam - which crank would be better for hilly road riding?

My wife has an XS size Specialized Sequoia Comp which has a 170mm crank arm length. She is 5 foot tall with a 26.5 inch inseam. Shouldn't bikes at opposite ends of the frame size spectrum have cranks with a greater difference in length?

I would appreciate any advice on the proper sizing of cranks arms. Thanks in advance.


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## Al1943 (Jun 23, 2003)

I doubt that you will notice the difference between the 175 and the 172.5 cranks. If you have trouble with hills look to your gearing.
There have been numerous forum threads about crank lengths, some recently, try searching for them.

Al


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## spookyload (Jan 30, 2004)

Get a tape measure out and look at how little 2.5 mm really is. If you can feel that difference, you are way too picky.


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## technocycle (Oct 29, 2004)

*Size Doesn't Matter*



ride4fun said:


> Hi all,
> 
> I just picked up a new 56cm Specialized Roubaix Comp triple and noticed the crank arm length is 172.5mm, and all my other bikes are 175mm. I am 6 foot tall with 34.5 inch inseam - which crank would be better for hilly road riding?
> 
> ...


I am 5'8 with a 29.5 inseam, I have always ridden a 172.5 and recently switched to a 175. I cannot tell the difference. If you measure it, it is very minimal.


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## everydaybike (Feb 25, 2005)

ride4fun said:


> Hi all,
> 
> I just picked up a new 56cm Specialized Roubaix Comp triple and noticed the crank arm length is 172.5mm, and all my other bikes are 175mm. I am 6 foot tall with 34.5 inch inseam - which crank would be better for hilly road riding?
> 
> ...


Take a look at a metric ruler... 2.5mm isn't a huge amount. There is a [slight] leverage advantage to a longer crank but you should not notice it at all... Even the 5mm difference between your cranks and your wife's is minimal. The difference in size between your frame and your wife's frame will explain the difference in crank length. A rider with shorter legs needs a shorter crank. I'd be more concerned about the frame size than crank length. If you do the math according to KOPS your frame seems a little small compared to your inseam length. But then, all that really matters is that you are comfortable on the bike and that you are happy with how it fits you. Enjoy your new ride... Spring is coming


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## TurboTurtle (Feb 4, 2004)

ride4fun said:


> Hi all,
> 
> I just picked up a new 56cm Specialized Roubaix Comp triple and noticed the crank arm length is 172.5mm, and all my other bikes are 175mm. I am 6 foot tall with 34.5 inch inseam - which crank would be better for hilly road riding?
> 
> ...


I've read "expert"opinions that say everything from it doesn't matter at all (had two dif lengths on right and left - no dif) and that the manufacturers actually should produce 150mm to 200mm. Both of these opinions were from the same "expert". - TF


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## neil0502 (Feb 11, 2005)

*Here's some science. Now ignore it!*

1) Measure from the top of your femur to the floor (centimeters). Multiply that by 1.85.

2) Measure the length of your femur (centimeters). Multiply that by 3.95.

Somewhere between those two results is your "ideal" crank length.

Now . . . as I said in the title . . . ignore those numbers (unless the average was 175mm) go back to your 175's where you were happy


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## BenR (Dec 14, 2001)

*If it bugs you, ask for 175's*

I'm about the same height as you and had 175's for a while and then switched back to 172.5's because I was having knee problems, but it turned out to not be the cranks. They were the same model crank on the same bike and pedals, and I noticed a substantial difference. Yes, it's only 2.5 mm difference but x2 for the circle diameter that your foot has to make, and I would argue that some people who ride a lot on one bike will notice a half centimeter change. I definitely liked the longer cranks for climbing and attacks where I tend to use torque rather than good spinning technique, but my average cadence was tiny bit slower (duh). If it weren't for the cost, I think I'd go back to 175's now that I have a bike that fits me. For a 6' person, conventional wisdom says that you're on the borderline between the two. Not that we have much in common with them, but a lot of pros run relatively long cranks. Of course they have their share of spinner too, whatever that's worth.

It really depends on your riding style. If you have a slow cadence anyway, you might want longer cranks. If you are a trackie or spinner, better to not bog yourself down. That said, lots of people claim to not notice a difference. It's your hobby and in the big scheme of things, cranks don't usually cost that much. If the 172.5's aren't working for you (or you need to exchange them now for 175's before you start using them), I'd stick with what YOU want.


