Dave Moulton

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Monday
Feb272017

Crank Length

A reader recently asked me a question about crank length and suggested I might write an article on the subject. 

There is a reason I haven’t touched on this subject before in over eleven years writing here. It is one of those subjects like "Knee over Pedal." I feel it is unimportant and irrelevant.

However, when I started to think about it, I realized I could maybe throw some logic on the fire, rather than adding to the huge pile of horse shit that is already out there. The whole reason to mess with anything like crank length is to improve performance. Go faster for the same amount of effort.

Were it that simple someone would have figured it out long before now and we would all be using something different than we have been using for the last 100 years. And if ever there was a case for the old adage, “If it ain’t broke, don’t fix it,” this is it.

The part that most fail to grasp is that increasing crank length increases leverage, meaning (In theory.) you can push a bigger gear, but unless you can pedal this higher gear at the same RPM for the same amount of effort, you are not going any faster.

Because you have increased crank length you have also increased the diameter of the pedaling circle. Any advantage in leverage is canceled out by the disadvantage of the greater turning circle.

Your feet, and the leg muscles that drive the feet, are having to move a greater distance (Therefore greater speed.) per revolution of the crank. You may as well stick with the standard length crank and pedal the lower gear. You are traveling at the same road speed, relative to the speed your feet and legs are moving.

Also if you are switching from a 170 to a 175mm. crank, that is one centimeter greater pedaling circle. Your saddle needs to stay in the same place. (Let’s assume for this argument that your saddle height was right to start with.) If you lower the saddle by 5mm. your knees will be coming up a full centimeter higher at the top of the stroke.

Better to leave the saddle where it is. Your crank and pedals will still be at the center of the turning circle you are used to. The extra length of the crank will be equally spread 5mm. extra reach at the bottom of the stroke, and 5mm. higher at the top of the stroke.

I notice longer cranks are being touted as a cure for various leg pains. Often leg and other pains are because the rider is not in the peak of physical condition. Start any exercise regimen, not just cycling, and the participant will often feel discomfort. All one can do is slowly and carefully work through it, until the body becomes accustomed to the extra stress being placed on it.

I fail to see where pedaling in a larger circle can help. It is placing more stress on the body, not less. It is akin to telling your doctor that walking is painful, and he suggests you walk faster and take longer strides. Just because long legs can accommodate longer cranks, doesn’t mean they should, or that there is necessarily an advantage in doing so. Try adjusting your saddle height first. It costs nothing and it is less of a shock on your system.

Here is another analogy. A person with long legs could climb stairs two steps at a time. He may get to the top of the building quicker, but one thing for sure, he has expended a lot more energy in doing so. Just because he can climb stairs two at a time, doesn’t mean he should.

Of course there is nothing stopping him climbing stairs two at a time, and there is nothing stopping him from fitting different length cranks, I am just pointing out that anyone saying there is some big advantage in doing so, is simply blowing smoke.

So how did we arrive at the crank lengths we use today? Let’s first look back in history to the forerunner of the chain driven bike, the high wheeler.

The big wheel was around 60 inches or five feet diameter, cranks had to be short in order to keep the wheel diameter as large as possible.

When the chain driven bike came on the scene in 1885, there were no restrictions on crank length. However, its invention was soon followed by mass production of bicycles and standards had to be set. It was England that started the bike industry and so set the early standards. Even today the world uses half inch pitch bicycle chain as standard when the most of the world uses metric measure for almost everything else.

The standard crank length was soon established at 6 1/2 inches for most bicycles. Because twice 6 1/2 is 13 inches, which is an average stride length for a leisurely walk. However, later it was found for racing bikes 6 3/4 worked better. 7 inches was too long for all but the tallest riders. That 1/4 inch either way made a big difference.

Do you ever wonder why Campagnolo offer a 172.5 mm. crank? Up until WWII Britain led the world in bicycles and components, including the high end racing equipment.  6 3/4 inch cranks were the standard for racing worldwide.

