Crank length: science!

reacher

i cant see that, if it means you spin faster on shorter cranks then surely at some level that's making a difference to the forces exerted or how the force is applied same with longer cranks which is what i use. i'v played around a lot with gearing and cadence and its quite a difference in how you can apply it, rightly or wrongly longer cranks do enable you to exert considerably more force climbing at a low cadence, or so it seems to me anyway

joncomelately

964Cup wrote:

This was a question about the underlying physics, which I hoped someone had already worked out, or could work out faster than me. I've done it now.

Sorry, I thought my first two posts (4th and 6th in the thread) had answered that question....

reacher

Right, seems I was completely wrong
Can anyone tell me, I'v been working to improve my cadence climbing, if I changed to a shorter crank would my cadence automatically go up ?
Or is it you are more a spinner or a grinder and thats it and you either spin a short or long crank fast or slow regardless

rower63

Power = torque x revs/min
Torque = pedalforce x crank length
IE Power = pedalforce x crank length x revs/min

To make sense of the little formulae above, consider keeping, say, power constant, and seeing how the other things need to change to preserve that power.
To maintain constant power after reducing the crank length, you need to either increase revs at same pedal pressure, or increase pedal pressure whilst holding revs constant.
Also consider an absurd extreme: if you double the crank length, to keep power constant you need to halve the pedalforce at constant revs, or halve the revs at constant pedalforce.

A little thought should reveal that crank length is a gearing issue: increasing the crank length simply reduces the gearing, but in a different place from where you normally think of it on a bike.


ETA ... so the "answer" to those who ask "which is better?" is, like all gearing questions, "it depends what you prefer". Tony Martin likes 58-tooth, Lance Armstrong used to like to spin.


NB Torque is NOT work, even though they have the same units. This is because they’re actually vectors, and the distance being multiplied by the pedalforce to produce torque is perpendicular to the force, whereas “work distance” must be in the same direction as the force.

rower63

964Cup wrote:

... The point, apart from comfort factors, of longer cranks is leverage, right? ...

so yes, right!

svetty

reacher wrote:

Or is it you are more a spinner or a grinder and thats it and you either spin a short or long crank fast or slow regardless

This

thomasmorris

In terms of the physics, energy / power cannot be created or destroyed. Crank length will not change the amount of energy wasted (drive train friction) so there is no benefit to smaller or longer cranks in terms of energy transfer (i.e. power).

Therefore no physics equation you can do will tell you which crank length is more "efficient".

However, there is human biology / physiology. This dictates your optimum foot speed and range of movement for power generation.

So the obvious conclusion is: changing crank length is like changing the size of any other lever in bike system (chainrings, cassette, wheel size) and will not magically generate more power. Ultimately the gearing of the complete bike system can be calculated as foot speed versus ground speed. Yet it would be totally illogical to alter you crank length just to get an easier or harder gear as you could far more easily just change gear!

SO with a full range of gears and leverage ratios, crank length become a single consideration for getting the right set up physiologically / bio-mechanically that will allow you to put more power in to the system.

Therefore It's impossible to answer the OPs question without turning it into a person specific choice based on individuals bio-mechanics, physiology and preferences.

thomasmorris

rower63 wrote:

964Cup wrote:

... The point, apart from comfort factors, of longer cranks is leverage, right? ...

so yes, right!

Longer cranks do increase leverage, but by a tiny amount when compared to the rest of the system. So increasing leverage is not really 'the point' of them. For example shifting down just one gear will effect you leverage much more greatly than 5 mm of crank length.

So it comes back to this: which crank length allow you to produce the most power. This will be a bio mechanical / physiological question regarding muscle firing patterns, range of effective movement etc. etc.

rick_chasey

In my experience, and iIve fiddled about a lot with cranks, it's not so much about "how much power" you can produce for a given crank length, but, as a short arse, how comfortable it is and how sore (or not) my hipflexors and back are.

