crank length for new bike

[jamisfan]

Hi, I'm a fella putting together a new bike for my g/f. Frame is a Jamis Ventura 48cm. Shoe size ~38/40. 165 mm are what comes with the bike stock, I would like to try 170s but unsure about wheel/foot clearance. Any one have 170s on their Jamis or similar sized frame? Thx.

[Triskeliongirl]

Hi, I'm a fella putting together a new bike for my g/f. Frame is a Jamis Ventura 48cm. Shoe size ~38/40. 165 mm are what comes with the bike stock, I would like to try 170s but unsure about wheel/foot clearance. Any one have 170s on their Jamis or similar sized frame? Thx.

I think 165 is appropriate for that frame size. I ride a similar frame size and use 165s. I tried 170s and they gave me terrible knee pain. I am 5'4" tall.

[emily_in_nc]

OTOH, I ride a small framed bike (46 cm) and use 170 mm cranks, and like them fine (no knee pain). 165s will help with faster spinning, but 170s give a bit more leverage. I have 170s on all my bikes (road and mountain). It's really a personal decision and depends on the particular person's biomechanics as to which would be optimal. If she has longer legs for her height, the 170s might be more appropriate and vice versa, but there are exceptions to every rule! I am 5'2", by the way.

Emily

[Zen]

You sure about that height, Emily ;) ?

I'm 5'2" (I think)
47 cm frame
size 39 Sidi's
170 cranks
no problems

[emily_in_nc]

You sure about that height, Emily ;) ?

I'm 5'2" (I think)
47 cm frame
size 39 Sidi's
170 cranks
no problems

Good one, zen - you read my "I'm Shrinking" thread! ;) That's why I said 5'2", when I usually say 5'2-1/2". I guess I'll drop the 1/2", but I'm not quite ready to compromise on the 5'2"! :D

I also wear 39 Sidis just like you!

Emily

[KnottedYet]

I prefer 175's, so don't nobody ask me!

[DirtDiva]

Yeah, but you're taller than everyone else in this thread except perhaps the fella who started it. :p

[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 :p

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? :D


- 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?!!:eek: :D ) 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.:D :p

[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.)