After WWII, Italy really moved into the component market. 172.5 mm. is pretty close to 6 3/4 inches. So this became the new standard. It did in the UK anyway. Everyone I knew, myself included, used 172.5, a few taller guys used 175. It is interesting that Campagnolo is the oldest established out of the big three companies. Campagnolo, Shimano, and Scram, and they still only offer 170. 172.5, and 175mm. crank lengths. Maybe it is all we need.

There may be a case for 180 cranks for someone with exceptionally long legs, say 36 inch or longer. Conversely, 165 cranks for a person with 29 or less inseam. But this whole range of crank lengths throughout the complete range of body sizes I feel is just hype put out there by the bike fitting industry.

Many of the best bike riders in the world range in height roughly between 5’ 7” and 5’ 11.” They ride small to mid-size bikes, and use standard length cranks. It has always been that way for years. Of course there are exceptions, but he day tall, long legged guys using long cranks, start dominating professional racing, is the day I will change my views on crank length.

Certain things in bicycle design were established many years ago, and remain the same because it happens to be right. Half inch pitch chain, already mentioned. 27 in. wheel diameter. (Measure your 700c tire.) 73 degrees is the best head angle for a road bike. The same with crank lengths.

The original crank lengths set over 100 years ago were: 6 1/2 inches (Almost exactly 165mm.) 6 3/4 inches, which is right in between 170 and 175mm. And 7 inches (Slightly less than 180mm.) That is all the range you need. It works, why fuck with it?

To sum up, yes there is a case for different crank lengths, but only over the relatively small range of a centimeter and a half. 165mm. to 180mm. This should accommodate the extreme range of leg lengths well beyond normal averages. Campagnolo's range, 170, 172.5, and 175 is fine for most of us.

Remember too, I don't have a pig in this market, I’m not trying to sell you anything. My final advice, just enjoy the bike, and stop trying to over-think it.

 

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Reader Comments (24)

I'm a touch over 6'5" and find that 175mm cranks are fine for me, I have them on all my road bikes. My MTB has 170mm cranks, and I can't tell the difference when riding them.

I think that the shoes I wear, and how tightly they are done up, are likely to have as great a difference in terms of the diameter that my feet travel as do the length of modern cranks.

February 27, 2017 | Unregistered CommenterSimon

While I agree crank length not a huge deal, it can help find a sweet spot for comfortable riding. Possibly too much emphasis is put on what racers do - they seem to use a lot of drugs and put up with a lot of pain for tiny gains - not really representative of your average recreational cyclist - especially those of use getting on in years.

Bike racers differ less than the average rider - you can be long-waisted or long-legged. I do have to argue with the "two steps at a time expends more energy" analogy. I think a physicist would claim the same amount of work was done regardless, while a physiologist might point out that big, long, heavy legs have to be accelerated and stopped for each step, so taking fewer, longer strides may very well be more efficient for longer-legged people.

I'm only 6'3" but have very long legs (sock feet floor-to-crotch 36") and large (US14) feet. (Dave'e frame sizing is one of the few that takes foot size into account - most suggest a saddle height much too low for me because of course pivoting a long foot adds much more to your leg extension than pivoting a small foot.)

I run 175mm cranks on most of my bikes because that's the longest available at normal costs. I do have some touring bikes with 180mm (MTB) cranks, and I find I prefer them. I think this is because they just work better for my long heavy legs - I also tend to push a lower gear longer than many smaller, lighter riders would. Imagine the difference between twirling a cheerleader's baton and twirling a dumbbell. One has just enough weight to get into a rhythm - the other takes forever to get moving and then you can't stop it.

February 27, 2017 | Unregistered CommenterSteveP

If you take a look at the chart here, http://www.customcranks.de/en/cranklength.html which lists the proportion crank length to inseam for a number of well known riders you get the impression that there's considerable leeway in what works.

