Crank length

General cycling advice ( NOT technical ! )
Mike Sales
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Re: Crank length

Postby Mike Sales » 7 Jan 2020, 2:44pm

Marcus Aurelius wrote:
Cyril Haearn wrote:Marginal gains, what length cranks do professional racing cyclists use?
Yes you’re correct. A lot of pro’s typically use 169 or even shorter on their race bikes ( Froome uses 168-169 mm during the TDF IIRC ).


Shurely shome mistake?
1 or 2 mills is a very marginal gain.
Must be specially made too.

recumbentpanda
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Re: Crank length

Postby recumbentpanda » 7 Jan 2020, 3:01pm

Remember the other two levers. A couple of mm off the crank can change the leg angles significantly enough to matter, especially over time/distance.

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The utility cyclist
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Re: Crank length

Postby The utility cyclist » 7 Jan 2020, 5:28pm

Mike Sales wrote:
Marcus Aurelius wrote:
Cyril Haearn wrote:Marginal gains, what length cranks do professional racing cyclists use?
Yes you’re correct. A lot of pro’s typically use 169 or even shorter on their race bikes ( Froome uses 168-169 mm during the TDF IIRC ).


Shurely shome mistake?
1 or 2 mills is a very marginal gain.
Must be specially made too.

Aero gains is the reason why Froome and other pros now use shorter cranks, this makes more of a difference than the crank length itself. Opens up the hips which lowers or allows the torso to be lowered more and thus also aids breathing when compared to a longer crank for same position.
It also apparently has an effect of lowering blood pressure.
Shorter cranks also help to eliminate dead spots in the pedalling action.

https://www.cyclingweekly.com/news/prod ... ter-188288
BUT for the 2017 Vuelta Froome rode 175mm cranks on his Pina dogma F10 Xlight according to GCN when they had his bike, this is in line with the thinking of more climbing or sprinting could benefit longer cranks. https://www.globalcyclingnetwork.com/vi ... spa-a-2017

Marcus Aurelius
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Re: Crank length

Postby Marcus Aurelius » 7 Jan 2020, 6:21pm

recumbentpanda wrote:Remember the other two levers. A couple of mm off the crank can change the leg angles significantly enough to matter, especially over time/distance.

Absolutely.

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Tigerbiten
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Re: Crank length

Postby Tigerbiten » 7 Jan 2020, 7:41pm

The effort needed to turn the pedals is made up from two components .......

The first is the force needed to push the pedal down. This goes up as cranks get shorter.
The second is how much energy it takes to spin your legs. This goes up at the square of the cadence. If your feet don't move as far due to shorter cranks then you can spin your feet faster for the same effort.

With shorter cranks and a slightly higher cadence then the second effect can counteract the first to maybe give you an overall energy saving as long as you don't run out of low gears on hills.
Once you run out of gears then it will be harder to climb.

Luck ......... :D

MikeF
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Re: Crank length

Postby MikeF » 7 Jan 2020, 8:06pm

RickH wrote:
MikeF wrote:
Cyril Haearn wrote:Shorter cranks effectively increase the gear, long cranks decrease the gear because the travel per revolution increases
But they don't actually alter the gear. That remains the same.

True. But shorter cranks make the effort (the amount of force you have to push on the pedals) greater for the same gear at the same speed. That will tend to make it feel like a higher gear.
Yes. Shorter cranks do increase the effort needed.
"It takes a genius to spot the obvious" - my old physics master

MikeF
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Re: Crank length

Postby MikeF » 7 Jan 2020, 8:08pm

Marcus Aurelius wrote:
RickH wrote:
MikeF wrote:But they don't actually alter the gear. That remains the same.

True. But shorter cranks make the effort (the amount of force you have to push on the pedals) greater for the same gear at the same speed. That will tend to make it feel like a higher gear.

Not really. The length of your leg, in relation to the crank makes this negligible. I swapped from 172.5 mm cranks, to 170, on all of my road bikes. I also use power meters. The shorter cranks make it very slightly easier to make the same power, at high enough cadence to improve and maintain efficiency, but it really is marginal. The biggest difference is that I find it far more comfortable to have the smaller pedal circle of the 170mm cranks. That’s the key ( for me anyway). I also find it easier to accelerate to high cadence with the shorter crank. Shorter cranks also make tight corners in Crits and such like safer, because your risk of pedals striking the deck with a big lean angle is reduced.
Interesting points.
"It takes a genius to spot the obvious" - my old physics master

Debs
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Re: Crank length

Postby Debs » 7 Jan 2020, 8:15pm

MikeF wrote:
RickH wrote:
MikeF wrote:But they don't actually alter the gear. That remains the same.