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## everydaybike (Feb 25, 2005)

BenR said:


> I'm about the same height as you and had 175's for a while and then switched back to 172.5's because I was having knee problems, but it turned out to not be the cranks. They were the same model crank on the same bike and pedals, and I noticed a substantial difference. Yes, it's only 2.5 mm difference but x2 for the circle diameter that your foot has to make, and I would argue that some people who ride a lot on one bike will notice a half centimeter change. I definitely liked the longer cranks for climbing and attacks where I tend to use torque rather than good spinning technique, but my average cadence was tiny bit slower (duh). If it weren't for the cost, I think I'd go back to 175's now that I have a bike that fits me. For a 6' person, conventional wisdom says that you're on the borderline between the two. Not that we have much in common with them, but a lot of pros run relatively long cranks. Of course they have their share of spinner too, whatever that's worth.
> 
> It really depends on your riding style. If you have a slow cadence anyway, you might want longer cranks. If you are a trackie or spinner, better to not bog yourself down. That said, lots of people claim to not notice a difference. It's your hobby and in the big scheme of things, cranks don't usually cost that much. If the 172.5's aren't working for you (or you need to exchange them now for 175's before you start using them), I'd stick with what YOU want.


Good advice! It all boils down to what works as an individual... Some like the extra leverage to push a bigger gear and some like to spin at a higher cadence. Hence a little longer crank versus a shorter crank. Even though I'm in the over fifty crowd, I usually ride 200 to 300 miles a week from spring to late fall then cut back when the weather makes a good argument for spending more time on the trainer. On the trainer, I use the shorter cranks because I'm more interested in spin and endurance than in stomping down with a little extra leverage and the fact that my trainer/beater bike has the 172.5mm crankset. I really don't notice a huge difference physically when I'm on the road even when the math explains the difference clearly. However, I do notice a difference when I turn to my mountain bike which sports 170mm versus the 175mm cranks. It's easier to spin but get less response when I really stomp on the pedals. The pros adjust accordingly simply because they can and the options are always abundant. But for my average, everyday self, I won't be spending any less time on a bike if the cranks happen to be 172.5mm or 175mm. I've also had knee problems and thought it was the cranks and that's how I came to change length. But I quickly found it wasn't the crank length as much as it was the position on the bike in relationship to my pedal spindle. I slid the seat back about 10mm and up about 5mm and it solved the problem... That said, it can make a difference to increase or decrease a crank length but for an everyday rider who doesn't measure watts or count the 2 gram difference between a 4mm titanium bolt and a stainless steel bolt, it shouldn't make a huge difference in performance or quality of the ride. But, in the end, it's all up to the individual...


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## harvey (Feb 27, 2005)

*Go for 175, probably*



neil0502 said:


> 1) Measure from the top of your femur to the floor (centimeters). Multiply that by 1.85.
> 
> 2) Measure the length of your femur (centimeters). Multiply that by 3.95.
> 
> Somewhere between those two results is your "ideal" crank length.


Did you see the web site http://www.peterwhitecycles.com/fitting.htm by Peter White, a local (NewEngland) frame builder? He discusses the "18.5 Rule" in detail.

Personally, my very first Klein MTB had a 180 mm crank (I had bought the bike used and at that time didn't pay attention to many numbers aside from wheel count (2) and gearing (3 by 6)). For the record, I'm 6' tall with a 34+" inseam. My presest MTB (an Ibis) has 175 mm cranks; I tend to be a spinner on tough trails where I can and also use the bike a lot on the road to ride to venues (rather than loading it into my car, a no-no).

My about-to-be-retired road bike has 172.5 mm cranks, but I just noticed this a couple of weeks ago after having had the bike for 12 years ( Duhh..). I had to decide if I wanted to stick with this shorter crank for the new bike or go to the 175 -- I tried to evoke the 18.5 Rule, had a little trouble finding the top of my femur, think I found it, and calculated a length closer to the 175.

My gut says it probably won't make much difference unless you are a pro TT racer trying to knock off those few seconds from your best PR.


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## Slip Stream (Jul 19, 2002)

http://www.cranklength.info/index.htm


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## Spinfinity (Feb 3, 2004)

*Could you measure within 2.5mm without help?*



neil0502 said:


> 1) Measure from the top of your femur to the floor (centimeters). Multiply that by 1.85.
> 
> 2) Measure the length of your femur (centimeters). Multiply that by 3.95.
> 
> ...


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## Spoke Wrench (Aug 20, 2001)

That's what I was thinking too. My son-in-law is a chiropractor so I have access to X-ray facilities and I'm still not confident I could measure my femur to 2.5mm accuracy.


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