Crank length

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mattsccm
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Crank length

Post by mattsccm »

This may be remarkably simple to some but I am a person who like to see something work and can't with this.
Lets think about differing crank lengths. To my mind a longer crank needs a greater pedal speed to maintain the same cadence as a short crank. IE the foot is further away from the centre thus having to go faster for maintain RPM. Bit like the waters of a river goig faster on the outside of a bend. Am I correct?
A long crank gives greater leverage thus making a bigger gear easier to pedal. Yes?
Thus if I want to push a bigger gear I need to pedal faster.Yes?
No idea about the percentages/watts etc involved.
Just wondering really. :roll:
Jdsk
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Re: Crank length

Post by Jdsk »

mattsccm wrote: 20 Jun 2021, 5:52pmLets think about differing crank lengths. To my mind a longer crank needs a greater pedal speed to maintain the same cadence as a short crank. IE the foot is further away from the centre thus having to go faster for maintain RPM.
Yes.

mattsccm wrote: 20 Jun 2021, 5:52pm long crank gives greater leverage thus making a bigger gear easier to pedal. Yes?
It gives greater leverage. "Easier to pedal" is subjective.

mattsccm wrote: 20 Jun 2021, 5:52pmThus if I want to push a bigger gear I need to pedal faster.Yes?
What are you keeping constant and what are you changing in this comparison?

Jonathan
peetee
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Re: Crank length

Post by peetee »

The greater leverage one is a lot more complicated as there are at least two fixed-length levers operating in each leg; the hip and knee (and, depending on your pedalling style, possibly the ankle too). Longer cranks can be detrimental if they force the body’s joints beyond their range of common or ‘learned’ swept angle.
The older I get the more I’m inclined to act my shoe size, not my age.
mattsccm
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Re: Crank length

Post by mattsccm »

Just musing. Not taking into account body mechanics etc. In a perfect world I would love something that gave me huge leverage to push much bigger gears yet allows me to keep the cadence down as low as possible. Of course the answer is either somewhat stronger legs or an engine. :roll: :D
Of course things like leverage or pedal speed are probably barely different unless we are talking about cranks that are 10cm different.
As I said , just wondering.
robc02
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Re: Crank length

Post by robc02 »

It does seem intuitive that longer legged riders would probably be better suited to slightly longer cranks and vice versa for shorter legged riders. However, over the years there have been various attempts to study and quantify this and as far as I know they haven't been conclusive. It seems most riders can adapt to quite different crank lengths - as far as power production is concerned anyway.

Variations in leverage and foot speed due to crank length must be considered along with gear ratio as in practice all these things combine.

I suppose personal preference and factors such as limited mobility are the main drivers for most people.

Despite all of this I can't help feeling that 175mm cranks are "right" for me and am reluctant to use anything else. All in the mind? - Maybe.

(Just a thought.... my Humber roadster has 165mm (well 6 1/2 inches actually) cranks and it feels fine! But then it gets ridden in a very much more relaxed style than any of my other bikes).
cycle tramp
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Re: Crank length

Post by cycle tramp »

mattsccm wrote: 20 Jun 2021, 5:52pm A long crank gives greater leverage thus making a bigger gear easier to pedal. Yes?
Thus if I want to push a bigger gear I need to pedal faster.Yes?
No idea about the percentages/watts etc involved.
Just wondering really. :roll:
My advice would be to google 'Mike Burrows Crank Length' because this gets complicated quickly.
The answer to your first question is yes, but a bigger crank length/ higher gear doesn't mean that you will go faster, and it may mean that you'll exhaust the power in your muscles more quickly.
Mike Burrows concluded that a shorter crank length coupled with lower gearing increase his cycling speed, described his legs as making short punching strokes instead of turning windmills... (look at Olympic speed walkers - are they making long strides or very small steps? Some of them are only placing half a foot steps in front of the other).
..However crank length is extremely personal and what may work for one person may not work for another...
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andrew_s
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Re: Crank length

Post by andrew_s »

mattsccm wrote: 20 Jun 2021, 5:52pmAm I correct?
You are correct, and in good company.

The late Sheldon Brown had a bit of a thing about crank length being included in gearing (he called it "gain ratio"), but never made much progress trying to get people to adopt it.

A longer crank gives what is effectively a lower gear (aka "more leverage").
Getting the lower gear by increasing the crank length does not increase your cadence in the same way that getting a lower gear by using a smaller chainring would.
Other differences are that your foot is moving in a larger circle, which can give knee problems due to the increased knee bend at the top of the pedal stoke, and that it's harder to keep up the same cadence that is possible with shorter cranks.

