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Deciding factor for crank length(15 posts)

Deciding factor for crank lengthbikefreax
Oct 20, 2003 4:01 AM
I need help on crank size. I am getting a new frame that is 1.5 cm smaller on standover. This means I will need to raise my seat, or maybe get longer cranks? I currently have 175mm cranks and was wondering what changing to the next size up might do? What is the determining factor as to size of crank arm?
not relevant to frame size...C-40
Oct 20, 2003 6:13 AM
You should use the exact same saddle height, regardless of the frame size that you choose. You would not raise your saddle (reletive to the bottom bracket) due to a change in frame size. There would be no reason to change crank length if you are satisfied with your current 175mm length.

The choice of crank length depends on your cycling inseam (not pants inseam) or saddle height and your pedaling style. A 5mm change in crank length only changes leverage by about 3% which is a very small amount. Using longer cranks increases leverage slightly but may reduce cadence.
not relevant to frame size...MShaw
Oct 20, 2003 10:03 AM
Somewhere out on the web there's a site that tells you what your "optimal" crank length is based on thigh length, etc. I found it once but can't remember where it went...

Since your legs don't change with frame size, run whatever's right for your legs not the bike size.

No optimal crank length formulaKerry Irons
Oct 20, 2003 5:09 PM
Every increment of crank length is about 2-3 rpm. Going from 170 to 175 is about 5 rpm. If your total range of cadence is 50-60 up a steep hill to 80 on the flats, you need to have a wider range of gears to get you everywhere you want to go. If you can comfortably spin up to 110, then you can go 35 in a 53/13. If 80 is your comfortable cadence, you need a 53/11 to just go 30.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. What little research has been done on crank length suggests that people adapt to different crank lengths and there is no optimum or formula related to body proportion. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.
Kerry, don’t understand your RPM thinking...TFerguson
Oct 21, 2003 1:43 PM
If you want to change your cadence, just shift. If I change my crank length from 165 to 175, all I have to do is shift one cog and I'm back to the same cadence. My legs can't tell why it's easier or harder.

It's about "foot speed"Kerry Irons
Oct 21, 2003 4:34 PM
A longer crank requires your feet to move faster to create the larger circle. Experience tells most people that they like their foot speed to remain roughly constant, so when they go to longer cranks, they reduce their cadence. The problem from this is longer duration muscle contractions and some additional stress on the knees. It's not many rpm so it's not a big issue, but it is a real difference for most people. Your legs can tell that they are making larger or smaller circles.
Got it. Thanks. (nm)TFerguson
Oct 21, 2003 6:04 PM
leverage & It's about "foot speed"charlieboy
Oct 22, 2003 12:58 AM
Longer cranks = more leverage, so while a longer crank may require higher footspeed less force is required to shift a given load than is required using a shorter crank. So my theory is that I find it easier to spin a longer crank than a shorter one - that's my theory and it probably only works for me!

Do we really care about all this physics, or is this just something we wrestle with while spending long dark winter nights plugging away on the turbo trainer?:)
Leverage I don't buy...TFerguson
Oct 22, 2003 5:10 AM
Your legs can't tell whether it takes less force to pedal because of longer crank arms or because you went to a lower gear.

Leverage I don't buy...charlieboy
Oct 22, 2003 6:05 AM
Mine can, in the same gear. Maybe yours can't, but the physics of levers still applies;)

I guess this all comes down to riding what you like, not what someone else says is good for you.

Still, it gives us all a chance to be clever dicks ( speaking as one myself, of course!)
It's you interpretation of the physics I don't buy...TFerguson
Oct 22, 2003 6:21 AM
As far as "feel", that I can't argue. But whether you add 10mm to your crank arms or shift your cogs one gear lower, the power required is the same. I know you said, "in the same gear", but that's the point. Why wouldn't you shift? It seems a lot easier than changing crank arms.

physics... It's you interpretation of the physics I don't buy...charlieboy
Oct 22, 2003 6:58 AM
Go here for the physics

Salient points...

'In words, the force passed to the road scales with the length of the pedal arm, if all other parameters remain fixed.

Therefore a longer pedal means more force to the road, so the bike accelerates faster for the same force provided by the cyclist.'


'So the cyclist can provide less force to achieve the same speed.

A longer pedal arm is an advantage from this simple argument.'

I've had enough for today - it's been emotional.
It's not just about force but also biomechanics....divve
Oct 22, 2003 7:25 AM
...a simple test would be to draw circles in the air with your increase the circle size until your whole arm has to move. Which is quicker? You'll probably find that with-in a certain range there's a happy medium. Go much smaller or larger and it becomes inefficient and uncomfortable to do.
Try tooMShaw
Oct 22, 2003 8:22 AM
They have a power given crank length section that was an eye opener!

Now I know why I'm having problems keeping up with Shaun Wallace!

go long young man re: Deciding factor for crank lengthcharlieboy
Oct 22, 2003 2:02 AM
For some really challenging stuff on crank length see here...

Please no flames on this - I only offer it for a widening of the argument!!;)