Road Bike, Cycling Forums banner
1 - 6 of 6 Posts

·
Registered
Joined
·
92 Posts
Discussion Starter · #1 ·
Can anyone offer an information source or advice on determining an appropriate crank arm length?

I am 5'10" with a 31.5" inseam...54cm frame

I currently run a 172.5mm crankset only because this is what came on the bike. My working cadence is 90 - 110.

I've heard 165mm to 170mm to "run whatever feels good"

Do a few millimeters make a difference?
 

·
Registered
Joined
·
8,123 Posts
Nurse_Flash said:
I've heard 165mm to 170mm to "run whatever feels good"

Do a few millimeters make a difference?
There are many opinions on this, but no definitive answers....which is why "Run whatever feels good" is in reality the end of the discussion.

Personally, my dimensions are not to dissimilar to yours...5'11" and 32.5" inseam however I have short femurs for my leg length. I run 170mm cranks and like them more than longer cranks and I've ridden 172.5's and 175's.

Part of the reason for 170's is because they allow me to move my seat back that extra 2.5mm to 5mm compared to longer cranks. I like steep Seat Tube Angles on my bikes due to my shorter femur length. Since most bikes that come close to fitting me have 73.5 degree STA's and I need a 74 - 74.5 degree STA to center my seat...going to the shorter crank allows this.

Another reason is the shorter cranks allow me to raise my seat by a few more mm's. This gives me more saddle to bar drop which makes the ride more comfortable for me. On all my bikes I run a -17 degree stem with zero spacers under it...and many times I want to go lower and spend more than 50% of the time in the drops. The shorter cranks help with this as well.

Last but not least, the shorter cranks reduce the angle my leg bends throughout a stroke, thus making it just a bit easier on my knees and in reality keeping my leg in a bit more powerful position.

In addition to all that...I have a bit more cornering clearance for crits.

The only thing I really lose is a longer lever to put torque on the cranks with when climbing...however, considering the benefits I gain from the shorter cranks, the slight gain in leverage just isn't worth it to me.
 

·
Registered
Joined
·
48 Posts
Before going further I'm just going to say that this is a highly opinionated subject. Many fitters use different methods and swear theirs is the best.
A couple of years back I had knee issues which forced me to look into this same topic and what it boiled down to for me was how much knee bend I had at the top of the stroke. Mine was way beyond 90 degrees which was putting undo pressure on my kneecap. I went down from 170mm to 155mm which might seem out of this world but it did fix my issue. Now I have 155mm cranks on all my bikes and I'm not looking back. My thread can be found here for reference:
http://forums.roadbikereview.com/showthread.php?t=66218

But in short to answer your question two formulas that I came upon which I don't remember who they're from are
1. inseam x 2.16 = 172.8 crank
2. (inseam x 1.25) + 65 = 165.01 crank

For the two formulas above I used the inseam you specified converted to cm = 80.01

In your scenario the both give different results, I went with formula #2 and sized down to 155mm. Hope this helps!
 

·
Steaming piles of opinion
Joined
·
10,503 Posts
Absent any particular condition causing a problem, the few mm's one way or another don't mean much. For your size - 170 - 172.5 would be considered 'normal.'

Lennard Zinn is a believer in proportionally sized cranks - at least he uses the concept to make custom crank sales. Some reading at the link might sway you one way or another. Personally, doing the reading and the math convinced me that it was BS, but different strokes.

Now, I wouldn't put my wife on my 175's, and I don't think much of getting on the spin bikes with the 155's. But one increment this way or that doesn't seem to mean much to me.
 

·
Registered
Joined
·
21,637 Posts
Crank length formulas

LtSpeed03 said:
But in short to answer your question two formulas that I came upon which I don't remember who they're from are
1. inseam x 2.16 = 172.8 crank
2. (inseam x 1.25) + 65 = 165.01 crank
The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
=============================
Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.
 

·
Registered
Joined
·
92 Posts
Discussion Starter · #6 ·
Kerry Irons said:
The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
=============================
Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.

Great response here...evidenced based.
 
1 - 6 of 6 Posts
Top