Bicycle Mechanics - Determining crank length
Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.
03-05-01, 02:00 PM
Is there a guideline for determining appropriate crank length or is it a matter of personal taste? I read recently where some cyclists use crank lengths as short as 167.5 mm to achieve faster cadence. Comments?
03-05-01, 05:53 PM
167.5 for small riders, 170 is pretty well standard.
I like 175 on the mt. bike for more grunt pushing up steep hills.
167.5 feel like rideing a windmill, you realy would not like them.
Besides forget about these high cadence freaks and ride what is comfortable for you, I havent worried about it since i quit raceing.
And i could top out on a egerometer at 160 +
03-06-01, 04:41 AM
I think a shorter crank arm is good for climbing, but for me i prefer the crank arm at 175 mm
03-06-01, 06:21 AM
I know some small people who ride 170 or larger, just because that came with the bike. Imagine if you increased the length of your cranks proportinately, it would be like an 6' guy riding 190mm or larger.
Short or long are relative terms, once you know what normal is for your size.
a few months ago I came up with the idea of using two well established formulas for seat post height and deriving the crank length from them : as follows :
1. Top of seat to centre of BB = 0.883 X your inseam* length
2. Top of seat to pedal at bottom of downstroke = 1.09 X your inseam* length
So, if you calculate your inseam and work out these two values, the difference between them is your crank length.
I never actually found anybody to confirm my idea and I did this out of common sense, however, taking into account that these are two well accepted formulas and that when I set up my bike to these specs I solved my knee problems, I think it works.
* write to me if you need instructions on how to measure your inseam.
03-09-01, 10:19 AM
OK, in an effort not so much to check crank length but to get a better idea of appropriate seat height. I came up with some puzzling results.
First, I measured my inseam several times according to your description. 32 inches. So far, so good.
Then, thinking it would be easier to do some of the calculations in metric, I divided by .03937 to get millimeters. 813 mm. OK.
813 X .883=718 center bb to seat top distance
813 X 1.09=886 low pedal to seat top distance
886-718=168 calculated crank length. Sounds reasonable. Standard 170 probably fine.
Everything AOK so far (somebody check my math).
Converted the 886 millimeters to inches just to ease measurement with standard yardstick. 886 X .03937= 34.88 inches.
My confusion is that when I go to the bike to see how this compares, my low pedal to seat top distance is now close to 37 inches, and I was thinking it felt kind of low. I am resetting it right now just to see how 35 inches feels on my ride home later. Have I miscalculated somewhere? Should I pedal more efficiently at 35 inches and get used to that height?
03-09-01, 06:27 PM
I am very sorry, I cannot help you with this one, I have failed my math subject when I was in School, so sorry buddy
03-10-01, 03:59 PM
Followup to my earlier post. I neglected to mention that I have a suspension seatpost. It compresses a good inch or so when I place my not insubstantial mass upon it. That plus additional compression when I move as I pedal probably accounts for the difference between calculation and perception. As my seat gets more conditioned and my mass decreases I may switch to a fixed seatpost.
The calculations do put calculated crank length right in the normal ball park. Good intuition, Claude.
03-10-01, 04:02 PM
Forgot to mention seat padding (bike and human) that probably contributes to the, uh, squishiness of the calculated vs perceived measurements.
it seems like the calculations you worked out are OK. Maybe you should experiment with 35inches, as it seems like that is the "correct" measurement, but then, as they always say when stating these measurement techniques is that finally it is your comfort which wins over the math.
03-13-01, 09:23 PM
Only people with really short legs and track sprinters (riding fixed gears) use 167.5s. A rule of thumb I read somewhere is that if you ride up to a 55 cm (c-t) road frame, you should use 170 mm cranks. Starting at 55 you might consider 172.5s... it becomes a matter of personal preference and physiology. I forget the rules above that point, since they have no bearing on me. ;)
There are plenty of great (like G. Lemond) and not so great (like me) cyclists who, for whatever reason, tend not to spin really high rpms. Which, by the way, doesn't necessarily mean we mash in low gears, either. I prefer the slightly longer cranks, since they don't interfere with my spin, and the give slightly better leverage (and hence torque) on climbs.
The biggest potential drawback is that the overall range of knee motion per rev is increased, which can cause knee distress. With longer cranks, when you get your seat height dialed-in, the top of your stroke is higher by 2x the crank length difference, i.e. your foot will be about .2 inch higher with 172.5s over 170s. Not much, but it can cause problems.
I guess the point of my diatribe is that for maybe 95% of riders, the cranks that came with the bike are probably fine, if the frame fits. For the others, there are a lot of factors to consider when deciding whether to go with longer or shorter cranks.
03-14-01, 05:44 AM
Thanks, Newtbob and everyone else who responded. Now I know I can probably just stick with 170 unless someone makes a strong case for some other length. I was not suggesting that I might go to something like 167.5, just using that as an example of an extreme apparently based on rider preference.
Powered by vBulletin® Version 4.1.12 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.