glenng

I`m trying to pick out parts for a new build. I`m not sure what crank arm length to go with. What are some of the determining factors for deciding crank length.

Many thanks
Glenn

CastIron

Customarily, inseam.
There are calculators on-line that'll dope it out for you. Perhaps someone here will offer a link.

msheron

Perfect fit................or that is what I believe in the whole scheme of things.

glenng

Thanks for the reply , My inseam is 34" . I googled and came up with conflicting info. Hope some folks here could shed mor light on this subject.

Thanks
Glenn

ac29593

I just swapped mine out for what felt comfortable. I was using 170mm until I got my new bike. I had 172.5mm on the new one, and that was according to "fit". But I didnt like them and went back to the shorter crank arms. Couldnt be happier. I would suggest to see what is the best "fit", then try a few different sizes to make sure you are buying what you like. I know some people say they cant tell the difference between a few mm's, but try riding them one at a time and I bet you will be able to tell the difference.

superdex

Peter White has a "formula" https://www.peterwhitecycles.com/fitting.htm and his word is pretty well respected. How tall are you, what's your inseam and shoe size? I'm 6'3", 33" inseam and a size 11.5, and I run 175mm cranks (but 165 on the fixed)....

the other basic question is, "what about your current crank length do you not like?"

VT to CA

I say go with the inseam. I use 175's (loooong legs) on a 53-55 frame (shoooort torso)... if I was to go with the system that uses frame size to determine crank length, I'd be on 170's... too short for me.

But then again, we're talking about 2.5 mm length difference between crank arms here... I have to admit I've always found this particular area of cycling tech to be sort of bogus... especially after comparing a set of 172.5 FSA Pro Team Issue cranks with my 175 FSA K-Force MegaExo cranks... the 172.5's were actually a hair longer. (measuring from the hole at the BB spindle to the hole for the pedal spindle)

I honestly think the length differences are so minute and precise, and the cycling industry's standards for measurements (and weights!) in production fabrications so imprecise, that having the "perfect length" in a crank arm is more of a placebo than anything.

msheron

Like I said the overall perfect fit..............let a professional fitter fit you then go from there. I know inseam and such gets you in the ball park but until your fitted and like one post stated you try a few sizes out then you will not know what is best.

blue_nose

+1

We are talking about the width of two dimes between a 170 and 172.5mm crank.

A couple of points to consider:

(1) There is usually an imbalance of leg length - most people actually have one short leg
(2) Imperfections on your pedal stroke are probably well within this 2mm margin.

glenng

Thats a great article.

I`m 6' , 34 i.s. , size 11 shoe.

I`ve always either purchased "off the rack" bikes or custom`s but in every case they were fitted by the LBS.

This is the first build were I`m doing the all the "foot-work".


My current bike has 172.5 cranks and they seem fine....but maybe 175 would seem fine too...maybe it wouldn`t make much difference. 2.5 mm is less than a 1/10th of an inch.

Glenn

superdex

is there a buddie's bike with that length you could try? perhaps the LBS has a demo bike they'll let you take a day or two?

bac

Upper leg length, and foot size are both real factors when choosing crank length. Having said that, I would still go with whatever feels the best for YOU. Good luck.

... Brad

ac29593

Lets forget about the width for a second, and compare the circumference of the circle you will be pedal. 172.5mm has a 1083.85mm circumference and the 170's have a 1068.14, a difference of 15.71mm. Now go back to the arguement where the difference is almost non-existent (assume that comfort, fit, and leverage are about equal), Id rather pedal the smaller circle. This variance will be the same for all 2.5mm increments.

blue_nose

There are actually two competing factors here:

(1) Shorter crank arms will allow you to spin faster. Faster cadence can result in better efficiency.

(2) In contrast, if you are comfortable pedaling the larger crank arm then this will create more leverage - generating more to power in contrast to the shorter crank arm.

You are still talking about a minimal difference – in a blind test very few (if any) would be able to tell the difference between a 170 and 172.5 mm crank. My two previous points abut leg length differences pedal stroke imbalances are also within the range of difference between the two pedal strokes you have outlined above.

AnthonyG

Yes your going to get conflicting information so you will have to make you own decision. I'm a fan of shorter cranks and I use custom 135 mm cranks and they're great. Now mindyou I'm quite short with proportionaly short legs but I still like cranks on the proportionaly shorter side. While shorter cranks may give you slightly less leverage on the plus side they make it easier to maintain an aerodynamic tuck because you knees aren't lifting as high at the top of the stroke and your muscles stay operating in their favored range.

See, https://www.cranklength.info for my favorite reference on the subject. If you read the first article you should note that he's not claiming that his reccomendations are gospel but a good place to start. EDIT: I just had a quick look at the site and he's making changes at the moment so its not there.

