crank length

[sgtiger]

5' 1 3/4" w/26" standover, the builders for my custom Rodriguez reccomended 170m cranks for me. Besides taking the standard standover measurement, the LBS guys took another measurement with me kneeling to get my femur-to-tibia ratio. Turns out my femurs are way longer than my tibia. Who knew? I think shoe size went into consideration too. My knees are definitely happier now than when I had 165m cranks. Just so you know my current bike's geometry is way different than the bike I had with the 165m cranks on them, so that may make a difference too.

[OakLeaf]

Good point. Short femurs here.

[madscot13]

how can you tell with the ratio of femur: tibia which crank you need? I've never heard about that and would like to hear more

[LBTC]

how can you tell with the ratio of femur: tibia which crank you need? I've never heard about that and would like to hear more

me too! not only that, how can you tell what the ratio is? What is way longer?

H&B
~T~

[sgtiger]

I don't know what the calculations are exactly. The following pic is from the LBS's site:



More here:
http://rodcycle.com/articles/fitting_example.html

The black tubing(Rodriguez on side) is fully adjustable. They used it take different height measurements. First, of course, was overall height. Then one just between my collarbones(like 3rd pic down), although that has nothing to do with the crank length. Stand-over where I was asked to place the tubing as high as I could get it: to the point where it was a bit uncomfortable, but my feet are still flat on the floor. The last kneeling: knees against the wooden part that juts up from base, again the tubing placed as high as it would go. They just subtract the kneeling measurement from the stand-over height to get the difference.

BTW, kneeling like this(pose 1), not with buttocks sitting on feet(Um....actually my buttocks don't sit on my feet. Heels come only to the bottom of my bum!):



Besides anatomy, your bike's geometry also plays into crank length:
-Seat tube angle: it can affect the distance from bottom bracket to saddle
-Angle of seat post: some have a bend that put the saddle farther back from BB than staight seat post
-??? I'm sure there's more, but I have no clue.

I also read somewhere(Bicycling maybe?) that pedaling style can affect one persons peference for a certain crank length vs. another person given that their measurements and bike specs are similar. Longer crack=more leverage(Whoops! I guess my mind is still on bums. Should be crank BTW). So the person who likes to spin at a higher RPM tends to prefer shorter cranks than someone who mashes.

But please don't just take my word for it, 'cause I'm still learning and I have MUCH to learn. Hehe!

Where's our PT(A) and bike fitting gurus when we need them? Knot? Wahine? Velogirl?

[KnottedYet]

I don't know nothin'.

There are some pretty wild equations you can use to figure out crank length. Don't know where they went, but if I find 'em I'll post 'em.

Honestly, that's too much math for me.

I have 170's on one bike and 175's on the other. I like my 175 a lot. I'm a masher with long femurs.

I guess you just start with what you've got, list out any problems you have with them (do they hurt your knees, can you get a good KOPS, do you feel like you are struggling against them, and so on) and then change out from there.

Before going after crank length, though, I'd get a really good bike fit. Make sure it isn't an issue of saddle placement or reach or cleats affecting your leggies.

[RoadRaven]

Hmmm... interesting reading...

My first bike had 175 cranks.
Then I got a smaller frame/bike (Giant) and it had 170 cranks. That is the bike I raced on.

Then we bought another race bike for me - also with 170 cranks on - and a women specific frame (EMC2) and turned my Giant into a time trial specific bike.

I have just reverted to 175 cranks on my time trial bike (Giant) to hopefully help my speed...

I have enjoyed reading this. Thank you for starting it.