True. But shorter cranks make the effort (the amount of force you have to push on the pedals) greater for the same gear at the same speed. That will tend to make it feel like a higher gear.
Yes. Shorter cranks do increase the effort needed.


But longer cranks require a larger circumference of peddling motion = your legs need to move further distance to complete each revolution.

Mike Sales
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Re: Crank length

Postby Mike Sales » 7 Jan 2020, 8:21pm

Perhaps it is because my legs are about 20 mm. different in length and one hip makes my thigh move out as it moves up that I find the idea that 1 mm. makes a perceptible difference incredible.
170 mm. cranks are used happily by riders whose legs must vary by much more in length.
I admit to not being a highly tuned racing machine, but I do ok.in the circs.

Marcus Aurelius
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Re: Crank length

Postby Marcus Aurelius » 7 Jan 2020, 9:06pm

Debs wrote:
But longer cranks require a larger circumference of peddling motion = your legs need to move further distance to complete each revolution.


Quite right. Your legs are quite badly designed levers, as far as cycling is concerned as well. Smaller pedal circles are normally favourable, if you can.

MikeF
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Re: Crank length

Postby MikeF » 7 Jan 2020, 9:10pm

Debs wrote:
MikeF wrote:
RickH wrote:True. But shorter cranks make the effort (the amount of force you have to push on the pedals) greater for the same gear at the same speed. That will tend to make it feel like a higher gear.
Yes. Shorter cranks do increase the effort needed.


But longer cranks require a larger circumference of peddling motion = your legs need to move further distance to complete each revolution.

Agreed, but pedalling motion. No peddling is required. :wink:
"It takes a genius to spot the obvious" - my old physics master

KM2
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Re: Crank length

Postby KM2 » 7 Jan 2020, 9:24pm

Mike Sales wrote

Perhaps it is because my legs are about 20 mm. different in length and one hip makes my thigh move out as it moves up that I find the idea that 1 mm. makes a perceptible difference incredible.
170 mm. cranks are used happily by riders whose legs must vary by much more in length.
I admit to not being a highly tuned racing machine, but I do ok.in the circs.


If the difference is in the thigh, the crank length should be amended. Two different lengths.
If the lower leg is source of the difference then the pedal could be built up!! Bit like the TA pedals of old.
You would need to find your centres of rotation for each joint and measure accurately.

Mike Sales
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Re: Crank length

Postby Mike Sales » 7 Jan 2020, 9:32pm

KM2 wrote:
If the difference is in the thigh, the crank length should be amended. Two different lengths.
If the lower leg is source of the difference then the pedal could be built up!! Bit like the TA pedals of old.
You would need to find your centres of rotation for each joint and measure accurately.


Bit late for that now. I hope my hip recovers enough to ride, but I have my doubts.
Fifty years ago I damaged my hip, short leg one effect. Forty years ago I got on my bike and it did me much good, gave me strength and helped me walk again.
I did 160 k per day tours, and rode weekly in Eryri. I never bothered with crank length adjustment, never felt the need.

Bowedw
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Re: Crank length

Postby Bowedw » 7 Jan 2020, 10:51pm

I have 165mm cranks which I feel suits me as I have short legs. In theory it needs a lower gear but with a multi gear option and the fact that roads are seldom flat I could not pin point any difference in effort now. What really converted me was a pair of 160mm cranks that I had taken off a bike ,years later decided to try them and found them beneficial. Maybe sort of placebo effect. Still unable to erase the notion the longer the better, I settled on 165 ones.
Years ago a passionate strong lifelong cyclist, local to me,rode something like 180 or maybe 185 cranks. Years later when I saw him he could hardly bend his legs to walk. Maybe not the cranks fault but very suspicious really.

ChrisBL
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Re: Crank length

Postby ChrisBL » 8 Jan 2020, 9:34pm

I too have short legs, a small bike and 165mm cranks. My knees hurt less than when I use longer cranks (on our tandem).