Personaly I would reccomend staying with the slightly shorter cranks if you used to them as the supposed advantage of more leverage for the longer cranks will be nullified by making it harder to stay aerodynamic and ultimately wind resistance is the greatest force we have to face.

In closing I find I'm faster up hills with the shorter cranks as well. Any advantage of more leverage is nullified by being able to operate in a more powerful muscle range with the shorter cranks.

Regards, Anthony

cacatfish

Some say it is determined by your shoe size or by the size of your hands.
Others equate it with the mass of your nose. In reality I dont think there is any
real way to determine it. Mainly just genetics I guess.

ElJamoquio

...about 1.5%.

VegaVixen

Glenn, my inseam is 84cm, femur length 38cm, and the professional fitter said 172.5mm. FWIW.

oboeguy

Inseam is between 85cm and 86cm, IIRC and on one bike I have 170mm cranks, the other, 175mm. I only noticed that they were different within the last half year or so, too. One bike's a folder, the other a road bike; I've ridden centuries on both. Whoop-de-do.

aikigreg

What i did was buy a set of tandem crankarm shorteners. I rode the snot out of the bike and adjusted every few weeks until I found the sweet spot for both spin and speed. Surprisingly, shorter cranks did not affect speed until I went TOO short (below 155mm). However my knee pain/recovery time went down and my distances and average speed went up for shorter cranks. So I'll never go anything longer than 165s again, and I have a 33" inseeam.

mihlbach

I doubt this. I am fairly certain that the advertised length of the crank refers to the radius of the circle made by the crank, measured from a line going through the center of the BB to a line going through the midpoint of the pedal spindle. The actual distance from the center of the bb to the center of the pedal spindle will be slightly greater than that if the crankarm is angled out slightly..and many crank arms are angled outward slightly, so that they will clear the frame. So if you just compared crank arm lengths by holding them side-by-side or by simply putting a calipers or tape measure from the center of the BB to the center of the pedal hole, you are not necessarily measuring the actual radius of the circle made by the cranks, nor are you acounting for any differences in q-factor caused by the outward angle of the cranks.
Awhile back I measured and compared all the cranks I have, including all the stuff I have sitting around and I found them to be spot on identical to the length stamped on them. The cranks I measured include:
Shimano 105, 170mm
Shimano 105, 175mm
Sugino 75 track crank, 167.5mm
Shimano Ultegra, 175mm
Shimano Tiagra, 175
Sugino impel, 175mm
some ancient French cranks, 170mm
a Sugino road crank of some sort, 170mm
and a few other cheapo crank arms I had lying around.

cyclotoine

In my opinion the big companies (shimano, campagnolo) do not produce and acceptable range, you can only get 180m in record or dura-ace. why? I don't know, probably production costs. there are those out there who will produce a crank from 150mm to well over 200mm... Zinn for example. TA makes a nice range. There are many different schools of thought but I will tell you this.. I have used 165mm, 170mm, 172.5mm, 175mm, 180mm and 185mm soon to try out some 177.5mm cranks. SO far 180mm suits me best though Zinns estimation would put me around 195-200mm, I can't afford the custom bike that would require and I like 180mm the best. 94cm inseam.

MichaelW

If you are building your frame from the ground up to accomodate longer cranks, have a read of Zinn who not only builds large frames but supplies custom long cranks for very tall riders.
The alterations you need for longer-than-normal cranks are a higher bottom bracket to prevent grounding and a longer BB-front hub distance to prevent toe-clip overlap. Neither of these should be problematic for a tall rider. You may also want to do something about heel-clip at the chainstays, either make the stays longer and/or curve them in.

A difference in crank length of a few mm is marginal but many riders at the extremes of size are riding cranks that are out by a cm or two.

The other point is that changes in length do not affect power. Power = force distance and assuming constant pedalling force, you can pedal for 1 min and your feet will cover the same distance with fewer rotations needed with longer cranks.
Longer cranks do increase torque at the BB which is used to turn a higher gear ratio. The resultant speed depends only on power.

StalkerZERO

So far I haven't seen any links posted here besides that peter white article. So, does anybody have any links to fit calculators?
I believe my inseam is 30 inches. My height is around 5' 8" or 5' 9" I'm not sure at the moment. And right now I'm running a fsa crankset at 170mm. Over the months of use I have decided that it does feel uncomfortable and yes I do have the correct saddle height and position......for myself. So I have a feeling an adjustment to a new crankset might be the way to go.
I want to change out the triple for a compact double and the crank length I want is going to be different than 170mm. Its just that I'm not sure if it should be longer or shorter.
My upper torso is generally longer than my legs and my upper thighs are longer than the calves. Should I go with a 165mm or a 175mm?

StalkerZERO

Please help? I'm going to go talk to the lbs and I don't want to seem like a TOTAL idiot. Well.......not totally anyway.