crank length for new bike

[boy in a kilt]

At worst, she may get a tad bit of toe overlap. As long as she isn't prone to doing lots of track stands, she'll probably be ok. A quick way to check is measure from the center of the crank spindle out about 210mm (I'm guessing on the spindle to end of toe-box measurement). If you haven't gotten to the tire yet, she'll be ok.

The main difference she'll notice is the extra bit of torque she'll get when she pedals. The extra 5mm of lever arm may not seem like much, but it does make a bit of difference.

[VeloVT]

I'll share my experience, for what it's worth (maybe not very much, since I'm not the same size, and it doesn't address toe overlap issues at all). I'm 5'6". I have a 52 cm cx frame and a 54 cm road frame, both still with stock cranks. The 54 cm frame has 172.5mm cranks, and the 52 cm frame has 170mm cranks.

I notice the difference. Really. I wouldn't have believed I'd feel a 2.5mm difference but I do. I definitely prefer the 172.5 mm cranks. I feel I get considerably more power, especially on hills. Since I'm also thinking of switching the cx bikes triple for a compact double with slightly higher gearing, I'm considering getting longer cranks at the same time.

FWIW.

Oh, I'm also a size 39 shoe.

[Crankin]

Well, the shop put 170 cranks on my Kuota, so I tried them, after having 165s on my Trek. I hated them. I felt like I had to use a lot more energy, so off they went after 2 weeks. I am 5' 1" and my bike is 51 cm, with a sloping top tube. The shop owner tried to tell me that the longer cranks would turn me into a "climbing machine," but I am a spinner who always uses the easiest gear possible! Well, I do use the big ring at times, but too much just makes my knees hurt.
I guess it depends on what kind of riding she will be doing.

[Beane]

here's something interesting:

http://www.nettally.com/palmk/crankset.html

...But I think it's a personal preference/trial-and-error kind of thing.

[Eden]

I'm thinking if you are short with longer legs (especially thighs) that long cranks would be worse - my reasoning is, I have this body type and I already have to push my saddle back almost all of the way to get good knee position. If I had a longer crank I'd have to try to find even more set back. I'd also have to lower my saddle some, which depending on the kind of fit you are looking for could be good or bad. For me, I want to preserve any drop that I have so I don't want to have to put my saddle down at all.

btw I have 165's on all my bikes except for the kiddie cross bike that I recently aquired, which as 150 somethings on it.

[boy in a kilt]

There are formulas for determining crank length. Some involve the length of your whole leg, some involve just the length of your thigh. I think some involve a third order tensor describing space-time in the general vicinity of your leg.

Go with what feels right.

[OakLeaf]

I think it's the femur and the femur/tibia ratio. Whole leg length doesn't tell you much. But it's kind of theoretical since you can't measure any of that accurately without an X-ray. Then I guess pedaling style goes into it too - whether she ankles a lot, and if so, foot size probably matters as well.

The main thing you want to look for IMO is, when you set the seat height so her foot is in the appropriate position at the bottom of the pedal stroke, is her knee bent more than 90 degrees at the top of the pedal stroke? I'm pretty strong about that, myself. You hear it over and over and over again when you do squats, lunges, step-ups or step aerobics: do not bend your knee more than 90 degrees when it's weight bearing. Then nobody pays attention to that when fitting a bicycle

Does she have the luxury of trying out various sized cranks, or a fitment machine with variable crankarm length? What RPM can she spin smoothly with the longer cranks vs. the shorter ones? Has she ever had knee trouble in the past?

... and if she does wind up wanting a 170 mm triple, does she want the Shimano 105 that came stock with my bike?


- Oak, 5'3", 51 cm Cannondale feminin road frame (which is slightly tall per the standover height, but good top tube length and perfect for my pedal stroke with just over 11 cm of seatpost showing), sz 40 Specialized shoes, 165 mm cranks, knee problems if I use longer ones.

[KnottedYet]

There are formulas for determining crank length. Some involve the length of your whole leg, some involve just the length of your thigh. I think some involve a third order tensor describing space-time in the general vicinity of your leg.

Go with what feels right.

I think I like the "3rd order tensor function describing space-time" better than the old "18.5% of your length from greater trochanter to bottom of heel."

Much more exciting! (but does it give you a length in parsecs? and will it cause distruptions in the spacetime continuum?)