February 27, 2017 | Unregistered Commenterdave_f

I'm one of those tall riders, but with 36" inseams my length is in my legs. Combine that with a less-than-ideal lower back and I end up wanting a tall bike with relatively little reach. I went as far as a 20mm quill stem on my old road bike!

And yes, I often climb stairs two at a time. My road climbing is dreadful though - I have a triple with a biggest ratio less than 1 ! And still struggle up steeper grades that others of my age do in half the time.

I'd really like to try longer cranks to see if they make a difference, but they're rarer than rockinghorse poop. Zinn has 190+ mm custom cranks but they're $800 USD a set. That's about 7x what my whole road bike is worth, and is far too much for a test.

February 27, 2017 | Unregistered CommenterCriggie

I notice you only looked at 170 mm or longer, what about shorter options? You wouldn't insist kids ride 170's would you? 150 is common, and I stuck some on a MTB I was using for commuting. My knees seemed to hurt a little bit but maybe I should have been spinning more in the lower gears. (Mike Burrows discusses this in his book bicycle design). I have seen pictures of ordinaries with slotted crank arms to allow adjustable effective length. I have a Sugino Maxy 171 RH crank, but have yet to see if it causes a problem as I haven't got the matching LH crank.

February 27, 2017 | Unregistered Commenterpeter

Switching from a 170 to a 175 mm crank changes the pedal circle to be 3.14 cm larger, not 1 cm larger. Pi x Diameter= circumference.

Jim Townsend

February 27, 2017 | Unregistered CommenterJim Townsend

The two bikes that ride the most have 177.5 and 175 cranks.
Considering that the frames are 62cm ctc I figured that something a little longer was keeping it proportional.
When I was building them I was young and flexible. And back then everyone offered optional length at the same price.
If I ride short cranks (165) I notice it, but I am sure that I couldn't tell 170 from 175.
I think that your foot position on the peddle makes as much if not more difference than the crank arm length.

February 27, 2017 | Unregistered Commenteredstainless

To reiterate what Dave said changing crank length changes gearing. The size of wheels, chainrings, rear cogs, and cranks in the drive train all affect your gear size. While smoothness derived from pushing a bigger gear is an advantage for time trials (longer cranks) most riders (particularly racing cyclists) would benifit from increased agility. Muscles are more effecient at higher rates of contraction. Technique wise its faster to develop strength than finesse so, many try to take the easier path and limit their development. Another very important issue not covered is range of motion. There is a ideal between maximum extension and maximum compression. A short rider with a long crank will reach at the bottom of the stroke while be overcompressed at the top of the stroke. When overcompressed the pelvis must be rotated back towards vertical disengaging the glutieus, compromising the largest muscles in the body. This also comprimises breathing and weight distribution. The frame designer always works within the limits of equipment. Small frames are built with steeper seat angles both to move the pelvis forward and the limits of the 700c wheel.This hurts women who's thighs are generally longer with shorter lower leg and feet than men of the same leg length. Very tall riders should be aware of bicycle design limits with the use of very long crankarms for marginal gains. Very short riders should be aware that by keeping more muscle groups engaged they certainly might have gains using crank lengths below 170mm. The Fact the Campagnolo only offers 170, 172.5, and 175mm is an inventory consideration not a design consideration.

February 27, 2017 | Unregistered CommenterTerence Shaw

hello Dave!
i'm Mircea from Bucharest, Romania.
thank you for your blog!

here is my take on crank length:

Adam Hansen, 1.86m height, long legs, having the record of riding 16th times consecutive grand tours, is riding with 180mm cranks. unfortunately, for his team, Campagnolo do not offer that crank length so he had to search somewhere else...
it sounds long those 180mm but in reality they are not at all for Adam. in fact Nairo Quintana, 1.67m height, using 172.5mm cranks, is riding massive long cranks for his body, meaning PROPORTION WISE!
in cycling there is a big problem, people use numbers of linear distance instead of using relations, of using PROPORTIONS.

