Lengthening and shortening cranks - Have you ever experimented with changing your crank length?

[Bimble]

I briefly skimmed an article about Pro Tour teams tending towards shorter cranks in the belief it was more efficient / aerodynamic.

My very basic understanding of physics tells me a longer crank will lead to a larger circumference in the pedal turning circle, and for a single turn in any one gear will require slightly less power than a shorter crank (tighter circle) in the same gear. A shorter crank, on the other hand, will require slightly more power for the same gear but will turn quicker (due to the smaller pedal turning circumference). [This may be completely wrong ... ]

I imagine your body / leg length will have some impact too, and it possibly doesn't make much difference for a short ride, but can crank length improve the quality of your ride or give you a better "bike fit"? And have you ever experimented to find crank lengths that better suit you and your body / riding style or needs?

 

[midlife]

The work you do is the same (force times distance). At higher cadence there may be benefit in shorter cranks as the distance traveled by the foot is less, I've used cranks if all lengths and can't say I've noticed a difference.

Shaun

 

[S-Express]

but can crank length improve the quality of your ride or give you a better "bike fit"?

Crank length is all about leg length in relation bike fit. That's about all there is to it.

http://myworldfromabicycle.blogspot.co.uk/2010/08/dude-your-crank-lengths-fine-you-just.html

Summary: Common crank sizes are nearly all equal in efficiencies. Cranks size can be chosen for reasons such as ground clearance for cornering/obstacles (shorter), aerodynamics (shorter), or rehabilitation/flexibility (longer). Sprinting 120 rpm is best. 60 rpm is better than 100 rpm aerobically (generally lower cadences are more efficient than higher). Natural pedal stroke is best (do not pull up), crank length has no effect on fatigue, no effect on metabolic efficiencies and very small effect on maximum power. A big gear sprint is better than a small gear sprint (for 30seconds) .

 

[Sharky]

Yes. Currently ride with 145 or 150mm. No real impact to leverage as you just ride a lower gear and increase cadence. But raising the saddle by 2 to 3 cm makes the angle at the hips and knees less severe, so more comfortable and because you can bend further down, potentailly more aero.

 

[Ian H]

Mike Burrows has been an advocate of very (by modern standards) short cranks. The pros have ridden different lengths according to whatever current fashion research indicated.

Merckx rode 180s for much of his career.

 

[S-Express]

Merckx rode 180s for much of his career.

So did Indurain. Their height and leg length justified it.

 

[Ian H]

So did Indurain. Their height and leg length justified it.

Well, yes—according to current orthodoxy. The science behind most of this stuff is shaky.

 

[boydj]

As I've said on another thread, crank length affects the range of motion that your legs go through. If you are short, long cranks will have your knees banging your chest, with the saddle sitting on the top tube.

 

[Bimble]

It's got me wondering whether I ought to try shorter cranks at some point in the future?! Whilst I'm nearly 6ft, I have a long torso and short legs by comparison with guys of a similar height, and on a couple of bikes have had the saddle at what seems a much lower height than those guys (sitting on the top tube whilst they have seemingly masses of seat stem on show). I wonder, too, if I might benefit from a slightly smaller frame size?

Mind you, the short legs thing never seemed to get me much sympathy from the PE teacher when I invariably came last on the cross-country runs.

 

[byegad]

Very much of this is opinion over science. I've seen different people argue they were way faster on either very long and very short cranks.

I'm 5'7" with a long torso and short legs and use standard 170mm cranks. The reason being that, when I run out of low gears as I climb very steep hills*. I keep moving by pushing on the pedal rather than spinning. On a recumbent trike my climbing speed can be <2mph and getting off and pushing is not an option unless I walk doubled over as I push! A short crank would mean I'd have to push harder as I'd have less leverage.

* For very steep, think >20% gradient.

 

[S-Express]

A short crank would mean I'd have to push harder as I'd have less leverage.

That's the theory, but the differences in leverage are so miniscule as to be practically irrelevant.

 

[fossyant]

Makes no difference. I have 170's on my road bikes and 175's on the MTB's.

 

[byegad]

That's the theory, but the differences in leverage are so miniscule as to be practically irrelevant.

Between 150 and 175mm the difference is enough!

 

[S-Express]

Between 150 and 175mm the difference is enough!

Presumably you've quantified it then. What is the difference?

 

[fossyant]

Between 150 and 175mm the difference is enough!

That's probably enough to make your knees go pop.

I've always stuck with 170 as it allows all my bikes to be set up the same. I did get a replacement crank set for my MTB, that was only available in 175 - didn't notice any difference, and then my next MTB came with them already. 175 seems pretty standard on MTB's. A mate always had 175's but he was over 6 foot tall.