In terms of numbers, the difference between a 170 and 175 crank is about the same as between a 34 and 35 chainring.
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Tigerbiten
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Re: Crank length

Post by Tigerbiten »

My understanding is .........

Different crank length affects you in two opposite ways.

Longer cranks gives you a better mechanical advantage.
So when you run out of gears down and get out the saddle to grind up a hill, it's easier to push longer cranks down.

But biomechanically it's more efficient to spin near the top of your power band.
As shorter cranks tend to let you spin faster.
At speed shorter cranks tend to be more biomechanically efficient.
So if you don't run out of gears down then shorter cranks may well be more efficient.

Normal crank lengths tend to be a compromise between these two effects.
Short enough to let you spin efficiently on the flat but long enough to let you grind slowly up hill.

Luck ........ :D
rogerzilla
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Re: Crank length

Post by rogerzilla »

There are other considerations that may push you to shorter cranks: knee problems and ground clearance, mainly. Most fixies (should) have 165mm cranks because of the risk of pedal strike when cornering.
iandusud
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Re: Crank length

Post by iandusud »

rogerzilla wrote: 21 Jun 2021, 6:22am There are other considerations that may push you to shorter cranks: knee problems and ground clearance, mainly. Most fixies (should) have 165mm cranks because of the risk of pedal strike when cornering.
Or a higher bottom bracket :)
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Tigerbiten
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Re: Crank length

Post by Tigerbiten »

Another class of bikes that can benefit from short cranks is the recumbent.
You cannot get out of the seat to grind up hills, so you're better gearing right down to spin up them.
That's exactly the situation where short cranks can work out better on a long run.

Luck ........ :D
Bmblbzzz
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Re: Crank length

Post by Bmblbzzz »

By increasing the crank length you are effectively lowering the gearing for the same chain ring, sprocket and tyre size, as others have said. Although the ratio between rpm at the crank and rpm at the back wheel has not changed, the ratio between speed of pedal and speed tyre has. This is the "easier to pedal" part.

But biomechanical factors make it often easier to pedal shorter cranks; more or less a factor of your leg length, just as your strike while walking is.
rogerzilla
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Re: Crank length

Post by rogerzilla »

Agree- you're increasing the gearing with shorter cranks. For a given rpm and road speed, you need more force on the pedals and the distance travelled by the pedals is less. This doesn't contradict the fact that shorter cranks are more "spinnable" (less articulation of the leg), which is another reason to have them on fixies. 175rpm downhill is easier on 165s than on 175s.
mattsccm
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Re: Crank length

Post by mattsccm »

Now there is an idea. I would love to slow my legs down on the fixed in descent but equally make live easier on the climbs. Got it. Tomorrow I take 2 chainsets to work and swap half way.
JohnW
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Re: Crank length

Post by JohnW »

mattsccm wrote: 21 Jun 2021, 6:41pm Now there is an idea. I would love to slow my legs down on the fixed in descent but equally make live easier on the climbs. Got it. Tomorrow I take 2 chainsets to work and swap half way.
Matt - get some advice from a decent LBS to start with, find what you're comfortable with and get used to it - and stop worrying.

My first cotterless alloy crankset (about 1960) was a 'Stronglight' set that I bought from Johnny Mapplebeck. He had a 172.5 mm set in stock and I bought that - and the same length subsequently.. Eventually the 172.5mm cranks became difficult to get, and I bought a 170mm set and ........and never noticed the difference.

There will probably be a formula for crank length, based on upper leg length, lower leg length, diameter of your kneecap, length of your big toe................but I'll tell you this story about myself : for most of my working life (27 years of it, anyway) I rode to /from work every day - 10 miles each way - I mean every day - sun, snow, rain, hail.....and I rode on bikes that I'd built up from used components that I didn't mind getting wet, mucky, salted up.............you get the idea.

I re-built my work-bike each year, once the snows had finished, and did little to it except oil it and keep the brakes in good fettle, until after the following year's snow. Parts would be worn, creaking - a real mess - I even re-used last years chain and freewheel block from my best bike. To be honest, I replaced block and chain more than that because with the kind of looking after my work-bike got the blocks and chains used to wear out more than once a year!

One spring I disassembled the bike and checked to see what components would be cleaned and re-used, and found that I'd been riding one 172.5mm crank and one 170 mm crank for at least a year - and never noticed!
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