when a big guy and small guy walk or run side by side they will have a completely different step length, all in relation with own body, with own range of movement. same should be with the crank length, bigger pedal circle for the bigger rider, smaller pedal circle for the smaller rider.
if all runners would do the step length as Usain Bolt does will they be faster?!.. if Usain Bolt would run in smaller step length will he be faster?!.. definitely not! RANGE OF MOVEMENT is a big missing link in cycling.

i believe there should be more crank length options out there, each with different inner and outer ring size so that the gear gets adjusted.

best regards,
Mircea

February 28, 2017 | Unregistered CommenterMircea Andrei Ghinea

I have one bike with 175's and one with 170's. I don't ride the bike with 170's all that often, but when I do, it's not as if the first thing I think is "Wow, these cranks sure are short!" In fact, I don't think I can tell the difference. Maybe if I was racing and needed to squeeze out that last 1% of efficiency or power a pro fitter would tell me to use one or the other (or maybe 172.5's). But just going for a ride--what the heck.

February 28, 2017 | Unregistered CommenterEric

I see why you never touched Crank Length. Probably shouldn’t have; It’s all Spin.

Longer cranks do not give you more leverage, just as shorter ones don’t allow you to spin easier. Look at where the (supposed) fulcrum point is. A rider is spinning circles, not riding a see-saw. You are converting circular motion into linear progress. We have already figured this out long ago.

That same “fulcrum” can influence you into buying something different. Ride what you have.
Steve

February 28, 2017 | Unregistered CommenterSteve

" Over - thinking " is probably what we've periodically all been prone to in our quest for the perfect bicycle. At 75 I'm still not totally cured !

February 28, 2017 | Unregistered CommenterAnthony C.

"... I realized I could maybe throw some logic on the fire, rather than adding to the huge pile of horse shit that is already out there." That sentiment is a big part of the reason why I keep coming back to your blog, Dave. Thanks for encouraging us to just enjoy the ride.

February 28, 2017 | Unregistered Commentermorlamweb

Hi Dave, crank length for big guys is a subject of high interest for me. I'm 6'4" and it's all in the legs. I've been stuck on 175 cranks but last year had the opportunity to have a three month demo ride on a Leonard Zinn designed KHS Flite 747 with 200mm cranks. I thought it would be a potentially difficult transition – maybe some knee pain or at the very least some adjustment to the new crank length. In fact, it was super easy and felt natural from the first minutes of the ride. I also have to say that I’m not sure your analogy of walking up steps is a useful one. Your argument being that just because somebody with long legs and take the stairs two steps at a time doesn’t mean that’s a good thing or biomechanically efficient. I could argue the other way that having long legs (and the long feet that go with them) it’s annoying and inefficient for me to take smaller steps. The analogy that Zinn uses is the sewing machine needle effect. Somebody with long legs and a 170-175 crank is having to spin tat crank up and down like crazy. Maybe I’m just wrong to think of this on an intuitive level but my bike it full custom with every dimension fit to my measurements. So it seems only natural that I’d have a longer crank, too. Just my thoughts and thanks for outlining the conversation. The only thing that holds me back is the significant cost of the 200 crank.

March 2, 2017 | Unregistered CommenterMatt Walsh

@Matt Walsh I totally agree. There's heaps of opinion in cycling, with experts on both sides. Sometimes the best fix is to just try it for yourself and see how it goes.

Personally I've tried a 1.4 metre super long wheelbase bike, and a SWB recumbent, as well as clipless/toeclips/platform pedals to find which is best for me.

I have access to plenty of old parts through the local bike cooperative, so my plan is to rough up something by somehow fastening two pairs of sacrificial crankarms together for a test.

March 2, 2017 | Unregistered CommenterCriggie

A few years ago I bought a stunning DeRosa, 56 CM. Perfect glove fit for me as I am bog average 5"10". Except....something felt weird, and I kept scuffing my toe clips when launching. AHA! they must be 172.5 cranks! They were.... 180's!! Traded for a pair of 170's and it is now truly a glove fit. At the same time I also scored 170's for JH 155 who is going to get built soon - ish. I was taught, and teach, that crank arm length is not a big deal, but for collectors who have several bikes, keeping them all the same is a good idea.

