Crank length

General cycling advice ( NOT technical ! )
densmall
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Joined: 29 Jan 2015, 10:20am

Crank length

Postby densmall » 6 Jan 2020, 2:10pm

Does fitting shorter cranks make pedalling any harder?

Jdsk
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Re: Crank length

Postby Jdsk » 6 Jan 2020, 3:58pm

You'll need more force on the pedal to get the same effect. Whether that's "harder" for you is difficult to predict.

I suggest getting cranks that suit your size and desired movement regardless of force.

Jonathan

Darkman
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Re: Crank length

Postby Darkman » 6 Jan 2020, 4:06pm

In physics terms, the crank is just a lever than amplifies the input force. Think trying to undo a bolt with a regular spanner, vs an 18-inch long breaker bar. The longer the spanner, the easier the job is (potential bolt-shearing shenanigans aside).

That said, the noticeable difference between one crank length and the next, would likely be negligible as most are in the same ballpark.

mattheus
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Re: Crank length

Postby mattheus » 6 Jan 2020, 4:19pm

MORE FORCE REQUIRED is only the case if you're in the same gear!

(we could have a long discussion about force, power, torque, Work Done … in fact, knowing this forum, I'd be surprised if we don't … (: ...

There have been some credible* studies showing that - after sufficient adaptation - most cyclists would be lot more efficient on cranks much shorter than the standard 170mm. I think they looked at 135mm, something like that.

If it was something I could try for £5 and 10-minutes of spannering/faffing, I'd do it.


*to me

Jdsk
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Re: Crank length

Postby Jdsk » 6 Jan 2020, 4:23pm

mattheus wrote:MORE FORCE REQUIRED is only the case if you're in the same gear!

Of course. That's why I suggested getting the crank length right for size and leg action and not for force. Then you can sort the gearing.

Jonathan

recumbentpanda
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Joined: 6 Apr 2009, 12:13pm

Re: Crank length

Postby recumbentpanda » 6 Jan 2020, 4:52pm

Here in Recumbentworld many of us use short cranks.

The objection of ‘reduced leverage’ is perfectly correct -except that at least two other levers are involved in the system: the upper and lower leg. Those of us recumbenteers who use short cranks generally do so because it puts the ‘human levers’ in a better position during the power stroke parts of the pedalling circle. Imagine squatting with feet flat on the floor. Now try to stand up. First few inches are hard, because the angle of the ‘levers’ gives poor leverage for the muscles. The more upright you get, the easier it becomes to straighten the leg.

Then there is the question of cadence. Not everyone finds a high cadence to their liking, and not everyone finds short cranks conducive to a high cadence -but many do and use them for that reason.

With some recumbent bike/rider combinations, knee ligament strain can be an issue. I have found that short cranks overcome this, probably by reducing the maximum angle of bend and thus improving muscle leverage and reducing strain.

Incidentally, if you want to experiment, the BMX marques like Sinz do nice square taper cranks with 110 bcd spiders, in a range of lengths. Alternatively Sugino do lovely old school triple touring cranksets, again quite a wide range of lengths. Bear in mind though that you will need to increase saddle height on an upright bike, and boom extension on a ‘bent as in both cases your furthest away pedal will be coming closer to you!

brynpoeth
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Re: Crank length

Postby brynpoeth » 6 Jan 2020, 4:57pm

Shorter cranks effectively increase the gear, long cranks decrease the gear because the travel per revolution increases
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Steve
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Re: Crank length

Postby Steve » 6 Jan 2020, 7:52pm

Far more complicated than just simple physics, as it involves the human body and in particular the knee joint! I'm over 6 feet but prefer 170mm cranks to anything longer, as they're much easier on the knees.

Brucey
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Re: Crank length

Postby Brucey » 6 Jan 2020, 9:21pm

densmall wrote:Does fitting shorter cranks make pedalling any harder?


in tests, preferred crank length have been shown to only vary weakly with leg length. Preferred cadence and pedal pressure likewise. I happen to think that habituation counts more than anything else for all these things. You only need to watch a group of cyclists going up a hill to realise that they don't all pedal the same; some would think it hard work to spin a small gear, others to mash a big one. Same deal with crank lengths really.

cheers
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MikeF
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Re: Crank length

Postby MikeF » 7 Jan 2020, 1:31pm

brynpoeth 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.
"It takes a genius to spot the obvious" - my old physics master

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RickH
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Re: Crank length

Postby RickH » 7 Jan 2020, 1:43pm

MikeF wrote:
brynpoeth 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.

brynpoeth
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Re: Crank length

Postby brynpoeth » 7 Jan 2020, 1:54pm

Would it be worth having adjustable cranks to help going uphill on fixed?

Probably someone has already thought of that
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Marcus Aurelius
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Re: Crank length

Postby Marcus Aurelius » 7 Jan 2020, 2:26pm

RickH wrote:
MikeF wrote:
brynpoeth 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.

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.
Last edited by Marcus Aurelius on 7 Jan 2020, 2:31pm, edited 1 time in total.

brynpoeth
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Re: Crank length

Postby brynpoeth » 7 Jan 2020, 2:30pm

Marginal gains, what length cranks do professional racing cyclists use?
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Marcus Aurelius
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Re: Crank length

Postby Marcus Aurelius » 7 Jan 2020, 2:32pm

brynpoeth 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 ).