Crank Lengths - What Impact?

[e-rider]

I used to use 170mm cranks until I bought a bike that came with 175mm cranks.

Now 170mm cranks feel too short and I'd never consider anything other than 175s for normal use (I can tell the difference between 170 and 175s)

I do have long legs though!

 

[e-rider]

the circumference of the circle you pedal increases by about 3.2cm with each revolution (changing between 170 and 175) if you had one crank of each length, everyone would be able to notice this.

 

[Amanda P]

If you are so petite that you can't get the boom of your 'bent in far enough to fit, short cranks (really short, like 150mm) will help.

That's what Mrs Uncle Phil and I have learned, anyway. (She's the small one).

 

[jimboalee]

I have a bike in my shed that weighs 24lb. It has a single speed 48" gear.

Now you would think it would zip up hills no problem. Wrong! It has 150mm crank length.
Getting up even an 8% is a struggle.

I need a 170mm cottered set with a 52 tooth ring for a 1/8" chain.

 

[brodie]

There's that bloke in the States who's a great advocate of matching crank length to leg length, but to be meaningful, he reckons the size option in cranksets would have to be from at least 150 to 200 not the 165/170/175 options that are around. He says those sorts of variations are just fashion.

That's a good point. A tall rider using even 180 cranks (supposedly outrageously long) would be using relatively shorter cranks than a short person using 165 cranks.

 

[asterix]

My frames are 60-62cm but I don't notice that one has 175mm cranks and the others 170mm.

 

[Crackle]

If you want to know the difference, try riding a bike with one crank 5mm longer than another. Many years ago I did just that for a few months. Times were harder then than now. The difference is appreciable, easily noticed. I used to favour longer cranks which seemed to suit my slower cadence. These days I spin more and shorter cranks seem better for that. Suiting it to leg length makes more sense. I know my Audax came with different length cranks on different frame sizes.

 

[montage]

I have heard longer cranks can damage your knees? This a myth?

 

[jayce]

I have 3 bikes and my dolan has 175 my cannondale 170 and the u6 is 170 im 5 9. 30" leg and they are all hard to pedal up hill ,i need a little engine on the back to help me along

 

[asterix]

I have heard longer cranks can damage your knees? This a myth?

No, it's quite correct. Although the cranks would obviously have to be extraordinarily long.

 

[jimboalee]

Its similar to putting a 'stroker' crank and shorter rods in a motorcar engine.
The pressure generated by the combustion is applied at a wider radius, therefore resulting in a slightly higher torque.
The penalty is the increased moment of inertia, so top end revs are pulled down.


Translating to bicycle language, the pressure applied by your foot is applied at a wider radius - giving more torque.
There IS a Moment of inertia penalty resulting in an inability to spin at the revs you did on shorter cranks.

Another unspoken tactic on the pro circuit is changing crank length to suit the terrain being ridden. Mountains = longer cranks.