crank length for new bike

[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.

[lph]

question off the top of my head: does anybody know if women in general have a different femur/tibia length ratio than men?

Just curious, because the leg warmers I just got (with a clear articulated knee) do not cover my ankle bone, which is a bit irritating, but go all the way (!) up to the top of my thigh. Otherwise they're well made, but maybe I just have long tibias. Or short femurs. Or something.

[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?)

[sgtiger]

The LBS that did the fitting for my custom bike, measured my in-seam both standing and kneeling to determine femur/tibia ratio. And, yes, shoe size was taken into account too. I don't know the exact formula they used; they supposedly developed an exclusive program using the measurements of past fittings that they have done. Anyhow, at 5'1 1/2" and with an inseam of 27" my bike was spec'd with 170mm cranks (which works for me BTW). Surprising because everyone else always suggested 165mm, but their recommendations were based on in-seam length alone. And for me, my femur is longer than my tibia. In fact, when I'm stretching my quads, my heel barely touches the bottom of my bum. (Did I mention, I'm built like a hobbit?!! ) Of course, they did offer to swap out any parts that didn't work out or fit right, but I haven't had to do that. I love, love, love my bike!


Hey BinaK, apparently Knot knows her math-geek humor.

[emily_in_nc]

The LBS that did the fitting for my custom bike, measured my in-seam both standing and kneeling to determine femur/tibia ratio. And, yes, shoe size was taken into account too. I don't know the exact formula they used; they supposedly developed an exclusive program using the measurements of past fittings that they have done. Anyhow, at 5'1 1/2" and with an inseam of 27" my bike was spec'd with 170mm cranks (which works for me BTW).

Your experience is comforting for me b/c I am your height, slightly longer inseam, long femurs, and also have 170mm cranks and am fine with it, but sometimes I feel like a freak having such long cranks for my small size. Glad I'm not the only one!

Emily

[boy in a kilt]

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?)

If you rotate your crank too fast, the tensor describing it is diagonal which means that everyone and everything else around disappears...

Wait, damn. Dropped a sign.

You explode.

[KnottedYet]

Long cranks for long femurs is a good thing. (think about the quasi-mythical KOPS: a longer crank makes much more sense for a longer femur.)

Women tend to have longer femurs relative to tibias, so groovy on the longer cranks! BUT, we also tend to have shorter legs than men, so groovy on the shorter cranks.

Lots of fitters poo-poo the KOPS method of fitting saddle-position/cranks, but I quite like it from a PT standpoint.

Whatever works is cool.

If you need your saddle pushed back for your weight distribution you like between bars and saddle, then adjust the crank length to get the KOPS or to make your legs happy.

( to the OP: building a bike for your gf might actually require letting her ride it a bit before you set anything in stone.)