I do find shorter cranks spinnier, and that also helps the back issues I suspect.

darkhairedlord

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

StillGoing

darkhairedlord wrote:

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

Not exactly correct. The crank length will part determine the hip angle at the top of the pedal stroke. If someone is on a bike slightly too big, a shorter crank will help achieve the correct angle for the hips at the top of the stroke and allow them to get on the power stroke less restricted.

darkhairedlord

philthy3 wrote:

darkhairedlord wrote:

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

Not exactly correct. The crank length will part determine the hip angle at the top of the pedal stroke. If someone is on a bike slightly too big, a shorter crank will help achieve the correct angle for the hips at the top of the stroke and allow them to get on the power stroke less restricted.

Crank length has nothing to do with frame size.

StillGoing

darkhairedlord wrote:

philthy3 wrote:

darkhairedlord wrote:

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

Not exactly correct. The crank length will part determine the hip angle at the top of the pedal stroke. If someone is on a bike slightly too big, a shorter crank will help achieve the correct angle for the hips at the top of the stroke and allow them to get on the power stroke less restricted.

Crank length has nothing to do with frame size.

You're completely missing the point that was made. It didn't mention anywhere that crank length had anything to do with frame size, merely that the crank length can make a difference in achieving the hip angle at the top of the pedal stroke where the frame is slightly too big.

pbassred

The limiting factor in a TT position is generally hip angle and 'thigh hitting abdomen/chest' issues. These are eased with shorter cranks so shorter cranks are often advocated by 'marginal gains' time trial riders to enable a lower position.

A lot of people here are focusing on physics and marginal gains. The difference between the 2 extremes is 10/175*100 = 5.7% You haven't changed the power in your legs. You've swapped leverage for speed - which is exactly how your gears work

I guess I must be the only overweight cyclist here. I swapped my 175 for 165mm last November. I did it for an entirely different reason. My belly is a reality. My legs are fixed. With the pedal at the bottom of the stroke. My saddle was now 10mm too low. so I raised it 10mm. However with the pedal at the top of the stroke my foot was now lower, so 10mm (short crank) raised PLUS another 10mm (raised saddle). I gained 20mm for my legs against my midriff.

My ride position is far more comfortable - even on the drops. not straining against my own body is far more efficient = Further/faster.

964cup

thomasmorris wrote:

Therefore It's impossible to answer the OPs question without turning it into a person specific choice based on individuals bio-mechanics, physiology and preferences.

Except that it's perfectly possible to answer my question. I've answered it. Foot speed is constant for a given power output irrespective of crank length assuming constant pedal pressure.

Bonus answer: perceived effort for a given power output at a given cadence is inversely proportional to crank length as a result of leverage (assuming perceived effort = pedal pressure).

Others may have used this as a retread of the interminable which crank length/what gearing/which bike etc threads that we all know and presumably love, but I asked a question because I couldn't be arsed to do some maths, then answered it because it turned out I could be bothered after all.

Coming soon - included hip and knee angles, trig edition.

lesfirth

philthy3 wrote:

darkhairedlord wrote:

philthy3 wrote:

darkhairedlord wrote:

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

Not exactly correct. The crank length will part determine the hip angle at the top of the pedal stroke. If someone is on a bike slightly too big, a shorter crank will help achieve the correct angle for the hips at the top of the stroke and allow them to get on the power stroke less restricted.

Crank length has nothing to do with frame size.

You're completely missing the point that was made. It didn't mention anywhere that crank length had anything to do with frame size, merely that the crank length can make a difference in achieving the hip angle at the top of the pedal stroke where the frame is slightly too big.

Oh yes you did and you have just done it again!

StillGoing

lesfirth wrote:

philthy3 wrote:

darkhairedlord wrote:

philthy3 wrote:

darkhairedlord wrote:

Cranks as long as your knees will let you. If in doubt 172.5 will suffice.

Not exactly correct. The crank length will part determine the hip angle at the top of the pedal stroke. If someone is on a bike slightly too big, a shorter crank will help achieve the correct angle for the hips at the top of the stroke and allow them to get on the power stroke less restricted.

Crank length has nothing to do with frame size.

You're completely missing the point that was made. It didn't mention anywhere that crank length had anything to do with frame size, merely that the crank length can make a difference in achieving the hip angle at the top of the pedal stroke where the frame is slightly too big.

Oh yes you did and you have just done it again!

Oh no I didn't. :roll:

Moonbiker

Won't shorter cranks mean you have to raise your saddle so you are less aero?