March 2, 2017 | Unregistered CommenterClyde Putman

back with my second reply here.

about your analogy of a person with long legs climbing stairs two steps at a time. again, it's about proportions. two steps at a time means 2 times more, means one step being 2 times bigger. the question is: is that tall person 2 times taller?? the answer is NO.. so, yes, for a tall person climbing stairs two steps at a time in the end it might be too much..

BUT if he can adjust the stairs in relation with own body, then he will be more efficient. if this tall guy is about 15% taller than average then he just needs to adjust the height of the stair with about 15% more... and then he will become more efficient.

THIS is happening (as i wrote in my first reply here) when walking, running. the body naturally adjust the length of the step. as i said, Usain Bolt (1.95m height, with very long legs) can not run with smaller steps, can not run with the steps of a top athlete of 1.80m height... i mean he can but he will be for sure inefficient.

best regards,
Mircea

March 3, 2017 | Unregistered CommenterMircea Andrei Ghinea

Dave-I am surprised that you would say Britain led the world in bikes and components up to WWII. Frank Berto and Jan Hiene would certainly not agree.
I think one of the most interesting aspects of bicycle history, is the different paths taken by the French and the English. There seems to have been little sharing of knowledge. For example the saddle bag vs. the front bags for club bikes. And the
choice of how gears should be managed.
When did you first see the French touring and randonneur bikes?

March 3, 2017 | Unregistered Commenterdvenable

"the different paths taken by the French and the English"

Nothing changes. We have a French cat - a Chartreux - whose breed is not recognized by the English. The English have a similar breed - the British Blue - that is not recognised by the French. But we've got pubs!

March 3, 2017 | Unregistered CommenterSteveP

It's all a load of bollocks (qv) innit?

March 12, 2017 | Unregistered CommenterMartin H,

If you're very tall or short, it's certainly worth seeking out and trying proportional cranks. When I (at 1.95m) first rode 180mm cranks, it felt like I could finally breathe fully. When I went to 200mm, it felt like flying. Now when I go back to 180s it feels like I'm pedaling with my toes!

There's probably no perfect answer but pedaling is pretty important for cycling; find something that's comfortable!

March 17, 2017 | Unregistered CommenterBob

There is a brief mention of 27" wheels in this article. What has been the reason why 27" wheels have been replaced by 700c?

May 16, 2017 | Unregistered CommenterAlexander Lopez

You start by calling it all horseshit, then proceed to explain your thoughts on the matter with the assumption that the saddle height is correct. Then in a few short sentences you tell the reader that perhaps the saddle height should be adjusted. Somewhere along the way you reminisce about riding on 170 cranks while taller folk used 175's. You then concede that perhaps those with longer legs might benefit from 180's. Perhaps you haven't ventured into this subject because you cannot make a coherent argument against it. But you're certain it makes no difference.

Personally, with a 38.5" cycling inseam, while I can certainly ride and enjoy a bike with 175's, I'm really looking forward to the arrival of my KHS 747 with 200 mm cranks.

July 22, 2017 | Unregistered CommenterUnger Robert

Robert - that is exactly why I'm trying to make some longer cranks to see if it makes a difference to me. Sadly its a high-pressure area and all my temporary test rigs have failed so far.

Doesn't help that cranks tend to be fairly resistant to drilling too. My current plan is 4 (scrappy) crank arms with the outer ones overlapped by about half their length, and then bolted through such the bolts don't interfere with my feet or with the chain.

I've been trialing some 175mm cranks on a hack bike, and it has a low BB so I get pedal scrape easily. That is no fun, so long cranks will necessitate a bike with a high bottom bracket, which means even higher off the grouund. And the UCI Mandated maximum wheel base makes the bike even sketchier on super-steep grades.

Just can't win as a tall person :-\

July 22, 2017 | Unregistered CommenterCriggie

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